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ATMOSPHERIC CORROSION AND ELEVATED TEMPERATURE OXIDATION :red_flag: -…
ATMOSPHERIC CORROSION AND ELEVATED TEMPERATURE OXIDATION
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Atmospheric Corrosion
Corrosion occurs by the combined effect of water and oxygen and continues as long as water and oxygen are both supplied.
Therefore, corrosion can be suppressed or prevented by avoiding water and oxygen from direct contact with the surface of a steel sheet.
Atmospheric environments are most commonly divided into four categories:
rural
urban
industrial
marine
Important Variables in atmospheric corrosion
Contaminants:
SOx, atmospheric salinity (NaCl), NOx
Temperature:
High temperature increase electrochemical corrosion rates (-)
Increase electrolyte evaporation (+)
Increase the critical humidity to form electrolyte (+)
Time of wetness:
Relative humidity, hygroscopic nature of the corrosion products.
Atmospheric parameters affecting corrosion
Humidity
A thin layer of condensed water is necessary for atmospheric corrosion
corrosion of uncontaminated surfaces is often relatively low
Pollutants or other contaminants
SO2 has been identified as one of the most important air pollutants
NOx , CO2 , H2S are considered less corrosive than SO2.
Airborne Salinity
Airborne salinity refers to the content of gaseous and suspended salt in the atmosphere.
Remedial Measures
Reduction of Relative Humidity.
Use of Vapor - Phase Inhibitors
Use of Organic, Inorganic, or Metallic Coatings
Use of Alloys
Atmospheric Corrosion Monitoring
Direct monitoring of corrosion damage
Indirect monitoring of corrosion damage with corrosion sensors
High Temperature Corrosion
High temperature corrosion is a form of corrosion that does not require the presence of a liquid electrolyte. Sometimes, this type of damage is called "dry corrosion" or "scaling".
High Temperature Hot Corrosion - An aggressive attack of metal by molten salts, particularly sodium sulfates, that occurs between 870 and 980oC (1600 and 1800 oF).
Hot corrosion - An accelerated corrosion of metal surfaces that results from the combined effect of oxidation and reactions with sulfur compounds and other contaminants
Carburization
Carburization can have an influence on the corrosion behavior as well, as carbon can form carbides (like Cr23C6, Cr3C2 or Cr7C3 ), depleting the metal matrix locally of chromium and making it more sensitive to corrosion
Carburization: Prevention
Minor alloying elements can exert an influence on the susceptibility to carburization of various alloys.
preoxidation and the subsequent formation of an oxide film will increase the resistance against decarburization
Carburization is the increase of the carbon content of (the surface of) a steel due to interactions withthe environment at elevated temperatures.
Decarburization
Decarburization is the decrease of the carbon content of (the surface of) a steel due to interactions with the environment at elevated temperatures.
Decarburization by Hydrogen
Surface decarburization
Internal decarburization
