The effect of salt composition on the chlorine corrosion of low alloy steels

Material wastage of superheater tubes made of low alloy steels is greatly accelerated under normal operating conditions if alkali chlorides are stable in the deposits. The effect of salt composition on the corrosion behavior especially that of the alkali cation, Na or K was investigated using crucible tests in a thermobalance under a flowing N₂-O₂-CO₂-atmosphere, followed by SEM/EDS analyses and thermodynamic calculations. For both salts, rapid corrosion occurred due to the known mechanism of active oxidation. In the case of sodium chloride, a major part of the metal loss was caused by volatilization of metal chlorides even at temperatures as low as 480 °C, while in the case of potassium chloride, enhanced oxide growth almost conclusively determined the corrosion rates. Further, no evidence of significant iron or potassium evaporation was found up to a temperature of 530 °C. The different behavior of the steels when exposed to the two alkali chlorides has been explained by the difference in the thermodynamic stability of the corresponding alkali ferrites. However, the difference in the thermodynamic stability and volatility of the complex chlorides of these alkali metals and iron or various other transition metals present as alloying elements in these steels may also be important. Moreover, applying temperature cycles typical for superheater tubes in boilers enhanced the metal loss rate compared to isothermal exposures. Also, temperature cycling affects the corrosion scale morphology.