Chromate or Phosphate??

[stonerabroad]

i'm looking at specifying a conversion coating on some mild steel vessels. currently, our supplier is using an iron phosphate as the 'primer' then is powdercoating the exterior, a common practice. the corrosion resistance is insufficient though, because after storing or operating these vessels, the interior of the vessels rusts.

i've noted that zinc phosphate is a slightly more expensive, yet more durable and effective coating than the iron phosphate currently used, but is unavailable in my area. zinc chromate is readily available though and i was wondering if the corrosion resistance is better or worse?

 

[CoryPad]

"Zinc chromate" will be much better than either iron phosphate or zinc phosphate for corrosion resistance if you consider these coatings alone, but it may not promote adhesion of other materials like the phosphate treatments do, which may result in worse performance. You must work on the entire system - talk to the powder coating supplier about acceptable primers.

 

[stonerabroad]

can you zinc chromate mild steel?

our local shops have only really done it on aluminum.

 

[CoryPad]

Mild steel is easy to plate with zinc plus chormate conversion coatings.