A comparative study on atmospheric oxidation of reduced activation ferritic martensitic steel and grade 91 steel

SEM micrographs showing the thickness of oxide film at a location with less spallation after∼8 months of atmospheric exposure for (a) RAFM Steel and (b) P91Steel. [Display omitted] •Oxidation resistance of RAFM steel is inferior to that of P91 steel in the coastal environments.•Presence of Cl- promotes the formation of less protective flaky Fe rich oxides.•Replacement of Mo in P91steel with W to form RAFM steel caused the difference in chemistry of oxides formed.•This difference led to variation in oxidation resistance properties of both the steels. Oxidation behaviour of Reduced Activation Ferritic Martensitic Steel (RAFMS), the structural material for fusion reactors, exposed to atmosphere for ∼2 years has been studied giving special emphasis on the morphology and chemistry of the oxides formed. The results obtained are compared with those of Grade 91(P91) steel, the composition of which was modified to develop RAFM steel; replacing Mo and Nb with W and Ta respectively. The oxidation products formed on RAFMS and P91 steel plates exposed to atmosphere were analyzed using of different material characterization techniques includes confocal microscopy, scanning electron microscopy, energy dispersive spectroscopy, X-ray photo electron spectroscopy and laser Raman spectroscopy. It is found that rate of oxidation in RAFM steel is faster than in P91 steel. The chloride ions in the environment and difference in surface oxide chemistry play roles in deteriorating the oxidation resistance of RAFM steel. Oxides formed on RAFM steel contain W while those formed on P91 steel contain Mo and difference in the oxide layers seems to affect their adherence to the substrate. Electrochemical corrosion studies on these steels using impedance spectroscopy showed corrosion resistance of RAFM steel in chloride containing acidic medium is lower than that of P91steel.