FR2655060A1 - Process and compositions for improving the corrosion resistance of metals and alloys - Google Patents

Abstract

Process for preventing the surface corrosion of articles made of metals or metal alloys. The process is characterised in that the surface to be protected, optionally passivated or phosphated, is coated with a composition comprising either an aqueous solution of at least one water-soluble silicate to which has been added a freshly prepared silica gel or sol, or a solution in a gel or sol of freshly prepared silica of a silicate which is insoluble or poorly soluble in water, and in that drying of this surface is then performed.

Description

 The present invention relates to a method for improving the corrosion resistance of metals and alloys. The invention also relates to compositions which can be used in the implementation of this process.

 Corrosion of metals is a serious problem, which appeared as soon as the first metallic objects appeared. The extent of this problem and its implications, ranging from the simple aesthetics of objects to real security problems, have inspired researchers and engineers from the earliest times.

 Among the most significant inventions in this field, we will mention the development of stainless steels and the introduction on the industrial market of new metals, such as titanium, with remarkable properties. All these solutions are however very expensive.

 The metal that best meets the requirements for strength, flexibility, price and availability is ordinary steel. To resist corrosion, the steel is galvanized, which does not affect its cost beyond measure, if the thickness of the zinc layer is limited.

 However, zinc, despite its special surface properties, still remains a more electronegative metal than steel. It is said to be "less noble", that is to say more oxidizable. It will therefore be more or less quickly corroded, in the form of more or less oxidized salts, and white streaks, blisters, then testify to the disappearance of the zinc metal.

 Passivations by conversion layer considerably delay the appearance of "white rust", the magnitude of which obviously depends on the type of exposure to which they are subjected. Both to quantify and to standardize this resistance, an accelerated corrosion test is used, using the so-called "salt spray" technique.

 According to the American standard ASTM "8, to which reference will be made in the remainder of this description, the parts or objects to be tested are placed in a closed enclosure, in which is maintained an atmosphere of salt spray, constantly sprayed, based on pure water at 400C containing 5% by weight of sodium chloride.

 Experience teaches that the best types of protection range from so-called yellow passivations, based on chromium VI, with a resistance of 150 hours, to so-called blue passivations, based on chromium III, whose resistance reaches 24 hours, by the way. through all intermediaries.

 These results, however significant they may be, are no longer sufficient, particularly in view of the anti-rust guarantee required from automobile manufacturers.

 It is under these conditions that we have witnessed in recent years the appearance of innumerable passivation protection products based on organic polymers such as epoxides, polyvinyl chloride, urethanes, alkylene, etc. ... The most effective are based on silicones (silanes and other siloxanes).

 All these products are in fact only disguised paintings and have their advantages and defects, the latter being nevertheless accentuated.

 These compositions are of delicate implementation, requiring both an apparatus and a space exclusively dedicated to this purpose. They are polluting, making wastewater treatment equipment compulsory. Some are toxic, none are resistant to shot blasting, few allow sufficient adhesion to a traditional paint finish and, above all, none resist well to "thermal shock", an additional obstacle, also required by the automotive industry.

 Thus, a passive surface, sheathed with a layer of polymer, containing or not containing oxidized chromium derivatives, after treatment with thermal shock, will resist salt spray less well than the same passivation, but without overcoating. This is naturally due to cracking and flaking of the overlay, causing corrosion "under deposition", always favored under such conditions. But above all, even if they were adequate - which they are not - these finishes would remain expensive, thus canceling out the final advantage linked to galvanization.

 Indeed, if the use of ordinary steels, followed by multiple anti-corrosion interventions, is more expensive than stainless steel, it is clear that this type of protection must be abandoned.

 The present invention aims to remedy these drawbacks of the known technique by proposing a method for preventing corrosion of metals and alloys which is easy to implement, which is less expensive than those of the prior art and which ensures none protection at least as good as that of known methods.

 The invention also aims to provide compositions which can be used in the implementation of this process, which are inexpensive and easy to produce.

 The invention finally aims to provide a process and compositions of this type which can be used to improve the corrosion resistance of surfaces of various metals or alloys, whether these surfaces are passivated, or phosphated.

