US3077421A

US3077421A - Processes of producing tin-nickelphosphorus coatings

Info

Publication number: US3077421A
Application number: US95262A
Authority: US
Inventors: Budininkas Pranas
Original assignee: General American Transportation Corp
Current assignee: General American Transportation Corp
Priority date: 1961-03-13
Filing date: 1961-03-13
Publication date: 1963-02-12
Anticipated expiration: 1980-02-12
Also published as: CH441924A; LU40878A1; GB982307A; ES272010A1; BE611315A

Description

Feb. 12, 1963 P. EBUDININKAS 3,077,421

PROCESSES OF PRODUCING TiN-NICKEL-PHOSPHORUS COATINGS Filed March 1s, 1961 FIG. @5455 mm m m/ CHEM/64L NICKEL Fur/Iva marg Q INTERFACE mar.- FQ-NI-P Fe BASE ME 74L DIFFUSION rm/ PLAT/N6 011m? 510m 2 PORT/0N Ava 0J5 MIL CHEM/64L NICKEL PLAT/N6 ALLOY DISTRIBUTED //v M F 4 03 MIL. r//v o/sm/az/rm m/ Ill '03}.

CHEM/64L NICKEL PLAT/N6 ALLOX /.4 M/LS CHEM/64L NICKEL PLAT/N6 may 92 M 8 P 02 MIL INTERFACE ALLOY Fe-N/-P Fe-BASE METAL INV EN TOR. Pranas Budminlras PROCESSES OF PRODUCING TIN-NICKEL-PHOSPHORUS COATINGS Filed March 13, 1961 Feb. 12, 1963 P. BUDININKAS 4 Sheets-Sheet 3 REACTION TEMP 630C, Sn C/z, TEMP 480 C, Nitrogen-Hydrogen 2"! MIXTURE TOTAL GAS FLOW 60cc/min.

FIG: 6

0. P m M T E T 2. 2 m mnwu w A? c 6 C615 C502 65 0 W awi vw F 6 RF A M M ESEO T T l G M 0 N N 6 o A mmm wm 0 RNWn u A m 5 A J z m 0 0. O O 0 -QE Q wit 63.599 .3 32x25 TIME OF DEPOSIT/0N, HOURS 0 TOTAL GAS FLOW 60 cc/min.

7 5 0 m m F N. 1. U T 15 mm 5 m a C c 0 1 0 mm WW mm 6 FF 0 g N 66 R L C MM M 00 n 0 TT m A 5. MW -m W N N M w u 0 n". Y Y M H 6 m N m M N Q 0 853338 -35 satqg 6 @3525 United States Patent 6 3,677,421 PROtIESSES F PRGDUCING THN-NHCKEL- PHGESPHQRUS COATINGS Pranas Budininkas, Gary, Ind, assignor to General American Transportation Corporation, Chicago, ill, a

corporation of New York Filed Mar. 13, that, filer. No. 95,262 26 Claims. (Cl. 117-7'1) This invention relates to processes of producing protective metal coatings upon articles, and particularly tinnickel-phosphorus alloy coatings upon the articles.

i-ieretoiore, articles have been provided with nickelphosphorus coatings upon the surfaces thereof by chemical deposition from a plating bath of the nickel cationhypophosphite anion type and such coatings have been particularly advantageous because they can be applied to articles having a variety of compositions, sizes, shapes and configurations. Although such nickel-phosphorus coatings attord good protection in a variety of uses and a degree of protection at least equal to that of electrolytic-ally deposited nickel, efforts have been made to improve the protective properties thereof because of the great convenience in producing such nickel-phosphorus coatings on a wide variety of base elements having substantially any desired shape. For example, various physical treatments of the nickel'phosphorus coating have been developed to improve the protective properties thereof, such as the heat treatment disclosed in US. Patent No. 2,908,419, granted on October 13, 1959, to Paul Talmey and William J. Crehan.

Thus, it is a general object or" the present invention to provide an improved process of producing a protective coating for an article, and particularly an improved tinnicke phosphorus coating, wherein a nickel-phosphorus coating is first produced by chemical deposition trom a plating bath of the nickel cation-hypophosphite anion type and then modified by diffusion tin plating.

Another object of the invention is to provide an improved process of producing an article of manufacture comprising a body carrying an improved protective coating of nickel-phosphorus alloy intimately bonded thereto, the outer skin of the coating having tin diffused therein.

Yet another object of the invention is to provide an improved process of producing an article of manufacture comprising a body having a heat-hardened protective coating intimately bonded thereto, wherein the coating comprises a nickel-phosphorus alloy carrying substantial tin thermally diiiused in the outer skin portion thereof.

Still another object of the invention is to provide an improved process of producing an article of manufacture including a body having an improved protective coating thereon, wherein the coating comprises a tin-nickel-phosphorus alloy.

Yet another object of the invention is to provide an improved process of producing on an article of manufacture an improved protective coating of tin-nickel-phosphorus alloy which shows corrosion resistance toward basic solutions, neutral solutions and acidic solutions superior to that of electrolytically deposited nickel, nickelphosphorus coatings, and electrolytically formed codeposits of tin and nickel.

Still another object of the invention is to provide an improved process of producing a protective coating including a nickel-phosphoms alloy having a vapor deposited tin coating thereon.

Yet -another object of the invention is to provide an improved process for enhancing the protective properties of a nickel-phosphorus coating chemically deposited from a plating bath of the nickel-cation-hypophosphite anion type.

Still another object of the invention is to provide an improved process of providing a tin diffusion coating upon a nickel-phosphorus alloy.

Yet another object of the invention is to provide an improved process of converting the outer skin of a nickelphosphorus coating to a tin-nickel-phosphorus alloy.

A further object of the invention is to provide an im proved process of forming protective coatings upon base metal workpieces whereby to produce tin-niekel-phosphorus alloy coatings thereon.

Further features of the invention pertain to the particular arrangement of the steps of the processes of producing protective coating on articles or workpieces, whereby the above-outlined and additional operating features thereof are attained.

This invention, both as to its organization and method of operation, together with further objects and advantages thereof, will best be understood by reference to the following specification, taken in connection with the accompanyirg drawings, in which:

FIGURE 1 is a view in section through a typical article that can be coated in accordance with the present invention, the article being illustrated as comprising a base metal such as iron or the like;

FIG. 2 is a view in cross section similar to FIG. 1 and showing a chemical nickel plating upon the upper surface of the base metal;

FIG. 3 is a view similar to FIG. 2 showing a diffusion tin plating outer skin portion on the chemical nickel plating and being of the character and made in accordance with the principles of the present invention;

FIG. 4 is a view similar to FIG. 3 but on a large scale and illustrating the coating obtained by one preferred embodiment of the present invention, the outer skin portion of the coating having separated into three discrete layers;

FIG. 5 is a view partly diagrammatic and partly in cross section of an apparatus suitable for carrying out the process of the present invention to produce an article having a' protective coating in accordance with the principles of the present invention;

FIG. 6 is a graph showing the relation between the time of deposition of tin dilfusion coatings on nickelphosphorus coatings and the calculated thickness of the tin dilfusion coatings, this relationship being illustrated for three separate combinations of the process variables;

FIG. 7 is a graph showing the relationship between the calculated thickness of tin diilusion coatings on nickelphosphorus coatings and the ratio of nitrogen to hydrogen by volume in the reducing gas;

FIG. 8 is a view partly in cross section and partly schematic illustrating the manner in which the principles of the present invention can be applied to a hollow article made from several separate pieces whereby to produce a protective coating in accordance with the present invention;

FIG. 9 is a side elevational view of a railway tank car provided with a tank body incorporating a liner produced in accordance with the process of the present invention;

FIG. 10 is a greatly enlarged fragmentary view of a portion of a wall of the tank body of the railway car,

taken in the direction of the arrows along the line -10 in FIG. 9; and

FIG. 11 is a greatly enlarged fragmentary sectional view of another portion of the wall of the tank body of the railway car, taken in the direction of the arrows along the line l1ll in FIG. 9.

There is shown in FIG. 1 of the drawings a diagrammatic representation of an article generally designated by the numeral 10 which may be made of a base metal such as iron or the like. in accordance with the present invention, a nickel-phosphorus protective coating is formed thereon and after heat treatment there is produced the article illustrated in FIG. 2 in which a chemical nickel plating layer 22 containing, for example, about 92% nickel and 8% phosphorus by weight is shown on the exposed surface of the base metal 10 and intimately bonded thereto by means of an interface alloy layer 21 comprising essentially iron and nickel and phosphorus. In accordance with the present invention, the outer skin of the chemical nickel plating layer 22 can have tin applied thereto and diifused thereinto to produce a new article of FIG. 3 having an alloy la or 31 on the outer surface thereof, the alloy layer 31 being a tin-nickelphosphorus alloy having a variable content of tin therethrough with the tin being more highly concentrated adjacent to the outer surface thereof and gradually decreasing in concentration toward the layer 22. For example, the alloy layer 31 may have an average composition of approximately 45% tin, 51% nickel and 4% phosphorus by weight, thereby to provide the new article 30 having corrosion resistance properties superior to that of the article 20 illustrated in FIG. 2. As will be described more fully hereinafter, under certain operating conditions and in accordance with one preferred embodiment of the present invention, the diifusion tin plating outer skin portion 31 of FIG. 3 can be transformed so that it in fact contains three separate and distinct layers which are diagrammatically illustrated in FIG. 4 in the article 50. More specifically, the article comprises, for example, a base metal 10 on which is superimposed the interface alloy layer 31 which is approximately 0.2 mil thick and comprises essentially iron and nickel and phosphorus. Upon the alloy layer 21 is the chemical nickel plating alloy layer which may have a thickness in the order of 1.4 mils and has a typical composition of 92% nickel and 8% phosphorus by weight. Disposed upon the alloy layer 22 is the layer 31 which in fact includes three separate layers, namely, an outer layer 41 approximately 0.25 mil thick high tin alloy, an intermediate layer 42 approximately 0.l5 mil thick and comprising the nickelphosphorus chemical nickel plating alloy distributed in tin, and a lower layer 43 approximately 0.3 mil thick and comprising tin distributed in the nickel-phosphorus nickel chemical plating alloy, the three

layers

41, 42, and 43 corresponding to three different phases of the tin-nickelphosphorus system.

