US2463010A - Method of reducing corrosion - Google Patents

Description

Patented Mar. 1, 1949 nm'rnon or aanuomc poaaosrou John 0. Almen, Royal Oak, Mich., assignor to' General Motors Corporation, Detroit, Mich, a

corporation of Delaware No Drawing.

1 Shot peening or blasting a surface with smooth steel balls for compressively stressing the skin of a metal article increases materially resistance to fatigue failure. peening can be effected for the purpose of resisting corrosion of several kinds. a magnesium strip one-sixteenth of an inchv in thickness stressed by bending will fail by stress corrosion in approximately two minutes time when immersed in a salt solution. On the other hand the same specimen will endure indefinitely after the surface has been compressively stressed by light shot peening in the manner to be described.

Magnesium or other similar metals when closely fitted to a dissimilar metal, such as iron, are likely to corrode badly. Corrosion which occurs between bolted surfaces, onsplines, press fits and other like closely fitted parts is known under the general name of fretting corrosion. Such corrosion is retarded and its penetration reduced by compressively prestressing the surface. Stress corrosion is severe in magnesium'and its avoidance is of special importance in aircraft construction largely comprised of magnesium metals in critical parts.

Rapid electrolytic corrosion occurs on such metals as aluminum, magnesium, etc., when iron dust is retained on the surface of the metal from shot blasting. Such corrosion occurs on exposed surfaces particularly .when immersed in salt solutions although it will occur in the atmosphere particularly in damp weather, such as rusting of steel parts. 7

Stress corrosion is the more rapid attack of normal atmospheric corrosion. The penetration is rapid, presumably because when a tension stress is present the gaps formed'by corrosive severance of the bonds of adjacent particles will This corrosion results from very small motions between the tightly fitted parts. It may fundamentally be an electrolytic action since we'are told that all corrosion is electrolytic, but electrolytic action may occur without the presence of I- have discovered that proper For example,

Application January 5, 1944, Serial No. 517,102

'3 Claims. (01. 29-148) metal between regions of dissimilar stress. Fretting corrosion, therefore, is not dependent-on the contamination due to shot peening whether the fretting corrosion occurs between similar or dissimilar metals.

Compressively prestressing the surface of parts 7 subject to corrosion damage has proved itself highly effective.

However, care must be exercised to see that the metal surface to be compressed is free from contact with a dissimilar metal during the prestressing operation including dust or flake particles from the shot which tend to accumulate at the shot blasting station and to settle on articles being operated upon and under the hammering to be embedded in and to contaminate the metal surface. Thus iron dust from steel shot would contaminate the surface of a magnesium article and must be removed or prevented from' adhering in order to avoid subsequent electrolytic action between the dissimilar metals which electrolytic action would undo the effect of prestressing in reducing corrosion.

Magnesium and similar materials corrode very rapidly when stressed in tension and less rapidly when unstressed. The object of compressive prestressing is to place a thin surface layer of the material in a state of compression even though underlying layers are stressed in tension.

To accomplish the result, I have used the expedient 'of coating the surface of the article to be treated with a strong paint or lacquer and then after the coating has dried and set, peening through the coating. A strongly adherent and elastic paint is required to avoid flaking during the peening operation but the coating should be thin so as not to unduly cushion the impact 01' the shot. Thus the impacts are transmitted to the metal surface by means of a nonmetallic agency and the surface is maintained free from contamination. Once the surface has been densifled the paint, if desired, can be removed and at dissimilar metals. Burns of the Bell Laboratories states that electrolytic action may occur in a any rate the magnesium may be joined or press fitted to an iron part without fear of serious corrosion problems. plish the same result in the absence of contact between dissimilar metals, it is proposed to use smooth shot of a nonmetallic material, such as glass or a relatively strong plastic.

Glass shot has been found to be satisfactory for this purpose.

In my study of the problem I have noted resistance'to other forms of corrosion results-from shot peening. Fretting corrosion has been great- This has As an alternative to accomdue to the surface roughness of the peened specimen but penetration of corrosion is apparently greatly reduced. Similar results-have been obtained in fretting corrosion between dissimilar metals in airplane engines.

In addition to shot peening, the results described abo've may be obtained by other prestressing means such as rolling with small rollers or balls. It is only necessary that the surface the metal be stressed in compression.

I claim:

1. The method of reducing surface corrosion between tightly fitted partsof dissimilar metals, comprising subjecting one ofthe parts, before they are tightly fitted, to a conditioning treatment which includes preh ating the joint surface with a thin elastic film capable of transmitting impact therethrough and incapable of electrolytic action with the material of said surface and compressively stressing the surface beneath said film by repetitive blows of an impacting body against the film covered skin of the joint surface which precompresses the film covered skin of the joint surface and thereafter tightly fitting together said parts with the joint surfaces of one in contact with the precompressed skin of the joint surface of the other. Y

2. The method of decreasing the tendency toward surface corrosion in articles containing a magnesium metal, including applying on the surface to be protected, a coating of strongly adherent and elastic paint, allowing the paint to set and then peening the metal through the coating to stress the surface of the metal in compression.

5 uniformly spherical shot.

3. The method of prestressing in compression the surface of an article formed of largely magnesium including peening said surface by blasting the same with nonmetallic smooth and JOHN O. ALMEN.

file of this patent:

UNITED STATES PATENTS Number Name Date 326,469 Youngs Sept. 15, 1885 489,642 Reich Jan. 10, 1893 1,433,213 Lowell Oct. 24, 1922 2,043,481 Krissiep June 9, 1936 2,058,841 Thomson Oct. 2'7, 1936 2,149,253 Cooper Mar. 7, 1938 2,185,262 Lupo Jan. 2, 1940 2,208,732 Powell July 23, 1940 2,234,491 Hayden Mar. 11, 1941 2,349,395 Zublin' May 23, 1944 FOREIGN PATENTS Number Country Date 284,313 Great Britain Mar. 1, 1928 288,637 Germany Nov. 9, 1915 OTHER REFERENCES Pub., The Technology of Magnesium and Its Alloys, F. A. Hughes & Co. Ltd., London, 1940, pages 231, 233. i

Metals Handbook, 1939 'ed., Am. Society for Metals, Cleveland, Ohio, page 1614.