Formation of Corrosion Cells

2 Causes of Corrosion Danger 18.2.1 Formation of Corrosion Cells 

The usual practice in old wells of only partially cementing the outer pipe can lead to cell formation (steel in the cement-steel in the soil) in the transition regions to the uncoated sections (see Sections 4.2 and 4.3). In contrast to the well-known cathode steel-soil in the vicinity of the ground surface, the cathodic activity of the 

Further cell currents flow between the wells as a result of electrical connections established between them by the flow lines and of the different free corrosion potentials, thereby allowing them to behave as anode or cathode. The currents can amount to a few amps so that considerable corrosion damage can arise. The action of these cells can be prevented by building in insulators between the drilling and the field cable. 

Sections of the borehole casing threatened by corrosion can be located with the help of the profile measurement technique described in Section 18.3.1. In general, the profile measurement cannot identify which factors are the main cause of corrosion danger. 

---

Different microstructural regions in a material which has an almost uniform composition can also lead to the formation of corrosion cells (e.g., in the vicinity of welds). Basically, corrosion cells can be successfully overcome by cathodic protection. However, in practice, care has to be taken to avoid electrical shielding by large current-consuming cathode surfaces by keeping the area as small as possible. In general, with mixed installations of different metals, it must be remembered that the protection potentials and the protection range depend on the materials (Section 2.4). This can restrict the use of cathodic protection or make special potential control necessary. 

Figure 12.21 Mechanisms of the degradation of paint fihns (a) degradation of a polymer due to ultraviolet radiation or chemical attack (b) the formation of blisters by osmosis and (c) cathodic delamination due to the formation of corrosion cells.

The formation of thick biofilms could in principle be beneficial if a compact film would uniformly cover a metal, preventing access of oxygen to the surface. However, biofilms usually do not form uniformly over a metal surface but usually in patches. Therefore they stimulate the formation of corrosion cells between covered and non-covered areas. The phenomenon is further enhanced if anaerobic conditions prevail at the metal surface in the areas covered by the biofUm, creating conditions for the development of anaerobic bacteria such as SRB that inhibit passivation. The following two examples illustrate the corrosive effect of microorganisms. 

Welds and mechanical joints can be areas where accelerated corrosion occurs for several reasons. Field coatings applied to joints are sometimes not of the same quality as shop-applied coatings on the remainder of the pipeline. This can lead to the environment penetrating to the metal and the formation of corrosion cells, Mechanical joints and rough welds can lead to crevice corrosion. Weld metallurgy also plays a role in corrosion. Welds containing high levels of sulfide inclusions can lead to localized corrosion of the weld metal. 

The porous structure has a very strong adsorbing ability. This permits the surface of an anodic film to be dyed, but it can also be contaminated. To prevent the formation of corrosion cells, the surface must be sealed. This is accomplished using hot water or steam, which seals the pores by the formation of boehmite (AI2O3 H2O) or bayerite (ALO, H2O). 

The solubility of phosphate films varies with pH, the type of phosphate, and its morphology. In the alkaline range of pH >10 the solubility of phosphate film is directly related to the adhesion of paint film. Formation of corrosion cells under the paint film causes an increase in pH at the cathodic area since the cathodic reactions produce OH ions and pH at the interface between the steel and paint film reaches 13.5. During the cathodic electrodeposition process for primers, pH at the surfaces reaches a value of 10-12, whereas in the anodic electrodeposition process pH at the surface is 3-4. 

The following are general recommendations to avoid the initiation of see due to the formation of corrosion cells in service. 

After candidate materials have been selected, the next step is to analyze the environment in which a system will operate. When considering the operational environment many may automatically consider exposure to atmospheric, industrial, or marine conditions as the corrosion inducing factors. While important, such a global or "macro" view may be too limited since conditions imposed by the configuration and operation of a system may result in the formation of corrosion cells and of particularly corrosive microenvironments. 

