Passive stray currents

Anodic polarization can occur in the presence of stray currents. Oxygen is evolved on the passive steel according to ... 

In structures affected by electrical fields, DC stray current in the concrete can enter the reinforcement in some areas (i. e. it passes from the concrete to the steel) and return to the concrete in a remote site. The passive layer can be destroyed in those areas where the current leaves the steel (Chapter 9). 

Above the passivity range, that is for potentials above about +600 mV, the steel is brought to conditions known as transpassivity. oxygen may be produced on its surface according to the anodic reaction of oxygen evolution 2H2O —> O2 + 4H + 4e, which produces acidity. Steel reaches these conditions only in the presence of an external polarization (for example in the presence of stray currents). Since the anodic reaction is oxygen evolution, dissolution of iron and consequent corrosion of the steel does not take place (i. e. the passive film is not destroyed). Nevertheless, if these conditions persist until the quantity of acidity produced is sufficient to neutralize the aLkaHnity in the concrete in contact with the steel, the passive film will be destroyed and corrosion will initiate. This aspect wiU be dealt with in Chapter 9. 

First precondition. Passive reinforcement in non-carbonated and chloride-free concrete offers a high intrinsic resistance to stray current, since the driving force required to produce the circulation of an appreciable current density in the anodic areas (i. e. >2 niA/m ) is at least 600 mV [2]. 

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. 

Figure 9.7 Schematic representation of electrochemical conditions in the cathodic and anodic zones of passive reinforcement in chloride-contaminated concrete that is subject to stray current...

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. 

We have seen that stray current can hardly induce corrosion on passive steel in non-carbonated and chloride-free concrete. However, the potential adverse effects of stray current on concrete structures may become increasingly important with the increased use of underground concrete construction. Stray-current effects are rarely recognised as such. The importance increases further due to the increase of the required service lives (i. e. 100 y or more). 

In principle, the cathodic process can be supported also from interference currents or stray currents. If the concrete is not degraded, iron is passive, and the anodic process is not the iron dissolution but the oxygen evolution without corrosion of the iron itself. If chlorides are present at a concentration sufficient to depassivate iron, oxygen evolution does not occur, but there is iron corrosion at much lower potential differences. 

In maskless EMM, there will not be any photoresist mask on the surface of the workpiece sample. Metal dissolution takes place from the desired area of the workpiece in a very selective and controlled way. Highly localized selective metal dissolution from the surface of the workpiece can generate a designed pattern or shape in a 2D or 3D scale. Anodic dissolution in maskless EMM is controlled by the current density, which depends on various predominant machining parameters and anodic reaction pattern. An lEG between the workpiece surface and tool is maintained at a very low value such that stray current effect is minimized. Passivating electrolyte is suitable for maskless EMM due to its ability to form transpassive oxide films and evolve oxygen in the stray current zone. Here, current efficiency varies as a function of the distance from the machining area hence, metal removal is more selective and localized across the zone of the workpiece surface where it faces the tool. 

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