Corrosion and its inhibition in the carbon steel induction-pressure weldments used in power plants

摘要

IN RECENT times a considerable effort has been made to undertake a systematic and fundamental approach in the study the corrosion behaviour of he steel weldments used in power boilers. The induction-pressure welding process (IPW) is widely used, especially for tubular products, for three important reasons: (i) cost effectiveness; (ii) Fit is faster than any other fusion methods; and (iii) it is suitable for mass production. This technical paper describes experimental work carried out to evaluate and compare corrosion and its inhibition in SA 210 Gr AI carbon steel boiler steel weldments prepared by induction pressure welding process in HCI at 0.1 M, 0.5 M, and 1.0 M concentrations. The parent metal and the weld root were chosen as the regions of exposure for the study, which was made at room temperature and at 100 deg C after all the test specimens were suitably stress relieved. Electrochemical polarization techniques such as Tafel line extrapolation and linear polarization resistance were used to measure the corrosion current. The role of hexamine, thiourea (in 0.5 M HCI), and mixed inhibitor (thiourea + hexamine in 0.5 M HCI), each at 100 ppm concentration, was also studied in all the experiments. Microstructural observation and surface characterization and morphology using SEM, analysis of FTIR spectra, and XRD studies were also made in order to highlight the nature and extent of the film formation. The following conclusions are drawn: the Tafel technique registers the minimum corrosion rate. The use of hexamine or mixed inhibitor is found beneficial compared to thiourea at high temperature. At 500 x, the SEM picture shows a film in the form of spheres with a nest-like structure at the centre, and including both sulphur and nitrogen compounds. The FTIR spectrum shows the presence of a peak corresponding to the -C=S functional group, which is found to disappear, while the -NH_2 stretching band is found to be broader. The XRD studies do not show peaks of appreciable sharpness, implying that the film on the surface is amorphous in nature.