Corrosion local

Electromagnetic NDT instruments allow to increase the test records reliability. They are most efficient for detection of the rope durability loss as a result of loss of metallic cross-sectional area (LMA) localized faults (LF) like broken wires, localized corrosion etc. 

In fig, 4 local corrosion by erosion is shown in a pipe with a bore of 100 mm behind a welding. In this case only the nominal wall thickness of the pipe is known (6.3 mm). To calibrate the obtained density changes into wall thickness changes a step wedge exposure with a nominal wall thickness of 13 mm (double wall penetration in the pipe exposure) and the same source / film system combination was used. From this a pcff = 1-30 1/cm can be expected which is used for the wall thickness estimation of the pipe image according to equation (4). 

The passive state of a metal can, under certain circumstances, be prone to localized instabilities. Most investigated is the case of localized dissolution events on oxide-passivated surfaces [51, 106, 107, 108, 109, 110, ill, 112, 113, 114, 115, 116, 117 and 118]. The essence of localized corrosion is that distinct anodic sites on the surface can be identified where the metal oxidation reaction (e.g. Fe —> Fe + 2e ) dominates, surrounded by a cathodic zone where the reduction reaction takes place (e.g. 2Fi + 2e —> Fi2). The result is the fonnation of an active pit in the metal, an example of which is illustrated in figure C2.8.6(a) and (b). 

In all cases of localized corrosion, tlie ratio of the catliodic to tlie anodic area plays a major role in tlie localized dissolution rate. A large catliodic area provides high catliodic currents and, due to electroneutrality requirements, tlie small anodic area must provide a high anodic current. Hence, tlie local current density, i.e., local corrosion rate, becomes higher witli a larger catliode/anode-ratio. 

Localized corrosion is far more treacherous in nature and far less readily predictable and controllable tlian unifonn corrosion and it is, moreover, capable of leading to unexpected damage witli disastrous consequences, especially since inspection of corrosion damage is in many cases difficult. 

Fline F, Komai K and Yamakawa K (eds) 1988 Localized Corrosion (London Elsevier)... 

Tousek J 1985 Theoretical Aspects of the Localized Corrosion of Metals (Rockport, MA TransTech) Boehni H 1987 Corrosion Mechanisms ed F Mansfeld (New York Dekker)... 

Brown B F, Kruger J and Staehle R W (eds) 1974 Localized Corrosion (Flouston, TX NACE)... 

Frankenthal R P and Kruger J (eds) 1984 Equilibrium Diagrams of Localized Corrosion Proc. vol 84-9 (Pennington, NJ Electrochemical Society)... 

Localized corrosion, which occurs when the anodic sites remain stationary, is a more serious industrial problem. Forms of localized corrosion include pitting, selective leaching (eg, dezincification), galvanic corrosion, crevice or underdeposit corrosion, intergranular corrosion, stress corrosion cracking, and microbiologicaHy influenced corrosion. Another form of corrosion, which caimot be accurately categorized as either uniform or localized, is erosion corrosion. 

As discussed above, deposits can cause accelerated localized corrosion by creating differential aeration cells. This same phenomenon occurs with a biofilm. The nonuniform nature of biofilm formation creates an inherent differential, which is enhanced by the oxygen consumption of organisms in the biofilm. 

Many of the by-products of microbial metaboHsm, including organic acids and hydrogen sulfide, are corrosive. These materials can concentrate in the biofilm, causing accelerated metal attack. Corrosion tends to be self-limiting due to the buildup of corrosion reaction products. However, microbes can absorb some of these materials in their metaboHsm, thereby removing them from the anodic or cathodic site. The removal of reaction products, termed depolari tion stimulates further corrosion. Figure 10 shows a typical result of microbial corrosion. The surface exhibits scattered areas of localized corrosion, unrelated to flow pattern. The corrosion appears to spread in a somewhat circular pattern from the site of initial colonization. 

Corrosion control requires a change in either the metal or the environment. The first approach, changing the metal, is expensive. Also, highly alloyed materials, which are resistant to general corrosion, are more prone to failure by localized corrosion mechanisms such as stress corrosion cracking. 

An especially insidious type of corrosion is localized corrosion (1—3,5) which occurs at distinct sites on the surface of a metal while the remainder of the metal is either not attacked or attacked much more slowly. Localized corrosion is usually seen on metals that are passivated, ie, protected from corrosion by oxide films, and occurs as a result of the breakdown of the oxide film. Generally the oxide film breakdown requires the presence of an aggressive anion, the most common of which is chloride. Localized corrosion can cause considerable damage to a metal stmcture without the metal exhibiting any appreciable loss in weight. Localized corrosion occurs on a number of technologically important materials such as stainless steels, nickel-base alloys, aluminum, titanium, and copper (see Aluminumand ALUMINUM ALLOYS Nickel AND nickel alloys Steel and Titaniumand titanium alloys). 

