Geoffrey M. Scamans

The influence of surface treatment on filiform corrosion resistance of painted aluminium alloy sheet

Abstract Filiform corrosion of AA 5005 H14 aluminium alloy sheet has been investigated. Painted and scribed panels, with different surface treatments, were inoculated in HCl and exposed in a constant humidity cabinet maintained at 40 °C and 75–85% RH for 1000 h. After exposure, the panels were examined by optical and electron microscopy. It is evident that filiform corrosion susceptibility is determined largely by the near-surface microstructure. Heavily deformed layers, comprising oxide-decorated fine grains and dispersoids on as-rolled and mechanically ground alloy surfaces, are readily susceptible to filiform corrosion. Removal of these deformed layers, by caustic etching and acid cleaning, results in a high filiform corrosion resistance. On such surfaces, underfilm corrosion progresses by localized corrosion of the substrate; with comparatively slowly growing filaments propagating by repeated blistering of the overlying lacquer.

Environmental Degradation of Marine Aluminum Alloys—Past, Present, and Future

Aluminum alloy usage in maritime environments has fluctuated over the past 125 years with localized corrosion or environment sensitive cracking during service repeatedly preventing their full commercial exploitation, despite a series of innovations. Following a historical overview, discussion will focus on aluminum-magnesium alloys, highlighting current corrosion-related issues (e.g., intergranular corrosion, environment sensitive cracking) and potential opportunities to eliminate these problems in future commercial alloys.

The role of crack branching in stress corrosion cracking of aluminium alloys

Abstract Stress corrosion cracks of all types are characterised by extensive crack branching, and this is frequently used as the key failure analysis characteristic to identify this type of cracking. For aluminium alloys, stress corrosion cracking (SCC) is almost exclusively an intergranular failure mechanism. For plate and extruded components, this had led to the development of test procedures using double cantilever beam and compact tension precracked specimens that rely on the pancake grain shape to constrain cracking, so that fracture mechanics can be applied to the analysis of stress intensity and crack velocity and the evolution of a characteristic performance curve. We have used X-ray computed tomography to examine in detail SCC in aluminium alloys in three dimensions for the first time. We have found that crack branching limits the stress intensity at the crack tip as the applied stress is shared amongst a number of cracks that are held together by uncracked ligaments. We propose that the plateau region observed in the v-K curve is an artefact due to crack branching, and at the crack tips of the many crack branches, cracking essentially occurs at constant K almost irrespective of the crack length. We have amplified the crack branching effect by examining a sample where the long axis of the pancake grains was inclined to the applied stressing direction. Our results have profound implications for the future use of precracked specimens for SCC susceptibility testing and the interpretation of results from these tests.

The Influence of Prolonged Natural Aging on the Subsequent Artificial Aging Response of the AA6111 Automotive Alloy

The influence of prolonged ambient temperature storage on the subsequent precipitation behavior of the AA6111-T4P automotive alloy during artificial aging has been investigated using hardness testing and high-resolution transmission electron microscopy. The results indicate that growth of atomic co-clusters and GP zones during ambient storage suppress the onset of Q′ and Q precipitation, and retard the formation of elemental silicon particles. However, the overall hardening response of the stored AA6111 T4P alloy to subsequent peak-aging and over-aging treatments is not altered by the significant reduction in the volume fraction of Q′ and Q precipitates due to their lower strengthening effects compared with GP zones and the Q″ phase precipitates.

Pre-exposure embrittlement of an Al-Cu-Mg alloy, AA2024-T351

Abstract When Al-Cu-Mg (AA2024-T351) alloy plate is immersed in 0.53 m NaCl solution, intergranular corrosion occurs, which initially takes the form of narrow fissures at the surface. Although growth of these fissures is initially rapid, it soon slows and the penetration depth is limited. However, these intergranular fissures can give rise to intergranular subcritical crack growth and eventual catastrophic fracture if specimens are strained slowly in laboratory air after pre-exposure to the NaCl solution. In our experiments, tensile specimens of the alloy were strained at strain rates in the range 10-7–10-4/s, and the cracking propensity was found to increase with pre-exposure times up to around 300 h. However, cracking was much reduced with longer pre-exposure times and became negligible after 500 h. This rather unexpected result can be explained by the observation that the intergranular corrosion fissure becomes increasingly broad and un-crack-like with extended pre-exposure times, failing to meet the prerequisite initiation conditions required for intergranular stress corrosion cracking. A further unexpected result was that straining specimens in the same 0.53 m NaCl solution after pre-exposure produced no intergranular crack growth. To investigate this further, tensile tests were conducted under electrochemical control, and this showed that the free corrosion potential in the test solution was too low in 0.53 m NaCl to allow cracking to proceed.