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

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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