[1]United States Patent Office 3@4282441 3,428,441 ARTICLE COATED WITH A COMPOSITE PARTICULATE, MICROPOROUS CHROMIUM COATING AND METHOD OF PRODUCING SAID ARTICLE Arthur H. Du Rose, Richmond Heights, Karl S. 'WiIIson, Cleveland, and Gustavo C. Tejada, Euclid, Ohio, assignors, by mesne assignments, to Kewanee Oil Company, Bryn Mawr, Pa., a corporation of Delaware No Drawing. Filed JuIy 28, 1965, Ser. No. 475,556 U.S. C]. 29-183.5 10 Claims Jut. Cl. C23b 5106; B32b 3110,15116 ABSTRACT OF THE DISCLOSURE This invention comprises a composite metal coating, and a method for preparation thereof, comprising a metal substrate having thereon a discontinuous particulate electrodeposited coating partially covering the substrate, the particles of this coating being discrete and having an average particle thickness of 0.003-0.05 mil and containing 1,000-1,000,000 particles per square inch, and a microporous chromium layer overlaying and adherent to the discontinuous layer and the exposed substrate, the chromium layer having 1,000-1,000,000 micropores per square inch and a thickness of 0.003-0.05 mil. The method of preparing the composite coating comprises the steps of electrodepositing on the metal substrate a coating of particles until the number and size gives the desired discontinuous layer, then discontinuing this electrodeposition, and subsequently electrodepositing thereon a chromium layer to the desired dimension. This invention relates to corrosion-resistant coatings and more especially to composite coatings made up of a substrate to be protected, a discontinuous coating made up of discrete particles, and a microporous layer of chromium overlaying and adherent to said discontinuous coating-substrate composite. Such coatings may be produced by a series of steps, suitably first electrodepositing a discontinuous coating of discrete particles, and second, plating a layer of chromium onto the substrate through said discontinuous coating. The substrate may be a metal, preferably bright nickel, or semibright nickel, or bright over semibright nickel. Suitably also we may use as substrate cobalt, bronze, copper, or silver. Yet other metals rnay be used. The discontinuous layer-forrning solution is electrolyzed and the result partially coats the substrate selected with a discontinuous coating of discrete particles, there being from about a thousand to about a million particles per square inch. The resulting article is then plated in a chormium solution, the chromium plating between the particles of the discontinuous layer a microporous coating of chromium with good resistance to corrosion of the composite. The prior art contains a series of patents based upon sedimentation and embedding within a metallic matrix of finely divided, insoluble materials. Examples are: U.S. 3,152,971; 3,152,972; 3,152,973. In some instances, these are said to give a good satin finish; while fully bright deposits are secured in some instances by modifications of these techniques, as for example, as seen in Belgian Patent No. 616,567. In the prior art there are two layers deposited over the substrate with the final (top) layer being discontinuous due to insoluble particles embedded in the metal. The insoluble particles may be suspended in the bath from whic@h the first layer is to be deposited, or in the bath Patented Feb. 18, 1969 2 used for depositing the top layer. Commercial practice to date makes use of the insoluble particles deposited in a thin layer of nickel over which cbromium is plated. At the points wbere the embedded insoluble, nonmetallic particles protrude above the nickel matrix, the chromium does not electrodeposit mrith the result that there are discontinuities in the chromium deposit. In our approach to the formation of the micropores in the chromium, we avoid the problems of keeping in10 soluble particles suspended in the plating bath. We also avoid the problem of having the particles settle in large numbers on a horizontol portion and sparsely on a vertical surface. We use a bath containing no insoluble particles (ex15 cept for trivial amounts of contaminants.) From this solution, we deposit on the substrate by electrochemical means, insoluble materials only, which are not embedded in a metal matrix as is the case according to the prior art. Rather, the electrochernically formed insoluble particles 20 are formed at the surface of the substrate and remain attached tbereto by chemical forces. These chemical forces catising adherence to the surface need be strong enough only to hold the insoluble material to the surface during normal handling and rinsing operations. 