Abstract
Aluminum-magnesium AA5XXX alloys are widely used in marine applications due to beneficial properties such as expense, strength, and weldability. However, when these alloys have greater than 3 wt% Mg, they are susceptible to sensitization at standard temperatures over time. With sensitization, more anodic β-phase (Al3Mg2) precipitates at the grain boundaries, resulting in a direct pathway for intergranular corrosion (IGC) propagation. With mechanical stress, intergranular stress corrosion cracking (IGSCC) can also occur. In addition to AA5XXX alloys, many marine vessels utilize CDA 706, a 90/10 cupronickel alloy, piping for water and heat transfer. CDA 706 directly couple to the AA5XXX superstructure. In addition, as copper corrosion products form, they can be carried through these pipes and deposit on exterior of the vessel’s superstructure. These deposited copper corrosion products can be reduced to elemental copper, forming a galvanic couple that accelerates IGC propagation of the sensitized AA5XXX alloys.
Previous work has shown that both the cathode:anode ratio and increasing DoS accelerates IGC, as well as the anode size influences IGC. To understand the effects for these alloy system, AA5456-H116 was coupled to CDA 706 under full immersion conditions at varying cathode:anode ratios, with alterations in degree of sensitization (DoS) and anode area. Outdoor exposure testing was also utilized with AA5XXX alloys (AA5456-H116 and AA5083-H116) sensitized to varying levels and coupled to CDA 706 of different sizes to investigate IGC propagation of samples exposed to service marine environments. Exposure samples were mounted on the R/V Endeavor and R/V Kilo Moana, two University-National Oceanographic Laboratory System (UNOLS) vessels ported out of Rhode Island and Hawaii respectively. These exposure samples were exposed to diurnal weather conditions and salt spray as the vessels were at sea. In addition, galvanically coupled samples were also exposed in accelerated testing with ASTM G85-A2 wet bottom (WB) and G85 WB modified version of G85-A2. Cross-sectional analysis was used to quantitatively assess the spatial and temporal distribution of the damage. Modelling was also used to correlate damage to water layer thickness. These results are correlated to outdoor exposure retrievals to compare IGC damage and connect service exposures to laboratory testing.
Based on initial results, DoS is shown to increase IGC propagation in both full immersion and outdoor exposure samples. As more β-phase is present at grain boundaries, IGC accelerates propagation. Mixed potential theory was used to explain the role of cathodic kinetics as cathode:anode ratio increases IGC following full immersion testing. Outdoor exposure returns show the effect of not only DoS, but cathode:anode ratio, geolocation, and relative humidity (RH) conditions under which samples were exposed. Following accelerated testing in G85-A2, damage follows throwing power trends modeled by Finite Element Modeling with COMSOL. A water layer thickness of 3000 um was found to be most comparable to damage seen experimentally.
