US3834880A

US3834880A - Chromium alloy coated ferrous metal products

Info

Publication number: US3834880A
Application number: US00203566A
Authority: US
Inventors: C Vessey
Original assignee: Individual
Current assignee: Individual
Priority date: 1967-02-27
Filing date: 1971-12-01
Publication date: 1974-09-10
Anticipated expiration: 1991-09-10

Abstract

A FERROUS METAL SUBSTRATE HAVING COMPACTED ON TO THE SURFACE THEREOF PARTICLES OF AN IRON-CHROMIUM ALLOY CONTAINING A TOTAL OF FROM 2 TO 20% BY WEIGHT.BASED ON THE TOTAL WEIGHT OF THE ALLOY OF ALUMINUM AND/OR SILICON IS PARTICULARLY SUITABLE FOR BEING SUBJECTED TO A HEAT TREATMENT TO PRODUCE A CHROMISED PRODUCT. CHROMISED PRODUCTS OBTAINED FROM THE SUBSTRATE ACCORDING TO THE INVENTION ARE SUITABLE FOR REDUCTION BY COLD ROLLING.

Description

Sept. 10, 1914 A. VESSEY 3,834,830

CHROIIUM ALLOY COATED FERROUS METAL PRODUCTS Filed Dec. 1. 1971 United States Patent O M 3,834,880 CHROMIUM ALLOY COATED FERROUS METAL PRODUCTS Clifford Alfred Vessey, Harrogate, England Continuation-impart of abandoned applications Ser. No. 707,292, Feb. 21, 1968, Ser. No. 788,039, Dec. 30, 1968, and Ser. No. 797,269, Feb. 6, 1969. This application Dec. 1, 1971, Ser. No. 203,566 Claims priority, application Great Britain, Feb. 27, 1967, 9,239/ 67; Apr. 25, 1967, 19,024/67; July 5, 1967, 31,043/67; Jan. 12, 1968, 1,938/68; Feb. 9, 1968, 6,646/68; May 20, 1968, 24,029/ 68 Int. Cl. B32b 15/00 US. Cl. 29-1912 16 Claims ABSTRACT OF THE DISCLOSURE A ferrous metal substrate having compacted on to the surface thereof particles of an iron-chromium alloy containing a total of from 2 to 20% by weight based on the total weight of the alloy of aluminum and/or silicon is particularly suitable for being subjected to a heat treatment to produce a chromised product. Chromised products obtained from the substrate according to the invention are suitable for reduction by cold rolling.

BACKGROUND OF THE INVENTION The present application is a continuation-in-part application of abandoned applications Ser. No. 707,292 dated Feb. 21, 1968, application Ser. No. 788,039 dated Dec. 30, 1968 and application Ser. No. 797,269 dated Feb. 6, 1969.

It relates to a ferrous metal substrate having compacted on to at least one surface chromium containing particles. Such substrates may be converted to chromised articles by subjecting them to a heat treatment to bring about diffusion of chromium atoms present in the particles into the ferrous substrate thereby resulting in the formation of a layer of chrome steel or stainless steel on the substrate. This process is known as chromising.

In US. Pat. No. 3,594,135 there is described and claimed a product of the type with. which the present invention is concerned.

The products are obtained by applying a layer of chromium metal-containing powder, such as pure chromium metal or an iron-chromium alloy and then applying pressure to compact the powder into the substrate.

The overall process in which a chromium-containing powder is compacted on to a ferrous substrate and is then subjected to a diffusion heat treatment will hereinafter be termed as a compaction chromising process.

Such processes are described in US. Pat. Nos. 3,585,068, 3,589,927, 3,312,546 and 3,340,054. In a'l of these the material employed as the chromium-containing powder is either pure chromium or is an iron-chromium alloy, which may contain small amounts of impurities common in such alloys. For example US. Pat. No. 3,340,054 describes the use of an alloy of the composition 71.3% chromium, 0.35% manganese, 1.42% silicon, 0.01% carbon with a balance of iron.

