US2172428A - Corrosion-resistant ferrous alloys - Google Patents

Description

Patented Sept. 12, 1939 UNITED STATES PATENT OFFICE bridge, Mass.,

assignors The Chemical Foundation, Incorporated, New York, N. Y., a corporation of Delaware No Drawing. Application May 15, 1937,

Serial No. 142,807

17 Claims.

This invention relates to the production of passive steel articles which are characterized by an inherent marked resistance to pit corrosion.

The present application relates to the type of 5 subject matter described in prior application Serial No. 141,921, filed May 11, 1937, by Wulfi, and application Serial No. 142,829, filed May 15, 1937, by Uhlig.

As is fully explained in the earlier applications,

10 the passive ferrous alloys of the type of chrome steels, chome-nickel steels, and the like, while possessing many desirable characteristics and particularly a high resistance to general surface corrosion, nevertheless are susceptible to a spel5 cial form of corrosion, known as pit corrosion.

This particular form of corrosion, unlike general surface corrosion, is localized in its attack and progresses perpendicularly into the body of the steel. The attack initiates apparently at unpredictable segregated points on the surface and then eats directly into the body of the metal, rather than over the surface area. The pitting is particularly engeneered in special media such as saline solutions.

Those skilled in the art are generally agreed that these points of attack or initiation of pitting are attributable to two main causes which, for the sake of a term, may be called the extraneous cause and inherent cause. The extraneous cause of the pitting appears to be the adhesion of foreign substances, of organic or inorganic origin, on the surface of the metal. These interfaces between the metal and the adherent material establish a locus for electrochemical actions which ultimately 35 result in dissolution of the metal at or adjacent the region of adhesion and the beginning or the establishment of a pit. Thereafter the corrosion proceeds into the body of the metal, due apparently to the establishment of an oxygen-re- 40 duction electrolytic cell, preferably termed an oxidation-reduction cell.

Pitting may also be instituted by some inherent characteristic of the steel. As pointed out in the prior applications, the earlier investigators in this field were not in agreement as to the particular characteristic of the alloy which conduced to the development of pitting. Most of the prior workers assumed that the locus of the pitting attack was some inhomogeneity of the metal. 50 These workers were inclined to attribute the cause of the attack to a special or specific inhomogeneity such as carbide inclusions, oxide inclusions, and the like. These explanations, however, always were presented with many reservations, for,

55 as a matter of fact, it was recognized that, as a broad proposition, dirty steels were not especially more susceptible to pitting than the clean steels. As pointed out in the earlier applications, the complexity of factors potentially involved in the pitting phenomena rendered the problem par- 5 ticularly bafiling. The difiiculty of choosing any rational approach to the problem was therefore accentuated by the disagreements and conflicts among the investigators as to the most important factor involved.

For these reasons, the whole problem of the inherent susceptibility of these passive steels to pit corrosion was broadly investigated. Each factor which might be of potential importance in the pitting, either from a metallurgical or electrochemical aspect, was carefully investigated. Wherever possible, the individual factors were segregated and their influence on pit susceptibility was determined. As is carefully explained in the earlier applications, the purpose of the investigation was to determine the relative importance of the several factors which might have a bearing on pitting and to thereby ascertain the salient or predominant cause of pitting.

As shown in the earlier applications, particularly in the application of John Wulif, Serial No. 141,921, it was found that of the many factors involved, the cold working of the metal, and especially the surface mechanical effect of the cold Work, was the predominant element in determining the inherent susceptibility of the steels to pit corrosion. v

As a result of the basic investigations, two methods of procedure were developed which neutralized or alleviated the effects of cold working and increased the pitting resistance of the treated steels. While the major function of each treatment was esentially to heal the mechanicallsy imperfect surface, each method did improve the steeLby reducing the possible effect of other factors, such as nonmetallic inclusions and other surface inhomogeneities, adsorbed gases, and the like.

