US3247086A - Method for enhancing corrosion resistance of stainless steels and products thereof - Google Patents

Description

April 19, 1966 Filed May 25, 196

D. GOLDSTEIN ETAL 3,247,086 METHOD FOR ENHANCING CORROSION RESISTANCE OF STAINLESS STEELS AND PRODUCTS THEREOF 4 Sheets-Sheet 1 6 a O Q Q 0 G 0 INVENTORS Dam'dCyO/asin Alf/7m Hask ll ZZZ ATTO NE) April 19, 1966 D. GOLDSTEIN ETAL 3,247,036

METHOD FOR ENHANCING CORROSION RESISTANCE OF STAINLESS STEELS AND PRODUCTS THEREOF 4 Sheets-Sheet 2 Filed May 25, 1961 AQZ/wc Masha/ZZZ,

BY ATT Aprll 1966 D. sows-ram ETAL 9 3 METHOD FOR ENHANCING CORROSION RESISTANCE 0F STAINLESS STEELS AND PRODUCTS THEREOF Filed May 25, 1961 4 Sheets-Sheet '5 v W UNTREATED B TREATED V UNTREATED A TREATED UNTREATED TREATED Ali/EN T6193 mm Galiieim f Am/19$,{oui&

. M Y M535? April 1966 n. GOLDSTEIN ETAL 3 METHOD FOR ENHANCING CORROSION RESISTANCE OF STAINLESS STEELS AND PRODUCTS THEREOF Filed May 25, 1961 4 Sheets-Sheet 4s United States Patent METHOD FOR ENHANCING CQRROSION RESIST- ANCE 6F STAINLESS STEELS AND PRODUCTS THEREOF David Goldstein, Carnegie, and Arthur Moslrowitz, Pittsburgh, Pa., assignors to Crucible Steel Company of America, Pittsburgh, Pa., a corporation of New .lersey Filed May 25, 1961, Ser. No. 112,688 8 Claims. (Cl. 204-140) This invention pertains to improved stainless steels and methods of making the same, and in particular to methods of enhancing the corrosion resistance of bright annealed stainless steels, especially the straight chromium chromium-nickel, chromium-manganese, and thechromiumnickel-manganese stainless steels and modifications thereof.

The so-called stainless steels have a wide variety of useful applications primarily because of their property of stainlessness, that is, resistance to corrosive and oxidative attack in a variety of environments. This property, however, is not an invariable attribute of the stainless steels, it being exhibited invarying degrees, depending upon the nature of the environment and the composition of the steel. The usual stainless steels comprise an iron base with varying amounts of alloying constituents, primarily chromium, with or without nickel and/ or manganese, to confer the stainless properties thereon. These steels also frequently contain, and generally in lesser amounts, additional alloying elements, such as molybdenum, copper, titanium, columbium, tantalum, zirconium, silicon, nitrogen, etc., for various purposes. The stainless steels usually contain from about 12% up to 27% or more of chromium, at least about 11.5 or 12% of this element being required for useful corrosion resistance. The straight chromium steels are especially important-due to their low cost (by reason of the absence of the high cost element nickel)-in many constructional and decorative applications, such as automotive trim, architectural structures and elements, etc.

A difiiculty recently encountered in connection with the use of the straight chromium steels, e.g., American Iron and Steel Institute (AISI) Type 430 stainless steel, having about 14 to 18% chromium, in automotive trim application, has been the rapid appearance, after subjection -to corrosive service atmosphere environments, of a corrosive attack manifested by the appearance of a disfiguring spottiness or discoloration of the steel surface. This discoloration is characterized partly by the appearance of rust-like streaks and, to an even greater degree, by rela: tively extensive etching of the steel surface in isolated but profusely occurring spots or patches. Recognition of this problem, and a discussion of its probable origin, together with a suitable test for evaluating propensity of the steel to corrode in this manner, is given in an article appearing in Corrosion, June 1960, pages 105408.

Following the work presented in the above-mentioned publication, it is now generally recognized that the spot or etching corrosion of stainless steels, such as Type 430, is due to massive chromium depletion of the steel surface caused by the high temperature, short term, air anneal to which the steels are subjected immediately prior .to finishing. The aforesaid type of etching corrosion of these stainless steels has now been generally overcome by eliminating the aforementioned high temperature air anneal and by substituting therefor the so-called bright anneal, i.e., a final anneal of the steel article, e.g. in sheet or strip form, in the substantial absence of oxygen, for example, in a vacuum, or in the presence of an inert or reducing fluid medium such as liquid sodium or a suitdfi i'ifiiifi Patented Apr. 19, 19%6 able gas such as hydrogen, nitrogen, argon or cracked ammonia.

The bright annealing process possesses advantages in addition to preventing chromium depletion and consequent etch corrosion of the steel. Thus, the product of the bright annealing process possesses a bright, highly finished surface substantially free of disfiguring oxide or scale and which, therefore, has been thought to be useful directly with little or no further processing such as pickling, polishing, bufiing, etc. Elimination of such additional steps reduces the cost of the product and, hence, adds to the value of the bright annealing process to both the manufacture and the user.

