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February 18 - February 21, 2020

ASTM A780 says zinc-rich paints for repairing hot-dip galvanized damage must contain either between 65-69% metallic zinc by weight or greater than 92% metallic zinc by weight in dry film. Why is only 65-69% or above 92% accepted? Is any zinc-rich paint out of these ranges not suitable for HDG repair?

Selected Answers

From Marco Fabio Ramenzoni of ZINGA METALL BRASIL on February 29, 2020:
In my understanding, the text "between 65-69% metallic zinc by weight" is a measure of solids by weight in the wet product. On the other hand, the text "greater than 92% metallic zinc by weight in dry film" is a measure of the zinc contents in the dry film. Two different measures. The important number for galvanic activity and cathodic protection is the zinc contents in the dry film. Not all of the products with 65-69% metallic zinc by weight produce more than 92% of zinc on the dry film.

From Marco Fabio Ramenzoni of ZINGA METALL BRASIL on February 26, 2020:
Although not totally clear, ASTM A780 mentions organic binder paints with zinc contents in the range of 65% to 69%. My understanding is that this number would be the concentration of zinc + the binder inside the paint package, That would mean the solids by volume, (SBV), taking the binder off. Top quality zinc coatings with that sort of SBV produce a film with 92%, or more, of zinc in the dry film. And this is the most important target to be obtained so as to make the equivalent galvanic protection as the hot dip galvanizing provides.

From Julian Hay of JhayCon Inspection and Consulting on February 26, 2020:
The post from Rob Francis regarding zinc has a typo that should be corrected... .presumably he means 69 to 92%....not 69 to 72 %.

From Rob Francis of R A Francis Consulting on February 19, 2020:
According to the AGA web site, the gap is because tests showed that adhesion of the 69-72% zinc coatings was not as good. Given A780 allows surface preparation from near white blast, through SP11 to SP2, I would have thought cleanliness would be a far more important factor.

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Tagged categories: Asia Pacific; Coating Application; EMEA (Europe, Middle East and Africa); Latin America; North America; Zinc; Zinc-rich (inorganic); Zinc-rich (organic)

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