Oxygen corrosion locations

Oxygen corrosion only occurs on metal surfaces exposed to oxygenated waters. Many commonly used industrial alloys react with dissolved oxygen in water, forming a variety of oxides and hydroxides. However, alloys most seriously affected are cast irons, galvanized steel, and non-stainless steels. Attack occurs in locations where tuberculation also occurs (see Chap. 3). Often, oxygen corrosion is a precursor to tubercle development. 

The tendency of oxygen-deprived locations to become anodic is the cause of many commonly-observed patterns of corrosion. 

Atmospheric corrosion results from a metal s ambient-temperature reaction, with the earth s atmosphere as the corrosive environment. Atmospheric corrosion is electrochemical in nature, but differs from corrosion in aqueous solutions in that the electrochemical reactions occur under very thin layers of electrolyte on the metal surface. This influences the amount of oxygen present on the metal surface, since diffusion of oxygen from the atmosphere/electrolyte solution interface to the solution/metal interface is rapid. Atmospheric corrosion rates of metals are strongly influenced by moisture, temperature and presence of contaminants (e.g., NaCl, SO2,. ..). Hence, significantly different resistances to atmospheric corrosion are observed depending on the geographical location, whether mral, urban or marine. 

A large number of parameters are involved in the choice of the corrosion protection system and the provision of the proteetion eurrent these are deseribed elsewhere (see Chapters 6 and 17). In partieular, for new locations of fixed production platforms, a knowledge of, for example, water temperature, oxygen content, conductivity, flow rate, chemical composition, biological activity, and abrasion by sand is useful. Measurements must be carried out at the sea location over a long period, so that an increased margin of safety can be calculated. 

The composition given in Table 2.8 is global and, for most components, is reasonably constant for all locations, but the water vapour content will obviously vary according to the climatic region, season of the year, time of the day, etc. However, only oxygen, carbon dioxide and water vapour need to be considered in the context of atmospheric corrosion. 

Apart from caustic cracking, the main forms of corrosion in low-pressure plant are oxygen pitting and scab formation. The residence of oxygen at sensitive locations on-load, possibly following off-load initiation, means that... 

The corrosion of iron occurs particularly rapidly when an aqueous solution is present. This is because water that contains ions provides an oxidation pathway with an activation energy that is much lower than the activation energy for the direct reaction of iron with oxygen gas. As illustrated schematically in Figure 19-21. oxidation and reduction occur at different locations on the metal surface. In the absence of dissolved ions to act as charge carriers, a complete electrical circuit is missing, so the redox reaction is slow, hi contrast, when dissolved ions are present, such as in salt water and acidic water, corrosion can be quite rapid. 

Metals are subject to electrochemical corrosion in the presence of water Metal atoms lose electrons to become positively charged metal ions that go into solution. These then react with other chemical species in the soil ground-water to form solid corrosion products (e.g., metal oxides, hydroxides, sulfates). It is these solid corrosion products that often form a colored matrix with soil particles around the corroding object (Cronyn 1990). The initial formation of the metal ions takes place at a site on the metal known as the anode, whereas the electrons produced consumed by another reaction with an electron acceptor (the cathode). Due to the electrical conductivity of metals the location of the anode and cathode can be at different locations on the metal surface. In the presence of water and oxygen the cathodic reaction is... 

Corrosion often begins at a location where the metal is stressed in some way or isolated from oxygen, such as between joints or under a paint film. The metal ions dissolve in the moisture film and the electrons migrate to another location where they are taken up by a depolarizer . Oxygen is the most common depolarizer the resulting hydroxide ions react with the Fe2+ to form the mixture of hydrous iron oxides known as rust . 

Oxygen/aluminum ratios were also determined at various locations in the cracks as indicated in Figure 7.11 and results are given in Table 7.2. The oxygen/aluminum ratio increases from 1.5 to 2.0 as the crack location moves towards the crack tip. The ratios of 1.5 and 2.0 likely represent AI2O3 and AIO2, respectively. This observation tends to indicate that corrosion induced cracking took place. 

EDS study at location B, at the bottom of a pit showed that location was mainly composed of Cu and Ni with a small amount of Fe, which could be attributed to contamination since Fe was not detected in some other pits. The EDS result indicates no denickelification inside the pit since both Ni and Cu were found, and the crystals appear to be compact with no evidence of any copper crystal deposit or selective nickel dissolution leaving a porous structure. It should also be noted that there was no corrosion product at the bottom of the pit, and there was clear evidence of copper redeposit at the edge of the pit, as indicated in EDS of the copper and oxygen peaks. 

Secondly, we focused on the role of oxygen impurities in element-selective corrosion of austenitic stainless steels in liquid sodium. Calculation was carried out for an oxygen impurity associated with an Na atom located at the site on top of a Cr atom at the Fe(OOl) surface in sodium. The result shows that a positively charged Cr atom will be released selectively into sodium by an 0 anion. 

Let us consider the example of a piece of iron with a barnacle growing on it Fig. 7-15). Underneath the barnacle the oxygen concentration is lower than outside the confines of the barnacle shell. The ferrous iron and pH levels in solution at both places are assumed to be identical. We will examine the tendency of the corrosion reaction to proceed at both locations (A under the barnacle and B outside the barnacle). 

Corrosion potential. Any embedded reference electrode allows the electrochemical potential of the adjacent rebars to be measured. This allows depassivation of the rebars or of any other steel sensor element put at different depths to be detected by a drop in half-cell potential. The corrosion potential will be influenced by concrete humidity and oxygen content (Chapter 7). The depassivation of the steel probe located in the outermost cover concrete will present an early warning and suitable in-depth distribution of a set of steel probes allows the corrosion risk to be evaluated or the time of depassivation of the rebars to be calculated. 

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