PROCESS FOR FORMING ADHERENT CHROMIUM ELECTRODEPOSITS FROM A HIGH ENERGY EFFICIENT BATH

Abstract: The use of high efficiency chromium plating processes has been hampered by the inability to obtain adequate coating adhesion. The invention relates to a method of electrodepositing chromium on a metal substrate such as ferrous metal, stainless steel or cast iron which includes the step of activating the substrate by electrolytically etching in an activating solution. The activating solution is selected from a) a substantially neutral solution, b) a mixture of sulfuric acid, added sulfate ion and an inorganic oxidizing agent c) a mixture of chromic acid and chloride ion or d) an anodic oxidizing polarizing solution. It has been found that the use of such activating solutions overcomes the previously encountered adhesion problems.

Description: PROCESS FOR FORMING ADHERENT CHROMIUM ELECTRODEPOSITS FROM A HIGH ENERGY EFFICIENT BATH This invention relates to electrodeposition of chromium on basis metals, and, more particularly, it is concerned with an activation process by which an adherent chromium electrodeposit is formed on metal substrates, such as ferrous metals, stainless steel or cast iron, using a high energy e±ticient chromium plating bath. The use of high efficiency chromium plating processes has been hampered by the inability to obtain adequate coating adhesion to certain basis metals. Such baths are o£ the types disclosed Dillenberg U.S. Patent No. 4,093,522; Perakh et al. , U.S. Patent No. 4,2134,396; and Chessin, U.S. Patent 4,450,050 and 4,472,249. The problem has been • attributed to the presence of the halide ion in the bath, which may intertere with initiation ot chromium deposition. An adequate bond with conventional chromium plating solutions, such as those using a solution ot chromic acid and catalysts, such as sulfate, or sulfate m combination with various fluorides, can be obtained by reverse or anodically etching the substrate m the plating solution or in a separate chromium acid containing solution at a prede¬ termined current density for a predetermined time. A table which lists the time lengths for such an etching process is found in "Metal Finishing" 80 (5) 65-8 (1982) by C. H. Peger. The use of certain sulfuric acid and hydrofluoric acid etches for specific stainless steels is also suggested in the publication. Anodic chromic acid treatments for 400 stainless steel alloys and for low and high carbon steels is disclosed in "48th Metal Finishing Guidebook-Directory" /8, 188-202 (1980) by A. Logozzo. Also recommended are cathod c treat- ments in sulfuric-tluoride solutions for 300 stainless, for nickel alloys and for cast iron. A further alternative which is found in the prior art is the use of a Woods nickel strike for certain special nickel and cobalt-based alloys. The use of a ferric chloride-hydrochloric acid solution as a clearing agent tor the smut produced by anodic sulfuric etches is disclosed at page 137 of "Hard Chromium Plating" Robert Draper Ltd., Teddington, 1964 by J. D. Greenwood. Brune and McEnally in "Plating" 42., 1127-32 (1955) describe the use ot a magnesium sulfate sulfuric acid anodic etch solution for preparing ferrous parts for plating. Similarly, ASTM Specification B-242-49T suggests the appli¬ cation of an anodic etch using a sulfuric acid solution containing sodium sulfate. ASTM B1//-68 described the use of sulfuric acid or chromic acid as activators for chromium electroplating on steel tor engineering use. Chessin in U.S. Patent 4,450,050 described an activa¬ tion pretreatment for bonding high efficiency chromium electrodeposits on a metal substrate which is characterized by the step ot first plating the substrate metal with iron or an iron alloy from an iron salt containing bath. Herrmann, in U.S. Patent 4,416,758, activates metal substrates in an aqueous alkaline cyanide containing solu¬ tion using current which is periodically reversed, followed by rinsing and chromium plating. It has been found that when these procedures are employed w th the halide containing high energy efficient chromium plating baths, most metal substrates are not adequately plated because the chromium deposits from these baths have inadequate adhesion. It can be speculated that the reducing conditions at the cathode at the initiation of deposition cause the halide ion to be reduced to a form which interferes with the molecular bonding of the chromium to the substrate. in any event the use of high efficiency chromium