US2458073A - Coating magnesium and magnesium alloys - Google Patents

Description

Patented Jan. 4, 1949 COATING MAGNESIUM AND MAGNESIUM ALLOYS Herman-W. Hemker, Euclid, and Donald L. Ep-

pink, Oleveland,hio, assignors to The Parker Appliance Company, Cleveland, Ohio, a corpo ration of Ohio No Drawing. Application January 6, 1944,

Serial No. 517,262

4 Claims. 1

It is known that magnesium metal, because of its low specific gravity, is highly advantageous for employment where weights should be kept at a minimum. The strength and serviceability of articles have been improved by employing the magnesium in the form of alloys.

Furthermore, many proposals have been made for so treating articles formed of magnesium or magnesium alloys that they are resistant to corrosion, which is known to be highly and quickly damaging by reason of the high reactivity of magnesium metal to oxygen, particularly in the presence of saline solutions. Upon accelerated corrosion tests, however, it has been found that the best of the prior proposals give a maximum life not exceeding 100 to 150 hours and most of the proposed treatments lead to failure in much shorter times.

In accordance with the present invention, an article formed of magnesium or magnesium alloy is given a superficial treatment which produces a condition of high resistance to corrosion, whereby the article remains stable over periods, under accelerated corrosion tests, amounting to many hundred hours.

Fundamentally, the non-corrodability is produced by treating the article superficially with an aqueous solution at high temperature, this solution being highly alkaline and containing dissolved therein a strong oxidiz ng agent. In addition. it is preferred to introduce additional soluble substances into the treating solution which operate jointly with the other ingredients of the solution to produce a high resistance to corrosion, such additional substances including soluble borates, ferrocyanides, acetates, antimonates, and aluminates.

In preparing the solution, and for the purpose of having a low vapor tension at the operating temperature, it is desirable to have essential saturation of the solvent water by the solutes; and in particular it is preferred to select mixtures of oxidizing agents in order to avoid limitation of effective concentration by the mass action effect and limiting solubility of a singleoxidizing agent. It will be understood that the additional solutes introduced have, in addition to chemical and physical protective effects upon the article, also an effect of increasing the boiling point of the aqueous solution and therewith permitting operation in liquid phase at a high temperature.

It has been found that advantageous treating solutions can be prepared atsolution densities whichrepresent'frorn 20 to 25 per cent'of water, with the remainderin the formof dissolved matotal dissociation of the dissolved material can:

not occur and hence there is always present'arr excess of undissociated. alkali.

It has been found that the procedure is effective upon magnesium metal and uponcomnier cially available alloys thereof. Among such alloys; are the following, which are identifiedby a'lbi-i trary designations in lieu of trademark names; f

Alloy A: 1.5% manganese, 0.3 'silicon ,balance essentially magnesium.

Alloy B: 0.15% manganese, 0.4% to 1% zinc, 0.3% silicon, 5.8 to 7.2% aluminum, balance essentiallymagnesium. v I

Alloy C: 0.3% manganese, 1.0% zinc, 2 .7%- aluminum, balance essentially magnesium.

Alloy D: 0. manganese, 0.6% zinc, 9.0% aluminum, balance essentially magnesium.-

As specific examples of 1 practice under this invention, the following may be given:

. 800 hours. i

. mate and 40 grams of sodium nitrate. The an Example I A bath is prepared which comprises a water, solution containing 1200 grams of sodium hydroxide, 720 grams of sodium bichromate, 600 grams of sodium nitrate, 240 4 grams of borax (Na2B4O-1-10H2O), 300 grams of sodium acetate, together with a quantity of water which gives a boiling temperature in excess of C., as for example 1200 milliliters. f

This solution has a water content of about 28% by Weight. It is brought to the maximum temperature short of ebullition, and the articles formed of alloy B are suspended inthe bath and held immersed for 45 minutes. Upon removal, the articles are rinsed in water at 20' Q. and are then found to have gained. a. condition of resist ance to accelerated salt spray. treatmentwhich prevents objectionable corrosion for as long as Example II A similar treatment, at maximum temperature short of ebullition, was given in a treating solu: tion'made from grams of water, 125 grams of sodium hydroxide, 48 grams of sodium bichroticles of alloyA demonstrated resistance to the? accelerated salt spraytestfor 350 hours? articleso'f'alloy B for 300 hours; articles of alloy q for hours.

asoaove Example III When 20 grams of borax, or quantities of boric acid or sodium. metaborate containing boron equivalent to that of the said quantity of borax, was added to'the bath of Example II, articles of alloy A resisted the accelerated test for 350 hours and articles of alloy B for 500 hours.