it has now been found that the protective layers of the article 30 in FIG. 3 and the objects and advantages set forth above can be obtained by first producing a nickelphosphorus chemical nickel plating coating upon the surface of the base material 10 by chemical deposition from a plating bath of the nickel cation-hypophosphite anion type and then converting to a tin-nickel-phosphorus alloy the outer skin of the coating by simultaneously depositing metallic tin upon the outer surface of the coating and by diffusing and alloying the tin into the outer skin of the coating. The tin is preferably deposited upon the nickelphosphorus alloy by heating the coating to a temperature above the melting point of tin and below the melting point of the nickel-phosphorus coating and reducing to metallic tin a compound of tin upon the outer surface of the heated coating. The compounds of tin useful in the present invention are the tin halides, either stannous or stannic compounds being useful for this purpose, the preferred compound being stannous chloride. The tin compound is preferably reduced by means of a reducing gas which contains hydrogen and can be produced by mixing nitrogen and hydrogen, by cracking ammonia or by thermally cracking natural gas. It has been found that coatings having improved appearances are obtained if sufficient hydrogen is present in the reducing gas, gray coatings of tin being obtained if the ratio of nitrogen to hydrogen by volume is above 3.5 and more desirable semi-bright tin deposits being obtained if the ratio of nitrogen to hydrogen by volume is less than 3.5, a preferred concentration of the hydrogen in the reducing gas being in the range of 25% to about 40% by volume. The reaction is carried out at a temperature preferably above the melting point of tin, i.e., 332 C., and below the melting point of the nickel-phosphorus alloy, i.e., 880 C., the preferred range of temperature being from about 400 C. to about 630 (1., the optimum operating temperature being 630 C. In carrying out the process the tin compound is volatilized and mixed with the reducing gas and the resultant mixture applied to the heated nickelphosphorus surface, the various reaction variables being selected so that the rate of deposition of metallic tin upon the nickel-phosphorus surface is less than the rate of diffusion of metallic tin into the nickel-phosphorus coating. It also has been found that it is desirable to have a substantial nickel-phosphorus coating to give good cor rosion protection and preferably the coating should be at least about 2 mils thick and may be even thicker to give optimum corrosion resistance for the tin-nickel-phosphorus alloy coating formed.

The nickel-phosphorus layer 22 may be produced from any of the well-known nickel cation-hypophosphite anion plating baths. More particularly, the chemical plating bath employed may be any one of a number of available compositions, such, for example, as disclosed in US. Patent No. 2,532,283, granted on December 5, 1950, to Abner Brenner and Grace E. Riddell; US. Patent No. 2,658,841, granted on November 10, 1953, to Gregoire. Gutzeit and Abraham Krieg; or U.S. Patent No. 2,658,842, granted on November 10, 1953, to Gregoire Gutzeit and Ernest J. Ramirez. However, it is preferable that this chemical plating bath be of the composition of that disclosed in US Patent No. 2,822,294, granted on February 4, 1958, to Gre oire Gutzeit, Paul Talmey and Warren G. Lee, since this particular plating bath is admirably suited to a continuous plating process. The chemical plating bath of the Gutzeit, Talmey and Lee patent mentioned essentially comprises an aqueous solution of a. nickel salt, a hypophosphite, a complexing agent selected from the group consisting of lactic acid and salts thereof, and an exalting additive selected from the group consist ing of propionic acid and salts thereof. In this plating bath, the absolute concentration of hypophosphite ions is within the range 0.15 to 1.20 moles per liter, the ratio between the concentrations of nickel ions and hypophosphite ions is within the range 0.25 to 1.60, the absolute concentration of lactic ions is within the range 0.25 to 0.60 mole per liter, the absolute concentration of propionic ions is within the range 0.025 to 0.060 mole per liter, and the pH is Within the approximate range 4.0 to 5.6.

In the chemical plating of the upper exposed surface of the base metal 10, the plating bath is continuously circulated across the exposed surface and through the associated continuous plating system, not shown, with regeneration of the plating bath, as time proceeds, in order to maintain substantially the composition thereof set forth, as is disclosed in US Patent No. 2,717,2 8, granted on September 6, 1955, to Paul Talmey and William J. Crehan. In this method, the temperature of the plating bath contacting the base metal 10 is maintained near the boiling point thereof, at about 210 F., so as to obtain a high plating rate in the production of the coating 22; and the plating step is continued throughout an appropriate time interval in order to obtain the desired thickness of the coating 22, the plating rate of the plating bath men tioned being about 1 mil per hour. Normally the thickness of the coating 22 is at least about /2 mil and usually in the approximate range 1 to 5 mils, a thickness of about 1.5 to 2.0 mils being recommended for general utility.

The coating 22, as chemically deposited, is in the form of a layer intimately bonded to the surface of the base metal It and comprises an amorphous solid material consisting essentially of a metastable undercooled solution of phosphorus in nickel, and including about 88 to 94% nickel, and 6 to 12% phosphorus by weight, the coating 22 being characterized by adhesion, wear resistance, and resistance to corrosive attack by ordinary acids, bases, and other reagents, comparable to electro-deposited nickel. As chemically desposited, the coating 22 has a hardness corresponding to a Vickers hardness number (V.H.N.) of about 525. The variable composition of the coating 22 with respect to the inclusion of nickel and phosphorus is dependent on pH and, to a limited extent, upon the concentration of the hypophosphite in the plating bath, and also upon the concentration of phosphite in the plating bath, it being understood that as the plating reactions proceed at the catalytic surfaces of the base metal 10, the hypophosphite ions are oxidized to phosphite anions as the nickel cations are correspondingly reduced to metallic nickel and deposited upon the catalytic surface of the metal comprising the workpiece it With certain types of chemical nickel plating baths and utilizing certain systems of deposition, it is possible to obtain a coating 22 having nickel and phosphorus content outside of the ranges specified above and more particularly it is possible to obtain coatings including from about 85% to 97% nickel and from about 3% to 15% phosphorus by weight.

As noted above, the chemical deposition of the coating 22 upon the workpiece It) involves the catalytic plating reactions mentioned, whereby the workpiece 10 must be formed of catalytic material or must have growth nuclei of catalytic material thereon. While there are a great number of catalytic materials upon which the chem ical deposition may take place, the ordinary catalytic materials conventionally comprise iron and its alloys, copper and its alloys, and aluminum and its alloys. For example, the material of the workpiece might be: iron, carbon steel, chrome steel, cobalt steel, silicon steel, manganese steel, nickel steel, molybdenum steel, nickelcobalt steel, nickel-chrome steel, chrome-manganese steel, manganese-molybdenum steel, chrcme-copper-nickel steel, copper, brass, bronze, silicon bronze, Phosphor bronze, beryllium-copper, cadmium-copper, chromium-copper, nickel-copper, aluminum, aluminum-brass and aluminumbronze. If the workpiece it) is not formed of one of the above materials, it may be desirable to affix to the exposed surface thereof growth centers of catalytic metal, the growth centers being applied, for example, by means of the process set forth in US. Patent No. 2,690,401, granted on September 28, 1954, to Gregoire Gutzeit, William J. Crehan and Abraham Krieg, and US. Patent No. 2,699,402, granted on September 28, 1954, to William I. Crehan. On the other hand, if the workpiece It) is formed of certain metals such as magnesium or titanium, it must be treated in a particular manner to obtain a satisfactory coating thereon, the method of treating titanium, zirconium and hafnium being set forth in US. Patent No. 2,928,757, granted on March 15, 1960, to Warren G. Lee and Emilian Browar, and the method of treating articles made of magnesium and its alloys being set forth in the copending application of Pranas Budininkas, Serial No. 734,823, filed May 13, 1958, now Patent No. 2,983,634, granted on May 9, 1961.

