The effects of metal cations on corrosion of mild steel in model fresh water were investigated by electrochemical techniques and immersion tests. Analysis of X-ray photoelectron spectroscopy showed that metal cations have large hardness of cations, X, are incorporated in passive films. The electrochemical and immersion results showed that X is not suitable as a corrosion indicator of mild steel. A novel corrosion indicator, "corrosion inhibitory effect of cations Y", that consists of X and molar volume ratio, ∆V, is proposed, and it is shown that the novel indicator can estimate the corrosion rate of mild steel in fresh water.
The effect of the metal cations, Na + , K + , Ca 2+ , Mg 2+ , Zn 2+ , and Ni 2+ , on the oxide film structure and morphology changes during long term immersion corrosion tests of aluminum alloy (A3003) in model tap waters was investigated by X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy. The effect of the metal cations on the corrosion behavior was also investigated with mass change and electrochemical tests. The hardness of the metal cations, X, based on the Hard and Soft Acids and Bases (HSAB) concept was applied to explain the effect of metal cations on the passive oxide film structure and corrosion resistance. The mass change rate and corrosion current density decreased with increasing metal cation hardness. The XPS results showed that hard metal cations like Zn 2+ and Ni 2+ were incorporated in the oxide films, while the four soft metal cations were not incorporated in the oxide films.The results are in good agreement with those which could be expected from the HSAB hardness of the metal cations.
Inhibition ability of gluconates for fresh water corrosion of mild steel changed with metal cations were investigated by immersion tests and electrochemical impedance spectroscopy (EIS) tests. The results of the tests showed that the inhibition ability of gluconates was enhanced by metal cations that have large "corrosion inhibitory effect of cation, Y", in model fresh water. Surface analysis showed that gluconate ligands and metal cations that have large Y can bond on mild steel and that they form a protective film on the steel. EIS analysis with an equivalent circuit suggested that metal cations that have large Y would reduce the area of defects in the protective film of gluconates.
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