Types of hot corrosion

Hot corrosion can occur at high temperatures where the deposit is in the liquid state right from the beginning or the solid deposit turns into liquid during the exposure as a result of reaction with the enviromnent. These two types of hot corrosion processes are termed High Temperature Hot Corrosion (HTHC) or Type L and Low Temperature Hot Corrosion (LTHC) or Type IL respectively (Khanna and Jha, 1998). 

High temperature (Type 1) hot corrosion (HTHC) is normally observed in the temperature range of about 825-950°C when the condensed phase is clearly liquid. The typical microstructure for HTHC shows the formation of sulphides and a corresponding depletion of the reactive components in the alloy substrate. The external corrosion products consist of oxide precipitates dispersed in the salt film. The presence of the pore, crevice or crack across a protective film can lead to the sulphidation of the alloy substrate. This results in a significant shift in the basicity of the salt film. Once the fused salt contacts the alloy substrate, the rate and duration of the rapid corrosion kinetics are decided by the magnitude and gradient of salt basicity relative to the local solubilities for the oxide scale phases (Rapp and Zhang, 1994). 

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). 

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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. 

Usually, there are two types of hot corrosion mechanisms (Figure 5.9) ... 

FIGURE 5.9 Types of hot corrosion in terms of the metal loss as a function of temperature. (From Pokluda J. and Kianicova M., Damage and Performance Assessment of Protective Coatings on Turbine Blades, Gas Turbines, Gurrappa Injeti (Ed.), InTech. ISBN 978-953-307-146-6. Available from http //www.intechopen. com/books/gas-turbines/damage-and-performance-assessment-of-protective-coatings-on-turbine-blades, 2010. With permission.)... 

Thin molten deposits of Na2S04 are formed on alloys used in gas turbines. Depending upon conditions, different types of hot corrosion attack can occur. High-temperature hot corrosion is observed during operation of gas turbines when deposits of Na2S04 accumulate on alloys at temperatures above... 

Generally, two types of hot corrosion are distinguished in the literature talking about the corrosion situation induced by Na and S [6] ... 

There are now two distinct forms of hot corrosion recognized by the industry, although the end result is the same. These two types are high-temperature (Type 1) and low-temperature (Type 2) hot corrosion. 

Since the reliability of gas turbines in the power industry has been lower than desired in recent years because of hot-corrosion problems, techniques have been developed to detect and control the parameters that cause these problems. By monitoring the water content and corrosive contaminant in the fuel line, any changes in fuel quality can be noted and corrective measures initiated. The concept here is that Na contaminants in the fuel are caused from external sources such as seawater thus, by monitoring water content, Na content is automatically being monitored. This on-line technique is adequate for lighter distillate fuels. For heavier fuels, a more complete analysis of the fuel should be carried out at least once a month using the batch-type system. The data should be input directly to the computer. The water and corrosion detecting systems also operate in conjunction with the batch analysis for the heavier fuels. 

Hence, the hot-dip compounds, or greases smeared cold, are better for assemblies with non-metallic parts masked if necessary. Solvent-containing protectives therefore find greater application in the protection of simple parts or components. The available means of application, the nature of any additional packaging and the economics and scale of the protective treatment are further factors that influence the choice of type of temporary corrosion preventive. 

In such reactions as the tarnishing of silver in air, the oxidation of aluminum in air, or attack of lead in sulfate-containing environments, thin, tightly adherent protective films are formed, and the metal surface remains smooth. It should be mentioned that underground corrosion is frequently observed as localized corrosion. Oxidation, sulfidation, carburization, hydrogen effects, and hot corrosion can be considered as types of general corrosion.20... 

In this article, some fundamental aspects of Hot Corrosion are explained. For practical reasons, most of the investigations were done on sulfate and carbonate melts, and the article will focus on these two types of melts. For sulfate melt-induced Hot Corrosion, the excellent review article by Rapp [1] is recommended for further reading. 

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 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]. 

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]. 

The characteristics of individual forms of corrosion are taken into consideration by providing appropriate corrosion specimens. Welded coupons having the surface quality of the material used later in practice are sufficient for determining uniform corrosion rates and acquiring general information on the type of local corrosion. Resistance to crevice corrosion can be determined by using specimens as described in ASTM G 78-83. Conditions of heat transfer can be simulated by using hot-wall/cool-wall specimens under temperature-controlled conditions. 

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... 

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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