 To this end, the subject of the invention is a process for preventing surface corrosion of objects made of metals or metal alloys, characterized in that the surface to be protected, optionally passivated or phosphated, is coated with a composition comprising either an aqueous solution of at least one water-soluble silicate supplemented with a freshly prepared silica gel or sol, or a solution in a freshly prepared silica gel or sol of a silicate insoluble or sparingly soluble in water, and in that one then proceeds to the drying of this surface.

 The coating of the surface to be protected may be carried out by any means known in the art and, possibly, by dipping the object in said composition. This coating may be carried out at room temperature.

 The subject of the invention is also a composition for preventing surface corrosion of objects made of metals or metal alloys, characterized in that it comprises an aqueous solution of at least one soluble silicate added with a gel or sol of silica sharply obtained.

 Alternatively, the subject of the invention is finally a composition for the prevention of surface corrosion of objects made of metals or metal alloys, characterized in that it comprises a solution in a silica gel or sol freshly obtained from silicate insoluble or sparingly soluble in water.

 Advantageously, as will be explained below, said composition will contain 1 to 5% by weight of an oxygenated compound such as a borate or an aluminate, and / or titanium, to reinforce the mechanical properties of the protective layer obtained. after drying.

 The composition in accordance with the invention may also contain a minimum quantity of polyethylene glycol, in particular with an average molecular weight close to 5000, for example in an amount of the order of 0.1 to 2% by weight, in order to '' increase the flexibility of the protective layer.

 Likewise, polytetrafluoroethylene may be incorporated into the composition to improve the coefficient of friction of the protective layer obtained by drying.

 The invention therefore aims to protect metal surfaces by depositing t and solidifying on their surface a layer of silicates dissolved in water or in a gel, having a protective effect close to that of a layer of vitreous silica.

 The way in which the silicates are bonded together is of great importance in order to obtain satisfactory protection of the treated surfaces against corrosion, since as much as possible, the structure of the vitreous silica must be brought as close as possible.

 We know that the basic structural unit of silica and all silicate derivatives is a tetragonal arrangement of the four oxygen atoms around a central silicon atom.

 The stoichiometry imposed by the formula SiO2 requires that, on the whole, each oxygen atom is pooled between two tetrahedra, while the valence rules exclude that this oxygen atom is common to more than two tetrahedra.

 Sharing a corner of tetrahedrons in structures is the most common, edge sharing is exceptional, and face sharing almost never occurs, due to electrostatic repulsion.

 Structures with individual tetrahedra as the basic cell are called neosilicates, Spi042 '. The double tetrahedra corresponding to the sorosilicates, 6-Si207, and the corresponding open chains are the inosilicates. The paralleling of two inosilicate chains recalls the amphiboles and brings up a new basic motif, the Si401l6-. With more elaborate structures, we reach the phyllosilicates, each tetrahedron of which shares three of its oxygen atoms in the same plane.

 Finally, silica is a borderline case, in which an infinite three-dimensional network is formed by sharing all the oxygen of a given tetrahedron with neighboring groups.

 To get closer to it, it is therefore necessary to move, as far as possible, from inosilicates, of pronounced saline character, to three-dimensional networks very much less ionized.

Silicon, like all the other elements of the group
IV of Mendéléiev's table, forms acid oxides of general formula M02, but, unlike the oxides of other elements, which form defined salts with basic oxides, for silicon a simple relationship between its acid oxide and counterions basics is often no longer discernible and the number of forms of inosilicates is infinite.

 They are also classified according to the SiO2 / Na2O ratio, which usually varies from 1/2 to 4 in industrial materials.

 Those skilled in the art were therefore led to attribute a high protection efficiency to inosilicates with a high SiO2 / Na2O ratio, or even to SiO2 gels (infinite ratio).

 Contrary to this expectation and completely unexpectedly, the tests carried out by the Applicant have shown that the protective layers produced from soluble silicates with a high SiO2 / Na20 ratio, greater than or equal to 4, prove to be clearly less resistant to fog. saline than those made from a traditional commercial soda silicate (ratio equal to 3.3).

