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topic 4844

Salt Spray performance on galvanized steel


(2000)

When salt spray testing galvanized powder coated parts can anyone identify the critical factors that effect salt spray results. Please have data to back up information.

When scribing parts should the scribe penetrate the galvanized, please advise.

We have seen results where the galvanized coating has been eroded during the salt spray test. Is this normal?

Marc Burger
- Louisville, Ky USA


(2000)

Marc,

Hopefully you will get more technical replies from others, but you are experiencing the exact reason why cyclical testing or prohesion testing is favored by those who coat over galvanized material.

Simply stated, the galvanization does not have a chance to "repair itself" while under constant exposure in the salt spray. In real life (field exposure), at some time the coating will dry out, allowing this repair process to occur and therefore improving corrosion resistance with galvanized material.

Glen Larson
- Redmond, Washington


(2000)

Marc

The s/s test requires that you scribe thru the paint film to the zinc surface. You do not cut thru to the steel. You will find that the zinc will chemically erode until red rust forms in the scribe. The performance of the paint will depend upon pretreatment. Suitable yellow chromate will give >1000 hours ASTM B117 [link by ed. to spec at TechStreet] salt spray. So will correctly applied zinc phosphate. Cleaner-coaters ("iron phosphate") depend heavily upon the final rinse used for performance but are unlikely to exceed 500 hours. Painting HDG steel is not straightforward as the zinc corrodes readily in salt and hence the paint film will be undercut from scribe or damaged areas, hence pretreat is vital for a durable finish. R

Roger Bridger
- Croydon, UK



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