Hot corrosion is a complex phenomenon involving sulphidation, oxidation or both. Hot corrosion is a form of accelerated oxidation which affects the surfaces exposed to high-temperature gases contaminated with sulphur and alkali metal salts. These contaminants combine in the gas phase to form alkali metal sulphates. If the temperature of the alloy or coating surface is below the dew point of the alkali sulphate vapours and above the sulphate melting points, molten sulphate deposits are formed. Molten sodium sulphate is the principal agent in causing hot corrosion.
Metallic corrosion is the surface wastage that occurs when metals are exposed to a reactive environment. The chemical compounds that constitute the products of such wastage are close cousins of the metalliferous mineral rocks that we find in the Earth’s crust. In other words, corrosion reactions cause metals to revert to their original ores. At temperatures above 200°C there is usually significant reaction of most metals in dry air, and the rate and extent of reaction progressively increase either as the temperature is raised or the air is contaminated by other gases. In general, it may be said that the degree of wastage is largely governed by the ionic conducting properties of the corrosion product when it is present as a solid scale by its mechanical strength and adherence to the underlying metal. The study of high temperature corrosion products is therefore a study of semiconducting oxides, sulphides and so on and the influences of temperature, pressure and ionic contaminants on their mechanical coherence, stability and permeability.
According to Hancock (1987), hot corrosion is an accelerated form of oxidation which occurs when metals are heated in the temperature range 700–900°C in the presence of sulphate deposits formed as a result of the reaction between sodium chloride and sulphur compounds in the gas phase around the metals. For more information see https://doi.org/10.1533/9781845694258.1.164
