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Letter 8094
Tin Plating Finish Problem on Aluminium
Castings - Fungus Formation
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I have a finishing problem of tin plating on machined aluminium
castings (shape & size : cylindrical - ~6 cm. dia. x 6 cm.
height). The surface finish of the aluminium castings after machining
is about 3 to 4 microns Ra. The method of plating followed is
detailed below :
1. The machined castings are washed in a solution of soap oil to
remove excess oil & burrs and degreased in a trichloroethylene
bath and wiped clean with a soft cloth before plating.
2. The tin plating bath is prepared with sodium stannate powder
diluted with water. The pH of the above sodium stannate solution as
measured with Litmus paper is found to be between 10 and 11.
3. The cleansed aluminum castings are dipped in the sodium
stannate bath for about 2 minutes, then immediately dipped in a cold
water bath and again dipped in a hot water bath for 3 to 5 seconds
each. The castings are wiped clean with soft cloth again.
- The problem is : Fungus like formation (black spots) on the
surface on the tin plated aluminium after about 24 hours when kept
in normal factory atmosphere.
- Environmental Conditions : Temp. : 30 to 36ƒ C Humidity : 75
to 80 %
- Factory with aluminium foundry and machine shop inside the
same building.
Any procedural flaws in the tin plating method or environment may
please be pointed out. Can any additives or catalysts improve the tin
plating longevity?
B C Pai
- Chennai, TN, India
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Read your problem, Fungs type marks are observed due to two main
components left unattended. Ist Platig time is less and 2ndly while
in around Chennai because of Higher TDS in water, you need water
rinse followed by DM Water Rinse Followed by Hot DM Water
Rinse.
Further now a days a newer concept of Tin Plating is Acidic Tin
Plating has come into figure stands much better in Asthetic Look as
well corrosion resistance.
Arun Kumar Ralhan
Tinchem Enterprises - Patiala, Punjab, India
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