Evaluation of chromate passivation and chromate conversion coating on 55% Al–Zn coated steel

Steel sheets coated with hot-dip 55% Al–Zn is used in situations where better atmospheric corrosion resistance is needed. This type of alloy coating exhibits greater durability due to aluminum barrier protection together with zinc galvanic protection. The surface treatments for this alloy are based on chromate. Usually, two different cromating coatings are deposited on the alloy. The first is used to improve the corrosion resistance on it and the second to increase the adherence of paint films. The aim of the present work is to compare the corrosion resistance and electrochemical behavior of the 55% Al–Zn after the first and the second chromate conversion coatings treatment. Studies of cyclic voltammetry, atomic absorption and Auger spectroscopy showed that the second treatment used to increase the adherence of the paint, turns the surface more susceptible to corrosion.