HIGH-RATE CHROMIUM ALLOY PLATING

Abstract: High-speed plating of corrosion-resistant, chromium alloy coatings from divalent/trivalent chromium solutions is feasible with high current densities, rapid solution flow and careful control of pH. Chromium-iron-nickel alloy coatings are plated on copper cathodes, for example, from trivalent chromium baths at 30 microns per minute and 160 A/dm<s2>s. Current densities in the range of about 75-400 A/dm<s2>s (5-26 A/in<s2>s) are most useful.

Description: HIGH-RATE CHROMIUM ALLO •PLATING Background of the Invention Extensive use of relatively scarce materi¬ als / such as nickel and chromium/ in corrosive envi¬ ronments may be reduced by an acceptable plating pro- cess which may form a corrosion- resistant coating of, say, 25 μ of a chromium alloy, on an inexpensive substrate, such as steel or brass. A bright, decora¬ tive coating of chromium alloy is also valued in some uses. In the past, most commercial plating of bright chromium has been carried out from solutions of hexavalent chromium, such as chromic acid. Unfortu¬ nately, these baths, where chromium is complexed as an anion, are historically ineffective for plating al- loys. Efforts at plating from divalent and/or tri¬ valent chromium solutions have allowed the production of some alloy plate, but at low deposition rates and typically at current densities below about 1 A/in*-*-* (15 A/dm2) , and often much lower. Moreover, in electrodepositing an alloy from a solution containing metal ions, it is well known that a less active metal will deposit in preference to a more active metal. Considering chromium alloys containing iron and/or nickel, the relative nickel, iron and chromium reduction potentials would be expected to result in deposits which are rich in nickel and iron. The chromium is clearly more active with a potential of about -0.74 volts for the Cr +3 to Cr° reduction. Summary of the Invention I is an object of the present invention to provide a method of plating chromium alloy with iron and, optionally nickel and/or cobalt. It is also an object to plate such alloy composition which may substantially approximate the metal ratio in the electrolyte, in spite of the differ¬ ence in activity of the metals. It is also an object to provide a high-rate plating process for chromium alloy. It is further an object to provide such electrodeposition process for producing chromium alloy from solutions comprising divalent and trivalent chro¬ mium. It is finally an object that such process be controllable to yield a thick, dense chromium alloy deposit. In accordance with the objectives, the in¬ vention is a method for high-rate plating of chromium alloy from an electrolyte solution containing divalent and trivalent chromium ions, ions of iron and, option¬ ally, ions of nickel and/or cobalt as additional al- loying constituents. The high-rate plating is carried out at a. current density of at least about 75 A/dm 2 (preferably at least about 150 A/dm 2 ) , a pH of between about 0.5 and 2.0 and with relative motion between the cathode and thre plating solution of at least about 1 /sec (preferably 1-8 m/sec) . Deposits of composition 5-80% (by weight) chromium, 20-95% iron and 0-50% nickel are preferably formed by electrolyzing an electrolyte solution having metal ion concentrations of 20 g/1 to saturation diva- lent/trivalent chromium, 1-50 g/1 iron and 0-50 g/1 nickel. Complexing anions of sulfuric, sulfamic, hy¬ drochloric, phosphoric and boric acids are preferred in the electrolyte. When using insoluble anodes, a porous barrier is typically positioned around the cathode to prevent migration of anode oxidation reaction products to the cathode where they would otherwise oxidize the divalent/trivalent chromium to the hexavalent state. Within the general conditions stated above, the inventor has also discovered that the best deposits of chromium alloy may be obtained by strictly main¬ taining the free acid of the electrolyte within a narrow range corresponding to a pH of about 1.7 to 1.8. Very accurate metering must be used to monitor pH or a titration may be necessary to establish the amount of free-acid in the bath. The invention also comprises the novel ague- ous electroplating solution which comprises of from 20 g/1 to saturation divalent/trivalent chromium ions, 1-50 g/1 iron ions and 0-50 g/1 total nickel ions, with a pH adjusted to between about 1.7 and 1.8. Complexing anions of mineral acids may be used in the electrolyte solution. Description