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

Abstract

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 (Rp) of 0.3 Cr and 0.5 Cr steels was higher than that of 0 Cr steel, and the Rp 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. FeCl2 and CrCl3 are formed from the Cl− nest developed in the early stage, and the pitting at CrCl3-formed regions are locally accelerated because Cr is strongly hydrolyzed to a very low pH.

New Accelerated Corrosion Test Method Simulating Atmospheric Corrosion of Complex Phase Steel Combining Cyclic Corrosion Test and Electrochemically Accelerated Corrosion Test

“…When β-FeOOH comes into contact with water, it releases Cl − ions. The dissolved Cl − ions then undergo hydrolysis reactions with Fe 2+ and Cr 3+ , causing a reduction in the pH of the steel surface [8]. Due to these characteristics, the region where β-FeOOH is formed can be considered a Cl-concentrated region.…”

“…Cr-goethite blocked the diffusion of ions from the outside of Cr-goethite to the inside and vice versa. Therefore, Cl − ions that were transported through the defects of Cr-goethite or released when β-FeOOH was wet accelerated localized corrosion via autocatalytic reaction [8]. In addition, Cr 3+ has vital hydrolyzing characteristics, which induce a further pH decrease, causing the fast progression of localized corrosion [10].…”

Effect of Frequency and Ratio of Wet/Dry Stages in Cyclic Corrosion Tests on Localized Corrosion of Complex-Phase High-Strength Steel

“…At this time, the slope of the mass loss regression equation for nth condition is called An, and the slope of the corrosion depth regression equation for nth condition is called Bn. The uniform corrosion acceleration coefficient (α1) and localized corrosion acceleration coefficient (α2) are calculated using Equations ( 4) and (5).…”

“…These variations can lead to micro-galvanic and localized corrosion issues in CP steel under atmospheric conditions. Notably, recent studies have reported that chromium (Cr) can increase localized corrosion in low-alloy steels under atmospheric corrosion conditions [5][6][7]. To address this concern, CP steel variants without Cr have been developed.…”