Effect of Cr on Aqueous and Atmospheric Corrosion of Automotive Carbon Steel.

This study investigated the effect of Cr alloying element on the corrosion properties of automotive carbon steel (0.1C, 0.5Si, 2.5Mn, Fe Bal., composition given in wt.%) in aqueous and atmospheric conditions using electrochemical measurement and cyclic corrosion tests. Three steels with 0, 0.3, and 0.5 wt.% Cr were studied by electrochemical impedance spectroscopy. Polarization resistance (R) of 0.3 Cr and 0.5 Cr steels was higher than that of 0 Cr steel, and the R also increased as the Cr content increased. Therefore, Cr increases the corrosion resistance of automotive carbon steel immersed in a chloride ion (Cl)-containing aqueous solution. In the cyclic corrosion test results, Cl was concentrated at the metal/rust interface in all of the steels regardless of Cr content. The Cl was uniformly concentrated and distributed on the 0 Cr steel, but locally and non-uniformly concentrated on the Cr-added steels. The inner rust layer consisted of β-FeOOH containing Cl and Cr-goethite, while the outer rust layer was composed of amorphous iron oxyhydroxide mixed with various types of rust. FeCl and CrCl are formed from the Cl nest developed in the early stage, and the pitting at CrCl-formed regions are locally accelerated because Cr is strongly hydrolyzed to a very low pH.