In the first method which was developed, the surface blemishes, which resulted from the cold work, and which were found to be the prolific source of pits, were removed by annealing the article under avacuum and in the recrystallization range of the alloy. In these circumstances, a considerable amount of occluded, adsorbed or absorbed gases were removed, nonmetallic inclusions were broken down and/or reduced, and the blemished surface of the article was healed or refinished. As there explained, at the elevated temperatures of the operation, the atoms of the metal present a relatively high degree of mobility and difiusibiiity so that the sharp points or spicules of the mechanical imperfections were removed or thermally burnished, so to speak, and a substantially submacroscopically smooth surface was produced. It was also explained that the resulting steel was cleaner by reason of the removal of the gases and the breakdown and/or removal of the nonmetallic inclusions, so that the whole body, and not only the surface, of the steel was characterized by a greatly increased poten- I tial resistance to pitting.

- In the second application referred to, namely, H. H. Uhlig, Serial No. 142,829, another method of treatment was developed and described. In this method, the surface of the steel was healed by a chemical rather than a thermal resurfacing treatment. As there explained, a special bath was developed which was characterized by an ability not only to dissolve off the surface inhomogeneities such as oxides, sulphides, selenides and the like, but also to slough off the metal of the peaks of the striae, scarifications, pits and other mechanical imperfections. This treatment therefore produced a surface which not only was mechanically healed, but also one which was chemically clean, thus permitting a further marked improvement in pit resistance by simultaneous or subsequent passivizing.

As a result of further experimentation, it has been found desirable to treat passive steel articles, of the character contemplated herein, by a special two stage method so as not only to permanently beneficiate it, but also to chemically resurface the steel.' In other words, the present invention relates to a method of accomplishing the major advantages of each of the treatments described in Q the earlier applications by a simple and economical method. As will be seen, the treatment renders the steel peculiarly amenable to passivation to thereby further enhance its resistance to pit corrosion and permanently improve the body or subsurface portion of the article so as to establish an increased potential resistance to pit corrosion throughout the body of the metal.

In the course of the general investigation todetermine the relative importance of the different factors involved in the pit corrosion phenomenon, the nonmetal inhomogeneities were considered. It has been suggested in the past that these inhomogeneities, such for example as the carbides or oxide inclusions, might be a prolific source of pitting because of the possibility of electrochemical action, i. e., the establishment of an electrical cell by reason of the presence of the heterogeneous inclusion adjacent the metallic grains.

As was explained in the prior applications, the potential effect of carbide inclusions in inducin pit corrosion was studied. It was found that while such carbides may play a role in this type of corrosion, their importance, relatively to the mechanical defects, was very slight.

Similarly, experiments were conducted to determine theeifect, and toevaluate the importance of, other major inclusions such as oxides, sulphides, selenides and slag. Since as pointed out above, these constitute heterogeneous dispersions in an otherwise substantially homogeneous structure, they must, for purposes of complete consideration of the phenomenon, be regarded of at least presumptive importance.

In these experiments, the same type of stock specimens or samples were employed as are described in the earlier applications. It was found that the sulphide and selenide inclusions could be.

annealed specimens. conclusively that althoughan oxide inclusion (by 75 microscopically identified without undue difiicuity, particularly at magnifications of the order or from 500 to 2,0007diameters. It was further determined that when such an inclusion was wetted with the special corrodent (i. e., the FeCla solution) pitting quickly initiated at the interface of the inclusion and the metal matrix. These tests were extended to samples of typical commercial 18-8 alloys which were not specified as free machining stock, i. e., relativelydirty stock. Here also the inclusions under consider ation could be identified and were found to induce pitting at the inclusion-metal interface. It was ascertained additionally that the oxide and slag inclusions functioned in an equivalent manher, that is to say, under drastic corrosion conditions, pitting tended to initiated at the interface of these inclusions and the metal.

With the effect of the inclusions thus qualitatively established, tests were conducted to determine the quantitative effects of the several inhomogeneities and thus to evaluate or determine their relative importance in the phenomenon of pitting.

For this purpose, the various sulphides, oxides and slag particles, which constitute the usual sonims of stainless steels, were obtained and after reduction to a fine particle size, were distributed over the surface of standard specimens. These particles were then wetted with the corrodent and the action was observed microscopically.

