CA1195645A - High-rate chromium alloy plating - Google Patents

Abstract

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 alloy (optionally further containing nickel and/or cobalt) coatings are plated on copper cathodes, for example, from trivalent chromium baths at 30 microns per minute and 160 A/dm2.
Current densities in the range of about 75-400 A/dm2 (5-26 A/in2) are most useful.

Description

HIGH-RATE CHROMIUM ALLOY PLATING
Back~round 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-5 cess which may form a corrosion- resistant coating of, say, 25 um 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 oE
hexavalent chromium, such as chromic acid. Unfortu~
nately, these baths, where chromium is complexed as an anion, are historically ineffective for plating al~
15 loys. Efforts at plating from divalent and/or tri valent chromium solutions have allowed the production of some alloy plate, but at low deposi~ion rates and typically at current densities below abo~t 1 A/in2 (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 25 chromium reduction potentials would be expectecl 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 _ 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 al~o an object to plate such alloy composition ~hich may substantially approximate the 35 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 ~hick, 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/dm2 (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 l 20 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/l to saturation diva--25 lent/trivalent chromium~ l-S0 g/l iron and 0-50 g/l nickel. Complexing anions of sulfuric, sulEarnic, hy-drochloricl phosphoric and boric acids are preferred in the electrolyte. When using insoluble anodes, a porous barrier is typically positioned around the cathode to 30 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 35 of chromium alloy may be obtained by strictly main-tainir.g the free acid of the electrolyte within a narrow range corresponding to a pH of about 1.7 to 1.8.

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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 aque-5 ous electroplating solution which comprises of from 20 g/l to saturation divalent/trivalent chromium ions, 1-50 g/l iron ions and 0 50 g/l 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 elec~rolyte 10 solution.

DescrLption 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 15 fall in the range (by weight) of 5-30~ chromium, 2~-95~
iron and 0~50~ total nickel and/or cobalt. We have found that alloys outside of this range may be pJated according to the invention, but for the desired cor-rosion-resistance of the coatings, at least about 5-10~
20 chromium is neces~sary. 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 25 stainless steels are preferred. Type 304 stainless (18~ Cr-8% Ni-~ Mn-balance Fe) is one desirable com-posi~lon. ~lowever, examples and discussion regarding chromium-irorl-nickel alloys are intended to include alloys wherein cobalt may be substituted, as known in 30 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 deposi~ed without harm. Man-ganese, silicon and copper are examples.
The alloy coatin~ is formed on a conventional 35 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/l to saturation of chromium ions, 1-50 9/l iron ions and a total of 0-50 g/l of nickel and/or cobalt ions. The trivalent chromium may be converted to the divalent 10 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 refererlce to trivalent chromium is also intended to include the lower ~e~e which coexists in the bath.
15 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 20 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 25 particular, these include the anions from the mineral acids: sulfuric, sulfamic, hydrochloric, phosphoric and boric acids.
The pH oE the electrolyte has been found to be a critical factor in depositing thiclc~ bright and 30 semi-bright coatings. Within the pH range of 0.5~2.0, good chromium alloy coatings can be deposited which are matte t~xtured, but which are still useful in some applications of corrosion and wear resistance These coatings are generally limited in thickness ~o about 12 35 to 25 ~m. 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 5 crack-free, even a~ thicknesses above 125 ~m. 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 10 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 15 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 usiny this titration method is about 0.5 ml - 1.5 ml NaOH, 20 corresponding to the pH of about 1.8 - 1.7, respect-ively.
The temperature of the plating solution is preferably in the range of 25-75C.