 Equally surprisingly, the corrosion protection of the SiO2 layers prepared from a SiO2 sol or gel has not been found to be better.

 On the contrary, the addition of a freshly prepared SiO2 gel from a stock solution to a solution of silicates of the same composition, in which the gel eventually dissolves entirely, provides a composition for the production of layers. particularly effective protective coatings and clearly superior to the protective layers prepared with a similar commercial silicate.

 The amount of freshly prepared SiO2 gel or sol incorporated into the composition according to the invention may represent between 5 and 50% by weight of this composition.

 This silica gel or sol can be prepared by methods known per se (acidification, ion-releasing resins, precipitation using solvents, etc.).

 It will be noted that a composition based on a corresponding lithiated derivative can be prepared in an analogous manner solubilized directly in silica gel, when it is known that ordinary lithium silicate is insoluble in water. Although more expensive, this composition gives even better results than with sodium silicate.

 Without linking the validity of the present invention to a scientific explanation, the Applicant considers that the way of preparing the compositions in accordance with the invention, intended to provide effective anticorrosion layers, allows the polymerization of silica acid forming siloxane bonds , to develop under the best conditions, in particular of pH and thermodynamic stability.

 It is desirable to avoid extensive polymerizations of the organic type and to seek a solidification effect, as in the setting chemistry of cements, by the addition of suitable additives, imitating certain glasses.

The best additives are borates, aluminates and other oxygen derivatives, especially titanium. Not only do they not interfere with networks, but they also form excellent attachment points for the "tectosilicates" thus formed.

The protective layer obtained with the compositions according to the invention has a completely crosslinked network, with partitioning by about three oxygen atoms, comparable to those obtained with vitreous silica, but infinitely more flexible. It has considerable resistance to thermal shock, great transparency, good refractory qualities, good abrasion resistance and very good adhesion quality of a subsequent layer of paint for which it truly acts as a primary layer or prevail
In addition to the adjuvants already mentioned, the addition of small quantities, for example 0.1 to 2% by weight, of polyethylene glycol, preferably of average molecular weight equal to 6000, is to be envisaged, which further strengthens the flexibility of the resulting protective layer.

 The composition according to the invention can be colored, to give a color to the protective layer. The dyes used must naturally be resistant to the alkalinity of the composition.

 Likewise, the possible addition of polytetrafluoroethylene to the composition, using suitable dispersants, may give the protective layer a better coefficient of friction. The latter additive will also give a more matte appearance to the resulting protective layer.

 The ready-to-use composition is able to self-purify, most metal silicates being insoluble and easily filterable. For certain uses, it is useful to complex the metals remaining in solution with EDTA (ethylene diamine tetra-acetic acid) or any other complexing agent of this kind.

 The thickness of the protective layer will vary depending on the desired effect and can be adjusted by adjusting the dilution of the mother solution. Thus, a layer of 5 to 10 microns will be obtained by diluting the mother solution five times with tap water.

 The following Comparative Examples are intended to illustrate the invention. They are not limiting.

 In all these examples, a carefully degreased 10 cm × 20 cm steel plate is used, then galvanized in an electrolytic bath containing Zn ++, Cl ions and organic brightening products, at a voltage of 2 V, with an intensity of 3A / dm2, for 10 minutes.

 The shiny zinc layer obtained has an average thickness of 8 microns.

EXAMPLE 1
The steel plate having undergone the anticorrosion treatment mentioned above is rinsed, then passivated in a bichromate solution for 30 seconds. The conversion thus obtained has an iridescent yellow appearance.

 The resulting plate is exposed to salt spray under the ASTM test conditions mentioned above. The first traces of "white rust" appear after 120 hours.

EXAMPLE 2
The dichromated passivated plate of Example 1 is coated by dipping a layer of silica gel.

 After drying, it is exposed to salt spray under the same conditions as in Example 1. It reveals traces of corrosion after 150 hours.

EXAMPLE 3
Two dichromate passivated steel plates of Example 1 are coated by dipping a layer of sodium silicate, one with a SiO2 / Na2O ratio equal to 4 and the other with a ratio equal to 3.3.

