Aeration buried structures

Differential aeration in buried structures. A clear example of macrocell action was documented in diaphragm walls in Berlin, illustrated in Figure 8.1 [6]. In this case, anodic areas had formed at the lower, non-aerated parts of the reinforcement at the ground side, while steel on the free side and higher up acted as cathode. Large amounts of corrosion products were found inside the concrete at various distances from the anodes and in the soil, suggesting that relatively soluble iron(II) oxides had formed that were able to move away from the anodes. Chlorides originated... 

Before any remedial action can be taken, it is necessary to identify the t) e of bacteria involved in corrosion. Aeration of water in a closed recirculating system reduces the activity of anerobic bacteria. Chlorination and treatment with biocides help control populations of some bacteria, though they are not effective in all cases. Also, the bacteriocides fail to reach the areas underneath deposits where the bacteria thrive. Coating a buried structure with tar, enamel, plastic, or the like is often an effective means to... 

In the above cell, HCl is in two different concentrations. The activity (molality x activity coefficient) ai is greater than activity 02 fli > <12-Several types of concentration cells are encountered in corrosion. For example, a concentration cell is formed if one end of a pipe is exposed to soil and the other end to air. The end of the pipe in air is exposed to a high concentration of oxygen than the end of the pipe in the soil. The formation of a concentration cell leads to differential aeration corrosion in buried structures in the soil. 

Galvanised steelwork buried in the soil in the form of service pipes or structural steelwork withstands attack better than bare steel, except when the soil is more alkaline than pH 9-4 or more acid than pH 2-6. Poorly aerated soils are corrosive to zinc, although they do not necessarily cause pitting. However, soils with fair to good aeration containing high concentrations of chlorides and sulphates may do so. Bare iron may be attacked five... 

Underground structures and pitting. The bottom of a metallic pipe or hose buried in the earth, with a relatively limited surface of metal poorly aerated, has the tendency to... 

The action of macrocells in structures buried in the soil or immersed in water is different from that of structures exposed to the atmosphere two circumstances promote macrocell effects while another reduces them. First, concrete is wetter than in aerated structures and its resistivity is lower, particularly in structures immersed in seawater. This reduces the ohmic drop in the concrete and increases the size of the effective cathodic area in relation to the anodic one. Secondly, the soil or the seawater around the concrete is an electrolyte of low resistivity, and the macro-cell current can also flow outside the concrete. This further reduces the ohmic resistance between the anodic area and passive reinforcement. Thirdly, there is, however, a mitigating aspect. Oxygen can only diffuse with great difficulty through wet concrete and thus it hardly reaches the surface of the embedded steel. Depletion of oxygen at the surface of the rebar that is observed in this case makes initiation of corrosion very difficult, and, even when corrosion initiates, the driving voltage for the macrocell is very low. 

The danger of corrosion on buried installations in industrial plants is increased by various soils and by cell formation with cathodes of steel in concrete. The rest potentials of these foreign cathodes are between t/cu-cuS04 = and -0.5 V [4-6]. Factors that affect cell formation are the type of cement, the water/cement ratio and the aeration of the concrete [6]. Figure 12-1 shows schematically the cell action and the variation of the pipe/soil potential where there is contact with a steel-concrete structure. The cell current density is determined by the large area of the cathode [see Fig. 2-6 and Eq. (2-44)]. In industrial installations the area of steel surface in concrete is usually greater than 10 m ... 

Differential aeration cells are frequently encountered in practice. They form in particular on carbon steel in neutral environments, whenever oxygen accesses certain parts of the structure with more difficulty. Figure 7.19 gives a few examples (a) presence of a crevice between a poorly tightened nut and a plate (b) accumulation of muddy residue on a steel plate (c) a steel structure partially immersed in water (d) a buried pipe that is not entirely surrounded by aerated backfill. 

Zinc may be used as a reference half-cell in soils. However, the main application of zinc in buried applications is in galvanized steel, for example, in the fabrication of culverts. Performance may be adequate unless soils are poorly aerated, acidic, or highly contaminated with chlorides, sulfides, and other corrosive ions. Well-drained soils with a coarse texture (the sandy type) provide a high degree of aeration. It should also be borne in mind that zinc corrodes rapidly under highly alkaline conditions. Such conditions can arise on the surface of cathodically overprotected structures. The degree of corrosion protection afforded by galvanizing obviously increases with the... 

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