The effect of sodium peroxide on the corrosion behaviour of some stainless steel alloys in a carbonate melt

The corrosion behaviour of a ferritic (alloy 1) and two austenitic stainless steel alloys (alloys 2 and 3) was studied in a molten Li2CO3-Na2CO3-K2CO3 ternary mixture in the presence of Na2O2 additions at temperatures of 475, 500, 525 and 550degreesC. The techniques of measurements were open circuit potential, galvanostatic anodic polarisation and cyclic voltametry. The addition of Na2O2 increased the concentration of oxide ions in the carbonate melt. There is a tendency for oxidation and passivation of the alloys to commence immediately on their immersion in the melt, and end at the passivity breakdown, where the decomposition of carbonate ions occurs with the formation Of CO2 and O-2 gases. The oxide scales of a ferritic alloy are less protective than those formed on the austenitic alloys. The oxide scales, in most cases, are multilayered, and the presence of Na2O2 in the carbonate melt gives rise to the formation of a more protective inner layer of oxide scales on the surface of the austenitic alloys.