Study Of SO₄^2- And Cl^- Ions On The Pitting Corrosion Of Boiler Water-wall Tubes

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 SO₄^2- 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 SO₄^2- 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 SO₄^2- ions on the pitting corrosion of water-wall tube 20G, 15CrMoG and 12Cr1MoVG. The effect of Cl^- and SO₄^2- 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. SO₄^2- accelerates dissolution of unpassivated and passivated 20G, but enhances corrosion resistance of 15CrMoG and 12Cr1MoVG. Cl^-/SO₄^2- 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, SO₄^2- accelerates dissolution of unpassivated and passivated 20G, 15CrMoG and 12Cr1MoVG, and the pitting with jagged edge are formed. On the unit actual operating condition, SO₄^2- 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^-/ SO₄^2- increases, the amount of Cl^- and SO₄^2- reduce, mass gain rate of unpassivated and passivated 20G increases, and mass gain rate of unpassivated and passivated 15CrMoG is similar. As Cl^-/ SO₄^2- 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 SO₄^2-, when Cl^-/SO₄^2- is low, SO₄^2- content is high, the adsorbed Cl^- concentrations reduce, corrosion resistance of 15CrMoG and 12Cr1MoVG enhance. For 20G specimens, the total erosive ions ( Cl^- + SO₄^2- ) 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 SO₄^2- ions is the local acidification theory, which is similar to the mechanism of Cl^-.The result indicated that SO₄^2- ions inhibited pit corrosion at room temperature, and accelerated corrosion at high temperature, the critical susceptive SO₄^2- ions concentrations was 1 mg·L^-1.