of the Invention The invention is a method for electroplating a chromium alloy containing iron and, optionally, nick¬ el and/or cobalt. The alloy compositions preferably fall in the range (by weight) of 5-80% chromium, 20*95% iron and 0-50% total nickel and/or cobalt. We have found that alloys outside of this range- may be plated according to the invention, but for the desired cor¬ rosion-resistance of the coatings, at least about 5-10% chromium is necessary. Chromium and nickel contents above the preferred range unduly raise the cost of the alloys and are, therefore not preferred. Chromium- nickel-iron alloys are the preferred coating composi¬ tions and, in particular, the 300 and 400 classes of stainless steels are preferred. Type 304 stainless (18% Cr-8% Ni-2% Mn-balance Fe) is one desirable com¬ position. However, examples and discussion regarding chromium-iron-nickel alloys are intended to include alloys wherein cobalt may be substituted, as known in the art, for all or a portion of the nickel. Other impurities which may enter the deposit from the anode, for example, may also be deposited without harm. Man¬ ganese, silicon and copper are examples. The alloy coating is formed on a conventional cathode surface of, for example, steel, iron, aluminum, brass or copper. Insoluble anodes, such as made from lead, may be used, although soluble alloy anodes of iron and chromium have been most useful in the inven¬ tive process. Plating Solution The electrolyte is a divalent/trivalent chromium salt solution preferably containing 20 g/1 to saturation of chromium ions, 1-50 g/1 iron ions and a total of 0-50 g/1 of nickel and/or cobalt ions. The trivalent chromium may be converted to the divalent form and vice versa .so that the exact ratio thereof was not clearly identified. Therefore, the two species are believed to both be present and necessary, and the reference to trivalent chromium is also intended to include the lower specie which coexists in the bath. Excess divalent form can adversely-affect nickel depo¬ sition because it tends to reduce the nickel ions to the metal, resulting in precipitation or plating on the walls, etc. of the cell. Some electrolyte solutions require a period of stabilization before yielding superior product. This may be due to a need to produce some particular minimum quantity of divalent chromium in the bath. Conventional complexing anions for chromium plating are also necessary in the inventive method. In particular, these include the anions from the mineral acids: sulfuric, sulfamic, hydrochloric, phosphoric and boric acids. The pH of the electrolyte has been found to be a critical factor in depositing thick, bright and semi-bright coatings. Within the pH range of 0.5-2.0, good chromium alloy coatings can be deposited which are matte textured, but which are still useful in some applications of corrosion and wear resistance. These coatings are generally limited in thickness to about 12 to 25 jam. Thicker coatings tend to crack or peel as a result of increasing internal stresses. OMP It has been found, however, that when the acidity of the electrolyte corresponds to a pH of between about 1.7 and 1.8, bright and semi-bright coatings can be obtained which are adherent, dense and crack-free, even at thicknesses above 125 jam. The reason for -this phenomenon is not understood at this point, but the result is dramatic over this range. The acidity range is so narrow that diffi¬ culty may be encountered in accurately measuring and maintaining it throughout the solution. Certainly, sensitive instruments exist for measuring the pH, and in practice a pH meter might be used for convenience. However, for accuracy, we prefer to determine the acidity by measuring the amount of "free acid" by titration against a standard basic solution. We define the "free acid" content as the quantity of 0.1 N NaOH solution needed to bring a 1.0 ml aliquot of electro¬ lyte to pH 3.5. The preferred range of free acid using this titration method is about 0.5 ml - 1.5 ml NaOH, corresponding to the pH of about 1.8 - 1.7, respect¬ ively. The temperature of the plating solution is preferably in the range of 25-75°C. Operating Conditions Along with acidity, the most critical oper¬ ating parameters to obtaining crack-free, adherent coatings are the current density and the agitation or solution flow. The acidity and solution flow particu¬ larly affect the deposition rate and the density of the coating, but acidity does not significantly