Ask The Expert: Powder on Tri-Chrome Treatment

By: Rodger Talbert 28. February 2012

 

Q. We have been coating aluminum alloys for some time with inconsistent results as far as field performance of the coating. We had some corrosion and adhesion failure when our product was used in a harsh environment. Following your advice in an earlier inquiry, we are now having a tri-chrome pretreatment process being done at a local vendor on all of our aluminum castings. We considered bringing this process into our own shop, but it ended up not being feasible for us to do in-house. So far our performance is excellent.

My question is about one casting we have in particular on a new product. The current alloy is A413. This is a structural part, and we have seen failures in testing. Our casting vendor tells us A380 would be much more suitable as its strength properties are better. I also know that the copper content is much higher in the alloy. Do you see any issues in powder coating this alloy now that we are doing the tri-chrome pretreatment process? T.S.

A. Copper content in an aluminum alloy does influence performance, but it does not tell the whole story. With a single powder layer and standard cleaning method (clean, iron phosphate with fluoride) the 380 will not have as good corrosion resistance as the 413. With chrome it will be very close to the same. If you want more confidence you could prime the surface and then topcoat. That will perform very well.

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ASK AN EXPERT: Corrision Resistance

By: Rodger Talbert 13. February 2012

 

Q. We need some reasonable corrosion protection for out parts, and our powder coating is failing in salt spray testing after nearly 300 hours. What should we change to enhance our salt spray results? B.P.

 

A. First, let’s be clear on the difference between salt spray results and corrosion resistance in field use. Salt spray is a measurement of how well the coating holds up in a salt spray cabinet. It cannot predict the real resistance to a particular environment. It is useful for comparison of different treatment and coating options, but it does not predict field life. If you need to understand the potential field life you should use some type of cyclic testing.

 

When a coating does not have good resistance in a corrosion test it can be from several different causes:

 

  • -The surface that the powder is applied to is not clean enough or lacks a sufficient conversion coating. For example, iron phosphate on steel with a single coat of powder will usually provide around 250 to 500 hours of salt spray resistance; zinc phosphate can increase that to between 500 and 1,000 hours; a primer coat and topcoat combination can give you well over 1,000 hours of salt spray.
  •  
  • -Your coating may not have very good performance properties. If you need corrosion resistance, be sure to let your powder supplier know it.
  •  
  • -The coating could be applied too thin. Be sure you have at least 3 mils of coating if you need corrosion resistance.
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  • -The coating may be under-cured. Be sure that the film is fully cured.

 

If you have a satisfactory cleaning and pretreatment setup, a good powder, and good coverage at the necessary thickness you should be able to meet your salt spray requirements. 

 

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POWDER COATING: Conversion to Zirconium Pretreatment

By: Rodger Talbert 28. October 2011

 

Q. We’re considering conversion of our washer from iron phosphate to a new zirconium product. We’ve been told it will save energy and reduce maintenance with no loss of performance. Is that true? What things do we need to look for as far as the washer is concerned to make sure we can run this new product? B.A.

A. In general, the transitional metal products that include zirconium oxide do save energy and reduce maintenance. They do not need heat, and they do not generate significant sludge. You should be aware of several issues. You must have stainless steel tanks, pumps and tunnel. These products require very clean substrates, so make sure you have excellent cleaning with oil removal or solids removal if you have a lot of oil or grit. Rinsing is critical, because alkalinity will cause major problems in the treatment tank. Two rinses are better than one. These are the critical issues.
 

Talk to your chemical supplier and do some testing. I think you will find it is a real benefit.

 

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