It has recently been discovered, however, that the bright annealed products possess certain disadvantages 'not previously recognized. Thus, it is now known that, although the bright annealed stainless steels are resistant to etch corrosion due to surface chromium depletion, those products, unfortunately, have an enhanced susceptibility to corrosion of a different sort. Thus, such steels have been found to exhibit a so-called pit corrosion when exposed to rigorous corrosive environments. This latter corrosion phenomenon is different from the etch corrosion eliminated by the bright annealing process and manifests itself by the appearance on the bright annealed steel surface of a multitude of relatively deep, small diameter, individual pits. It appears that the susceptibility of the bright annealed steel surface to the pit-type corrosion tends to be inversely proportional to the resistance of the steel to the spot-type corrosion, that is to say, the more resistant the steel to spot corrosion, the more susceptible it is to.

the pit-type corrosion. Appearance of pit corrosion on relatively thin gauge articles, such as sheet and strip, is highly disadvantageous and effectively precludes use of such steel for many applications. The severity of the pitting makes elimination of the pits quite difficult since the pits are relatively deep and, accordingly, a relatively large quantity of surface metal ordinarily must be removed in order to eliminate the pits. This is obviously impossible in the case of finished articles of greatly reduced thickness.

The pit corrosion phenomenon, together with a suitable test for determining susceptibility to such corrosion is set forth in The Corrosion Handbook, John Wiley & Sons, 1948, page 1022.

Accordingly, it is an object of the present invention to provide bright finished stainless steel articles, especially thin gauge forms thereof, which possess enhanced resistance to pit corrosion.

It is another object of the invention to provide a method of substantially inhibiting pit corrosion susceptibility of stainless steels.

It is still another object of the invention to provide a method of producing thin gauge articles, especially of the straight chromium, chromium-nickel, chromium-manganese and chromium-nickel manganese varieties, which have bright, highly finished surfaces which are highly corrosion resistant.

Yet another object of the invention is the provision of means to produce elongated, thin gauge articles of stainless steel having a highly finished surface possessing enhanced corrosion resistance.

In accordance with the above objects, a preferred embodiment of the method of the invention comprises pro duction of an elongated article, such as stainless steel sheet or strip, as by cold rolling, forming, etc., to substantially a final desired thickness, subjecting the article to an elevated temperature anneal in the substantial absence of oxygen, and thereafter subjecting the heat treated article to an electrolytic treatment comprising passing the article through an electrolytic bath and simultaneously impressing an electrical current upon the article, which is maintained as the anode, while refraining from removing substantial quantities of metal therefrom.

The foregoing and other objects will be made readily apparent and a fuller understanding of the invention will be had by reference to the following description and v appended drawings wherein:

FIG. 1 (la and 1!) taken together) is a schematic, elevational view, partly in cross section, of one form of apparatus, in accordance with the invention, for producing bright finished, corrosion resistant stainless steel articles, and

FIGS. 2-6 are photographic illustrations of stainless steel samples of various compositions showing the highly advantageous effect of the method of the invention visa-vis corrosion resistance of the tested steels.

Referring now to the drawings, and in particular to FIG. 1 thereof, the numeral 11 designates generally a furnace for the bright annealing of an elongated strip or sheet 12 of stainless steel. The furance 11, of usual construction, comprises an outer housing 13, a first furnace guide roll 14, a second furnace guide roll 16 and a third furnace guide roll 17 for conveying and guiding the strip 12 through the furnace. As illustrated, there may be provided within the furnace and about the downwardly traveling leg of the strip 12 a heating compartment, designated generally by the numeral 18, for heating the strip to a desired elevated temperature. Heating is accomplished, for example, by means of electrical radiant heater elements 19 spaced from opposite sides of the traveling strip 12 and adapted to heat the same by radiation. The strip 12 enters the heating compartment 18 through a substantially gas-tight first seal 21 and exits from the heating compartment through a narrow slot 22 in the wall of the heating compartment. Provision is made for means (not shown) for introducing into the interior of the heating compartment a suitable inert or reducing gas such as hydrogen, nitrogen, argon, cracked ammonia or the like in order to exclude the surface of the heated strip 12 from contact with oxygen.

Disposed downwardly from the heating compartment 18 and about the lower portion of the travel path of the strip 12 is a cooling compartment designated generally by the numeral 23. Entry of the strip 12 into the cooling compartment 23 is through a narrow slot 24 in the upper extremity of the compartment 23. Within the cooling compartment 23, there is provided, on opposite sides of the strip 12, a plurality of manifolds 26 provided with a number of apertures or jets 27 directed normally to the surface of the strip 12. Means (not shown) are provided for propelling through the manifolds 26 and jets 27 a cooling fluid such as the same inert gas utilized in the heating compartment 18. After extraction of heat from the moving strip 12, the cooling fluid is removed from the cooling compartment 23 through exit ducts 28 whence the heated cooling fluid is conveyed to a suitable heat exchanger (not shown) for recooling. The strip 12, now

cooled to substantially room temperature exits from the furnace through a gas-tight second seal 29. The thereby bright annealed strip 12 is conveyed over a first strip guide roll 31 and thence under first and second bath guide rolls 32 and 33 whereby the strip is submerged in and conveyed through an electrolytic bath medium contained within an electrolytic bath tank 34. Within the bath tank 34 there is provided a first series of pairs of electrodes 36 adjacent the entry extremity of the tank 34 and a second series of pairs of electrodes 37 adjacent an exit extremity of the tank 34.