In accordance with the present invention the workpiece it with the protective coating 22 of nickel-phosphorus thereon can be treated to increase the corrosion resistance of the coating 22 by forming a diffusion coating 31 of tin on the outer skin portion thereof. The diffusion tin coating process can conveniently be carried out in the apparatus 5% illustrated in FIG. 5 of the drawings. In

the system 5% ammonia gas may be used as asource of hydrogen that serves as the reducing agent and a halide of tin may be used as the source of tin. The ammonia gas is fed from a line 561 to a flow meter 502 from which the measured stream of ammonia gas flows through a line 5433 to an inlet of a ceramic tube 504 disposed within a furnace 565 and containing therein a mass 5% of steel wool; the steel wool when heated to about 930 C. catalyzes the cracking of ammonia gas to produce free nitrogen and free hydrogen. The mixture of nitrogen and hydrogen together with any other uncracked ammonia is fed by a line 597 to a first manually operable valve 5% and a second manually operable valve 519. The other side of the valve 51% connects with a line 5439 which is connected to a container or chamber 510 for the tin halide through an inlet connection 511 therefor. In order to heat the tin halide within the container 510 to the necessary vaporizin temperature, a suitable heater 512 which may be electrically operated surrounds the container 516. An outlet connection 513 is provided for the container 514? and is adapted to receive therethrough the stream of reducing gas that enters at the inlet 523, the stream of reducing gas sweeping across the surface of the tin halide in the container 510 to entrain and mix therein quantities of the vaporized tin halide. The outlet connection 513 is one leg of a Y-connection, another leg of the Y-connection being an elongated tube 514 extending through a heat exchanger 5% and into substantially the center of a reaction chamber 531. The heat exchanger 52th is of the counter-current type and includes a cylindrical housing 521 enclosing a substantial portion of the tube 514, a gas inlet 5'23 disposed within the container 531 and an exhaust 524 at the other end of the housing 521. Exhaust gases from the reaction chamber 531 can flow through the inlet connection 523-, through the space 522 between the tube 514 and the housing 521 and out through the exhaust 524, the outgoing gases giving up a substantial portion of the sensible heat therein to the incoming reaction gases to aid in raising the temperature of the reaction gases to that within the reaction chamber 531. The reaction chamber 531 is disposed within a furnace 53E capable of maintaining the reaction chamber 531 and the contents thereof at the desired reaction temperature and may be, for example, a Waltz furnace which is an automatic resistance type electric furnace. Means is provided to suspend one or more of the workpieces Zl) therein and a thermocouple well 534 is also provided to receive therein a thermocouple 535 connected to the controller for the furnace 539.

EXAMPLE 1 Utilizing the system 5% of FIGURE 5, nickel-phosphorus coatings were converted to tin-nickel-phosphorus coatings on mild steel specimens rectangular in shape and having approximately 20 sq. cm. of surface area. First a nickel-phosphorus coating was applied to the steel speci mens utilizing the method disclosed above and explained in greater detail in US. Patent No. 2,822,294 to provide thereon a nickel-phosphorus coating having a thickness of approximately 2 mils. A quantity of anhydrous stannous chloride was placed in the container 510 and the heater 512 placed in operation. The valve 568 was closed and a bypass valve 519 opened, the valve 519 interconnecting the line 507 with the reaction gas tube 514 via a line 516 and the third leg 515 of the Y-connection. The furnace 5% is then placed in operation and heated to about 930 C. after which ammonia gas is introduced into the tube 564 where about 99% of the ammonia gas is cracked to form a mixture of hydrogen and ammonia containing about 75% of hydrogen by volume. The gases are fed through the line 597 to the valve 519 to the line 516 to the inlet 515 and the conduit 514 into the reaction chamber 531 in order to purge the air from the reaction chamber 531 while the furnace 530 is being heated. After about 30 minutes of purging by means of the reducing am er gas, all of the furnaces are at the operating temperature, the furnace 505 being operated at approximately 530 C. and the furnace 512 at about 480 C. and the furnace 5.30 at about 630 C. The valve 508 is then opened and the valve 19 closed whereby the reducing gas is now conveyed by means of the line 509 to the input connection 511 of the chamber 510 whereby the reducing gas is mixed with the stannous chloride vapors within the chamber 10, the mixture being conveyed through the outlet conduit 513 and the tube 514 to the interior of the re-- action chamber 531. The mixture of the stannous chloride and the reducing gas impinges upon the surface of the workpiece 20 whereupon a portion of the stannous chloride is reduced to metallic tin, the metallic tin being; well above its melting point of 332 C. The molten tin proceeds to alloy with and diffuse into the nickel-phosphorus coating 2'1; upon the workpiece 20. The reaction gases then pass into the inlet connection 523 to the heat exchanger 520 and through the passage 522 therein and out through the exhaust 524, the exhaust gases serving to heat the incoming reaction gases whereby to conserve energy within the system. Preferably, the exhaust 52 4 is held under a pressure equal to approximately 2 inches of water so that the pressure within the reaction chamber 531 is slightly higher than atmospheric pressure. The reaction is continued for a suitable eriod of time and in a typical example the reaction proceeded for two hours. The workpiece 20 was then removed and was found to have gained 0.0721 gram in weight and it was found that the resultant diffusion tin plating had a thickness of 0.197 mil. The diffusion tin coating 31 was semi-bright and gray in color, was evenly applied and thoroughly covered the workpiece 20.

t was found that when the coating 31 was applied to a nickel-phosphorus coating 22 containing 92% nickel and 8% phosphorus by weight, the composition of the layer 31 was within the following ranges: from about 40% to about 50% tin, from about 46% to about 56% nickel, and from about 4% to about 5% phosphorus by weight. However, the composition of the coating 22 may vary substantially as has been explained above and may contain from about 85% to about 97% nickel and from about 3% to about 15% phosphorus by weight, and accordingly, the layer 31 may have a composition which varies substantially and may contain from about 1% toabout 50% tin and from about 46% to about 93% nickel and from about 3% to about 12% phosphorus by weight.

It has been found that there are several competing reactions which may be taking place within the reaction chamber 531 as follows:

(1) Catalytic reduction (when hydrogen is present).

A SnCls Hz Sn 2H0] (2) Autoreduction-oxidation.

A 2811012 :1." Sn SnCl (3) Replacement of nickel with tin.

A Ni snort Sn NiOlz When hydrogen is present, reaction No. 1 above predominates and there is substantially no tin deposited by means of the mechanisms of reactions No. 2 and No. 3. In the absence of hydrogen, the reaction No. 2 tends to dominate, deposition proceeding by the autoreductionoxidation process. In no event is reaction No. 3 of any substantial significance. None of the reactions have any substantial conversion at equilibrium but reasonable deposition rates are obtained under non-equilibrium conditions where reactants are provided in a surplus and the reaction products are continuously removed. The nickelphosphorus coating 22. has also been found to be a cata lyst for the reduction of tin in accordance with reaction g No. 1 above and is a substantially other metals including tin.

The ditiusion tin coating 31 is substantially superior to the nickel-phosphorus coating 22 as regards corrosion 'esistance to common chemicals and there is set forth in Table 1 a comparison of the corrosion resistance of the workpiece 30 with the corrosion resistance of the workpiece 20, the workpiece 30 having as the outer skin portion thereof a tin-nickel-phosphorus alloy and the workpiece 20 having as the outer skin portion thereof a nickelphosphorus coating as plated, the figures given being the corrosion rate in mils per year.

Table 1 [Corrosion rate, mils per year] etter catalyst than Niclrel- Tin-Nickel- Coinmodit-y Phosphorus Phosphorus Coating Coating Ammoniaterl Ammonium Nitrate, 30% ammonia and 40% ammonium nitrate, by Weight 2. 20 0.180 Ammonium Hydro do, 28 3091, ammonia, by

Weight O. 98 0. 020 Ammonium Nitrate 30%, by Weight 8.24 0. 053 Citric Acid, 5% by Weight 2. 65 0. 610 Dry Sherry Wine 0. 94 0.000 Ferric Sulfate, 1% by Weight 25. 0. 520 Lactic Acid, 50% by 'Weigbt 0v 03 0. 179 Lactic Acid, 80% by Weigl1t 0. 37 0. 006 Sautorne Wine 1. 36 0. 080 Sulfuric Acid, 10% by volume 16. 20 3. 450

The above corrosion rates were obtained by test procedure which included immersing the test specimens in the various solutions at 30 C. with complete immersion and no aeration. All the test specimens used had 20 sq. cm. of surface area and were immersed in 100 ml. or solution either by suspending the specimen from a glass hook or by resting the two lower corners thereof on the bottom of a test tube. Tests with very volatile liquids were performed in sealed tubes; tests with less volatile solutions were performed with the tubes closed with rubber stoppers equipped with condenser tubes; and tests with non-volatile solutions were performed in open test tubes. All dilute solutions were changed once a week. The weight loss and the appearance of the solutions were checked periodically, and at least once a week. If no earlier failure was observed, the tests were continued for a total of three to six weeks. Whenever penetration through the nickel-phosphorus coating or the tin-nickelphosphorus coating was observed, the test was discontinued and the corrosion rate was calculated in mils per year using the weight loss from the time of termination of each test; however, for specimens failing before the termination of the tests, the corrosion rate was calculated using the time at the inspection prior to the failure. The density of the tin-nickel-phosphorus alloy lies between the density of tin and the density for the nickel-phosphorus coating which is heavier than tin but for the purpose of determining the corrosion rates, the density of tin was used in the calculations, thereby to obtain conservative estimates of the corrosion rates, i.e., the corrosion rates obtained in this manner are slightly higher than if they would be using the actual density of the tin-nickel-phosphorus alloy.

The corrosion tests results consistently indicated that the tin-nickel-phosphorus alloy possessed corrosion resistance superior to the nickel-phosphorus coating in the following solutions: ammonium hydroxide 28-30% ammonia by weight; ammoniated ammonium nitrate, 30% ammonia and 40% ammonium nitrate by weight; ammonium nitrate, 30% by weight; acetaldehyde; formaldehyde; aeetic anhydride; glacial acetic acid; acetic acid, 5% by weight; lactic acid, 50% and 80% by weight; citric acid, 5% by weight; ferric sulfate, 1% by weight; sulfuric acid, 10% by volume; nitric acid, concentrated (70% HNO by weight), and 20% by volume; dry sherry wine; and sauterne wine. The tin-nickel-phosphorus al- 10; provides sufficient protection in ammoniacal and 9. weakly basic solutions that it can be used in commercial applications where the nickel-phosphorus alloy has not been used heretofore because of the relatively high corrosion rate thereof. In general, the tin diffusion coating 31 shows good corrosion resistance toward basic solutions, neutral solutions, and acidic solutions, the tinnickehphosphorus alloy thereof being readily soluble only in aqua regia.

