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Ask the Expert Question-and-Answer Archive

by Ron Joseph

September, 2004

Steel Preparation Before Applying Zinc Chromate Primer

Q. Before applying zinc chromate to an ASTM A-36 steel (blasted to a commercial finish), is any "solution" required to further prepare the surface? After the zinc chromate is applied (and cured), is it necessary to "treat" the 2-mil layer of zinc chromate before applying a finish coat of Alkyd paint? If so, what is this substance called? This is for corrosion protection of mining haul truck rims subjected to muddy environments.

The most important aspect of the surface preparation is the degree to which you blast clean the steel surface. Obviously, the higher the degree of cleanliness as defined by ASTM, SSPC, NACE and other organizations, the better corrosion protection you will achieve.

The next important criterion is the abrasive blasted surface profile. If the abrasive blast profile is too deep, the peaks will protrude through the zinc chromate primer. Since you intend to apply a 2-mil layer of primer you should ensure that the surface profile is no more than 1.5 mils from peak to valley.

If the primer is applied in a relatively dry environment and soon after abrasive blasting has been completed, you should not have a need for an additional "wash primer", or as you have referred to it in your e-mail "solution". Similarly, if the surface cleanliness of the steel meets the definition of near-white or white metal you will also not need to apply a wash primer.

On the other hand, if the metal cleanliness is questionable or if you are particularly concerned about corrosion and intercoat adhesion, you should consider to apply a vinyl butyral wash primer but the application must yield a dry film thickness of 0.3 - 0.5 mils. If the film thickness exceeds this range, delamination between the zinc chromate primer and the substrate can be expected. The wash primer contains a solution of phosphoric acid which will further passivate the steel and protect it from corrosion.

Most industrial paint manufacturers sell vinyl butyral wash primers.

You must follow the manufacturer's mixing and application instructions quite rigorously before you apply this coating.

You can apply the alkyd topcoat directly over the zinc chromate primer.

In my opinion the dry film thickness for the complete coating system should not be less than 3.0 mils measured over the peaks of the blast profile.

I hope this answer will be of value.

Best wishes,

Ron Joseph

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