Pitting stray currents

This chapter has discussed the mechanism of what happens at the steel surface. The chemical reactions, formation of oxides, pitting, stray currents, bacterial corrosion, anodes, cathodes and reference electrode potentials (half cells) have been reviewed. A more detailed account of the electrochemistry of corrosion and corrosion of steel in concrete is given in Appendix B. Chapter 3 will discuss the processes that lead to the corrosion and the consequences in terms of damage to structures. We will then move on to the measurement of the problem and how to deal with it. 

All metals will corrode under certain conditions. Internal corrosion is caused by galvanic corrosion, pitting, corrosion fatigue, stress corrosion cracking, stray currents, etc. 

DC stray currents may have more serious consequences in chloride-contaminated concrete. On passive reinforcement in concrete containing chloride in a quantity below the critical content and thus in itself insufficient to initiate localized corrosion, the driving voltage AE required for current to flow through the reinforcement is lower than in chloride-free concrete and decreases as the chloride content increases (Figure 9.7). This is a consequence of less perfect passivity, and in particular a lower pitting potential. 

Protection that concrete offers to steel against stray current ceases when corrosion of the reinforcement has initiated, e. g. due to carbonation, chloride contamination, or the stray current itself In this case, any current flowing through the steel will increase the corrosion rate at the anodic site, similarly as in buried steel structures. Figure 9.8 shows that even small driving voltages can lead to an increase in the corrosion rate on the anodic area (from to Furthermore, it has been observed that if steel is subjected to pitting corrosion in chloride-contaminated concrete, the anodic current increases the size of the attacked area [5]. 

The best protection against stray current is, therefore, provided by concrete. Those methods that can improve the resistance of concrete to carbonation or chloride contamination, which are illustrated in Chapters 11 and 12, are also beneficial with regard to stray-current-induced corrosion. It should be observed that this may not be the same for preventative techniques, since conditions leading to corrosion initiation due to stray current are different, in terms of potential, from those leading to corrosion initiation due to carbonation or chloride contamination. For instance, the use of stainless steel or galvanized-steel bars, which improves the resistance to pitting corrosion in chloride-contaminated concrete (Chapter 15), does not substantially improve the resistance to stray current in chloride-free and non-carbonated concrete [4]. In any case, a high concrete resistivity will reduce the current flow due to stray current. 

The first chapter constitutes an introduction to corrosion and various forms of corrosion such as general or uniform or quasi-uniform corrosion, galvanic corrosion, stray current corrosion, localized corrosion, such as pitting and crevice corrosion, metallurgically influenced and microbiologically influenced corrosion, mechanically assisted corrosion and environmentally induced cracking. 

Carbon and low-alloy steels are probably the most commonly used materials for pipes handling water, petroleum products, and some chemicals. Reference 1 provides a summaiy of the different specifications used for pipelines. Steel tends to corrode by both pitting and uniform surface deterioration [2]. Steel must usually be protected from corrosion both on the inside from the material being carried and on the outside from corrosion by the atmosphere, soil, or water that surrounds the pipe. External corrosion protection is provided by material selection, selective backfill, barrier coatings, stray current control, and cathodic protection. Internal corrosion protection can be provided by inhibitors, coatings, design process control, and materials selection. 

Coupons are relatively small pieces of metal, the same composition as the structure being evaluated, that are buried in the soil next to the structure. They are intended to measure the corrosivity of the soil where the structure is buried or is intended to be placed. If the structure tdready exists, the coupon should be connected to the structure in order to expose it to galvanic and stray currents, as well as the soil. Coupons can be evaluated after a period of time using mass loss (ASTM G 112) or polarization as previously discussed. Coupons should be large enough for pitting to develop. All coupons should be evaluated visually after exposure. 

Service life can also be affected by galvanic contact with a dissimilar metal. The less resistant material tends to be dissolved and may experience general corrosion, pitting/crevice corrosion, or SCC. Hydrogen may be liberated at the more resistant metal, making hydrogen embrittlement an issue if the material is susceptible. Stray currents, e.g., from a DC power source, may have the same effect as dissimilar metal contact. 

Additionally, stray-current corrosion is common on seagoing vessels, such as boats. For example, powered battery chargers generate dc stray currents, which may flow indefinitely if proper precautions are ignored or overlooked. This type of corrosion can manifest as pitting in confined ares, metal surface discoloration, rust formation on steel parts or weakening of batteries. 

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