Upon its exit from the electrolytic bath tank 34, the strip 12 is passed between a pair of second strip guide rolls 38, whence it is conveyed, under a first spray guide roll 39, over a sump pit, designated generally by the numeral 41, constructed with an outer shell 42 and an inner, acid resistant liner 43. While over the pit 41, the strip 12 is subjected to a water spray, as at 44, to remove from the strip acid remaining on the strip from its immersion in the tank 34. The acid wash water is collected in the pit 41 and either may be neutralized and discharged as waste or subjected to suitable processing to recover the acid values therein. The strip 12 is then passed through one or more wiper elements 46, over a second spray guide roll 47 and thence to a scrubber designated generally by the numeral 48.

In the scrubber 48, the strip 12 is passed between a plurality of pairs of strip-flexing rolls such as roll pairs 49, 51 and 52. Before, between and after passage through the roll pairs 49, 51 and 52, the strip is subjected to the action of high pressure water scrubbing sprays as at 53. The wash water from the scrubbing operation is collected in the bottom of the scrubber unit 48 and discharged therefrom as waste by suitable means (not shown).

Following scrubbing of the strip it is passed over a first rinse guide roll 54 and then immersed in a hot water rinse bath in a container 56. The strip is held in the rinse water in a submerged position by means of a second rinse guide roll 57 from which the strip is conveyed out of the rinse bath, given a final clear water rinse as at 58, and then conveyed over a third strip guide roll 59 to a hot air dryer, designated generally by the numeral 60. Heated air is supplied to the dryer, at a temperature sufliciently low to avoid oxidation of the strip, by means of a blower 61.

After emergence of the strip from the dryer 60 it is subjected to such further processing as may be desired, as flattening, etc., and then conveyed to a coiler unit where the strip is sheared and packaged into coils for shipment.

In operation of the above-described apparatus, the bright annealed strip 12, after emergence from the annealing furnace 11, is passed into the electrolytic bath in tank 34. The first series of electrode pairs 36, connected to a suitable source of electromotive energy, are anodic and the second series of electrode pairs 37, connected to the same electromotive source, are cathodic. The strip 12, as it enters the bath medium and passes between the electrode pairs 36, acquires a negative polarity charge, that is, it is cathodic. Upon passage of the strip between the second series of electrode pairs 37, however, the strip acquires a positive polarity charge, that is, it becomes anodic.

The electrode construction illustrated in FIG. 1 is preferred since in this manner no direct electromechanical contact is effected between the electrodes and the strip and hence danger of arcing and thereby disfiguring the strip, with consequent yield loss, is avoided. It is to be understood, however, that the invention is not limited to this particular construction but also contemplates the provision of direct contact means between the electrodes and the moving strip. In the latter case, the strip is to be maintained anodic throughout its travel through the electrolytic bath. A procedure wherein the strip is first made anodic and then cathodic, although it might be utilized in certain instances, with a proper balancing of other process factors, is not preferred since it has been found that, for production of articles having highly enhanced pit corrosion resistance, final anodic treatment is necessary. Consequently, the strip 12 should be anodic during at least a terminal portion of its travel through the electrolytic bath.

The effectiveness of the invention to enhance the pit corrosion resistance of the contemplated stainless steels depends, to a more or less great degree, upon a proper combination of a number of process variables, such as current density and nature and temperature of the electrolyte. These variables in turn, of course, must be selected with a view toward the composition, surface area and thickness of the article being treated. Consequently, a wide variety of combinations of process factors may be utilized in the performance of the inventive method, but the above-mentioned and other factors must be selected within certain broad limitations in order to produce a product which retains a high degree of surface brightness and which also possesses the desired corrosion resistance. Thus, although the total electrical energy which is impressed upon the article in the electrolytic bath may, in unusual circumstances, be as low as about 5 ampereseconds per square foot (hereinafter referred to current density), a current density below that value does not produce the requisite properties in the treated stainless steels. Accordingly, it is required that the current density impressed upon the treated article be at least about 5 to 50 and preferably about 250 ampere-seconds per square foot of article surface. Although there has not been observed any detrimental decrease of corrosion resistance by reason of the use of excessively high current density, current densities over about 500 ampere-seconds per square foot, especially if utilized in an electrolytic bath having a temperature over 95100 F. tend to result in a product of decreased surface brightness. However, current densities of up to about 1200 ampere-seconds per square foot have been utilized and in a nitric acid electrolyte, with the production of steel articles having excellent corrosion resistance despite the presence of a visible surface cloudiness. However, such cloudiness, it has been found, can be eliminated in part by subjecting the upper bath temperature is set at about 100 F. when current densities in excess of about 500 ampere-seconds per square foot are used. Preferably the electrolytic bath temperature is maintained at about room temperature.