After making a number of such tests, it was determined that the heterogeneous particle or that while the investigations reported herein do show that heterogeneous inclusions tend to establish a pit, yet the relative magnitude or extent of pitting, from this source, is slight as compared to that induced by cold work.

In order further to check the function of the inclusions, particularly the oxides, and to eliminate their effect, specimens of stainless steel were cold worked and were submitted to a low temperature vacuum anneal. More specifically, a series of samples were differentially cold worked and then annealed at temperatures at approximately 450 C. for 41 hours while under a vacuum of about 10- mm. of mercury and then radiation quenched. This treatment, as will be appreciated, tended to relieve the strains established during cold working and also to reduce the oxide inclusions.

Since it was previously found that oxide inclusions, as such, did tend to induce pitting, it would seem to be expectable that the annealed, degassed and deoxidized specimens would present a greater resistance to pitting than equivalent specimens which were unannealed. The low temperature vacuum annealed specimens were subjected tothe accelerated-corrosion test, that is, each specimen wascontacted for a period of four hours with an aqueous solution containing 10% of FeCl: and 22 cc. of 2.45 NHCl per litre.

It was found, rather unexpectedly, that the loss in weight due to the formation of pits was greater in these annealed samples than in equivalent un- These tests show quite electrochemical action) does tend to induce pitting, yet in the quantitative sense, the effect on pitting is relatively minor. Such tests also show that reduction of the oxides and degassing, per se, not only do not reduce the tendency to pitting, but actually increase it.

The explanation of the apparent paradox, however, is clear when all factors are considered. While under the temperatre and vacuum conditions of the anneal, gases are withdrawn in rather copious quantities and the oxides and other inclusions are broken down and reduced, yet the temperatures involved are not sufficiently high to impart the necessary mobility to the metal to insure the self-healing action. Under the conditions obtaining in such a low temperature anneal, the gases withdrawn tend to follow the paths of least resistance and therefore tend to expirate through the fissures and cracks in the surface of the metal. The removal of the dissolved, adsorbed, and absorbed gases naturally tends to leave corresponding voids in the metal. The establishment of voids, and the enlargement of existing pores or cracks, is, of course, greatly accentuated by the reduction of the oxides and other metallic compounds, for here truly molecular magnitudes are involved. As the oxide, and particularly the sulphide inclusion, are reduced,

the space occupied by the residual metal is, of

course, less than that of the original matrix, so that, in point of fact, the metal body itself is more vesicular after such an anneal than before. Since, as was explained in the earlier applications, such cracks or declivities are the prolific sources of pitting, the increased susceptibility of such low temperature annealed metals is not surprising. I

It may be observed, in passing, that this establishment of the porous or vesicular structure as a result of degassification and reduction of sulphides and oxides, in situ, is precisely the method used to produce highly active -metal catalyst. Thus it has been suggested to produce a highly porous and hence active catalytic mass by fusing copper oxide, solidifying, and then reducing in hydrogen, so as to establish a sponge-like mass of copper metal.

It has been found, as explained in the application of John Wulff, Serial No. 141,921, that a process which at the one time combines the valuable effects of degassing, reduction of inclusions, and resurfacing, positively insures the production of a steel which is eminently resistant to pit corrosion. In the process described in such earlier application, the steel was vacuumized in the austenitic range of the alloy and. quenched so that concomitantly with the reduction of the inclusions and degassing, there was achieved a resurfacing of the skin or surface of the metal, i. e., a healing of the pores and cracks established initially by the cold Work.

It has now been found that substantially the same ultimate results may be achieved by utilizing a plural stage treatment. In this novel treatment, the undesired effects of the inclusions are largely reduced or eliminated by a vacuum or hydrogen anneal at relatively low temperature, and the metal is then resurfaced, preferably by a chemical method, so as to form a substantially smooth, healed surface. As a result of such treatment, the mechanical imperfections of the surface, which have been found to be the important foci of pitting, are eliminated; furthermore, the body as well as the surface of the metal is cleaned of the deleterious inclusions. Insh ort, with such a treatment, the body or mass of the metal itself, as well as the surface, is rendered inherently more resistant to pit corrosion.

