Degradation of a corrosion and heat resistant steel pipe

Degradation process of a cracking furnace pipe made of the austenitic 25Cr–35Ni alloy was investigated using SEM equipped with energy dispersive probe and full area and selective area X-ray diffraction techniques. The results revealed that the bulk diffusion of C into the tube resulted in a complex process of carbide formation at the grain boundaries, Cr depletion of the matrix and thus the appearance of the ferromagnetic behaviour. The degradation was remarkably enhanced by the abrasive environment and the periodic decoking (elimination of deposed carbon by means of an air–steam mixture) process leading to the formation of spongy Cr-oxide.

► The abrasion effect of the gas flow led to the pinning of the Cr-oxide layer.
► Recovering the Cr-oxide layer caused Cr depletion in the matrix.
► A bulk C diffusion from the diluted gas led to the growth of the Cr-carbides.
► The Cr-carbides transformed to Cr-oxides due to the applied steam cleanings.
► The resulting microstructure was susceptible for cracking.