Investigation of Surface Corrosion Morphology and Mass Change of Three Aluminum Aerospace Alloys As It Pertains to Corrosion Rate at Various Atmospheric Exposure Sites
Authors: Hansen Douglas , Borth David , Wolters Gregory , Truong Thao , Hlavin Brant , Cole Farrah , Wickman Braden 
Published: December 2020

Replicate coupons of bare aluminum alloys AA2024-T3, AA6061-T6, and AA7075-T6 are being exposed at two coastal atmospheric test sites, Kennedy Space Center (KSC) and the US Naval Research Laboratory in Key West, FL (NRL-KW). Various analyses including mass loss/mass gain determinations, elemental composition analysis (via scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS)), as well as pit density and pit volume measurements (via 3-dimensional laser/optical microscopy) are being performed to determine if there is a correlation between corrosion rate, corrosion morphology, and surface chemistries of the coupons exposed at the outdoor sites. The sample coupons are being retrieved at 3-month intervals. The aluminum alloy coupons are cleaned until a consistent change in mass between cleaning cycles has been reached per ASTM G1 (Table A1). At this point of the investigation, a cumulative exposure time of 6 months will be discussed and presented. Mass loss data for the bare metal substrates at the two field locations indicate that different mass losses occur at the two locations for similar exposure lengths for all three alloys, but at overlapping time intervals. For the AA7075-T6 alloy, mass gain (negative mass loss values) was measured for the 1Q (3 month) and 2Q (6 month) exposure at the NRL-KW site, whereas mass loss values (positive mass loss values) were determined for the KSC site for similar time exposures; however, replacement replicate AA7075-T6 coupon exposures at the NRL-KW site for the same length of exposure but at a different time interval, exhibited mass loss values (Figure 1).
While the observed corrosion rates were different, it was determined that the morphology and elemental composition of the alloy substrates were different as well. Due to the extreme localized pitting attack on the 6061 and 7075 alloys, optical analyses of the coupons indicate that the resulting variability of the mass determinations may be due to incomplete removal of corrosion product from within the subsurface horizontal exfoliating pits populating the exposed coupons. Indeed, the populating of the coupons with these occluded pits was observed to increase with field exposure time at both field exposure sites, resulting in increased variability in mass loss values. These results reinforce the importance of deploying more than a single sample coupon of these aluminum alloys for corrosion rate calculations using mass loss determinations. Pit morphology and elemental analysis of the exposed alloy coupons as a function of exposure time and location will also be presented and discussed.
These findings are important in developing accelerated test protocols. In previous laboratory exposure tests it was found that similar mass losses were observed at an accelerated rate in the modified chamber for these alloys versus the field exposures, but the surface morphology of the substrates exposed in the chamber were markedly different from the substrates in the field. Therefore, it may be more informative to consider the elemental composition, pit density and pit morphology of the corroded surface in addition to the mass loss, rather than rely on mass loss alone.

Figure 1. Plot of mass loss comparison for aluminum alloys coupons exposed at KSC and NRL-KW for 3 and 6 months.

Figure 1