Fundamental Study of Corrosion Preventive Compounds: Part II—Effects on Galvanic Corrosion of Coated Al Alloy Panels Coupled to Noble Fasteners

Abstract

The effects of corrosion preventive compounds (CPCs) on galvanic corrosion of coated Al alloy panels coupled to noble material fasteners were studied using a test specimen consisting of a painted AA7075-T6 panel and uncoated through-hole noble material fasteners. This specimen provides accelerated degradation of multi-layered coatings through galvanic attack. Homemade hard film CPCs formulated based on the general composition of a commercial hard film CPC were applied on the test specimens. The galvanic current between the panel and each fastener was monitored using a zero-resistance ammeter during exposure of the test panels in an ASTM B117 chamber for 3 weeks. The corrosion morphology, depth, and volume of the test panels after exposure and removal of the coating systems were investigated using optical profilometry (OP). The equivalent mass losses determined by two means, OP analysis and charge calculations by integration of galvanic currents with exposure time, were compared. For comparison with the homemade CPCs, test specimens without CPC and with commercial hard and soft film CPCs were also studied. CPCs reduced the galvanic current to a large extent and correspondingly mitigated galvanic attack. In particular, the commercial CPCs and a homemade CPC without corrosion inhibitor exhibited the best performance. This homemade CPC was also applied on a pre-corroded test panel. The galvanic current decreased after the CPC was applied on the pre-corroded test panel, indicating that the homemade CPC inhibited both the initiation and growth of galvanic corrosion.