Hydrogen stray currents

Finally, it is necessary to point out that although a particular method of corrosion control may be quite effective for the structure under consideration it can introduce unforeseen corrosion hazards elsewhere. Perhaps the best example is provided by cathodic protection in which stray currents (interaction) result in the corrosion of an adjacent unprotected structure or of steel-reinforcement bars embedded in concrete a further hazard is when the cathodically protected steel is fastened with high-strength steel bolts, since cathodic protection of the tatter could result in hydrogen absorption and hydrogen cracking. 

Stray current dumpers (SCD) with proprietary coating applied in interlayers. The system and the coating composition is designed to promote hydrogen evolution at low overpotentials and to protect the titanium metal phase. 

In the case of direct current (DC) interference, a cathodic reaction (e. g. oxygen reduction or hydrogen evolution) takes place where the current enters the buried structure, while an anodic reaction (e. g. metal dissolution) occurs where the current returns to the original path, through the soil (Figure 9.1). Metal loss results in the anodic points, where the current leaves the structure usually, the attack is localised and can have serious consequences especially on pipelines. Effects of AC stray current are more complex however, alternate currents are known to be much less dangerous than direct ones. 

In general, steel in concrete operates in the interval of potential and pH outside the critical ranges for hydrogen evolution. Under particular conditions, however, the situation may be different. Situations that make it jxtssible for hydrogen to develop are localized corrosion on the reinforcement that lead to oxygen depletion (and thus depresses the potential), acidity production at the anodic zones, and external cathodic polarization applied to the steel (due to, for example, excessive cathodic protection or stray currents). 

Since the maximum voltage that can be generated with zinc anodes is extremely unlikely to generate hydrogen embritdement, galvanic systems have been used to protect prestressed concrete members. They are also used on fusion bonded epoxy coated steel reinforced piles as the effects of electrical discontinuity between bars is unlikely to lead to significant stray current induced corrosion as the currents and potentials are low. 

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. 

The corrosion effects of stray current can be easily demonstrated with a simple laboratory setup such as shown in Fig. 7.30(cell containing a dilute saline solution, the formation of hydrogen bubbles is readily visible on the steel nail connected to the negative post of the DC power supply [Fig. 7.30(b)], while the nail connected to the positive post shows signs of rapid corrosion a few minutes later [Fig. 7.30(c)]. 

A very low-frequency alternating or commutated/direct current, typically 50-100 mA, is normally used to avoid ac stray or dc polarization (i.e., accumulation of hydrogen ions at the negative electrode) effects. 

Although the anode and cathode reactions are independent, they are clearly coupled to each other by the necessity to balance the overall reaction, so that the electrons produced in the HOR are consumed in the ORR. Note that the overall balanced chemical equation has no stray charged species and is identical to the chemical combustion of hydrogen in air. However, in the electrochemical reaction, the anode oxidation and cathode reduction reactions are separate and produce or consume the charged species that make up the current. 

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