US3411995A - Process and product for plating on cast,malleable,carburized and carbonitrided irons - Google Patents

Description

United States Patent PROCESS AND PRODUCT FOR PLATING ON CAST, MALLEABLE, CARBURIZED AND CARBONITRIDED IRONS Edward B. Saubestre, Hamden, and Theophil J.

Wieczorek, West Haven, Conn., assignors to Enthone, Incorporated, New Haven, Conn.

No Drawing. Filed Mar. 15, 1965, Ser. No. 439,987

6 Claims. (Cl. 20438) ABSTRACT OF THE DISCLOSURE Process for cyanide zinc electroplating ferrous metal surfaces having discrete carboniferous particles in their surfaces and which enables a sufficiently high hydrogen overvolatge to be maintained during the electroplating to allow the zinc electroplating to occur. The process involves immersing the ferrous metal surface in an aqueous acid solution comprising about 2 to 36 ounces per gallon of phosphate ion, 0.15 to 3 ounces per gallon of at least one wetting agent from the group of nonionic and cationic surfactants, and about 4 to 20 ounces per gallon of a miscible solvent from the group of compounds of the formulae:

wherein R is H, -CO-CH -PO(OH) or --SO (OH), R is H or H(CH n is 1-2 and m is 1-4, and optionally an organic thio compound as corrosion inhibitor, the aqueous acid solution having a pH of up to 3.0 inclusive and being at room temperature or an elevated temperature up to 180 F., for a time sufficient to form the ferrous surface a thin continuous phosphate-containing film. The thus-coated ferrous metal surface is then zinc electroplated in a cyanide zinc electroplating bath.

The present invention relates to an improved process for plating on ,cast, malleable, carburized and carbonitrided irons and to the resulting product.

The general term cast iron includes gray irons, white cast irons, chilled (white face) cast irons and malleable irons. Cast irons are alloys of iron, carbon and silicon in which more carbon is present than can be retained in solid solutions in austenite at the eutectic temperature. The carbon content of cast irons is generally 1.5 to 4.5 percent. Gray iron is the most widely used of the cast irons.

The term gray iron covers a series of eutectiferous alloys that offer a wide selection of mechanical properties, with the composition and processing so adjusted that the matrix structure is largely pearlite (a lamellar mixture of ferrite and cementite) with many graphitic flakes dispersed throughout. This presence of graphite flakes imparts the characteristic gray fracture of these alloys. As a sub-group, we may consider the austenitic cast irons, which contain sufficient amounts of alloying elements to lower the eutectoid transformation temperature to such an extent that austenite is retained as the matrix at room temperature, with graphite flakes dispersed throughout the structure. (Nickel is commonly used for this purpose.) 'In white cast iron, almost all of the carbon is in the combined form. The presence of ferrite and free cementite is accompanied by only very Patented Nov. 19, 1968 small amounts of graphitic matter. Malleable cast iron refers to White cast irons which have been heat treated so as to decompose most of the cementite into ferrite and free (or temper) carbon which is usually in the form of nodularized graphite particles.

Carburizing is the process of increasing the carbon content of the ferrous surface by exposing it at high temperature to an atmosphere of CO +CO with or without hydrocarbon gases so that when quenched, the surface portion thus carburized will be substantially harder than the underlying metal. A typical carburized steel may have 1.5 percent C in the surface layers. The combined proc esses of carburizing and hardening have long been known as case hardening. Carbonitriding is that specific example of carburizing in which ammonia and hydrocarbon gases are decomposed to provide simultaneous addition of carbon and nitrogen to the surface being case hardened. Another example of a similar operation is cyaniding, in which molten potassium cyanide is decomposed to achieve the same ends.

