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Effect of mechanical pre-loadings on corrosion resistance of chromium-electroplated steel rods in marine environment

Abstract : Providing high hardness, low friction coefficient, as well as, relatively good corrosion resistance, chromium-plated coatings (similar to 20 mu m) are widely used for steel cylinder rods in marine environment. However, the standardized corrosion test method (ISO 9227, NSS) used to evaluate efficiency of this type of coatings does not take into account in-service mechanical loadings on cylinder rods. Nevertheless, the uniform initial network of microcracks in chromium coating is changing under mechanical loadings. Propagation of these microcracks explains premature corrosion of the steel substrate. The aim of the study was to evaluate relationship between mechanical loadings, propagation of microcracks network and corrosion resistance of chromium coatings. After monotonic pre-loading tests, it was demonstrated by microscopic observations that the microcracks propagation started at stress levels higher than the substrate yield stress (520 MPa). The microcracks become effective, i.e. they have instantly undergone through the whole coating thickness to reach the steel substrate. The density of effective microcracks increases with the total macroscopic level, i.e. the intercrack distance goes from 60 +/- 5 mu m at 1% of total strain to approximately 27 +/- 2 mu m at 10%. Electrochemical measurements have shown that the higher the plastic strain level applied during mechanical loading, the more the corrosion potential of the sample decreased until reaching the steel substrate value of approximately -0.65 V/SCE after 2 h of immersion. The polarization curves have also highligthed an increase in the corrosion current density with the strain level. Therefore, electrochemical measurements could be used to realize quick and comprehensive assesment of the effect of monotonic preloadings on corrosion properties of the chromium coating.
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Submitted on : Wednesday, May 2, 2018 - 3:32:40 PM
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Varvara Shubina Helbert, Matthieu Dhondt, Rémi Homette, Shabnam Arbab Chirani, Sylvain Calloch. Effect of mechanical pre-loadings on corrosion resistance of chromium-electroplated steel rods in marine environment. Materials Research Express, IOP Publishing Ltd, 2018, 5 (3), pp.036522. ⟨10.1088/2053-1591/aab477⟩. ⟨hal-01783645⟩



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