Perforation Resistance of Precoated Steels in an Accelerated Automotive Test 860270

Samples of electrogalvanized, hot-dipped galvanized, galvannealed, single and dual layer Zn-Fe (19-25%) and Zn-Ni (13%) alloy electroplated, zinc-rich primed and cold-rolled steel were cathodic e-coat primed and exposed for 250 cycles in an accelerated automotive-corrosion test environment. By extending the test period, it became possible to determine the number of cycles to perforation for all materials and to collect comparable data on scribe creepback corrosion, under paint corrosion, and overall material degradation with time. During the first 100 cycles, results were primarily indicative of the cosmetic corrosion behavior of test materials, showing that Zn-Fe and dual layer coatings with a chromium plus chromium oxide top-coating to be the best coatings. However, after 250 cycles, results clearly show that electrolytic pure zinc coatings offer the best perforation protection, i.e., better than heavier hot-dipped coatings or electrolytic Zn-Fe alloy coatings of equal coating weights. A correlation was evident between available zinc in the electrolytic coatings and time to perforation, but with an apparent diminishing return above 40 g/m². Zinc-rich primed and Zn-Ni (13%) alloy coatings did not offer a substantial increase in time to perforation in comparison to cold-rolled steel.