US2264310A - Magnesium base alloy - Google Patents

Description

Patented Dec. 2, 1941 MAGNESIUM BASE ALLOY Joseph D. Hanawalt and Charles E. Nelson, Midland, Mich., assignors to The Dow Chemical Company, Midland, Mich., a corporation of Michigan No Drawing.

Application March 9, 1940,

Serial No. 323,116

1 Claim.

This invention relates to alloys of magnesium with aluminum, manganese, and zinc. It relates in particular to an alloy containing primarily magnesium with from 1 to 12 per cent of aluminum, from 0.01 to 0.5 per cent of manganese, and from 2 to 4 per cent of zinc.

Among the most desirable commercially available alloys of magnesium from the standpoint of its general physical characteristics is the alloy of commercial magnesium with from 1 to 12 per cent industry for many years has only fair corrosion resistance (.4 to milligrams per square centimeter per day) compared to ultra pure magnesium which has a corrosion resistance value of not to exceed 0.2 milligramsper square centimeter per day and the commercial quaternary alloy cannot be utilized for many purposes for which it is otherwise well suited.

As is the case of many other pure metals, magnesium itself is not a desirable structural metal and industry has resorted to alloys, among the commonest of which is that of magnesium with aluminum, manganese and zinc above described. The commercial alloy of magnesium, aluminum, manganese, and from 2 to 4 per cent of zinc is comparatively easily corroded in aqueous sodium chloride solution or by brine spray and is therefore unsuited, for example, for use along the seaboard or on board ship where it might be exposed to prevailing atmospheric conditions.'- The comparatively poor corrosion resistance of the commercial magnesium-aluminum-manganese-zinc alloy is not due to the presence of the aluminum or of the manganese or of the zinc but it is due to the existence ofadditional elements in actually small but still disadvantageously high proportions.- Thus, an alloy consisting of very pure magnesium, pure aluminum, pure manganese, and pure zinc and containing no other elements has a corrosion resistance at least equal to that of magnesium alone. Such a pure alloy. however, is not as workable in all respects as the commercial alloy and-further it is improbable that it could be made generally available economically.

It is an object of the present invention to pro-.

vide an alloy of magnesium with from 1 to 12 per cent of aluminum, from 0.01 to 0.5 per cent of manganese, and from 2 to 4 per cent of zinc which will have a corrosion resistance equal to that of the pure alloy and which, in addition to high corrosion resistance, exhibits workability at least comparable with that of the alloy heretofore commercially available.

The above-mentioned corrosion resistance values of magnesium and its alloys are determined by means of a now standard procedure known as the alternate immersion method. This method as applied to magnesium alloys comprises immersing a weighed sample of the material of measured area into a 3 per cent aqueous sodium chloride solution at room temperature for 2' minutes, withdrawing the sample and holding it in air for 1 minute, repeating this cycle for a protracted period, and computing the loss in weight of the sample per square centimeter of surface area per day. In the standard tests herein reported, the corrosion resistance values are expressed in terms of average weight loss per day per unit area over a testing period of 112 days.

It has now been found that the foregoing and related objects may be attained and that a. magnesium-aluminum-manganese-zinc alloy can be produced which will have a corrosion resistance, as measured by the alternate immersion method, at least as good as that of ultra pure magnesium and which exhibits the desired physical properties which industry requires by including in the said alloy a very small amount of at least one of the elements iron, nickel, copper, lead and silicon. It has been recognized that the presence of iron in various magnesium alloys is disadvantageous and that the amount of iron should be kept as low as possible to minimize corrosion. This is not the whole story, however, and we have found that the mere elimination of iron or reduction of the iron content to a very low value is not suificient to produce a non-corrosive alloy when other elements are present. It has been found that not only must the amount of iron be minimized but also that the amount of numerous other elements must be controlled with'a rather narrow range. The presence of at leastone of the elements iron, nickel, copper, lead and silicon is distinctly advantageous from some standpoints, provided the amount thereof is below the tolerance limit, as will be more fully described hereinafter.

The invention, then, resides in an alloy charactcrized. by a degree ofcorrosion resistance substantially equal to that of pure magnesium consisting of from 1 to 12 per cent of aluminum, from-0.01 to 0.5 per cent of manganese, from 2 to 4 per cent of zinc and containing at least one of the elements iron, nickel, copper, lead and silicon in amount not to exceed 0.003 per cent of iron, not to exceed 0.002 per cent of nickel, not

to exceed 0.5 per cent of copper, and preferably not to exceed 0.7 per cent of lead, and not to exceed 1.0 per cent of silicon; the balance being magnesium. An alloy composed as above is characterized by having a corrosion rate, in the alternate immersion method in brine, of not to exceed 0.2 milligram per square centimeter per day.

The foregoing tolerance limits for the added elements iron, nickel, copper, lead and silicon apply to the herein claimed composition, whether these elements are present individually or in groups. In the case of certain magnesium base alloys, it has been determined, and is now being taught in concurrently filed applications directed to such subject matter, that certain of the said added elements exhibit a co-operati-ve corrosion efiect when present together with iron such'that the amount of iron and of the co-operative element must be reduced below the tolerance limit when both are present. This limitation does not apply to the present compositions which are, therefore, more readily producible in quantity and of quality suitable for industrial purposes.

The following table illustrates the critical nature of the tolerance limit for nickel given in the preceding discussion of corrosion. The elements named in the tables are the only ones present in significant amounts. The amounts of the respective elements in the alloy being tested are expressed in per cent by weight of the alloy, the balance being magnesium, and the corrosion rates determined by the alternate immersion method 40 are expressed in milligrams weight loss per square centimeter per day based on the average loss over a period of 112 days.

The herein claimed improved alloy in addition to its improved corrosion resistance is at least as hard and strong as the commercial alloy. The alloy herein claimed differs from that claimed in our co-pending application Serial No. 323,115, filed concurrently herewith, both in the change in the amount of zinc present, both in the corrosion tolerance limits, a change in which has been found necessary due to the efiects of the larger amount of zinc recited in the present claims. Thus, the presence of from 2 to 4 per cent of zinc in a magnesium-aluminum-manganese zinc alloy somewhat increases the absolute values of the corrosion tolerance limits for the various added elements herein recited as well as lessening the injurious eflect of the said added elements when these are present in amounts greater than the tolerance limits.

We claim:

A magnesium base alloy including from' 1 to 12 per centum of aluminum, from 2.0 to 4.0 per centum of zinc, from 0.01 to 0.5 per centum of manganese, and containing nickel in a positive amount not exceeding 0.002 per centum, said alloy having a corrosion resistance below about 0.2 milligram per square centimeter per day in alternate immersion in three per centum aqueous sodium chloride.

JOSEPH D. HANAWALT.

CHARLES E. NELSON.