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Cast iron parts rusting before powder coat

March 22, 2012

Q. We are starting a new product line that includes ductile cast iron castings that we machine in house using a water soluble coolant. Before sending any parts out for powder coating we would usually run them through our parts washer (warm water and detergent) then blow them dry. We've found that the parts can sit around with the coolant on them for days and not rust, but as soon as they are cleaned they start to rust. If you're fast about drying them it helps, but they still rust a little, and our powder coater isn't sure how to deal with that rust. Currently we're sending the powder coater the parts with the coolant residue still on them and they are solvent washing them by hand, (they don't have a parts washer.) But they can't do as good a job as our parts washer.

Ductile iron's been around forever, what do other people do?

Paul Carlson
Plant engineer - Kent, Washington, USA

March 23, 2012

Hi Paul. Iron and steel are not corrosion resistant, and they rust. There is not anything unusual about your situation except your good luck in finding that the machining coolant is sufficient to deter the rust for a long enough time.

The answer is already in your question. Keep on keeping on, and if the powder coater is expending too much labor, they should get a parts washer like yours :-)


Ted Mooney, finishing.com
Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey

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