Whilst the use of chromium metal for this purpose is satisfactory it is an expensive material. The use of conventional iron-chromium alloys as described above, whilst suitable for use under certain operating conditions on some ferrous substrates, tend to be more difiicult to cause to compact in the substrate than pure chromium. This can result in operating difficulties in carrying out the chromising process particularly on hard substrates since it is essental that the application of pressure in the compaction forms an adherent porous layer on the substrate. In this layer there exists mechanical bonds at least Patented Sept. 10, 1974 between some of the particles with each other and the substrate. The degree of adhesion required is such that the product can be handled in subsequent operations, for example in being coiled for the heat treatment. If the degree of adhesion is not sufliciently good then chromium-containing powder may become detached from the substrate and this will result in a rough finish to the coating formed on the substrate thus resulting in a poor appearance in the final product.

It is an object of the present invention to provide a ferrous substrate on at least one surface of which there is compacted a chromium containing alloy of improved degree of compactability over normal iron-chromium alloys.

It is a further object of the invention to provide a process for chromising ferrous metal substrates which comprises applying to at least one surface of the substrate a chromium-containing alloy having improved adhesive properties on compaction in comparison with conventional iron-chromium alloys, compacting such an alloy on to the substrate and subjecting the so coated substrate to a diffusion heat treatment.

A yet further object of the invention is to provide a chromised ferrous metal substrate which may be cold rolled.

Accordingly, the present invention provides a product capable of conversion by heat treatment to a chromised ferrous article which comprises a ferrous metal substrate having compacted thereon on at least one surface a layer of particles of an alloy comprising at least 60% by weight of chromium and a total of from 2 to 20% by weight of aluminium and/or silicon the remainder being substantially iron, at least some of the particles having mechanical bonds with each other or the substrate.

DETAILED DESCRIPTION OF THE INVENTION INCLUDING PREFERRED EMBODIMENTS Products having the iron-chromium alloy containing aluminium and/or silicon particles compacted on to at least one of their surfaces are often in the form of steel strips or plates.

The steels preferably employed are conventionally available mild carbon steels such as rimming steel or capped steel not containing a carbide stabiliser. However, forms of ferrous substrates may be employed if desired. Normally the substrate employed will be a piece of strip which has been both hot and cold rolled down to a thickness which is roughly the same as that which is desired for the chromised product. However, it may be desired subsequently to cold roll the final chromised product, for example to improve its surface properties, or to effect a substantial reduction of the thickness of the strip, for example of up to an elongation. If this is the case then it may be possible to employ strip or sheet which has only been hot rolled, for example to a thickness of up to 0.25", although more usually it will be in the range of 0.03 to 0.18 inches. In certain circumstances this may be a preferred method of carrying out the chromising process.

The particles of alloy compacted on to at least one surface of the product have a chromium composition of at least 60% and a total aluminium and/or silicon content of from 2 to 20% by Weight, the remainder is substantially iron, although a small amount of other additive elements as hereinafter described may also be present if desired. In such cases the total amount of such an additive element and aluminium and/or silicon will not exceed 20% by weight of the alloy. Inevitably small quantities of impurities are normally present in ferrochromes and these may also be present in the alloys employed in the product of the present invention. Frequently the chromium content of the alloy will be greater than the lower limit of 60%. For example, it is often in the range 71 to 85%. Indeed I have found that when the chromium content of the alloy rises above 74% this in itself has some effect in increasing the degree of adhesion possible on compaction of the alloy particles into the substrate. Thus the use of such alloys may sometimes be preferred, particularly those having a chromium content in excess of 8l% by weight, e.g. 81% to 85% by weight. Preferably alloys for present use have a total aluminium and/or silicon content of at least 3% by weight. Often the total amount of these elements present will be less than 10% by weight of the whole alloy composition. Thus the alloy employed commonly has a total aluminium'and/or silicon content in the range 3l0% by weight. The alloy normally has a carbon content of less than 0.1% by weight often it is in the range of 0.02 to 0.1% by weight. If desired other addi five elements such as molybdenum, niobium, vanadium or titanium may also be present. The particles of the alloy are normally of such a size that they will pass through a British Standard sieve of mesh 100. Preferably the part'i-' cles are such that they will pass through a sieve of British Standard mesh 200. I

time in a given furnace. If this arrangement of the substrate for the heat treatment is employed, it is necessary to introduce the halide to the environment in some other method than as a gas. This can be accomplished by applying the'halide to the substrate prior to arranging it as a close coil or stack or other arrangement precluding good contact with the furnace atmosphere. A process of this type is described in U.S.'Pat. No. 3,585,068.

