Type II hot corrosion

Type II Hot Corrosion takes place at temperatures below the melting point of Na2S04. As the reaction proceeds, dissolution of corrosion products occurs and a melt is formed. In the case of Ni, NiS04 and/or a solid solution of Na2S04 + NiS04 is gradually formed through the reaction of NiO + SO3 ... 

Type II hot corrosion occurs between 600 and 850 °C and involves base-metal sulfates that require a certain concentration of sulfur trioxide for stabilization. These sulfates, when stable, react with alkali metals to form salts with low melting points and impede protective oxide layer formation [17]. 

Reaction (2-48) is shifted to the left, leading to a salt melt of low 0 activity which corresponds to low Na20 activity. This form of hot corrosion is usually encountered at lower temperatures of between 600 and 800 °C. It is also often called type II hot corrosion or low temperature hot corrosion. Type I hot corrosion has been assigned to the situation at high temperatures, which means above the melting point of Na2S04 of 884 °C. In particular, low temperature hot corrosion can lead to complex surface layers consisting of solid and liquid phases. This... 

Low-temperature hot corrosion in the temperature range up to 800°C, which is also called type II hot corrosion... 

This reaction sequence that characterizes type II hot corrosion represents an acidic fluxing mechanism since positively charged metal cations are formed. As a summary reaction for nickel the following equation can be given ... 

Low temperature (Type II) hot corrosion (LTHC) occurs well below the melting point of Na2S04 (884°C). The reaction product morphology is characterized by a non-uniform attack in the form of pits, with only little sulphide formation close to the alloy/scale interface and little depletion of Cr or A1 in the alloy substrate (Rapp and Zhang, 1994). 

Hot corrosion refers to corrosion between a metal-oxide and a molten salt deposit. It occurs at the solid-gas interface. Molten salts are extremely corrosive and their presence increases the rate of corrosion by two orders of magnitude when compared to high-temperature corrosion at similar temperatures and conditions [27—29]. They act as solvents, preventing the formation of a stable oxide, or they chemically react with the oxide layers. By transporting through, the salts may damage the protective oxide layers. Two different types of hot corrosion exist, namely. Type I and II. 

Figure 8.40 Schematic diagram to show the relationship of the different hot corrosion mechanisms as a function of temperature and SO3 pressure, (i) Type II, gas-phase induced acidic fluxing, (ii) At high SO3 pressures (PSO3 > 10 atm) pronounced sulphide formation accompanied by oxidation of sulphides and fluxing reactions, (iii) Type I (alloy-induced acidic fluxing basic fluxing, sulphidation).

Two forms of hot-corrosion are encountered (a) Type I (high-temperature hot-salt corrosion) is prevalent in aircraft turbine engines operating at temperatures over 850 C (b) type II (low-temperature hot-salt corrosion) occurs especially in industrial and marine turbine engines over the temperature range 700-800°C. 

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