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

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.

Larry Hanke materials testing laboratory Minneapolis, Minnesota

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

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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