In the preferred form of the process, the steel to be protected, preferably after the final stage of cold work, is subjected to a low temperature anneal under conditions which insure a substantial reduction of the-nonmetal inclusions, such as the oxides, sulphides, and selenides. This anneal may be carried out at any desired temperature, preferably from about 300 C. to 450 C., and

for a time suflicient to partially or completely relieve the strains made by cold work. Thereafter, the cleaned or clarified metal is subjected to a treatment, preferably in a chemical bath, which insures the formation of a submacroscopically smooth surface, free from the mechanical foci which engender'pitting.

In one form of the invention, the preliminary treatment may comprise an annealing step in a hydrogen or other reducing atmosphere and at a temperature of the order of from about 300C. to

450 C., more or less. The treatment may be continued for that period of time sufficient to secure the desired normalizing and chemical reductions.

After such treatment, the article which now I comprises a cleaner steel, inherently less susceptible to pit corrosion, is then treated in a chemical resurfacing bath, preferably of the type described in the application of H. H. Uhlig, Serial No. 142,829. This treatment may comprise an immersion for a period of five minutes, more or less, in a strong acidic solution, preferably of the following formula:

Per cent HCl 30 H2304 40 T1014 5.5 H20 24 5 The bath may maintained at an elevated temperature of the order of" C., more or less.

In these circumstances, as explained in the earlier application, the peaks of the striae or other mechanical imperfections on the steel are dissolved or sloughed off so as to produce a substantially smooth metal surface, free from pits or depressions which would provide potential areas for the accumulation and stagnation of the corrodent. This type of treatment, therefore, positively reduces and eliminates occlusions such as metallic oxides, which themselves induce corrosion, as well as the mechanical imperfections which constitute the more important factors in pit susceptibility. It will be observed, additionally, that such a treatment insures the production perature range so as to insure the optimum reduction of oxides, sulphides and selenides. Thereafter, the preliminary treated steel may be subjected to the action of a chemical resurfacing bath to establish the substantially submac-roscopically smooth surface of decreased pit susceptibility.

As pointed out in the application of H. H. Uhlig,

Serial No. 142,829, this chemical resurfacing treatment, in addition to providing a smooth,

sealed surface, also insures a chemically clean,

and hence chemically homogenous surface. This active surface is peculiarly amenable to passivation by treatment with suitable passivizing agents, such as solutions of nitric acid or chromium trioxide. A further refinement of the invention therefore comprises the treatment of those steels which are susceptible to pit corrosion by a series of steps so as to reduce or eliminate the nonmetallic inclusions, to resurface the metal and to effectively passivize the resurfaced metal. This may be achieved by adding a passivizing agent, such as nitric acid, to the resurfacing bath, or by washing the article, after resurfacing treatment, and then while the metal is still wet subjecting it to a separate passivation treatment.

This type of combined treatment insures a product which is particularly resistant to pit corrosion. The combination of the preliminary anneal and the resurfacing treatment improves the ultimate steel to an unpredictable degree. For example, and by way of direct comparison, a

- specimen of 188 stainless steel, which after reduction by cold working, followed by a chemical resurfacingtreatment, when tested bythe accelerated method, showed a loss of weight of substantially 0.5 mg. per sq. cm. This loss is due substantially completely to pitting and not to general corrosion. A specimen of the same composition which was worked to the same degree, and was then treated according to the invention, that is, was heated in a vacuum at about 400 C. and was then chemically resurfaced, when tested by the accelerated method, showed a loss in weight of less than 0.12 mg. per sq. cm. This result is the more unexpected when it is considered that the vacuum anneal alone not only does not decrease pitting but greatly increases it. By way of further comparison, a specimen of the same steel which was reduced to the same degree, that is, 10%, but which was untreated, when tested for pitting exhibited a loss in weight of substantially 1.25 mg. per sq. cm. The improved low temperature anneal-chemical resurfacing treatment therefore imparts to the steel a resistance to pit corrosion substantially ten times as great as that of untreated stock. This resistance to pitting, as already explained, can be further enhanced by passivation of the resurfaced article.

