Electrochemical corrosion protection of steel in soil and concrete – experimental study

Background

Cathodic protection (CP) is an electrochemical technique used to reduce and control the corrosion of metals. This technique is applied worldwide to protect underground structures, reinforced concrete structures, marine exposed structures, ship hulls and heat exchangers.

In the recent years, different researchers concluded that the increase in pH, promoted by the generation of OH− at the steel surface as a consequence of the protection current, is fundamental for achieving corrosion protection of the steel in soil, namely passivation. Nowadays, it is well-accepted that the effectiveness of the CP technology is achieved by steel passivation, rather than activation polarisation of the steel in the immunity domain. The environment at the steel surface during CP is subjected to changes due to removal (consumption) of oxygen, as a consequence of the cathodic reaction; migration of anions (chlorides), due to the electric field generated by the CP system; and generation and accumulation of hydroxyl ions at the steel surface as product of the cathodic reactions.

Aims and objectives

The aim of the project is to deliver a well-documented study on the temporospatial variation of pH and oxygen concentration in a well-characterized soil during CP. The project contributes to understanding the role of pH and oxygen in achieving steel protection and to establishing knowledge-based criteria for ensuring CP effectiveness.

Methodology

Carbon steel samples are buried in quartz sand, saturated with various simulated soil solutions. The pH variation is determined by means of planar optical sensors (optodes) and iridium oxide (IrOx) sensors. The oxygen variation is determined by means of fiber optical oxygen sensors. Different soil porosities are obtained by dosing quartz sand with various granulometry. The cathodic protection is applied in potentiostatic condition. Different levels of cathodic protection are studied by varying the ON-potentials (EON).

The following video shows an example of the temporospatial changes in pH if a cathodic protection current is applied to a working electrode (WE) located on the right (grey bar) in the figure. (The video will come soon) 

Additionally, situations of heterogeneous corrosion will be studied, namely situations where the coupon subject to protection current is partially active and partially passive.