Process of diffusion coating

United States Patent Office

3,184,331

Patented May 18, 1965

Some improvement in the control of carbon concentration in iron-chromium coatings on ferrous articles has been obtained. U.S. Patent 2,791,517 to Becker et al. discloses that the segregation of carbon to the coatings may be re5 duced by the use of carbon stabilizing elements, such as titanium, niobium, and tantalum in the substrate. Benneck, Koch, and Tofaute in “Stahl Und Eisen” for April 27, 1944, pages 265 to 270, teach that the concentration of carbon in the coating should be less than the concen10 tration of carbon in the substrate to obtain good corrosion resistance. These workers allegedly were able to attain such a goal only by the use of carbon stabilizing elements, notably titanium, in the substrate, as taught by Becker et al. However, these prior art attempts to solve the problem 15 of controlling carbon concentration involve a severe economic penalty in that typical carbon steels of commerce cannot be used. As a result, these attempts have not provided a practical solution to the problem from an industrial point of view.

A further goal for the control of carbon in chromium diffusion coatings having a chromium concentration at the surface in excess of about 12% by weight has been to eliminate the presence of iron-chromium carbides at the surface of the coating. The presence of such carbides has 25 been recognized as reducing the formability of chromium diffusion coatings as well as impairing the decorative value of an otherwise bright metallic surface. While it may be possible to avoid the problem of forming iron-chromium carbides at the surface of the coating by using as the 30 base metal very low carbon irons, such base metals are economically impractical to use due to the costly step that is required in removing carbon in its production. Moreover the chromium diffusion coated articles prepared from such base metals are extremely soft and have un35 suitable strength characteristics for most applications.

A further desirable goal in the control of carbon in coatings of chromium diffusion coatings on ferrous metal articles has been to prevent the occurrence of intergranular precipitates of iron-chromium carbides in the coating.

It is an object of the present invention to provide a practical method for diffusion coating of ferrous articles with one or more of the elements selected from the group consisting of chromium, nickel, manganese, and cobalt.

It is another object of the present invention to provide a novel article of manufacture comprising an unstabilized ferrous metal substrate having a diffusion coating of a ferritic iron-chromium alloy, wherein the carbon concentration of said coating is less than the carbon concentration of said substrate.

It is still another object of the present invention to provide a novel article of manufacture comprising an unstabilized ferrous metal substrate containing an amount of carbon in excess of 0.01 % by weight, having a diffusion coating of a ferritic iron-chromium alloy containing at least 12% by weight chromium at the surface thereof, wherein the surface of said coating is free of ironchromium carbides.

It is still another object of the present invention to provide a novel article of manufacture comprising an unstabilized ferrous metal substrate having a diffusion coating of a ferritic iron-chromium alloy containing at least 12% by weight chromium at the surface thereof, wherein said coating is characterized in being free of 65 intergranular precipitates of iron-chromium carbides.

The above and other objects are accomplished in accordance with the present invention by contacting a ferrous metal article with a substantially carbon free molten bath containing a transfer agent selected from a group 70 consisting of calcium, barium, magnesium, and strontium having incorporated therein at least one diffusing element selected from the group consisting of chromium, nickel,

3,184,331 PROCESS OF DIFFUSION COATING

Giles F. Carter, Wilmington, Del., assignor to E. I. du Pont de Nemours and Company, Wilmington, Del., a corporation of Delaware

No Drawing. Filed Dec. 16,1963, Ser. No. 330,540

Claims. (Cl. 117—114)

The present application is a continuation-in-part of application Serial No. 139,369, filed September 20, 1961, which is a continuation-in-part of application Serial No. 44,015, filed July 20,1960, which in turn is a continuationin-part of application Serial No. 835,171, filed August 21, 1959, all now abandoned.

This invention relates to a novel metal diffusion process in which various elements are diffused into a ferrous metal article from a melt of certain metal transfer agents and to novel articles of manufacture comprising a ferrous metal substrate having a ferritic iron-chromium alloy coating prepared by said metal diffusion process.

The primary purpose of metal coatings is for surface protection. Coated metals are commonly used materials which have their surfaces protected against corrosion, oxidation, and wear. Many of these metal coatings are produced by electroplating, by hot dipping, or metal spraying. These coatings as a class are alike in that the deposit is a distinct adjunct to the metal base and adhere essentially by means of mechanical bonds. Such coatings usually suffer from the disadvantage that the adhesive forces are not great enough to maintain an integral bond between the coating and base metal when deformation is applied. Consequently, these coatings are generally applied only after the base metal is formed into the desired shape.

Coatings can also be applied to metal surfaces through diffusion processes. The object of such processes is to enrich or alloy the surface of a metal with certain elements which provide desirable properties such as corrosion resistance not possessed by the base metal itself at a minimum expenditure of the foreign or enriching element. For example, by chromium diffusion in accordance with 40 such techniques, plain carbon steel can be made corrosion resistant to certain depths. An outstanding feature of articles prepared by diffusion processes is that the coating is metallurgically bonded to the substrate metal of the article so that an integral bond is formed between coating 45 and base metal. Unfortunately, however, diffusion methods in the past have been found to be of limited commercial value due to apparatus limitations, inferiority of the coatings obtained, or economic reasons.

The prior art chromium diffusion methods for treating 50 ferrous articles generally employ solid packs or gaseous treatment schemes in which chromium transfer is accomplished by gaseous compounds of chromium. As soon as chromium is deposited on the substrate surface, it diffuses inward to form a diffusion coating. Articles produced by 55 such prior art processes have found limited commercial acceptance in areas where resistance to wear or high temperature oxidation is paramount. However, articles prepared by such processes have attained no significant commercial acceptance in the industrially more important go area where corrosion resistance and formability are the foremost criteria for performance. Skilled workers in this art have long recognized the control of carbon in the coatings of articles prepared as being essential to gaining these property requirements. It has been found nevertheless that controlling the carbon concentration in chromium diffusion coatings formed on a ferrous metal substrate presents a most formidable obstacle due to the strong affinity of carbon for chromium which causes the carbon invariably present in the ferrous metal base to migrate at the processing temperature toward the chromium and thus concentrate in the coating.

3,184,331 transfer bath may be completely in the molten state with the diffusing element in solution in the molten transfer agent. Alternatively, in the case where the diffusing element has a limited solubility in the transfer agent such 5 as, for example, chromium, an excess of the solid diffusing element may be incorporated so that there is both a solid and liquid phase present. . It is to be understood that it is the transfer agent in . the liquid phase of the bath that is the medium for the movement of the dif10 fusing elements into the ferrous metal article treated.

Solid phases that may be present in the bath, either are present as inert diluents or as a convenient reservoir of the diffusing elements. When the amount of solid phase in the transfer bath becomes excessive an undesirable result is the embedding of particles in the surface of the coating. . It will be within the skill of one in the art to determine what portion of the bath should be present as liquid phase for any particular, bath composition to avoid the above-mentioned problem. Usually, however, the liquid phase will constitute over 50% by weight of the bath. While the content of transfer . agent in the bath may vary between wide limits, a practical, lower limit for most coating operations within the, invention will be above about 10% by weight.