In that specification, in which the particles stated to be compacted on to the substrate were either pure chromium or a straight ironchromium alloy (the use of latter subsequently being found to cause the difficulties previously referred to). the halides stated to be suitable were ferrous, ferric, cobalt, nickel and manganous halides usually in hydrated form. Whilst such halides may be employed when the particles of the chromium-containing material compacted on the substrate are of the alloy previously de- The amount of alloy which is compacted on the sur'- face depends upon the thickness of the desired chromised coating. will also'depend upon the number of surfaces on which the alloy is compacted. For example ifthe sub-,

strate employed is steel strip, that so long as this is arranged for the heat treatment in a suitable form, for example in a coil, it may only be necessary to have the alloy particles compacted in one surface of the strip in order eventually to produce a chromised coating on both surfaces. However, in the case where only one surface does have the alloy coating compacted on it it will normally be desirable to apply'approximately twice the amount of alloy per square foot to that surface as wouldbe required if both surfaces were so covered. Normally the alloy will be present on the surface in an amount to provide at least 2 g. of chromium per square foot of surface. Frequently it will be in the range 5 to 20 grams of chromium per square foot of surface preferably it is applied in an amount of about 11 g. per square foot of surface if the coating is applied to all effective surfaces of the substrate.

The alloy particles are normally applied to the substrate by first wetting it with a suitable material and then applying the chromium-containing alloy powder to it, for example using an electrostatic precipitation in order to obtain an even coating. The powder present on the substrate is then caused to adhere to it by the application of pressure. For example it may be passed through rollers set to give a linear pressure of several tons, commonly in the range 7 to 10 tons, per linear inch.

In order to convert this product into a chromised prodnot having a relatively non-porous diffusion coating, as opposed to the substrate simply having chromium-containing alloy particles compacted on it, it is necessary to subject the substrate with its alloy particles compacted on it to a heat treatment to bring about the formation of a chromium diffusion layer on the surface of the substrate. This heat treatment will be carried out in the presence of a halide. If the substrate is subjected to the heat treatment in such a manner that its surfaces are open to the atmosphere of the furnace in which the heat treatment is carried out it will be possible simply to introduce the halide as a gas into the furnace. For example this is described in US. Pats. Nos. 3,589,927, 3,312,546 and 3,340,054 which describe different processes for carrying out the chromising of steel strip by carrying out the heat treatment stage on strip in the form of an open coil. In such a process the halide employed is normally a hydrogen halide, commonly hydrogen chloride.

However, it is often more convenient to be able to place adjacent surfaces of the substrate in contact during the heat treatment, for example in the form of a close coil or stack since this reduces labour costs and also permits a greater amount of substrate to be heat treated at any one scribed, we have found that there is a tendency for them to result in formation of a final diffusion coating in which part'of theicouting is continuous but is surmounted by a region which is porous. For some applications this may be tolerated. However, for many applications it may not.

We believed that such porosity is a result of the aluminium and/or silicon readily forming oxides at'temperatures below the temperature used to bring about chromising which are not reducible at the temperatures used for chromising. This may result inthe formation of an'inert oxide film in the surface layer and so cause porosity.

We'have now found that we may obtain satisfactory non-porous chromium diffusion coatings on ferrous'metal substrates by a compaction chromising process when aluminium and/0r silicon are present in the chromiumcontaining layer, if we employ as the halide one which is anhydrous and will react with the additive element during the heat treatment stage to form its halide. Such halides will hereinafter be referred to as anhydrous reactive halides.

While the scope of our invention is in no way limited by any particulartheory we believe that anhydrous reactive halides are successful in the presence of aluminium and/or silicon whilst hydrated ones are less so because water of crystallisation of the hydrated compounds becomes released during the heat treatment and reacts with the aluminium or silicon to form an oxide or oxyhalide.

