Understanding pitting corrosion in aluminium and its alloys

We are often shocked when we read media reports about catastrophes like air crashes, shipwrecks, bridge collapses, or explosionof gas pipelines. Investigations invariably point towards environmental cracking, a stress corrosion induced mechanical failure, as the apparent cause.Not many of us are aware, however, that deep down such cracks emanate from tiny corrosion pits. Pits are minuscule trenches that form when a defective local site on a metal surface corrodes due to environmental exposure while rest of the surface is protected by a barrier-like passive film. While pitting corrosion alone can cause a major failure, they also serve as initiation sites for secondary modes of corrosion such as Stress Corrosion Cracking (SCC), Inter-Granular Corrosion (IGC) or corrosion fatigue.

Aluminium is an important class of light metalalloy system that is indispensable in the manufacture of aircraft and shipbuilding components as it has desirable properties that aid in fuel efficiency,which in turn reduces greenhouse effects. Pitting is an imperative form of corrosion in aluminium wherein microstructures that are carefully tailored to meet engineering requirements, are often heterogeneous and unfortunately form the basis for initiation of corrosion pits. However, design of microstructurally complex alloys is possible with an in-depth understanding of pitting mechanism that would enable adoption of appropriate mitigation strategies.            

This is where I am hoping to make a difference. IITB-MonashResearch Academy, where I have enrolled for a PhD, is a collaboration between India and Australia that endeavours to strengthen scientific relationshipsbetween the two countries. Graduate research scholars study for a dually-badged PhD from both IIT Bombay and Monash University, spending time at both institutions to enrich their research experience.

Pits can cause irreversible and persistent damage accumulation. Thankfully, however, not all pits are detrimental unless they reach stability. An initiated pit switches between an active and dormant state several times, called metastable pitting, attempting to establish a conducivepit chemistry before it transforms into a stable pit; else the pit perishes.Thus, my prime focus is to study metastable pitting characteristics in order to understand the critical factors that influence the transition of a pit to stability.

The study of pits is challenging as pit events at any instant are numerous, dynamic and stochastic (Fig 2). For instance, it is complex to determine when and where a pit would occur, wherein the susceptible sites are characteristic to the microstructure of an alloy and other metallurgical parameters. To overcome this difficulty, we employ in-situ analytical characterisation of specifically fabricated microelectrodes, which has enabled real-time imaging of the surface, during electrochemical metastable pitting studies. Successful isolation of single metastable pit events enabled a detailed real-time investigation of their behavioural characteristics during growth and decay of a metastable current transient (Fig 3) and their transition to stable pits, which in turn have provided significant insights on pitting mechanism.

This research work has many potential benefits such as rational alloy design and additive manufacturing with safety as the prime focus to provide reliable corrosion-resistant materials for the manufacture of vehicles and in construction. Additionally, advancing the current knowledge in pitting would provide a stronger basis for understanding secondary modes of corrosion and development of mitigation strategies.