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Corrosion data on superheater alloys for waste and biomass boilers: Specific effects of CO2 and deposits

Corrosion data collected from tests on superheater materials (X10, X20, 2.25Cr1Mo, AC66, Sanicro28, Esshette 1250 TP347H) in simulated waste/biomass environments in the vicinity of superheaters have shown a significant influence of the concentration of CO2 in the environment on corrosion behaviour.

The tasks of establishing the individual influences of CO2 and H2O were divided between 2 CORBI partners (MPIE addresses the effect of H2O). Using a gas composition based on biomass combustion conditions containing N2+22%H2O+5%O2+xCO2, where the concentration of CO2 was varied in the range 0 to 25 vol. %. The experiments were carried out at isothermal temperature ¿ 535C for the samples without deposit and with (Cl-containing filter/cyclone ash deposits).

The results showed that:
- Corrosion rate, for the alloys without the deposit, increase with increasing CO2 content, especially for the ferritic steels.

- Corrosion rate for samples with the deposit increase significantly and in this case increased internal oxidation was observed.

- Various carbides of metallic alloying elements become less stable at the oxide scale-metallic alloy phase boundary with increasing partial pressure of carbon dioxide.

- Carbides and oxides of various alloying elements (Fe, Cr, Mo, Mn...) are unstable in their mixtures with alkali metal carbonates or model ash deposits.

- However, some of the mixed alkali oxide-transition metal oxides that are predicted to be formed as reaction products, may dissociate under atmospheres with appreciable carbon dioxide and/or water vapour content.

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