The metal transfer bath can be prepared in a number of suitable ways. The transfer agent and one or more of the diffusing elements can be heated up together to process temperature. Alternatively, one or more of the diffusing elements in selected concentrations can be prepared and added to the molten transfer agent; the mixture then being heated to, process temperature. The diffusing elements may be added periodically to replenish the bath or added continuously in controlled amounts to facilitate .prolonged coating operation. The diffusing elements may be added in almost any particle form. It has been found, however, at least in the instance of those diffusing elements that are slightly soluble in the metal transfer agent, such as, chromium, manganese, and cobalt that improved results are obtained if the diffusing element is added in the form of -a finely divided powder. The diffusing elements are generally introduced to the diffusing bath in their elemental form, the metals as commercially available being fully satisfactory in the process. Sources of the diffusing elements other than the elemental form of the, metals may also be. used, such as, for example, a Cr-Ni alloy or a Fe-Cr alloy may serve as a suitable source of chromium in the process of the invention. In addition, compounds which are reducible by the transfer agent to the metallic form of the diffusing element may be employed, such as, for example, CrO_s or Cr₂O₃ which may be readily reduced to chromium. .

The use of a blanket of inert gas over the metal transfer bath is desirable but not necessary since the bath may be. operated under carefully controlled conditions in the open atmosphere. It is desirable to agitate the bath during operation by mechanical or some other means but this again is not’essential.

. The operating temperature of the bath for the process is selected to favorably affect the. rate of diffusion of the cements and to maintain the transfer agent or combinations thereof in the molten state. Generally, temperatures less; than, about 800° C. are not considered practical for metal diffusion because the rate of diffusion is too slow. However, since the transfer agents either alone or in combination can be maintained in a molten state at 800°: 0., this may be considered .as an: approximate minimum practical . operating temperature for the process. A preferred, operating temperature for the process, particularly when calcium is employed as the transfer agent, is from, about 1000 1200¹¹ C. The maximum practical operating temperature may be considered to be the normal boiling point of the transfer agent used but, in any event, the temperature of operation must, be maintained below the normal melting point of the solid ferrous metal article treated, . -.

manganese, and cobalt, wherein said contacting is carried out at a temperature between'about 800° C. and the melting point of said article.

The diffusion method hereof is applicable to any ferrous metal article such as cast iron, mild steel, stainless steel, or the like, wherein iron is the predominant element. With the proper contacting time and control of process factors disclosed hereinafter any of the named diffusing elements or combinations thereof can be diffused into the ferrous article from the molten transfer agent to form alloy coatings of predetermined compositions distinct from the original alloy material. In addition to diffusing one or more of the diffusion elements into a ferrous article, the process can also be adapted to remove or. decrease the amount of any of the named diffusing elements 15 present in a ferrous article treated in order, to alter its surface alloy composition.

Metals that serve as suitable transfer agents in the process are calcium, barium, strontium, and magnesium. Usually these agents are used separately although they 20 may be used satisfactorily in combination with one another. Calcium is the preferred transfer, agent exhibiting a relatively low vapor pressure at operating temperatures found to favor metal diffusion into the solid ferrous articles treated. The transfer agent in the process may be ? 25 replaced in part with various diluent materials so as to reduce the amount of transfer agent required and to modify the transfer properties of the diffusing elements. Illustrative examples of such diluents are copper, lead, tin, and calcium nitride. 30

Although it is not intended to limit the invention to any particular theory of operation, it is believed that the process of diffusing the named elements is. best .explained in terms of an isothermal liquid-to-solid transfer in which the molten transfer agent acts principally as a solvent and 35 transfer medium to bring the diffusing elements in contact with the solid ferrous metal article accompanied by an isothermal, solid-state diffusion process of coating growth.

The greatest thermodynamic tendency for liquid-to- ⁴⁰solid transfer occurs when the transfer agent in the molten bath is saturated with the diffusing element and when the diffusing element is not present in the solid article, though capable of complete solution therein. As an example, the liquid-to-solid transfer of chromium to iron in a Ca- 45 Cr melt readily reaches an optimum thermodynamic tendency since chromium is completely miscible in iron and only slightly soluble in the transfer agent, the amount of chromium soluble in calcium, for example, at 1100° C. being less than about 0.1% by weight. 50

Cobalt and manganese are similar to chromium in having a relatively low solubility in the transfer agent and a high solubility in ferrous metal articles. Nickel, on the_:other hand, is an example of a diffusing element that is highly soluble in both the transfer agents and the ferrous 55 metal articles. Consequently, higher concentrations of this latter element in a melt of the transfer agent is required in order to reach the greatest. thermodynamic tendency for liquid-to-solid transfer to occur.

Liquid-to-solid transfer results in the incorporation of 60 the diffusing element into the substrate surface. At the . high temperatures employed in the contacting, further inward diffusion of the element then causes coating growth. The rate of coating growth is dictated by the well-known laws of solid-state diffusion and varies for the particular .65 diffusing element involved. .

As illustrative of attainable surface concentrations for alloy coatings of a single diffusing element, alloy coatings may be prepared on a ferrous metal article such coatings containing up to about 85-90% by -weight cobalt;, up to 70 about 60% by weight chromium or manganese, and up to about 50 55% by weight nickel. .

The metal transfer bath comprises^ the metal· transfer agent or agents, the diffusing element or elements, and? any diluent materials; vyhieft may be present,,: The metal 75

3,184,331

The residence time of the ferrous article in the metal transfer bath for diffusing in any particular diffusing element influences the thickness of the coating obtained and may vary widely. Coatings of appreciable thickness for some of the diffusing elements may be formed in as little as one minute of treatment in the metal transfer bath. For example, in the case of a calcium bath operated at 1100° C. which is saturated with chromium, a 0.3 mil coating may be obtained in one minute.

Depending on the size of the metal transfer bath and the treating time necessary for a desired thickness of coating of a particular diffusing element or combination thereof, coiled steel sheet or shaped ferrous metal articles may be passed continuously through the metal transfer bath at a rate to provide the required residence time for a desired coating, or articles may be immersed batchwise in the metal transfer bath to provide the required residence time for a desired coating and then withdrawn.

No special pretreatment of the ferrous metal articles is required before immersion in the metal transfer bath. Good coatings have been obtained by the process even in the presence of scale or thin films of oil on the surface of the base metal. It is, of course, desirable that the surface of .the ferrous base metal be clean and for optimum results, it is preferable that the metal articles be subjected to conventional degreasing treatments. For optimum results, it is also desirable to remove any burrs or sharp surface irregularities from the base metal since these may later be mechanically dislodged during use, thus exposing the uncoated base metal to a corrosive environment. Furthermore, the presence of reentrant angles as scratches and burrs permits the entrapment of liquid corrodants which can be unusually aggressive. For these reasons, it is preferred to precondition the base metal by polishing to remove deep scratches or rough edges. Surprisingly, it has also been found that somewhat improved results are obtained by using ferrous base metals prepared by the well-known basic oxygen refining process rather than those refined by older open-hearth methods. It is not clear whether this results from an improved surface or from minor compositional effects of the refining procedure; but, nevertheless, such improvements are noted with these steels, particularly in the case of forming chromium diffusion coated ferrous articles.

The ferrous articles treated in accordance with the hereinbefore described method of the invention are termed coated articles, although it must be appreciated the diffusing elements migrate into the solid surface of the ferrous articles and thus alter the characteristics of the articles. For the usual treating times, the coating is characterized by different concentrations of the diffusing elements at its outer surface than are found in the interior. When the process involved is metal transfer from the liquid to the solid then the coating is characterized by greater concentrations of the diffusing elements at its outer surface and a decreasing concentration of the difusing elements with increasing distance from the surface.

