The Effect of Tensile Stress on the Corrosion of Aluminium and Some Aluminium Alloys

The effect of externally applied tensile stress on the rate of corrosion of A1 in 0.1N HCl and 0.1N HCl containing 3% and 5% NaCl has been determined by measurement of the evolved. Elastic stress and overstrain up to 10% has little effect. 20% overstrain apparently reduces the rate of attack, but owing to the difficulty of assessing the true surface area of overstrained metal, the effect of this stress is difficult to ascertain. Similar tests conducted on Al-7% Mg alloy show that stress has no effect on the resistance of the annealed alloy, but increases the rate of attack of the strain-aged alloy, probably due to exposure of the anodic phase at the grain boundaries. Potential measurements on Al and Al-7% Mg alloy confirm these results. The effect of stress on the corrosion of strain-aged Al-7% Mg in 3% NaCl has been determined by measurement of the O2 absorbed and H2 evolved. There is no characteristic difference between the rate of corrosion - time curves for the stressed and unstressed alloy. A technique for investigating the stress-corrosion susceptibility of materials has been developed and experiments conducted on Al-7% Mg and Al-5% Mg alloys. From the curves of applied stress v. percentage of the loss of strength at failure which is due to stress-corrosion, a method is developed whereby the stress-corrosion susceptibility of materials may be expressed on a comparative basis. The theory that stress concentrations at the base of cracks are responsible for mechanical failure is discussed and an alternative theory of the mechanical role of stress during stress-corrosion, based on eccentric loading, is put forward