Effect of Heat Treatment on Dealloying Corrosion in Manganese Aluminium Bronze

Manganese aluminium bronze (MAB) used for ship propeller was deteriorated by de-alloying corrosion due to the difference in chemical composition between 𝛼 and 𝛽 phases or 𝛼 - 𝛽 galvanic coupling. This work, heat treatment by heating at a temperature of 800 ºC and 910 ºC for 1 hour followed by water-quenching was performed to reduce the difference in chemical composition between the two phases and to study de-alloying corrosion compared with the as-received alloy. The optical micrographs indicated that after heat treatment the amount of 𝛽 phase increased, revealed by large 𝛽 grains, from 27 % in an as-received sample to 77 % and 96 % in 800 ºC and 910 ºC heat-treated samples respectively. The chemical compositions analyzed by Energy Dispersive X-ray Spectrometer revealed that Al and Mn content in 𝛽 phase decreased from about 10 wt.% to about 7-8 wt.% while those in 𝛼 phase remained at about 6 wt.% after heat treatment; hence, the difference of elements content between phases reduced. Immersion test in 3.5% NaCl solution denoted de-alloying corrosion preferential occurred in 𝛽 phase. In the most attacked 800 ºC heat-treated sample, corrosion not only occurred within the 𝛽 grain but propagated from grain to grain as well. The extension of attacks caused from galvanic coupling between a - b adjacent to the grain boundary which provides deeper corrosion attack than that in the as-received sample. In the 910 ºC heat-treated sample, its microstructure was more homogenised; therefore, de-alloying corrosion was comparatively less.