Local-action corrosion cells

Attack associated with nonuniformity of the aqueous environments at a surface is called concentration cell corrosion. Corrosion occurs when the environment near the metal surface differs from region to region. These differences create anodes and cathodes (regions differing in electrochemical potential). Local-action corrosion cells are established, and anodic areas lose metal by corrosion. Shielded areas are particularly susceptible to attack, as they often act as anodes (Fig. 2.1). Differences in concentration of dissolved ions such as hydrogen, oxygen, chloride, sulfate, etc. eventually develop between shielded and nearby regions. 

The driving force of a thermogalvanic corrosion cell is therefore the e.m.f. attributable to these four effects, but modified by anodic and cathodic polarisation of the metal electrodes as a result of local action corrosion processes. 

In extreme cases irritant chemicals can have a corrosive action. Corrosive substances can attack and weaken materials of construction, as mentioned in Chapter 3. They can also attack living tissue (e.g. to cause skin ulceration and in severe cases chemical burns), kill cells and possibly predispose to secondary bacterial invasion. Thus while acute irritation is a local and reversible response, corrosion is irreversible destruction at the site of the contact. The outcome is influenced by the nature of the compound, the concentration, duration of exposure, the pH (see Figure 4.1) etc. Thus dilute mineral acids may be irritant whereas at higher concentrations they may cause corrosion. 

Under the appropriate conditions most metals are subject to corrosion, i.e. the gradual destruction of the metal by chemical means. Corrosion commonly occurs at metal surfaces in the presence of oxygen and moisture and involves electrochemical reactions (B-78MI11505, B-63MI11501, B-76MI11501). A metal surface can be regarded as a composite of localized electrodes connected through the bulk of the metal. In the presence of an electrolyte, for example surface moisture, these local-action cells are responsible for the chemical conversion of the metal to corrosion products. Reduction occurs at the cathodic sites (equation 16), and oxidation takes place at anodic sites (equation 17). 

EXPERIMENTAL PROCEDURES OF LABORATORY STUDY ON LOCAL ACTION CELL CORROSION... 

Experimental procedures of laboratory study on local action cell corrosion 367... 

Corrosion cells can be produced by the interaction of small, local, adjacent anodes and cathodes on the same piece of metal. These so-called "local-action cells" form because the surface of a piece of metal is not uniform. Small variations in composition, local environment, orientation of the grain structure, and differences in the amount of stress and surface imperfections all may contribute to the creation of tiny areas of... 

Any metal surface, similar to the situation for zinc, is a composite of electrodes electrically short-circuited through the body of the metal itself (Fig. 2.2). So long as the metal remains dry, local-action current and corrosion are not observed. But on exposure of the metal to water or aqueous solutions, local-action cells are able to function and are accompanied by chemical conversion of the metal to corrosion products. Local-action current, in other words, may... 

Both resistance of the electrolyte and polarization of the electrodes limit the magnitude of current produced by a galvanic cell. For local-action cells on the surface of a metal, electrodes are in close proximity to each other consequently, resistance of the electrolyte is usually a secondary factor compared to the more important factor of polarization. When polarization occurs mostly at the anodes, the corrosion reaction is said to be anodically controlled (see Fig. 5.7). Under anodic control, the corrosion potential is close to the thermodynamic potential of the cathode. A practical example is impure lead immersed in sulfuric add, where a lead sulfate film covers the anodic areas and exposes cathodic impurities, such as copper. Other examples are magnesium exposed to natural waters and iron immersed in a chromate solution. 

General Description. Uniform or general corrosion, as the name implies, results in a fairly uniform penetration (or thinning) over the entire exposed metal surface. The general attack results from local corrosion-cell action that is, multiple anodes and cathodes are operating on the metal surface at any given time. The location of the anodic and cathodic areas continues to move about on the surface, resulting in uniform corrosion. Uniform corrosion often results from atmospheric exposure (especially polluted industrial environments) exposure in fresh, brackish, and salt waters or exposure in soils and chemicals. 

Corrosion susceptibility in aqueous media is assessed on the basis of the rating numbers [3, 14], which are different from those of soils. An increased likelihood of corrosion is in general found only in the splash zone. Particularly severe local corrosion can occur in tidal regions, due to the intensive cathodic action of rust components [23, 24]. Since cathodic protection cannot be effective in such areas, the only possibility for corrosion protection measures in the splash zone is increased thickness of protective coatings (see Chapter 16). In contrast to their behavior in soils, horizontal cells have practically no significance. 

Fig. 19.16 Schematic E — I diagrams of local cell action on stainless steel in CUSO4 + H2SO4 solution showing the effect of metallic copper on corrosion rate. C and A are the open-circuit potentials of the local cathodic and anodic areas and / is the corrosion current. The electrode potentials of a platinised-platinum electrode and metallic copper immersed in the same solution as the stainless steel are indicated by arrows, (a) represents the corrosion of stainless steel in CUSO4 -I- H2 SO4, (b) the rate when copper is introduced into the acid, but is not in contact with the steel, and (c) the rate when copper is in contact with the stainless steel...

The idea that metal corrosion could be due to local-cell action was put forward in 1830 by Auguste Arthur de la Rive, and became very popular. An extreme view derived from this idea is the assertion that perfectly pure metals lacking all foreign inclusions will not corrode. However, it does not correspond to reality. It was established long ago... 

It follows that the corrosion potential on a heterogeneous metal corroding by local-cell action is virtually equal to the mixed potential at an electrode on which electronation and deelectronation reactions are occurring on spatially separated sinks and sources and is identical to a mixed potential when the metal is corroding homogeneously by a Wagner-Traud mechanism. The concept of the corrosion current /corr and the corrosion potential 40corr will now be treated quantitatively. 

A good starting point is the basic picture of corrosion by local-cell action, according to which a corroding metal consists of electron-sink areas at which metal dissolution takes place and electron-source areas at which an electronation reaction occurs. Under conditions where there is an exponential current-potential relationship... 

Other hypotheses invoke corrosion in Grignard reactions by direct chemical action l.l()[. No evidence that distinguishes local-cell corrosion from direct chemical action is known to us. However, there is only weak evidence of the discrete Mg(l) intermediates that are required by the hypothesis of direct chemical action (Section 7.3.K). In addition, pitting is a characteristic ol local-cell corrosion, which may also be easier to integrate with the I) model that direct... 

Local Cell Action in High-temperature Corrosion... 

Fig. 15 Local cell action in high-temperature corrosion [118,122], (a) Growth of Agl on an Ag—Ta couple in I2 (gas), (b) Corrosion of Ni in a borate melt, reactions on a Ni—Pt couple.

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