Interpretation of the corrosion process of a galvannealed coating layer on dual-phase steel

The dissolution process of a galvannealed coating layer on dual-phase steel was examined by correlating a stripping test, metallographic observations and a polarisation test in an acidified chloride solution. The galvannealed coating layer was composed of several Fe–Zn intermetallic phases, namely the gamma, delta, and zeta phases, from the substrate. The dissolution began from the outermost zeta phase and proceeded to the gamma and then the delta phase. The dissolution rates for each intermetallic phase and galvanic couples were measured and estimated through a polarisation test, and the gamma phase in the gamma-substrate galvanic couple exhibited the highest corrosion rate.

► The dissolution of the GA coating began from the zeta phase and proceeded to the gamma and then the delta phase.
► The gamma phase in the gamma-substrate galvanic couple exhibited the highest corrosion rate.
► The zeta phase should be kept uniform and thick to protect the substrate from corrosion.