plating is limited by the problem of inadequate adhesion. Cast iron substrates present a particular problem with respect to forming adherent chromium deposits. Several references describe pretreatments for case iron but neither recommends them for use in chromium plating. Canning Handbook on Electroplating (W. Canning, Ltd., Birmingham, England) 1973, p. 338 and 345. Graham, Electroplating and Engineering Handbook (Reinhold) 1955, p. 167. Graham's method is a pickling or immersion process and does not use electrolysis. Canning proposes a solution which contains chromium acid, which is disadvantageous because it creates a waste treatment problem. Chen and Baldauf, in U.S. Patent 4,412,892, uses an anodic treatment in a sulfuric acid-hydrochloric acid solution. It has now been found that the adherence problems encountered with the prior art systems for electrodepositing chromium on metal substrates from high energy efficient chromium baths can be overcome by activating the substrate by electrolytic etching m an activation solution (1) which is a substantially neutral (pH 5-10) alkali metal sulfate solution. The activation solution does not require sulfuric or chromic acids to perform its activating function. Alternatively, an activation solution (2) which in¬ cludes sulfuric acid, added sulfate ion, and an inorganic oxidizing agent, may be used. Suitably, the activation solution contains 10 to S Q7, by volume of concentrated sulturic acid, 0.25 to 2 moles/1 of added sulfate ion, and 3.5 x 10 -3 moles/1 to 140 x 10-3 moles/1 ot an inorganic oxidizing agent, such as nitrate, permanganate, perchlorate or persulfate. (3) An activation solution also may consist of chromic acid and chloride ion, optionally with bromide ion, parti¬ cularly for stainless steel substrates. For electrodepositing chromium on cast iron metal substrates, the use of a two-step activation process in which the cast iron substrate is electrolytically activated by an anodic treatment followed sequentially by a catho-dic treatment, is recommended. While the process of the invention can be utilized for electrodepositing chromium from any chromium bath, it can be used advantageously tor forming adherent chromium on a metal substrate from a high energy efficient chromium bath which includes a hal-ide ion as an essential constituent. The. presence of the halide ion can cause problems with adherence of the chromium deposit unless the metal substrate is given the pretreatment of the invention. EMBODIMENT (1) The process comprises subjecting the metal substrate to electrolytic etching, preferably anodic etching, in a substantially neutral (pH 5-10) solution of an alkali metal sulfate, and electrodepositing chromium thereon. The electrolytic etching step is carried out for about 10 seconds to 10 minutes at a current density of about 0.1 asi to 10 asi, and at a temperature from about room tempera¬ ture to 50°C. After the activation step the current is turned off and the activated metal substrate is transferred to a rinsing vessel where it is rinsed free of the activation solution. -S- Then the metal substrate is placed in the chromium electro¬ plating bath and chromium metal is deposited thereon. Optionally, a reverse (i.e. the workpiece is made the anode) in a chromic acid-containing solution, for example, the chromium plating solution itself, may be used preceding the actual chromium plating operation. The activation solution, while free of added sulfuric acid or chromic acid, may contain one or more of the follow¬ ing additives: a buffer, such as borax, to maintain the pH of the solution at the desired level; an oxidizing salt, such as sodium molybdate or sodium chromate, for faster polarization; an attack metal compound, such as sodium chloride or sodium nitrate, to aid in attacking the surface of the substrate; a complexor such as citrate or gluconate, to stabilize the products of the electrolytic reaction; and a surfactant to reduce surface tension between solution and substrate. This activating solution is particularly useful tor ferrous metals such as cast irons and alloy and high carbon steels. EXAMPLE 1 An activation solution was prepared from 45 g/1 of sodium sulfate and 20 g/1 of sodium molybdate hexahydrate. The pH was 7. A cast iron substrate was placed in the solution at room temperature and the substrate was anodi¬ cally etched at 0.5 asi for 30 seconds. After transfer to a rinsing bath, the activated metal was chromium plated in a high energy efficient bath containing