In all of the cases described above, the electroplater is faced with a similar problem: the surface of the metal to be plated contains free carbon, generally in the form of graphite. The production of a satisfactory plate on cast irons depends upon a high cathodic hydrogen overvoltage on the iron being plated. The graphite or free carbon on the surface presents two problems; it leads to undissolved smuts on the surface of the part to be plated in the conventional cleaning cycles frequently employed prior to plating and it lowers significantly the hydrogen overvoltage at the site of such graphitic inclusions. The latter effect can be quite serious when plating such objects with zinc from cyanide solutions. In such solutions, standard electrode potentials favor the cathodic reduction of hydrogen rather than zinc. The only reason that zinc can be plated at all from cyanide solutions is that on most substrates the hydrogen overvoltage is large, while the zinc overvoltage is negligible, thus permitting the cathodic reduction of zinc preferentially. As noted above, however, surfaces containing free carbon, especially in graphitic form, greatly lower hydrogen overvoltage so that zinc and other metals do not platedirectly on such surfaces from cyanide electrolytes.

Free carbon and graphite deposits occur to a large extent in ordinary untreated cast irons and in other cases are produced by heat treatment, such as case hardening to improve the strength and resistability of the product. In either case, discrete particles of carbon and/or graphite on the surface either occurring naturally or by the processing of the iron by carburizing or carbonitriding lower the hydrogen overvoltage values to such an extent that the evolution of hydrogen and deposition of a continuous plate on the iron surface is either severely affected or precluded completely.

ASTM Recommended Practice B320-60 summarizes current thinking regarding methods for preparation of malleable, gray, nodular and white iron castings for electroplating. The preparation cycle recommended by ASTM involves four basic steps:

(1) Removal of oils, greases, residual polishing and bufiing compounds and shop dirt by cleaning.

(2) Removal of oxide films and scale and the loosening of surface carbon by pickling or by salt bath treatment.

(3) Removal of any surface smut caused by step (2).

(4) Activation of the casting for subsequent plating. For racked parts, ASTM recommends the following cycle:

(1) Soak cleaning,

(2) Rinse,

(3) Anodic cleaning,

(4) Rinse,

(5) Acid Pickling,

(6) Rinse,

(7) Anodic cleaning,

(8) Rinse,

(9) Neutralization (activation) in 10 percent H 80 if acid plating is to follow. If alkaline or cyanide plating is to follow, this step is eliminated.

(10) Rinse.

The critical step is the acid pickling step. If the subsequent electroplating is to be done under conditions causing sufliciently high hydrogen overvoltage (most acid solutions, and such alkaline solution as copper, cadmium, or tin), a brief dip (less than seconds) in a room temperature solution of percent (vol.) HCl (37 percent by weight) or 5 to 10 percent (vol.) H SO (98 percent by weight) is usually adequate. If the plating is to be done in an alkaline solution of low hydrogen overvoltage such as cyanide zinc, however, anodic treatment in acid to remove surface carbon is preferred. This is done by making the part the anode in a solution of to percent (vol.) sulfuric acid (98 percent by weight) for at least 30 seconds, preferably more, at 100 a.s.f. While a black film of carbon smut will form during the first 15 to 30 seconds, the part will become passive and the oxygen evolved at the part will remove the carbon by a combination of scrubbing and oxidation, leaving the casting relatively clean. Even then, plating may be incomplete in cyanide type zinc baths, so that ASTM recommends a preliminary electrolytic strike in acid zinc, cyanide cadmium, or alkaline tin baths.

While not an integral part of this disclosure, it should be noted that the inventors have claimed for some time that the above cycle could be significantly improved by substituting the use of typical alkaline descalers for the acid pickling step recommended by ASTM. For example, in step 5 of the above cycle, we have previously recommended the use of a solution containing, for example, about 10 oz. per gallon of an iron chelate, such as EDTA (ethylenedia-minetetraacetic acid), gluconate, oxalate, citrate, heptogluconate and the like, 22 oz. per gallon of casutic soda or potash, and 16 oz. per gallon of cyanide of soda or potash. We have recognized that such solutions may be efiectively used at room temperature and up to 140 F. using periodic reverse current, 5 seconds anodic, 9 seconds cathodic, as an example, for a sufiicient period of time, 5 minutes is exemplary to clean the surface to be plated. The use of such materials, in the manner prescribed above, minimizes both attack on the substrate, and the raising of carbon smuts thereon. Generally speaking, it may be said that parts treated by this revised procedure plate in a brighter and more uniform manner than those plated by the standard ASTM procedure. Nonetheless, even using these improved methods, we recognized that in many cases, substrates of the type discussed above would still yield inferior quality electrodeposits, due to smuts left behind after cleaning, but most seriously might plate incompletely (especially in low current density areas) in cyanide zinc solutions due to the overvoltage phenomenon previously referred to.

