Chromate inhibition of environmentally assisted fatigue crack propagation of aluminium alloy AA 2024-T3 in 3.5% NaCl solution

Aluminium alloy AA 2024-T3 has been used in aircraft construction for over the years due to its high strength to weight ratio, extremely good damage tolerance and high resistance to fatigue crack propagation. However, this material is susceptible to environmetally assisted fatigue crack propagation (EAFCP) when it is exposed to water vapour and chloride solution. In aerospace applications, a control of EAFCP is usually carried out by a protective coating system that contains a corrosion inhibitor. In the present investigation, the behaviour of EAFCP of aluminium alloy AA 2024-T3 in 3.5% NaCl solution containing various levels of chromate (Cr2O42-) inhibitor, typically 0.1%, 0.3% and 0.5% has been studied using centre-cracked tension (CCT) specimens in the longitudinal-transverse (LT) orientation. The fatigue experiment was carried out using a constant amplitude with stress ratio (R) of 0.1 and frequency of 11 Hz. The nature of chromate inhibitor in chloride environment was studied using electrochemical polarisation method in combination with optical microscopy and SEM equipped with EDX-spectrometer. Results of this investigation show that the additions of chromate inhibitor to the bulk 3.5% NaCl solution tend to reduce corrosion rate, increase passivity as indicated by the presence of anodic passivation plateaus in potentiodynamic polarisation curves and lower fatigue crack propagation rate (da/dN) of AA 2024-T3 at low ΔK. The mechanism of chromate inhibition on AA 2024-T3 aluminium alloy is complex involving mechanical, chemical and microstructural interactions and is discussed in the present investigation.