Combined Corrosion/Weathering Testing of Coated Steel Products for Automotive Applications

An accelerated test method comprising wet/dry corrosion cycling using a 0.05 wt% NaCI/0.35 wt% [NH₄]₂SO₄ electrolyte alternating with exposures in a UV-condensation chamber using UVA-340 fluorescent bulbs has recently been developed primarily for assessing Industrial maintenance paint performance. In the present work, this method has been applied to a series of 10 coated sheet steel materials intended for automotive applications; the same materials under test in a joint AISI/SAE study. The results indicate that the test weathering component caused little degradation to the automotive paint. Paint creepback from scribe lines on phosphate/cold-rolled steel materials was accelerated by factors ranging between 5 - 7.6 when compared with 2 year field data obtained by the AISI/SAE. Equivalent acceleration factors for zinc coated steels were an order of magnitude greater implying that at least two distinct degradation mechanisms occurred during testing depending on the presence or absence of a zinc precoat. Suggested alterations to the test method for these materials include a more aggressive UV component and an electrolyte modified in terms of its composition and concentration.