US2172422A - Corrosion-resistant ferrous alloys - Google Patents

Description

Patented Sept. 12, 1939 UNITED STATES v 2,172,422 connosron-nnsrsrm FERROUS ALLOYS Herbert Henry Uhlig, Cambridge, Mass, assignor to The Chemical Foundation, Incorporated, New York, N. Y., a corporation of Delaware No Drawing. Application May 15, 1937, Serial N0. 142,830

13 Claims. (01. 148 7)" This invention relates to the production of improved passive ferrous alloys, more particularly to stainless steels of the 18-8 type, which are characterized by a high resistance to certain types of corrosion. ,The invention isfurther concerned with a novel method of rolling or otherwise fabricating articles from such alloys and the products thereby produced.

As has been explained in copending applica- 10 tions of A. L. Kaye et 8.1., Serial No. 91,096, filed July 17, 1936, and Serial No. 137,930, filed April 20, 1937, even the passiveferrous alloys, such as stainless steels, are susceptible to certain specific forms of corrosion. As there explained, these steels are susceptible to pit corrosion which may be engendered either by some inherent characteristic of the steel itself, or by some extraneous factor. In the earlier applications, it was pointed out that the inherent susceptibility of these 81- loys to put corrosion may be minimized by incorporating in the original composition certain components such as silver which are soluble in one or more of the alloy constituents such as the nickel or manganese of the steel and which ultimately may form a film or skin of insoluble chlorides. This insoluble chloride film functions to protect the subjacent metal from pitting attack.

In these earlier disclosures, it was indicated 3 that an advantageous method of incorporating the insoluble chloride-forming constituents was to add them to the melt. It was further explained that of such chloride-forming constituents silver operated very eflectively.

While the methods disclosed in the earlier applications do produce a steel which is inherently more resistant to pit corrosion, such methods do present certain disadvantages, particularly in that a relatively large amount of the expensive silver ingredient must be incorporated in order to provide a suflicient quantity, available at the surface, for the formation of the protective chloride film.

The present method relates to the production of novel types of passive ferrous alloys generally characterizedby the same high resistance to pit corrosion as the earlier products. Under the new method, however, this particular resistance is secured by using minimal amounts of the expensive so alloying constituent and utilizing it under such circumstances that other benefits are secured.

As is known, the shaping of stainless steel articles is a -diflicult problem. It. is well known to those skilled in the art that stainless steel compositions are fiimi ltly workable and require special handling and treatment during rolling and forming. In rolling mill practice, the great bulk of the throughput is generally restricted to low carbon steels. This is not due to any lack of demand for the rolled alloy steel stock, but pri- 5 marily to the high wear on the. alloy rolls and the consequent high cost of the harder alloys. In rolling mill practice, the rolls tend to become engraved or embossed by cracks in the ingot or strips which are being rolled, and especially by .10 the hard scale forming on the surface of the strip or metal undergoing reduction. This condition is so pronounced that the footage or throughput which may be expected of a given roll, or pair of rolls, wholly is a matter of conjecture and is not, at the present time, susceptible to any rational predetermination. These difllculties of rolling are further accentuated by the tendency of the stock, and even low carbon strips,

to break up in the rolls with the result that 20 the torn edges mar or scarify the roll. When the rolls are scarred or marked to a point where such markings are rolled into the surface of the strip, such rolls must be removed and reground to give a smooth surface. Even here, the greatest 25 care must be exercised so as not to exceed the permissible variations in the curvature of the rolls when the surface metal is removed. For this reason the total quantity of metal which may be removed from the rolls is only a few thousandths E of an inch, for any greater amount will alter the curvature of the rolls to such an extent to render them unsuitable for further use in the stand for which they are designed. When this ultimate condition obtains, the rolls must be 5 junked.

Because of these di fliculties, even when operating upon low carbon stock, a given pair of rolls may last only for half a coil of strip, although in other circumstances, as many as ten coils may 40 'be rolled without mishap. These difficulties which obtain in the rolling of the easily workable low carbon steels are, of course, sharply accentu ated in the case of fabrication of stainless steels, and ferrous alloys of similar characteristics. 45 Such steels, due to the high percentage of the hard constituents, such as chromium, are more brittle and are more diflicult to reduce. This condition is so marked that the commercial rolling of stainless steels is substantially restricted to a relatively few rolling mills which make a speciality of the product. Even here a distinct price differential obtains on account of the excessive cost involved in fabrication.