Operatin~ Conditions ~long with acidity, the most critical oper-ating parameters to obtaining craclc-free, adherent coatings are the current density and the agitation or solution flow. The acidity and solution flow particu-larly affect the depositiorlrate and the density oE the 30 coatinc3, but acidity does not significantly affect composition of the deposit except at very low pH where nickel and iron plati~g reactions decrease in effi-ciency. Compositio~ s~ m~rè particularly afected 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-famate solutions, even for high-chromium depositsO
Current densities for the inventive method lO 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 there-with. I'urbulent action near the cathode, resulting 20 from cathode motion or solution flow, creates a trans-port mechanism for replacing depleted electroly~e with cation-rich solution. Relative motion of at least l m/sec bet~een the cathode surface and the plating solution is generally sufficient to create the ~.urbu-25 lent conditions necessary for good deposits~ Typi-cally, velocities of l-~0 m/sec could be usedr but 1-8 m/sec i9 preferred.
With the agitation and other means Eor mi-gration of anode products to the region of the cathode, 30 :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 cathodeO Conventional porous mem-branes (ceramic cups) may be used around the cathode 3S for this purposeO

Examples of the Preferred Embodiments Example 1 - Iron-Chromium Alloy Composition Compa~able 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~l Cr) and 0.75 molar iron (42 g/l Fe). The current density was 160 amps/dm2 and the rod-shaped steel 15 cathode was rotated with a 2.5 m/sec surface velocity.
A lead anode was utilized and was isolated Erom the cathode by a porous alumina diaphragm. Temperatures were between about 37 and 49C.
Sample 43F used a 10 minute deposition at pH
20 1.6 while sample 52 plated for 5 minu~es at pH 1.7. In both cases the alloy composition weight ratio was substantially ~he 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 ~n the sulEamate bath. To avoid this in longer deposi~iolls, a platinum or graph-ite anode or, preEerably, a soluble anode could be us~ .

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 (0O4 moles~ and sulfuric 35 "free acid" ~1.3 ml)~ pH was measured at 1~75. A

~ 8--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 5 matte textured as the pH increased ~o about 1.8 (free acid of 0.5 ml). Sulfuric acid was added ~o bring the free acid to about 1.1 ml and adherent, bright plates were again deposited.
The kright plates were tested and found to be 10 extremely adherent, corrosion resistant to nitric ac.id and resistant to high-temperature oxidation. Hardness was on the order of 410 ~Knoop) with a 100 gram load, equiva:Lent to Vickers DPH=360 or Rockwell C ~ 39.

Example 3 - Preferred Alloy Composi~ions 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:

~0 Concentration Chem_cal Added As: (g/l) Chromium Chromium chloride 39 49 0 0 Chromium Chromium sulfate 0 0 ~6 39 25 Iron Ferrous sulfate 3 1 6.6 Nickel Nickelous sulEate 45 1]. 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~

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

Table_ _ _ _ Deposition Conditions Alloy Composition ~ _ . .
Sample pH C.D. Temp. Agita- Solu~ Cr Ni Fe No. (~/dm2) (C) tion tion (w/o) ~/o) (w/o) (rn/sec) No.
__ _ 203/4-13A 0.45 160 62 2 1 53O7 1.6 44.7 -14C 1.4 310 62 2 2 26.5 1.9 71.6 l9E 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 -20~ 160 50 2 4 31 7 62 The chromium content in the al:loy cleposit is dependent on several operating conditions, including current densi-ty, a~itation, pH, ratio of metal ions in solution and type of anion used to complex the metal ~0 ionC~. Comparing samples 13A and 14C, the difference in p~1 is the ma~or variable and the chromium content is higher when the pH was lower (higher acid content)O
This is reasonable because the coulombic (cathode) efficiency for plating both iron and nickel is known to 25 be poor at the lower pH values.
Samples 18D and 18F were plated under similar conditions with the exception of current density. The ~s~

results show ~hat 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 l9E and l9L, 5 the temperature was increased ~rom 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 temper-atures, higher pEI, higher concentration of chromium and lower concentrations of both nickel and iron, the alloy deposit may be pushed to a highe~ chromium and a lower 15 ni.ckel 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 sub~ect to cracking in thicker deposits.

Example 4 - Cr-Fe-Ni Alloy in Mixed Chloride/SulEate Bath Sample 202/98--14E was plated in a conven-tional cell using a soluble Type 304 stainless steel anode and a solution oE:

25 Chromium (chlori.de salt) 4808 g/l Iron (sulfate salt) 1.0 g/l ~ickel (sulfate salt)11.2 g/l ~ori.c acid 35.0 g/l Ammonium Sulfate 55.0 g/1 30 The temperature was 62C and the pH was 1.4.