25 In metal plating operations, the first atoms of metal generally deposit not to provide a continuous sheet of metal but rather at isolated nuclei. From these nuclei, the metal grain or crystal gro@ws both horizontally and vertically. T-he horizontal growth (i.e. growth parallel to 3 0 the substrate) from many nuclei continues until a continuous covering of the substrate results. Since we desire only to form the insoluble particles at the substrate surface father than to embed them in a supporting metal matrix, the time required for our 35 operation is usually a matter of seconds compared to several minutes for the embedding process. In most instances, formation of the insoluble material is secured by making the substrate cathodic in the bath. In same instances, the substrate is made anodic, or a 40 sequential combination of both polarities is used toadvantage. After the formation of the insoluble particles on the substrate, the article is rinsed and is then plated with chromium from any typical chromium plating bath, with 45 a chromium deposit -of the order of .003 to 0.05, and preferably from 0.01 to 0.03 inil in thickness. With increasing thickness of the chromium, the chromium tends to bridge over the insoluble particles with a decrease in porosity. We prefer a deposit of the order of 0.01 mil 50 thickness, such thickness being customary commercialiy for deposits in which the substrate does not bave the insoluble particles. For evaluating porosity, the well known Dubpernell test is used, in which the article serves as catbode in an 55 acid copper sulfate bath with a low current density. The chromium is believed to be covered by an oxide film onto which the copper does not deposit except at breaks in the chromium oxide where the underlaying substrate nickel is exposed. 60 Tests have indicated that when a lar-ge number of pores in the chromium deposit are secured as evidenced by a Dubpernell test, articles exhibit improved resistance to atmospheric or accelerated corrosion t:esting. 65 The insoluble materials can be electrochemically deposited from solutions containing any one of a variety of chemical compounds or mixtures of compounds, and the operating condition may vary considerably. The following Table I contains a number of solutions which may be used for the production of the discontinuous 70 coating of particles on the substrate.
[2]3)4282441 4 TABLE I Example Bath Composition and Operating Characteristics I ---- 50 g./l. sodium chloride (NaCI). 1 g./l. lead acetate (Pb[02C2H3]2-3H2,0. 5 140' F., pH 6.5, 10 sec. cathodic at 5 a.s.f. ---- 50 g./l. barium chloride (BaCl2.5H20)- 1 g./l. lead acetate (Pb[02C2H3]2-3H20)- 140' F., pH 5.5, 10 see. cathodic at 10 a.s.f. 3 ---- 75 g./l. lead acetate (Pb[02C2H3]2.3H20)- 10 200 g./l. sodium hydroxide (NaOH). 185' F., 10 sec. cathodic at 5 a.s.f. 4 ---- 0.1 g./l. lead acetate (Pb[02C2H3]2-3H20)- g./l. sodium acetate (NaO2C2H3). 140' F., pH 7.2, 10 sec. cathodic at 5 a.s.f. 15 ---- 5 g./l. bismuth nitrate (BiLN0312-5H20)- 140' F., 10 see. cathodic at 5 a.s.f. 6 ---- 50 g./l. magnesium chloride (MgCI2) - 140' F., 40 a.s.f., 10,-60 sec. cathodically then 10 sec. anodically. 20 7 ---- 5,0 g./l. magnesium chloride (MgCI2)- 1 g./l. lead acetate (PbLO2C2H3]2-3H20)- 140' F., 10-60 sec. cathodic at 40 a.s.f. 8 ---- Same as 7 but 10-60 sec. cathodic, then 10 sec. anodic at 40 a.s.f. 25 ---- 50 g./l. molybdic acid (H2MO04) - 140' F., 10 sec. cathodic at 20 a.s.f. 10 ---- Same as 9 except 10 sec. cathodic, then 5 sec. anodic at 20 a.s.f 1 1 ---- 50 g.11. antimony trichloride (SbCI3). 30 140' F., 10-30 see. cathodic at 20, a.s.f. 12 ---- 3 g./l. aluminum chloride (AlCI3)- 140' F., pH 4.3, 10 sec. cathodic, then 10 sec. anodic at 10 a.s.f. 13 ---- 3 g./l. aluminum chloride (AIC13) NAOH to 35 pH 11.4. 140' F., 10 sec. cathodic, then 10 sec. anodic at 20 a.s.f. 14 ---- 3 g./l. aluminum chloride (AIC13)- 140' F., pH 4, 10 see. anodic then 10 sec. ca- 40 thodic at 20 a.s.f. 15 ---- Same as 14 except 30 see. anodic then 30 sec. cathodic. 