Southern Metal Finishing

Re: Troubleshooting Hexavalent Chromium Baths

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Troubleshooting Hexavalent Chromium Baths   
By: Shane Moore of Scovill Fasteners, Clarksville, GA
Many defects and difficulties often arise when plating hexavalent chromium.  Many of these defects can often be prevented with routine maintenance of the plating solution.

First I will begin with the basics.  The concentrations of the chromic acid, sulfate, trivalent chrome and secondary catalyst (if applicable) must be analyzed frequently.  After the analysis is complete check to ensure that they (including the chromic acid/sulfate ratio) are within the recommended operating parameters of the technical data sheets.  If they are not in compliance, make the necessary adjustments.

Next, a chromium solution will often build chlorides, usually from poor rinse water and drag in from a nickel tank rinse (containing Nickel Chloride) a hydrochloric acid activate rinse, or a woods nickel strike rinse.  To remove chloride contamination from a chromium bath, dummy plate at 100-ASF with an anode/ cathode ratio of 1:1.  Make sure your ventilation is adequate as the chlorides will be driven out of the plating solution as the chloride gas.  Analyze the plating solution for the chloride concentration after a few hours of this electrolysis.

Another contaminant to a chromium bath is an excessive amount of trivalent chromium.  The recommended procedure for reducing the trivalent concentration  is by dummy plating with a large anode to a small cathode ratio (approximately 10:1).  This re-oxidizes the trivalent chromium back into hexavalent chromium as a result of the oxygen being generated at the anode surface.

Both of the above conditions (trivalent chromium increase and chloride contamination) can be dramatically reduced with the frequent use of a porous pot.  Ask you local equipment distributor (CPC) for more information on this equipment.

Now a word about anodes.  Typically the anodes should be made of lead, alloyed with either 6% tin, or 6% antimony.   Some anodes contain an alloy of all three elements, but the former is the most common.  The anode surface should be checked periodically for yellow scale build up.  This yellow chromate film normally forms during inactive periods. Eventually, this build up will severely decrease the plating speed of the bath and will need to be cleaned.

A few other preventative maintenance procedures that I will mention include checking to ensure you are operating at the correct temperature and current density, observing the meters on the rectifier to make sure you have no interruptions of current, periodic cleaning of all contacts such as anodes and cathodes as well as the hooks.  It is always a good idea to dummy plate for 5 minutes at mid - high current before the beginning of each shift.

Article Contributed By: Shane Moore (Scovill Fasteners)

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rajaram60

Re: Troubleshooting Hexavalent Chromium Baths

Dear Mr Shane Moore
Your article troubleshooting in Hard chrome is very useful.being a hard chrome plater I am yet to learn to remove chloride contamination.Can you expand 100ASF in your above mentioned article (removal of chloride)Does this process completely remove chloride.Please do help me.
Rajaram(rajaram60@hotmail.com)

Labrat

Re: Troubleshooting Hexavalent Chromium Baths

This electrolysis process does in fact remove all of the chloride contamination. I have used this process in the past and was always successful. By 100 ASF, I am referring to 100 amps per square feet of cathode surface area. For example, I normally removed all of the anodes from the plating tank, except for one on each anode bar. I then positioned a small flat stainless steel part hanging from the cathode bar, approximately 4" x 16" being submerged in solution. Total area of this part is approximately 0.9 square feet (4x16)x2/144, so requires 90 amps. Check the chloride concentration (mercuric chloride titration method will suffice) before beginning the electrolysis treatment, then again four hours later. The progression will allow you to calculate the time needed to complete the chloride removal.

rajaram60

Re: Troubleshooting Hexavalent Chromium Baths

Dear member(Mr Labrat)

