Explorative investigation on the corrosion behaviour of aluminium crossover alloys

This work covers an explorative analysis about novel crossover aluminium alloys’ corrosion behaviour. As testing materials, four different wrought heat-treatable aluminium alloys containing magnesium and zinc (Al-Mg-Zn), with copper (Cu) and silver (Ag) in different amounts, were used. In particular, this new type of alloy is characterised by Mg/Zn>1 (in particular for this work, around 1,3) and heat-treatable by the precipitation of the T-phase (Mg32(Zn,Al)49). Each alloy was manufactured on laboratory scale, following the conventional processing method for heat-treatable aluminium alloys: raw materials preparation, casting, homogenisation, chemical composition analysis, hot and cold rolling (with an intermediate annealing step), solution annealing treatment, quenching and age-hardening. Five different ageing conditions were used for the alloys: artificial aging (T6) in one and two steps, artificial over-aging (T7) in one and two steps, and natural aging (T4). The manufactured slabs (in 2 mm sheets shape) were then cut into 2 cm x 1 cm specimens, and polished. After these manufacturing steps, the cleaning procedure designed for the corrosion tests, was performed by using a NaOH and HNO3 cleaning solutions. A Design Of Experiments was planned (considering BS EN ISO 11846:2008 as a reference for both the pre- and post-cleaning procedures, and testing parameters) to perform various immersion tests, in NaCl solutions with different pH (3, 6 and 9) and concentration (0,25, 0,5 and 1 M) values, for 72 hours. One single replicate of each test was made. After the tests, microscopic observations of the specimens’ surfaces and cross-sections, and measurements (mass loss, maximum depth of the corrosion attack and average dimension of the largest pits observable in the most attacked zones and sections) were performed to understand the influence of the test factors, the alloying systems, and the ageing treatments on corrosion. Also, the results were used to understand not only the corrosion performance of the alloys (with their ageing conditions), but considerations were done in order to understand which type of corrosion (mainly uniform, pitting, or intergranular, considering the testing conditions) is the most significant one in which conditions. In particular, for Al-Mg-Zn alloy pH was found to be the key factor for corrosion, leading to the presence of mainly uniform corrosion for neutral conditions and pitting for acidic and alkaline environments. Traces of intergranular corrosion (IGC) were minimal and observed only for samples treated with T6-single-step and T7-single-step for neutral pH. NaCl concentration and pH of the test solutions were both significant for the results on Al-Mg-Zn-Ag alloys, with mainly uniform corrosion observed for intermediate values of the test factors, IGC for low and pitting for high values. The transition from pitting to uniform corrosion was observed for the Al-Mg-Zn-Cu alloys in the T4 and T6-double-step conditions by increasing the pH of the solutions, while for the T7-double-step condition pitting was the predominant mechanism with pits morphology and density modifying by varying the value of the test parameter. For the T6- and T7-single-step conditions of this alloy evidence of IGC was observed for all pH values, and in general a modification in the extension of pitting and IGC was observed by increasing the NaCl concentration of the test solutions. Finally, for the Al-Mg-Zn-Cu-Ag alloy evidence of localized corrosion (IGC and pitting) was observed for all NaCl concentration and pH conditions tested.