Corrosion Evolution of weathering steel in Qinghai Salt Lake Atmospheric Environment

摘要

Salt lakes on the earth, especially the west region of China, are widely distributed. The atmospheric environment in Qinghai salt lake region is different from common marine atmospheric environment, and there is a lack of research about the atmospheric corrosiveness and metal corrosion data. With the implement of the national western development project, it is exigent to investigate the atmospheric corrosiveness and metal corrosion situation of the salt lake region. Steels as the most fundamental materials are widely employed in traffic, energy and buildings, etc. In order to supply the fundamental data and technical support to the construction in the salt lake region, and make clear the main impact factors of steel corrosion, further supply the theoretical basis for developing economical and practical weathering steel, the exposure test with duration of 30 months for CortenA weathering steel was performed. The corrosion evolution of the weathering steel was investigated by XRD, IRS, SEM, EDX, EPMA and electrochemical techniques. The influence of Mg to the corrosion process is clarified and the atmospheric corrosion model of the steel in Qinghai salt lake region is proposed. The weight loss results of the exposure test showed that the embarking season had a great influence on the initial corrosion stage of the weathering steel, and the relative humility was the key impact factor. In the whole exposure duration, the weathering steel showed stepladder corrosion dynamics behavior, indicating that the rust layers on the steel were not protective. The average corrosion rate showed a fluctuation tendency with time. The rust composition was mainly changed from γ-FeOOH and β-FeOOH, via β-FeOOH and magnetite(Fe_3O_4), to magnesioferrite(MgFe_2O_4) and iowaite (Mg_4Fe(OH)_8OCl·xH_2O). Correspondingly, the rust structure altered from loose and porous, via relative dense and adherent, to incompact and discontinuous again. Mg not only stayed in the rust as salt deposits, decreasing the critical relative humility greatly and increasing the time of wetness, but also took part in the rusting evolution process and accumulated in the rust, forming corrosion products MgFe_2O_4 and Mg_4Fe(OH)_8OCl·xH_2O, which enhanced the conductivity of the rust, meantime made the rust layer anion-selective, promoted the diffusion of Cl ions, changed the rust layers structure and properties, and further influenced the following corrosion process. The electrochemical results showed that both rust resistance R_r and charge transfer resistance R_t were small, indicating that the rust layer were rough, porous and conductive, facilitated the transportation of corrosive ions in the rust layer and accelerated the corrosion of the matrix. The rust capacitance C_r displayed a fluctuation tendency with exposure time, indicating that the rust was not stable and hard to form a density structure, which had poor protectiveness.