EFFECT OF pH VALUE ON THE CORROSION EVOLUTION OF Q235B STEEL IN SIMULATED COASTAL-INDUSTRIAL ATMOSPHERES

The atmosphere in many cities along the coastal lines such as Qingdao in China has been polluted with SO2 due to the development of industry, and the atmosphere therefore has been changed to coastal-industrial atmosphere. The corrosion behavior and mechanism of steels in coastal-industrial atmosphere with the co-existence of SO2 and Cl- are different from that in the coastal atmosphere containing only Cl- or the industrial atmosphere containing only SO2. In addition, pH value is diverse in different coastal-industrial atmosphere. However, there are only few researches on the effect of pH value on the corrosion evolution of steels in the coastal-industrial atmosphere. Almost all the atmospheric corrosion data of steels were obtained by the field exposure test, which could not reflect the dependence of the atmospheric corrosion evolution of steels on pH value due to the difficulties in controlling the field conditions. In this work, the effect of pH value on the corrosion evolution of Q235B steel in the simulated coastal-industrial atmospheres has been investigated by the dry/wet cyclic corrosion test (CCT), XRD and EIS. The results indicate that, when the content of SO(2)is lower, changing pH value has no effect on the corrosion of the steel. When the content of SO2 is higher, the corrosion rate of Q235B steel influenced by changing pH value shows an extreme phenomenon, that is, when the pH value being a certain value between the "higher" and the "lower", the corrosion rate of Q235B steel reaches the maximum value. When the SO(2)content is certain, changing pH value almost has no effect on the rust composition. To some extent, the existence of SO(2)inhibits the formation of beta-FeOOH. With the increasing of SO2 content, the relative contents of beta-FeOOH and gamma-FeOOH are decreasing, and gamma-FeOOH maybe reduced back to Fe3O4 or transform to alpha- FeOOH. With the corrosion process prolongs, the rust evolution shows almost the same trend. In addition, when the content of SO(2)in the simulated coastal-industrial atmosphere is lower, the Q235B steel mainly follows Cl- corrosion mechanism, and the influence of pH value on corrosion behavior of the steel is not obvious. When the content of SO(2)is higher, the Q235B steel also follows cr corrosion mechanism in the early stage; with prolonging the dry/wet cyclic corrosion test number, H2SO4 regeneration mechanism accelerates corrosion of the steel as the effect of SO2 on corrosion increasing significantly.