A distinct advantage of the method of the invention resides in the fact that several diffusing elements may be effectively diffused into a ferrous base metal simultaneously. Therefore, by selecting various amounts of diffusing elements and by proper control of process conditions, the concentrations of the elements present in the coatings can be controlled and ferrous alloys containing two· or more of the diffusing elements of predetermined composition may be formed. Other diffusion techniques such as commercial vapor diffusion processes are usually limited in that the diffusion of more than one element is not practical unless diffusion of each element is carried out separately. Step-wise diffusion in this manner makes it difficult if not impossible to control alloy composition at the surface of the coating since during diffusion of each succeeding element, the earlier diffused elements continue to diffuse inwardly from the surface under the influence of heat.

It is possible, .therefore, to form in a one step operation iron-chromium-nickel alloy coatings on ferrous substrates which will protect the substrate against corrosive attack. For example, coatings of iron-chromium-nickel alloy may be formed on inexpensive ferrous base metals which will impart surface alloy compositions which are equivalent to the commercial iron-chromium-nickel austenitic stainless steels. Coatings may also be formed of iron-chromium-nickel alloys which are fully equivalent in surface composition to nickel bearing ferritic stainless steels.

It is also possible to treat commercially known stainless steels by the process to alter the surface compositions and properties of such steel by adjusting the nickel and chromium surface concentrations. Austenitic stainless steels such as type 304 stainless containing 18% chromium-8% nickel are well known to be susceptible to stress corrosion cracking. By treating such a steel in a metal transfer bath containing chromium, the chromium content of the surface of the treated steel is enriched while simultaneously some of the nickel is transferred to the metal transfer bath resulting in a ferritic alloy forming at the surface of the treated steel which markedly improves the resistance of the treated steel article to stress corrosion cracking.

A salient feature of the method of the invention is the potent purifying action on the coatings by the transfer agent particularly when calcium is used, which occurs simultaneously with the formation of coatings on ferrous base metals whereby the concentration of carbon and nitrogen are effectively reduced to limits in the coatings lower than heretofore obtained. Other interstitial impurities, such as, oxygen, sulfur, phosphorous, etc., are also reduced in concentration.

In view of the wide variety of articles of manufacture which can be prepared by the process of .the invention, it is to be appreciated that a variety of steps subsequent to coating may be used depending on the properties desired in the final article for any particular use. Thus, it may be desirable to rapidly quench the article after coating or subsequently apply any of a variety of heat treatments to develop particular properties in either the coating or the substrate. Similarly, the step of polishing or buffing the coated surface to improve reflectivity or color may be desirable. It also may be desirable to selectively dissolve away the ferrous substrate material containing less than about 12% by weight chromium from .the article of manufacture in order to obtain as a useful article in itself a thin film of the unsupported diffusion alloy coating.

A better understanding of the process of the invention will be gained from the following working examples illustrating preferred modes of operation. Throughout the examples, the amount of the various ingredients are given in terms of percent by weight unless otherwise indicated. In all of the examples the transfer agent comprises more than 60% by weight of the diffusing bath. The thicknesses of .the coatings formed on the ferrous metal articles were determined by metallographic examination or by measuring the thickness of the strip film after the substrate has been dissolved away. The compositions reported for the surface of the coating were determined by X-ray fluorescence.

A first series of experiments demonstrates the preparation of ferrous alloy coatings with a single diffusing element in the diffusing bath.

EXAMPLE 1

An iron container holding a bath composed of 2400 grams of calcium and 100 grams of powdered chromium ( — 100 mesh) was heated to 1100° C. The bath was agitated by a mechanical stirrer and was protected from the atmosphere by an inert atmosphere of argon. A mild steel coupon (4 x 1 x .06) containing about 0.06% carbon was introduced and withdrawn after 60 minutes treatment. A coating 2.1 mils thick was formed having a surface concentration of 40% chromium.

3,184,331

Using the same bath composition three additional runs were made on identical mild steel coupons with identical treating times but with varying treating temperatures. The first of these coupons was treated with the bath temperature being 990° C., the second with the bath tern- 5 perature-being 1080° C., and the third with the bath temperature being 1125° C. The coating for the first coupon was 0.85 mil thick having a surface concentration of 33% Cr, the coating of the second coupon was 1.7 mils thick having a surface concentration of 39% Or, io and the coating for the third coupon was 2,7 mils thick having a surface concentration of 42% Cr. Thus, it will be seen that as the treating temperature is increased, the thickness of the coating increases, provided other conditions are the same. 15

EXAMPLE 2

An iron container holding a bath prepared from 1500 grams of calcium and 50 grams of CrO₃ was heated to 1100° C. The bath was agitated by a mechanical stirrer _2Qand was covered by an inert atmosphere of argon. A mild steel coupon containing about 0.06% carbon was treated in the bath for 45 minutes. A coating of 1.6 mils was formed, having a surface concentration of 36% chromium. C'r₂O₃ may be substituted for CrO₃ as the ₂g source for chromium with substantially equivalent results.

EXAMPLE 3

An iron container holding a bath composed of 100 grams magnesium and 10 grams chromium was heated to 1100° C. The bath was again agitated by a mechanical stirrer and was protected from the atmosphere by an inert atmosphere of argon: A mild steel coupon identical to that used in the. runs of Example 1 was introduced and withdrawn after two hours of treatment. Analysis showed 35 that the surface of the coating contained 22% chromium.

EXAMPLE 4

A bath comprising 500 grams calcium and 100 grams ferrochrome (—100 mesh) Was heated to 1060° C. in argon. A high carbon (about 0.8% carbon) steel sample (% x 2) was treated for 16 minutes and was found to have a chromized coating 0,40 mil thick. The surface concentration of chromium was 18%.

EXAMPLE 5

An iron container holding a bath prepared from 800 grams of calcium and 325 grams of nickel shot was heated to 1110° C. The bath was agitated by a mechanical stirrer and was protected from the atmosphere by an inert atmosphere of argon. Four mild steel coupons (3 x ¥2 x 0.04) containing 0.06% carbon were introduced and one coupon was withdrawn after 0.57 hour, the second withdrawn after 1.14 hours, the third withdrawn after 2.28 hours, and the fourth withdrawn after 4.55 hours. The . coating in sample 1 was 0.21 mil thick having a_ surface concentration of 33% Ni; sample 2 was 0.36 mil thick, having a surface concentration of 36% Ni; sample 3 was 0.62 mil thick, having a surface concentration of .37.% Ni; and sample 4 was 0.81 mil thick, having a surface concentration of 39% Ni.

EXAMPLE 6

A bath was formed containing 500 grams calcium and 20 grams of powdered manganese. Mild steel was treated in this bath for 30 minutes at 1100° C. forming a coating of 0.7 mil having a surface concentration of 50% manganese.

; EXAMPLE 7

A bath was formed containing 500 grams calcium, and. a ten gram lump of cobalt. Mild steel ; coupons were immersed in this bath for one hour at 1050—1100° C. and developed a coating having a surface concentration of approximately 6% cobalt. When 20 grams of cobalt powder (200 mesh) were added to the bath, the samples gained 60 mg. in 30 minutes and the surface of the coating contained approximately 67% cobalt.

EXAMPLES

A bath was formed in an iron crucible containing 150 grams barium and 5 grams of cobalt powder. The bath was agitated and operated under argon, A sample of iron (0.0025% C) was immersed in this bath for 15 minutes at 1100° C. A coating 0.2 mil thick was obtained on the base metal containing approxiamtely 30% cobalt at the surface.

