High-temperature corrosion

Two types of sulfate-induced hot corrosion are generally distinguished: Type I takes place above the melting point of sodium sulfate, whereas Type II occurs below the melting point of sodium sulfate but in the presence of small amounts of SO3. In Type I, the protective oxide scale is dissolved by the molten salt. Sulfur is released from the salt and diffuses into the metal substrate, forming grey- or blue-colored aluminum or chromium sulfides. With the aluminum or chromium sequestered, after the salt layer has been removed, the steel cannot rebuild a new protective oxide layer. Alkali sulfates are formed from sulfur trioxide and sodium-containing compounds. As the formation of vanadates is preferred, sulfates are formed only if the amount of alkali metals is higher than the corresponding amount of vanadium. The same kind of attack has been observed for potassium sulfate and magnesium sulfate. ==Vanadium==

Vanadium is present in petroleum, especially from Canada, western United States, Venezuela and the Caribbean region, often bound to porphyrine in organometallic complexes. These complexes get concentrated on the higher-boiling fractions, which are then form the base of heavy residual fuel oils. Residues of sodium, primarily from sodium chloride and spent oil treatment chemicals, are also present in this petroleum fraction. Combusting any amount more than 100 ppm of sodium and vanadium will yield ash capable of causing fuel ash corrosion. At high temperatures or when there is a lower availability of oxygen, refractory oxidesvanadium dioxide and vanadium trioxideform. These more reduced forms of vanadium do not promote corrosion. However, at conditions most common for burning, vanadium pentoxide gets formed. Together with sodium oxide, vanadates of various composition ratios are formed. Vanadates of composition approximating Na2O.6 V2O5 have the highest corrosion rates at the temperatures between 593 °C and 816 °C; at lower temperatures, the vanadate is in solid state, and at higher temperatures, vanadates with higher proportion of vanadium contribute the most to higher corrosion rates. The presence of sodium in a ratio of 1:3 gives the lowest melting point and must be avoided. This melting point of 535 °C can cause problems on the hot spots of the engine like piston crowns, valve seats, and turbochargers. ==Lead==

Lead can form a low-melting slag capable of fluxing protective oxide scales. Lead is more often known for causing stress corrosion cracking in common materials that are exposed to molten lead. The cracking tendency of lead has been known for some time, since most iron based alloys, including those used in steel containers and vessels for molten lead baths, usually fail due to cracking. Lead can also induce liquid metal embrittlement, a rapid brittle fracture under tensile stress where liquid lead penetrates grain boundaries. Lead also causes "hot shortness" in stainless steel, leading to a loss of ductility at high temperatures as lead segregates to grain boundaries. Additionally, in nuclear reactor environments, lead impurities can concentrate in steam generator tubes, linked to stress corrosion cracking, particularly aggravated in caustic conditions. ==See also==