Protective Effect of Intermittent-Current Cathodic Protection on a Steel Bar in Concrete
Authors: Otani Shunsuke , Wakabayashi Toru , Mochizuki Noriyasu 
Published: February 2020

Introduction

Cathodic protection (CP) controls the corrosion current of steel bars in concrete by polarizing the steel potential in the less noble direction. CP is accompanied by spin-off reactions that generate OH- ions (thereby increasing the pH) and decrease the chloride ion concentration near the steel surface. In addition, concrete is a solid-phase material with a small diffusion rate. Therefore, the Cl-/OH-molar concentration ratio remains low at the steel surface even when the CP expires after a sufficiently long time. A repeated intermittent-current CP is thought to control steel corrosion.

This study examines the preventive effect of intermittent-current CP against the corrosion of steel bars in concrete specimens.

Methods

The lean concrete contained a water-to-cement ratio of 70% and 15 kg/m3chloride ions. The high water-to-cement ratio and the chloride cause progressive corrosion, enabling short-term evaluation of corrosion prevention. The concrete specimens were cast as prisms with dimensions of 100 mm × 100 mm × 200 mm. A plain steel bar and an insoluble anode were placed in the concrete specimens.

The CP test evaluated two types of intermittent-current CP, a continuous current CP and a No CP specimen. To promote steel corrosion, the CP test was conducted at 40° C and 70% RH. From days 164–218, the environment was changed to 20° C and 70% RH.

Results and discussion

The corrosion weight loss and corrosion areas of CP specimens were lower than that of No CP specimens at the end of the test. However, the corrosion weight loss and corrosion area of the CP specimens had increased. Therefore, it was thought that the intermittent-current condition must be appropriately set to suit the exposure environment and the characteristic concrete conditions.

　The CP specimens were analyzed in a simulation study. The simulation analysis considered the electrophoresis and diffusion of chloride ions, and the electrophoresis, diffusion and cathodic generation of hydroxide ions, which depend on the CP condition (such as provision or stoppage of current).

Figure 1 indicates the relationship between the estimated Cl-/OH- molar ratio and the corrosion rate of the steel bars in the CP specimens. The Cl-/OH- molar ratio largely depended on the current density. In addition, a threshold between the corrosion and passivity areas of the steel bars appeared at an approximate Cl-/OH- molar ratio of 0.6. Similar thresholds have been reported in past studies [1]. However, large corrosion rates were observed even at Cl-/OH- molar ratios below 0.6. This suggests that although the CP decreased the Cl-/OH- molar ratio, the re-passivation of the steel bars was in an insufficient state. On the other hand, low corrosion rates in the passivity area indicated by ○ in Figure 2, which plots the steel potential under the rest (stop current) CP condition versus the estimated Cl-/OH- molar ratio. Raising the steel potential, which manifests from the re-passivation behavior of the steel bars, appears to be necessarily with dropping the Cl-/OH-molar ratio. Therefore, this condition is potentially applicable to intermittent-current CP.

References

[1] Hausmann, D.A.: Materials Protection, Vo.6, No.11, p.19, 1967

Figure 1