| 其他摘要 | Carbon steel and weathering steel as important structural materials have been widely used in buildings, machines, vehicles, etc. These equipments are mainly operated under atmospheric environment. Particles in the air often induce corrosion of the steels and significantly influence their service lives. Cl- is one of the most important factors that facilitate the corrosion of both kinds of steels. 316L stainless steel has been widely applied as a building material in marine and industrial environments due to its excellent corrosion resistance. However, in chloride-containing environments, stainless steels are susceptible to pitting corrosion, resulting in the damage of appearances and the shortening of service lives. When the same steel is exposed to different environments, the corrosion behavior and the characteristics of the rusts vary due to the complexity and diverse nature of the contributing parameters. Therefore, it is difficult to predict the atmospheric corrosiveness for steels according to the meteorological parameters and pollution data of environment. Qinghai Salt Lake is located in the northwestern China and high content of Cl- is contained in its atmosphere. With the fast development of local economy and the implement of the west development project, the corrosiveness of the salt lake atmosphere for steels has received enormous attention from construction department of goverment, industry and academia world.
In this work, the corrosion tests of carbon steel, weathering steel and stainless steel were carried out in outdoor atmosphere, indoor acceleration environment, and the dilute Salt Lake water, respectively. The rusts were characterized by scanning electron microscopy (SEM), electron probe micro analyzer (EPMA), X-ray diffraction (XRD), infrared transmission spectroscopy (IRS), X-ray photoelectron spectroscopy (XPS) and electrochemical techniques. The corrosion process and mechanism in salt rich atmosphere were investigated. The following conclusions were drawn:
1. After outdoor 25 months and indoor 96 hours of exposure, Corten A weathering steel showed the similar corrosion weight loss to Q235 carbon steel and did not exhibit better corrosion resistance.
2. The indoor accelerated corrosion tests of weathering and carbon steel were relatively well correlated with the outdoor exposure tests. In the salt-rich environment, the corrosion weight loss after the accelerated test of 12 hours was close to that after the outdoor exposure of 6 months. The rusts were mainly composed of β-FeOOH, Fe8(O,OH)16Cl1.3 and a little γ-FeOOH. Cl and Mg foreign elements were rich in the rust. The alloyed elements Cr, Ni and Cu in weathering steel were detected in the rusts, however, these elements showed no remarkable protective ability. Anodic dissolution was suppressed and cathodic reaction was accelerated due to the rusts formed on both kinds of the steels. The very small value of the rust resistance RR displayed the weak protective ability of rusts.
3. The initial atmospheric corrosion of carbon steel and weathering steel were controlled by the micro-cell mechanism. And with the thickening of the rust layer, the electrochemical mechanism began to dominate. The corrosion products β-FeOOH and Fe8(O,OH)16Cl1.3 were transformed into γ-FeOOH, δ-FeOOH and Fe3O4 by various chemical and electrochemical reactions.
4. In the range of 1-100 dilution times of the Salt Lake water, with increasing the dilution times, the anodic polarization process of weathering and carbon steel changed from general corrosion to pitting corrosion. And the cathodic polarization was converted from hydrogen evolution to oxygen reductive reaction. The values of the reaction resistance Rt decreased with dilution times, implying that the attack of the solution became stronger. For 316L stainless steel, with the decrease of the concentration of the Salt Lake water, the anodic polarization was converted from general corrosion to pitting corrosion, which was similar to that of weathering steels and carbon steels. The anodic and cathodic current density showed the decrease and increase, respectively. The values of Rt increased with dilution times, indicating the weakening of the attack of the solution, which was different from that of weathering steel and carbon steel.
5. In the case of indoor corrosion test, under vertical exposure, the specimens with rolling surface showed better corrosion resistance than that with polished surface because the thick protective oxide film containing Fe(III) and Cr(VI) on rolling surface could effectively suppress the attack of Cl- ions. And for polished surface, the samples under vertical exposure were corroded more severely in comparison with those under horizontal exposure, which may be because of less removal action of the solution and higher concentration of retained on the surface.
6. The accelerated corrosion test of 316L stainless steel showed some correlation with that of exposure test. Cl- facilitated the pitting corrosion. The weight loss increased and the corrosion rate decreased with exposure time. The corrosion products, which were mainly composed of the compounds containing Fe(II), Fe(III), Cr(III)-ox, Cr(III)-hy and Cr(VI), hindered the corrosion of stainless steel. The rust layer suppressed the anodic and cathodic reactions and caused the increase of the value of charge-transfer resistance Rt. However, the corrosion weight loss after the indoor corrosion test of 30 days was much lower than that after the outdoor exposure of 6 months. Thus, the accelerated corrosion test was not an effective way to simulate the corrosion of 316 L stainless steel in salt rich atmosphere. |
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