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Phosphate Coating Weights & Powder Thickness

How much will the corrosion performance of a component be affected if the coating weight is reduced from 120 to 80 GSM?

Rodger Talbert

Columnist, Products Finishing magazine

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Q. I understand that coating weights and film thickness can have an impact on corrosion performance of powder coated steel. My question is: how much will the corrosion performance of a component be affected if the coating weight is reduced from 120 to 80 GSM? Similarly, how will increasing the DFT from 50 to 70 or decreasing it from 70 to 50? The substrate for both these scenarios is galvanized steel. J.S.

 
A. I think you mean mg/ft2. For iron phosphate, the normal range is 25–50 mg/ft2 or 0.25–5 g/m2. Zinc phosphate coatings can be higher and may run from 50 to 125 mg/ft2 or 0.5–1.4 g/m2. Now, how much does coating weight affect performance? It depends on how consistent the structure is. If the coating is inconsistent, it is more damaging than lower weights. However, lower coating weights do reduce performance. It is not possible to say just how much because of the variables associated with the metal and the coating. The primary goal is to always produce a consistent coating weight well within the manufacture’s specifications.
 
Film thickness will also impact performance. For most indoor applications a 50 micron coating thickness (2 mils) is adequate as long as it will hide the metal. For outdoor applications a higher film thickness is generally recommended (75 micron or more, 3 mils). The critical thing to understand is that ultimate performance is achieved by the correct treatment and coating thickness. They both contribute to corrosion performance. Generally speaking, if you run at the low end of both ranges (low coating weights and low film build) you are vulnerable to failure. If you run from the center to the top of recommended ranges you are probably not going to fail unless you are using the wrong products.
 
For galvanized surfaces you should focus on cleaning and coverage more than coating weights.
 

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