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The Protection Performance of Polypyrrole Coating Doped with Different Counter Anions for the Corrosion of NiTi Alloy

The electrochemical corrosion behavior of NiTi alloy electrodeposited conductive polypyrrole (PPy) coatings doped with small size sulfate (SO42 ￻ ), medium size para-toluene sulfonate (pTS ￻ ) and large size dodecylbenzene sulfonate (DBS ￻ ) counter anions has been studied by electrochemical measurements and surface analysis. It is demonstrated that PPy coatings inhibit the corrosion of NiTi substrate in chloride solution by shifting the corrosion potential and pitting potential positively and increasing the impedance of corrosion process. The protective performance of PPy coatings for NiTi alloy is in the order: PPy/pTS < PPy/SO4 < PPy/DBS. The protective performance of the doped PPy coatings for NiTi alloy is related to their anions exchange abilities. The worse anion exchange ability PPy coating has, the better protective performance. The increase of solution pH increases the corrosion rate of the coated NiTi substrate.