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Blistering on heat treated products?

July 27, 2008

We have a regular problem with some spring steel products, and not so regular problem on high tensile steel products (e.g., ISO 10.9 cap screws or SAE grade 8 bolts).

In the cases with the spring steel, especially where it is a flat plate (one we do is 73mm x 50mm x 2mm), it blisters nearly every first attempt at plating. The blistering occurs during the heat treating phase.

With bolts and cap screws this generally only occurs when the chloride levels are low.

One particular problem batch of spring plates has been replated a number of times in our acid zinc barrel and also in our cyanide zinc rack line. Some were taken away be the manufacturer and linished, they came out fine when they were replated. A batch of thinner plates from the same type (but not the same sheet) of spring steel also came out fine second time in the barrel.

I would like to know if there is anything I can do to prevent the regular blistering first time round. Our process for these springs with the barrel is: Minimum 5 mins cleaner (IP Cleaner 301 SE), rinse, 30secs - 1min 30 hydrochloric dip (depending on age of acid), rinse, drag in/out, plate, drag in/out, rinse, silver passivate (Tripass PK3), rinse, dry and bake (1hr at 180 deg C).

The cap screws etc get much the same process as above, but they are often pickled in sulfuric acid before going into the barrel, and I also skip the rinse and drag in/out between the hydrochloric and the plating tank (this is on advice from MacDermid, and does seem to give an improved finish, it has not affected the blistering problem).

Thanks in advance,

David Chord
Plating shop employee - Lower Hutt, New Zealand

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I think that it is the high surface carbon in the spring steel. It is very easy to over clean and over etch this type of material. The result is normally lots of tiny blisters.
Try a minimum amount of clean and etch. To verify, try a ver light blast of say 5 parts and then only long enough in the caustic to clean off the blast dust and then about 1/10th the normal etch. Should not blister. If it does not, try a rack with minimal clean and etch. Consider a very short vapor degrease first. then a very short etch. and plate immediately.

James Watts
- Navarre, Florida
July 30, 2008

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August 1, 2008

I think mild alkaline electrocleaner before plating will improve and after that 2 water swill and neutralizie dip before plating might reduce blistring effectively.

Alternatively try Hot potassium permanganate ⇦ this on eBay or Amazon [affil links] treatment for removal of heat treated scales with usual cleaning after that.

P. Gurumoorthi
electroplating process chemicals - Chennai, Tamilnadu, India

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Hi down there,

well as previous collegues have written, most of the blistering problem is the pre-treatment. Please check with your provider also the content of inhibitors in the etching process. Also you can do a test in your lab how good your pickling is inhibated with this agents. Consider as well the composition of your acid zinc bath (chloride, ph, etc).

Regards,
Dominik

Dominik Michalek
- Mexico City, Mexico
August 4, 2008