Although a wide variety of electrolytes may be utilized in the electrolytic treatment of the invention, we prefer an acid bath such as nitric acid or sulfuric acid. Although treatment in a sulfuric acid bath, e.g., from 0.1 to 10% or more of sulfuric acid has been found to render the contemplated steels quite resistant to pit corrosion, a sulfuric acid bath tends to give a product having a surface of less brightness than that which can be achieved by utilization of a nitric acid bath. Therefore, a nitric acid-containing electrolyte is preferred. Exemplary of other electrolytes of utility are other acids, as phosphoric acid, salts, as sodium sulfate, and bases, such as sodium hydroxide. The minimal electrolyte concentration is to be understood to be that amount of electrolyte effective to conduct the current density required for operation of the contemplated process. Higher electrolyte concentrations may, of course, be productive of enhanced results.

The results of standard iron chloride pit and etching or spot corrosion tests on samples of AISI 430 stainless steel both untreated and treated with a number of electrolytes 25 treated article to a temper rolling operation. For exin accordance with the invention, is set forth in the folample, Type 430 stainless steel, treated in 6% nitric ac d lowing Table I:

' TABLE I Current Pit Test Results 1 Test Current/ Time, Density, Etch Test Surface No. Electrolyte Unit Area, Seconds at Amp. Sec./ Rating 2 Brightness 3 Amp/Ft? Polarity Ft. Surface Appearance Weight Loss, Mg.

379 1-2 32 4 2 54 1-2 33 4 8 34 1-2 33 4 14 35 1-2 34 4 33 1-2 34 10 2 31 1-2 33 10 8 38 1-2 34 10 14 44 1-2 36 10 20 47 1-2 37 2 57 1-2 34 25 8 57 1-2 37 25 14 53 1-2 41 25 20 59 1-2 42 40 2 54 1-2 8 40 1-2 39 40 14 39 1-2 42 40 20 39 1-2 48 2 61 1-2 35 55 4 46 1-2 40 55 8 35 1-2 44 55 20 40 1-2 51 10 5 93 1-2 10 10 118 1-2 40 2. 5 100 No pits 138 1-2 40 5 1-2 10 5 101 1-2 10 10 do 87 1-2 40 5 200 Few large pits, both sides--. 104 1-2 40 10 400 No pits 1-2 10% NaOH. 10 5 5 Several pits, one side only 127 1-2 31 10% NaOH 10 10 100 Line1 type pitting, one side 114 1-2 011 y. 32 10% NaoH 40 5 200 Moderatepitting,both sides- 124 1-2 33 10% NaOH 40 10 400 Verg few large pits, both 104 1-2 si es.

Motlification of test as set forth in Corrosion Handbook, edited by H. H. Uhlig, John Wiley & Sons, Inc., New York, 1948, page 1022. Test specimens approximately 2 x 1 inches were arranged ad acent an aerator near the bottom of a cylindrical jar 6 inches in diameter and 12 inches in height. The jar was filled with a test solution comprising 108 grams per liter of FeCl3.6H2O and 4.15 ml. per liter of commercial, eonccntrated (approximately 37-38 percent by weight H01) hydrochloric acid. Specimens were supported by glass holders hooked oyer the rim of the jar. The lower 3% inches of the jar was immersed in a 30 C. constant temperature bath. The test solution was continually aerated during the test period of four (4) hours. Upon completion of the test, the specimens were removed from the solution, rinsed, dried and visually inspected for number of pits. Weight determination before and after exposure indicated weight loss due to corrosion.

2 Test as set forth in Con csion June 1960, pages -108, i.e., exposure of sample surface under one drop of Solution applied for 5 minutes at room temperature. Solution consists of 10 grams of FeCheHzO, 5 grams N aCl in 2.5 m1. reagent grade (37-38%) H01 and 200 ml. H O. Rating as follows: lno attack, 2very light attack, 3light attack, 4moderate attack, 5heavy attack.

3 Determined by means of Photovolt Corporation reflectometer, Model No. 610, using a difiuse reflectance head and a blue filter. Increasing reflectometer reading indicates greater surface dullness.

The electrolytic treatment of each sample was conducted at room temperature and the sample was first made cathodic for the time shown in Table I and then anodic for an equal length of time. After treatment, the 3 inch x 1 inch samples were rinsed, dried and cut into 2 inch x 1 inch specimens for the pit corrosion tests as aforesaid. The remaning 1 inch x 1 inch specimens were ampere-seconds per square foot of article surface and an 75 utilized in the etch corrosion tests as aforesaid.

It will be seen from Table I that electrolytic treatment with a 6% nitric acid electrolyte, and with current densities of from 8 to 1100 amp. sec./ft. was found to be effective to drastically reduce or to eliminate entirely pit corrosion of the steel samples. It will be noted, in Tests 1 21, that current densities of up to about 50-100 ampere seconds per square foot did not materially decrease the surface brightness of the treated steel, as compared to that of the untreated steel (Test No. 1), whereas use of increasingly greater current densities, particularly from about 800-1100 amp. sec./ft. did tend to decrease surface brightness somewhat. It has been found that nitric acid concentrations of about 0.1 to about by weight of aqueous bath, and especially about 0.5 to 6 or 7%, are especially desirable and that nitric acid concentrations as high as 50-70% are useful in the application of the invention.

