This invention relates to the high-speed plating of chromium alloys.

Extensive use of relatively scarce materials, such as nickel and chromium, in corrosive environments may be reduced by an acceptable plating process which may form a corrosion-resistant coating of, say, 25 µm of a chromium alloy, on an inexpensive substrate, such as steel or brass. A bright, decorative 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. Unfortunately, these baths, where chromium is complexed as an anion, are historically ineffective for plating alloys. Efforts at plating from divalent and/or trivalent chromium solutions have allowed the production of some alloy plate, but at low deposition rates and typically at current densities below about 1 Alin² (15 Aldm²), 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.

US-A-3,917,517 discloses hypophosphite containing chromium alloys electroplating baths providing chromium platings at current densities of up to 100 A/dm^2. US-A-4,142,948 discloses chromium electroplating baths containing trivalent chromium and operating at pH below 5 and at current densities up to 100 Aldm² due to the presence, as additives, of amino- or hydroxy-substituted carboxylic acids.

It 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 difference 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 chromium.

It is finally an object that such process be controllable to yield a thick, dense chromium alloy deposit.

In accordance with the objectives, the invention disclosed in claim 1 is a method for high-rate plating of chromium alloy from an electrolyte solution containing divalent and trivalent chromium ions, ions of iron and, optionally, ions of nickel and/or cobalt as additional alloying constitutents. The high-rate plating is carried out at a current density of at least about 75 Aldm² (preferably at least about 150 A/dm2), a pH of between about 0.5 and 2.0 and with relative motion between the cathode and the plating solution of at least about 1 m/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/I to saturation divalent/trivalent chromium, 1-50 g/I iron and 0-50 g/I nickel. Complexing anions of sulfuric, sulfamic, hydrochloric, 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 maintaining 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 is a method for electroplating a chromium alloy containing iron and, optionally, nickel 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 corrosion-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 compositions 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 composition. 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. Manganese, 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 inventive process.

The electrolyte is a divalent/trivalent chromium salt solution preferably containing 20 g/I to saturation of chromium ions, 1-50 g/I iron ions and a total of 0-50 g/I 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 deposition 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 µm. Thicker coatings tend to crack or peel as a result of increasing internal stresses.

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 pm. 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 difficulty 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 electrolyte 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, respectively.

The temperature of the plating solution is preferably in the range of 25­75°C.

Along with acidity, the most critical operating parameters to obtaining crack-free, adherent coatings are the current density and the agitation or solution flow. The acidity and solution flow particularly 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 efficiency. 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 sulfamate solutions, even for high-chromium deposits.

Current densities for the inventive method are at least 75 amps/dm², but preferably within the range of about 150-400 amps/dm². 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 therewith. Turbulent action near the cathode, resulting from cathode motion or solution flow, creates a transport 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 turbulent conditions necessary for good deposits. Typically, velocities of 1-80 m/sec could be used, but 1-8 m/sec is preferred.

With the agitation and other means for migration 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 membranes (ceramic cups) may be used around the cathode for this purpose.

According to the invention, an alloy may be 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 and chromium sulfamate electrolyte was made by dissolving the metals in an acid solution of sulfamic acid. The concentrations were 0.25 molar chromium (13 g/I Cr) and 0.75 molar iron (42 g/I Fe). The current density was 160 amps/dm² and the rod-shaped steel 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 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%.

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 graphite anode or, preferably, a soluble anode could be used.

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 "free acid" (1.3 ml). pH was measured at 1.75. A lustrous deposit having a few matte spots was plated at 160 amps/dm² 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.

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:

A Type 304 stainless steel alloy anode was used. Operating parameters are given in Table 1.

Manganese content in the alloy samples was less than 1% and is, therefore, not reported.

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 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 coulombic (cathode) efficiency for plating both iron and nickel is known to 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, 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.

It is evident that by making several changes in the plating parameters, for example, lower temperatures, 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 nickel content.

Generally, good bright and semi-bright coatings 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.

Sample 202/98-14e was plated in a conventional cell using a soluble Type 304 stainless steel anode and a solution of:

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/dm², a 125 µm coating was applied in 30 minutes. The relatively dense coating was matte textured on the surface but otherwise generally crack free and had a composition of 16 Cr-21 Ni-63 Fe.

Iron-chromium alloy coatings were deposited from an electrolyte solution of the chromium (56 g/I) and iron (52 g/I) 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.

These samples were made prior to our recognition 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 agitation 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 adherence 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.

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 solution compositions were as follows:

Alloy coatings were deposited at 50°C as shown in Table 3.

Some early results (Samples 61-80) were taken 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 outside 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 using 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 current density, e.g. from 32 to 62% Cr with an increase in current density from 160 to 390 amps/dm².