High-temperature corrosion
High-temperature corrosion is a mechanism of
Sulfates
Two types of sulfate-induced hot corrosion are generally distinguished: Type I takes place above the melting point of sodium sulfate, whereas Type II occurs below the melting point of sodium sulfate but in the presence of small amounts of SO3.[2][3]
In Type I, the protective oxide scale is dissolved by the molten salt. Sulfur is released from the salt and diffuses into the metal substrate, forming grey- or blue-colored aluminum or chromium sulfides. With the aluminum or chromium sequestered, after the salt layer has been removed, the steel cannot rebuild a new protective oxide layer. Alkali sulfates are formed from sulfur trioxide and sodium-containing compounds. As the formation of vanadates is preferred, sulfates are formed only if the amount of alkali metals is higher than the corresponding amount of vanadium.[3]
The same kind of attack has been observed for potassium sulfate and magnesium sulfate.
Vanadium
Vanadium is present in
Most fuels contain small traces of
At high temperatures or when there is a lower availability of oxygen,
The solubility of the passivation layer oxides in the molten vanadates depends on the composition of the oxide layer. Iron(III) oxide is readily soluble in vanadates between Na2O.6 V2O5 and 6 Na2O.V2O5, at temperatures below 705 °C in amounts up to equal to the mass of the vanadate. This composition range is common for ashes, which aggravates the problem. Chromium(III) oxide, nickel(II) oxide, and cobalt(II) oxide are less soluble in vanadates; they convert the vanadates to the less corrosive ionic form and their vanadates are tightly adherent, refractory, and act as oxygen barriers.[5][3]
The rate of corrosion caused by vanadates can be lowered by reducing the amount of excess air available for combustion to preferentially form the refractory oxides, using refractory coatings on the exposed surfaces, or using high-chromium alloys, such as 50% Ni/50% Cr or 40% Ni/60% Cr. [6]
The presence of sodium in a ratio of 1:3 gives the lowest melting point and must be avoided. This melting point of 535 °C can cause problems on the hot spots of the engine like piston crowns, valve seats, and turbochargers.[5][3]
Lead
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Lead can form a low-melting slag capable of fluxing protective oxide scales.[7][8] Lead is more often known for causing stress corrosion cracking in common materials that are exposed to molten lead. The cracking tendency of lead has been known for some time, since most iron based alloys, including those used in steel containers and vessels for molten lead baths, usually fail due to cracking.[9]
See also
- Internal oxidation
- Deal-Grove model
- Thermal oxidation
- Corrosion engineering
References
- OCLC 77562951.
- ISBN 978-0-08-044587-8.
- ^ ISBN 978-0-87170-853-3.
- ^ ISBN 978-0-7506-7856-8. Archivedfrom the original on 2018-04-18. Retrieved 2021-02-08.
- ^ ISBN 978-0-444-41619-3.
- ISBN 0-7506-7856-9p. 294
- ^ Schriner, Doug. "A Review of Slag Chemistry in Lead Recycling" (PDF).
- ISBN 9780080969886.
- OCLC 77545140.