The Influence of Non-Passivating and Passivating Alloys on the Galvanic and Local Corrosion Rates of 6061-T6 Al

Abstract

Galvanic corrosion of 6061-T6 aluminum-coupled metals was studied in marine, volcanic, and rainforest environments. In addition to outdoor exposure, the galvanic couples were subjected to the chloride-containing GM 9540 accelerated corrosion test. The galvanic couple types included 6061-T6 Al paired with Ti-6Al-4V, 316 stainless steel, silver, copper, 1018 steel, or Mg AZ31B using fiberglass-epoxy fasteners. In this experiment, galvanic corrosion currents were measured through portable data loggers connected to each metal in the aluminum-coupled specimens. The total corrosion on the 6061-T6 Al in the galvanic couple resulted from the galvanic corrosion between the 6061-T6 Al anode and the cathode plus additional simultaneous local corrosion on the 6061-T6 Al caused by local cathodic reactions occurring on the 6061-T6 Al. The value of the total corrosion rate (i.e., local corrosion plus galvanic corrosion) was determined by mass loss of the galvanically coupled aluminum coupons. The local corrosion rate was determined using the difference between the total corrosion rate and the galvanic corrosion rate, as determined from the galvanic current data and Faraday’s law. The corrosion rates of the coupled 6061-T6 Al coupons were up to approximately 20 times greater than the uncoupled 6061-T6 Al coupons. Interestingly, less than 30% of the mass loss of the coupled 6061-T6 Al was directly due to the galvanic current, and accordingly 70% or more was due to local corrosion on the 6061-T6 Al. This implied that corrosion-product contaminants from the cathodic coupons (i.e., Ti-6Al-4V, 316 stainless steel, silver, copper, and 1018 steel) or byproducts such as excess H+ on the anodic 6061-T6 Al and excess OH- on the adjacent cathode may have had a major influence on the local corrosion of the 6061-T6 Al. In some cases, the type of cathodic coupon (i.e., Ti-6Al-4V, 316 stainless steel, silver, copper, and 1018 steel) made a significant difference on the corrosion rates of the 6061-T6 Al. Potentiodynamic polarization experiments were also conducted to study the mechanisms of galvanic corrosion for the couples described above.