Minimizing Rust

Article From: Products Finishing, , from Powder Coating Consultants, Div. of Ninan, Inc.

Posted on: 12/1/2002

Question: What are the best processes for minimizing rust of powder coated steel poles?

Question:

What are the best processes for minimizing rust of powder coated steel poles? B.A.

Answer:

Rusting on steel can be tested in the laboratory using a salt spray test (ASTM B117). This accelerated test is used to determine corrosion limits (rusting) of a particular substrate (steel) using a specific pretreatment and coating. Normally, the test product is scribed (scratched to the base metal) before it is put into the salt spray test chamber. Periodically the test part is removed from the chamber and the coating adjacent to the scribe is examined for "creep" (corrosion under the coating). When a given amount of creep is achieved, then the test is concluded, and the time in the chamber is noted in hours. This is how you determine which coating and pretreatment should be used on your particular substrate to obtain the corrosion resistance you desire. What is "best" for your fielded conditions may be different from what is best for someone else's fielded conditions. Therefore, only you can determine what corrosion resistance is best suited to meet your objectives. You must discuss this corrosion requirement (in salt spray hours) with both your pretreatment and powder coating suppliers.

Following are some examples to guide you in determining what is best for your application (substrate = cold rolled steel and creep is rated at level 6 or 0.125 inch):

· Mild corrosion resistance 250 salt spray hours; requires good cleaning and a single powder coating.
· Good corrosion resistance = 250 to 500 salt spray hours; requires 70 mg/sq ft of iron phosphate and a single powder coating.
· Better corrosion resistance = 500 to 750 salt spray hours; requires 70 mg/sq ft of zinc phosphate and a single powder coating.
· Superior corrosion resistance = 750 to 1,000 salt spray hours; requires 70 mg/sq ft of iron or zinc phosphate and a zinc-rich powder primer or e-coat epoxy primer and a powder topcoat (two coats in total).

 


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