It will be further noted from the results of Table I that all specimens had good resistance to etch corrosion and that electrolytic treatment with both phosphoric acid and sodium sulfate as electrolyte gave substantial protection against pit corrosion. Although the treatment using sodium hydroxide was less effective, it did, nevertheless, give a product having much greater resistance to pitting than an untreated product. Increasing milkiness of the steel surfaces was noted during Test Nos. 22-33 with increasing total coulombs delivered to the samples.

Illustrative of the aforementioned advantages and disadvantages of using sulfuric acid as the electrolyte in the contemplated electrolytic treatment are a number of etch or spot tests conducted on 3 inch X 2 inch samples of commercial heats of hydrogen bright annealed 430 type stain- 8 from the treatment of samples of commercial air annealed AISI 430 stainless steel with a nitric acid electrolyte, -as compared to a sulfuric acid electrolyte, is illustrated by the results of tests given in Table III.

1 Determined by means of Photovolt Corporation refiectometer, Model No. 610, using a diffuse reflectance head and a blue filter. Increasing refiectorneter reading indicates greater surface dullness.

From Table III it will be seen that the surface brightness of the steel treated in a nitric acid electrolyte is significantly greater (lower reflectance reading) than that of the sulfuric acid-treated steels. Moreover, it is evident from the data of Table III that increase in the temperature of the electrolytic bath and in the applied current density reduces surface brightness.

less steel. The results of these tests are given m Table II: It has been found that the bulld up of dissolved salts TABLE II Annealing Treatment Treat- Test Electrolytic Bath ment, Amp./ Weight Reflectom- Etch Test No. Conditions Time/ Ft. Loss, eter Rating Atmos. T nIr Seconds Mg. Reading 1 1, 450 Many pits. 1, 500 34 Do. 1,550 35 DD. 1, G00 32 D0. 1, 650 d 43 D0. 1,450 6% H2804, 83 F 5 50 1.2 41 1-2 (N0 pits). 1, 500 6% H2504, 83 F 5 50 2. 1 42 0. 1, 550 6% H2804, 83 F 5 50 1. 7 1 (No pits). 1,600 6% H 804, 8 5 1.9 37 Do. 1, 050 6% H2804, 83 F 5 50 2. 1 49 Do.

1 Determined by means of Photovolt Corporation reflectometer, Model No. 610, using a diffuse reflectance head and a blue filter. Increasing refiectorneter reading indicates greater surface dullness.

From the foregoing tabulated data of Table II it will be seen that, whereas the hydrogen bright annealed material of Test No. 14 was relatively bright, as indicated by the lower refiectometer readings, the surface was, as indicated, defective by reason of the appearance of numerous pits after application of the spot test. On the other hand, the surface of the samples of Test No. 22, while retaining a substantial degree of brightness, was also substantially free of pits. bright annealing temperature has relatively little or no effect upon the etch corrosion of the product following the electrolytic treatment. A comparison of the reflectometer readings given in Test No. 14 for the bright annealed material with those given in connection with Test No. 22 for the material which was bright annealed and subsequently electrolytically treated in a sulfuric acid solution shows that, although no pitting was observed and although the etch test rating of the sulfuric acid electrolytically treated material was quite good, the reflectometer readings for the latter materials are higher than the reading for the bright annealed material which was not electrolytically treated. The diminution of surface brightness, encountered with the use of the contemplated electrolytic baths, is minimized by the use of a nitric acid-containing electrolyte. Thus, the enhanced surface brightness resulting It will be further noted from these tests thatsuch as salts of iron, chromium, etc., in the electrolytic bath is not highly critical to the operation of the electrolytic treatment. Thus, build up of iron and chromium cations to a total concentration of about 5% has been observed to have no discernible effect upon the properties of the treated steels.

The improvement of the contemplated stainless-steels, vis-a-vis corrosion resistance after treatment thereof in accordance with the invention, will be clearly seen by reference to FIGS. 2A-2F which illustrate the results achieved in subjecting strip-type samples of approximately inch X 1% inches in size, to the pit corrosion test as heretofore described. Thus, FIGS. 2A, 2B and 2D represent samples of commercial A151 430 stainless steel of a first manufacturer. FIG. 2B represents a sample of a commercial lot of the same steel of a second manufacturer, FIG. 2C represents a sample of a commercial lot of A181 430 type stainless steel modified by the addition of about 0.6% of molybdenum and 1% of copper, and FIG. 2F represents a sample of Type 430 stainless steel containing 1% each of molybdenum and copper. These latter samples are illustrative of compositional modifications made in an effort to overcome the pit corrosion susceptibility of 430 type stainless steel. All of the samples illustrated in FIGS. 2A-2F were bright annealed, in accordance with'usual commercial practice, in a cracked ammonia atmosphere. The samples illustrated in FIGS. 2A and 2B were then subjected to the electrolytic treatment of the invention in an aqueous electrolytic bath containing 6% by weight of nitric acid and wherein a current density of 250 ampere-seconds per square foot was impressed upon the samples. The two samples of FIGS. 2A and 2B were then rinsed and dried and, together with the samples of FIGS. 2C, 2D, 2E and 2F, were subjected to the standard pit corrosion test as aforesaid. The samples were then removed from the-test solution, rinsed, dried and weighed to determine weight loss of each, with the results set out hereinbelow in Table IV:

TABLE IV Sample: Weight loss, grams 2A 010 The samples were then visually inspected for the appearance of pit corrosion with the results illustrated in FIG. 2. It will be seen that, whereas samples 20, 2D, 2E and 2F comprising both standard AISI 430 type stainless steel and the modified molybdenum-copper compositions, were molybdenum-copper modified composition, is clearly illustrated in FIG. 4. It will be seen from inspection of that figure that the single sample corroded badly over a portion thereof which was not treated in accordance with the invention but suffered substantially no corrosion whatsoever over the remaining portion which had been treated electrolytically in accordance with the invention (250 ampere-seconds per square foot in 6% nitric acid).

The exact mechanism by means of which the inventive method exerts its beneficial influence is not known. It is believed, however, although the invention is not restricted by this hypothesis, that bright annealing, at the high temperature involved, and in the exclusion of oxygen, as by annealing in a vacuum or in a reducing or inert fluid medium, tends to adversely affect a thin, oxide film on the steel article, which film is of material benefit in the resistance of the steel base to corrosion. It is further believed that the electrolytic treatment herein contemplated serves to restore to the steel a condition of surface passivity and that this property may be due, at least in part, to the production, upon the steel surface, during the electrolytic treatment of a thin, impervious and passive highly corroded with the typical pit corrosion appearance,

samples 2A and 2B, which had been treated in accordance with the invention, exhibited substantially no pitting and, as shown by the data of Table IV, much less total weight loss.

Similar treatment (250 ampere-seconds per square foot in 6% nitric acid), following bright annealing of other stainless steels, as AISI Types 201, containing 16-18% chromium, 3.55.5% nickel and 55-75% manganese, and 301, containing 16-18% chromium, 68% nickel, and 2% maximum manganese, were found to prevent pit corrosion of such other steels also. The similar steels AISI Types 202 and 302 are similarly amenable to enhancement of corrosion resistance in accordance with the invention.

The appearance of stainless steels treated in accordance with the invention is not altered in any discernible way. Thus, subjection of the steels to the electrolytic treatment, in accordance with the principle of the invention, does not destroy the bright surface finish of the product issuing from the bright annealing furnace. Thus, the unaltered appearance of the surface of steels treated, as herein described, anodically in an electrolytic 6% nitric acid solution, at a current density of 250 ampere-seconds per square foot is illustrated in FIG. 3A. The single sample of commercial Type 430 stainless steel illustrated in that figure was treated, as aforesaid, over one portion thereof, as indicated, whereas the other portion was untreated. It will be seen by reference to FIG. 3A that there is no discernible difference in surface appearance between the treated and untreated portions of the sample. However, value of the electrolytic treatment in enhancing corrosion resistance is readily seen by reference to FIG. 313 wherein the same sample as illustrated in FIG. 3A was subjected to the standard pit corrosion test. It will-be noted that the untreated portion of the sample bears a multitude of the typical corrosion pits whereas the treated portion is completely free of such pits.

As noted hereinabove, attempts have been made to overcome pit corrosion susceptibility of straight chromium steels by modifying the chemistry thereof as by the addition of molybdenum and copper. FIG. 4 represents a further sample of commercial 430 type stainless steel modified by incorporation of molybdenum (0.6%) and copper (1% The failure of such additives to adequately protect these steels against pitting corrosion, as well as the beneficial effects of the method of the invention in enhancing the pit corrosion resistance of the film. Thus, as noted hereinabove, it has been found that to be most effective, the electrolytic treatment must include anodic treatment of the stainless steel article. That is, a treatment wherein the article is made the cathode during the entire duration of the electrolytic treatment does not suffice to produce an enhancement of corrosion resistance in the steels. Thus, FIGS. SA-SD illustrate the results obtained in a series of tests wherein a number of samples were made, respectively, the anode (FIG. 5A), the cathode (FIG. 5B), first the cathode and then the anode (FIG. 5C) and, lastly, the anode and then the cathode (FIG. 5D). It is evident from the series of four samples illustrated in FIGS. SA-SD that anodic treatment of the steel article is necessary. Thus, FIG. 5A shows a sample of commercial 430 type stainless steel which was anodic during the full course of the electrolytic treatment and which was observed to be substantially completely free of pit corrosion. On the other hand, the sample illustrated in FIG. 5B, made of the same material but which was made the cathode during the entire duration of the electrolytic treatment, is extensively pitted. The utility of an electrolytic treatment wherein the treated article is first cathodic and then anodic is illustrated in FIG. 5C wherein the sample, of the same material of construction, also was observed to be substantially free of pitting. In those instances wherein the article to be treated is alternately anodic and cathodic, the necessity of terminating the treatment with the treated article as the anode is illustrated in FIG. 5D wherein the 430 type stainless steel sample was first made anodic and then cathodic. It will be seen that, under such circumstances, although there is definite enhancement of corrosion resistance, the steel is nevertheless substantially pitted. In the case of each sample illustrated in FIG. 5, the electrolytic treatment consisted of immersion in 6% nitric acid and application of 250 ampere-seconds per square foot. In each case, the corrosion test applied was in accordance with the standard pit corrosion test procedure referred to hereinabove.