Two types of localized corrosion are pitting and crevice corrosion. Pitting corrosion occurs on exposed metal surfaces, whereas crevice corrosion occurs within occluded areas on the surfaces of metals such as the areas under rivets or gaskets, or beneath silt or dirt deposits. Crevice corrosion is usually associated with stagnant conditions within the crevices. A common example of pitting corrosion is evident on household storm window frames made from aluminum alloys. 

Charge Transport. Side reactions can occur if the current distribution (electrode potential) along an electrode is not uniform. The side reactions can take the form of unwanted by-product formation or localized corrosion of the electrode. The problem of current distribution is addressed by the analysis of charge transport ia cell design. The path of current flow ia a cell is dependent on cell geometry, activation overpotential, concentration overpotential, and conductivity of the electrolyte and electrodes. Three types of current distribution can be described (48) when these factors are analyzed, a nontrivial exercise even for simple geometries (11). 

Insoluble corrosion prodiic ts may be completely impeivious to the corroding liquid and, therefore, completely protective or they may be quite permeable and allow local or general corrosion to proceed unhindered. Films that are nonuniform or discontinuous may tend to localize corrosion in particular areas or to induce accelerated corrosion at certain points by initiating electrolytic effects of the concentration-cell type. Films may tend to retain or absorb moisture and thus, by delaying the time of drying, increase the extent of corrosion resulting from exposure to the atmosphere or to corrosive vapors. 

Cleaning Specimens after Test Before specimens are cleaned, their appearance should be observed and recorded. Locations of deposits, variations in types of deposits, and variations in corrosion produces are extremely important in evaluating localized corrosion such as pitting and concentration-cell attack. 

Depth of localized corrosion should be reported for the actual test period and not interpolated or extrapolated to an annual rate. The rate of initiation or propagation of pits is seldom uniform. The size, shape, and distribution or pits should oe noted. A distinction should be made between those occurring underneath the supporting devices (concentration cells) and those on the surfaces that were freely exposed to the test solution. An excellent discussion of pitting corrosion has been pubhshed [Corro.sion, 25t (January 1950)]. 

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. 

As with all elec trochemical studies, the environment must be electrically conduc tive. The corrosion rate is direc tly dependent on the Tafel slope. The Tafel slope varies quite widely with the particular corroding system and generally with the metal under test. As with the Tafel extrapolation technique, the Tafel slope generally used is an assumed, more or less average value. Again, as with the Tafel technique, the method is not sensitive to local corrosion. 

FIG. 28-13 Logic sequence Diagram 2 used to evaluate localized corrosion resistance from a CBD. 

Multiinformational Prohes Corrosion probes can provide more information than just corrosion rate. The next three types of probes yield information about the type of corrosion, the kinetics of the corrosion reaction, as well as the local corrosion rate. 

A final type of measurement is the detection of localized corrosion, such as pitting or crevice attack. Several corrosion-measuring probes can be used to detec t localized corrosion. Some can detect locahzed corrosion instantaneously and others only its result. These types of corrosion may contribute little to the actual mass loss, but can be devastating to equipment and piping. Detec tion and measurement of localized corrosion is one of the areas with the greatest potential for the use of some of the newest electrochemicaUy Based corrosion monitoring probes. 

This is also the case with methods that yield information on localized corrosion. The overall corrosion rate may be small when locahzed attack occurs, but failure due to perforation or loss of function may be the consequence of locahzed attack. 

Evidence of localized corrosion can be obtained from polarization methods such as potentiodynamic polarization, EIS, and electrochemical noise measurements, which are particularly well suited to providing data on localized corrosion. When evidence of localized attack is obtained, the engineer needs to perform a careful analysis of the conditions that may lead to such attack. Correlation with process conditions can provide additional data about the susceptibility of the equipment to locaHzed attack and can potentially help prevent failures due to pitting or crevice corrosion. Since pitting may have a delayed initiation phase, careful consideration of the cause of the localized attack is critical. Laboratory testing and involvement of an... 

For example, consider localized corrosion. Although data from corrosion probes indicate corrosion rate, it is not possible to tell that localized corrosion is the problem. 

Good Fair 60 10.3 Possibility of developing localized corrosion in seawater... 

Heat resistance and gas corrosion resistance depends on chemical, phase compositions and stmcture of an alloy. The local corrosion destmction (LCD) of heat resisting alloys (HRS), especially a cast condition, probably, is determined by sweat of alloying elements. 

The corroded tubercle floor is almost always a dish-shaped depression, much wider than it is deep (Fig. 3.23). Undercutting is very rare. The metal-loss width almost exactly matches the tubercular mound width. Corrosion rates exceeding 50 mil per year are rare, except when tubercles are young. Average local corrosion rates are usually 20 mil per year or less. 

Pitting can occur when normally protective corrosion-product or deposit layers are locally breached. Localized attack occurs during upsets or when protracted idle periods change water conditions abruptly. Regions adjacent to localized corrosion sites often remain... 

Passive attack involving underdeposit corrosion tends to involve large system surface areas and, hence, accounts for the greatest amount of metal loss, by weight, in cooling water systems. Active attack tends to produce intense localized corrosion and, as such, a greater incidence of perforations. 

---