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01462 MODELLING THE EFFECTS OF CHLORIDE CONTENTS, PH AND CATHODIC POLARIZATION ON CREVICE CORROSION RATES AND CORROSION START TIMES OF PURE CHROMIUM

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Product Number: 51300-01462-SG
ISBN: 01462 2001 CP
Author: Hans Hoffmeister
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A crevice corrosion model is presented for pure chromium aiming at providing basic information on the mechanisms leading to localized corrosion in an already deoxygenated crevice. Based on selected cathodic and anodic polarization curves the model is operating in successive l 0 s time steps assuming thermodynamic equilibria for the dissolution of chromium, the initial formation and final dissolution by chloride ions of the passivating chromiumhydroxide followed by formation of chromiumchloride and hydrochloric acid. While assuming no diffusion of the chromium ions from the crevice, their positive charges are equilibrated immediately by respective negative chloride ions. For mass calculation of chromiumhydroxyde and chromiumchloride the four component equilibrium diagram water-chromium-hydrogen-chloride was tentatively established from the water-chromium-hydrogen Pourbaix diagram and sporadic literature information on the effect of chlorides on solubilities of chromiumhydroxyde and chromiumchloride. As a result of the calculations for constant chloride, pH, and cathodic polarization the corrosion potentials and corrosion currents are reflecting the formation and dissolution of the passivating chromiumhydroxide with increasing times as was shown in earlier experiments. Based on the above rigorous assumptions the identified times for the start of depassivation and the fmal breakdown of the passive layer should represent worst case life time assessments since any slower diffusion would only slow down the process. The effects of various cathodic polarization was calculated from increasing levels of the initial corrosion potentials and slopes of the cathodic polarization curve, both providing reduced depassivation start and final passive layer break down times as well as increasing corrosion rates. For constant cathodic polarization, the increase of chloride and hydrogen ions provides severe reductions in break down times documenting critical contents above which, due to the absence of stable chromiumhydroxyde no passive layer will be formed at all. The calculations do so far, however, not yet consider possible effects of hydrogen and chloride ions on the cathodic polarization curve. As a summarizing result, a chloride- pH- domaine diagram for given cathodic polarization is established. Keywords: Crevice corrosion modelling, chromium, chloride, pH, polarization.
A crevice corrosion model is presented for pure chromium aiming at providing basic information on the mechanisms leading to localized corrosion in an already deoxygenated crevice. Based on selected cathodic and anodic polarization curves the model is operating in successive l 0 s time steps assuming thermodynamic equilibria for the dissolution of chromium, the initial formation and final dissolution by chloride ions of the passivating chromiumhydroxide followed by formation of chromiumchloride and hydrochloric acid. While assuming no diffusion of the chromium ions from the crevice, their positive charges are equilibrated immediately by respective negative chloride ions. For mass calculation of chromiumhydroxyde and chromiumchloride the four component equilibrium diagram water-chromium-hydrogen-chloride was tentatively established from the water-chromium-hydrogen Pourbaix diagram and sporadic literature information on the effect of chlorides on solubilities of chromiumhydroxyde and chromiumchloride. As a result of the calculations for constant chloride, pH, and cathodic polarization the corrosion potentials and corrosion currents are reflecting the formation and dissolution of the passivating chromiumhydroxide with increasing times as was shown in earlier experiments. Based on the above rigorous assumptions the identified times for the start of depassivation and the fmal breakdown of the passive layer should represent worst case life time assessments since any slower diffusion would only slow down the process. The effects of various cathodic polarization was calculated from increasing levels of the initial corrosion potentials and slopes of the cathodic polarization curve, both providing reduced depassivation start and final passive layer break down times as well as increasing corrosion rates. For constant cathodic polarization, the increase of chloride and hydrogen ions provides severe reductions in break down times documenting critical contents above which, due to the absence of stable chromiumhydroxyde no passive layer will be formed at all. The calculations do so far, however, not yet consider possible effects of hydrogen and chloride ions on the cathodic polarization curve. As a summarizing result, a chloride- pH- domaine diagram for given cathodic polarization is established. Keywords: Crevice corrosion modelling, chromium, chloride, pH, polarization.
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