EXAMPLE 9

A bath was formed in an iron crucible containing 154 grams barium , and 5 grams of manganese powder. The bath was agitated and operated under argon. A sample of iron (0,0025% C) was immersed in this bath for one hour at 1100° C. A coating 0.4 mil thick was obtained on the base metal containing approximately 35% Mn at the surface.

EXAMPLE 10

A bath was formed in a molybdenum crucible containing 90 grams strontium and 10 grams powdered nickel. The bath was agitated and operated under argon. A sample of iron (0.0025% C) was immersed in this bath for 15 minutes at 1100° C. A coating 0.9 mil thick was obtained on the base metal containing approxiamtely 55% nickel at the surface.

EXAMPLE 11

A bath was formed in an iron crucible containing 157 grams strontium and 5 grams of cobalt powder. The bath was agitated and operated under argon. A sample of iron (0.0025% C) was immersed in this bath for 75 minutes at 1100° C. A coating 0.5 mil thick was obtained on the base metal containing approximately 40% cobalt at the surface.

As has been shown in the foregoing examples, calcium, barium, strontium, and magnesium are effective as trans₄₀ fer media in the diffusion of the named diffusing elements into a ferrous base metal. For simplification, therefore, only calcium will be employed as the transfer agent in the remaining working examples.

A further series of experiments shows as an embodiment of the process of the invention, the formation of 45 coatings on a ferrous base metal involving the simultaneous diffusion of at least two diffusing elements.

^: EXAMPLE 12

An iron container holding a bath composed of 500 50 grams calcium and 50 grams of powdered chromium was heated to .1100° C. and 88 grams of nickel shot was introduced. The bath was agitated by a mechanical stirrer and the bath was covered by an inert atmosphere of argon. Four mild steel coupons (3 x ¥2 x 0.04) 55 containing. 0.06% carbon were introduced and one coupon was withdrawn after one hour, one after two hours, one after three hours, and one after four hours. Analysis of the surfaces of each coupon showed that ferrous alloys of about 45% chromium and 6% nickel (aver60. age) had formed. The coating thickness was 1.1 mils, 1.7 mils, 2.2 mils, and 2.5 mils for the one, two, three and four hour treatments respectively.

Under the same conditions, four additional samples of mild steel were treated in a bath of 500 grams calcium, 65 70 grams powdered chromium, and 210 grams nickel shot. Analysis of the surfaces of each coupon showed that ferrous alloys of about 24% chromium and 45% nickel average had formed. The coating thickness was 0.2 mil, 0.2 mil, 0.3 mil and 0.3 mil for the one, two, 70 three, and four hour treatments respectively.

EXAMPLE 13

A thick alloy diffusion coating (6 mils) was applied to mild steel upon immersion for a total of 40 hours 75 at 1100° C. in a bath containing 350 grams calcium and

3,184,331 grams ferrochrome (about 70% Cr) and 20 grams nickel. Another 20 grams nickel was added about ten hours before the sample was removed from the bath. The chromium concentration was 19% at the surface of the sample, and it diminished gradually to 15% at 5 mils from the outer surface. However, the nickel concentration dropped sharply from over 10% at the surface to 3.4% at 5 mils beneath the surface.

EXAMPLE 14

A bath of 500 grams calcium and 5 grams chromium powder was heated to 1100° C. Then 88 grams nickel shot were added after the calcium-chromium had been stirred (under argon) for 20 minutes. The temperature was 1100° C. After 30 minutes additional stirring, a mild steel sample was treated for one hour, then removed, water quenched, and cleaned. A coating of 0.51 mil was formed. The surface composition was 17% chromium, 23% nickel, and 60% iron. X-ray diffraction data showed that the surface was pure austenite.

EXAMPLE 15

A bath containing 500 grams calcium, 30 grams powdered chromium and 70 grams nickel shot was heated to 1100° C. A mild steel sample immersed in this bath for 4 hours had a coating 2.78 mils thick. The diffusion was carried out under an argon atmosphere with agitation.

X-ray diffraction analysis showed that austenite and ferrite were both present in the surface 0.1-0.2 mil layer. Slight polishing completely removed the austenite. This indicated that the structure of the coating was basically ferritic but with a thin austenitic skin. The thin austenitic surface layer formed because the sample was not quenched after treatment. Samples from a bath of the same composition, if water quenched, have a ferritic sur-

EXAMPLE 16

In a bath prepared from calcium, powdered chromium and nickel shot in the weight ratio of 400:40:70, a mild steel sample was treated for one hour. The surface composition was analyzed as about 45% chromium and 5% nickel. X-ray diffraction analysis and metallographic cross section examination showed that the coating contained no austenite but only ferrite.

Examples 12-16 show that concentrations of the elements present in the alloy coatings can be controlled and that alloys of widely varying contents can be formed by varying the amounts of the diffusing elements in the bath while the thickness of the coating formed is primarily dependent on the time of treatment at a given temperature and whether the coating is ferritic or austenitic at the treatment temperature.

EXAMPLE 17

A bath of 500 grams calcium and a ten-gram lump of cobalt was heated to 1050-1100° C. 20 grams of chromium powder was added after the calcium-cobalt had been stirred under argon for one hour. After 30 minutes’ additional stirring, a mild steel sample was treated for 30 minutes at 1050-1100° C. and a coating containing cobalt and chromium was formed. The outer surface of the coating analyzed about 27% cobalt and 14% chromium.

A new bath was made up having 500 grams calcium, 5 grams cobalt powder, and 20 grams chromium powder. After being immersed for five hours in the bath at 10501100° C., the surface of a sample was analyzed. The alloy had about 20% chromium and about 35% cobalt. Under the same conditions a bath having 500 grams calcium, 40 grams chromium, and 5 grams cobalt yielded coatings having a surface concentration of about 22% chromium and 16% cobalt after one hour.

Another set of experiments demonstrate further applications of the diffusion process of the invention.

EXAMPLE 18

The diffusion process can be advantageously employed to alter the surface characteristics of austenitic stainless steel so as to prevent stress corrosion cracking. Stress ^,J corrosion cracking generally occurs under residual or applied stress and the cracking of austenitic stainless steel can be observed when the steel is stressed and its surface is exposed to solutions containing chlorides. Although annealing may relieve prior residual stresses, additional stresses introduced by quenching -from the annealing treatment can cause cracking under corrosive conditions.

Since stress corrosion cracking is well known to start at the surface of an austenitic steel, a practical approach to solving the problem of stress corrosion cracking can be ¹⁵ made by altering the nickel and chromium surface concentrations through the use of diffusion coating without altering the underlying -bulk of the steel. In a series of tests, a number of 304 type stainless steel coupons were treated using calcium-chromium and calcium-chromium²⁰ nickel baths at 1050-1100° C. and were then removed from the bath and rapidly cooled. After treatment, the coupons were bent into a U-shape and immersed in boiling 42% aqueous magnesium chloride solutions which is the severest commonly-used accelerated test for determining ²⁵ stress corrosion cracking. The results below clearly show that even ten minutes of treatment time in the various baths markedly enhance the resistance of -the coupons to this type of cracking.