iodide ion at 5 asi for 60 minutes. The chromium deposit exhibited excellent adherence to the substrate and was smooth. A similar process m the absence of application of any reverse activation current provided only very poor adhesion ot chromium on the cast iron metal substrate. -6- EXAMPLE 2 The activation solution consisted ot 120 g/1 sodium sulfate and 40 g/1 of sodium nitrate. The pH was 6 ' . The substrate was strut steel, 1024, an induction hardened steel. Activation was carried out at 23 υ C at 1 asi for 3 minutes, followed by rinsing and high energy chromium reverse at 5 asi for 3 minutes, and high energy chromium plating at 5 asi for 15 minutes. The adherence of the chromium to the substrate was excellent and the deposit was quite smooth. EXAMPLE 3 The process of Example 2 was repeated using addi¬ tionally 10 g/1 ot boric acid in the activation solution, a pH of 7, and anodic etching at.2 asi for- 2 minutes. A similar excellent adhering deposit was obtained. EXAMPLE 4 The process of Example 2 was repeated using an activa¬ tion solution comprising 40 g/I sodium sulfate, 20 g/i sodium nitrate, 20 g/1 sodium citrate dihydr de, 5 g/1 sodium chloride, and 20 g/1 sodium tetraborate hexahydrate. The pH was 8. Activation was carried out at 1 asi tor 4 minutes , reverse etching in a high energy bath at 5 asi for 3 minutes and chromium plating at 5 asi for 15 minutes. An adherent, smooth chromium deposit was obtained. -7- EXAMPLE 5 The process of the above examples was repeated using potassium sulfate in place ot sodium sulfate with similar advantageous results. While the mechanism ot action of the activation solu¬ tion and process of the present invention is unknown at present, it s believed to reside m the formation ot a passive layer during the anodic etching step, which layer remains intact in the absence of any strong acid in the activation solution. EXAMPLE 6 The process of Example 2 may be repeated using addi¬ tionally 10 g/I of boric acid in the activation solution, a pH .of 7 and etching employing alternating current at 2 asi for 2 minutes. A similar excellent adhering deposit will result. EMBODIMENT (2) The process of the present invention may consist of subjecting the metal substrate to -electrolytic etching, preferably anodic etching, in a solution of sulfuric acid, added sulfate ion, and an oxidizing agent, rinsing the activated substrate, and electrodepositing chromium thereon, preferably from a high energy efficient chromium bath. Suitably the activation solution consists essentially of 10 to 50% by volume of concentrated sulfuric acid, 0.25 to 2 moles/1, of added sulfate ion, and 3.5 x 10 " to 14U x iU -3 moles/1 of an inorganic oxidizing agent. Preferably, the activation solution consists essen¬ tially of 20 to 0% by volume concentrated sulfuric acid, -8- _3 0.4 to 1 moie/1 ot added sulfate ion and 7.0 x 10 to 70 x _3 10 moles/1 of oxidizing agent. Typically, the activation solution consists essentially of 20 to 40% by volume of concentrated sulfuric acid, 100 to 200 g/1 of magnesium sulfate and 1 to 2 g/1 of sodium nitrate. Suitable inorganic oxidizing agents for use in the activation solution of the invention include oxygenated inorganic oxidizing agents, such as nitrate, permanganate, perchlorate, persulfate and molybdate ions, usually added as the sodium or potassium salts. Chromate is not as useful since it gives a rough deposit. The added sulfate ion usually is present as magnesium sulfate heptahydrate which is very soluble m the activation solution. The electrolytic etching step is carried out for about 10 seconds to 10 minutes, preferably 15 seconds to 1 minute, at a current density of about 1.5 amps/dm to 155 amps/dm 2 (asd) , preferably 1 to 8 amps/dm and at a temperature from about 10°C to 40°C, preferably at room temperature. After the activation step, the current is turned off and the activated metal substrate is transferred to a rinsing vessel where t is rinsed free of the activation solution, preferably within 1 minute after activation. Then the metal substrate is placed in the chromium electroplating bath and chromium metal is deposited thereon. Optionally, a reverse (i.e. the workpiece is made the anode) in a chromic acid-containing solution, for example, the chromium plating solution itself, may be used preceding the actual chromium plating operation. Typical metal substrates include ferrous metals such as cast irons and alloy and high carbon steels. -9- EXAMPLE 7 An activation solution was prepared