High temperature corrosion
Encyclopedia
Hot corrosion is a mechanism of "high temperature corrosion", others include high temperature oxidation, sulfidation and carbonization.
-induced hot corrosion are generally distinguished: Type I takes place above the melting point
of sodium sulfate
and Type II occurs below the melting point of sodium sulfate but in the presence of small amounts of SO3.
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 discrete grey/blue colored aluminum or chromium sulfides so that, after the salt layer has been removed, the steel cannot rebuilt 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.
The same kind of attack has been observed for potassium
and magnesium sulfate
.
, especially from Canada
, western United States
, Venezuela
and the Caribbean region, in the form of porphyrine complexes. These complexes get concentrated on the higher-boiling fractions, which are the base of heavy residual fuel oil
s. Residues of sodium, primarily from sodium chloride
and spent oil treatment chemicals, are also present. More than 100 ppm of sodium and vanadium will yield ash capable of causing fuel ash corrosion.
Most fuels contain small traces of vanadium
. The vanadium is oxidized to different vanadate
s. Molten vanadates present as deposits on metal can flux
oxide
scales
and passivation layers. Furthermore, the presence of vanadium accelerates the diffusion of oxygen
through the fused salt layer to the metal substrate; vanadates can be present in semiconducting or ionic form, where the semiconducting form has significantly higher corrosivity as the oxygen is transported via oxygen vacancies. Ionic form in contrast transports oxygen by diffusion of the vanadates, which is significantly slower. The semiconducting form is rich on vanadium pentoxide.
At high temperatures or lower availability of oxygen, refractory
oxides - vanadium dioxide and vanadium trioxide - form. These do not promote corrosion. However, at conditions most common for burning, vanadium pentoxide gets formed. Together with sodium oxide
, vanadates of various composition ratios are formed. Vanadates of composition approximating Na2O.6 V2O5 have the highest corrosion rates at the temperatures between 593 °C and 816 °C; at lower temperatures the vanadate is in solid state, at higher temperatures vanadates with higher proportion of vanadium provide higher corrosion rates.
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 less corrosive ionic form and their vanadates are tightly adherent, refractory, and acting as oxygen barriers.
The corrosion rate by vanadates can be lowered by lowering the amount of excess air for combustion (thus forming preferentially the refractory oxides), refractory coatings of the exposed surfaces, or use of high-chromium alloys, e.g. 50% Ni/50% Cr or 40% Ni/60% Cr.
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 seat
s, and turbocharger
s.
can form a low melting slag capable of fluxing protective oxide scales.
Hot corrosion by sulfates
Two types of sulfateSulfate
In inorganic chemistry, a sulfate is a salt of sulfuric acid.-Chemical properties:...
-induced hot corrosion are generally distinguished: Type I takes place above the melting point
Melting point
The melting point of a solid is the temperature at which it changes state from solid to liquid. At the melting point the solid and liquid phase exist in equilibrium. The melting point of a substance depends on pressure and is usually specified at standard atmospheric pressure...
of sodium sulfate
Sodium sulfate
Sodium sulfate is the sodium salt of sulfuric acid. When anhydrous, it is a white crystalline solid of formula Na2SO4 known as the mineral thenardite; the decahydrate Na2SO4·10H2O has been known as Glauber's salt or, historically, sal mirabilis since the 17th century. Another solid is the...
and Type II occurs below the melting point of sodium sulfate but in the presence of small amounts of SO3.
In Type I the protective oxide scale is dissolved by the molten salt. Sulfur
Sulfur
Sulfur or sulphur is the chemical element with atomic number 16. In the periodic table it is represented by the symbol S. It is an abundant, multivalent non-metal. Under normal conditions, sulfur atoms form cyclic octatomic molecules with chemical formula S8. Elemental sulfur is a bright yellow...
is released from the salt and diffuses into the metal substrate forming discrete grey/blue colored aluminum or chromium sulfides so that, after the salt layer has been removed, the steel cannot rebuilt a new protective oxide layer. Alkali sulfates are formed from sulfur trioxide
Sulfur trioxide
Sulfur trioxide is the chemical compound with the formula SO3. In the gaseous form, this species is a significant pollutant, being the primary agent in acid rain. It is prepared on massive scales as a precursor to sulfuric acid.-Structure and bonding:Gaseous SO3 is a trigonal planar molecule of...
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.
The same kind of attack has been observed for potassium
Potassium sulfate
Potassium sulfate is a non-flammable white crystalline salt which is soluble in water...
and magnesium sulfate
Magnesium sulfate
Magnesium sulfate is a chemical compound containing magnesium, sulfur and oxygen, with the formula MgSO4. It is often encountered as the heptahydrate epsomite , commonly called Epsom salt, from the town of Epsom in Surrey, England, where the salt was distilled from the springs that arise where the...
.
Hot corrosion by vanadium
Vanadium is present in petroleumPetroleum
Petroleum or crude oil is a naturally occurring, flammable liquid consisting of a complex mixture of hydrocarbons of various molecular weights and other liquid organic compounds, that are found in geologic formations beneath the Earth's surface. Petroleum is recovered mostly through oil drilling...
, especially from Canada
Canada
Canada is a North American country consisting of ten provinces and three territories. Located in the northern part of the continent, it extends from the Atlantic Ocean in the east to the Pacific Ocean in the west, and northward into the Arctic Ocean...
, western United States
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...
