US3622402A

US3622402A - Erosion-corrosion resistant coating

Info

Publication number: US3622402A
Application number: US796566A
Authority: US
Inventors: Sanford Baranow; Richard B Andrews
Original assignee: Avco Corp
Current assignee: Avco Corp
Priority date: 1969-02-04
Filing date: 1969-02-04
Publication date: 1971-11-23
Anticipated expiration: 1988-11-23

Abstract

A process for improving the erosion resistance and corrosion resistance of steel articles which comprises boronizing said articles and thereafter coating said boronized articles with chromium under conditions such that the chromium combines with the boron to produce an article bearing a coating containing a boride rich in chromium on the surface of said article.

Description

United States Patent [72] Inventors Sanlord Baranow Woodhridge; Richard B. Andrews, Milford, both of Conn. [21 Appl. No. 796,566 [22] Filed Feb. 4, I969 [45] Patented Nov. 23, 1971 [73] Assignee Avco Corporation Stratford, Conn.

[ 54] EROSION-CORROSION RESISTANT COATING 117/71, 106, 107.2, D10. 10; 29/1966, l9, l 66 Primary Examiner-Ralph S. Kendall Attorneys-Charles M. Hogan, Irwin P. Garfinkle and Lawrence 1. Field ABSTRACT: A process for improving the erosion resistance and corrosion resistance of steel articles which comprises boronizing said articles and thereafter coating said boronized articles with chromium under conditions such that the chromium combines with the boron to produce an article bearing a coating containing a boride rich in chromium on the surface of said article.

EROSION-CORROSION RESISTANT COATING This invention relates to articles of steel, including alloyed steels such as stainless steel, provided with a chromium-boron coating and to the process whereby such articles are produced. I

Many steels are known which possess the strength necessary for use in service under conditions where the steel is subjected to considerable stress. However, these alloys are often deficient in resistance to the erosion and/or corrosion encountered in their intended service.

One object of the present invention is to provide improved articles such as compressor blades which when exposed to the severe environmental conditions incidental to the operation of a gas turbine engine, exhibit a high degree of erosion resistance and a high degree of corrosion resistance and a fatigue life not substantially lower than that of the article before the coatings of the present invention are provided thereon.

Gas turbines are presently used to power helicopters and land vehicles adapted to be utilized on sandy beaches and in other erosive environments. The present invention provides erosion protection for the components of said turbines without sacrificing any substantial portion of the fatigue life of the steel from which the parts are fabricated.

As is well known, steels can be boronized by various techniques including pack cementation and chemical vapor deposition to obtain outer surface layers which contain ironboron compounds. Further, it is known that such coatings when properly formed possess extremely high-erosion resistance and markedly inferior corrosion resistance. With some materials, e.g., AM350 Stainless Steel (nominal composition: 16.5 percent Cr, 4.5 percent Ni, 3 percent M0, 0.1 percent N, balance Fe), the fatigue life of the resulting boronized article may be diminished by as much as 50 percent, as compared with the fatigue life on the article before boronizing.

in one effort to avoid this impairment of corrosion resistance it has been proposed to chromize the base material prior to boronizing, as described, e.g., in US. Pat. No. 3,029,162. The surface of the resulting article retains the higherosion resistance possessed by simple boronized surfaces, and possesses an improved corrosion resistance, as compared with such boronized articles, although the corrosion resistance of such chromized, boronized articles is only moderate. The fatigue life of such articles is still only about 50 percent of that of the original, uncoated material.

When the chromized, boronized article is examined under a metallurgical microscope, the microstructure of the coating is observed to be similar to that of a simple boron coating, except that the chromium-boron coating is found to be richer in chromium than the simple boron coating.

It has now been foundthat the improvement in erosion resistance resulting from a boron coating on the article can be retained and the improvement in corrosion resistance resulting from a chromium-boron coating can be increased by a simple, but fundamentally significant, change in the procedure described in US. Pat. No. 3,029,162, specifically by changing the order of deposition of the coating constituents. In the present invention, the boron is applied first and then the chromium is deposited on the articles to be protected.

When an iron or steel article is boronized, the coating formed is an outer layer comprising FeB and an inner layer of Fe B. Continued intrusion of boron produces increasingly thick layers of FeB and Fe B.

If, as in Samuels US. Pat. No. 3,029,162, the article is coated with chromium, silicon, titanium, etc., and then boronized, the resulting articles are found to comprise one or more FeCr-B compounds, as the surface coating. The

result is an improvement in the corrosion resistance, as com pared with a boronized, but not previously chromium-coated article.

Now, in the present invention, the boron is deposited first and then the chromium is deposited under such conditions that it is able to combine with the alread deposited boron. As a result, the outer layer of the article IS a most entirely Cr B or Cr B and very little Fe appears to be present in the outermost layers of the coating. This high chromium, boride coating is remarkably corrosion resistant as well as erosion resistant. in addition, the fatigue life of blades coated according to the present invention is at least percent that of uncoated blades and, therefore, the resulting articles are serviceable in many uses where low-fatigue life bars use of the prior art coated articles.

The following example will serve to further illustrate the benefits conferred by the present invention.

Twenty AM350 stainless steel alloy compressor blades were placed in a retort containing boron powder and 0.5 weight percent potassium fluoride. The retort was heated in an inert atmosphere for 1 hour at l,900 F. in which time an iron boride coating of 0.00l-inch nominal thickness was formed. The blades were then packed in a retort containing chromium powder and heated in a vacuum for 4 hours at l,950 F. After being permitted to cool to room temperature, the blades were removed from the retort. Then various tests, including sand erosion, corrosion and fatigue, were performed. The erosion rate of B-Cr coated blades was found to be about 20 times lower than that for uncoated blades. No attack was noted in salt-spray corrosion tests. The fatigue life of the blades was affected negligibly.

Other steels and other stainless steel alloys may be used in place of the specific alloy of the foregoing example and the boron and chromium may be deposited on the articles by any other known methods, provided that the order of deposition is as specified and that the boron-coated article is exposed to an elevated temperature for a sufficient time interval for the Cr to combine with the B already present.

It will also be understood that unless there is sufficient boron to form the desired high-chromium boride, the full benefits of the invention will not be realized. Hence the boronizing step should be carried out to form a boron-containing layer which is at least about 0.000 1 -inch thick.

Similarly, the amount of chromium applied to the boronized articles must sufiice to form the intended high-chromium boride-coating on the final article. Deposition may be by any known method, provided it does not remove or adversely affect the boron deposited on the articles.

Having now described a preferred embodiment of the invention, it is not intended that it be limited, except as may be required by the appended claims.

We claim:

1. A process for improving the erosion resistance and corrosion resistance of steel articles which comprises:

boronizing said articles. and thereafter chromizing said boronized articles to produce articles bearing a highchromium boride-containing coating on the surface of said articles.

2. The process of claim 1 wherein the boron-containing coating first laid down is at least approximately 0.000l-inch thick.

3. The process of claim 1 wherein the chromium is diffused into the earlier deposited boron by subjecting the boronized articles to temperatures between about 1,800" F. and 2,l00 F. for between about 1 and 16 hours.

4. An erosion resistant and corrosion resistant article produced by the process of claim 1.

Claims

2. The process of claim 1 wherein the boron-containing coating first laid down is at least approximately 0.0001-inches thick.
3. The process of claim 1 wherein the chromium is diffused into the earlier deposited boron by subjecting the boronized articles to temperatures between about 1,800* F. and 2,100* F. for between about 1 and 16 hours.
4. An erosion resistant and corrosion resistant article produced by the process of claim 1.