We have also surprisingly found that certain anhydrous halides which would not be expected to react particularly well with iron to form ferrous halides as is required for the process described in US. Pat. No. 3,585,068 are especially suited for use in the present invention. Among these are the alkali metal halides. It may be, although again we do not intend to limit ourselves to this theory, that the halide of the aluminium or silicon acts as an intermediate and reacts in its turn with iron to form ferrous halide.

From a practical point of view the anhydrous reactive halides for use in the present process fall into two classes, namely those which must be applied to the ferrous metal substrate, bearing a chromium-containing layer, in a dry form; suitably a powder and those which may be applied either as a solution or in dry form. The latter class consists of water soluble halides which form anhydrous halide coating on the substrate when dried.

Anhydrous reactive halides which cannot be employed in a solution may be applied to the ferrous metal substrate onto which a chromium-containing layer has been compacted in cases where the subsequent treatment of the substrate is such that the powder does not become detached during handling. Such halides include anhydrous aluminium, manganous, nickel and cobalt fluoride and anhydrous manganous, nickel and cobalt chlorides, but are preferably anhydrous aluminium fluoride and anhydrous manganous chloride.

Halides which do not have to be employed in dry form are normally applied to the substrate by dipping this into a solution of the halide and then drying it, for example,

by an infra-red drier. Halides having no hydrated form which are particularly suited to application in this manner are the alkali metal halides.

For simplicity of application and storage it is normally preferred to employ halides which do not need to be used in dry form.

Anhydrous reactive halides for use in the present invention are normally applied direct to the substrate as powders or solutions since this permits the substrate to be treated to be arranged for the heat treatment stage having adjacent surfaces in contact, for example, in the case of steel strip as a closed coil. It is, however, possible, if the substrate is to be heated in an arrangement wherein there is spacing between adjacent surfaces, to introduce a halide which is volatile at a temperature reached during the heat treatment directly to the furnace. For example aluminium trichloride may be introduced directly into the furnace rather than be applied to the substrate.

If desired mixtures of two or more anhydrous reactive halides, whether falling in the same class or not may be employed. We have also found that we may overcome problems associated with the formation of intermetallic compounds in the surface layer by employing fluorides as the anhydrous reactive halides. These problems are particularly pronounced in the case when aluminium is included in the surface layer, since this forms especially stable intermetallic compounds in the form of chromium aluminides. Fluorides being more reactive than chlorides are better able to break down intermetallic compounds and so release the chromium bound in these compounds for the diffusion process. We have found that fluorides which are of use in this way are those listed above as being of use to prevent the formation of oxides in the coating.

Suitable alkali metal halides are the fluorides, chlorides, bromides, and iodides of sodium and potassium. These may be employed either singly or in mixtures, such as mixtures of potassium fluoride and chloride and of sodium and potassium chlorides. We have found, however, that the incorporation of alkali metal halides, especially the bromides or iodides, particularly of potassium, is especially beneficial. We believe this to be because even in systems where there is no obvious source of oxygen it is possible for some residual oxygen to be retained in or on the surface of the substrate or the applied coating and to form areas of porosity in the final coating. The inclusion of an alkali metal (preferably potassium) bromide or iodide in the halide coating, alleviates this problem. The bromide or iodide partially dissociates at the operating temperature and the free alkali metal scavenges any residual oxygen, forming an oxide which is volatile and/or harmless. Alkali metal halides used in this type of chromising process as oxygen scavengers may be the only source of halide present or they may be present additionally to other halides normally used to promote chromising action.

Particularly useful alkali metal halides for use in the present invention are potassium bromide, sodium and potassium iodides especially preferred, however, is a mixture of potassium fluoride with a little potassium iodide.

Although, as indicated above, it is normally preferred for economic reasons to carry out the heat treatment required for processes of the invention in such manner that their adjacent surfaces are in contact, this being possible if the anhydrous halide coating is applied direct to the porous, adherent, chromium-containing surface layer, it is also possible to employ a technique wherein the substrate is heated in a form in which adjacent surfaces are not in contact, for example, as an open coil of steel strip.