The formation of 804 in the rust leads to the formation of corrosion cells at the rust Fe804 (sulfate nests) interface. Corrosion would continue to take place as long as the supply of 804 is abimdant [1]. A simplified diagram showing the contribution of an electrochemical cycle to atmospheric corrosion is shown in Fig. 10.8. The formation of Fe804 nests is illustrated in Fig. 10.9. To maintain corrosion, the corrosion cell requires an electronically conducting path which is provided by ferrous sulfate. Corrosion would slow down if the resistance of either of the paths is increased. 

Figure 12.7a Formation of corrosion cell due to porosity of concrete. The porosity affect the level of oxygen in the concrete...

Figure 12.7 b Formation of corrosion cell by difference in stress levels of the steel bar... 

Local variations in temperature and crevices that permit the accumulation of corrosion products are capable of allowing the formation of concentration cells, with the result of accelerated local corrosion. 

Coatings of less noble metals than the substrate metal (e.g., Zn on Fe) are only protective if the corrosion product of the metal coating restricts the corrosion process. At the same time, the formation of aeration cells is hindered by the metal coating. No corrosion occurs at defects. Additional cathodic protection to reduce the corrosion of the metal coating can be advantageous. Favorable polarization properties and low protection current requirements are possible but need to be tested in individual cases. The possibility of damage due to blistering and cathodic corrosion must be heeded. 

Galvanic anodes must not be backfilled with coke as with impressed current anodes. A strong corrosion cell would arise from the potential difference between the anode and the coke, which would lead to rapid destruction of the anode. In addition, the driving voltage would immediately collapse and finally the protected object would be seriously damaged by corrosion through the formation of a cell between it and the coke. 

Differences in temperature and concentration can in principle lead to corrosion cell formation, but have little effect below the water line. On the other hand, they have to be taken into account in the interior corrosion of containers and tanks in relation to their service operation (see Section 2.2.4.2). Generally the action of corrosion cells can be reduced or eliminated by cathodic protection. 

Bimetallic corrosion between two different metals (see also Section 1.7) embedded in damp wood, e.g. in the hull of a boat, can occur in two ways". If the metals are joined by a metallic conductor, then the formation of the cell... 

The precautions generally applicable to the preparation, exposure, cleaning and assessment of metal test specimens in tests in other environments will also apply in the case of field tests in the soil, but there will be additional precautions because of the nature of this environment. Whereas in the case of aqueous, particularly sea-water, and atmospheric environments the physical and chemical characteristics will be reasonably constant over distances covering individual test sites, this will not necessarily be the case in soils, which will almost inevitably be of a less homogeneous nature. The principal factors responsible for the corrosive nature of soils are the presence of bacteria, the chemistry (pH and salt content), the redox potential, electrical resistance, stray currents and the formation of concentration cells. Several of these factors are interrelated. 

Combustion products can affect sensitive electronic equipment. For example, hydrogen chloride (HCI) is formed by the combustion of PVC cables. Corrosion due to combusted PVC cable can be a substantial problem. This may result in increased contact resistance of electronic components. Condensed acids may result in the formation of electrolytic cells on surfaces. Certain wire and cable insulation, particularly silicone rubber, can be degraded on exposure to HCI. A methodology for classifying contamination levels and ease of restoration is presented in the SFPE Handbook... 

The surface finish of the component also has an impact on the mode and severity of the corrosion that can occur. Rough surfaces or tight crevices can facilitate the formation of concentration cells. Surface cleanliness can also be an issue with deposits or films acting as initiation sites. Biological growths can behave as deposits, or can change the underlying surface chemistry to promote corrosion. 

Bacterial activity on synthetic pyrite films deposited on glass resulted in the formation of corrosion areas around the cells, which could be seen as halos under a light microscope (9). Immunofluorescence techniques showed the bacteria to be preferentially accumulated along defect sites and regions of structural imperfections in the sulfide film. Bacteria were seen to be attached to the rim of craters. Acidic solutions of Fe(III) did not form pits but interacted with the entire exposed pyrite surface. 

---