Corrosion behavior and resistance of hot-dip Al−Cr coated steel sheet under a salt corrosive environment

Abstract

The corrosion behavior and resistance of hot-dip Al and Al−Cr coated steel sheets (SS) were investigated via scanning electron microscopy (SEM), energy dispersive X-ray spectrometry (EDX), electron probe micro analysis (EPMA), glow discharge light spectroscopy (GDLS), and X-ray diffraction patterns (XRD). The Al−Cr coated layer was composed of the following three phases. Al phase comprised the surface layer, Al₁₃Cr₂ the middle layer, and Al₁₃Fe₄ and Al₅Fe₂ phases the interfacial layer between the Fe substrate and the coated layer. The corrosion behavior of the Al−Cr coated layer showed different aspects compared with the Al coated layer. In the Al−Cr coated layer, Cr was observed at an intermediate layer having a band shape. From the analysis of the polarization curve, the initial corrosion current of the Al−Cr coated SS was 10 times lower than that of Al in the early stage, and the corrosion resistance was superior to that of Al. The Al coated SS formed an Al−Fe−Si IMC layer, and the coated layer was almost destroyed. Many cracks were produced, and the corroded parts were enlarged from the cracks. The upper part of the Al−Cr coated SS, an Al−Si coated layer, was corroded. In contrast, the Cr-rich intermediate layer was not destroyed. Consequently, the high corrosion resistance was attributed to the densely covered Al(OH)₃ and the intermetallic compound layer of Al₁₃Cr₂. These results significantly contribute toward attaining a detailed understanding of the corrosion behavior and resistance Al−Cr coated steel sheets.