Stability of spontaneous passive films on high strength Mo-containing stainless steels in aqueous solutions |
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Authors: | F El-Taib Heakal AA Ghoneim AM Fekry |
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Affiliation: | (1) Chemistry Department, Faculty of Science, Cairo University, Giza, Egypt |
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Abstract: | The stability of naturally grown passive films on some Mo-containing stainless steel specimens was examined in aerated and
deaerated universal buffer solutions with different pH (2–12) as well as in sulphate and chloride solutions. Open circuit
potential (E
oc) and electrochemical impedance spectroscopy (EIS) were used as measuring techniques. In all cases, E
oc shifts towards less negative values with time until the potential reaches its steady-state (E
ss) value. The E
ss value is found to be more positive with decrease in solution pH or increase in Mo content in the alloy and becomes less positive
in deaerated buffer solutions. Also, the thickening rate of the outer layer for the duplex passive film increases with increasing
extent of Mo in the steel substrate or pH of the test solution. For a given alloy, E
ss decreases linearly with the anion concentration (C), and is always more positive in Cl− than in SO42− media for C ≥ 0.05 M. Analysis of the EIS data showed that the total resistance (R
T) of the passive film has higher values in aerated solutions, and is generally lower in basic solutions. This indicates that
lower solution pH favours the formation of oxide films offering better protection. Furthermore, the higher values of R
T in Na2SO4 solutions suggest the formation of more stable passive films in sulphate than in chloride solutions. This is discussed on
the basis of the relative degree of anion incorporation into the passive films. |
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Keywords: | chloride CPE EIS Mo-containing steels passive films pH spontaneous growth sulphate |
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