Chromate conversion coating (CCC) protection of untreated and acid-pretreated 7xxx Al alloys with various Cu content in aerated chloride solution as well as the polarization behavior of as-polished alloys in deaerated chloride solution were investigated using polarization techniques in conjunction with metallography, scanning electron microscopy (SEM), and x-ray photoelectron spectroscopy (XPS). Only one breakdown potential was found in all alloys, and the value was slightly higher than the second breakdown potential for bare alloys. Breakdown of CCCs occurred on the matrix region. Electrochemical impedance spectroscopy (EIS) tests were used to show that alloyed Cu improved the CCC protection of polished AA7xxx-T6 samples, but Cu enrichment on the surface degraded the CCC protection of AA7xxx-T6 samples. The relevance of polarization and EIS results to salt spray testing is also addressed.

Subject
Acids, Scanning electron microscopy, Electrochemical impedance spectroscopy, Metal surfaces, Sodium chloride, Copper alloys, X-ray spectroscopy, Pretreatment, Immersion, Corrosion protection, Aluminum alloys, Electrical resistance, Breakdown potential
Keywords:
aluminum alloy, chromate conversion coating, copper enrichment, corrosion protection, electrochemical impedance spectroscopy, metallography, polarization techniques, salt spray testing, scanning electron microscopy, x-ray photoelectron spectroscopy
NACE International
2004
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