from 40% by volume of concentrated sulfuric acid, 100 g/1 magnesium sulfate heptahydrate, and 0.5 g/1 of sodium nitrate. A cast iron substrate was placed in the solution and the substrate was anodically etched at 15.5 asd for 1 minute at 16°C. After transfer to a cold water rinsing bath, the activated metal was chromium plated in a high energy efficient bath contain¬ ing iodide ion at 77.5 asd for 60 minutes. The chromium deposit exhibited excellent adherence to the substrate and was bright, smooth and hard. A similar process in the absence of sodium nitrate provided only very poor adhesion of chromium on the cast iron metal substrate. EXAMPLE 8 The activation solution consisted of 40% by volume of concentrated sulfuric acid, 100 g/1 of magnesium sulfate heptahydrate and 2 g/1 ot sodium nitrate. The substrate was cast iron. Activation was carried out at 21 U C at 1 1 . _ asd for 20 seconds, followed by rinsing and h gh energy chromium plating at 77.5 for 15 minutes. The adherence of the chromium to the substrate was excellent, the deposit was quite smooth, bright and hard. EXAMPLE 9 The process of Example 2 was repeated using 40% by volume of concentrated sulfuric acid, 100 g/1 of magnesium sulfate heptahydrate, and 5 g/1 of sodium nitrate as the activation solution, and anodically etching at 77.5 asd for 15 seconds at 12°C. An excellent adhering deposit was obtained. EXAMPLE 10 The process ot Example 2 was repeated using an activa¬ tion solution of 40% by volume of concentrated sulfuric acid, 100 g/1 of magnesium sulfate heptahydrate, and 1 g/1 sodium perchlorate. Activation was carried out at 77.5 asd tor 15 seconds. An adherent, smooth chromium deposit was obtained. EXAMPLE 11 The process of the above examples was repeated using similar molar amounts of potassium permanganate, sodium persulfate and sodium molybdate in place ot sodium nitrate. A similar advantageous adherent deposit of chromium was obtained. EMBODIMENT (3) In this embodiment, a stainless steel substrate is subjected to electrolytic etching, preferably anodic etch¬ ing, m a solution of chromic acid and chloride ion, or w th a mixture of chloride and bromide ions, and then electro¬ depositing chromium thereon from a " high energy efficient chromium bath. Preferably, the chloride ion should not exceed about 0.3M. The electrolytic etching step is carried out for about 10 seconds to 10 minutes, suitably 15 seconds to 1 minute, at a current density of about 0.5 to 155 amps/dm (asd), preferably 1-8 asd, and at a suitable temperature, prefer¬ ably at least about 40°C, and most preferably at about 55°- 60°C. After the activation step, the current is turned off and the activated metal substrate is transferred to a rinsing vessel where it can be rinsed free of the activation solution, if desired. Then the activated substrate s placed in the chromium electroplating bath and chromium metal is deposited thereon. Optionally, a cathodic or reverse etching step may follow the anodic etching as part of the activation process. Typical stainless steels include stainless steels numbers 304, 316, 316L and 410. EXAMPLE 12 An activation solution was prepared comprising 0.15 M chromic acid containing 0.14M chloride ion. A 304 stainless steel substrate was placed in the solution and the substrate was anodically etched at 2.5- asi for 1 minute at 57°C. After transfer to a cold water rinsing bath, the activated metal was chromium plated in a high energy efficient bath containing iodide ion at 77.5 asd for 60 minutes. The chromium deposit exhibited excellent adherence to the substrate and was bright, smooth and hard. A similar process without chloride ion present in the activation solution resulted m very poor adhesion ot chromium on the stainless steel substrate. EXAMPLE 13 The activation solution consisted of 0.15 M chromic acid, 0.14M chloride ion and 0.025M bromide ion. The substrate was 304 stainless steel. Activation was carried out at 5 U C at 2.5 asi for 1 minute, followed by rinsing and high energy chromium plating at 77.5 asd for 15 minutes. Adherence ot the chromium deposit to the substrate was excellent, and it was smooth, bright and hard. EXAMPLE 14 The process of Example 2 was repeated without the rinsing step. An excellent adhering deposit also was obtained. EXAMPLE 15 The activation step of Example 2 was repeated at a temperature of 26 α C. The chromium deposit did not adhere well to the substrate. EXAMPLE 16 The process of Example 2 was repeated using an activa¬ tion solution