The authoritative public forum
  for Metal Finishing since 1989
  

  -----

Powder coating 17-4 steel

I am applying an epoxy powder to a 17-4 steel centrifugal casting. Powder adhesion on this part has always been problematic. This part is machined, sandblasted (150 grit, 80-115psi), passivated, nickel plated (only in certain areas), and powder coated. The process occurs in that specific order. Maskant materials used in the nickel plate process are silicone free. Changing the type of powder is not an option. I could add a zinc chromate wash primer, but am trying to stay away from that.

Is anyone familiar with powder coating steel centrifugal castings? Is there anything obviously wrong with the process as it is laid out? I have scrutinized every step of this process and still cannot achieve good adhesion.

Lindsey Bradley
Process Engineer - Dallas, Texas
2007

---

Lindsey, an undercured paint can give the appearance of an adhesion problem, make sure that the thickest part of the piece is achieving the suggested cure temperature for the manufacturers specified amount of time. If this part is solid and fairly large it could take longer than usual to achieve the cure that you want. If you want to test to see if the paint is cured, soak a Q-tips ⇦ this on eBay or Amazon [affil links] with MEK / methyl ethyl ketone and rub it on the painted surface, again, do the test on the thickest part of the casting, if the paint comes off it's not cured. For proper curing the metal needs to be at the spec'd temp for the spec'd time, if it takes 20 minutes for the metal to achieve the spec'd temp, you add 20 minutes to the cure time to achieve the proper cure.

Sheldon Taylor
supply chain electronics
Wake Forest, North Carolina
2007

---

2007

There is a wealth of information that can be found if you type "waterborne primer for powder coating" into the Google search engine.
You state you have considered wash/etch primers -unfortunately these contain chromates and under current legislation these products are now considered environmentally unfriendly.

Terry Hickling
Birmingham, United Kingdom