With a ca~hode surface velocity of 2 m~sec and a current density of 155 amps/dm2~ a 125 ~m coatlng was applied in 30 minutesO The relatively dense coat-ing was matte textured on the surface but otherwise 5 generally crack free and had a compositi.on of 16 Cr-21 Ni-63 Fe.

Example 5 - Fe-Cr Alloy Coatings from Chloride Bath Iron-chromium alloy coatings were deposited 10 from an electrolyte solution of the chromium ~56 g/l) and iron (52 g/l) chloride salts at about 30C. I'he 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 15 conventionally defined as the percentage of the applied current used to deposit the chromium a].loy.

Table 2 Deposition Conditions Alloy Composition 20 ~ample Eiciency pH C.D. Agitation Cr Fe No. (A/dm2) m/sec 97A 5 0.83 0 2 98 96C 22 0.640 2.4 ~ 94 96B 36 0.680 2O4 8 92 25 96D 40 0.7160 2.4 18 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 p~ range.
Nevertheless, the effect of current density and agita-5 tion upon efficiency and the final alloy compositionwas 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 seén by observing 10 sample ~7~ wherein the efficiency and percent chromium in -the deposit were both low because of low current d~nsity and low agitation. The deposit was also limited to a very thin section hecause of poor adher-ence and cracking in tllicker deposits. Because the 15 coating was thin and low in chromium it had poor corrosion resistance.
Samples 96B, 96C and 96D were marginally cracked but were otherwise suitab]e coatings similar to conventional hard chromium plates deposited in 20 catalyzed chromic acid solutions. These cracks in the deposits may not be detrimental where wear resistance in the main property desired in a coating.

~xample 6 - Fe-Cr Alloy Deposits from Sulfate Baths A 30/70 chromium-iron anode was again used in a sul~ate solution to plate alloy coatings on a copper-coated, steel-ring cathode. I'he plating solu-.ion compositions were as follows:
Concentration, (g/l) Iron metal ions 2~ 14 21 Chromium metal ions 26 39 ~7 _13-Alloy coatings were deposited at 50C as shown in Table 3.

Table 3 . ~ ..__ ... . . _ .............. _ . ... _ Deposition Conditions Alloy Composition _ . . _ , . . _ _ Sample pH C~D. Agitation Solution Cr Fe No. (A/dm2) (m/sec)No.(w/o) (w/o~

.

61~ 1.5 160 2 1 20 80 62P 2.0 160 2 1 26 74 10 80D 1.9 230 3 2 24 76 80E 1.~ 310 3 2 48 52 80F 1.8 390 3 2 42 58 67C 1.7 160 3 2 65 35 11~ 1.8 160 3 3 29 71 15 llA 1.6:L 160 3 3 26 74 12A 1.745 160 3 3 32 68 12C ~.. 7~5 390 3 3 62 28 ~ ~._ _ _. . . = _, , _ _~ ==== =

Some early results (Samples 61-80) were tak-en before the importance of pH was ascertained. Hence, 20a pH meter without extreme accuracy was used. Later, the meter was replaced by a more accurate instrument.
Nevertheless, thin, matte coat:ings were obtained out-side of the preferred pH range using the divalent/-trivalent chromium electrolyte. These coatings may be 2smade with high chromium contents by use of the hiyh _14-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 5 sufficient accuracy in measuring pH and in maintaining that pH throughout the bathD When ~sing 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 10 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/dm2.

Claims (7)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A method for high rate electrodeposition of a chromium alloy coating on a cathode which com-prises electrolyzing an aqueous plating solution com-prising iron ions and both divalent and trivalent chromium ions at a current density of at least about 75 amps/dm2, 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/l to saturation of diva-lent and trivalent chromium ions, from about 1-50 g/l iron ions and from about 0-50 g/l 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 claim 1 which comprises maintaining the pH of the aqueous plating solution at between about 1.7 and 1.8.

6. The electrodeposition method of claim 5 wherein the anode is insoluble which further comprises preventing the oxidation of diva-lent and trivalent chromium near the cathode by inhibiting the migra-tion of oxidation agents to the cathode.

7. The electrodeposition method of claim 1 wherein the current density is between about 150 and 400 amps/dm2.