containing 0.34M chloride ion. The chromium deposit did not adhere well. EMBODIMENT (4) The anodic step is carried out in a polarizing solution in which the surface of the cast iron substrate is dissolved without causing surface roughness. Suitable polarizing solutions include oxidizing agents, such as a concentrated sulfuric acid solution, chromic acid, dichromate, disulfate and the like, preferably in a medium of high ionic strength, such as is provided by electrolyte salts such as sodium sulfate, potassium sulfate and magnesium sulfate. The anodic step is carried out for a period of time sufficient to effect polarization in the solution, whereupon passivation sets in, and is terminated before surface roughness ensues. Suitably, this period is 15 seconds to 10 2 minutes, at a current density ot about 15 to 45 amps/dm (asd) , preferably 20 to 30 asd. When a concentrated sulfuric acid solution is used as the anodic solution, the step is preferably carried out at a lower operating temperature than with other oxidizing agents. For example, at a concentration of about 30% sul¬ furic acid, the temperature of the bath preferably should be less than 20"C. At concentrations of sulfuric acid higher than 30%, however, the operating temperature can be 20 U C. and higher. The cathodic treatment step is effective to reduce the passive layer (oxide layer) formed on the substrate during the anodic step. During this cathodic treatment, nascent hydrogen is produced which interacts with the oxide layer to condition it to receive the chromium electrodeposit as an adherent film. Suitably, the cathodic activation solution is an elec¬ trolyte, such as is provided by the anodic solution itself, or an acid or alkaline medium. An alkaline solution is preferred. A typical anodic activation solution comprises about 10 to 50% sulfuric acid, optionally with about 20 g/1 or more ot dichromate, and 180 g/1 or more " of magnesium sulfate. A typical cathodic solution comprises a strong sodium hydroxide solution. Preferably, between and after the activation steps, the current is turned off and the activated metal substrate is transferred to a rinsing vessel where it is rinsed free of the activation solution. After both activation steps, the thus-activated sub¬ strate is placed in the chromium electroplating bath and chromium metal is deposited thereon. While the process of the invention can be utilized for electrodepositing chromium -14- from any chromium bath, it is used most advantageously for forming adherent chromium deposits on cast iron substrates from a high energy efficient chromium bath, such as is described in U.S. Patent 4,472,249. Typical cast irons for use herein include nodular, grey and malleable cast irons. EXAMPLE 17 An anodic polarizing solution was prepared from a 30% sulfuric acid solution to which was added 24 g/1 of sodium dichromate and 216 g/1 of magnesium sulfate. A nodular cast iron substrate was placed in this solution and the substrate was treated anodically at 15 asd for 2 minutes at lb u . After rinsing, the substrate was subjected to a cathodic treatment in a strongly allcaline solution (50 g/I caustic) at 15 asd for 1 minute at 64°C. , and rinsed again. The thus-activated substrate then was placed m a high energy efficient chromium bath containing iodide ion and chromium was deposited thereon at 77.5 asd for 60 minutes. The chromium layer which formed exhibited excellent adherence to the activated cast iron and was bright, smooth and hard. In contrast, when only either one of the activation steps was used, very poor adhesion was obtained. EXAMPLE 18 The anodic polarizing solution consisted ot a 30% sulfuric acid solution containing 100 g/1 of chromic acid. The substrate was grey cast iron. The anodic treatment was carried out at 15 asd for 2 minutes at 16°C. , followed by rinsing. The cathodic treatment was performed in the strongly alkaline solution of Example 1 at 15 asd for 1 minute at 40°C. After rinsing, chromium was deposited at 77.5 asd for 15 minutes. Adherence of the chromium deposit to the activated substrate was excellent, and it was smooth, bright and hard. EXAMPLE 19 The two-step activation process ot Examples 1 and 2 was repeated except that the cathodic treatment was carried out in the polarizing solution itself. An excellent adhering deposit was obtained. EXAMPLE 20 The processes of Examples 1-3 were repeated with the cathodic step preceding the anodic step. The chromium deposit was observed to adhere poorly to the substrate.