We have discovered an improved procedure for preparing carbon and graphite-containing ferrous surfaces for electroplating, and more specifically, for preparing such surfaces for electroplating with zinc and other metals from cyanide-containing electrolytes without use of prior electrolytic striking steps, as suggested by ASTM. This invention then reveals a method whereby such carbon and graphite-containing ferrous surfaces may be directly plated in a cyanide Zinc el ctroyl e Wi out smuts being present on the surface to be plated, and with hydrogen overvoltage in the plating bath so controlled that both throwing and covering power of the zinciferous electrolyte will be maximized relative to any previously known methods of preparation of such surfaces for such plating. It is common commercial practice to cadmium plate cast irons, etc. to avoid the difiiculties of zinc plating such substrates when a sacrificially protective coating is required. Since cadmium is both scarce and expensive, it follows that it is a further object of this invention to reveal a more economical method for plating such substrates wth a sacrificially protective corrosion resistant coating. Finally in some cases, cadmium plating is restored to for solderability purposes, since zinc coatings are not readily solderable. It has been noted in previous patents (US. 2,884,350 and 2,898,274) that it is possible to produce sacrificially protective readily solderable zinc alloy coatings from cyanide electrolytes. However, cast irons, etc. do not plate well in the baths of the cited patents either, using previously known preparation methods. Hence, a final purpose of the invention is to reveal an improved method of preparing cast iron surfaces whereby such substrates may be coated conveniently with a readily solderable, sacrificially protective corrosion resistant coating using the baths of US. 2,884,350 and 2,898,274, for example.

We have found that, quite contrary to normal practice, the improved methods which are the subject of our invention result in the formation of a thin continuous chemical film on the part to be plated. This thin chemical film overcomes the initial problem of low hydrogen overvoltage when commencing to plate in a cyanide zinc electrolyte. Conventional deoxidizing and preparation methods, by contrast, normally aim at making the surface to be plated as film-free as possible. We have also found that by addition of suitable wetter-s to the filmforming bath it is possible to sweep away all graphiticbearing particles from the surface, permitting a uniform formation of the desired film.

The film which we have found to be so desirable is a phosphate film produced from an aqueous acidic deoxidizing solution containing phosphate ions, together with nonionic, cationic surfactants or both. The film-forming solution may be used in one of three ways in the preparation for plating procedure for the substrates which are the subject of our invention. First, it may be used after cleaning as the sole deoxidizing step prior to plating, in which case the disclosed solution acts both to deoxidize and to form the desired film. Secondly, in the case of heavily oxidized substrates, it may be used after cleaning and acid pickling such as recommended in the ASTM procedure previously noted. Thirdly, and optimally, it may be used following alkaline descaling as described in the recently improved methods noted above.

The use of film-forming materials containing phosphates, surfactants and various other additives has previously been employed as a pre-cleaning step followed by the usual conventional methods for preparing such iron substrates for subsequent plating. These prior art procedures involve the immersion of the iron article in an acidic solution containing phosphates and the part is further treated in either an alkaline descaler or a combination of alkaline electrocleaners. Such materials, however, are completely removed from the surface by a pickling treatment or otherwise prior to the application of the usual plating procedures to the film free surface of the iron.

The prior art applications of such film-forming baths did not result in a surface which could be satisfactorily electroplated, particularly to the application of cyanide zinc plate to the ferriferous surface of the type which are the subject of this invention. As entirely distinguished from such prior art procedures, the present invention involves the application of a thin continuous film to the surface of the iron to be plated which remains in place on the ferriferous surface and the metal from the electrolyte is deposited directly on the thin filmed surface of the iron. Surprisingly, the result is that high overvoltage is maintained and a remarkably attractive and firmly adherent electroplate results.

