Study of hot corrosion of alloy 800 using linear polarisation resistance and weight loss measurement

Abstract

A study of the hot corrosion of alloy 800 at 700 and 900°C in 80Na₂SO₄+20V₂O₅ (wt-%) has been carried out. The techniques included linear polarisation resistance (LPR) and weight loss measurement during 10 days of experiments, supported by X-ray measurements and electron microscopy. The degradation of the alloy was discussed in terms of the dissolution of the external Cr₂O₃ layer with subsequent oxidation and sulfidation of the alloy. Both LPR and weight loss results showed that the corrosion rate is slightly higher at 900 than at 700°C. At the beginning of the experiments, the corrosion rates obtained using the two techniques increase at both temperatures, but decrease later on until they reach steady values. The measured corrosion rates were within the same order of magnitude for both techniques, but the rates obtained with the electrochemical technique were lower than those obtained using weight loss at 700°C, while at 900°C, the opposite was observed. This is because at 700°C the step that controls the corrosion process is the diffusion of species in the alloy or in the melt, whereas at 900°C, the step that controls the reaction is either charge transfer or there is a mixed control. It is evident that electrochemical techniques are powerful tools for monitoring on line hot corrosion processes when the corrosion rate is controlled by an electrochemical reaction, but may induce some errors when the molten salt changes from an ionic to an electronic conductor.