While the preferred form of the invention comprises a preliminary anneal to clarify the steel and minimize the ultimate effect of the inclusions, followedby the chemical resurfacing bath to remove the mechanical foci of pitting, it is to be understood that the broad principle of the invention comprehends the ultimate improvement of the steel by the correlation of these two treatments, in whatever sequence utilized and at any stage in the fabrication of the steel and/or use of the article.

In recapitulation, it will be noted that this type of treatment markedly increases the inherent resistance of steel articles to pit corrosion. It is particularly to be observed that not only is the surface improved, and immediate pitting therefore diminished, but also the body of the steel itself is beneficiated. Since the steel is clarified and the inclusions are largely reduced, the pitting that would have developed from these sources is substantially .eliminatecL- Such a steel, therefore, may periodically be resurfaced (and if desired, repassivized) with the assurance of achieving the pit corrosion resistance of new stock. This is to say that the improved resistance characterizes the whole body of the steel and is not simply an evanescent improvement. As mentioned hereinbefore, even though the inherent pitting resistance of a steel may be increased, pitting nevertheless may be induced by extraneous factors. A typical prolific source of such pitting is found in the growths or heterogeneous adhesions which, in service, may form on the surface of the article to be protected. According to the present invention, pitting due to such extraneous factors may be greatly minimized by periodically resurfacing the article, particularly where, as here, the body of the article itself, as well as the surface, is amenable to such periodic treatments. Since such resurfacing is accomplished by a chemical solution, in most circumstances the article can be periodically treated at the installation, and without dismantling, by washing, spraying, painting, and the like. Since the resurfacing solution is, in effect, an exceptionally strong solvent, it serves to dissolve such ad-v hesions as well as to heal the surface blemishes or mechanical imperfection which might have developed during use of the article. In suchcases of rehabilitation of the article at this installation, the chemically resurfaced article may be repassivized by contact with a suitable passivizing agent, sucli as nitric acid.

As will be appreciated, the novel methods of stainless steels containing columbium or equivalent additions.

It is clearly to be understood that the expressions chemical resurfacing bath and chemically resurfacing as used in the specification and claims are carefully to be distinguished from older methods of pickling and passivizing. In the typical pickling operation, a steel article containing surface oxides is subjected to a relatively Weak acid solution for the purpose of preferentially attacking the oxides with the minimal attack on the metal body. Indeed, in many commercial operations, organic and inorganic adjuvants are utilized so as to positively inhibit the action of the acid on the metal and to limit it to action on the oxides. Passivation, on the other hand, is usually accomplished by utilizing solutions of nitric acid and chromium trioxide and for the purpose of modifying chemical characteristics of the metal and not primarily for the purpose of dissolving such metal. In the present process, the chemical resurfacing bath is advisedly chosen of a high activity, i. e., ahigh acid strength, for the purpose of positively insuring a solvent or dissolving action on the metal body itself, so as to slough off or dissolve the mechanical imperfections in the surface of the steel,

which have been discovered to be a salient cause of pit corrosion.

While preferred modifications of the invensusceptibility of articles'composed of passive ferrous alloys which comprises, subjecting the articles to a low temperature anneal under conditions which are eifective to substantially reduce nonmetallic inclusions, and subsequently treating the article with a highly active metal solvent comprising an aqueous solution containing over of a mixture of sulphuric and hydrochloric acids for a period of time sufliciently prolonged to dissolve a sufiicient amount of surface metal to chemically resurface the articles and to substantially eliminate the foci of pit corrosion.

2. That method of reducing the pit corrosion susceptibility of articles composed of passive ferrous alloys which comprises, subjecting the articles to a vacuum anneal, below the carbide forming range, and for a period of time sufiicient to largely reduce nonmetallic inclusions and effectively degas the alloy, and subsequently treating the articles with a highly active metal solvent comprising an aqueous solution containing over 50% of a mixture of sulphuric and hydrochloric acids for a period of time suificiently prolonged to dissolve a sufiicient amount of the surface metal to chemically resurface the articles and to substantially eliminate the foci of pit corrosion.

