National University of Science and Technology MISiS, Moscow, Russia:

O. V. Volkova, Research Fellow at the Department of Steel Metallurgy, New Production Technology and Metal Protection, e-mail: expertcorr@gmail.com
A. G. Rakoch, Professor at the Department of Steel Metallurgy, New Production Technology and Metal Protection
A. V. Dub, Professor at the Department of Steel Metallurgy, New Production Technology and Metal Protection

The low-alloy aluminium alloys 6060Т5, 6063Т6 and АD31Т1 widely used in construction have been tested for susceptibility to pitting corrosion. Three different techniques were applied: electrochemical testing per GOST 9.912–89; specimens were continuously kept in a 3% NaCl solution for 90 days; cyclic testing when specimens were immersed in electrolyte for 10 min followed by a 50-minute exposure to air. It is shown that, for consistent analysis of the above alloys’ susceptibility to pitting corrosion, cyclic corrosion testing should be applied consisting of immersion in water solution containing chloride ions alternated with air exposure. When aluminium alloy specimens are exposed to air, the surface layer of moisture becomes thinner causing a greater diffusion of oxygen to the cathode surface. This creates a positive offset of the potential before the pitting potentials or past them while the electrolytes that remained on the alloys see a higher concentration of chloride ions. Pitting spots occur around cathode inclusions, then they combine to form visible localised corrosion damage – i.e. pits, which researchers often mistake for one isolated pitting spot. In the 6063Т6 and АD31Т1 alloys, cathode inclusions include segregated numerous particles containing iron — such as -(Fe,Si)Al and (Mn,Fe)Al, AlFeMnSi, whereas the 6060Т5 alloy has titanium and silicon cathode inclusions, which are not as effective as the above mentioned intermetallides, in particular, β-(Fe,Si)Al.

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