Metal finishing Q&As since 1989

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Phosphating or Galvanizing --which is better before finishing it with powder coating

January 28, 2010

Dear Sir,

We are manufacturers-exporters of metal handicrafts from New Delhi, India .

Presently we are trying to develop a new range of iron item like sundials , garden baskets, etc.

In our first attempt to make iron sundials weather resistance and avoid rusting when exposed to hard weather conditions , we used readymade galvanized iron strips to make sundials. We first cut the strips in sizes required and then got them electric welded. After that lumps of welding was smoothened out using hand grinding with buff wheels. To get them ready for powder coating , cold phosphating was done by dipping the items in appropriate chemicals. and then electro powder coating was done by usual method.

To send them to our foreign buyers we sent samples in normal packaging using bubble wrap and corrugated cartons.

Surprisingly the sample got rusted within 15 days at our buyer's place in Netherlands Whereas here (New Delhi, India ) at our factory we kept the sundial outside and we did not had rusting or corrosion at all.

In our second attempt we bought plain MS sheets and prepared necessary rings out of them. and then got them galvanized. But after galvanizing there were lumps on the edges of rings and while smoothening out the lumps the layer of galvanizing was rubbed off and raw iron started showing off. Then we were suggested that if we need to powder coat the item we should used 2 or 7 tank hot phosphating process to avoid rusting in powder-coated items.

Please guide us the right and better way to avoid rusting in a powder coated item

Awaiting your earliest response.

Jaya Bhatia
- New Delhi, India

The pretreatment is important very much. You should phosphate it properly with zinc phosphate chemicals. After proper phosphating , go for pure polyester powder coating or polyurethrene powder coating. This finish will provide high outdoor life and salt spay life. Hot dip galvanising before powder coating is not suitable for low adherence properties.

DR. DIPANKAR SEN
- WEST BENGAL, INDIA
February 7, 2010