Abstract
The rare earth (RE) modified chromizing coating was obtained on P110 oil casing tube steel (P110 steel) substrate by means of pack cementation technique to enhance the resistance against corrosion of P110 steel. Scanning Electron Microscopy (SEM), Energy Dispersive X-ray analysis (EDX) and X-ray diffraction (XRD) were employed to research microstructure, composition distribution and phase constitution of the chromizing coating. The effect of minor addition of RE on the microstructure of chromizing was discussed. Corrosion resistance of chromizing coating was investigated and compared with that of bare P110 steel via electrochemical corrosion and immersion corrosion in simulated oilfield brine solution, respectively. The results showed that a uniform, continuous and compact coating was formed on P110 steel. The coating with RE addition was more compact than that of the coating added no RE, and a small amount of RE addition could promote the chromizing procedure notably. From SEM and EDX investigation, it had been confirmed that the coating was composed of two different layers, an out layer and an inner layer; the coating mainly contains Fe and Cr; the concentration of Cr decreased as the distance from the surface increased, yet Fe presented the inverse trend. XRD analysis indicated the coating was built up by (Cr, Fe)23C6 referring to the out layer, (Cr, Fe)7C3, Cr7C3 and α-(Cr, Fe) corresponding to the inner layer. Electrochemical corrosion consequence was obtained as follows: the self-corroding electric potential of chromizing coating was higher, and the corrosion current density was lower than that of bare P110 steel, which revealed that chromizing coating had better anti-corrosion performance; immersion corrosion results demonstrated the mass loss of chromized P110 steel was lower, and this meant that chromizing coating had a better corrosion resistance than that of bare P110 steel on the experimental condition. A compact (Cr, Fe)xCy coating can be fabricated by pack cementation technique. As a result of minor RE addition, microstructure and corrosion resistance of the chromizing coating are improved obviously.
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