Table I

<td rowspan="2"> Bath</td><td rowspan="2"> Treatment time (min.)</td><td colspan="2"> Final surface concentration</td><td rowspan="2"> Boiling 42% MgCh (aq.) test time to cracking (hours)</td>

<td> Percent</td><td> Percent</td>

<td> 35</td><td></td><td> Cr</td><td> Ni</td><td></td>

60^z

180

180

130

881

200

100

180

Control______________ (1) Ca-Cr___________ ¢2) Ca-Cr___________ (3) Ca-Cr-Nl________

Diffusing baths used in 1 and 2 of Table I contained 600 grams calcium and 60 grams chromium and were operated at about 1060° C. In 1 and 2 it will be seen that the nickel concentration in the steel article decreased. Since no nickel was present in these baths, the distribution of nickel in the -baths and in the articles -was not an equilibrium distribution. Thus part of the nickel diffused -from the article’s surface into the bath.

The bath used in 3 contained 800 grams calcium, 112 grams powdered chromium and 336 grams nickel shot and was operated at about 1100° C. The stainless steel sam_rr pies of 2 and 3 were treated separately in the order listed in Table I. The samples treated in baths of 1-3 were rapidly cooled and the most resistant sample to cracking had a -ferritic layer exposed to MgCl₂.

EXAMPLE 19 ⁶⁰ The diffusion process can be advantageously employed to obtain intricately shaped forms by diffusion coating shaped or machined -ferrous parts for a sufficient length of time to impart -a coating (usually 3-10 mils) of the desired thickness. The coating can be punctured and the 65 ferrous substrate containing less than 12% chromium can be removed by dissolving. In this way, light-weight complex parts can be produced. The essentially infinite throwing power (i.e., the ability to coat inside small recesses or cavities) of the metal transfer baths permits intricate 70 shapes to be evenly coated.

A %-inch section of one inch diameter iron bar had a 14-inch hole drilled -through the axis of the bar and three 14-inch holes drilled radially perpendicular to the axis. This part was then immersed in a diffusion bath 75 containing molten calcium and an excess of chromium and

3,1

Π the bath was maintained at 1060° C. for 45 hours. All exposed surfaces were uniformly alloyed with chromium and the coating was found to extend more than ten mils in depth from the surface. The top of the part was sawed off and the ferrous base metal completely dissolved out by hot nitric acid leaving a thin shell of stainless steel having an intricate shape. It has been found that hot nitric acid removes the ferrous metal containing less than about 12% ±2% chromium.

In the manner similar to that shown above, various coat- j q ings containing chromium and nickel-chromium can be formed on ferrous articles with the subsequent removal of that ferrous metal containing less than about 12% chromium. Thus various shapes that -are now extremely difficult or impossible to make by conventional methods can 15 be readily prepared. Depending on the elements diffused, intricate forms having properties such as; corrosion resistance, high strength to weight ratio, and oxidation and wear resistance are obtainable.

As a part of the present invention, novel articles of 20 manufacture have been prepared comprising a ferrous metal substrate having a diffusion coating of a ferritic iron-chromium alloy which exhibit physical and metallurgical characteristics long sought in the art that lead to significant improvements in corrosion resistance, ap- 25 pearance, and formability over previously known chromium diffusion articles.

As mentioned hereinabove, control of carbon concentration in the chromium diffusion coating on a ferrous metal substrate in the past has presented a special problem ₃₀due to the strong tendency for the carbon in the sub-, strate metal to migrate, at the temperatures necessary for forming a diffusion coating, into the chromium-rich coating and concentrate there as carbides of iron and chromium. Owing to this difficulty, chromium diffusion coat- 35 ings formed on an unstabilized ferrous metal substrate prior to the present invention yielded articles of manufacture wherein the carbon concentration in the coating is greater than the carbon concentration in the substrate metal. Articles meeting this characterization, even in 40 the case of articles wherein the carbon concentration in the substrate is low, jeopardize the attainment of maximum corrosion resistance for a chromium-rich alloy coating and in the case of articles wherein the carbon concentration in the substrate is at a level practical and desirable _4gfor strength and economical considerations, such as above 0.01% by weight, this relationship in which the carbon concentration in the coating is greater than that in the substrate_: manifests itself in properties which restrict or eliminate such articles from serving in many important ₅₀end uses. For example, articles, prepared by prior art means of chromium diffusion coating common commercial grades of mild steel, have not found extensive use in decorative bright ware because they exhibited iron-chromium carbides at the surface of the coating or . formed 55 intergranular carbides which greatly detracted from corrosion resistance and the ability of such articles to be subjected to significant deformation without cracking of the chromium diffusion coating.

In accordance with the present invention, the foregoing go described process of the invention can be operated to produce a chromium diffusion coating on an unstabilized ferrous metal article which overcomes the above-described problems. More specifically, a novel article of manufacture can be prepared comprising an unstabilized ferrous 55 metal substrate having a diffusion coating of an ironchromium alloy, wherein the carbon concentration of the coating is less than the carbon concentration of the substrate. Moreover, a novel article of manufacture can be prepared comprising an uhstabilized ferrous metal sub- 70 strate which contains an amount of carbon (practical for: strength considerations, preferably above. 0.01% by weight, having a diffusion coating of an iron-chromium alloy containing at least 12% by weight chromium at the surf ace thereof, wherein the surface of said coating is 75 :,331 free of iron-chromium carbides. When the process of the invention is operated in combination with a subsequent rapid-quench step under properly controlled conditions either of the novel articles of manufacture mentioned hereinabove can be prepared wherein the iron-chromium alloy coating is further characterized in being substantially free of intergranular precipitates of iron-chromium carbides so that the maximum benefits of a corrosion resistant ferritic iron-chromium alloy coating can be attained.

Use of the term “unstabilized ferrous metal substrate” herein refers to ferrous metal substrates which have not been alloyed with carbon stabilizing elements and must therefore be interpreted in light of the residual amounts of metallic elements that can be found in steels as a consequence of usual refining procedures. Thus carbon stabilizing elements such as titanium, niobium, tantalum, zirconium, and vanadium are typically present in only small amounts unless purposefully added to steel since they are preferentially oxidized and slagged away during refining. On the other hand, elements such as chromium, manganese, molybdenum, and tungsten which may stabilize carbon can be found in larger amounts introduced from scrap iron and incompletely removed during refining. Therefore, the term “unstabilized ferrous metal substrate” as used herein and in the claims is intended to include ferrous metal substrates having no more than about 0.2% by weight of titanium, niobium, tantalum, zirconium, or vanadium or combinations thereof and no more than about 2% by weight of chromium, manganese, molybdenum, or tungsten, or combinations thereof.

A better understanding of the product aspects of the present invention will be gained from the following working examples and description. In all the examples hereafter, the carbon values of the coating and the bulk carbon concentration of the article were determined by analysis and the carbon value of the substrate calculated therefrom. The bulk carbon concentration was obtained by running an analysis on the article as prepared consisting of the coating and substrate material. The coating carbon concentration: was obtained by stripping off the coating from the article as prepared and running an analysis on the coating material thus isolated. The stripping technique employed involved cutting along one edge of the article as prepared to expose the substrate material and then immersing in boiling 30% nitric acid for 1 to 4 hours. The acid reagent dissolves away the substrate material leaving the chromium diffusion coating material which is submitted to carbon analysis. The thickness of the article (T_a) as prepared was measured and the thickness of the coating (T_c) determined after removal of the substrate. With this information and the determinations of coating carbon concentration (C_c) and bulk carbon concentration (C_b), as indicated above, the carbon concentration of the substrate (C_s) in all cases was calculated from the following relationship

T ^: τ ...7’

Examples 20 to 23 below illustrate the preparation of novel articles of the invention comprising an unstabilized ferrous metal substrate having a diffusion coating of an iron-chromium alloy wherein the carbon concentration in the coating is less than the carbon concentration of the substrate.