Example IV When grams of potassium ferrocyanide were added to the bath of Example II, articles of alloy A treated therein showed resistance for 269 hours, of alloy B for 310 hours, and of alloy C for 170 hours.

Example V and of alloy C for 150 hours.

Example VII When 9' grams of borax, 5 grams of trisodium phosphate, 2 grams of potassium ferrocyanide and 5 grams of sodium alumin'atewere added to the bath ofExample II, articles of alloy A showed resistance for'225 hours, of alloy B for 300 hours and of alloy ('2' for 250 l n'mrs. I

Example VIII When-m gramsof bor-axand grams of sodium acetate were added to thebatn of Example 11, articles of alloy showed a resistance for 200 hours of'all-oy B for too hours, and of alloy C for 200 hours. The water content is about '34 per cent. I

Example-IX I When 9 grams of *borax and 5 grams of trisodium phosphate were add'eol'to the bath of Example II, articles of alloy B showed resistance for200 hours.

Example X Employing thebathof Example II, but with grams of bich-romate, and also adding 1-0 grams of *borax, '5 grams of antimony trioxide and 25 grams of -sodiumacetate, and treating asand '7 g'rams of sodium aluminate, articles'of alloy A endured for 500 hours, and articles of alloy B for 200 hours.

The foregoing statements of times of resistance are'basedupon the salt spray'test, but are not necessarily the maxim-um for the particular Th accelerated salt spray test, referred to above, :is the standard test which comprises spray ingothe coated piece with a20% salt solution at a temperature of degrees I as set-forth in Governmental Specification AN-QQ-S-9l-.

The illustrative baths have the common features of composition and behavior in that they are used at high temperature, exhibit a high alkalinity, and contain a soluble oxidizing agent. In addition, many of the baths also comprise an addition agent or agents selected from the water soluble borates, ferrocyanides, acetates, aluminates, alkaline phosphates and antimonates.

The temperature should be above degrees C. and can be 170 degrees C., which is permitted without ebullition or excess loss of vapor, by reason of the high content of freely soluble solutes in thebath. In particular, it has been found advantageous to operate in the liquid phase at a temperature close to the boiling point of the se- I lected mixture, usually between and $.45 de- The oxidizing agent employed for the bath includes freely soluble nitrates, as salts of alkali metals, for the reason that such salts are highly soluble and do not become ineffective through.

forming precipitates from reaction with other ingredients of the bath or from reaction with the metals of the magnesium alloy itself. In addition, theoxidizing agent of the'bath should include a highly soluble chromate, such as an alkali metal chr-omate, inclusive of the sodium, potassium and lithium salts. The chromate may be introduced in the form of 'dichro-mate as this is converted to chromate in the bathitsel'f. I

Among the addition agents which have been found valuable for modifying the feasible operating temperature, and for co-action with the alkaline bath containing the nitrate or other highly oxidizing salt and a clironiate, itis preferred to employ a soluble borate by introducing borax, boric acid, or sodium metab'orate into the solution. -Other feasible ad'ditions'in'clude other soluble'borates, potassium and other soluble fer r-ocyani-des; antimony trioxide (which is amphetoxic and forms antimonate in the alkaline bath) (sodium and other soluble acetates, sodium and other soluble aluminates, trisodium and other soluble phosphates, together with mixtures of more than one of such ingredients, as pointed out in the examples above,

'Ith'as often been found advantageous to em.- ploymoreth'an one such agent, in order to get an effect greater than that by use of a corresponding total quantity of only one such agent.