, Venezuela
Venezuela
Venezuela , officially called the Bolivarian Republic of Venezuela , is a tropical country on the northern coast of South America. It borders Colombia to the west, Guyana to the east, and Brazil to the south...
and the Caribbean region, in the form of porphyrine complexes. These complexes get concentrated on the higher-boiling fractions, which are the base of heavy residual fuel oil
Fuel oil
Fuel oil is a fraction obtained from petroleum distillation, either as a distillate or a residue. Broadly speaking, fuel oil is any liquid petroleum product that is burned in a furnace or boiler for the generation of heat or used in an engine for the generation of power, except oils having a flash...
s. Residues of sodium, primarily from sodium chloride
Sodium chloride
Sodium chloride, also known as salt, common salt, table salt or halite, is an inorganic compound with the formula NaCl. Sodium chloride is the salt most responsible for the salinity of the ocean and of the extracellular fluid of many multicellular organisms...
and spent oil treatment chemicals, are also present. More than 100 ppm of sodium and vanadium will yield ash capable of causing fuel ash corrosion.
Most fuels contain small traces of vanadium
Vanadium
Vanadium is a chemical element with the symbol V and atomic number 23. It is a hard, silvery gray, ductile and malleable transition metal. The formation of an oxide layer stabilizes the metal against oxidation. The element is found only in chemically combined form in nature...
. The vanadium is oxidized to different vanadate
Vanadate
In chemistry, a vanadate is a compound containing an oxoanion of vanadium generally in its highest oxidation state of +5. The simplest vanadate ion is the tetrahedral, orthovanadate, VO43− anion, which is present in e.g. sodium orthovanadate and in solutions of V2O5 in strong base...
s. Molten vanadates present as deposits on metal can flux
Flux (metallurgy)
In metallurgy, a flux , is a chemical cleaning agent, flowing agent, or purifying agent. Fluxes may have more than one function at a time...
oxide
Oxide
An oxide is a chemical compound that contains at least one oxygen atom in its chemical formula. Metal oxides typically contain an anion of oxygen in the oxidation state of −2....
scales
Mill scale
Mill scale, often shortened to just scale, is the flaky surface of hot rolled steel, iron oxides consisting of Iron oxide, hematite, and magnetite....
and passivation layers. Furthermore, the presence of vanadium accelerates the diffusion of oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
through the fused salt layer to the metal substrate; vanadates can be present in semiconducting or ionic form, where the semiconducting form has significantly higher corrosivity as the oxygen is transported via oxygen vacancies. Ionic form in contrast transports oxygen by diffusion of the vanadates, which is significantly slower. The semiconducting form is rich on vanadium pentoxide.
At high temperatures or lower availability of oxygen, refractory
Refractory
A refractory material is one that retains its strength at high temperatures. ASTM C71 defines refractories as "non-metallic materials having those chemical and physical properties that make them applicable for structures, or as components of systems, that are exposed to environments above...
oxides - vanadium dioxide and vanadium trioxide - form. These do not promote corrosion. However, at conditions most common for burning, vanadium pentoxide gets formed. Together with sodium oxide
Sodium oxide
Sodium oxide is a chemical compound with the formula Na2O. It is used in ceramics and glasses, though not in a raw form. Treatment with water affords sodium hydroxide....
, vanadates of various composition ratios are formed. Vanadates of composition approximating Na2O.6 V2O5 have the highest corrosion rates at the temperatures between 593 °C and 816 °C; at lower temperatures the vanadate is in solid state, at higher temperatures vanadates with higher proportion of vanadium provide higher corrosion rates.
The solubility of the passivation layer oxides in the molten vanadates depends on the composition of the oxide layer. Iron(III) oxide
Iron(III) oxide
Iron oxide or ferric oxide is the inorganic compound with the formula Fe2O3. It is one of the three main oxides of iron, the other two being iron oxide , which is rare, and iron oxide , which also occurs naturally as the mineral magnetite. As the mineral known as hematite, Fe2O3 is the main...
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
Chromium(III) oxide
Chromium oxide is the inorganic compound of the formula Cr2O3. It is one of principal oxides of chromium and is used as a pigment. In nature, it occurs as the rare mineral eskolaite.-Structure and properties:...
, nickel(II) oxide
Nickel(II) oxide
Nickel oxide is the chemical compound with the formula NiO. It is notable as being the only well characterized oxide of nickel . The mineralogical form of NiO, bunsenite, is very rare. It is classified as a basic metal oxide...
, and cobalt(II) oxide
Cobalt(II) oxide
Cobalt oxide or cobalt monoxide is an inorganic compound that appears as olive-green to red crystals, or as a greyish or black powder...
are less soluble in vanadates; they convert the vanadates to less corrosive ionic form and their vanadates are tightly adherent, refractory, and acting as oxygen barriers.
The corrosion rate by vanadates can be lowered by lowering the amount of excess air for combustion (thus forming preferentially the refractory oxides), refractory coatings of the exposed surfaces, or use of high-chromium alloys, e.g. 50% Ni/50% Cr or 40% Ni/60% Cr.
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 seat
Valve seat
The valve seat in an internal combustion gasoline or diesel engine is the surface against which an intake or an exhaust valve rests during the portion of the engine operating cycle when that valve is closed...
s, and turbocharger
Turbocharger
A turbocharger, or turbo , from the Greek "τύρβη" is a centrifugal compressor powered by a turbine that is driven by an engine's exhaust gases. Its benefit lies with the compressor increasing the mass of air entering the engine , thereby resulting in greater performance...
s.
Hot corrosion by lead
LeadLead
Lead is a main-group element in the carbon group with the symbol Pb and atomic number 82. Lead is a soft, malleable poor metal. It is also counted as one of the heavy metals. Metallic lead has a bluish-white color after being freshly cut, but it soon tarnishes to a dull grayish color when exposed...
can form a low melting slag capable of fluxing protective oxide scales.
External links
- Robert A. Rapp "Hot corrosion of materials" Pure & Appl. Chem., Vol. 62, No. 1, pp. 113–122, 1990.
- Hot corrosion information