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Ask the Expert Question-and-Answer Archive
(Hard Chrome Plating)

by Larry Zitko, ChromeTech, Inc.
January, 2002

Stripping Chrome Plating

Q. Over time, the chemical composition of a chrome stripping bath has changed from 75 g/l NaOH and 0 g/l Na2CO3 to 38 g/l NaOH and 92 g/l Na2CO3. Also, the chrome part is 6 volts anodic in the bath, room temperature, in both cases. Would you expect the chrome stripping rate to decrease and why? I am seeking a scientific understanding what factors affect chrome plating stripping rates.

A. Hello Paul,

Thank you for posting your question to the "Ask The Expert" feature at the National Metal Finishing Resource Center website.

If I understand you correctly, the new strip bath was made up of about 10 oz/gal of sodium hydroxide and no sodium carbonate. As the bath ages, you detect a decrease in the sodium hydroxide, and you think you see a large formation of sodium carbonate.

I'm not a chemist by trade, but it is my understanding that the alkaline electrostripping of chromium, in a medium containing sodium, creates the soluble compound sodium chromate, Na2CrO4, in the bath as metallic chromium is removed from the workpiece. Sodium (from the NaOH) is utilized in this process. Thus it is commonplace to make regular bath additions of NaOH to replenish the sodium ions needed for the formation of sodium chromate during subsequent stripping operations.

I have read one text that refers to a conversion of sodium hydroxide to carbonate during electrostripping, but I can't find supporting data. I'm wondering if part of the 92 g/L of Na2CO3 is really a quantity of Na2CrO4 instead. Perhaps you have a qualified chemist at CTC to verify this. When we analyze strip baths here at ChromeTech, we typically express the buildup of chromium "as Cr" for convenience.

The buildup of the chromate is detrimental to the process, and without purification, the bath must ultimately be discarded and remade. Slow stripping rates, high energy consumption and damage to the part substrates are the warning signs.

Stripping baths made of various blends of sodium hydroxide and sodium carbonate are commonplace in our industry.

I hope this information has helped.

Additional response from Mike McGinness: I noticed your comments about sodium hydroxide and sodium carbonate. Sodium hydroxide will readily react with CO2 in the air, or in solution with water to form carbonic acid (H2O + CO2 = H2CO3). The carbonic acid then immediately reacts with any strong base (OH) such as sodium hydroxide to form (sodium...) Bi-carbonate (H2CO3 + NaOH = NaHCO3 + H20, the in a second step it reacts (NaHCO3 + NAOH = NA2CO3 + H20). There is not much one can do about it except avoid using air agitation in a caustic bath.

Follow-up comments:

Thank-you for your help, Larry!

Yes, my understanding had been that the carbonate buildup had been originating from the air agitation. Here is my current understanding of the contribution of carbonates to stripping rate slowing:

The facility electrostrips hard chromium plating from steel parts, for its refurbishment, according to MIL-STD-871, solution #6 (68 to 82 grams sodium hydroxide per liter of water, at 6 volts anodic, and room temperature - 60 - 90 degrees F). This process (also known as reverse current stripping, as well as anodic stripping) oxidizes the metallic chrome plating to hexavalent chromium. The process requires either part or bath agitation so that the chemicals are mixed at the part surface. These chemicals transport the hexavalent chromium from the surface of the part and into the bulk fluid. The hydroxide anion complexes the chromate from the part surface, and dissolves it into the solution as sodium chromate (Na2CrO4), which gives the solution a distinctive yellow color. Although the exact mechanism of this reaction could not be found in the literature, the following pathway is possible, leading to the final products:

Cathode Reaction: 6H2O + 6e- = 6OH- + 3H2(g)
Anode Reaction: Cr(s) = Cr+6 + 6e-
Hydroxide Reaction: Cr+6 + 8OH- + 2Na+ = Na2CrO4(aq.) + 4H2O
Overall: Cr(s) + 2H2O + 2NaOH(aq.) = Na2CrO4(aq.) + 3H2(g)

Good agitation of the hard chromium plated parts and/or solution is necessary, because pockets or blind holes containing hexavalent chromium can react into its fairly strong acid form [i.e., Cr+6 + 6OH- = H2CrO4(aq.) + 2H2O, with PKa 0.74] which can cause severe etching of the basis metals. Sodium carbonates can act as a buffer to reduce the total impact of this reaction. The build-up of other salts besides chromates, such as carbonates will reduce stripping rates [Guffie, Robert K., The Handbook of Hard Chromium Plating, Gardner Publications, Cincinnati, Ohio, 1986]. Note that carbonates are not involved in the reaction, and are meant only to inhibit the potential for etching the basis metal, which would slow the stripping rate.

The stripped chromium dissolves into solution, thus changing the bath salt concentration. The buildup of chromate salts is detrimental to the process, and without purification, the bath must ultimately be discarded and remade. The warning signs are slow stripping rates, high-energy consumption, and damage to the part substrate [reference personal communication with Larry Zitko of ChromeTech Inc., on 08 January 2002]. As the time spent to strip the chrome plating increases, the likelihood for uneven stripping occurs. While the work is progressing to anodically strip the remaining plating, the exposed basis metal areas are at risk of etching to an unacceptable degree. Hard steels can withstand the etching action of anodic stripping longer than unhardened steels.

A caustic solution, containing 75 grams per liter of sodium hydroxide, used for the stripping of chromium plating, has a pH of 14.0. This solution, while in use, is being mixed by air, which contains small amounts (350 ppm max.) of carbon dioxide (CO2). The CO2 present in the air reacts with the sodium hydroxide in the caustic solution and converts it into sodium carbonate over a period of time:

2NaOH(aq.) + CO2(g) = Na2CO3(aq.) + H2O

Because of the above chemical reactions (with carbon dioxide as well as chromium), the amount of sodium hydroxide in the stripping solution depletes progressively. CTC used EPA and Standard Methods to analyze the grab sample from a hard chromium stripping tank. The analytical results indicate the hydroxyl alkalinity of the stripping solution dropped as shown by the sample solution's pH (13.2), which now contains only 37.7 grams per liter of the sodium hydroxide left in the stripping solution. The stripping rate, therefore, also drops.

Based upon anecdotal data in the literature, CTC suggests that the presence of carbonate (92.2 grams/liter in the sample solution) is also lowering the stripping rate, but is not certain of its significance. CTC would, therefore, recommend that a bench-scale test may be performed to determine the effect of various concentrations of sodium carbonate in the caustic stripping solution on the rate of reverse current stripping chromium plating.

Also, under the highly alkaline conditions, and with time, some of the chromium will reduce valence to trivalent chromium and thereby precipitate to the bottom of the tank as hydroxide sludge. As the bottom of the tank fills with precipitants, the working volume of the bath decreases. This affects the rate of bath make-up and ultimately restricts the size of the part to be anodically stripped.




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