Many many thanks for your reply on removal of chloride from hard chrome baths.Your reply was very informative and I am sure it will help me a lot since I was quarantaining the chloride contaminated acids.
With regards
Rajaram60

rajaram60

Re: Troubleshooting Hexavalent Chromium Baths

Dear Mr Labrat
I wish to thank you for your information on chloride removal.Recently I have
found that in one of the hard chrome baths of a friend of mine who is doing hard plating of engine valves  the chloride content in chrome bath is 86ppm
and in etching tank(seperate chrome etch) is 460ppm.All of a sudden we are getting pitting in unplated areas like groove(in the cotter pin area of valve)
Do you think that chloride is the culprit causing the pitting due to overgassing??Please kindly reply me
Regards
Rajaram60 (rajaramseshadri@yahoo.com& rajaram60@hotmail.com)

DustinGebhardt

Re: Troubleshooting Hexavalent Chromium Baths

In my experience, high chlorides normally cause burning in the HCD areas of the parts.  However, it is not unlikely to imagine that the chloride could cause substrate attack in the highly-acidic and highly-agressive chromic acid bath. 

Were you getting plating inside the recess previously?

What are the parameters of the bath currently (chromic acid, sulfate, temperature, chloride, catalyst, Baume/specific gravity, mettalic impurities, cycle time)?

Are you having this problem with other parts with similar recesses?

-Dustin Gebhardt, CEF

Advanced Manufacturing/Finishing Engineer

Moen

Sanford, NC

rajaram60

Re: Troubleshooting Hexavalent Chromium Baths

Dear Mr Gebhardt
Many thanks for your prompt reply.The parameters of the hard chrome bath are Chromic Acid :254 gm/lt; Sulphate: 3.3 gm/lt; trivalent: 3.5
Iron: 1.6; HEF catalyst :14.5  CHLORIDE : 85ppm Temp: 55-60 degrees c
Our main job is plating of engine valves for GM Motors.We get plating in recess only if masking is not ok.But we are getting peel off often.Current applied is 7amps per sq.in(high??)Is alkaline etch required in Hard chrome prior to chrome etch done seperately in chrome bath .It is an automatic plating unit wherein the supplier has provided alkaline etch with two rinse water and then chrome etch (chloride :440ppm)prior to plating.The second rinse is highly blue to PH & I suppose alkali is getting carried to chrome etch.I think we will stop alkali etch since the valves highly polished after centreless grind.
Can you please expand HCD
Many thanks
Rajaram60

Shane Moore

Re: Troubleshooting Hexavalent Chromium Baths

Rajaram,
By alkaline etch, do you mean an alkaline electrocleaner? You may want to check for chloride contamination here as well, as this will definitely cause attack on the substrate.
I also noticed that your chrome to sulfate ratio is around 75:1. A hard chrome solution typically operates at 100:1 - may want to check your product technical data sheet to verify. If this is the case, may need to add Barium Carbonate to decrease the sulfate or increase the chrome concentration to bring the ratio up to 100:1. This will increase your throwing power to the lower curresnt density areas of the part.
I also agree with Dustin's comments. What he means by HCD is the high current density areas of the parts - the area that receives the most current (closest to the anode) or outer edges of the part.

Shane Moore
MacDermid, Inc
Technical Service Rep

rajaram60

Re: Troubleshooting Hexavalent Chromium Baths

dear Mr Shane Moore
Many thanks for UR reply.Yes the engine valves are degreased and given alkaline etch prior to chrome reverse etch.I think this can be eliminated since chrome reverse etch is enough if I am correct.Both the rinse waters are dark blue on litmus test.The chrome and sulphate ratio has been prescribed by Auto Tech whose chrome salt is being used.Will try increasing chrome content and make 100:1 ratio.
Thanks & Regards

Rajaraam

DustinGebhardt

Re: Troubleshooting Hexavalent Chromium Baths

The alkaline etch and chrome etch each play a different part in the cleaning cycle.  I would keep both of them.

-Dustin Gebhardt, CEF

Advanced Manufacturing/Finishing Engineer

Moen

Sanford, NC