Effect of Surface Pretreatment on Galvanic Attack of Coated Al Alloy Panels

Abstract

A test assembly consisting of a coated and scribed aluminum alloy 7075-T6 panel galvanically connected to uncoated through-hole noble fasteners has recently been utilized to quantify corrosion attack during corrosion testing in laboratory chambers. In this work, the corrosion morphology and extent of attack of Al alloy panels with five different surface pretreatments prior to painting and scribing were compared after exposure to ASTM B117 with galvanic connection to uncoated stainless steel fasteners. The nature of the attack for samples with different surface pretreatment samples was found to exhibit two different trends: they either penetrated deeply into the substrate at the scribes or spread out under coatings. The galvanic currents between the coated Al alloy panel and the bare 316 stainless steel fasteners were monitored during 21 days exposure in an ASTM B117 chamber. The current trends measured for different surface pretreatment panels were consistent with optical profilometry (OP) results. However, the galvanic current and OP techniques both underestimated the extent of corrosion attack because of local H2 evolution and undercut attack, respectively. The results from both techniques were modified to compensate for these deficiencies. Acceleration factors associated with the galvanic interaction for the different surface pretreatment panels were determined.

Evaluation of Coated Al Alloy Using the Breakpoint Frequency Method

“…The width, depth and length of each as-formed scribe were about 250 μm, 20 μm and 20 mm, respectively. In that study [33], the corrosion attack of the sample with NCP pretreatment was worse than the samples with other pretreatments. The panel with AP pretreatment exhibited no obvious corrosion attack on both "X" scribes.…”

“…Topographic analysis of the stripped panel was then conducted using optical profilometry (OP, Veeco Contour GT-K). The topographic images produced by OP were analyzed using a software package (Vision64TM v5.30 from Bruker) to quantify the corroded area under coatings [33].…”

“…Therefore, it is valid to evaluate coating protectiveness by monitoring the breakpoint frequency. The same chromate primer coated Al alloys with the same surface pretreatments, but with artificial scribes, were exposed to ASTM B117 for 100 days in a previous study [33]. Two "X" patterns were manually scribed into the coated aluminum alloy panels making sure to expose fresh Al metal at the bottom of the scribes.…”

“…A combination of Al alloy panel and noble metal fastener is also similar to structures used in real applications. A coated Al alloy panel couple with bare stainless steel 316 fasteners has been recently been studied to provide an accelerated response to lab and field environments [5,6,33]. A serial EIS measurement can be conducted on the coated area near the fasteners after pre-exposure.…”

Structure and Formation of Trivalent Chromium Conversion Coatings Containing Cobalt on Zinc Plated Steel

“…Therefore, the formulation of a conversion coating might vary depending on the substrate used. The TCC bath designed for Al substrate is usually based on zirconium (Zr) and/or titanium (Ti), 25,30,43,44 which is why there is less Cr in the resulting layer. Furthermore, the level of Cr in the TCC coating is proportional to the Cr concentration in the treatment solution, and many of the TCP coatings formed on aluminum were done using proprietary passivating baths.…”