Effects of Chromium, Cobalt, Copper, Nickel, and Calcium on the Corrosion Behavior of Low-Carbon Steel in Synthetic Groundwater

The aqueous corrosion characteristics of low-carbon steel (CS) with small amounts of Cr, Co, Cu, Ni, and Ca in synthetic groundwater were studied using electrochemical corrosion tests (potentiodynamic test and electrochemical impedance spectroscopy [EIS] measurements) and analytical techniques. Neither CS nor new alloy steels showed passive behavior in this synthetic groundwater. New alloy steels containing Cr-Co, Cr-Cu-Ni, and Cr-Cu-Ni-Ca showed higher corrosion resistance than CS in the potentiodynamic tests. EIS measurements showed that the Nyquist plot presented two time constants. The high-frequency resistance component (R_rust) and low-frequency resistance component (R_ct) were affected by the alloying elements. The polarization resistance (R_p = R_rust + R_ct) of steels could be clearly ranked as Cr-Cu-Ni-Ca steel >> Cr-Cu-Ni steel > Cr-Co steel > CS. Results of surface analyses (x-ray photoelectron spectroscopy [XPS] and electron probe microanalysis [EPMA]) showed that Cr and Cu were concentrated in the inner region of rust. However, Co and Ni were uniformly distributed all over the rust layer. These alloying elements improved corrosion resistance by the rust film formation during the initial stage of corrosion of low-alloy steel in synthetic groundwater. Furthermore, Ca induced strong alkalinity in the electrolyte near the steel surface, reduced the dissolution of steel, and finally led to a suppression of the formation of the rust layer in the aqueous condition.