The tin-nickel-phosphorus alloy in the coating 31 differs in other physical properties from the nickel-phospohorus alloy in the coating 22 and from the coatings of tin and nickel electroplated in a manner such that the tin and nickel are laid down simultaneously to form a single homogeneous coating. For example, the tinnickel-phosphorus alloy of the coating 31 is a solid at temperatures well above the melting point of tin. The tin-nickel-phosphorus alloy in a typical specimen has a hardness corresponding to a point within the range Vl-LN. 750 to 950; whereas the electroplated tin-nickel coating has a hardness corresponding to about V.H.N. 700.

It is essential in producing a satisfactory tin diffusion coating 31 that the metllic tin be deposited upon the surface of the nickel-phosphorus coating at a rate lower than the diffusion rate of tin into the nickel-phosphorus coating. If the metallic tin is in fact deposited at a rate higher than the diffusion rate of tin into the nickelphosphorus coating, the excess tin either covers the surface in a manner to prevent further catalytic reduction of tin thereon or balls up and rolls from the surface whereby to remove the metallic tin from contact with the nickelphosphorus coating. In this regard it is noted again that although the nickel-phosphorus coating is a good catalyst for the reduction of stannous chloride by hydrogen, tin itself is not a good catalyst and the reaction will not take place upon a surface of tin. In fact, a convenient way of treating the various parts of the reaction chamber 519, the heat exchanger 52% and reaction chamber 531 to minimize loss of tin by spurious reduction thereof is to coat these parts with the tin-nickel-phosphorus alloy. This can be conveniently done by first applying a nickelphosphorus coating as explained above and then carrying out the reaction of the present invention therein whereby to form upon the nickel-phosphorus coating a tin-nickel-phosphorus alloy.

The rate of deposition of tin upon the surface of the article being coated increases with an increase in the temperature within the reaction chamber 531. In order to determine the relationship between the reaction temperature and the amount of tin deposited upon the Workpiece, workpieces were utilized having three square inches of surface area and were coated in the reaction chamber 531 using cracked ammonia gas as the reducing agent, the ammonia being 99% cracked an being supplied at the rate of 1600 cc. per minute. The temperature of the stannous chloride was maintained at 480 C. and the coating was carried out for a period of three hours. At the end of three hours the specimens were removed from the reaction chamber 531 and weighed to determine the increase in weight thereof. The following is a summary of the weight gains ascertained for a plurality of reaction temperatures in Examples 2 to 6:

The rate of diffusion of atomic tin into the nickel-phospohorus alloy also increases as the temperature of the workpiece increases and for this additional reason the preferred operating temperature is the higher temperature of 630 C. Even higher rates of deposition of atomic tin can be obtained at temperatures above about 630 C., but it has been found that in general the base metal It) should not be heated above this temperature and the rate of diffusion does not increase with temperature as rapidly as does the rate of deposition of tin, and as a result metallic tin would be deposited at a rate grater than that at which it can be diffused into the nickel-phosphorus alloy and, accordingly, the additional tin would be lost from the coating operation, the excess tin balling up and rolling off of all inclined surfaces and coating and stopping the reaction on surfaces from which the balled up tin cannot drain.

The rate of deposition of the metallic tin is also a function of the partial pressure of hydrogen in the gases flowing into the reaction chamber 531 and the partial pressure of the tin compound in those gases as well as the reaction temperature in the reaction chamber 531. The effect of the partial pressure of hydrogen in the reducing gas is best illustrated in FIG. 7 of the drawings wherein there issummarized the results of a group of examples of coating operations carried out in the system 5% of FIGURE 5. Each of the examples plotted in FIG- URE 7 was carried out at a reaction temperature of 630 C. and the partial pressure of the stannous chloride in the reaction gases was maintained constant by heating the stannous chloride to a temperature of 480 C. The coating reactions were carried out for a time period of two hours on specimens having a surface area of sq. cm. The weight gain of the specimens was determined and a thickness of the tin-nickel-pho'sphorus alloy coating calculated and plotted on the vertical axis of FIGURE 7. The partial pressure of the hydrogen gas in the reducing gas was expressed as the ratio by volume of nitrogen to hydrogen and plotted on the horizontal axis in FIGURE 7. In order accurately to control the ratio of nitrogen to hydrogen in the reaction gases, a mixture of hydrogen and nitrogen gases was used in the place of cracked ammonia gas. To this end the system 5th} in FIGURE 5 is provided with a connection 541 to a source of hydrogen (not shown), the connection 541 communicating with a flow meter 542 which in turn is connected through a line 543 to a furnace 545 in which the hydrogen gas is heated. The outlet from the furnace 545 is connected through a line 547 to two manually operable control valves 548 and 549, the outlet of the control valve 543 being connected to the line 509 and the outlet of the control valve 549 being connected to the line 516, whereby all or a portion of the heated hydrogen gas can be passed through the chamber 510 to pick up stannous chloride vapors for inclusion in the reaction gases. A connection 551 is provided and adapted to be connected to a source of nitrogen gas (not shown), the connection 551 communicating with a flow meter 552 having the output thereof connected to a furnace 555 through a line 553. The furnace 555 is adapted to heat the incoming nitrogen gas and the output of the furnace 55 is connected to a line 557 which in turn connects to two manually

operable control valves

558 and 559, the control valve 558 connecting to the line 509 and the control valve 559 connecting to the line 516. Any desired portion of the heated stream of nitrogen gas can be passed through the chamber 51a to sweep stannous chloride vapors therewith into the reaction chamber 531.

Three rates of total gas flow were also utilized to obtain the data plotted in FIG. 7, the data indicated by a circle being obtained using a total gas flow of 60 cc. per minute, the data designated by a triangle being obtained using a total gas flow of cc. per minute and the data designated by a square being obtained by a gas flow of cc. per minute. The following table lists the thick ness of the tin-nickel-phosphorus alloy coating calculated Table 2 Thickness of Tin-Nickel- Phosphorus Coating in Mils Ratio of Nitrogen to Hydrogen By Volume 60 cc./ min.

150 cc./ min.

mill.

The thickest deposit of tin-niekel-phosphorus alloy coating was obtained when the reducing gas comprised only hydrogen (the nitrogen to hydrogen ratio being a coating of 0.2 mil having been obtained at a gas flow of only 60 cc./min. This coating was gray in color and semi-bright in luster as were all coatings obtained with a nitrogen to hydrogen ratio by volume less than about 3.5. When using nitrogen to hydrogen ratios by volume greater than about 3.5, the deposits obtained were dull in luster and gray in color and had a generally less desirable appearance than those coatings obtained with nitrogen to hydrogen ratios less than about 3.5. It is noted that the thickness of the coatings did not decrease substantially with further dilution of the hydrogen gas after a ratio of approximately and it is believed that the autoreductionoxidation reaction becomes a significant if not the dominant reaction taking place when the partial pressure of the hydrogen gas becomes so small. Accordingly, it is preferred that the partial pressure of hydrogen in the reaction gases correspond to a nitrogen to a hydrogen ratio by volume of less than about 3.5 and that the hydrogen constitutes approximately by volume of the reducing gas and up to about 40% by volume, the preferred amount being about 33% by volume of the reducing gas. Other reducing gases can be used in the place of cracked ammonia and the nitrogen-hydrogen mixtures discussed above. For example, anhydrous ammonia may be used as the reducing gas without prior cracking thereof, the ammonia gas being heated, mixed with the stannous chloride and the mixture applied against the surface of the workpiece in the reaction chamber 531. When the gas strikes the nickel-phosphorus coating, part of the ammonia is disassociated producing hydrogen which reduces the stannous chloride vapor to metallic tin. The deposited tin then diffuses into the nickel-phosphorus coating forming the nickel-tin alloy described above. The'hydrogen chloride which is produced as a result of the reaction reacts with the excess ammonia present forming ammonium chloride which is removed from the reaction zone by the sweep of the reaction gases. The total reaction can be expressed as follows:

Thus theoretically 2 /3 moles of ammonia are required to deposit one mole of tin. The following is a preferred example utilizing heated anhydrous ammonia as the reducing gas.

EXAMPLE 7 The reaction chamber 531 is heated to a temperature of 630 C. and purged of air by means of ammonia gas min. and the reaction continued for four hours. Each coupon had a weight gain of 0.0963 gram corresponding to a calculated thickness of the tin-nickel-phosphorus coating of 0.261 mil. The coating was gray in color and uniform throughout the surface of the coupon.

Cracked natural gas can also be used as the reducing gas in the present reaction. The following is an example of this reaction:

EXAMPLE 8 Natural gas was thermally cracked using an air to gas ratio of two to one to produce a resultant gas containing about 30% hydrogen by volume. The cracked natural gases were utilized to coat a workpiece, the reaction being carried out at a temperature of 630 C. with the stannous chloride maintained at a temperature of 555 C. and using a gas flow of 55 cc./min. for four hours. A workpiece having a surface area of 20 sq. cm. had a weight gain of 0.0788 gram corresponding to a calculated thickness for the tin-nickel-phosphorus alloy of 0.21 mil. The resultant coating was gray in color and continuous throughout the surface area of the workpiece and was generally brighter than the coatings obtained using anhydrous ammonia as the reducing gas.