As indicated, in addition to acid and phosphate ion, the film-forming solution contains one or more nonionic or cationic surfactants which will preferentially wet the surfaces being prepared for plating. Optionally and in addition to the wetters or surfactants, it may be advantageous in the case of substrates containing unusually large amounts of graphitic-bearing particles on the surface to add solvents to help sweep away all particles from the surface. These solvents should be miscible in all desired proportions with the acidic medium used. Examples of such solvents are ethylene glycol and many ester and ether derivatives thereof and various ketones.

Optionally and in cases where extensive amounts of deoxidation of the substrate are to be performed in the filmforming solution, thereby requiring long immersion times in the solution, it may be advantageous to add iron corrosion inhibitors to minimize attack on the substrate, thereby also minimizing the amount of smut raised on the surface. Common examples of such inhibitors are organic thio-compounds.

In essence, the invention disclosed and claimed herein comprises a cast iron product, whose surface contains discrete carboniferous particles and a relatively thin continuous phosphate-containing pre-plate film, essentially covering a surface of said product and a metal plate firmly adherent to said pre-plate film. It has been found that, contrary to expectations, this pre-plate film precludes the depression of the hydrogen overvoltage normally due to the presence of carboniferous particles on the surface of the cast iron which interferes with or entirely prevents the formation of a continuous and satisfactory plate on the article.

Although the invention is particularly applicable and will be described with respect to the electroplating of metals, particularly zinc, it is equally applicable to the electroless deposition of metals on iron surfaces, pretreated in the indicated manner and has been found to greatly reduce the corrosiveness of the resulting products. The thin continuous pre-plate film preferably is produced from a solution of a phosphate ion containing material and a surfactant substance selected from the class con sisting of nonionic and cationic surfactants.

As will appear hereinafter, the invention also contemplates a process for the application of this pre-plate film to the cast iron surface and the production of an electroplating film directly over and firmly attached to the thin phosphate containing layer.

Preferably, the pre-plate film created on the iron surface is formed from an aqueous solution which comprises from about 2 parts to 36 parts of the phosphate ion and from about 0.15 part to 3.0 parts of the surfactant substance. Optionally, the film-forming solution may contain miscible solvents, especially in the case of substrates containing unusually large amounts of carbon bearing particles, in an amount of from about 4.0 parts to 20.0 parts based upon the phosphate surfactant content of the solution.

Also optionally the film-forming solution in cases where extensive amounts of deoxidation of the substrate are to be performed by the film-forming solution, it may be advantageous in order to reduce unusually long immersion periods to add iron corrosion inhibitors to minimize attack on the substrate in an amount of from about 0.01 part to 1.0 parts also based upon the phosphate surfactant content of the preparation.

In order to deoxidize effectively the surface of the substrate to be plated, the film-forming solution should be acidic in nature, of a pH ranging from between 0.0 and 3.0 and optimally 0.6 to 0.8. It should also contain 2 to 36 ounces per gallon of the phosphate ion (PO optimally, it should contain 10 to 16 ounces per gallon of this ion. The simplest way to provide both the required acidity and the required amount of phosphate ion is to use phosphoric acid in the film-forming solution. In terms of the figures above, this would correspond to the use of 2.5 to 44 ounces per gallon (optimally 12 to 20 ounces per gallon) of 85 percent (wt.) phosphoric acid, as commercially available. Alternatively, but not preferably, the required acidity could be provided by the use of an acid other than phosphoric (such as sulfuric), combined with the use of a soluble phosphate salt. The salts which may be used are the orthophosphates, monohydrogen phosphates and dihydrogen phosphate (PO4 3, HPOK H2PO4 respectively) of sodium, potassium and ammonium ion. In addition, it is within the scope of this disclosure to include as a source of the required phosphate ion free acids and the sodium, potassium and ammonium salts thereof of other forms of the phosphate ion, such as pyrophosphoric (P 0 metaphosphoric (PO hexametaphosphoric ((PO tetraphosphates (P O and tripolyphosphates (P O Yet another source of the required phosphate ion is the use of organic phosphates, which, generally speaking, are phosphate esters. The following general reaction shows how phosphoric acid may be readily esterified with organic bases:

