What Does Hot Corrosion Mean?
This is the most aggressive method of degradation of metals, alloys and ceramics, especially when they operate at very high temperatures. Rapid degradation occurs due to lack of a solid layer found on the substrate. Components that are affected by hot corrosion attack include gas turbines, industrial plants and jet engines.
This can also be defined as the accelerated oxidation of materials that is induced by a film deposit of salt at elevated temperatures. The fused alkali sulfates are deposited on the hot substrates by the oxidation of heavy metal contaminants like vanadium in the fuel.
Corrosionpedia Explains Hot Corrosion
In a gas turbine, for example, sodium chloride from the air reacts with sulfur from the fuel to form sodium sulfate. The sulfate then deposits on the hot sections like the rotor blades, which in turn results in accelerated oxidation attack. Although the sulfur in the fuel on commercial jet engines and the marine gas turbines is very low in percentage, it is capable of corroding a good portion of the metal surface.
All the high-temperature alloys that suffer from hot corrosion attack exhibit both sulfidation and oxidation. The oxide layer that results is thick and porous, and the underlying alloy matrix is depleted in chromium. The deposit of the molten sodium sulfate is responsible for the initiation of hot corrosion attack.
There are two types of hot corrosion: type I and type II. A type I hot corrosion attack happens at a temperature range of 800°C to 900°C, with the minimum threshold being the melting point of the salt deposit. The upper temperature is taken to be the salt dew point. This happens in two stages: an incubation period where the rate of attack is slow as the oxide layer forms, and then the rate accelerates quickly. Type II hot corrosion has a different temperature range of 670°C to 750°C and is characterized by a pitting attack with little attack underneath. Type II hot corrosion is common in industrial gas and marine turbines.
There are many tests that can be used to study hot corrosion. They include the immersion/crucible testing, salt-coated method and burner rig testing used by manufacturers.