 After drying, the two plates are subjected to the salt spray test mentioned above.

On the plate having a protective layer with an SiO2 / Na2O ratio equal to 4, the first traces of corrosion appear after 150 hours, while with the protective layer corresponding to the ratio
SiO2 / Na2O equal to 3.3, the first traces of white rust appear after 200 hours.

EXAMPLE 4
The dichromate passivated plate of Example 2 is then soaked at room temperature for 30 seconds in a sodium silicate composition according to the invention containing 15% by weight of freshly prepared SiO2 gel.

 It is allowed to dry completely and then exposed to salt spray, under the conditions mentioned above. It resists 400 hours before the appearance of white rust, twice as long as in the best of the previous examples (Example 3).

EXAMPLE 5
The test of Example 1 is repeated, but passivating the steel plate coated with a layer of zinc by soaking for 30 seconds in a solution based on trivalent chromium (blue passivation).

 Subjected to the same salt spray test, the plate reveals traces of corrosion after 48 hours.

EXAMPLE 6
The test of Example 2 is repeated, but using instead of a dichromate passivated plate, the plate having undergone the blue passivation of Example 5.

 The plate coated with a protective layer of silica gel shows the first traces of corrosion after 96 hours.

EXAMPLE 7
The tests of Example 3 are repeated, but using, instead of dichromate-passivated plates, plates having undergone the blue passivation of Example 5.

 The plate coated with a layer of sodium silicate with an SiO2 / Na2O ratio equal to 4 reveals traces of white rust after 96 hours, while the plate coated with the silicate layer corresponding to the ratio equal to 3, 3 resists salt spray for 120 hours.

EXAMPLE 8
The test of Example 4 is repeated, using, instead of the dichromate passivated plate, the plate having undergone the blue passivation of Example 5.

 Exposed to salt spray under the same conditions as above, the plate coated with a protective layer produced with the composition in accordance with the invention resists salt spray for 240 hours, or twice as long as the plate of the Example. 7, which exhibited the best resistance.

 The results of the above examples therefore show the remarkable efficiency of the corrosion protection process according to the invention and the appreciable technical progress which it brings compared to the prior art.

Claims (13)

 1.- A method for preventing surface corrosion of objects made of metals or metal alloys, characterized in that the surface to be protected, optionally passivated or phosphated, is coated with a composition comprising either an aqueous solution of at least a water-soluble silicate supplemented with a freshly prepared silica gel or sol, or a solution in a freshly prepared silica gel or sol of a silicate insoluble or poorly soluble in water, and in that the this surface is then dried.  2.- Method according to claim 1, characterized in that the coating of the surface to be protected is carried out by dipping in said composition.  3.- Method according to one of claims 1 and 2, characterized in that the coating is carried out at room temperature.  4.- Composition for the prevention of surface corrosion of objects made of metals or metal alloys, characterized in that it comprises an aqueous solution of at least one soluble silicate added with a gel or silica sol that has been obtained frankly.  5. A composition according to claim 4, characterized in that the silica gel or sol represents between 5 and 50% by weight of the composition.  6.- Composition according to one of claims 4 to 6, characterized in that said silicate is a sodium silicate.  7.- Composition for the prevention of surface corrosion of objects made of metals or metal alloys, characterized in that it comprises a solution in a freshly obtained silica gel or sol of a silicate insoluble or sparingly soluble in water.  8.- Composition according to claim 7, characterized in that said silicate is a lithium silicate.  9.- Composition according to one of claims 4 to 8, characterized in that it comprises from 1 to 5% by weight of an oxygenated compound such as a borate or an aluminate, and / or titanium.  10.- Composition according to one of claims 5 to 9, characterized in that it comprises polyethylene glycol, preferably with an average molecular weight close to 6000.  11.- Composition according to one of claims 5 to 10, characterized in that it comprises polytetrafluoroethylene.  12.- Composition according to one of claims 5 to 11, characterized in that it contains dyes.  13.- Composition according to one of claims 5 to 12, characterized in that it contains ethylene diamine tetra-acetic acid (EDTA).