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As the increase of unit parameters, the quality of feed-water require more and more strict. If feed-water contains impurity ions exceeding allowed concentrations, boiler scale and corrosion accelerate, the waterside of water-wall tube corrodes, causing bursting of water-wall tube. Researching the influence of SO42- and Cl- ions on the pitting corrosion of water-wall tube, and analyzing corrosion mechanisms will provide theoretical basis for relevant standards, in order to control SO42- and Cl- ions concentrations in feed-water, and provider theoretical foundation for protection technology, which is valuable on safe and economic operation of power plants.The purpose of this study is to investigate the influence of Cl- and SO42- ions on the pitting corrosion of water-wall tube 20G, 15CrMoG and 12Cr1MoVG. The effect of Cl- and SO42- ions on pitting corrosion at room temperature and high temperature were assessed by electrochemical and high-temperature weight loss experiments. The specimen surfaces were examined by transmission reflection metallurgical microscopy, scanning electron microscope （SEM）, energy disperse spectroscopy （EDS）, and X-ray diffraction （XRD）. The main conclusions are as follows:（1） At room temperature, Cl- accelerates dissolution of unpassivated and passivated 20G, 15CrMoG and 12Cr1MoVG. SO42- accelerates dissolution of unpassivated and passivated 20G, but enhances corrosion resistance of 15CrMoG and 12Cr1MoVG. Cl-/SO42- enhance corrosion resistance of unpassivated and passivated 20G, but lows corrosion resistance of unpassivated and passivated 15CrMoG and 12Cr1MoVG.（2） At high temperature, Cl- accelerates dissolution of unpassivated and passivated 20G, 15CrMoG and 12Cr1MoVG. On the unit actual operating condition, Cl- concentrations must be controlled under 0.2 mg·L-1, 0.2 mg·L-1 and 0.6 mg·L-1 for unpassivated 20G, unpassivated 15CrMoG and unpassivated 12Cr1MoVG, and 0.2 mg·L-1, 0.4 mg·L-1 and 0.6 mg·L-1 for passivated 20G, passivated 15CrMoG and passivated 12Cr1MoVGrespectively.（3） At high temperature, SO42- accelerates dissolution of unpassivated and passivated 20G, 15CrMoG and 12Cr1MoVG, and the pitting with jagged edge are formed. On the unit actual operating condition, SO42- concentrations must be controlled under 0.2 mg·L-1, 0.6 mg·L-1 and 1 mg·L-1 for unpassivated 20G, unpassivated 15CrMoG and unpassivated 12Cr1MoVG, and 0.6 mg·L-1, 1 mg·L-1 and 1 mg·L-1 for passivated 20G, passivated 15CrMoG and passivated 12Cr1MoVG respectively.（4） At high temperature, along with Cl-/ SO42- increases, the amount of Cl- and SO42- reduce, mass gain rate of unpassivated and passivated 20G increases, and mass gain rate of unpassivated and passivated 15CrMoG is similar. As Cl-/ SO42- increases, mass gain rate of unpassivated and passivated 12Cr1MoVG increases and then reduces.（5） At room temperature, pitting corrosion mechanism of Cl- ions is competition adsorption of Cl- and OH-, and the adsorbed Cl- can form an intermediate complex, the passivated film dissolves. Because of competition adsorption of Cl- and SO42-, when Cl-/SO42- is low, SO42- content is high, the adsorbed Cl- concentrations reduce, corrosion resistance of 15CrMoG and 12Cr1MoVG enhance. For 20G specimens, the total erosive ions ( Cl- + SO42- ) concentrations increase, specimens dissolve. The corrosion mechanism of Cl- is proposed to accelerate Fe2+ hydrolysis, and loosely packed Fe3O4 is unprotected, H+ concentrations increase, local acidification can form, pitting corrosion intensifies. The mechanism of SO42- ions is the local acidification theory, which is similar to the mechanism of Cl-.The result indicated that SO42- ions inhibited pit corrosion at room temperature, and accelerated corrosion at high temperature, the critical susceptive SO42- ions concentrations was 1 mg·L-1.
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