1 6 ---- 100 g./l. lead fluoborate (Pb[BF4]2)- 80' F., 5 sec. cathodic at 5 a.s.f. 45 17 ---- Same as 16 except 10 sec. cathodic at 5 a.s.f. While, as indicated above, numerous substrate metals in the metallic state can be used, nickel is preferred. The nickel may be of a semibright or bright character. Where ro bright coatings are desirable the substrate should be bright. Since the particle forming solution yields a very thin discontinuous coating with adhering particles of average thickness, from about 0.003 to 0.05 mil; and since the chromium coating is very thin, preferable 0.01 to 0 ' 03 mil ' r the brightness tends to follow the brightness of th,, sub- )rl strate. The substrate A preferred substrate is bright nickel which may vary considerably in thickness. An ordinary coating such as is 60 in commercial use may be of a thickness of about one mil. This substrate may be made in the manner of ainy one of a number of so-called bright plates. A solution suitable, by way of example, for this substrate may be as follows: Gram s 65 NiSO4-6H20 ----------------------------- 250 NiCl, - 6H20 ----------------------------- 50 Boric, acid -------------------------------- 50 (C,H,S02NHSO,C,H,C,H4)1 ------------ --- 2 70 Triaminotriphenyl methane ------------ ----- 00.0001 Water to make one liter. The following patents contain other examples of brighteners and sulfo-oxygen carriers, U.S. Patents, 2,994,648; 3,000,799; 3,152,975. 75 Another preferred substrate, which when used in conjunction with the discontinous layer-microporous chromium composite of this invention, comprises a deposit on steel of semibright layer of nickel followed by a layer of bright nickel. The semibright nickel layer may be deposited from a bath such as described in U.S. Patent 2,683,115. This semibright deposit of nickel may vary in thickness but preferably is of the order of 0.5 to 1.0 mil. The subsequent bright nickel, deposited from a suitable bath has discussed above, is preferably of the order of 0.3 to 0.5 mil or even more in thickness. Use of this substrate in conjunction with the microporous composite of this invention enhances the resistance to corrosion over that of the substrate using the same total thickness of bright nickel only. In Table I we have indicated the times of electrolysis of the various solutions. These are not necessarily the optimum measure of electrolytir, treatment. The number and major dimension of the particles are together the best measure. Chromium plating Chromium plating is accomplished by any usual commercial chromium plating solution. For example, we may use a chromium plating bath containing Grams Chromic acid --------------------------------- 250 Sulf ate ion ---------------------------------- 2.5 Water to make one liter. We plate chromium through the discontinuities between the deposited particles of the discontinuous coating and thereby produce a microporous chromium coating. The resulting article of manufacture has the property of high resistance to corrosion. This is especially valuable when the substrate is nickel over steel. It may be valuable also when the substrate is cobalt over steel or a mixture of nickel and cobalt over steel. Cobalt is equivalent to nickel and wherever nickel is mentioned herein it is understood to include cobalt or mixtures of nickel and cobalt. Other chromium plating baths may be used wherein, for example, some or all of the sulf uric acid may be replaced by fluosilicate or other suitable catalyst ions. The sulfuric acid referred to is present in commercial chromium plating solutions as a source of sulfate ion. Solutions for making discontinuous deposits In Table I above we have shown a number of solutions for making discontinuous deposits on the metal substrates. We show there the conditions under which the discontinuous coatings preferably are produced on the substrate. We prefer to utilize, in the production of the substrate coated with a discontinuous coating, a compound of the ass consisting of: Bismuth nitrate, magnesium chloride, c' Molybdic acid, antimony trichloride, aluminum trichloride, lead acetate, and lead fluoborate. We prefer to make a composite coating by the followin-, process: Electrodepositing on a metal, preferably bright nickel, a discontinuous coating comprisin- particles of a thickness from 0.003 to 0.05 mil. We prefer a thickness of particle from 0.01 to 0.03 mil. The average thickness of the discontinuous coating particles serves to establish the degree of essential function in terms of 'erage thickness ranges of the particles. For deposition of the particles as a discontinuous, particulate layer on the substrate, we use a bath containing bath-soluble compounds of one or more metallic elements. We believe the electrodeposited particles to consist of insoluble oxides, hydroxides, and basic salts. In the case of deposits from the lead and antimony solutions, we believe that metallic lead or antimony may be deposited during the plating operation but that, on subsequent rinsing, a superficial layer of oxide, hydroxide, or basic salt is formed.