According to the present invention, stainless,

and equivalent steels, may be produced, which I the rolling operation, markedly increasing the life of the rolls, and hence reduce the cost of the stock being treated According to the present" invention, the ultimate incorporation of the insoluble chlorideforming constituent is secured by adding this constituent to the stainless steel or alloy after the melt and in the earlier stages of fabrication. The added, protective, constituent preferably is applied to the stock in such circumstances that it functions beneficially during the rolling operations to facilitate such operations.

Considered more in detail, in operating under the present invention, a stainless, or equivalent steel, member or billet is first cleansed and pickled in the usual manner. It is then provided with an electrodeposited surface of the desired component, such as silver. The electrodeposited plate or surface may be of any desired depth and may be applied in the manner well known to those skilled in the art. Thus typical solutions, such as solutions of KAg(CN)2 or NaAg(CN)2 may be utilized. Such solutions may be electrolyzed in the typical manner, as by using a silver anode. The electrolysis maybe carried out'at typical current densities, that is, about 3 to 8 ams. per sq. ft. with bath temperatures of from about 15 C. to- 0., more or less, and

i with a potential of from 1 to 2 volts. It is particularly to be observed at this point that since the'ultimate result here sought is not a smooth adherent silver plate, the electrolyte may be made up of simplersilver salts and the silver deposited in the form of the coarser crystals. Since, as will be seen more fully hereinafter, the applied silvercoating is mechanically intruded and thermally diffused through the surface case of the steel, other methods of applying the silver may be utilized, such for example, as dip coating.

After the application of the silver coat in any desired manner, the ingot or billet is then washed, heated, and hot rolled to a sheet, bar, or other form desired. It is to be observed that in the hot rolling process, the silver of the surface is at a temperature at which the atoms are especially mobile. This high mobility, combined with the plasticity induced by the mechanical working,

makes it possible for the silver atoms to diffuse readily into the ferrous alloy. Since the silver constituent, as noted hereinbefore, is soluble in one of the constituents of the alloy, such dissemination or intrusion of the silver is further facilitated so that the silver constituent is diffused into the base metal considerably deeper than the superficial layer. a

It will also be appreciated that in these circumstances, that is to say, where a stainless steel is provided with a continuous surface of silver,

decided mechanical advantages are secured. Due to'its mobility under the thermal conditions obtaining, the silver acts or functions as a selflubricator, thus markedly decreasing the friction between the rolls and the surface of the metal whichis undergoing reduction. In other words, the coating of the silver acts as a relatively soft buffer between the hard stock being rolled and the rolls themselves. This self-lubrication,

together with the usual lubrication used in commercial practice, markedly decreases friction and its attendant disadvantages.

The incorporation of the silver by this 3 631115 of applying a continuous coat and mechanically working it into the basis metal presents other advantages. As will be appreciated, during the hot rolling or forming operations, the silver coating isqsubstantiallypure because of its substantially non-oxidizing character and hence it serves as a means of protecting the formation of oxide scale. A double advantage is secured for a concomitant of the use of silver is that the working surfaces of the rolls are not mechanically scored or scarified to as great adegree as in ordinary practice, and their operative or working life is greatly increased.

Finally, the establishment of a smooth working and rolling surface, due to the fact that the silver itself constitutes the contact surface, insures a greater throughput between the grindings and resurfacings, and tearing and cracking of the metals is minimized since no substantial pullingor grabbing is encountered. It will be thus observed that while a metal coating after extenuation, due to, rolling, may be only a few molecules thick, it is nevertheless of sufiicient thickness to insure the enumerated beneficial results.

During the several rolling operations, the coating of the metal is progressively extenuated and the depth of the skin or coating is commensurately reduced. For this reason, it is desirable to apply one or more coatings between the several reduction steps. This is especially desirable when the initial materials which are plated are billets or strips of appreciable bulk. For this reason, it is advantageous, in some circumstances, to surface or electroplate the rolled metal a second time and continue the hot rolling after such replating. In these circumstances, the rolled sheet or bar is, of course, subjected to the usual cleaning steps prior to the replating operation. This second treatment, or replating, is particularly efficacious in securing protective cases of high silver content and of uniform composition. Where the exigencies of a particular operation so require, the replating-rerolling process may be repeated a number of times and, in some-cases, it is desirable that the alternation be continued over several cycles.

