Metal finishing Q&As since 1989

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Dull finish

2007

Sir,

The composition of Steel Section is

C-0.2055, Mn-0.4783,S-0.0391,P-0.0766,Si-0.2065,Cr-0.0509,Ni-0.0087, Mo-0.0019,V-0.0030,Ti-0.0027,Al-0.0156,Cu-0.0062.

I am getting Dull/Grey finish after galvanizing. Please advice what shall be the reason of dull finish?. Which Chemical is responsible for the same?

ROHIT GUPTA
Structures & Towers - Raipur, Chattisgarh, India

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2007

Dear Rohit,

Your steel is very reactive toward zinc and the alloy layer (e.g. 6% iron and 94% zinc) is most of the coating including the outer layer. The steel grays as it cools. Perhaps you could cool it faster, but this may cause distortion of product.

The Silicon 0.2065%; the phosphorus 0.0766%, and in a smaller way the aluminum 0.0156% are all problems. Since changing steel is an unlikely prospect, there are other ways to get a silver colored coating that is thinner than you have now. Nickel in the zinc would likely be too costly and not very effective because you are just at the high end of the Sandlin curve.

Regards,

Dr. Thomas H. Cook
Galvanizing Consultant - Hot Springs, South Dakota, USA