EXAMPLE 20

An iron container holding a bath composed to 1800 g. Ca and 72 g. of powdered chromium (—325 mesh) was heated to. 1140 ° C. Type 1008 Al-killed steel was.placed in the bath for a treating time of 9 minutes. The coated coupon was then removed from the bath and rapidly quenched from a temperature of approximately 1000° C. A coating 1.0 mil was formed on the base steel analyzing 38% Cr at the surface of the coating. The coating and bulk carbon concentration were determined by analysis

3,184, 13 to be 93 and 401 p.p.m., respectively. The carbon concentration of the substrate was calculated to be 407 p.p.m.

EXAMPLE 21

An iron container holding a bath composed of 2300 g. ^aCa and 115 g. of powdered chromium (—325 mesh) was heated to 1140° C. The bath was agitated by a mechanical stirrer. A coupon 20 mils thick of SAE type 1070 steel was placed in the bath for a treating time of 5 minutes. The coated coupon was then removed from ¹⁰the bath and rapidly quenched from a temperature of approximately 1000° C. A coating 0.65 mil was formed on the base steel. The coating and bulk carbon concentrations were determined by analysis to be 240 and 1860 p.p.m., respectively. The carbon concentration of the ¹⁵substrate was calculated to be 1973 p.p.m.

EXAMPLE 22

An iron container having a bath composed of 2000 g. 20 of Ca and 100 g. of powdered chromium (—100 mesh) was heated to 1100° C. The bath was agitated by a mechanical stirrer. A coupon 60 mils thick of SAE type 1008 rimmed steel was placed in the bath for a treatment time of 45 minutes. The coated coupon was then re- 25 moved from the bath and rapidly quenched from a temperature of approximately 1000° C. A coating of 1.7 mils was formed on the base metal analyzing 43% Cr at the surface of the coating. The coating and bulk carbon concentrations were determined by analysis to be 110 and 30 336 p.p.m., respectively. The substrate carbon concentration was calculated to contain 350 p.p.m.

EXAMPLE 23

A variety of other samples were coated in a similar manner after which the carbon concentration of the chromium diffusion coating and the chromium diffusion coating plus the substrate were analyzed. From these values, the carbon concentration of the substrate was cal- ₄₀culated. The results are reported in Table II below and show that while there may be a wide variation in substrate carbon concentration, the coating carbon concentration of these samples is invariably less than the substrate carbon concentration. 45

Table 11

331 able to rapidly quench the article recovered from the coating bath. To obtain optimum benefits the coating process will be operated above 1000° C. so that the article can be quenched from at least above 900° C. Therefore, “rapidly quenched” has reference to a quick transfer of the article, for example, in the order of from 3 to 40 seconds, from the molten bath to a cooling medium which will quickly dissipate heat. Oil provides a suitable cooling medium. While most quenching oils are satisfactory in this respect, it is desirable to test any particular oil to insure that it does not carbonize in contact with the hot metal surface and thereby introduce additional carbon into the coating. Immersion of the article in a fluidized bed or in a high velocity stream of gas, such as helium, has also proven successful for rapidly dissipating heat after coating. Water is also satisfactory as a cooling medium provided that rigorous precautions are taken to avoid the ignition of hydrogen liberated when water contacts calcium.

To illustrate the novel articles of the invention involving an unstabilized ferrous metal substrate containing an amount of carbon in excess of 0.01% by weight having a diffusion coating of a ferritic iron-chromium alloy wherein the surface of said coating is free of iron chromium carbides, the samples prepared in Examples 20-23 were examined by X-ray diffraction techniques for the presence of iron-chromium carbides on the surface of the coating and the results compared to a similar examination conducted on chromium diffusion coatings formed on similar unstabilized steels by prior art chromium diffusing techniques.

The application of X-ray diffraction is fully described in various textbooks such as “X-ray Metallography,” by A. Taylor, John Wiley &amp; Sons, Inc., 1961. In a typical example of applying this technique to determine the presence of iron-chromium carbides, a piece of 1008 Alkilled steel 20 mils thick and chromized in a prior art pack system containing CrCl₃, A1₂O₃, and Cr powder was placed in a North American Philip’s High Angle Spectrometer Goniometer. The counting circuit was set at a threshold voltage of 1.5 volts and run at a scaling factor of 200 times. After scanning the coating surface, the relative intensities of the diffracted beam at the given angles of diffraction were measured. From the diffraction

Run No.

Steel type

Steel thickness (mils)

1____________

2____________

3_____________

4_____________

7_____________

8_____________

9_____________

1070 1008

1070

1008

1008

1070

1008 1008 1008

Coating thickness (mils)

<td rowspan="2"> Percent Cr at surface of coating</td><td colspan="4"> Carbon concentration</td>

<td> Bulk</td><td> Substrate</td><td> Coating</td><td> Ratio Cc/Cg</td>

<td></td><td> 1,860</td><td rowspan="2"> 1,973 368</td><td> 240</td><td> 0.12</td>

<td> 38.2</td><td> '363</td><td> 67</td><td> 0.18</td>

<td> 43.2</td><td> 820</td><td> 850</td><td> 152</td><td> 0.18</td>

<td> 37.0</td><td> 401</td><td> 407</td><td> 93</td><td> 0. 23</td>

<td> 43.0</td><td> 336</td><td> 350</td><td> 110</td><td> 0.31</td>

<td></td><td> 634</td><td> 676</td><td> 248</td><td> 0. 37</td>

<td> 39.5</td><td> 368</td><td> 394</td><td> 182</td><td> 0. 46</td>

<td> 34.0</td><td rowspan="2"> 125 419</td><td rowspan="2"> 133 424</td><td rowspan="2"> 65 298</td><td rowspan="2"> 0. 49 0. 70</td>

<td></td>

It is obvious from the runs of the foregoing Examples 20 to 23 that the novel articles of the invention may be readily prepared by the process hereof when calcium is employed as the transfer agent. In view of the desire to retain sufficient carbon to confer useful strength to the substrate and in light of the strong decarburizing influence of the calcium baths, it is to be appreciated that excessive coating times should be avoided. The amount of carbon retained in the substrate as affected by its initial carbon concentration, its thickness, and the thickness of coating can be appreciated from the foregoing table. A simple trial with any selected type and thickness of base metal will indicate the limiting coating time and temperature for retaining any particular carbon level in the substrate and maintaining the carbon concentration of the coating below that of the substrate. It is prefer ⁶⁰ angle and Bragg’s equation, the interplanar (d) spacings were calculated. By referring to the ASTM Index of X-Ray Powder Data File (1962) all diffiracton peaks from the surface of the specimen could be readily indexed. In this instance, the major component was θδ identified as the iron-chromium carbide (Cr, Fe)₂₃C₆while the minor component was identified as the body centered cubic ferrite matrix.

It will be apparent to one skilled in conducting X-ray diffraction analysis that one single operating procedure 76 cannot be set forth in full detail which will be generally applicable for analysis of all samples and, therefore, in order to obtain a high order of sensitivity in determining whether or not iron-chromium carbides are present at the surface of chromium diffusion coatings on unstabilized 75 steels, the particular details of operation of the technique

3,184,331 cedure for application of this test to the analysis of ferritic chromium diffusion coatings for purposes of determining the preferred articles of the invention involves the use of anodic current densities in the range of from 0.5 to 1.0 amp./cm.² for 30 to 60 seconds. The etched surface after thorough washing is viewed under a metallographic microscope at magnifications of χ250ίο X500. As detailed in the ASTM test description, the type of microstructure in the field of view is carefully observed. For purposes of the characterization made herein and in the claims, the coating is considered to be free of intergranular precipitates of iron-chromium carbides if no grain within the field of view is found to be completely surrounded · by a “ditched” grain boundary. Coatings having a “ditched” grained boundary are found to exhibit significantly inferior corrosion resistance compared to the other types of microstructures observed in the application of the test.

