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

A study of the hot corrosion of alloy 800 at 700 and 900 degreesC in 80Na(2 )SO(4)+20V(2)O(5) (wt-%) has been carried out. The techniques included line ar polarisation resistance (LPR) and weight loss measurement during 10 days of experiments, supported by X-ray measurements and electron microscopy. T he degradation of the alloy was discussed in terms of the dissolution of th e external Cr2O3 layer with subsequent oxidation and sulfidation of the all oy. Both LPR and weight loss results showed that the corrosion rate is slig htly higher at 900 than at 700 degreesC. At the beginning of the experiment s, the corrosion rates obtained using the two techniques increase at both t emperatures, but decrease later on until they reach steady values. The meas ured corrosion rates were within the same order of magnitude for both techn iques, but the rates obtained with the electrochemical technique were lower than those obtained using weight loss at 700 degreesC, while at 900 degree sC, the opposite was observed. This is because at 700 degreesC the step tha t controls the corrosion process is the diffusion of species in the alloy o r in the melt, whereas at 900 degreesC, 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 co rrosion processes when the corrosion rate is controlled by an electrochemic al reaction, but may induce some errors when the molten salt changes from a n ionic to an electronic conductor. MST/4490.