Corrosion behavior of heat treated nickel-aluminium bronze alloy in artificial seawater

The effect of microstructure of nickel-aluminum bronze alloy (NAB) on the corrosion behavior in artificial seawater is studied using linear polarization, impedance and electrochemical noise tests. The alloy was heat treated in different heating cycles including quenching, normalizing and annealing. Microstructure of the specimens was characterized before and after heat treatment by optical microscopy and scanning electron microscopy. Results showed that the value of pearlite phase in the normalized alloy is much more than other specimens, leading to higher corrosion resistance. Polarization test showed that starting point of passivation in the polarization of the normalized alloy is lower than other specimens. The dissolution of Mn and Fe rich phases increased the Mn and Fe contents in solid solution, and this enhanced the passivation power of the surface of the alloy. The effect of the alloying elements was seen by a lower corrosion potential and an inflexion at around 280 mV (SCE) in the polarization curve, indicating the preferential dissolution of some elements beyond that potential. The polarization curve showed that the anodic polarization behavior of the alloy in the solution was essentially controlled by the intermetallic phases, mainly containing Cu. Two types of corrosion, pitting and selective corrosion, were observed in the specimens after being exposed to artificial seawater.