CORROSION-RESISTANCE OF ZN-AL ALLOY COATED STEELS INVESTIGATED USING ELECTROCHEMICAL IMPEDANCE SPECTROSCOPY

Electrochemical impedance spectroscopy and electron microscopy are use d to study corrosion processes affecting hot dipped Zn-Al alloy coated steels in aerated aqueous sodium chloride. Coatings containing 43 and 55 wt-%Al are allowed to corrode freely at pH 3, 6, and 9 for periods of 24 h. Scanning electron micrographs of corroded samples show pitti ng in the 43%Al coating, and areas of localised dezincification and po rosity development in the 55%Al coating. Electrochemical impedance spe ctra for the 4.3%Al coating vary little with time and pH and are consi stent with oxygen transport to the corroding surface occurring by semi -infinite diffusion through solution. Similar results are found for th e 55%Al coating at pH 6, where dezincification is limited. At pH 3 and 9 dezincification of the 55%Al coating is more extensive, and the imp edance spectra show marked, time dependent increases in diffusional im pedance contributions; It is proposed that these changes result from a progressive increase in the length and tortuosity of oxygen diffusion paths accompanying pore development as dezincification proceeds.