These preferred novel articles of the invention wherein the control of microstructure in the chromium diffusion coating is achieved to avoid intergranular precipitates of carbides can be prepared by the process of the invention employing calcium as the transfer agent when said process is conducted in conjunction with the subsequent step of rapid quenching while exercising careful control to limit the time interval between removal of the article from the molten bath and quenching. It will be obvious that if the articles removed from the bath are allowed to remain for any substantial period of time at the highly elevated temperatures imparted thereto by treatment in the molten bath, redistribution of carbon in the article may occur which can result in intergranular precipitates of carbides in the coating as well as the; carbon concentration in the coating rising above the carbon concentration in the underlying, substrate;_:

While the coated article should generally be immersed in the quenching medium within a time of about 3 to 40 seconds after it is removed from the coating bath, it will be within the skill of one versed in the art of heat treating •10 to determine the optimum condition using the microstructural characterization and the carbon analysis described herein. The thickness of the article as well as the effective thermal conductivity of the transfer environment and the quenching medium will determine in any 45 particular case the limiting cooling condition to avoid intergranular precipitation and to insure that the carbon concentration of the coating is maintained below that of the underlying substrate.

The further improvement in quality of the coatings of 50 t!ie novel articles of the invention available when this added characterization of freedom of intergranular precipitates of carbides in the coating is met is illustrated in the following example.

- EXAMPLE 24 ,

A series of five samples, each comprising an unstabilized ferrous metal substrate having a diffusion coating of an iron-chromium alloy, were prepared by the process of the invention in a manner similar to the samples de60 scribed in; Examples 20-22. After removal from the molten bath at 1140° C., the samples were held for varying times as indicated at temperatures above 500° C. The samples were then characterized in several ways. The microstructural oxalic acid etch characteristic was determined for each sample to determine whether or not it was free from intergranular precipitates of carbides.

Tn addition, each of the samples was evaluated for corrosion resistance, in the .Copper Acetic Acid Salt Spray (CASS) Test.; This test was, run in accordance with the procedure and apparatus published November 14, 1960, by the Chemical and Metallurgical Dept., Quality Control Office of the Ford Motor Company, identified as Quality Laboratory; and Chemical Engineering and Physical Test Methods, BQ5-1. The description of the procedure and apparatus for this test is quite lengthy and

IS must be determined in the light of the operating characteristics of the instrument used.

Using this analytical technique in this' manner, a very clear distinction can be noted between prior art articles and articles of the invention. Articles prepared by prior 5 art chromizing techniques wherein the carbon concentration in the unstabilized substrate exceeds about 0.01% by weight invariably exhibit significant amounts of an iron-chromium carbide, at the surface of the chromium diffusion coating while, by contrast, the articles of the 10 invention exhibit no iron-chromium carbides at the surface of the coating.

Examination of the samples prepared in Examples 2023 by X-ray diffraction within the bounds of the procedure described above to attain a high order of sensi-, 15 tivity produced diffraction peaks which were all indexed as belonging to the body centered cubic ferrite matrix. No peaks corresponding to an iron-chromium carbide were observed.

Examination of the corresponding prior, art samples in . 20 every instance showed diffraction peaks which were identified as an iron-chromium carbide. Of these, samples prepared by coating steeis of lower initial carbon content showed relatively weaker patterns of iron-chromium carbides but in all instances and in all other examinations 25 conducted on prior art articles in which the carbon content in the unstabilized substrate was greater than 0.01% by weight, positive identification of an iron-chromium carbide at the surface of the coating was made.

Articles of the invention wherein no iron-chromium 30 carbides are present at the surface of the chromium diffusion coating appear to be generally produced from the process of the invention when calcium is employed as the transfer agent over the range of conditions, set forth hereinabove as being suitable. 35

It has been found that an even further improvement in the quality of the novel articles of the invention, particularly with respect to corrosion resistance, can be obtained by control of the miscrostructure in the chromium diffusion coating whereby said coating is characterized in being free of intergranular precipitates of iron-chromium carbides. . '

To determine if the chromium diffusion coating of a particular sample meets this microstructural characterization, an established testing procedure for determining the degree, of intergranular precipitation of carbide phases in stainless steels is applied to the sample prepared for testing in a particular manner.

The essential details of the testing procedure are described in ASTM Standards No. 3, 1958, pages 292-298, under the title “Electrolytic. Oxalic Acid Etching Test.” This test has heretofore been used to distinguish, between “stepped” and “ditched” grain boundaries as they affect corrosion resistance of austenitic: stainless steel. It has now been determined that correlation can be made between corrosion resistance_: and coating microstructure of a ferritic chromium diffusion coating if this test is applied to such a coating between the 18 to 25% by weight chromium concentration levels within the coating. This appears to be necessitated by the peculiar etch character of the high chromium region of the coatings in oxalic acid and by the presence of surface iron-chromium carbides on chromium diffusion Coatings of the prior art.

The sample preparation procedure before, the oxalic acid etching test is conducted, therefore, consists of elec-; 65 tropolishing the coating to remove so much of the coating as is necessary to expose a layer containing a chromium concentration from 18 to 25%' by weight as determined . by X-ray fluorescence.

In running the testing procedure, the samples serving 70 as an anode are clectrolytically etched in 10% by weight ' oxalic acid solution contained within a stainless steel.: beaker acting as a cathode.. While.the ASTM procedure . for austenitic stainless steel calls for anodic current; den- ; ,, sities of 1 amp./cm.² for 90 seconds, the preferred pro- 75

3,184,331 will not be repeated herein in view of the reference provided. In this test, the sample is subjected to an acetic acid salt spray solution to which small amounts of copper chloride are added to promote corrosion. The test is now in broad use throughout the portion of the chromium plating industry concerned with out-of-door durability being regarded as an excellent accelerated corrosive test which simulates the corrosion behavior and durability of chromium plated steel and zinc alloy parts in out-ofdoor service. The results reported in Table III below represent the number of rust spots observed after 112 hours of exposure to the test on each sample having 10 in.² of coating surface area.

Table 111 alloy were prepared by means of a calcium bath containing chromium. The chromium concentration at the surface of the coating varied from sample to sample so as to provide a wide chromium range for investigation.

These samples were subjected to the accelerated pit test as described in J. Electrochem. Soc. 103, 375 (1956). In this test an anodic current density of 3 milliamps per cm.² is impressed on a sample for 5 minutes when immersed in a 0.1 molar NaCl solution. After this treat¹⁰ ment, the samples are washed and the number of pits counted. The table below tabulates the number of pits on samples having 25 cm.² surface area.

