Ni-Cr-W-C cast alloy has been expected as the candidate material for high temperature air heater tube in gasification and ash melting system due to good corrosion resistance and high temperature strength. However the corrosion resistance of the conventional alloy had not been sufficient yet because of accelerative corrosion of cyclic oxidation and chlorination around 1173K.
In this work, to increase the heater tube life by enhancing the high temperature corrosion resistance, effects of Al and Si addition on the corrosion in air and N2-10%O2-1000ppmHCl gas was investigated and the following results were obtained.
(1) Al addition was most efficient to enhance the resistance of the gas corrosion due to inhibition of the cyclic oxidation and chlorination by AlCl3 formation and vaporization and by Al2O3 protect layer. But Al addition induced internal AlN formation over 1373K.
(2) Although effect of only Si addition on the enhancement of high temperature corrosion was little, in case of combined addition with Al, Si was efficient to enhance gas corrosion moreover and inhibition the AlN formation. However, Si induced decrease of oxidation scale adherence and matrix ductility.
Furthermore, since carbide and the neighborhood were preferentially attacked by gas corrosion, effects of Cr and W, which were composition of carbide, were investigated. It was found that Cr was effective but excess addition induced decrease of gas corrosion resistance because of α-Cr precipitation. On the other hand, W was effective to alkaline fused salt corrosion, but not to gas corrosion.
On the basis of the above results and for consideration of manufacturable properties, Ni-27Cr-10W-2Al-2Si-0.3C has been developed as high temperature air heater tube material. The developed alloy is superior in high temperature corrosion resistance to conventional alloys and is expected to contribute for increasing the heater tube life.