When the substrate is arranged for the heat treatment stage in such manner that its surfaces are in contact the furnace will normally be purged by passing dry hydrogen, or other inert gas through it up to a temperature of about 250 C. to remove atmospheric oxygen and other harmful impurities at which temperature the purging will be stopped and the furnace then raised to the chromising temperature. If desired a compound which volatalises during the heat treatment to give a non-harmful product which may assist the purging can be applied to the substrate, but we have found that in general when employing anhydrous halides such purging aids are not required.

If the substrate is packed for the heat treatment in a spaced manner the furnace is heated with a purging stream of hydrogen gas passing through it to a temperature below that at which the halide of the additive oxygen-getting element is volatile and once this temperature is reached the heating to the chromising temperature is continued in a reducing or inert atmosphere such as a hydrogen or inert gas (e.g., argon) atmosphere which is not purged or recycled outside the furnace.

Whatever form of heat treatment is employed we have found that it is normally preferable to purge the furnace for a period of about 13 hours, normally heating it up to the purging temperature over a period of about 3 hours and holding it there for about 10 hours. The substrate will normally be held at the chromising temperature, that is a temperature in excess of 800 C. normally in the range 920-1000 C. for a period of about 16 hours.

Although the process of the invention has mainly been described in terms of chromising steel strip it is equally suited to other substrates onto which a porous adherent chromium-containing layer may be compacted. For example, coils of wire, sheets of steel, and steel pipes or tubes may be chromised in this manner.

The chromised coating formed on the substrate when the alloy particles initially caused to compact in the substrate contain aluminium and/or silicon tend to result in the formation of a chromium diffusion coating of increased thickness on the substrate over coatings obtained using pure chromium metal. As a result of this, it is, if desired, possible to effect a subsequent cold rolling of the chromised product without detracting from its corrosion resistant properties. Indeed it is possible to cold roll products obtained by the use of the aluminium and/or siliconcontaining alloy to effect reductions of 30% up to 80%, for example reductions in the range 45 to If such a substantial reduction is employed it will normally be necessary to anneal the product to render it workable. The temperature at which the annealing is carried out and its duration are determined by the use to which the chromised strip is to be put and the amount of reduction to which it has been subjected.

The following examples are given in order to illustrate the invention.

Example 1 A low carbon ferrochrome containing 62% chromium, 2 /2 silicon and 2 /2% aluminium, the remainder being substantially iron, was broken up and ground to a powder of particles size 200 ES. mesh.

This powder was applied to both surfaces of a piece of 20 gauge steel strip (0.2% carbon) which had been degreased in a solvent degreasing bath pickeled in 10% v./v. nitric acid for 10 seconds and washed with water, at a rate of 12 grams of chromium per square foot, and compacted by passing the strip between rolls. The coated steel strip was then passed through a solution of ferrous chloride tctrahydrate (4.2 parts) and ammonium chloride (0.97 parts) in water (6.5 parts) and dried bypassing the strip under an infra-red dryer. A length of the dried strip was wound at a tension of 800 lbs. on to a mandrel of 3 /2 inches external diameter and a free end clamped to retain the tension.

The coil was placed in a suitable furnace which was then purged with hydrogen for three hours at 250 C., after which time the temperature was raised to 400 C. for a period of 1 /2 hours. The flow of gas was continued in the furnace for 10 hours to ensure substantially complete removal of harmful products. The flow of gas was then stopped and the temperature raised to 900 C. over a period of hours and retained at this temperature for 16 hours.

After cooling the coil was removed from the furnace and washed in water to remove excess halide. Removal of a portion of the coating by filing and treatment with boiling 50% aqueous nitric acid to dissolve the steel core left a coating insoluble in nitric acid which was strong and self-supporting. Analysis of this coating showed 20.1% chromium. The thickness of the coating was in excess of 0.002 inches and revealed a diffusion alloy layer which contained some porosity but which had a nonporous layer adjacent to the steel substrate. A photomicrograph of the coating as shown by metallographic examination is shown in FIG. 1.

