ABSTRACT

The corrosion behaviour of Zn hot-dip galvanised steel and Zn galvannealed steel in 3.5% NaCl (to represent a marine environment) was studied with electrochemical and free corrosion tests. Potentiodynamic polarisation, electrochemical impedance and weight loss measurements were carried out on specimens after different immersion times at 25°C, in 3.5% NaCl solutions with a dissolved oxygen concentration of 6.5 ppm and a pH = 5.8.

XPS analysis was also made on different specimens before and after immersion in the corrosive solutions. The following were observed:

Zn galvannealed coatings (GA) have a lower average corrosion rate than hot dip galvanised Zn (HD) because of their more noble electrode potential;

during the first corrosion steps, the average corrosion rate of both GA and HD coatings is increased by microstructural heterogeneity;

the corrosion layer on HD and G A specimens is mainly formed by Zn oxidation compounds, having an atomic percentage of >90% in both specimens.

Al and Fe compounds are present as minor components in the surface layers; they could act as ‘dopants’ in the passive films by changing their chemical-physical properties and, hence, their passivating power.