where R is an organic radical. The above equations relate to the acid form of the phosphate esters, but for purposses of this disclosure, it should be considered that we will be tacitly referring as well to the sodium, potassium, and ammonium salts thereof. Therefore, we will write RO-PO(OM) for 1 esters, (RO) PO(OM) for 2 esters, where M is H+, New, or NH ion. For reasons of high cost, solubility, and viscosity of solution, if organic phosphates are to be used as a source of the phosphate ion, they should be used in conjunction with additional amounts of phosphate ion from inorganic phosphates, unless operating at the lower end of the phosphate ion range noted previously.

For 1, 2 and 3 esters as above defined, R may be either CH or C H For 1 esters only, R may further be H(CHOH) CH where n is 1 or 2.

Next, these film-forming solutions must contain surface-active agents, designed to wet preferentially all surfaces being prepared for plating. These may be present in an amount of 0.15 to 3 ounces per gallon; optimally 0.4 to 0.6 ounce per gallon. For reasons of solubility and compatibility, we generally prefer cationic or non-ionic surface-active agents (the former of the quaternary ammonium type, the latter of the poly (oxyethylenated) alkyl aryl type). More specifically, we generally prefer nonionic agents of the following general formula:

H (CH -Ar-O(CCH CH -O l-I where n is 6 to 20, m is 8 to 12 and Ar is an aryl ring of the phenyl or naphthalene type, and cationic agents of the following general formula:

The following are more specific examples of the general formula above.

where p:1 to 20, X=-H, CH where p=1 to 3 (only), X also X=C H or C l-I where 21:7 to 17 (odd only).

wherep=l to 3 and X=-H,

where X and X are as in II, IIA, HB.

where X X and X are as in II, IIA, HB.

8 In types I and II, R R and R are --H, -OH, CH;;, CH CH CH CH OH,

A- is Cl, Br, Ac, P0 H60 1 Where R=OH, A may also be OH.

III. Heterocyclic amines 0 Hz-- 0 H2 R R CH2CH2 where X is C H or C H n=8 to 18 (even only). A is as before.

IIIB

IIIA

where X is as in IIIA; R is H or CH A- is as before.

where X is C H or C H 11:7 to 17 (odd only); R is H or -CH A- is as before.

IIIE

X-N A- where X and A- are as in IIIC.

where X and A are as in IIID.

In addition to the above types, it has been found by us that it may be advantageous to combine the need for a surface-active agent with the need for phosphate ion (at least in part) by the use of phosphate-ester type surfactants. These surfactants are available as 1, 2 and 3 esters, in the same manner as noted above for the use of lower molecular weight organic phosphate esters:

In these formulae, M stands for H+, Na K+, NH,+ and R stands for an organic radical. While we have found that a number of such organic radicals are satisfactory for this purpose, we have obtained good results using agents in which R is a poly (oxyethylenated) alkyl aryl or a poly (oxyethylenated) aliphatic alcohol. These preferred types are of the following two general structures:

H CH -Ar{OCH CH equals R as above defined and where n is 6 to 20, m is 8 to 10 and Ar is the phenyl or naphthalene group; and

equals R as above defined and where n is 6 to 20, m is 9 to 13.