The stannous chloride utilized as the source of tin in all of the preceding Examples 1 through 8 has a boiling point of 620 C. and it is preferred to maintain the temperature of the stannous chloride in the chamber 510 well below the boiling point thereof in order to obtain the desired concentration oi hydrogen at the surface of the workpiece where the tin reduction reaction is to be carried out and in general it has been found desirable to maintain the temperature of the stannous chloride in the range from about 480 C. to about 500 C., the preferred temperature being about 480 C. When the stannous chloride is held at 480 C., a sufficient amount of stannous chloride vapor is present at the nickel-phosphorus reaction surface with manageable flow rates for the reducing gas and the partial pressure of hydrogen in the reducing gas or the hydrogen formed by the association of ammonia on the nickel-phosphorus surface is sufficient to reduce the stannous chloride at a rate to deposit metallic tin at a rate less than the rate of iffusion of tin into the nickel-phosphorus coating. In fact the amount of stannous chloride present is more than sufficient to provide an excess at the reaction surface under the conditions of the reaction set forth in Examples 1 to 8. The use of higher temperatures for the stannous chloride simply results in the recycling of even more stannous chloride, assuming that the stannous chloride is coooled at the exhaust outlet 524 and purified for reuse in the reaction.

It is believed that the reduction of tin compounds by hydrogen is catalytic in nature, the nickel-phosphorus coatings acting as a good catalyst and the tin-nickel-phosphorus alloy not appreciably catalyzing the tin reduction reaction. Accordingly, as the reaction progresses the rate of deposition of metallic tin decreases as the available nickel-phosphorus coating surface is covered by the tin-nickel-phosphorus alloy. There is shown in FIG. 6 of the drawings the results of three series of experiments illustrating that the rate of deposition of metallic tin decreases with time, the rate of deposition being more rapid at the beginning of the reaction period and steadily declining as the reaction proceeds. There is plotted in curve 601 the results of a series of runs in which a nitrogen-hydrogen gas mixture was utilized as the reducing gas, the ratio by volume of nitrogen to hydrogen being two to one. The reactions were carried out at 630 C., the stannous chloride being maintained at 480 C. and the reducing gas flow being 60 cc./min. It is clear from the curve 601 that the rate of deposition is maximum during the first part of the reaction period and then steadily decreases. Runs maintained for periods longer than 10 hours show substantially no further deposition of atomic tin upon the workpieces. On

the curve 692 are plotted the calculated thicknesses in mils of the tin-nickel-phophorus alloy coatings obtained using cracked natural gas as the reducing gas at a reaction temperature of 630 C., the stannous chloride being maintained at a temperature of 550 C. and the reducing gas flow being 55 cc./min. Again the rate of deposition of tin is higher at the beginning of the reaction and steadily declines. There are plotted on the curve 6% the results of utilizing anhydrous ammonia as the reducing gas at a reaction temperature of 510 C., the stannous chloride being maintained at 510 C. and the reducing gas flow being cc./min. Here again the reaction rate is highest at the beginning of the reaction and steadily decreases with time.

In all cases, the tin-nickel-phosphorus alloy coating ob tained at a reaction temperature of 630 C. during the reaction time of less than 4 hours shows substantially only a single layer as illustrated in FIG. 3 of the drawings, this being the layer 31 labeled Diffusion Tin Plating Outer Skin Portion. In a typical example, this alloy layer will have an average composition of 45% tin, 51% nickel and 4% phosphorus, by weight, it being understood that the amount of tin may be greater or less depending upon the various reaction conditions. After the tin reduction reaction has been carried on for four hoursor longer, the coating 31 forms into three separate layers illustrated in FIG. 4 of the drawings and designated by the numerals 41, 42 and 43'. In this case the outermost layer 41 consists essentially of tin with relatively small amounts of the nickel-phosphorus alloy distributed therein. The second layer 42 comprises predominantly tin in which is distributed a small amount of nickel-phosphorus chemical nickel plating alloy. The third layer 43 comprises predominantly the nickel-phosphorus chemical nickel plating alloy in which is distributed a small amount of tin. It is believed that the original unitary coating 31 of diffusion tin in a nickel-phosphorus coating approaches equilibrium after being heated for about four hours and separates into the

layers

41, 42 and 43 which represent the various possible combinations of ingredients for a system containing tin and nic 'el and phosphorus. In this regard it is noted that the tin definitely diffuses into the nickel-phosphorus coating inasmuch as tin is present at depths several times the thickness of the tin coating to be expected by calculation from the gain in weight. For example, in certain specimens in which the weight gain indicated a thickness of the tin layer of 0.49 mil, tin was present throughout a layer that Was 1.05 mils thick.

The protective coating of FIG. 4, Le, a coating in which the reaction has been carried out for a sufiicient length of time to produce three distinct layers containing tin, shows substantially better corrosion resistance than do those tinnickel-phosphorus protective coatings consisting of only one layer produced in the reaction carried out for less than four hours. Although the longer reaction time does deposit additional tin, the difference in corrosion rates indicates a change in character of the protection alforded in excess of that expected from the added tin deposited during the additional reaction time, particularly in view of the fact that the amount of tin deposited during the later portions of the reactions are substantially less than those deposited during the initial portion of the reaction. It is believed that the three layer tin-nickel-phosphorus alloy formation illustrated in FIG. 4 of the drawings serves further to cover any minute imperfections and thus greatly enhances the corrosion protection characteristics of the protective layer 31. Table 3 below summarizes the results of corrosion tests carried out on workpieces having thereon a nickel-phosphorus coating 2 mils thick to which is applied metallic tin by the process of the present invention, the tin being applied for periods of two hours, four hours and six hours, respectively, on the various specimens. The corrosion tests were carried out in the same manner described above when discussing generally the corrosion resistance of the article 3% The figures given for the corrosion rate data are in mils of corrosion per year.

Table 3 Corrosion Rate, Mils Per Year .Tin Commodity Deposit,

G/E O 2 Hour 4 Hour 6 Hour cm. Deposi- Deposi- Deposition tion tion Lactic Acid,

Acetic Acid, 5%

Ferric Sulfate, 1%

It has also been found that it is preferred to deposit the metallic tin upon nickel-phosphorus coatings having a thickness of about at least 2 mils in order to permit formation of the three

discrete layers

41, 42 and 43 illustrated in FIG. 4 and still provide a substantial layer 22 of a nickel-phosphorus coating in which no tin is present. Although tin-nickel-phosphorus alloy coatings produced upon nickel-phosphorus coatings that are less than 2 mils thick are entirely satisfactory for certain purposes, substantially increased corrosion resistance of the composite coating is realized if the nickel-phosphorus coating upon which the atomic tin is deposited has a thickness of at least 2 mils. There is set forth in Table 4 the results of corrosion tests conducted on specimens which have substantially the same amount of atomic tin deposited on nickel-phosphorus coatings having different thicknesses of 1 mil, 1.5 mils, and 2 mils, respectively. The corrosion rate data with respect to three common chemicals are given for illustrative purposes, the actual deposit of tin in grams for each 20 sq. cc. of specimen also being indicated. The corrosion rate data are in mils of corrosion per year.

1. Ammonium Nitrate, 30%

by weight 2. Formaldehyde Solution,

inhibited 1215% methanol.

3. Ferric Sulfate, 1% by weight It will be seen that in the case of corrosion by ammonium nitrate solution, the protective coating including tin diffused in 2 mils of nickel-phosphorus coating produces sub stantially lower corrosion rates than when the nickelphosphorus coating is only 1 mil thick. Likewise in the case of the formaldehyde solution and ferric sulfate solution, the 2 mils nickel-phosphorus coating treated with the diffusion tin process produces lower corrosion rates.

Other sources of tin can be utilized in the place of the stannous chloride utilized in the foregoing examples. Stannic chloride is a suitable source of tin for the reaction of the present invention although certain modifications must be made in the system 560 of FIG. in order to utilize stannic chloride in place of stannous chloride. Whereas stannous chloride is a solid at room temperature and has an appreciable vapor pressure only in the temperature rarnge 480 C. and up, stannic chloride is a liquid at room temperature and boils at approximately 113 C. In order to utilize stannic chloride in the system 5%, ammonia gas is fed from the conduit 551 to the furnace 555 without heating thereof and is then swept across the stannic chloride which is maintained in the chamber Slit} at room temperature. The nitrogen gas with the stannic chloride therein is fed through the line 513 to the tube 514-. Simultaneously cracked ammonia is fed from the line 5597 through the valve 519 and the line 516 to the tube 514, the precracked ammonia being maintained at an elevated temperature. The resultant mixture of nitrogen gas, stannic chloride vapors and cracked ammonia are then fed to the chamber 531 which is maintained at a suitable temperature such as 630 C. The reaction rate utilizing stannic chloride is comparable to that using stannous chloride and produces a tin-nickel-phosphorus coating which is attractive and comparable to or exceeds the qualities of the coating obtained using stannous chloride as a source of tin. The use of stannic chloride has the advantage over the use of stannous chloride that the furnace 512 need not be utilized when stannic chloride is utilized as the source of tin.

