Effect of sensitisation on the corrosion fatigue properties of AA5456-H116

Gunson, James (2011). Effect of sensitisation on the corrosion fatigue properties of AA5456-H116. University of Birmingham. M.Res.

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Abstract

The marine environment is highly aggressive towards most materials. However, the service life of aluminium in such aggressive marine environments can be exceptionally long. Aluminium sheets and plates of the Aluminium-Magnesium (5xxx) series are extensively used in naval structures. This is due to its good mechanical strength, formability, seawater corrosion resistance, and weldability. However, susceptibility to intergranular corrosion (IGC) and stress corrosion cracking (SCC) of 5xxx alloys with greater than 3wt.% of magnesium alloys can and does occur after prolonged thermal exposure to elevated temperatures, similar to those that occur in service. Various studies have shown that ‘sensitisation’ causes the formation and coarsening of electrochemically reactive β-phase at the grain boundaries, due to magnesium segregation, and that it does indeed cause the matrix to become susceptible to IGC. However, there has been little to no examination as to the effects of this grain boundary precipitation and the resultant corrosion susceptibility on the fatigue lifetimes of Al-Mg alloys. To establish the effects of sensitisation on the properties of AA5456-H116 standardised testing in the form of nitric acid mass loss testing (NAMLT) and electrochemical evaluation to confirm the increase in IGC and localised corrosion susceptibility as a result of increased precipitation of reactive β-phase at grain boundaries. The microstructure was also characterised metallographically, to observe grain boundary precipitation and the effects of sensitisation time. It was observed that increasing sensitisation time at 150\(^\circ\)C lead to an increase in β-phase precipitation and resultant decrease in IGC and localised corrosion susceptibility. In order to characterise the effects of sensitisation on the fatigue properties of pre-corroded AA5456-H116, S-N curves were generated and a systematic fractrographical analysis or fractured samples was undertaken. A decrease in fatigue lifetime as sensitisation time at 150\(^\circ\)C increased was shown. A complete examination of pit morphology using various microscopic techniques in order to investigate more clearly the morphology of pit that is likely to cause failure before that what is normal fatigue lifetime for as-received AA5456-H116. It was observed that there is an increase in the size, depth and density of pitting with more IGC present on and beneath the surface. Increased sensitisation time at 150\(^\circ\)C effectively renders the microstructure of AA5456-H116 susceptible to IGC and localised corrosion (in the form of pitting), which in turn modifies the local stress and ultimately shortens the fatigue life and lowers the threshold stress for crack initiation and propagation.

Type of Work:

Thesis (Masters by Research > M.Res.)

Award Type:

Masters by Research > M.Res.

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