Laboratory testing for evaluation of steels and alloys used in superheater area of boilers for biomass-based fuels

Short-term half-immersion tests in molten alkali salt of eutectic composition together with, e.g., thermogravimetry, autoclave tests and deposit crucible tests, were used to investigate the corrosion behaviour of various superheater tube steels and alloys for applications in superheater areas of boilers for biomass-based fuels. In order to simulate flue gas conditions where partial pressure of sulphur dioxide may be very low, air or wet air and various mixtures of N-2-CO2-H2O-O-2 were used as test atmospheres. Most tests were carried out around 530degreesC. The combined effect of carbon dioxide and water vapour was found to enhance the oxidation rates, even affecting the materials ranking. Water vapour was found to enhance alkali chloride attack. Sodium chloride was detrimental towards low alloy steels, whereas potassium chloride attacked preferentially alloys which were high in chromium. In the half-immersion tests, low-alloy steels and low-grade stainless steels were damaged initially by inter-granular attack. Subsequently, rapid oxide growth occurred on the specimen surface areas located above the immersion line. Highly alloyed materials suffered from pitting, and in some cases, from severe localized attack due to basic fluxing. Due to localized corrosion, the extent of corrosion attack for stainless or high-alloy steels approached or even exceeded that of low alloy steels in the half-immersion tests. The high-molybdenum-high-chromium nickel alloy (Alloy 625), and a stainless steel containing about 24 % chromium and 13 % nickel showed the best corrosion resistance. Corrosion morphologies obtained, as well as the ranking of the alloys tested, corresponded well with results from long-term corrosion tests in real plant.