In this paper, the statistical analysis of fluctuation characteristics of pipeline potential under dynamic DC stray current interference was carried out based on field test data. Then a self-designed experimental set-up was applied to study the corrosion behaviour of X70 steel in soil-simulating conditions. The statistical results showed that under the dynamic DC stray current interference, the pipeline potential was periodically fluctuated with dynamic periods between 10 and 300 s. The laboratory experiment results showed that there was a strong correlation between the corrosion rate and the dynamic period. When the dynamic period was less than 300 s, the corrosion rate was less than 22% of the predicted corrosion rate via Faraday's law. Then corrosion mechanism was discussed based on a series of experimental measurment results. The interface capacitance effect, as well as the charge consumption by extra reaction, acted a significant part in depressing metal dissolution under dynamic DC stray current interference.

To provide a reference for the evaluation of corrosion rate under dynamic metro stray current, both simulated experiments in laboratory and coupon tests in the field were conducted to explore the corrosion behavior and the relationship between corrosion rates and dynamic DC interference parameters of buried pipelines. The results show that the dynamic period has a significant influence on the corrosion rate and the ratio of actual corrosion rate to the theoretical corrosion rate. The statistical data of pipe to soil potentials or current densities of coupons electrically connected to pipelines obtained from the field in many cities of China show that the dynamic fluctuation periods under metro stray current are within the range of 0–300 s, among which the majority of fluctuation periods distribute from 50 to 200 s. The dynamic fluctuation of pipe to soil potentials and current densities of coupons exhibits symmetric or asymmetric characteristics. Under the situation of symmetric positive and negative current density, the ratio of the actual corrosion rate to the theoretical corrosion rate ranges from about 4% to 22% under the typical dynamic periods range from 50 to 200 s based on the simulated experiments results in the laboratory. Besides, two corrosion rate assessment methods, including the positive shifting and time ratios of off‐potentials and symmetrical or asymmetric characteristics of DC current density of coupons were proposed. In addition, the evaluation of the on‐potential criteria to assess the corrosion risk of time‐variant anodic interference was also discussed in the paper.

Corrosion behavior of X70 steel under the application of dynamic DC stray current was investigated in simulated soil solution by immersion tests and electrochemical measurements. Experimental results indicated that the corrosion rate of X70 steel under dynamic DC stray current was strongly influenced by the dynamic period. When the dynamic period was <80 s, the corrosion rate was only approximately 1–7% of that caused by an equivalent amount of steady direct current. However, as the dynamic period became >80 s, the corrosion rate increased significantly with the dynamic period. And finally when the dynamic period was >1 hr, the corrosion rate was nearly equal to 100% of that caused by an equivalent amount of steady direct current. Meanwhile, the surface examination showed that the specimens experienced uniform corrosion under the experimental conditions. On the basis of the experiment results, the dynamic DC stray current corrosion mechanism of X70 steel under the experimental conditions was discussed, which was closely related to the electric double‐layer structure at the electrode/solution interface and dynamic electrochemical reactions under dynamic stray current.

In this paper, an experimental investigation on X70 steel was performed with respect to dynamic DC stray current interference in a soil-simulated solution and in quartz sand. The results showed that the interference period had a huge impact on the corrosion rate. When the interference period was less than 100 s, corrosion rates in the two testing conditions were less than 12% of that induced by steady DC corrosion with an equivalent current. When the interference period was longer than 100 s, the corrosion rate in the soilsimulated solution increased more significantly than that in quartz sand. Also, when the interference period was longer than 1 h, the corrosion rate in the soil-simulated solution was approximately equal to the Faraday theory corrosion rate. However, the corrosion rate in quartz sand was still less than 81% of the Faraday theory corrosion rate, even though the interference period had been increased to 16 h. The difference of the corrosion rates in the two testing conditions was probably because of the different capacities of charge consumption by the capacitance effect of the electric double layer as well as the different charge consumption due to the extra reaction of species other than iron.