Example 2 Pieces of hot rolled steel (0.078 inches) having a carbon content of 0.06% carbon were degreased in an alkaline degreasing bath, pickled in hot inhibited sulphuric acid, pickled in v./v. nitric acid for 10 seconds then washed with water. An alloy consisting of 5% aluminium, 78% chromium, the balance iron was ground to 200 B5. mesh and applied to both surfaces of the steel at a rate of grams of chromium per square foot of surface. The alloy was then compacted into the surface of the steel by passing the strip between rolls. Anhydrous aluminum fluoride was then applied as a powder to one surface of the strips at a rate equivalent to 4 grams per square foot of surface. The strips complete with applied powder were then stacked together and bolted between thick mild steel plates to simulate a closed coil. The stack of plates were then placed in a suitable furnace which was then purged with 10% hydrogen in argon for 2 hours at 200 C. After this time hydrogen was introduced into the furnace and the temperature was raised to 400 C. for 10 hours to ensure removal of harmful contaminants. The flow of gas through the furnace was then ceased and the temperature of the furnace raised to 950 C. over a period of 6 hours and maintained at this temperature for 21 hours.

After cooling the strips were removed from the furnace and washed with water. The surfaces were silvery grey in colour and were resistant to corrosion by water, aqueous sodium chloride and aqueous nitric acid. Metallographic examination revealed the diffusion alloy layer to be free of pores. Removal of a portion of the coating by filing and dissolution of the steel core in 50% aqueous nitric acid revealed a coating 0.0030 ins. thick which was insoluble in nitric acid. Analysis of the coating after dissolution in hydrochloric acid showed an iron content of 74%. A similar experiment conducted in the way described, but using a solution of ferrous chloride tetrahy drate instead of anhydrous aluminium fluoride produced a coating which by metallographic examination was shown to have scattered pores and its thickness measured as above was 0.0019 inches.

Example 3 Pieces of hot rolled steel (0.078 inches) having a carbon content of 0.06% carbon were degreased in an alkaline degreasing bath, pickled in hot inhibited sulphuric acid, pickled in 10% v./v. nitric acid for 10 seconds then washed with water. An alloy consisting of 5% aluminium, 78% chromium, the balance iron was ground to 200 B5. mesh and applied to both surfaces of the steel at a rate of 15 grams of chromium per square foot of surface. The alloy was then compacted into the surface of the steel by passing the strip between rolls. The coated strips were then dipped into a solution of potassium fluoride (80 parts) in water (100 parts) and dried under an infra-red dryer to give a total pick-up of potassium fluoride of 4 grams per square foot of surface. The strips were then stacked together and bolted between thick mild steel plates to simulate a closed coil. The stack of plates were then placed in a suitable furnace which was then purged with 10% hydrogen in argon for 2 hours at 200 C. After this time hydrogen was introduced into the furnace and the temperature was raised to 400 C. for 10 hours to ensure removal of harmful contaminants. The flow of gas through the furnace was then ceased and the temperature of the furnace raised to 950 C. over a period of 6 hours and maintained at this temperature for 21 hours.

After cooling the strips were removed from the furnace and washed with water. The surfaces were silvery grey in colour and were resistant to corrosion by water, aqueous sodium chloride and aqueous nitric acid. Metallographic examination revealed the diflusion alloy to be free of pores. Removal of a portion of the coating by filing and dissolution of the steel core in 50% aqueous nitric acid revealed a coating 0.0030 inches thick was insoluble in nitric acid. Analysis of the coating after dissolution in hydrochloric acid showed an iron content of 74%. A similar experiment conducted in the Way described, but using a solution of ferrous chloride tetrahydrate instead of potassium fluoride produced a coating which by metallographic examination was shown to have scattered pores and its thickness measured as above was 0.0019 inches.

