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Corrosion Special Topical Papers |
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Protective Coatings: Performance Evaluation and Life Prediction*Dr. Jianhai Qiu
Electrochemical Impedance Spectroscopy (EIS)Fig.4 [4] is the impedance spectra for a rubberized fibre coating applied on a properly prepared steel surface. The pore resistance remained high even after 14 weeks immersion in 3.5% NaCl solution, whereas for the same coating applied on the steel substrate with mill scales, a noticeable decrease in the pore resistance is observed after 1 week immersion in NaCl solution (Fig.5).
Fig.4 The pore resistance remains high after 15 weeks immersion in 3.5% NaCl solution
Fig.5 The pore resistance decreased to below 108 after 5 weeks immersion in NaCl solution In general, when a coating system can maintain the impedance value above 109 Ohm.cm2 , corrosion at the metal/coating interface should not be an issue. When the impedance value drops below 107 Ohms.cm2 , one should be concerned about the corrosion activity at the metal/coating interface. The corrosion rate underneath the coating can be determined from Rt: CorrRate (mm/y) = k* (B/Rt) where k is the conversion factor and B is the proportionality constants determined by Tafel polarisation. |
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