Research On The Corrosion Behavior And Surface Modification Of T91/T92Steel

T91/T92steel used for power station boiler tubes exhibited excellent physicaland mechanical comprehensive properties, such as low thermal expansion coefficientsand good corrosion resistance. The aim of this paper is to study the initial oxidationand subsequent oxidation behaviors of T91/T92steel at high temperature under watervapor environment and explore the oxidation mechanism. The effect of shot blastingand superficially applied yttria on oxidation behavior of T91/T92steel in water vaporatmosphere at high temperature was investigated. The main results are as follows.The initial oxidation behavior of T91steel in90%Ar+10%H2O atmosphere at750℃was investigated. The morphology, microstructures and composition of oxidescales were examined by SEM/EDS and XRD. The results show that Cr-rich oxidefilm formed at the early oxidation stage. The main oxide was composed of Cr2O3,(Fe,Cr)2O3, FeCr2O4. This oxide film acted as a barrier of outward diffusion of ironand inward diffusion of oxygen, resulting in very slow oxidation rate. Iron oxideformed at the Cr-poor regions due to the depletion of chromium at the initial oxidationstage Fe diffusion outward very fast, which resulted in iron oxide layer formed on thesample surface.The initial oxidation behavior of T92steel in90%Ar+10%H2O atmosphere at750℃was investigated. It is shown that the weight gained during the oxidation tests.Slow oxidation process in initial stage was not observed, which is different from thatof T91steel. The main reason is related to W element existed in T92steel, whichresulted Cr equivalent decreased greatly. Thus a Cr-rich oxide film cannot be detectedon the sample surface in the early oxidation stage. The main oxide was composed ofFe2O3and FeCr2O4.The oxidation behaviors of T91/T92steel in90%Ar+10%H2O atmosphere at600℃,650℃,700℃and750℃were studied. The results show that at differenttemperatures, the oxidation scales of T91steel consisted of two layers, an porousFe2O3outer layer and an denser Cr-rich inner layer. Some cracks between the innerand outer layers were observed. The thermal stress originated in oxidation processled to exfoliation of the oxide layers. After the oxidation of T92steel, the oxidationscales also consisted of two layers, an porous Fe2O3outer layer and an Cr-rich innerlayer. Some voids existed in both outer and inner layers. Cracks between the inner and outer layers was also observed.T91/T92steel surfaces were treated by shot blasting. The oxidation tests of thetreated samples were conducted in90%Ar+10%H2O atmosphere at700℃. The resultsindicate that shot blasting could not promote the preferential formation of Cr oxidedue to low Cr-containing of T91/T92steels. Shot blasting on T91/T92steel surfacesdid not affect the oxidation behavior in high temperature water vapor environment.T91/T92steel surfaces were treated by superficially applied yttria. The oxidationtests of the treated samples were conducted in90%Ar+10%H2O atmosphere at650℃and700℃. It is shown that Y2O3promoted the outward diffusion of Cr to form anCr-rich oxide film. The Cr-rich film prevented the outward diffusion of Fe ions, whichresulted in reducing the oxidation rate greatly. Therefore, superficially applied yttriaimproved significantly the oxidation resistance of T91/T92steel in high temperaturewater vapor environment.