High-Temperature Oxidation and Corrosion of Metals

High-Temperature Oxidation and Corrosion of Metals is the chemical degradation of metals and alloys reacting directly with hot gases, forming oxide, sulphide or carbide layers that either protect the metal or allow it to waste away. It governs component life in furnaces, boilers, gas turbines, reformers and flare tips, where failure is driven by gas chemistry and temperature rather than by an aqueous electrolyte. Understanding how a protective scale forms, and why it eventually fails, is central to selecting alloys and coatings that survive the intended service life.

How protective oxide scales form

At elevated temperature a metal surface reacts with oxygen to grow an oxide scale. Whether that scale protects depends on its continuity and adhesion. A useful first indicator is the Pilling-Bedworth ratio (PBR), the volume of oxide formed per volume of metal consumed. A PBR below 1 gives a porous, non-protective film; a PBR between roughly 1 and 2 tends to give a dense, adherent scale; much above 2 the scale is highly stressed and prone to cracking. Protective scales thicken by slow diffusion through the oxide, so growth follows a parabolic law: the rate falls as the scale gets thicker. The three oxides that matter industrially are chromia, alumina and silica, because each is slow-growing, adherent and stable over a wide range.

Breakaway oxidation and spalling

A protective scale is only useful while it stays intact. Thermal cycling, mechanical strain and erosion crack or spall the oxide, exposing bare metal that re-oxidises rapidly. If the underlying alloy can no longer supply enough chromium or aluminium to reheal a continuous scale, protection is lost and the reaction accelerates, often switching from parabolic to linear kinetics. This runaway is called breakaway oxidation, and it consumes section thickness quickly. Thin components, sharp edges and repeatedly quenched parts are most at risk because the chromium or aluminium reservoir is limited and each spall event depletes it further. Small additions of reactive elements such as yttrium, cerium or hafnium improve scale adhesion and markedly delay breakaway.

Other modes of high-temperature attack

Oxidation rarely acts alone. Process gases introduce sulphur, carbon and molten deposits that attack the same alloys by different routes. Sulphidation forms metal sulphides that grow far faster than oxides and can produce low-melting eutectics; nickel-rich alloys are especially vulnerable because the nickel-sulphide eutectic melts near 645 degrees C. Carburisation and its aggressive low-temperature form, metal dusting, disintegrate the metal into coke and fine particles in carbon-rich gas. Hot corrosion (molten-salt attack) fluxes away the protective scale beneath sodium-sulphate or vanadate deposits.

The role of chromium, aluminium and silicon

Protective behaviour is engineered by adding elements that preferentially form a slow-growing scale. Chromium is the workhorse: roughly 18 to 25 percent gives a continuous chromia scale effective to about 900 degrees C, above which chromia can volatilise, particularly in moist gas. Aluminium forms alumina, the most stable of the three, extending protection above 1000 degrees C and underpinning the alumina-forming austenitic and FeCrAl alloys. Silicon assists both by forming a thin silica sub-layer that slows diffusion, though too much silicon harms weldability. Verifying that installed material actually meets the specified chromium and aluminium content is a routine integrity check, best confirmed by positive material identification rather than assumed from paperwork.

Where it occurs and how to mitigate it

High-temperature attack concentrates in reheat and reformer furnaces, boiler superheaters, waste-heat recovery, gas-turbine hot sections, and flare and incinerator tips. Mitigation follows three levers: alloy selection to match the gas chemistry and metal temperature; diffusion or overlay coatings to add a sacrificial reservoir of aluminium or chromium; and atmosphere control to hold oxygen, sulphur and carbon activities within safe limits. Diffusion coatings such as pack aluminising, chromising and siliconising enrich the surface, while MCrAlY overlays and yttria-stabilised zirconia thermal-barrier coatings protect turbine blades. External surfaces also degrade when hot lines are lagged, so a programme that couples hot-face alloy checks with attention to corrosion under insulation and periodic coating inspection gives the fullest picture of remaining life. Recording scale thickness, coating condition and metal temperatures against a maintenance schedule turns isolated inspections into a trendable dataset. A platform such as Fabrico can hold those inspection records and trigger the next assessment before breakaway sets in; you can Book a Fabrico demo to see how.

Frequently Asked Questions

What is the difference between oxidation and hot corrosion?

Oxidation is direct reaction with a gas to grow a solid oxide scale. Hot corrosion involves a molten salt deposit, typically sodium sulphate or vanadate, that dissolves or fluxes the protective scale, so metal loss is far faster and localised.

Why does breakaway oxidation happen suddenly?

The protective scale masks the true reaction rate while it holds. Once the alloy can no longer reheal a spalled or cracked scale, bare metal is exposed and kinetics shift from parabolic to near-linear, so the transition looks abrupt even though depletion was gradual.

Which oxide gives the best high-temperature protection?

Alumina is the most stable and slowest-growing, giving the best protection above 1000 degrees C. Chromia is effective to around 900 degrees C but can volatilise in hotter, moist or high-velocity gas.

Does adding more chromium always help?

Higher chromium improves oxidation and sulphidation resistance up to a point, but in strongly carburising or metal-dusting conditions an alumina-forming or high-silicon alloy often performs better, so the atmosphere must drive the choice.