Sound technical content, curated with aloha by
Ted Mooney, P.E. RET
Pine Beach, NJ
The authoritative public forum
for Metal Finishing since 1989
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Dark spots /staining after Potassium Stannate Tin Plating
June 14, 2011
We tin plate our power pistons to provide an initial (break-in) and continued (unit start-up and shut-down) bearing surface. On occasion within 12-24 hours after plating, small spots will appear at the plated surface. (see attached picture, 24 hr after plating).
Too, we have noticed "staining" spots (usually larger in size) appear after 4-6 weeks in inventory. A preservative is applied - light tectyl prior. (see attached picture, 6 weeks storage).
The piston material is either a ASTM A48
[affil. link] class 40 or ASTM A536 Ductile (65-45-12).
I'm looking for the possibilities for these causes. I do not think they are process related. The "spots, I'm inclined to believe are created from base material porosity & post plating solution bleed back out. The staining....... I'm unsure of but might be related to the preservative application.
Any thoughts....suggestions???? All Welcome!!
Manufacture - Houston, Texas
If these are aluminum pistons covered with immersion tin plate, then one possible cause is porosity in the aluminum. Some liquid gets into the pores and then leaches back out after plating. The immersion plate is fairly porous and has good wicking properties.
You can dye check the pistons for porosity before plating.
consultant - Cleveland Heights, Ohio
First of two simultaneous responses -- June 16, 2011
Alkaline Tin plating solution surfactants are very tenacious and will not plainly rinse away, thus causing post staining. After tin plating and rinsing, then go back into an alkaline cleaner, then rinse again.
Robert H Probert
Robert H Probert Technical Services
Garner, North Carolina
Second of two simultaneous responses -- June 16, 2011
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