Depassivating agents

Sulfate Sulfate ions have reactions similar to chloride. They are also depassivation agents (especially if allowed to concentrate sufficiently) and can greatly accelerate the risk of stress corrosion... 

Chloride is both a cathodic depolarizing agent (depolarizer) and a depassivating agent, and the presence of high chlorides in steam-water circuits significantly increases the risk of stress corrosion cracking of austenitic steels (type 300 stainless). 

Sulfate ions have reactions similar to those of chloride. They are corrosion-causative agents (similar to oxygen and hydrogen) of the various types of concentration cell corrosion. In addition, they also are depassivation agents and may greatly accelerate the risk of stress corrosion mechanisms. Saline corrosion pits resulting from high concentrations of chloride and sulfate salts also may be associated with low pH corrosion because hydrochloric acid and sulfuric acid can form within the pit, under deposits. 

The cracking process of a reinforced concrete structure can be divided into three phases the diffusion of the aggressive agent (chlorides in this case), the corrosion of the reinforcement steel and the crack propagation from the reinforcement steel towards the surface. The first phase is characterized by the diffusion of chlorides in the concrete. When the chloride concentration on the surface of the reinforcement steel exceeds a threshold concentration, we have steel depassivation followed by the initiation of the corrosion. The second phase is dominated by the expansion of corrosion products in which they slowly fill the pores surrounding the reinforcement steel. Finally, the propagation phase... 

The attack by external agents is more characteristic of steel fibres, where penetration of chlorides can depassivate the steel and lead to its corrosion [8-11]. Although this type of influence might be expected to be particularly harsh in steel fibre reinforced concrete because of the small cover over the fibres, experience has indicated otherwise [8], with such systems performing even better than those of conventional reinforcement. Several mechanisms have been proposed to account for this difference [10,11] and they are addressed in Chapter 7. 

---