Other halides of tin may be utilized as the source of tin for carrying out the process of the present invention and more particularly stannous fluoride, stannous bromide and stannous iodide may be utilized as a source of tin.

EXAMPLE The procedure of Example 1 was repeated utilizing stannous fluoride as the source of tin, the stannous fluoride being disposed in the chamber Sill and heated by the heater 512 to provide a substantial vapor pressure thereof whereby to mix a quantity of stannous fluoride vapors with the cracked ammonia gas. The reaction was carried out for a period of one hour at a temperature of 630 C. A satisfactory tin-nickel-phcsphorus alloy layer was formed having properties comparable to those produced by the process of Example 1 above.

EXAMPLE 1 l Workpieces were coated utilizing stannous bromide as as a source of tin and precracked anhydrous ammonia gas as the reducing agent in the system 5% illustrated in FIG. 5. The heater 5112 was operated to maintain the stannous bromide (Bl 623 C.) at a temperature of approximately 480 C. The reaction was carried out at 630 C. for a period of one hour which produces on the workpieces 2d a gray coating having the desirable characteristics described above for tin-nickel-phosphorus alloy coatings.

EXAMPLE 12 A coating reaction was carried out utilizing the system see in "HG. 5 and employing stannous iodide (Bl 720 C.) as a source of tin and precracked anhydrous ammonia gas as the reducing agent. The heater 512; was operated at 580 C. to supply sufficient stannous iodide vapors to provide the required amount of stannous chloride vapors on the reaction surface of the workpiece 'zZ-tl. The resulting tin-nit,rel-phosphorus coating had a bright metallic appearance, was uniform throughout, and possessed the desirable characteristics set forth above for such coatings.

The other stannic halides may also be used as a source of tin including stannic fluoride (sublimes at 705 C.), stannic bromide (13.1. 202 (1.), and stannic iodide (BJP. 340 C.), the temperature of the container 519 being adjusted to provide the suitable vapor pressure of the stannic halide therein.

In the preceding examples, the deposition of tin has been stated to have been upon nickel phosphorus coatings produced by chemical nickel plating from plating baths of the nickel cation-hypophosphite type in which the nickelphosphorus coating is utilized as plated. As plated, the nickel-phosphorus coating is an amorphous solid material consisting essentially of a metastable undercooled solution of phosphorus in nickel. The process of the present invention can also be applied to such nickel-phosphorus coatings which have been treated to form a nickel-phosphorus alloy, the method of producin the alloy from the amorphous solid material and the physical characteristics of the alloy being fully set forth in US. Patent No. 2,908,419, granted on Gctober 13, 1959, to Paul Talmey and William J. Crehan. As is pointed out in the aforementioned Talmey and Crehan Patent No. 2,908,419, the character of the nickel-phosphorus coating is completely altered upon heat treatment to a critical temperature of about 400 C. whereby to convert the amorphous solid material to a stable solid material consisting essentially of micro-crystals of nickel-phosphide (Ni l) dispersed in a matrix of nickel. The heat treatment is l referably carried out in an inert atmosphere such as nitrogen or in a reducing atmosphere such as cracked ammonia. The reaction is exothermic and proceeds with great rapidity throughout the motel-phosphorus coating when the critical temperature is obtained. The heat treated nickelphosphorus alloy has physical properties distinct from those of the ni kel-phosphorus coating as chemically plated and more particularly the hardness of the alloy is substantially greater than the plating in that the nickelphosphorus coating as plated has a hardness corresponding to Vl-LN. of about 525, whereas the alloy may have a hardness corresponding to a V.H.N. of 950 or higher. In general the hardness of the alloy is greatest when heated at substantially the critical temperature of 400 C. and gradually decreases the temperature of treatment increases so that the hardness after heat reatrnent at 630 C. would be from about V.l-l.N. 560 to 630.

The processes of the present invention can be readily applied to the heat treated nickel-phosphorus alloy Whereby to produce the desired protective coating 31 comprising the tin-nickel-phosphorus alloy described heretofore. In those cases wherein the nickel-phosphoru coating is in the as plated condition and has not been heat treated, the nickel-phosphorus coating is in fact heat treated during the carrying out of the process of the present invention inasmuch as the workpiece 26 including the coating 22 is heated to 630 C., i.e., to a temperature well above the critical heat treatment temperature of 400 C. As a result, although the chemical nickel plated layer 22. may initially be an amorphous solid material consisting essentially of a metastable undercooled solution of phosphorus and nickel, upon treatment in accordance with this invention, the coating 22 is converted to a stable solid material consisting essentially of micro-crystals of nickel phosphide dispersed in a matrix of: nickel and upon subjection or the coating 22 to a temperature of 630 C. for six hours would produce a hardness corresponding to a V.H.N. of about 575.

The process of the present invention is particularly suitable for treating hollow and tubular articles and there is shown in H61. 8 of the drawings a system particularly adapted for treating a hollow article 826. The article 32% may be called a container or a tank and it is to be understood that these terms as employed herein are intended to cover all those hollow structures that perform a retaining, storing, conveying, etc., function and embrace a great variety of hollow structures commonly referred to as tubes, pipes, drums, barrels, etc. The container 820 has been illustrated as being made from two cylindrical sections 321 and 831 which are suitably joined together. More particularly the cylinder $21 is provided at one end thereof with an outwardly directed flange 22 and at the other end thereof with a second outwardly directed flange 823, and the cylinder 831 is similarly provided at one end thereof with an outwardly directed flange 832 and at the 17 other end thereof with a second outwardly directed flange 833. The flanges 823 and 83-3 are placed in. abutting and contacting relationship and have aligned holes (not shown) formed-therein to receive therethrough a plurality of bolts 838 having threaded outer endsreceiving complementarily threaded nuts S39, whereby the bolts 838 and the nuts S39serve to clamp the

flanges

823 and 833 firmly against each other, a narrow crack or seam 837 being formed therebetween.

The container 820 has been shown mounted within an enclosure 801 including means to heat the contents thereof if desired whereby surfaces of the container 82% More particularly, the container 820 is mounted upon two pairs ofsupport

rollers

808 and 809 that are respectively supported uponframes 810 and 811 by means of axles 812 and 813-, respectively. A motor and gear box 815 has the output thereof connected to the shaft 812' whereby to drive the rollers 808 to thus to rotate the container 820 upon the

rollers

808 and 809. Coating materials can be admitted to the interior of the container 820 from an inlet connection 802 passing through a rotary connection and seal 803 to a head or manifold diagrammatically illustrated at 854', the manifold 8M sealing the adjacent end of the container 820. The other end of the container 820 is provided with an outlet manifold 805 sealing the adjacent end of the container 820 and communicating with a rotary connection and seal 806 which in turn connects with an outlet 867.

In providing a protective coating upon the interior surfaces of the container 320 in accordance with the present invention, the container is mounted as illustrated in FIG. 8 and a chemical nickel plating solution of the nickel cation-hypophosphite anion type described heretofore is pumped into the inlet connection 8412 and thus into the interior of the container 324). The container 820 is continuously rotated as the plating solution is continuously flowed therethrough from the manifold 864 to the manifold 805 thereby to produce upon the interior surface of the container 820 a nickel-phosphorus coating 840. If the material of construction of the container 820 is catalytic to the chemical nickel plating reaction, the coating 840 can be directly made thereupon; if the material of construction of the container 820 is not catalytic to the chemical nickel plating reaction, then the surface thereof can be treated to implant thereon catalytic growth centers whereby to permit deposition of the nickel-phosphorus coating 840 thereon.

The nickel-phosphorus coating 840 is of one piece and provides a continuous liner for both sections 821 and S31 and serves to fill and cover the joint 837 therebetween. At this point in the process the coating 84th is an amorphous solid consisting essentially of metastable undercoo'led solid solution of phosphorus and nickel and may comprise, for example, 92% nickel and 8% phosphorus by weight. It is possible at this point in the process to heat treat the coating 54h, butit is more economical to proceed directly with the deposition of tin thereon whereby to achieve heat treating of the coating 840 during the tin diffusion process.

After having put the coating 840 in place upon the intcrior surface of the container 820, the furnace 801 is operated to raise the temperature of the contents thereof including the container 820 to the operating temperature of 630 C. for the tin deposition reaction. Simultaneously nitrogen or the reducing gas is connected to the input connection 862 whereby to purge the interior of the container 320 of all air, water vapor and the like. After a suitable purging time, for example, of one-half hour, and after the container 820 has reached the operating temperature of 630 C., a mixture of reducing gas and a suitable compound of tin is introduced through the input connection 892, it being understood that this reaction mixture is the same as that produced in the con= duit 514 in FIG. 5. The reaction gases, including, for

to permit coating. of the interior .8-, example, stannous chloride and precracked anhydrous ammonia gas are continuously circulated through the container 820 and in contact with the coating 840' and the exhaust gases are removed through the outlet mania fold 805 to the outlet 807'. The reaction is carried out. for a period of time such as six hours whereby to provide upon the nickel-phosphorus coating 840 another coating 841 which is the tin-nickel-phosphorus alloy designated by the numeral 31 in FIG. 4; The container 820 is heated for six hours while passing the reaction gases therethrough, and accordingly the typical three layor outer protective coating 31 of FIG. 4 is produced and. simultaneously the nickel-phosphorus coating 22 is changed from an amorphous solid material to a stable. solid material consisting of micro-crystals of nickel phosphide dispersed in a matrix of nickel. After formation ofthe protective coating 841, the container 820 is cooled in an inert or reducing atmosphere to a. temperature of 200 C. after which it is removed from the furnace 801' and permitted to cool to the ambient temperature in the air. The resultant protective coating on the interior surface of the container $20 is continuous and of one piece and possesses the superior corrosion resistance properties discussed above with respect to the coating 31.

