Manabu Noguchi

Resistance of Coatings for Boiler Components of Waste-to-Energy Plants to Salt Melts Containing Copper Compounds

The accelerating effect of heavy metal compounds on the corrosive attack of boiler components like superheaters poses a severe problem in modern waste-to-energy plants (WTPs). Coatings are a possible solution to protect cheap, low alloyed steel substrates from heavy metal chloride and sulfate salts, which have a relatively low melting point. These salts dissolve many alloys, and therefore often are the limiting factor as far as the lifetime of superheater tubes is concerned. In this work the corrosion performance under artificial salt deposits of different coatings, manufactured by overlay welding, thermal spraying of self-fluxing as well as conventional systems was investigated. The results of our studies clearly demonstrate the importance of alloying elements such as molybdenum or silicon. Additionally, the coatings have to be dense and of a certain thickness in order to resist the corrosive attack under these severe conditions.

Effect of alloying elements on corrosion resistance of Ni-base cast alloys in waste incineration environments

After examining practical structural materials for use in the high-temperature environments of waste incinerators, it was found that a Ni-Cr-W casting alloy is promising. However, the corrosion reaches its peak in the vicinity of 900°C. This peak is related to the behavior of chlorides that form in the interior of the alloy. As the chlorides continuously generate and decompose, the Cl recycles, accelerating corrosion. It is thought that when the temperature rises, the chlorides vaporize outside the alloy, suppressing Cl recycling and thus reducing the amount of corrosion. Adding Al to the alloy generates highly volatile AlCl3, making it possible to prevent the accumulation of chlorides in the vicinity of 900°C and lower the corrosion peak. As a result of conducting exposure testing for a year at an actual incinerator, it was demonstrated that adding Al is an effective way to improve corrosion resistance.