AC Stray Current

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. 

Both DC and AC stray currents on a water line can cause corrosion. Stray current studies (18) show that the corrosion rate because of the dc current is generally greater than the corrosion rate because of ac current. General external... 

Sources of dc stray currents are commonly electric railways, grounded electric dc power lines, electric welding machines, cathodic protection systems, and electroplating plants. Sources of ac stray currents are usually grounded ac power lines or currents induced in a pipeline by parallel power lines. An example of dc stray current from an electric street railway system in which the steel rails are used for current return to the generating station is shown in Fig. 12.1. Because of poor... 

In harbors, central dc supply equipment is still occasionally used as welding equipment on wharves, travelling dc cranes, dc supply for ships in dock, etc. In Ref. 1 proposals for avoiding stray currents are detailed in which separate dc supply equipment is stipulated. In ac distribution networks in which the subsequent dc network is grounded in only one place, there is no possibility that the dc currents will occur as stray currents. 

Alternate stray currents are known to be much less dangerous than direct stray currents [8]. Even in neutral environments, such as soil, it was shown that AC with low current below 100 A/m will not give rise to noticeable effects on steel, even over... 

Although there is no experience, interaction between AC and DC stray currents cannot be excluded, since AC can influence the anodic behaviour of steel [8]. Therefore attention should be dedicated to possible synergistic effects of AC and DC stray currents that might, under specific circumstances, be able to stimulate the corrosion rate of depassivated steel or promote corrosion on passive steel. 

J. A. Gonzales, The determination of the corrosion rate of steel embedded in concrete by the polarization and AC impedance methods , in Corrosion Effect of Stray Currents and the Techniques for Evaluating Corrosion of Rebars in Concrete, ASTM STP 906, V. Chaker (Ed.), Philadelphia, 43-63, 1986. 

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. 

The resistivity of a soil can be measured by the four-electrode method with each electrode arranged in a straight line and each separated by the distance a (Fig. 12.4). Steady current I from a battery is passed through the two outer metal electrodes, and the potential difference, Ac ), of the two inner reference electrodes (e.g., Cu—CUSO4) is measured simultaneously. The measurement is usually repeated with current direction reversed in order to cancel out any stray currents. Then... 

Temperature rise in the windings and insulation due to losses. This includes losses due to excitation cnrrent in the field windings, ac current in the armature windings, the magnetic circuit, and any stray currents or magnetic fields that are generated. 

Stray-current corrosion, or stray-current electrolysis, is different fixim natural corrosion because it is caused by an externally induced electrical current (alternating, ac, or direct current, dc) and is basically independent of such environmental factors as oxygen concentration or pH. Environmental factors can enhance other corrosion mechanisms invcdved in the total corrosion process, but the stray-current corrosion portion of the mechanism is unaffected. 

Stray currents encountered in practice are usually dc (for example, fiom a welding generator) but can also be ac. For most metals, ac corrosion is negligible, but with aluminum it can be appreciable (Ref 33). Below a critical small ac current density, no corrosion of alumi-... 

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