affect composition of the deposit except at very low pH where nickel and iron plating reactions decrease in effi¬ ciency. Composition is more particularly affected by the current density and the electrolyte composition. It is well understood that the least active metal will deposit in preference to a more active metal. But in the inventive method, using high current density and solution flow, the composition of the deposit can be made to more closely approximate the electrolyte composition than in prior plating methods, especially for the iron-chromium binary alloy from sul- fa ate solutions, even for high-chromium deposits. Current densities for the inventive method are at least 75 amps/dm 2 , but preferably within the range of about 150-400 amps/dm 2 . The higher current densities favor deposition of chromium over the iron or other metals and are necessary for obtaining the high- chromium alloys from the trivalent chromium solutions. At such high current, the chromium, iron and particularly the nickel or cobalt, would be hard to plate in dense, adherent deposits were it not for high agitation or solution flow rates in conjunction there¬ with. Turbulent action near the cathode, resulting from cathode motion or solution flow, creates a trans¬ port mechanism for replacing depleted electrolyte with cation-rich solution. Relative .motion of at least 1 m/sec between the cathode surface and the plating solution is generally sufficient to create the turbu- lent conditions necessary for good deposits. Typi¬ cally, velocities of 1-80 m/sec could be used, but 1-8 m/sec is preferred. With the agitation and other means for mi¬ gration of anode products to the region of the cathode, it may be necessary to erect a barrier between an insoluble anode and the cathode to prevent the anode products from oxidizing the divalent and trivalent chromium near the cathode. Conventional porous mem¬ branes (ceramic cups) may be used around the cathode for this purpose. Exa ples of the Preferred Embodiments Example 1 - Iron-Chromium Alloy Composition Comparable to Bath Composition According to the invention, an alloy may be 5 deposited having a composition ratio virtually the same as the metal ratio in the electrolyte, despite the difference in reduction potentials of the chromium and iron plating reactions. In samples identified as 43F and 52A, an iron 10. and chromium sulfamate electrolyte was made by dis¬ solving the metals in an acid solution of sulfamic acid. The concentrations were 0.25 molar chromium (13 g/1 Cr) and 0.75 molar iron (42 g/1 Fe) . The current density was 160 amps/dm 2 and the rod-shaped steel 5 cathode was rotated with a 2.5 m/sec surface velocity. A lead anode was utilized and was isolated from the cathode by a porous alumina diaphragm. Temperatures were between about 37° and 49°C. Sample 43F used a 10 minute deposition at pH 0 1.6 while sample 52 plated for 5 minutes at pH 1.7. In both cases the alloy composition weight ratio was substantially the same as the electrolyte, 72 Fe - 28 Cr and 75 Fe - 25 Cr (+3%) respectively. Cathode efficiencies were about 26 - 27%. 5 At the end of the deposition, the lead anode showed signs of dissolving in the sulfamate bath. To avoid this in longer depositions, a platinum or graph¬ ite anode or, preferably, a soluble anode could be used. 0 Example 2 - pH Effects Recognition of the importance of pH occurred when plating several 47 mm-diameter rings (as cathodes) in succession in a bath containing chromium sulfate (0.9 moles) , iron sulfate (0.4 moles) and sulfuric 5 "free acid" (1.3 ml). pH was measured at 1.75. A __ OMP lustrous deposit having a few matte spots was plated at 160 amps/dm 2 and 3 m/sec cathode surface velocity. Deposits on successive carbon—steel rings improved to almost full bright plate and then began getting more matte textured as the pH increased to about 1.8 (free acid of 0.5 ml). Sulfuric acid was added to bring the free acid to about 1.1 ml and adherent, bright plates were again deposited. The bright plates were tested and found to be extremely adherent, corrosion resistant to nitric acid and resistant to high-temperature oxidation. Hardness was on the order of 410 (Knoop) with a 100 gram load, equivalent to Vickers DPH=360 or Rockwell C = 39. Example 3 - Preferred Alloy Compositions in Chloride and Sulfate Baths. A number of coatings were applied to 12.5 mm-diameter steel rods,.25 mm long, from sulfate and mixed sulfate/chloride solutions having the following compositions: