Under high temperature and molten deposits, oxygen and other corrosive gases act simultaneously. The corrosion produced is called thermal corrosion. For example, the corrosion of high temperature alloys in high temperature sulfur and salt containing gas.
In this case, the sulfide film formed by corrosion is loose and porous or cracked, and the concentration of crystal defects of the sulfide is large, which is conducive to the diffusion of metal, oxygen and sulfur through the film, and the melting point of the metal sulfide is low, which is easy to generate a melting point. For lower metal-metal sulfide eutectics (for example, the melting point of Ni3S2 is 787 ° C, while the melting point of Ni-Ni3S2 eutectic is only 645 ° C), for the above reasons, thermal corrosion is often more severe than simple high temperature oxidation. If the atmosphere contains elements such as vanadium and molybdenum, since the melting point of V2O3 is only 674 ° C, MoO3 is volatile at high temperatures, which will cause "catastrophic" high-temperature corrosion.
The main measures to prevent hot corrosion of seamless steel pipes are:
①Adjust alloy composition. The purpose is to form a dense oxide film and improve the oxidation resistance of the alloy. The Pirin-Bedworth ratio of the added alloy element should be greater than 1 (VMO / VM> 1). The Gibbs free energy of alloying elements should be negative compared to the base metal and can be oxidized first. Alloying elements should be selected according to Howe's law. In order to improve the thermal corrosion resistance of 42crmo seamless steel pipes, the commonly used alloying elements are chromium, aluminum, titanium, and rare earth. The role of chromium and a small amount of rare earth has been recognized, and the role of aluminum and titanium is still controversial.
③Change the environmental media conditions. Such as controlling the atmosphere to prevent oxidation, measures such as fuel desalination and desulfurization to prevent thermal corrosion.
