Interrelating electrochemical properties, salt-spray resistance and industrial performance of white-rust controlling passivators for galvanised surfaces

The relationship between electrochemical polarisation behaviour and corrosion resistance (imparted by passivators based on hexavalent chromium) of galvanised surfaces, is studied. A correlation exists between the actual field performance of passivators and the accelerated corrosion-tests carried out in the Laboratory. Effect of nitrate, fluoride, molybdate, sulphate, ions of aluminium and zinc, etc. has also been investigated. It has been noted that a pH between 3.5 to 4.5 provides the maximum salt spray resistance to the passivated surface. The morphology of protective films formed in the presence of additives, has been studied with the help of scanning electron microscopy. The mechanism of passivation has been described.