Claims: WHAT IS CLAIMED IS: 1. A method ot electrodepositing chromium on a metal substrate such as ferrous metals, stainless steel or a cast iron substrate, particularly from a high energy efficient chromium bath, which contains a halide ion, characterized by forming an adherent chromium deposit on said substrate, by including the step of activating said' substrate by electro- lytically etching said substrate in an activation solution which is selected from: a) a substantially neutral solution; b) a mixture of 10 to 50% by volume of concentrated sulfuric acid, 0.25 to 2 moles ot added sulfate ion, and 3.5 x 10 -3 to 140 x 10-3 moles/l of an inorganic oxidizing agent; c) a mixture of chromic acid and chloride ion, optionally with bromide ion; preferably, wherein the chloride ion concentration is between 0.10M to 0.30M; and preferably wherein activation is carried out at a temperature of at least 40°C; and d) a anodic oxidizing polarizing solution, suitably concentrated sulfuric acid, chromium acid, a dichromate, a d sulfate, or mixtures thereof; optionally with an added inorganic salt suitably sodium, potassium or magnesium sulfate, or mix¬ tures thereof; suitably tor a time sufficient to effect polarization and before surface roughness ensues, and, sequentially, cathodically treating said substrate in an electrolyte solution. 2. The method according to claim 1, further character¬ ized in that in that the activation solution (1) has a pH of 5-10; and does not contain added sulfuric acid or chromic acid; but, optionally, may include one or more of the following additives: a buffer, an oxidizing agent, an attack metal compound, a complexor or a surfactant; the activation step is carried out for about 10 seconds to 10 minutes at a current density of about 0.1 asi to 10 asi at between room temperature and 50°C. ; and the method may include one or more of these additional steps: water rinsing after activa¬ tion and reverse chromium etching. 3. The method according to claim 1 further character¬ ized m that activation solution (2) contains an inorganic oxidizing agent selected from nitrate ion, permanganate ion, perchlorate ion, persulfate ion or molybdate ion? said activation, step is carried out by anodic etching for 10 seconds to 10 minutes at a current density of 1.5 to 155 2 amps/dm at a temperature of 10°C. to 40°C. 4. The method according to claim 1 further character¬ ized in that the substrate m activation solution (3) is anodically etched for 10 seconds to 10 minutes at a current density of 2.5 asi tor 1-3 minutes at a temperature of 55°C. to 60 e C.; followed by cathodic etching. 5. The method according to claim 1 further character¬ ized m that the substrate in activation solution (4) is anodically etched in a 10 to 30% sulfuric acid solution, optionally with chromic acid and/or sodium, potassium or magnesium sulfate, at a temperature below 20°C. , for l seconds to 10 minutes at a current density ot 15 to 45 asd.

Inventors: CHESSIN HYMAN, JONES ALLEN R, KORBACH WILLIAM C, MCMULLEN WARREN H, CHESSIN, HYMAN, JONES, ALLEN, R., KORBACH, WILLIAM, C., MCMULLEN, WARREN, H.
Applicants: M & T CHEMICALS INC, M&T CHEMICALS INC.
Application year: 1986
Main classification: C25D5
IPC codes: C25D5