Two years pitting corrosion of AA5005-H34 aluminium alloy immersed in natural seawater: morphology characterisation

Abstract

Pitting corrosion of aluminium alloy AA5005-H34 immersed in natural sea water for up to 2 years is presented, together with new interpretations of the development of the pitting process over extended exposure periods. Trenching initiated at surface-located Fe-rich intermetallic (IM) particles. It occurred more extensively in sea water compared to atmospheric exposures. With increasing exposures, the pitting morphologies became more complex. Three types, i.e. hemispherical pits (Type 1), crystallographic pits (Type 2) and characteristic 'petal'-like pits (Type 3), were observed after 12 months. It is suggested that the formation of Type 2 and Type 3 pits occurs at an advanced corrosion stage, and are associated with large Fe-rich IM particles near the metal surface. The influence of marine growth on pitting was negligible.

Two years pitting corrosion of AA5005-H34 aluminium alloy immersed in natural seawater: data interpretation

“…This means the extremely deep pits should have the same or at least similar characteristics [34]. The results in Figure 4 and Table 2 further confirm that pitting morphology became more complex and the pitting mechanism within the local areas might have become more complex as exposure time increased as suggested earlier [15]. This complexity is very likely to have resulted in a non-homogeneous pit population.…”

“…In Figure 5(b), a concave trend appears to be involved at about 3 months (marked by a black circle)both on the data for maximum pit depth and the average of 15 deepest pits. Most likely the reason for this is that the AA5005-H34 aluminium alloy had incurred pitting (trenching) at Fe-rich intermetallic (IM) particles on the metal surface before exposure, as reported earlier [15]. Potentially this pre-trenching could have influenced the sites of pitting initiation as well as the time of initiation of new pits, and thus affected the depth of the pits in the short term.…”

“…In the earlier study it was observed [15] that in the early part of the exposure period trenching around Fe-rich IM particles tended to cause only shallow hemispherical pits (Type 1). Crystallographic pits (Type 2) and 'petal'-like pits (Type ).…”

“…As reported earlier [15] it was possible to categorise the pitting morphology of AA5005-H34 aluminium alloy after long-term exposure in natural seawater as falling into three types, i.e. hemispherical pits with relatively smooth pit walls (Type 1 = T1), crystallographic pits bound with half-cubic facets (Type 2 = T2) and 'petal'-like pits bound with repeated irregular convex-concave patterns (Type 3 = T3).…”

“…To further investigate the development of pitting corrosion and mass loss, additional longer-term corrosion data for 5000 series aluminium alloys was considered necessary, and in this study AA5005-H34 aluminium alloy was immersed in natural seawater for up to two years. As part of this work, pitting morphology development and the effects of marine fouling on pitting was observed and has been reported [15]. The present paper deals with the interpretation of the corrosion loss data, the relationships between the mass loss and pit depth and the exposure time.…”