A failure investigation was performed to decide the exact cause and suggest measures in order to allow a life-time improvement and/or increase the temperature. Three burners at different stages of degradation were used for the investigation.
The alloy used for the inner nozzle and the outer tube casing was 2.4879 (a Cr2O3 former). SEM/EDX analysis showed that Cr disappears from the steel in an alarming rate, most probably by water vapour enhanced evaporation. The protective Cr2O3 decomposes with catastrophic oxidation and spallation as a result. The thinning of the tube wall and the appearance of holes eventually causes the break.
This type of failure can maybe be avoided by choosing an alloy that forms Al2O3 instead of Cr2O3. One such candidate would be Centralloy 60HT that forms Al2O3 and has good mechanical strength up to 1200°C.
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