Example 4 Pieces of hot rolled steel 0.098 inches thick and containing 0.05% carbon were degreased in an alkaline degreasing bath, pickled in a hot inhibited sulphuric acid, pickled in 10% v./v. nitric acid for 10 seconds and washed with water. An alloy consisting of 4.8% silicon, 78.3% chromium and the balance iron was ground to 200 B5. mesh and applied to both surfaces of the steel at a rate of 15 grams of chromium per square foot of surface. The alloy was then compacted into the surface of the strip by passing the strip between rolls. The coated strips were then dipped into a solution of potassium iodide parts) in water (100 parts) and dried under an infra-red dryer to give a total pick-up of potassium iodide of 4 grams per square foot of surface. The strips of steel were stacked together bolted between thick mild steel plates to simulate a closed coil and placed in a suitable furnace which was purged with hydrogen for 2 hours at room temperature. The temperature was then raised to 400 C. over a period of 4 hours and maintained at this temperature for 10 hours. The gas flow through the furnace was then stopped and the temperature raised to 950 C. over a period of 6 hours and maintained at this temperature for 24 hours. Metallographic examination of the chromised steel produced by using potassium iodide showed complete non-porous diffusion alloy layers.

After cooling the strips were removed from the furnace and washed with water. The surfaces were silvery grey in colour and were resistant to corrosion by water, aqueous sodium chloride and aqueous nitric acid. Removal of a portion of the coating by filing and treatment with 50% aqueous nitric acid to dissolve the steel core revealed a coating of 0.0031 inches which was insoluble in nitric acid. Analysis of the coating after dissolution in hydrochloric acid showed an iron content of 74%.

What is claimed:

1. Ferrous metal work piece which comprises a ferrous metal substrate having compacted thereon on at least one surface a layer of particles of ferrochrome alloy consisting essentially of (a) between 60% and by weight of said ferrochrome alloy of chromium, (b) an additive metal selected from the group consisting of (i) aluminum and (ii) aluminum and silicon, said aluminum in said additive metal being at least 2% by weight of said ferrochrome alloy and is not in excess of 10% and the total of aluminum and silicon is not in excess of 10%, and (c) the balance essentially iron; at least some of said particles having mechanical bonds with each other or the substrate.

2. Ferrous metal work piece as claimed in claim 1 wherein the chromium content of the alloy is from 81% to 85% by weight.

3. Ferrous metal work piece as claimed in claim 1 wherein the alloy includes carbon in the range 0.02 to 0.1% by weight.

4. Ferrous metal work piece as claimed in claim 1 wherein chromium is present on at least one surface at least 2 grams of chromium per square foot.

5. Ferrous metal Work piece as claimed in claim 4 wherein chromium is present on at least one surface from 5 to 20 grams of chromium per square foot.

6. Ferrous metal work piece as claimed in claim 5 which is in the form of a steel sheet or strip.

7. Ferrous metal work piece as claimed in claim 6 which is in the form of a coil of steel strip.

8. Ferrous metal work piece as claimed in claim 7 which is in the form of a close coil.

9. Ferrous metal work piece as claimed in claim 8 wherein there is present on at least one surface of the strip a halide selected from the group consisting of alkali metal halides and anhydrous aluminium, manganous, cobalt and nickel fluorides and manganous, cobalt and nickel chlorides.

10. Ferrous metal work piece as claimed in claim 5 wherein said additive metal is in an amount of at least 3%.

11. Ferrous metal work piece as claimed in claim 10 which is 0.03 to 0.18 inches thick.

12. Ferrous metal work piece as claimed in claim 1 which is in the form of a steel sheet or strip.

13. Ferrous metal work piece as claimed in claim 1 wherein there is also present upon at least one surface an anhydrous halide.

References Cited UNITED STATES PATENTS 3,312,546 4/1967 Mayer et al. 29-1966 1,902,092 3/1933 Norwood 11722 3,589,927 6/1971 Holker et a1. 29-196.6 X 3,340,054 9/1967 Ward et al 29196.6 3,594,135 7/1971 Holker et al 29-1966 X 1,704,733 3/1929 Fahrenwald -176 1,853,369 4/1932 Marshall 11722 FOREIGN PATENTS 722,797 2/ 1955 Great Britain 117107.2

ALLEN B. CURTIS, Primary Examiner US. Cl. X.R.