In recapitulation, itwill be seen that a number of major advantages are attained by silver plating the ingots or billets and subsequently rolling them. In these circumstances, the silver coating of the ingot protects the basis metal from the chemical incidents of hot rolling and decreases the rolling friction. The silver so incorporated, because of its low melting point, readily diffuses throughout the surface of the alloy and such diffusion is, of course, accelerated or accentuated by the mechanical intrusive effects of the rolling. In these circumstances, a silver-rich alloy of substantial depth of case is formed on the surface so that maximum protection of the ultimate steel against pit corrosion is secured with the minimal amount of the expensive silver .component. It is therefore possible to secure a higher effective, concentration of silver in the stainless steels than by'the direct addition of the basis metal against The articles fabricated according to the 'prescribed procedure may be used directly in service. In these circumstances, when the article is subjected to a saline medium, the film or skin of insoluble chlorides forms on the surface and functions to protect the body of the metal against pit corrosion in the manner more particularly gefined in the copending application, Serial No.

Preferably, after the articles are so formed, they are subjected to a further treatment according to the disclosure in prior application Serial No. 137,930, so'as to preform the skin of the protective insoluble silver compound on the articles. For this purpose, the products or articles which are fabricated according to the invention may be subjected to. chemical reagents, either in the vapor or liquid form, to positively establish the continuous adherent coat of the insoluble silver compound. For example, as typifying the general method of procedure contemplated, a steel article fabricated from the improved silvercontaining stainless steel may be chloridized, in any suitable manner, to convert the silver in the surface of the alloy to the silver chloride. Thus, the steel article may first be cleaned, if necessary, by suitable methods, as by a dip in a relatively dilute sulfuric acid solution. After cleaning, the article may be rinsed and then subjected to the actionof a reagent which is effective to form the silver chloride. For this purpose, the article may be immersed in an aqueous solution of a suitable metal chloride, or a hypochlorite. The formation of the silver chloride may be accelerated by employing elevated temperatures, electrochemical action, and the like. The time of treatment 7 .may beprolonged to any desired degree to secure a corresponding depth of the protective silver chloride.

Again, such chloridizing treatment may be carried out in a suitable apparatus in which the articles are contacted with chlorine gas. Here, as will be understood, the temperature, pressure and moisture conditions of the gas, and the time of treament, may be controlled to secure the desired characteristics of the final film.

Preferably, the surface-insolublized sheet metals are used as such and neither machined, 3 nor trimmed, nor cut at the ends. This is to avoid exposure of the unprotected core of the metal, since such exposure would tend to nullify the. advantages obtained from the invention. However, in the case of formed articles, made by welding or otherwise joining cut surfaces or edges, the joints may be filled in with silver solder or even with fine silver, and this may be fused in place. Such additions may thereafter be compacted, as by peening. After such treatment of the raw edges, the whole article may be subjected to the insolublizing treatment.

Similarly, in the event that sheet stock having 'cut edges is desired to be utilized, these edges may be protected in the same manner, that is to say, by applying a thin coat of silver by any-definal protective compound may be comprised of any desired insoluble silver salt. Thus, in lieu of a chloridizing treatment, other similar functioning compounds may be produced in situ, such as silver cyanide, phosphate, chromate, bromide, oxide, and the like. The production of such special'insoluble silver compounds may be eifected by any'suitable method known to those skilled in the art. For the production of the phosphatized coating, for example, the special silvercontaining stainless steels may thoroughly be cleaned and then subjected to the action of a suitable phosphatizing medium, such as a solution of phosphoric acid, of suitable concentration, containing manganese dioxide or equivalent compounds. The solution may be boiled while in contact with the steel for a suflicient period of time to form the phosphate coating to the desired depth. Surfaces of the silver cyanide, chromate, bromide or oxide, may be produced by any suitable methods in which the silver coated steel is reacted with a reagent under conditions which insure the formation of the desired salt. It will thus be appreciated that according to the present invention, novel types of passive ferrous alloys in the form of sheets, rods, bars and the like, may be produced with a preformed, pit corrosionrcsisting surface. In addition, the sheet or strip metal may be made up into formed articles which likewise can be made corrosion-resistant at any place in'which the basis metal may have been exposed during the forming, treating, and shaping operations. Such articles may be of any desired size and shape, since the conformation does not impose any substantial limitations upon the final insolublizing treatment. While preferred modifications. of the invention have been described, it will be understood that these are given to explain the underlying principles, and to typify anymethod of protecting passive steels against pit corrosion by instituting or establishing a preformed, protective film produced from a reactive metal, which metal is diffused in the skin of the alloy and which is adapted immediately or ultimately to form the insoluble protective film.

I claim: I

1. The method of increasing the resistance of nickel-containing and manganese-containing stainless steels to pit corrosion which comprises, applying an adherent and compacted film of silver on the surface thereof and subsequently contacting the silver with a suitable reagent to form a continuous film of of the silver.

