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Zinc plated bolts failed Kesternich test

Our electrolytic zinc plated bolts failed in Kesternich test, but its salt spray resistance is 600 h. Our applied coating is: Zinc coating + Cr+6 YL passivation + T&T15 topcoat + VH302 GZ topcaoat. VH 302 topcoat is silica based. What could we do to increase the SO2 resistance?

Is it possible, instead of increase silica based topcoat decrease SO2 resistance? I need help.

Thank you.

Fatma Babacan
Engineer - Izmir, Turkey
February 14, 2011

In general, it is the zinc layer thickness that controls the resistance to SO2 in the Kesternich test. The silicate layer improves the corrosion resistance in the neutral salt spray test, but not in the SO2 test. If the thickness of the zinc layer is too low, or if there is any damage or defects, it will result in reduced resistance. What is the specific test requirement, and what were the results? Usually the requirement is 3 cycles (DIN 50018) for thin coatings (5-9 micrometers) and 5 cycles for thicker coatings (9-15 micrometers).

Toby Padfield
Automotive component & module supplier - Michigan, USA
February 25, 2011

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