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Letter 7863
Zinc plating on 12L14 carbon Steel
(Blistering)
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We are zinc electroplating on machined 12L14 steel and we are
experiencing blistering on the finish.
The zinc process is a rack operation, chloride zinc. When the
parts are processed for the first time, the small blisters can be
seen only with magnification (22X).
The parts had to be reprocessed due to the wrong chromate, and the
second time these blisters are much more visible, with the naked eye.
They are very small and uniform.
The chromate is yellow and we can see the blisters are grey in
color. Each time we process again the parts, more blisters appear.
The 12L14 steel is known as a free-cutting carbon steel,
resulferized and rephosphorized, with the following compositions in
%; Carbon 0.13 max., Manganese .85-1.15, Phosphorus .07-0.12, Sulfur
0.24-0.33 and Lead 0.15 to 0.35.
We are blaming the lead content on the steel. What is the best way
to clean and process new parts? What can we do with the reprocessed
parts? Are we bringing up (to the surface) the lead?
Any help will be greatly appreciated.
Enrique Segovia
- Monterrey, Mexico
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It could be the lead, but my guess is that you are etching out the
large manganese sulfide inclusions that will be in the microstructure
of this steel. Each time you reprocess, the inclusions are etched out
to a deeper depth - thus increasing the severity of the problem.
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Larry Hanke
materials testing laboratory
Minneapolis, Minnesota
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Larry,
Thank-you for your help. What preparation process do you recommend
if this inclusions are manganese sulfide, as you mention.
Thank you again for your help,
Enrique Segovia
- Monterrey, Mexico
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Plating on 12l14 leaded steel. You can generally tell when you are
dealing with it as any acid treatment to remove rust in a 50% HCL
solution will cause it to gas and stink. But that treatment is held
to a minimum and only as long as it takes to remove any rust. From
that point I use a solution of 10% fluoboric acid for about 2 minutes
to remove the oxidation from smeared lead on the surface.
Have had success plating electroless nickel direct on the surface
or cyanide copper strike or plate. The plated parts pass all Quality
tests including destructive testing.
Todd Huehn
- Blaine, MN
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