EFFECT OF PRE-AGEING CONDITIONS ON THE HARDNESS AND CORROSION CHARACTERISTICS OF DOUBLE-THERMALLY AGED Al-Si-Mg ALLOY

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Department of Engineering

ABSTRACT
The effect of pre-ageing condition on the hardness and corrosion characteristics of doublethermally aged treatment (DTAT) chill cast Al-Si-Mg alloys in simulated seawater environment using elemental sodium-modifier has been studied. This was with a view to improving the precipitation response and corrosion resistance of the Al-Si-Mg alloy. The temper conditions are DTAT at 90oC, 105oC and 120oC with variable pre-ageing time 1-5 h respectively. These were compared with the as-cast and single thermally aged (STA) treated samples. The hardness of the Al-Si-Mg alloy was determined using the Rockwell-F scale hardness tester, while the corrosion characteristics was evaluated using weight loss and linear polarisation corrosion techniques in 3.5% NaCl solution. Within the α- aluminium solid solution, the elements are indicated by the Energy Dispersive Spectrum (EDS). From the results DTAT samples showed better hardness values as compared with the STAT and as-cast samples. The DTA treatment produced increased hardness value of the samples more than 100%. These significant improvements were evidenced in the DTAT at 120oC for 3 h (with a hardness value of 18.2 HRF) and DTAT at 120oC for 4 h (with a hardness value of 9.8 HRF). Improvement in the hardness can be attributed to fine coherent clusters precipitates which serve as obstacle to dislocation movement. The corrosion of the DTAT samples in all temper conditions in the simulated seawater showed a higher improvement in corrosion resistance than the as-cast and conventional STAT alloy. From the Optical Microscope (OPM) results of as-corroded samples, it is obvious that the type of corrosion that occurred in this alloy is pitting/uniform corrosion. The hardness results of some selected samples after weight loss corrosion in simulated seawater vi also showed a decrease in hardness value as compared with its hardness before corrosion attack. The linear polarisation of some of the thermally treated samples demonstrated a higher corrosion potential and linear polarisation resistance with lower current density compared to the as-cast and STAT samples. It can be deduced that the thermal treatment have improved the hardness and corrosion resistance of the alloy under the studied condition.