3. That method of reducing the pit corrosion susceptibility of articles composed of passive ferrous alloys which comprises, subjecting the article to an annealing action at a temperature of between substantially 300 C. and substantially 450 C. and in a reducing atmosphere to largely reduce the nonmetallic inclusions, and subsequently treating the article with a highly active metal solvent comprising an aqueous solution containing over 50% of a mixture of sulphuric and hydrochloric acids for a period of time sufficiently prolonged to dissolve a sufficient amount of the surface metal to resurface the article and substantially eliminate the foci of pit corrosion.

4. That method of reducing the pit corrosion susceptibility of articles composed of passive ferrous alloys which comprises, subjecting the articles to a hydrogen anneal, below the carbide forming range, and for a period of time sufiicient to largely reduce the nonmetallic inclusions, and subsequently treating the articles with a highly active metal solvent comprising an aqueous solution containing over 50% of a mixture of sulphuric and hydrochloric acids for a period of time sufiiciently prolonged to dissolve a' suffcient amount of the surface metal to chemically resurface the articles and thereby substantially eliminate the foci of pit corrosion.

5. A method of reducing the pit corrosion sus. ceptibility of artcles composed of passive ferrous alloys which comprises, sequentially annealing the articles under a vacuum and in contact with hydrogen for a total period of time sufiicient to largely reduce the nonmetallic inclusions and effectively degas the alloy and subsequently treating the articles with a metal solvent comprising an aqueous solution containing over 50% of a aqueous solution containing over 50% of a mixture of sulphuric and hydrochloric acids for a period of time sufliciently prolonged to dissolve a sufiicient amount of the metal of the surface to chemically resurface the articles and thereby substantially eliminate the foci of pit corrosion.

7. That method of reducing the pit corrosion susceptibility of articles composed of passive ferrous alloys which comprises, subjecting the articles to a low temperature anneal under conditions which are eflective to reduce nonmetallic inclusions, subsequently treating the articles with a highly active metal solvent comprising an aqueous-solution containing over 50% of a mixture of sulphuric and hydrochloric acids for a period of time sufliciently prolonged .to dissolve a sufficient amount of metal from the surface to chemically resurface the articles and then passivizing the articles. by contact with a suitable passivizing agent.

8. A method of reducing the pit corrosion susceptibility of artcles composed of passive ferrous alloys which comprises, subjecting the articles to a vacuum anneal below the carbide forming range, and for a period of time sufficient to largely reduce nonmetallic inclusions and effectively degas the alloy, then treating the articles with an active metal solvent comprising an aqueous solution containing over 50% of a mixture of sulphuric and hydrochloric acids for a period of time sufficiently prolonged to dissolve a sumcient amount of metal from the surface to chemically resurface the articles and substantially eliminate the foci of pit corrosion and then passivizing the resurfaced articles by contact with a suitable passivizing agent.

9. That method of reducing the pit corrosion susceptibility of articles composed of passive ferrous alloys which comprises, subjecting the articles to an annealing action, at temperatures of between substantially 300 C. and 450 C., and in an atmosphere of hydrogen to largely reduce the nonmetallic inclusions, treating the articles with an active metal solvent comprising an aqueous solution containing over 50% of a mixture of sulphuric and hydrochloric acids for a period of time sufficiently prolonged to dissolve a sufiicient amount of surface metal to chemically resurface the articles and then passivizing the surface of the articles by contact with a suitable passivizing agent.