By the term freely soluble is meant a condition of solubility 'by which the total content of solutesexceeds the content of water in the bath. Thus; a maximum proportion of water is about 55 70 by weight, and it is preferred to operate with the waterw'content between 25 and 30 percent by weight, wherewith the bath is saturated at the operating temperature with dissolved compounds which preferably are the freely soluble alkali metal salts, alkaline salts and alkalies. Under these conditions, the coating procedure upon magnesium an'd'the commercial magnesium alloys containing 88% or more of magnesium, for example, may-beeff-ected in 45 minutes at a tom peratureof .135 to degrees C. for an optimum effect, butgood results are obtainable by treating for anylength of time between .15 and fio Preferred Range Grams Grams 1. Water 125 2. Freely soluble inorganic alkali (alkali metal alkali) l. 125 02. to 187. 5 3. oxidizing agents:

Alkali metal nitrate 40 20 to 60 (b) Alkali metal chromate or (lichromate 48 24 to 72 4. Addition agents 30 0 to 75 In metallographi-c investigation of the treated articles, it is found that there is no intergranular corrosion in the samples; but, with articles demonstrating relatively low salt spray resistance, some pitting appears which may be due to thin spots or pin holes in the protective coating.

It is preferred to employ the material at high concentration, under the conditions stated, with the solids present in quantity to effect saturation at the reaction temperature. An excess of the solute mixture is usually present at reaction temperature; and upon cooling to room temperature the bath forms a heterogeneous mass of crystals. The bath can be employed in an open pot, as the boiling point is so high that no ebullitio-n or bubbling occurs; and air contact with the solution has no essential influence upon its life.

A presently used proportion is that of employing three-fourths of a gallon of the bath in Example I in treating up to square feet of several alloys, but even after articles aggregating this superficial area have been passed through the bath there still appears no sign of depletion or exhaustion of the bath from capability of producing the desired result. Upon prolonged use,

precipitation of a non-watersoluble substance occurs but this doe not impair the efficiency of operation and can be removed if desired.

The precise chemical composition of the coating film is unknown. It is not electrically conductive and has a color range from gray to black, being a light to medium gray in most instances. The coating is so thin that it does not essentially change the dimensions of the article.

It is obvious that the invention can be emplayed in many ways other than those of the specific examples and within the scope of the appended claim-s.

We claim:

1. The method of providing a corrosion-resistant coating upon surfaces of magnesium and high-magnesium-content alloys, which comprises cont-acting the surface at a temperature of 1120 to 1'70 degrees C. for substantially 15 to minutes with an' aqueous solution prepared by dissolving in water, as the predominant constituents, 62.5 to 187.5 parts of alkali metal hydroxide, substantially 24 to '72 parts of alkali metal bichromate, and substantially 20 to 60 parts of alkali metal nitrate, the proportion of water in the solution being between 20 and 55 per cent, and the solution having a pH value in excess of 13.

2. The method as in claim 1, in which the aqueous solution in addition contains substantially 30 to parts of freely soluble alkali metal salts of Weak acids.

3. The method as in claim 1, in which the aqueous solution in addition contains substantially 30 to 75 parts of a mixture of borax and alkali metal acetate, at least one-third of the said mixture being borax and the amount of acetate being at least as great as the amount of borax.

4. The method of providing a corrosion-resistant coating upon surfaces of magnesium and high-magnesium-content alloys, which comprises contacting the surface at substantially maximum temperature short of ebullition for about 45 minutes with an aqueous solution prepared by dissolving in water, parts of alkali metal ydroxide, substantially 48 parts of alkali metal bichromate, and substantially so par-ts of alkali metal nitrate, and substantially 30 parts of a mixture of borax and alkali metal acetate, substantially one-third of the mixture being borax, the proportion of water in the solution being between substantially 25 to 3d per cent, and the solution having a pH value in excess of 13.

HERMAN W. HEMKER. DONALD L. EPPINK.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS FOREIGN PATENTS Country Date Austria July 25, 1923 Great Britain July 8, 1930 Germany May 23, 1941 Number Number