Type I hot corrosion

Type I hot corrosion is the transport of sulfur from a sulfur deposit (a molten salt) through an existing oxide layer, where it forms stable sulfides with chromium. As time progresses, chromium fully reacts with sulfur and can no longer move through the oxide layer to provide protection against oxidative attack. Type I hot corrosion occurs from 750 to 950 °C [16]. 

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... 

High-temperature hot corrosion in the temperature range between 800°C and 1000°C, which is also called type I hot corrosion... 

Schematic of type I hot corrosion (for explanation see text) (a) during the incubation period and (b) after the...

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).

High-temperature hot corrosion (HTHC), labeled as Type I (800°C-950°C)... 

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. 

Actually, in many cases strength and mechanical properties become of secondaiy importance in process applications, compared with resistance to the corrosive surroundings. All common heat-resistant alloys form oxides when exposed to hot oxidizing environments. Whether the alloy is resistant depends upon whether the oxide is stable and forms a protective film. Thus, mild steel is seldom used above 480°C (900°F) because of excessive scaling rates. Higher temperatures require chromium (see Fig. 28-25). Thus, type 502 steel, with 4 to 6 percent Cr, is acceptable to 620°C (I,I50°F). A 9 to 12 percent Cr steel will handle 730°C (I,350°F) 14 to 18 percent Cr extends the limit to 800°C (I,500°F) and 27 percent Cr to I,I00°C (2,000°F). 

There are many temporary protectives on the market and it would be impracticable to describe them individually. However, they may be classified according to the type of film formed, i.e. soft film, hard film and oil film the soft film may be further sub-divided into solvent-deposited thin film, hot-dip thick film, smearing and slushing types. All these types are removable with common petroleum solvents. There are also strippable types based on plastics (deposited by hot dipping or from solvents) or rubber latex (deposited from emulsions) these do not adhere to the metal surfaces and are removed by peeling. In addition there are volatile corrosion inhibitors (V.C.I.) consisting of substances, the vapour from which inhibits corrosion of ferrous metals. 

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