In metal components containing chromium that are heated above 300 °C and fitted with insulation materials such as blankets or shells, Cr(VI) can form. The metal and the insulation do not contain Cr(VI) to begin with.
The oxidation step from Cr(III) to a Cr(VI) species is essentially a process in which chromium (originally in the +3 state) takes up additional oxygen, reaching a higher oxidation state. The following is an explanation of the possible mechanisms and conditions:
- Initial state and environment:
- Cr(III) is often present in a solid phase as an oxide (e.g. Cr₂O₃) or as part of a complex matrix, in the passive layer of the metal component.
- In a system with CaO (or another basic oxide) and at elevated temperatures (from 300 °C), a basic environment develops that in many cases promotes the oxidation of Cr(III).
- Role of oxygen:
- Oxygen (O₂) is the oxidising agent. At elevated temperatures, the adsorption of oxygen on the surface of Cr(III)-rich particles increases.
- Once adsorbed on the surface, oxygen can act as an electron acceptor.
- Electron transfer and formation of intermediate species:
- The oxidation reaction requires electron transfer. Cr(III) must lose electrons to be transported from the +3 to the +6 oxidation state.
- During this process, an intermediate phase may form (possibly as oxyhydroxides or other intermediates), in which oxygen atoms are gradually incorporated into the chromium structure.
- Formation of Cr(VI) species and reaction with Ca²⁺:
- Full oxidation ultimately yields a Cr(VI) species, typically in the form of the chromate ion (CrO₄²⁻) or dichromate ion (Cr₂O₇²⁻), depending on the pH and the prevailing conditions.
- In a basic environment where Ca²⁺ is present (originating from CaO), the formed CrO₄²⁻ can for example react to produce calcium chromate.
- Kinetic and thermodynamic aspects:
- Thermodynamically: At 600 °C and in a basic, oxygen-rich environment, the formation of Cr(VI) and consequently the production of chromate is energetically more favourable than at lower temperatures.
- Kinetically: The elevated temperature lowers the activation energy, allowing electron transfer to proceed more rapidly. Even so, the oxidation of Cr(III) to Cr(VI) is not always extremely fast; considerable time may be required for complete conversion, depending on the precise conditions (such as the concentration of reactants, the degree of contact between components, and the diffusion of oxygen through the system).
- Summary:
- The oxidation step begins with the adsorption of oxygen on the Cr(III) surfaces.
- Electron transfer then converts Cr(III) gradually to Cr(VI).
- In a basic environment (e.g. due to the presence of CaO), the formed Cr(VI) species are subsequently stabilised, which can lead to the formation of calcium chromate where local conditions (such as Ca²⁺ concentration and pH) permit.
These sequential steps make clear that the oxidation of Cr(III) to Cr(VI) is technically feasible at temperatures from 300 °C in the presence of oxygen and a basic matrix, but that the extent and rate of this oxidation depend strongly on the specific material conditions and reaction time.