10. That method of reducing pit corrosion susceptibility of articles composed of passive ferrous alloys which comprises, subjecting the articles to a vacuum anneal, below the carbide forming range and for aperiod of time suflicient to largely reduce nonmetallic inclusions and effectively degas the metal alloy, subsequently treating the articles with a highly active metal solvent comprising an aqueous solution containing over 50% of a mixture of sulphuric and hydrochloric acids for a period of time sufficiently prolonged to dissolve a sufficient amount of metal from the surface to chemically resurface the articles and thereby substantially eliminate the foci of pit corrosion, and resurfacing the articles periodically during use, by contact with the said metal prisingan aqueous solution containing over of a mixture of sulphuric and hydrochloric acids and a passivizing agent, maintaining the articles in contact with such solution for a period of time sumciently prolonged to dissolve a sufficient amount of metal from the surface to chemically resurface the articles to thereby substantially eliminate the mechanical foci of pit corrosion and establish a substantially passive film thereon.

12. That method of reducing the pit corrosion susceptibility of articles composed of passive ferrous alloys which comprises, subjecting the articles to a vacuum anneal, at a temperature range of between substantially 300 C. and 450 C., and for a period of time sumcient to largely reduce nonmetallic inclusions, and subsequently treating the articles with a solution containing a highly active metal solvent comprising an aqueous solution containing over 50% of a mixture of sulphuric and hydrochloric acids and a passivizing agent, maintaining the articles in contact with such solution for a period of time sumciently prolonged to insure the dissolution of a substantial amount of surface metal and of the order of 0.0004 in. to chemically resurface the articles to thereby substantially eliminate the foci of pit corrosion and establish a substantially passive film thereon.

13. A method of reducing the pit corrosion susceptibility of articles composed of passive ferrous alloys which comprises, subjecting the articles to a hydrogen anneal, below the carbide forming range, and for a period of time sufflcient to largely reduce nonmetallic inclusions, and subsequently treating the articles with a solution which contains a strong metal solvent comprising an aqueous solution containing over 50% of a mixture of sulphuric and hydrochloric acids and a passivizing agent maintaining the articles in contact with the solution for a period of time sufliciently prolonged to dissolve off an appreciable quantity of the surface metal and to chemically resurface the articles to thereby substantially eliminate the mechanical foci of pit corrosion and establish a substantially passive film thereon.

14. 'A method of reducing the pit corrosion susceptibility of articles composed of passive ferrous alloys which comprises, subjecting the articles to a vacuum "anneal, below the carbide forming range, subsequently subjecting the treated articles to a hydrogen anneal in substantially the same temperature range, then contacting the articles with a solution which contains a highly active metal solvent comprising an aqueous solution containing over 50% of a mixture of sulphuric and hydrochloric acids and a passivizing agent maintaining the articles in contact with the solution for a period of time sufficiently prolonged to insure the dissolution of a substantial amount of surface metal to chemically resurface the articles to thereby substantially eliminate the mechanical foci of pit corrosion and establish a substantially passive film thereon.

15. A method of reducing the pit corrosion susceptibility of articles composed of stainless steels of the 18 chromium, 8 nickel type which comprises, subjecting the articles to a low temperature anneal, under conditions which are effective to reduce non-metallic inclusions, subsequently treating the article with a highly active metal solvent comprising an aqueous solution containing over'50% of a mixture of sulphuric and hydrochloric acids for aperiod of-time sufliciently prolonged to dissolve a suflicient amount of surface metal to chemically re'surface the article and to substantially eliminate mechanical foci of pit corrosion.

16. An austenitic chrome-nickel steel characterized by a striking inherent resistance to pit corrosion which has been vacuumized at elevated temperatures but below the carbide forming range, and which hasbeen chemically resurfaced in contact with a metal solvent comprising an aqueous solution containing more than 50% of a mixture of hydrochloric and sulphuric acidsfor a period of time suflicient to reduce the thickness of the steel to the order of 0.0004 in.

1'7. A chrome-nickel steel characterized bya. striking inherent resistance to pit corrosion which has been vacuumized at elevated temperatures but below the carbide forming range, and which has been chemically resurfaced in contact with a metal solvent comprising an aqueous solution containing more than 50% of a mixture of hydrochloric and sulphuric acids for a period of time suflicient to reduce the thickness of the steel to the order of 0.0004 in.

JOHN C. WULFF.

HERBERT HENRY UHLIG.