The superior corrosion resistance of the steels treated anodically, as illustrated in FIGS. SA-SC, is substantiated by the reduced weight loss, after pit testing, of the anodically treated specimens, as compared to the cathodically treated samples, as seen in Table V.

TABLE V Sample I Weight loss, grams 5A 0.08

Although the theory has been advanced that the electrolytic treatment herein contemplated results in the pro- I l vision of a passivating film upon the surface of the treated stainless steels, it does not follow that subjecting the steel to any one of a variety of oxidation processes will accomplish the results achieved by the invention. Thus FIG.

12; results of such tests, given in Table VII, show that steels, treated in accordance with the invention, are still highly superior to untreated steels, even after polishing and/ or buffing.

TABLE VII Eflect of mechanical polishing and buflfng upon corrosion resistance of treated steels AISI Type 430 bright annealed and subjected to electrolytic treatment in 6% nitri acid with 250 ampere-seconds per square foot.

2 Bright annealed AISI Type 430 stainless steel.

3 Light polish with polishing wheel using Linde A polishing abrasive.

4 Butl pass made with hard telt butting wheel using stainless bufling compound N 0. SCR (Devine Brothers Company, Utica, New York).

6 illustrates the results of a number of tests wherein samples of commercial 430 type stainless steel were subjected to a number of oxidizing processes. FIG. 6A represents such a sample which was subjected, in an untreated form, i.e., direct bright annealed condition, to the standard pit corrosion test. As illustrated, the sample surface presented a great multitude of pits. FIG. 6B illustrates such a sample which was subjected to the electrolytic treat ment as herein contemplated wherein the electrolytic bath comprised an aqueous solution containing 6% by weight I of sulfuric acid and which was subjected to 250 ampereseconds per square foot prior to subjection of the sample to the standard pit corrosion test. It will be seen from that figure that the sample was substantially completely free from pitting. A small surface area presenting a pitted appearance is seen at the bottom of the sample. This area corresponds to the sample area held by the clamping means used to support the sample in the electrolytic treating bath and which area was, therefore, not so treated. FIG. 6C illustrates a sample of the same material which, prior to the standard pit corrosion test, was treated by immersion for 2 minutes in a nonelectrolytic bath comprising an aqueous solution containing 10% by weight of nitric acid and held at a temperature of 180 F. The ineffectiveness of such a treatment to provide substantially enhanced corrosion resistance is obvious from an inspection of the sample as illustrated in FIG. 6C which shows the surface to be liberally pitted. A similar test was applied to the sample illustrated in FIG. 6D but in the latter case the nonelectrolytic nitric acid bath contained, in addition, a trace of hydrofluoric acid. As will be seen by reference to that figure, that treatment also failed to substantially enhance corrosion resistance of the sample. Metal loss from the samples of FIGS. 6A-6D after standard pit corrosion testing is reported in Table VI, wherein it will be noted that loss from the sample treated in accordance with the invention is markedly less than that from the samples treated otherwise.

TABLE VI Sample: Weight loss, gram Samples of treated and untreated steels were subjected to various polishing and boiling procedures, and then subjected to the standard spot and pit corrosion tests. The

The test samples utilized in the aforesaid tests consisted of elongated strips of AISI Type 430 stainless steel. Along each of the strips, zones were buffed to varying degrees as indicated in the table and the surface of each strip, except for half of the unbutfed zone was then polished as noted. A narrow portion was then cut from each specimen for spot testing of all zones and the remainder, containing all zones, was subjected to the immersion pit test.

It will be seen from the results given in Table VII that the resistance to pitting of the material tested in accordance with the invention is not lessend by buffing and/or polishing. 'Nor did bufiing and/ or polishing enhance the corrosion resistance of the experimental steels to a degree approaching the enhancement conferred on those steels by the inventive method.

If, as hypothesized, the effect of the electrolytic treatrnent is to provide the stainless steel base with a highly corrosion resistant surface film, and, if, as further hypothesized, that film is exceedingly thin, the film is, nevertheless, exceedingly resistant to mechanical destruction. Thus, additional evidence of the permanence of the result effected by the invention in conferring corrosion resistance is afforded by the results of still further tests. Thus, thin strip specimens of bright annealed AISI 430 stainless steel, both untreated and treated electrolytically in accordance with the invention, were bent about a Ai-inch-diameter mandrel and both treated and untreated specimens were then subjected to the pit test. The untreated specimens were profusely pitted whereas the treated specimens were free of pits. Similar samples were cupped (by means of a standard Olsen cup tester) and others were subjected to a 15% elongation in stand ard tensile test equipment. In all cases, the untreated samples pitted badly upon subsequent pit testing whereas the treated samples were free of pits.

