High-Temperature Corrosion and Metallurgical Degradation
High-temperature corrosion
At elevated temperature, corrosive species in the process attack metal at rates that depend on temperature, partial pressure of the aggressive species, and alloy content. Sulfidation occurs in H2S/H2 streams from roughly 500 to 1000 deg F and is resisted by increasing chromium. Oxidation (scaling) of carbon steel begins around 900 deg F. Naphthenic acid corrosion acts from roughly 350 to 750 deg F, depends on total acid number and velocity, and is more severe in high-velocity areas; steels with higher molybdenum resist it. Carburization and metal dusting occur in carbon-bearing gases at high temperature, with metal dusting producing catastrophic pitting and dust-like debris. Fuel ash corrosion involves vanadium and sodium in combustion gases. Hydrogen attack of steel by high-temperature, high-pressure hydrogen follows the Nelson curves and is time and temperature dependent.
Metallurgical degradation
Long-time exposure changes microstructure without adding corrosion: spheroidization of carbides above roughly 800 deg F in carbon steel, graphitization of carbon steel and C-0.5Mo (often in weld heat-affected zones) above roughly 800 deg F, sigma phase formation in 300-series stainless between roughly 1000 and 1700 deg F, 885 deg F embrittlement of ferritic and duplex stainless, and temper embrittlement of Cr-Mo steels after slow cooling through roughly 650 to 1070 deg F. Creep is a high-temperature, stress-driven failure with bulging and intergranular cavitation.
Traps and worked example
Questions test whether you know the controlling variable - temperature, partial pressure, or velocity - and whether the mechanism is internal (HTHA) or surface (sulfidation). Worked example: a carbon steel hydrogen line at 600 deg F with high hydrogen partial pressure is suspected of high-temperature hydrogen attack. Conventional thickness UT will not find internal fissures; use velocity-ratio and backscatter UT plus metallographic replication, and check the operating point against the Nelson curves.