The effect of NaCl on the corrosion of stainless steel alloys in a eutectic mixture of NaNO_3-KNO_3

摘要

THE CORROSION behaviour of a ferritic and an austenitic stainless steel alloy was investigated in a nitrate melt in presence of NaCl. The potential-and current-time measurements under open-circuit conditions, and galvanostatic anodic-polarization measurements, were carried out in 0.5, 1.0, and 1.5 M NaCl in the nitrate melt at different temperatures (400-600 deg C). At high temperatures (500 and 600 deg C), fluctuations were observed in the potential-time curves, which increase with the increase of NaCl concentration. These fluctuations are less for the austenitic stainless steel alloy than for the ferritic one. The presence of NaCl in the melt showed a detrimental effect on the stability of the protective oxide film. The values of activation energy, calculated from the Arrhenius plots, decreased with increasing NaCl concentration in the melt. The current-time curves in NaCl have the general features of those obtained in the pure me the main difference is the presence of fluctuations on the curves, which are greater for the ferritic stainless steel alloy than for the austenitic one. The galvanostatic anodic-polarization measurements show the onset of passivation at 400, 450, and 500 deg C for both alloys, and also at 550 deg C for the austenitic alloy. The values of the passivity potential range for the alloys were calculated; they increase with increasing NaCl concentration, and decrease with the decrease of temperature. The values of the passivity potential range were higher for the austenitic alloys than for those of the ferritic alloy, indicating the higher corrosion resistance of the austenite alloy. The corrosion parameters were deduced from the polarization curves, and the results obtained of the effect of NaCl are discussed on the basis of the accelerating effect of chloride ions and the competing process between Cl~ and O_2~ in the melt.