2. A method of increasing the pit corrosion resistance of nmkel-containing and manganesecontaining passive, and difficultl-y workable ferrous alloys which comprises, applying an adherent coating of silver to the'surface of the steel, hot rolling the steel to a predetermined reduction, then applying another continuous coating of silver to the reduced steel and hot rolling the so-treated steel to secure further reduction and to form a solid solution silver alloy.

3. A stainless steel article which comprises a core of nickel-containing and manganese-containing stainless steels and a thermally diffused case of substantial depth which contains a predetermined quantity of silver intergranularly incorporated in the steel the exposed surface of the article comprising a continuous film of an insoluble compoundof silver.

4. A nickel-containing and manganese-containing stainless steel article which comprises, a core of the steel and a case of substantial depth which contains a predetermined quantity of silver an insoluble compound in solid solution in the steel; the exposed surfaces of the article comprising a continuous film of an insoluble compound of silver.

5. A method of increasing the resistance of nickel-containing and manganese-containing passive ferrous alloys to electrochemical corrosion inducedv in saline media which comprises, surfacing such steels with a metal which is insoluble in chromium and iron but which is soluble in nickel or manganese and which metal forms a water-insoluble chloride; hot rolling the so-surfaced steels to effect penetration and diffusion of thesurfacing metal in the steels; and subjecting the steels to a chloridizing action to form an insoluble salt of said metal on the surface of the steels.

6. The method of producing nickel-containing and manganese-containing passive ferrous alloys which are characterized by a high resistance to pit corrosion which comprises, electrodepositing a surface of silver on the alloy, hot rolling the plated alloy to diffuse the silver into the surface; and subjecting the exposed surface to a chloridizing environment to form a continuous adherent film of silver chloride thereon.

'7. That method of producing nickel-containing and manganese-containing stainless steels to impart thereto a high resistance to pit corrosion whichcomprises, surfacing such steels with silver;

hot rolling the surfaced alloy to effect diffusion and solution of the silver in the alloy and to produce a substantially continuous surface of silver on the alloy; and thereafter subjecting the alloy, to a solution containing available chlorine to form a continuous adherent film of silver chloride on the exposedsurface'.

8. A stainless steel article for use in saline en vironment and characterized by a marked resistance to pit corrosion which comprises, iron, nickel and chromium and a rolled and diffused surfacing of silver, in which the'silver is alloyed with the nickel and which article has a preformed continuous adherent film of silver chloride on the surface.

9. A method of increasing the resistance of chrome-nickel and equivalent steels to electrochemical corrosion in saline media which-comprises, surfacing the steels with an alloying element insoluble in chromium and iron, but

.soluble with another ingredient of the steel and which is capable of forming a water insoluble chloride; and subjecting the surfaced steel to a chloridizing treatment to form on the surface thereof a continuous adherent film of insoluble chloride of said'element.

v 10. That method of increasing the resistance of chrome-nickel and equivalent stainless steels to pit corrosion in saline media which comprises, surfacing the steel with an alloying element insoluble in chromium and iron, but soluble in an: other ingredient of the steel and which element is capable offorming a water insoluble cyanide; hot rolling the so-surfaced steel to cause diffusion and alloying of the surfacing element; and subjecting the steel to a cyaniding treatment to form on the surface a continuous adherent film of an insoluble cyanide of said surfacing metal.

11. That method of increasing the resistance of chrome-nickel and equivalent stainless steels to pit corrosion which comprises, surfacing the steel with an alloying element insoluble in chromium and iron but soluble in other ingredients of the steeland which element is capable of forming a water insoluble phosphate; hot rolling the so surfaced steel to cause diffusion and alloying of the surfacing element; and subjecting the steel to a phosphatizing treatment to. form on the exposed surface thereof a continuous, adherent film of an insoluble phosphate of said element.

12. A stainless steel article for use in saline media which is characterized by a marked resist-- ance to pit corrosion which comprises, iron, nickel and chromium and a mechanically intruded and thermally diffused surfacing of a water-insoluble, salt-forming metal having the characteristics of silver, and which article has a preformed, continuous adherent film of an in-.

soluble salt of said metal, said salt being chosen from a group comprising chromates, phosphates and oxides.

13. A stainless steel article for use in saline media which is characterized by a marked resistance to pit corrosion whichcomprises, iron, nickel and chromium and a mechanically intruded and thermally diffused surfacing of silver,

and which article has a preformed, continuous adherent film of an insoluble halogen salt of silver.

' HERBERT HENRY UHLIG.