<td rowspan="2"> Run No.</td><td rowspan="2"> Hold time (sec.)</td><td rowspan="2"> Steel thickness (mils)</td><td colspan="2"> Coating</td><td colspan="3"> Carbon (p.p.m.)</td><td rowspan="2"> Cc/C,</td><td rowspan="2"> Etch character*</td><td rowspan="2"> CASS test (112 hrs.)</td>

<td> Thickness (mils)</td><td> Percent Cr</td><td> Bulk</td><td> Coating</td><td> Substrate</td>

<td> 1______</td><td> 600</td><td> 20</td><td> 1.2</td><td> 36</td><td> 72</td><td> 404</td><td> 26</td><td> 15.5</td><td> D</td><td> &gt;100</td>

<td> 2_____</td><td> 60</td><td> 90</td><td> 1.1</td><td> 38</td><td> 329</td><td> 399</td><td> 328</td><td> 1.2</td><td> D</td><td> 30</td>

<td> 3_______</td><td> 12</td><td> 90</td><td> 1.3</td><td> 34</td><td> 295</td><td> 161</td><td> 299</td><td> 0. 54</td><td> D</td><td> 11</td>

<td> 4_______</td><td> 4</td><td> 90</td><td> 1.3</td><td> 39</td><td> 269</td><td> 130</td><td> 273</td><td> 0. 48</td><td> S</td><td> 0</td>

<td> 5_______</td><td> 3</td><td> 90</td><td> 1.1</td><td> 36</td><td> 342</td><td> 132</td><td> 348</td><td> 0.38</td><td> S</td><td> 0</td>

*D—Ditched (having intergranular precipitates). S—Stepped (having no intergranular precipitates).

It is to be noted in the tabulated data that samples having both a C_c/C_s ratio greater than one and a “ditched” microstructure, show very inferior corrosion performance. An improvement is noted when the C_c/C_s ratio becomes very close to or less than one although the microstructure remains “ditched.” However, when the C_c/C_sratio is less than one and the microstructure is of the “stepped” type, the corrosion performance of the samples is outstanding after extended exposure to the accelerated CASS corrosion test.

It will be obvious from the foregoing that one of the outstanding advantages offered by the novel articles of the invention is that relatively inexpensive base metal may be provided the surface characteristics of a superior ferritic chromium steel with the use of only a very thin surface coating. A ferritic chromium coating of any finite thickness may be useful in improving the corrosion resistance of a base metal of mild steel, for example. Usually, however, the ferritic iron-chromium alloy coating will be of a thickness approximately 0.5 mil or greater.

The term “ferritic iron-chromium alloy” used to describe the diffusion coating of the novel articles of the invention is, of course, intended to include other alloying elements in addition to chromium and iron so long as the structure of the alloy coating remains essentially ferritic. For example, it will be clear from the foregoing description of the process that nickel or nickel in combination with other elements may be incorporated in the iron-chromium alloy coating formed.

Although articles of the invention may be formed having a wide range of chromium concentration, it is desirable that the chromium concentration at the surface of the coating be at least 12% by weight so as to impart a stainless quality to the surface. It is particularly preferred that the novel articles of the invention have a concentration of chromium at the surface of the diffusion coating in excess of 28% by weight. It has been found that articles of the invention in which the chromium concentration at the surface of the diffusion coating is in excess of 28% by weight are remarkably resistant to a well-known insidious type of corrosion, namely, “pitting corrosion” as illustrated by the following example.

EXAMPLE 25

A series of samples comprising an unstabilized steel having a diffusion coating of a ferritic iron-chromium

Table IV

Percent Cr at surface of coating: Number of pits

13.1 &gt;200

17.3 &gt;200

17.5 &gt;200

17.9 &gt;200

18.0 &gt;200

23.1 130

24.6 50

25.5 --------------------------------i 14, 37

25.9 20

26.5 13

27.1 17

28.0 -------------------------------- None

29.0 -------------------------------- None

29.3 -------------------------------- None

29.5 -------------------------------- None

30.3 -------------------------------- None

31.0 ------------------------------- None

32.1 -------------------------------- None

33.6 -------------------------------- None

34.1 -------------------------------- None

35.7 -------------------------------- None

37.0 -------------------------------- None

38.8 -------------------------------- None

41.7 -------------------------------- None ¹ 2 samples.

It is quite apparent from Table IV above that “pitting corrosion” is significantly reduced when the surface chromium concentration exceeds 24% by weight and essentially eliminated when the surface chromium concentration exceeds 28% by weight. The maximum chromium concentration at the surface of the coating for articles of the invention may well exceed the 41.7% shown above. Usually, however, there is little advantage ⁰ in exceeding 60% by weight chrominum which is approximately the maximum attaintable surface concentration by the process of the invention.

It is also to be appreciated that although outstanding corrosion resistance has been shown for the novel articles '° hereof even further improved corrosion resistance can be obtained by well-known post treatment techniques for this purpose. For example, corrosion resistance can be markedly improved by passivating the article after coating in 50% nitric acid or 20% nitric acid-2% sodium ⁷⁵ dichromate solutions.

3,184,331

In like manner, many well-known treatments can be employed to improve the surface appearance of the coated article, if desired. For example, an improved surface finish can be obtained by cold-rolling the base metal to a mirror-finish before coating, or alternatively, the coated article may be cold-rolled after being formed to improve the surface appearance of the article.

Owing to the outstanding corrosion resistance and formability properties of the iron-chromium alloy coating present thereon, the novel articles of the invention find utility in a wide variety of shaped forms and applications. The chromium diffusion coating may be formed on preshaped ferrous articles. In such manner, the articles of the invention can thus be made in the following shaped forms: automobile bumpers; automotive bright hardware, such as, brake handles, door hardware, radio antennae, roof racks, windshield wiper arms, dash board metal work, marine hardware; machinery, such as, business machine hardware, gears, spray nozzles, valves, pumps, cams, conveyor parts, wire cables, springs, nuts, 20 bolts, and screws; appliances, such as, irons, washing machine tubs, stationary tubs, and dish racks for dish washers; sporting equipment, such as, golf club heads, ice skates, fishing reel gears; and various consumer articles, such as, cutlery, screening, spades, flash light 25 cases, etc.

Alternatively, the chromium diffusion coating can be readily formed on a flat rolled sheet of formable iron or steel after which the coated sheet may be formed into shaped articles. In this manner, articles of the inven- 30 tion can be made into the following, forms: automobile bumpers, grilles, moldings, hub caps, wheel covers, mufflers, tail pipes; appliance trim, such as on refrigerators, ranges, toasters, coffee pots, etc.; kitchen cabinets; water heaters, water softeners, water cooler tops; shower 35 stalls; bath tubs; lavatories; sinks, splash boards, stove mats; gutters, downspouts; wall panels and tile; architectural moldings; door frames, window frames; cafeteria counters, counter trim, hoods, cabinets; food processing equipment; mail boxes, cooking utensils; camp ⁴⁰equipment; hospital equipment; drums and barrels; and industrial articles, such as -heat-exchanger plates, tubing and piping, structural steel work, and processing equipment.

By dissolving away the substrate metal from the novel _4garticles of the invention in the manner previously described, the original ferritic iron-chromium alloy diffusion coating can be obtained as a very thin unsupported film which per se has very unique properties. In this manner, films of ferritic iron-chromium alloy containing at least ₅₀12% chromium are formed having thicknesses in the Order of about 0.5 mil to 3.0 mils, or greater, if desired, which have , a concentration gradient of chromium throughout the film. Such films derived from the hovel articles of the invention are unique over films derived from any prior 55 art chromium diffusion coated ferrous article in that the surface of the film having the highest chromium concentration would be free of iron-chromium carbides. Furthermore, such films derived from the most preferred novel articles of the invention, when corrosion resistance is go of paramount importance, would be free from intergranular precipitates of iron-chromium carbides.

While other modifications of this invention, which may be employed within the scope of the invention have not been described, the invention is intended to include all 65 such as may be comprised within the following claims.