The effect of the Mn/Fe ratio on the corrosion resistance of aluminum alloys of the Al – Mn system

The effect of the chemical composition of aluminum alloys of the 3xxx series on corrosion resistance under conditions simulating the effects of the marine atmosphere has been studied. The object of the study was homogenized ingots from alloys of the Al – Mn – Mg – Fe – Si system containing constant concentrations of silicon and magnesium (0.1 and 0.25% (wt.), respectively) and variable concentrations of manganese and iron (0.5–1.5 and 0.15–0.30% (wt.), respectively). The research was aimed at establishing relationship between the Mn/Fe ratio and corrosion resistance. In order to study the phase composition of the alloys under consideration, thermodynamic calculations in the Thermo-Calc software package and the results of X-ray microanalysis were used. The microstructure and phase composition were analyzed using optical and electron microscopy. Corrosion tests were carried out for 40 days using the SWAAT method in accordance with ASTM G 85. It is shown that an increase in the Mn/Fe ratio leads to an increase in the proportion of the Al6Mn phase and an increase in the manganese content in the intermetallic phases at a constant concentration of iron. This pattern, along with an increase in the manganese content in the solid solution, probably affected a decrease in the intensity of both cathode and anode processes due to the convergence of matrix potentials and precipitates, which, in turn, had a positive effect on corrosion resistance in salt mist. All the studied alloys showed high resistance to the effects of the marine atmosphere, however, the alloy with Mn/Fe = 6.5 had the highest corrosion resistance and minimum pitting depth (110 microns). A decrease in this ratio is accompanied by an increase in the depth of corrosion damage and surface roughness.