Although some emphasis has hereinbefore been placed upon the great advantages of the invention in respect to the production of material especially suitable for automotive trim, it is to be understood that the enhancement of corrosion resistance conferred by the inventive method materially increases the usefulness of the treated steels for other applications as well. For example, in the photographic industries, ferrotype plates are utilized for the processing of film and, for that purpose, require metals of an extremely fine surface finish and which are highly resistant to the corrosive effects of the chemicals utilized in such procedures. Heretofore, a major defect in such ferrotype plates, prepared from bright annealed AISI Type 430 stainless steel, was their susceptibility to corrosion and the consequent marring of the photographic film in contact therewith. Consequently, stainless steels treated in accordance with the present invention find particular application in the construction of such ferrotype plates. The treated steels are also of great utility in the construction of exterior architectural structural and decorative articles and in the construction of processing equipment in the chemical and allied industries wherein the steels are exposed to rigorous corrosive environments.

It is to be understood that the foregoing description and drawings are only illustrative of the principles of the invention and that various modifications and additions may be made thereto by those skilled in the art Without departing from the spirit and scope of the invention.

What is claimed is:

1. A method of producing a bright finished stainless steel article of enhanced pit corrosion resistance comprising cold working the article to a desired final form and to impart a desired bright finish, annealing the Wrought article at an elevated temperature and thereafter cooling the annealed article in the substantial absence of oxygen to a temperature below that at which an oxide scale is formed on the article by exposure to air, immersing the article in an electrolytic bath, imposing upon the immersed article an electrical current density of from about to about 1200 ampere seconds per square foot of article surface, and making the article anodic during at least the terminal portion of its immersion in the bath.

2. The method of claim 1 wherein the electrolytic bath is an aqueous solution containing an electrolyte selected from the group consisting of nitric acid, sulfuric acid,

3. The method of claim 1 wherein the electrolytic bath is an aqueous solution containing from about 0.1 to about by weight of nitric acid.

4. The method of claim 3 wherein the bath contains from about 0.1 to about 10% by weight of nitric acid.

5. The method of claim 2 wherein the bath contains from about 0.1 to about 10% by weight of sulfuric acid.

6. The method of claim 1 wherein the bath temperature is maintained between room temperature and F.

7. The method of claim 4 wherein the current density is from about 50 to about 500 ampere seconds per square foot.

8. A bright finished, cold worked stainless steel article which has been bright annealed and thereafter provided with an anodized surface of a reflectivity substantially equivalent to that of the unanodized article and exhibiting enhanced chloride pit corrosion resistance as compared with a similar article not so provided.

References Cited by the Examiner UNITED STATES PATENTS 1,811,409 6/1931 Thormann 204210 1,961,752 6/1934 Fink 204-- 2,092,130 9/1937 Lyons 204-34 2,109,675 3/1938 Miller 204-34 2,174,841 10/1939 Robinson 204-207 2,606,866 8/ 1952 Neish 20429 2,786,003 3/ 1957 Hollinesworth 148-135 OTHER REFERENCES Zapffe: Stainless Steels, The American Society for Metals, 1949 (only pp. 2-69 and 270 relied upon).

phosphoric acid, and ionizable salts thereof, and mix- 35 JOHN MACK Primary Examiner tures thereof.

JOSEPH REBOLD, Examiner.

Claims (2)

1. A METHOD OF PRODUCING A BRIGHT FINISHED STAINLESS STEEL ARTICLE OF ENHANCED PIT CORROSION RESISTANCE COMPRISING COLD WORKING THE ARTICLE TO A DESIRED FINAL FORM AND TO IMPART A DESIRED BRIGHT FINISH, ANNEALING THE WROUGHT ARTICLE AT AN ELEVATED TEMPERATURE AND THEREAFTER COOLING THE ANNEALED ARTICLE IN THE SUBSTANTIAL ABSENCE OF OXYGEN TO A TEMPERATURE BELOW THAT AT WHICH AN OXIDE SCALE IS FORMED ON THE ARTICLE BY EXPOSURE TO AIR, IMMERSING THE ARTICLE IN AN ELECTROLYTIC BATH, IMPOSING UPON THE IMMERSED ARTICLE AN ELECTRICAL CURRENT DENSITY OF FROM ABOUT 5 TO ABOUT 1200 AMPERE SECONDS PER SQUARE FOOT OF ARTICLE SURFACE, AND MAKING THE ARTICLE ANODIC DURING AT LEAST THE TERMINAL PORTION OF ITS IMMERSION IN THE BATH.

8. A BRIGHT FINISHED, COLD WORKED STAINLESS STEEL ARTICLE WHICH HAS BEEN BRIGHT ANNEALED AND THEREAFTER PROVIDED WITH AN ANODIZED SURFACE OF A REFLECTIVITY SUBSTANTIALLY EQUIVALENT TO THAT OF THE UNANODIZED ARTICLE AND EXHIBITING ENHANCED CHLORIDE PIT CORROSION RESISTANCE AS COMPARED WITH A SIMILAR ARTICLE NOT SO PROVIDED.

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