Investigation of Surface Corrosion Morphology and Mass Change of Three Aluminum Aerospace Alloys As It Pertains to Corrosion Rate at Various Atmospheric Exposure Sites

摘要

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).