Concentration Chemical Added As: (g/1) Chromium Chromium chloride 39 49 0 0 Chromium Chromium sulfate 0 0 26 39 Iron Ferrous sulfate 3 1 6. ,6 1 Nickel Nickelous sulfate 45 11 11 5. ,6 Boric Acid Boric acid 35 35 23 35 Ammonia Ammonium sulfate 14 14 8. ,5 13 Manganese Manganous sulfate 0 0 2. ,2 3. ,3- A Type 304 stainless steel alloy anode was used. Operating parameters are given in Table 1. OM Manganese content in the alloy samples was less than 1% and is, therefore, not reported. Table 1 Deposition < Conditions Alloy 5 ' Ccmposition Sample pH CD. Temp. Agita¬ Solu¬ Cr Ni Fe No. (A/dm 2 ) <°C) tion tion (w/o) Wo) (w/ (πv/sec) No. 03/4-13 A 0.45 160 62 2 1 53.7 1.6 44 -14C 1.4 310 62 2 2 26.5 1.9 71 -19E 1.8 160 62 2 3 21 22 57 -18D 1.5 160 62 2 3 4 41 55 -18F 1.65 220 62 2 •3 16 36 48 -19L 1.8 160 65 2 3 8 48 44 -20G 1.8 160 50 2 4 31 7 62 The chromium content in the alloy deposit is dependent on several ' operating conditions, including current density, agitation, pH, ratio of metal ions in solution and type of anion used to complex the metal 0 ions. Comparing samples 13A and 14C, the difference in pH is the major variable and the chromium content is higher when the pH was lower (higher acid content) . This is reasonable because the coulo bic (cathode) efficiency for plating both iron and nickel is known to 5 be poor at the lower pH values. Samples 18D and 18F were plated under similar conditions with the exception of current density. The results show that the higher current density used for sample 18F resulted in a higher chromium content. Temperature also affects the percentage of chromium in a deposit. Comparing samples 19E and 19L, 5 the temperature was increased from 62 to 65C and the chromium content in the deposit was reduced from 21 to 8 percent. In general, the temperature does not appear to be quite this critical, but higher temperatures do not favor the chromium deposition. 0 It is evident that by making several changes in the plating parameters, for example, lower temper¬ atures, higher pH, higher concentration of chromium and lower concentrations of both nickel and iron, the alloy deposit may be pushed to a'higher chromium and a lower 5 nickel content. Generally, good bright and semi-bright coat¬ ings were obtained in the deposits plated between about pH 1.7 and pH 1.8 while the others were matte textured • and subject to cracking in thicker deposits. 0 Example 4 - Cr-Fe-Ni Alloy in Mixed Chloride/Sulfate Bath Sample 202/98-14E was plated in a conven¬ tional cell using a soluble Type 304 stainless steel anode and a solution of: 5 Chromium (chloride salt) 48.8 g/1 Iron (sulfate salt) 1.0 g/1 Nickel (sulfate salt) 11.2 g/1 Boric acid 35.0 g/1 .Ammonium Sulfate 55.0 g/1 0 The temperature was 62°C and the pH was 1.4 With a cathode surface velocity of 2 m/sec and a current density of 155 amps/dm 2 , a 125 jam coating was applied in 30 minutes. The relatively dense coat¬ ing was matte textured on the surface but otherwise generally crack free and had a composition of 16 Cr-21 Ni-63 Fe. Example 5 - Fe-Cr Alloy Coatings from Chloride Bath Iron-chromium alloy coatings were deposited from an electrolyte solution of the chromiuπ (56 g/1) and iron (52 g/1) chloride salts at about 30°C. The apparatus of example 1 was used (with the exception of a soluble 30/70 chromium-iron anode) to plate the alloy coatings shown in Table 2. Cathode efficiency is conventionally defined as the percentage of the applied current used to deposit the chromium alloy. Table 2 Depo;sition Conditions Alloy Composition Sample Ξfficiency pH CD. Agitation Cr Fe No. (A/dm 2 ) m/sec 97A 5 0.8 3 0 2 98 96C 22 0.6 40 2.4 6 94 96B 36 0.6 80 2.4 8 92 96D 40 0.7 160 2.4 IS 82 These samples were made prior to our recog¬ nition of the importance of pH and they are within our broad range, but outside of our preferred pH range. Nevertheless, the. effect of current density and agita- tion upon efficiency and the final alloy composition was clearly shown, wherein the chromium content and efficiency of the deposit were proportional to the current density. The importance of using high current densities and agitations can be seen by observing sample 97A wherein the efficiency and percent chromium in the deposit were both low because of low current density and low agitation. The deposit was also limited to a very thin section because of poor adher¬ ence and cracking in thicker deposits. Because the coating was thin and low in chromium it had poor corrosion resistance. Samples 