Referring now to FIGS. 9 to 11, inclusive, of the drawings, there is illustrated another form of a container or tank, namely, a railway tank car 910 com.- prising mobile structure 911 carrying a shipping container or tank 912 embodying the features of the present invention. The tank 912, as illustrated, comprises a horizontally extending substantially cylindrical hollow body 913, two end headers 914, and a centrally disposed upstanding substantially cylindrical hollow dome 915. The body 913 includes a number of tubular sections 913a, five being illustrated, that are formed of steel plate and are secured. by butt-welding at the meeting edges thereof to provide the seams or joints 916, as shown in FIG. 10; while the end headers 914 are also formed of steel plate and secured in lapped relationship by steel rivets 917 to the adjacent end sections 913a, as shown in FIG. 11. Further, the dome 915 is formed of steel plate and secured in a cooperating opening provided in the middle section 913a by arc welding, as indicated at 918. The construction of the tank 912, described above, and involving both welded and riveted joints between the various component elements thereof, is entirely conventional, and altogether arbitrary as a matter of structure,'in order clearly to demonstrate the broad application of the present invention. N

Continuing now with the construction of the tank 912, the dome 915 carries a removable steel cover 919, and the two end headers 914 are respectively provided with two steel fixtures 920 of tubular form, that, in turn, respectively carry two removable steel covers 921; which fixtures 920 may be employed in filling and in emptying the tank 912, when certain fluids are shipped or stored therein. Finally, the entire interior surfaces' of the tank 912 are provided with a smooth continuous seamless liner 922, comprising a solid layer of nickel-phosphorus material intimately bonded to the interior surfaces mentioned. Also, the liner 922 completely covers the welded seams or joints 916 at the meeting edges'of the 'se'c tions 913a, as illustrated in FIG. 10, and thelapped edges of the end sections 9132: and the end headers'9 1 4 at the riveted joints thcrebetween, together with the inner heads of the rivets 917, as illustrated in FIG. 11. Moreover, the liner 922 extends in covering relationship with the interior surfaces of the fixtures 920; whereby the liner 922 is of integral one piece construction throughout and is thoroughly devoid of cracks, scams or discontinuities of any kind whatsoever. Furthermore, the interior surfaces of the

covers

919 and 921 are respectivelyprovided with integral one piece liners, not shown, ofthe character of the liner 922; whereby the entire interior volume of the tank 912 is completely bounded by the one piece liner 922, and by theone piece liners, not

shown, respectively carried by the interior surfaces of the" covers 919 and 921.

The liner 922 may be applied in the same manner as the coating 84d described above and the liner 922 further has the surface thereof treated to diffuse tin thereinto to provide on the interior surface thereof a tinnickel-phosphorus coating of the same character as the coating 841 described above with respect to FIG. 8.

f In view of the foregoing, it will be appreciated that the coated workpieces 3i and 4t and the coated container 829 and the railway car 916 can be used in con tact with a wide variety of fluids that cannot be permitted to have direct contact with the base metal 15) or the

walls

821 and 831 of the container 82:"? and the car 9ft); whereby the range of utility of these workpieces and the container are greatly extended and are substantially wider than those attained by other types of protective coatings which have been employed heretofore and including such materials asrubber, glass, organic plastics, electrolytically deposited nickel, electrolytically deposited tin-nickel, chemically deposited nickel-phosphorus, etc., since it is obvious that many chemicals have selective corrosive or other deleterious actions on such materials aside from many other objectionable properties thereof. Also, it will be understoodthat the workpieces and container of the present invention are by no means limited to utilization in stationary chemical treatment or reaction apparatus but may be utilized in the fundamental transportation and distribution of such fluids, including corrosive reagents, and otherwise widely used in industry.

' The articles of manufacture resulting from the present process and described above in conjunction with FIGS. 8 to 10, inclusive, of the drawings, are disclosed and claimed in the copending continuation-impart application of Pranas Budininkas, Serial No. 138,443, filed September 15, 1961.

While there has been described what is at present considered to be certain embodiments of the present invention, it will be understood that various modifications may be made therein, and it is intended to cover in the appended claims all such modifieations that fall Within the true spirit and scope of the invention.

What is claimed is:

l. The process of converting to a tin-nickel-phosphorus layer the outer skin of a nickel-phosphorus coating carried by a workpiece, comprising heating the coating to a temperature above the melting point of tin and below the melting point of the nickel-phosphorus coating, and simultaneously reducing to metallic tin a compound of tin upon the outer surface of the heated coating and diffusing the tin into the outer skin of the coating, the tin being reduced upon the outer surface of the coating at a rate that is not higher than the rate of diffusing of the tin into the outer skin of the coating.

i 2. The process of converting to a tin-nickel-phosphorus layer the outer skin of a nickel-phosphorus coating carried by a workpiece, comprising heating the coating to a temperature above the melting point of tin and below the melting point of the nickel-phosphorus coating, and simultaneously reducing to metallic tin a compound of tin upon the outer surface of the heated coating and diffusing the tin into the outer skin of the coating, the tin being reduced upon the outer surface of the coating at a rate that is lower than the rate of diffusing of the tin into the coating.

- 3. The process of converting to a tin-nickel-phospho- -rus layer the outer skin of a nickel-phosphorus coating carried by a workpiece, comprising heating the coating to a temperature above the melting point of tin and below the melting point of the nickel-phosphorus coating, contacting the outer surface of the heated coating with a compound of tin in the presence of a reducing agent to form metallic tin upon the outer surface of the coating, and diffusing the tin into the outer skin of the coating as the tin isformed thereon, the tin being. formed upon the outer surface of the coating at a rate that is not higher than the rate of diffusing of the tin into the outer skin of the coating.

4. The process of converting to a tin-nickel-phosphorus layer the outer skin of a nickel-phosphorus coating carried by a workpiece, comprising heating the coating to a temperature above the melting point of tin and below the melting point of the nickel-phosphorus coating, contacting the outer surface of the heated coating with a compound of tin in the presence of a reducing mixture of gases including hydrogen to form metallic tin thereon, and diffusing the tin into the outer skin of the coating as the tin is formed thereon, the tin being formed upon the outer surface of the coating at a rate that is not higher than the rate of diffusing of the tin into the outer skin of the coating.

5. The process set forth in claim 4, wherein the con centration of hydrogen in the gas is in the range from about 25% to about 40% by volume.

6. The process set forth in claim 4, wherein the reducing gas consists essentially of hydrogen, nitrogen and ammonia.

7. The process set forth in claim 4, wherein the reducing gas consists essentially of thermally cracked natural gas.

8. The process of converting to a tin-nickel-phosphorus layer the outer skin of a nickel-phosphorus coating carried by a workpiece, comprising heating the coating to a temperature above the melting point of tin and below the melting point of the nickel-phosphorus coating, and simultaneously reducing to metallic tin upon the outer surface of the heated coating a tin halide and diffusing the tin into the outer skin of the coating, the tin being reduced upon the outer surface of the coating at a rate that is not higher than the rate of diffusing of the tin into the outer skin of the coating.

9. The process of converting to a tin-nickel-pho-sphorus layer the outer skin of a nickel-phosphorus coating carried by a workpiece, comprising heating the coating to a temperature above the melting point of tin and below the melting point of the nickel-phosphorus coating, and simultaneously reducing to metallic tin upon the outer surface of the heated coating a tin chloride and diffusing the tin into the outer skin of the coating, the tin being reduced upon the outer surface of the coating at a rate that is not higher than the rate of diifusing of the tin into the outer skin of the coating.

10. The process set forth in claim 9, wherein the tin chloride is stannous chloride.

11. The process of converting to a tin-nickel-phosphorus layer the outer skin of a nickel-phosphorus coating carried by a workpiece, comprising heating the coating to a temperature above the melting point of tin and below the melting point of the nickel-phosphorus coating, passing a stream of gases including a compound of tin and a reducing agent over the outer surface of the heated coating to reduce to metallic tin the compound of tin thereupon, and diifusiug the tin as it is formed into the outer skin of the coating, the tin being reduced upon the outer surface of the coating at a rate that is not higher than the rate of diffusing of the tin into the outer skin of the coating.

12. The process of converting to a tiu-nickel-phosphorus layer the outer skin of a nickel-phosphorus coating carried by a workpiece, comprising heating-the coating to a temperature above the melting point of tin and below the melting point of the nickel-phosphorus coating, passing a stream of gases including a compound of tin and a reducing agent over the outer surface of the heated coating to reduce to metallic tin the compound of tin thereupon, diffusing the tin as it is formed into the outer skin of the coating, the tin being reduced upon the outer surface of the coating at a rate that is not higher than the rate of diffusing of the tin into the outer skin of the coating, and continuously removing from the outer surface of the heated coating the side products of the reaction involved in reducing the compound of tin to metallic tin. v i V 13. The process of converting to a tin-nickel-phosphorus layer the outer skin of a nickel-phosphorus coating carried by a workpiece, comprising heating the coating to a temperature from about 400 C. to about 630 C., passing a stream of gases including a compound of tin and a reducing agent over the outer surface of the heated coating to reduce to metallic tin the compound of tin thereupon, and diffusing the tin as it is formed into the outer skin of the coating, the tin being formed upon the outer surface of the coating at a rate that is not higher than the rate of diffusing of the tin into the outer skin of the coating.