The solvent which is an optional part of the film-forming solution may be of either or both of two general types. The first type consists of ketones of the general structure CH3COR, WhCI'C R CH3-, C2H5, 01'

The second type consists of ethylene glycol ethers and/ or esters of the following general structure where n=1 or 2; R is H or an ester linkage of the following types: -COCH PO(OH) or SO (OH); R is H or an ether linkage of the type H(CH where 111:1-4. The total amount of combined solvents present should be 4 to 20 ounces per gallon; optimally, 8 to 12 ounces per gallon. Examples of solvents of the above type are:

Ketones:

Acetone (CH COCH Methyl ethyl ketone (CH COC H Diacetone alcohol In the above tabulation, the words Cellosolve and Carbitol are registered trademarks of Union Carbide. In addition, the correspondnig esters of each of the above is to be considered. As a typical example, this would be the formula for ethyl Cellosolve phosphate;

It should be noted that in the event that the particular glycol ether ester used is the corresponding phosphate ester, part of the phosphate requirement of our invention is correspondingly satisfied.

Finally, as a last optional ingredient, the film-forming solution may contain organic thio-compounds which act as iron corrosion inhibitors in acid solutions. Such materials may be required if the film-forming solution is used to do an appreciable amount of deoxidizing, leading to extended immersion times of ferriferous substrates in the bath. While not thereby limited, we have found the following compounds to be useful in this regard: NaSCN;

(C H NCSSNa. These inhibitors are generally used at concentrations of less than 1 ounce per gallon.

The film-forming solution is generally used at room temperature so as to minimize any attack on the ferriferous substrate, and so as to minimize heating costs. If desired, however, these solutions may be operated at temperatures up to 180 F. The time of immersion at room temperature is generally one to ten minutes, However, it is to be understood that this disclosure is not thereby limited, as particular circumstances could dictate immersion times outside of these ranges.

The following examples represent the preferred embodiments of our invention, but are not intended to be inclusive or restrictive in any way over the above scope. The examples represent actual trials in the laboratory on parts obtained from industrial plants, as representative of parts which have been previously impossible to plate directly in cyanide zinc electrolytes in a satisfactory manner.

10 EXAMPLE 1 Malleable iron parts which had been previously heat treated and quenched were rack plated directly in a cyanide zinc bath using the following cycle:

(1) Clean in an alkaline descaler for 5 minutes at room temperature, using periodic reverse current, 5 sec. anodic, 9 sec. cathodic cycle. The bath contained:

Ounces per gallon NaOH 21 Na gluconate ll NaCn 16 (2) Cold water rinse. (3) Immerse in film-forming solution at room temperature for 2 minutes. The bath contained:

Ounces per gallon Glycol ether 8 P0 7 13 Poly (oxyethylenated) nonylphenol 0.6 Diethylthiourea 0.3

pH (electrometric) 0.7.

EXAMPLE 2 Grey iron castings which were rather badly oxidized were barrel plated directly in a cyanide zinc bath using the following cycle:

(1) Clean in an alkaline descaler for 15 minutes at room temperature, using periodic reverse current, 5 seconds anodic, 8 seconds cathodic cycle. The bath contained:

Ounces per gallon NaOH 26 Na gluconate 6 NaCN 16 (2) Cold water rinse.

(3) Immerse in film-forming solution at room temperature for two minutes. The bath contained:

Ounces per gallon Glycol ether 8 PO4 3 Poly(oxyethylenated) nonylphenol 0.6 Diethylthiourea 0.3

pH (electrometric), 0.7.

(4) Cold water rinse.

(5) Barrel zinc plate in a cyanide-type proprietary bright zinc bath for one hour at 7 volts.

(6) Cold water rinse.

(7) Bright dip in chromate dip.

(8) Cold water rinse.

(9) Hot blast dry.

The castings were completely covered even in the lowest current density recesses with a uniformly bright zinc deposit.

EXAMPLE 3 Carbonitrided self-locking automotive bolts which were badly scaled and oiled were barrel plated directly in a cyanide zinc bath using the following cycle:

(1) Tumble clean in an alkaline heavy-duty soak Ounces per gallon NaOH 26 Na gluconate 6 NaCN 16 (4) Double cold water rinse. Immerse in film-forming solution at room temperature for two minutes. The bath contained:

Ounces per gallon Glycol ether 16 PO['3 Poly(oxyethylenated) nonylphenol 0.6 pH (electrometric) 0.4

EXAMPLE 4 Oxidized cast iron fittings were rack plated directly in a cyanide zinc bath using the following cycle:

(1) Clean in alkaline heavy duty soa-k cleaner, used at 8 ounces per gallon, 175 F for 6 minutes, to remove oil.

