US3378359A

US3378359A - Method of protecting austenitic stainless steel subject to stress corrosion

Info

Publication number: US3378359A
Application number: US613038A
Authority: US
Inventors: Kenneth R Walston; Alistair S Couper
Original assignee: Standard Oil Co
Current assignee: Standard Oil Co
Priority date: 1967-01-31
Filing date: 1967-01-31
Publication date: 1968-04-16
Anticipated expiration: 1985-04-16

Description

United States Patent 3,378,359 METHOD OF PROTECTING AUSTENITIC STAINLESS STEEL SUBJECT TO STRESS CORROSION v Kenneth R. Walston, Morristown, N.J., and Alistair S.

Couper, Chicago, Ill., assignors to Standard Oil Company, Chicago, 11]., a corporation of Indiana No Drawing. Continuation of application Ser. No. 378,445, June 26, 1964. This application Jan. 31, 1967, Ser. No. 613,038

4 Claims. (Cl. 29-1962) ABSTRACT OF THE DISCLOSURE Austenitic stainless steel, having its exposed surface partially coated with aluminum, is used in a process of the type wherein the surface area of the steel is to be exposed to a stress-corrosive environment.

This is a continuation of our copending application Ser. No. 378,445, filed June 26, 1964.

This invention relates to improvements in the resistance of austenitic stainless steels to stress-corrosion cracking and more particularly to a method of improving the resistance of austenitic stainless steels to stress-corrosion cracking by partially coating the stainless steel with aluminum and to the resultant stainless steel object.

Austenitic stainless steels are those with austenite and commonly contain appreciable amounts of chromium and nickel. Usually the combined chromium and nickel contentis greater than 24% with each element being present in an amount equal to about 7% or more. Usually the carbon is very low. One of the common austenitic stainless steels is the 18-8 type containing about 18% chromium and 8% nickel.

The austenitic stainless steels are known to have the problem of stress-corrosion cracking. This problem is discussed in such references as Samans, Carl H.: Engineering Metals and Their Alloys, third printing, New York, The MacMillan Company, 1952, p. 621 and Chemical Processing, Costly Stress-Corrosion Cracking Curbed, Oct. 21, 1963, issue. Generally the problem has been encountered when the stainless steel has been used in a chloride or sulfide environment.

Stress-corrosion cracking in austenitic stainless steels is particularly troublesome since the damage is caused by .cracks which in many instances are quite deep and significantly reduce the effective life of the stainless steel compared to the slower effects of ordinary corrosion. Because of the problems associated with stress-corrosion cracking in austenitic stainless steels, it is an object of this invention to improve their resistance to cracking. Other objects will become apparent from the detailed description below.

In our elforts to improve the resistance of austenitic stainless steels to stress-corrosion cracking, we have discovered that such resistance is remarkably improved when the stainless steel is partially coated with aluminum. It is not necessary that the aluminum cover the entire surface of the austenitic stainless steel to provide the improved resistance. However, it is advantageous to have the aluminum substantially distributed over the surface of the stainless steel to increase the elfectiveness of the protection.

Briefly, the invention is directed to a method of improving the resistance of austenitic stainless steel to stresscorrosion cracking, which method comprises partially coating stainless steel with aluminum. Useful forms of the coated stainless steel are vessels, exchangers, pipes, and the like.

In the method of this invention, the austenitic stainless steel is partially coated with aluminum. The coating process may be carried out by spraying, welding, dipping, or the like. Advantageously, the partial coating should be substantially distributed over the surface of the stainless steeL'A stainless steel which is particularly of interest for this protection contains about 18% chromium and about 8% nickel.

Advantageously, the coating process may include a subsequent heat treatment to improve the bonding of the aluminum to the stainless steel. The heat treatment is commonly carried out at temperatures up to about 1300 F. and in an inert atmosphere, although the latter is not essential. At 1300 F., the time for the treatment is commonly about one hour.

The resultant austenitic stainless steel with its partial coating of aluminum exhibits improved resistance to stress-corrosion cracking. The protected stainless steel may be utilized in numerous ways. One of particular interest is as a stainless steel vessel.

The particular benefits of the aluminum protection is of primary interest when the stainless steel is subjected to the environment of chloride or sulfide, since these environments are usually associated with stress-corrosion cracking.

The following examples are some embodiments illustrating the invention. It is to be understood that these are for illustrative purposes only and do not purport to be wholly definitive with respect to conditions or scope.

Example I A test was carried out in which three 18-8 stainless steel specimens in the form of horseshoes were subjected to boiling magnesium chloride. The first specimen was unprotected, the second specimen was attached to two pieces of aluminum, and the third specimen was partially coated with aluminum. Each specimen was prepared by bending a stainless steel bar into a horseshoe shape with a bolt then being inserted through the two ends of the horseshoe to prevent any outward movement of the ends. Each of the aluminum pieces for the second specimen was placed on the bolt adjacent to each end of the horseshoe. The third specimen was prepared by spraying aluminum on the stainless steel bar and then bending the bar into the horseshoe shape. This bending caused the aluminum layer to separate at the location of the maximum bend and expose the stainless steel.

The magnesium chloride solution was prepared from approximately 350 grams of .MgCl 6H O and 40 milliliters H O. The test was carried out in a reflux flask and at a temperature of approximately 307 F. (153 0.).

Each specimen was individually tested by placing it in the boiling solution, and the test was carried out until failure of the specimen occurred. The failure point occurred when the cracks in the stainless steel specimens were sufficiently deep to permit the collapse of the horseshoe shape when its two ends were manually moved inwardly.

In the test, it was observed that the unprotected specimen failed in about 4 hours, the specimen attached to the aluminum pieces failed in about 17 days, and the specimen partially coated with aluminum failed in about 38 days. These results demonstrate that the partial coating of aluminum more than doubled the life of the stainless steel specimen in the chloride environment.

Example II A second test was carried out in which stainless steel horseshoes, of the type-described in Example I, were subjected to polythionic acid at approximately F. The acid was prepared by bubbling H 8 through an aqueous solution saturated with S0 until the solution became saturated with respect to H S.

In the test the unprotected specimen failed after about an hour, the specimen attached to the aluminum pieces failed in about 1 hour also, and the specimen partially coated with aluminum failed in about 17 hours. The above results demonstrate that the partial coating of aluminum increased the life of the stainless steel in the sulfide environment by a multifold factor.

While the invention in its various aspects has been described with reference to particular embodiments thereof, it is apparent that these are by way of illustration only. Accordingly, it will be understood that modifications and variations thereof will be apparent to those skilled in the art, and it is thus intended to embrace all such modifications and embodiments as fall within the broad scope of the appended claims.

We claim:

1. In a method of utilizing austenitic stainless steel subjected to stress corrosion, the improvement providing the exposed surface with a partial aluminum coating in discontinuous portions distributed over the exposed surface of the steel.

2. The method of claim 1 wherein said stainless steel contains about 18% chromium and about 8% nickel.

3. The method of claim 1 wherein said environment is a chloride environment.

4. The method of claim 1 wherein said environment is a sulfide environment.

References Cited UNITED STATES PATENTS 2,171,040 8/1939 Merritt 29l96.2 2,908,073 10/1959 Dulin 29197 3,167,403 1/1965 Smith 29196.2 3,173,202 3/1965 Farber 29196.2 3,210,840 10/1965 Ulam 29-196.2

2g HYLAND BIZOT, Primary Examiner.