| Zinc is an optimal choice to protect steel for its lower corrosion potential and corrosion rate. However, different corrosion behavior of zinc coating in different environment was obviously observed. In order to investigate the different corrosion behavior, long term exposure method was commonly used, in which corrosion quantity of samples was measured and corrosion product was analyzed. But the efficiency and timeliness of this method has always been questioned due to the comlexity of atmospheric environment and long duration. So it works little in concluding the corrosion rules of zinc in natural atmospheric environment.The existing accelerating corrosion test and indirect test methods were disadvantaged of less data and lack of study on the corrosion process of zinc. Long term, continuous, real-time and in-situ detection has become the trend in developing new methods to study the metal corrosion behavior in atmospheric environment. And a new kind of atmospheric corrosion monitor (ACM) was developed in this paper to study zinc corrosion in atmospheric environment. Inert carbon film is used as cathode, which avoids the effect of cathode material caused by corrosion in traditional ACM. Cathode and anode are located in two parallel planes in three layered structure, which avoids the influence of anodic corrosion products on cathode and improves the consistency as well as service life of sensor. ACM was equipped with a battery and solar panels to power its function and GPRS module to realize long-range data transmission compared with traditional exposure methods. The information loss and data error can be avoided without repeated manual operation. And the stability of corrosion data is guaranteed. ACM was proved to be competent for long duration, continuous, real-time and in-situ corrosion detection after being tested in field environment, in the emulated environment and with zinc surface modification. ACM is an effective method to study the atmospheric corrosion of zinc.Multi factor coupling test is an important method to simulate the corrosion behavior in atmospheric environment with the characteristics of acceleration and reproducibility. This test has not yet formed a unified standard so far and no mature products to execute it. Then,5 factors composite emulated environment test instrument was developed, which can control the corrosion factors and environmental parameters precisely to accelerate the corrosion of zinc.Long-term, real-time and in-situ corrosion tests of Zn/C galvanic type ACM and hot-dip galvanized sample were carried out in field and emulated environment. Effects of temperature and humidity on corrosion of zinc, effects of Cl- on both of the corrosion mechanism and corrosion kinetics of zinc, effects of SO2 intermediate product on zinc corrosion and kinetics and characteristics were studied.Conclusions are listed as following:Temperature and humidity influence the corrosion rate of zinc in atmospheric environment. Corrosion rate of zinc increases when temperature or humidity increases. And the effect of humidity is stronger than that of temperature. More importantly, the effect of temperature and humidity on zinc corrosion is coupling, which means each of them affects with another. A binary quadric mathematical model is presented in this paper to fit the coupling effect of temperature and humidity on zinc corrosion, which is more accurate than TOW in illustrating the corrosion mechanism of zinc in atmospheric environment.The corrosion proceeds electrode reaction control, mixed control and diffusion control in dissolution of zinc in atmospheric environment without corrosive ions. The corrosion kinetics of zinc follows a power function model after long-term corrosion in this environment.Zinc has been kept in an active state in atmospheric environment containing Cl-. And the corrosion product was not an effective barrier layer. The corrosion kinetics of zinc is linear in the environment. Nonlinear potential distribution in the double layer is formed for adsorption of Cl- which changes the potential energy of the matrix and reduces the activation energy. The corrosion rate of zinc increased with the increase of Cl- concentration. When the deposited amount of NaCl was 40 μg/cm2, the corrosion rate of zinc was at maximum and the adsorption of Cl- reached the highest level. When the concentration of Cl- in the diffusion layer increased, zinc would be hindered from the matrix, and thus led to the decrease of dissolution rate.The corrosion rate of zinc is affected by the acidification and intermediate products such as SO32- and SO42- of SO2. Insoluble sulfite on zinc generated by SO32- is difficult to become sulfate and hindered the dissolution of zinc. SO42- could accelerate the dissolution of zinc with the same mechanism as Cl-.SO32- is oxidized into SO42- in large proportion when the concentration of SO2 in atmosphere is lower than 10 ppm. Both the adsorption of SO32- on zinc surface and zinc corrosion rate reached the maximum when the concentration of SO2 was 10 ppm. When the concentration of SO2 exceeded 10 ppm, pH value decreased sharply, sulfite dissolved and the corrosion rate of zinc increased.SO32- is oxidized into SO42- in high proportion in general atmosphere when the concentration of SO2 is less than 10 ppm. SO42- accelerate the dissolution of zinc, and remains the active state of zinc, which results in the linear kinetics of zinc corrosion. |
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