(2) Double cold 'water rinse.

(3) Immersion film-forming solution at room temperature for 3 minutes. The bath contained:

Ounces per gallon P04 Imidazolinium chloride (quaternary ammonium salt) 1 pH (electrometric) 0.4.

(4) Cold water rinse.

(5) Zinc plate in cyanide type proprietary bright zinc bath for minutes at 1 volt.

(6) Cold water rinse.

(7) Bright dip in chromate dip.

(8) Cold water rinse.

(9) Dry.

While we have shown and described some preferred embodiments of our invention, it will be understood that it is not to be limited to all of the details shown, but is capable of modification and variation Within the spirit of the invention and within the scope of the claims. For example, the method has been advantageously used in the plating of other metals such as cadmium, copper, nickel, chromium and electroless nickel.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A process for cyanide zinc electroplating a ferrous metal surface of an object having discrete carboniferous particles in said surface, which comprises immersing the object ferrous metal surface having the carboniferous particles in said surface in an aqueous acid solution comprising about 2 to 36 ounces per gallon of phosphate ion, 0.15 to 3 ounces per gallon of at least one lwetting agent selected from the group consisting of nonionic and cationic surfactants, about 4 to ounces per gallon of a miscible solvent selected from the group consisting of compounds of the formulae:

wherein R is CH;,, C H or whereinR is H,

COCH PO(OH) or SO (OH),

R is H or H(CH n is l-2 and m is 1-4, and an organic thio compound in a corrosion-inhibiting amount less than 1 ounce per gallon, said aqueous acid solution having a pH of up to 3.0 inclusive and being at a temperature in the range of room temperature to 180 F., for a time sufiicient to form thereon a thin continuous phosphatecontaining film, and electroplating a firmly adherent zinc plate onto the thus coated ferrous metal surface in a cyanide zinc electroplating bath whereby a sufiiciently high hydrogen overvoltage is maintained during the zinc electroplating to enable said electroplating to occur.

2. The process of claim 1 'wherein the ferrous metal surface having the discrete carboniferous particles therein is a carburized ferrous metal surface.

3. The process of claim 1 wherein the phosphate ions are supplied to the aqueous acid solution as phosphoric acid, and the organic thio compound is selected from the group consisting of sodium thiocyanate,

' 0.15 to 3 ounces per gallon of at least one wetting agent selected from the group consisting of nonionic and cationic surfactants, about 4 to 20 ounces per gallon of a miscible solvent selected from the group consisting of compounds of the formulae:

ll CH3C-R wherein R is wherein R is H, COCH -PO(OH) or R is H or H(CH n is l2 and m is 1-4, said aqueous acid solution having a pH of up to 3.0 inclusive and being at a temperature in the range of room temperature to F., for a time sufiicient to form thereon a thin continuous phosphate-containing film, and electroplating a rlirmly adherent zinc plate onto the thus coated ferrous metal surface in a cyanide zinc electroplating bath whereby a sufficiently high hydrogen overvoltage is maintained during the zinc electroplating to enable said electroplating to occur.

6. The process of claim 5 wherein the ferrous metal surface having the discrete carboniferous particles therein is a carburized ferrous metal surface.

(References on following page) References Cited UNITED STATES PATENTS Parker.

Benning et a1. 117134 X 5 Prutton 148-65 Douty et a1. 1486.15 Jernstedt 148-615 Dodd et a1. 1486.15 Snyder et a1 148-6.15

14 2,840,498 6/1958 Logue et a1 1486.15 3,133,005 5/1964 Ades 20434 X FOREIGN PATENTS 845,119 5/ 1939 France.

OTHER REFERENCES LoPresti, Metal Finishing, October 1942, pp. 533-536.

RALPH S. KENDALL, Primary Examiner.