14. The process of converting to a tin-nickel-phosphorus layer the outer skin of a nickel-phosphorus coating carried by a workpiece, comprising heating the coating to a temperature from about 400 C. to about 630 C., passing a stream of gases including a compound of tin and a reducing agent over the outer surface of the heated coating for about at least four hours to reduce the metallic tin to compound of tin thereupon, and diffusing the tin as it is formed into the outer skin of the coating, the tin being formed upon the outer surface of the coating at a rate that is not higher than the rate of diffusing of the tin into the outer skin of the coating.

15. The process of converting to a tin-nickel-phosphorus layer the outer skin of a nickel-phosphorus coating carried by a workpiece, comprising heating the coating to a temperature from about 400 C. to about 630 C., heating a quantity of tin halide to a temperature below the boiling point thereof, passing a stream of reducing gas over the heated tin halide and onto the outer surface of the heated coating to reduce to metallic tin the tin halide therein upon the heated coating, and diffusing the tin as it is formed into the outer skin of the coating, the tin being formed upon the outer surface of the coating at a rate that is not higher than the rate of diffusing of the tin into the outer skin of the coating.

16. The process of converting to a tin-nickel-phosphorus layer the outer skin of a nickel-phosphorus coating carried by a workpiece, comprising heating the coating to a temperature from about 400 C. to about 630 C., heating a body of stannous chloride to a temperature in the range of about 480 C. to about 500 C., passing for a period of about at least four hours a stream of reducing gas including at least 25% hydrogen by volume over the heated stannous chloride and onto the outer surface of the heating coating to reduce the stannous chloride to metallic tin thereupon, and diffusing the tin as it is formed into the outer skin of the coating, the tin being formed upon the outer surface of the coating at a rate that is not higher than the rate of diffusing of the tin into the outer skin of the coating.

17. The process comprising providing a workpiece carrying a nickel-phosphorus coating, heating the coating to a temperature above the melting point of tin and below the melting point of the nickel-phosphorus coating, and then converting to a tin-nickel-phosphorus alloy the outer skin of the coating by simultaneously reducing to metallic tin a compound of tin upon the outer surface of the heated coating and diffusing and alloying the tin into the outer skin of the coating, the tin being reduced upon the outer surface of the coating at a rate that is not higher than the rate of difi'using and alloying of the tin into the outer skin of the coating.

18. The process comprising providing a workpiece carrying a nickel-phosphorus coating, heating the coating to a temperature above the melting point of tin and below the melting point of the nickel-phosphorus coating, and then converting to a tin-nickel-phosphorus alloy the outer skin of the coating by simultaneously reducing to metallic tin a compound of tin upon the outer surface of the heated coating and diffusing and alloying the tin into the outer skin of the coating, the rate of reducing the compound of tin to metallic tin upon the outer surface of the coating being lowerthan the rate of diffusing and alloying of the tin into the outer skin of the coating. e V y H 19 The process comprising providing a workpiece carrying a nickel-phosphorus coating havinga thickness of at least about 2 mils, heating the coating to a temperature above the melting point of tin and below the melting point of the nickel-phosphorus coating, and then converting to a tin-nickel phosphorus alloy the outer skin of the coating by simultaneously reducing to metallic tin a compound of tin upon the outer surface of the heated coating and difiusing and alloying the tin into the outer skin of the coating, the rate of reducing the compound of tin to metallic tin upon the outersurface of said coating being lower than the rate of diffusing and alloying of the tin into the outer skin. of the coating.

20. The process comprising providing a workpiece carrying a nickel-phosphorus coating having a thickness of at least about 2 mils, heating the coating to a temperature above the melting point of tin and below the melting point of the nickel-phosphorus coating, and then converting to a tin-nickel-phosphorus alloy the outer skin of the coating by simultaneously reducing to metallic tin a compound of tin upon the outer surface of the heated coating and diffusing and alloying the tin into the outer skin of the coating through a period of at least about four hours, the rate of reducing the compound of tin to metallic tin upon the outer surface of the coating being lower than the rate of diffusing and alloying of the tin into the outer skin of the coating.

21. The process comprising providing a base metal workpiece, applying a nickel-phosphorus coating upon the exterior surface of the workpiece by chemical deposition from a plating bath of the nickel cation-hypophosphite anion type, and then converting to a tinnickel-phosphorus layer the outer skin of the coating by heating the coating to a temperature above the melting point of tin and below the melting point of the nickelphosphorus coating, and simultaneously reducing to metallic tin a compound of tin upon the outer surface of the heated coating and diffusing the tin into the outer skin of the coating, the tin being reduced upon the outer surface of the coating at a rate that is not higher than the rate of diffusing of the tin into the outer skin of the coating.

22. The process comprising providing a base metal workpiece, applying a nickel-phosphorus coating upon the exterior surface of the workpiece by chemical deposition from a plating bath of the nickel cation-hypophosphite anion type, and then converting to a tin-nickelphosphorus layer the outer skin of the coating by heating the coating to a temperature above the melting point of tin and below the melting point of the nickel-phosphorus coating, and simultaneously reducing to metallic tin a compound of tin upon the outer surface of the heated coating and diffusing the tin into the outer skin of the coating, the tin being reduced upon the outer surface of the coating at a rate that is lower than the rate of diffusing of the tin into the outer skin of the coating.

23. The process comprising providing a base metal workpiece, applying a nickel-phosphorus coating upon the exterior surface of the workpiece by chemical deposition from a plating bath of the nickel cation-hypophosphite anion type, heating the coating to a temperature above the melting point of tin and below the melting point of the nickel-phosphorus coating, contacting the outer surface of the heated coating with a compound of tin and a reducing agent to reduce the compound of tin to metallic tin thereon, and simultaneously diffusing and alloying the tin into the outer skin of the coating as the tin is deposited thereon, the tin being reduced upon the outer surface of the coating at a rate that 23 is not higher than the rate of diffusing and alloying of the tin into the outer skin of the coating.

24. The process comprising providing a base metal workpiece, applying a nickel-phosphorus coating upon the exterior surface of the workpiece by chemical deposition from a plating bath of the nickel cation-hypophosphite anion type, heating the coating to a temperature above the melting point of tin and below the melting point of the nickel-phosphorus coating, contacting the outer surface of the heated coating with a gas mixture including a tin halide and hydrogen to deposit thereon metallic tin by reduction of the tin halide, and simultaneously diffusing and alloying the tin into the outer skin of the heated coating as the tin is deposited thereon, the tin Ibeing deposited upon the outer surface of the coating at a rate that is not higher than the rate of diffusing and alloying of the tin into the outer skin of the coating.

25. The process comprising providing a base metal workpiece, applying a nickel-phosphorus coating having a thickness of at least about 2 mils upon the exterior Surface of the workpiece by chemical deposition'from a plating bath of the nickel cation-hypophosphite anion type, and then converting to a tin-nickel-phosphorus alloy the outer skin of the coating by heating the coating to a temperature above the melting point of tin and below the melting point of the nickel-phosphorus coating, and simultaneously reducing to metallic tin a compound of tin upon the outer surface of the heated coating and diffusing and alloying the tin into the outer skin of the coating, the tin being reduced upon the outer surface of the coating at a rate that is not higher than the rate of diifusing and alloying of the tin into the outer skin of the coating.

26. The process comprising providing a base metal workpiece, applying a nickel-phosphorus coating having a thickness of at least about 2 mils upon the exterior surface of the workpiece by chemical deposition from a plating bath of the nickel cation-hypophosphite anion type, and then converting to a tin-nickel-phosphorus alloy the outer skin of the coating by heating the coating to a temperature above the melting point of tin and beloW the melting point of the nickel-phosphorus coating, and simultaneously reducing to metallic tin a compound of tin upon the outer surface of the heated coating and diffusing and alloying the tin into the outer skin of the coating throughout a time interval of at least about four hours, the tin being reduced upon the outer surface of the coating at a rate that is not higher than the rate of diffusing and alloying of the tin into the outer skin of the coating.

References Cited in the file of this patent UNITED STATES PATENTS 1,975,818 Work Oct. 9, 1934 2,428,526 Osterheld Oct. 7, 1947 2,858,959 Bix by Nov. 4, 1958 2,867,550 Weber Jan. 6, 1959 2,947,639 Balden Aug. 2, 1960 2,955,944 Spaulding Oct. 11, 1960.

Claims

1. THE PROCESS OF CONVERTING TO A TIN-NICKEL-PHOSPHOROUS LAYER THE OUTER SKIN OF A NICKEL-PHOSPHOROUS COATING CARRIED BY A WORKPIECE, COMPRISING HEATING THE COATING TO A TEMPERATURE ABOVE THE MELTING POINT OF TIN AND BELOW THE MELTING POINT OF THE NICKEL-PHOSPHORUS COATING, AND SIMULTANEOUSLY REDUCING TO METALLIC TIN A COMPOUND OF TIN UPON THE OUTER SURFACE OF THE HEATED COATING AND DIFFUSING THE TIN INTO THE OUTER SKIN OF THE COATING, THE TIN BEING REDUCED UPON THE OUTER SURFACE OF THE COATING AT A RATE THAT IS NOT HIGHER THAN THE RATE OF DIFFUSING OF THE TIN INTO THE SKIN OF THE COATING.