High Temperature Corrosion of Some Stainless Steels

Doctoral thesis, 2009

The high temperature corrosion of the alloy Sanicro 28 (35Fe27Cr31Ni) is addressed in this thesis. The influence of water vapour and two potassium-containing salts was investigated. In addition, three other alloys were studied. Polished steel coupons were isothermally exposed in tube furnaces at 600–800°C in 5% O2 and in 5% O2 + 40% H2O. Exposure time was 1–672 hours. Some samples were coated with salt prior to exposure (0.1 mg/cm2 KCl(s) or K2CO3(s)). Other samples were exposed in situ to KCl(g), using a crucible containing KCl(s) positioned upstream of the samples. Besides laboratory studies, field exposures were conducted in full-scale power plants. The samples were investigated by gravimetry, XRD, SEM, EDX, FIB, TEM, AES and IC. Alloy Sanicro 28 forms a protective, duplex scale consisting of a corundum type chromium-rich oxide, (CrxFe1-x)2O3, and a spinel type oxide, in O2 and in O2 + H2O environments. In the presence of water vapour, chromic acid, CrO2(OH)2, is vaporized. However, the resulting chromia depletion of the outer part of the oxide does not lead to breakaway corrosion, due to the high chromium to iron ratio in the alloy. Potassium chloride strongly accelerates the high temperature corrosion of alloy Sanicro 28. Corrosion is initiated by the formation of potassium chromate, K2CrO4, through the reaction of KCl with the protective, chromium-rich oxide. Chromate formation acts as a sink for chromium in the oxide and leads to a deterioration of its protective properties. No evidence was found to support the formation of transition metal chlorides. Once the protective chromium-rich oxide has been depleted in chromium by chromate formation, the alloy becomes susceptible to direct attack by the remaining KCl(s). The KCl-induced corrosion of alloy Sanicro 28 is the same in principle regardless of whether KCl is in solid or gaseous form. Moreover, the reaction with K2CO3(s) is analogous to that with KCl(s). Alkali-induced corrosion of chromia-forming alloys is important, e.g. in fireside corrosion in biomass and waste fired power plants. The present results imply that the corrosivity in boilers can be mitigated by changing the fuel mix so as to convert alkali chlorides and carbonates, in the flue gas and in deposits, into compounds that do not react with chromia. This can be achieved by adding sulphur to the fuel, which converts alkali chlorides and carbonates to the corresponding sulphates.