96B, 96C and 96D were marginally cracked but were otherwise suitable coatings similar to conventional hard chromium plates deposited in catalyzed chromic acid solutions. These cracks in the deposits may not be detrimental where wear resistance in the main property desired in a coating. Example 6 - Fe-Cr Alloy Deposits from Sulfate Baths A 30/70 chromium-iron anode was again used in a sulfate solution to plate alloy coatings on a copper-coated, steel-ring cathode. The plating solu¬ tion compositions were as follows: Concentration, (g/1) 1 1 1 Iron metal ions 28 14 21 Chromium metal ions 26 39 47 Alloy coatings were deposited at 50°C as sho in Table 3. Table 3 Deposition Conditions Alloy Composition Sample pH . CD. Agitation Solution Cr Fe No. (A/dm 2 ) (m/sec) No. (w/o) (w/o) 61F 1.5 160 2 1 20 80 62P 2.0 160 2 1 26 74 80D 1.9 230 3 2 24 76 80E 1.9 310 3 2 48 52 80F 1.8 390 3 2 42 58 67C " 1.7 160 3 2 65 35 11B 1.8 160 3 3 29 71 11A 1.61 160 3 3 26 74 12A 1.745 160 3 3 32 68 12C 1.745 390 3 3 62 28 Some early results (Samples 61-80) were tak¬ en before the importance of pH was ascertained. Hence, a pH meter without extreme accuracy was used. Later, the meter was replaced by a more accurate instrument. Nevertheless, thin, matte*coatings were obtained out¬ side of the preferred pH range using the divalent/- trivalent chromium electrolyte. These coatings may be made with high chromium contents by use of the high current densities and agitation. Results were not always consistent when us¬ ing the less accurate pH meter as can be seen in the Table 3, however, we attribute this to the lack of sufficient accuracy in measuring pH and in maintaining that pH throughout the bath. When using the more accurate meter and when within the preferred pH range it may be seen that good control of the process can be had. For example, in samples 12A and 12C, the pH was within the preferred ' range and the chromium content of the deposit was increased greatly with increasing cur¬ rent density, eg. from 32 to 62% Cr with an increase in current density from 160 to 390 amps/dm 2 . - OM

Claims: I claim : 1. A method for high rate electr'odeposition of a chromium alloy coating on a cathode which com¬ prises electrolyzing an aqueous plating solution com- prising iron ions and divalent and trivalent chromium ions at a current density of at least about 75 amps/dm 2 , a pH of between about 0.5 and 2.0 and with relative motion between the cathode and the aqueous plating solution at the cathode surface of at least about 1 m/sec. 2. The electrodeposition method of claim 1 which comprises electrolyzing the aqueous plating so¬ lution further comprising additional alloying metal ions selected from nickel and/or cobalt. 3. The electrodeposition method of claim 2 for producing a chromium alloy coating consisting es-* sentially of 5-80 weight percent chromium, 20-95 weight percent iron and 0-50 weight percent nickel which comprises electrolyzing an aqueous plating solution comprising of from about 20 g/1 to saturation of diva¬ lent and trivalent chromium ions, from about 1-50 g/1 iron ions and from about 0-50 g/1 nickel ions. 4. The electrodeposition method of claims 1 or 2 comprising electrolyzing the aqueous, plating so- lution which further comprises complexing anions of mineral acids selected from sulfuric, sulfamic, hydro¬ chloric, phosphoric and boric acids. 5. The electrodeposition method of claims 1 or 2 which comprises maintaining the pH of the aqueous plating solution at between about 1.7 and 1.8. -16- 6. The electrodeposition method of claim 5 wherein the anode is insoluble which further comprises preventing the oxidation of divalent and trivalent chromium near the cathode by inhibiting the migration of oxidation agents to the cathode. 7. The electrodeposition method of claim 1 wherein the current density is between about 150 and 400 amps/dm 2 . 8. An aqueous chromium alloy electroplating solution which comprises of from 20 g/1 to saturation divalent and trivalent chromium ions, 1-50 g/1 iron ions and 0-50 g/1 nickel ions, and wherein the pH of the solution is within the range of 1.7 and 1.8. 9. The aqueous chromium alloy electroplat- ing solution of claim 8 which further ' includes com¬ plexing anions ofmineral acids selected from sulfuric, sulfamic, hydrochloric, phosphoric and boric acids. OMPI

Inventors: SCHAER GLENN R
Applicants: BATTELLE DEVELOPMENT CORP, BATTELLE DEV CORP
Application year: 1982
Main classification: C25D3
IPC codes: C25D3, C25D5