Pit corrosion

Laser-based profilometry is now being applied to a wide variety of both NDT and Quality Control gauging applications. In the world of NDT, the primary interest is in the details associated with surface topography or deformation of a particular component. Laser-based profilometry systems are commonly used to inspect surfaces for defects such as pitting, corrosion, deformation and cracking. Quality control gauges are used for absolute measurement of dimensions, such as the diameter and thickness of a given part. 

Pitting occurs witli many metals in halide containing solutions. Typical examples of metallic materials prone to pitting corrosion are Fe, stainless steels and Al. The process is autocatalytic, i.e., by initial dissolution, conditions are established which furtlier stimulate dissolution inside tire pit tire metal (Fe in tire example of figure C2.8.6 dissolves. 

Zsklarska-Smialowska Z 1986 Pitting Corrosion of Metals (Flouston, TX National Association of Corrosion Engineers)... 

The last example presented in this section deals with the pitting corrosion of Fe in CIO solutions. Perchlorate is less known as an aggressive ion but reveals some unique and remarkable characteristics with regard to pitting corrosion. For example, the critical pitting potential (1.46 V against a standard hydrogen electrode (SHE) for Fe/1 M NaClO ) can be measured with an accuracy of less than 4 mV [61] which is very unexpected if compared to... 

Figure C2.10.4. XPS Cl 2p signals of an iron specimen emersed from 1 M HCIO (a) after passivation at 1 V (SHE) (b) after 2 minutes pitting corrosion at 1.5 V (SHE). Contributions of CIOj at 208 eV and CE at 198 eV are visible in different amounts.

Prinz FI and Strehblow FI-FI 1998 Investigations on pitting corrosion of iron in perchlorate electrolytes Corn Scl. 40 1671-83... 

Fig. 9. Pitting corrosion is damaging because it can lead rapidly to equipment failure.

Pitting corrosion may occur generaHy over an entire aHoy surface or be localized in a specific area. The latter is the more serious circumstance. Such attack occurs usuaHy at surfaces on which incomplete protective films exist or at external surface contaminants such as dirt. PotentiaHy serious types of corrosion that have clearly defined causes include stress—corrosion cracking, deaHoying, and corrosion fatigue (27—34). 

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. 

The stainless steels contain appreciable amounts of Cr, Ni, or both. The straight chrome steels, types 410, 416, and 430, contain about 12, 13, and 16 wt % Cr respectively. The chrome—nickel steels include type 301 (18 wt % Cr and 9 wt % Ni), type 304 (19 wt % Cr and 10 wt % Ni), and type 316 (19 wt % Cr and 12 wt % Ni). Additionally, type 316 contains 2—3 wt % Mo which gready improves resistance to crevice corrosion in seawater as well as general corrosion resistance. AH of the stainless steels offer exceptional improvement in atmospheric conditions. The corrosion resistance results from the formation of a passive film and, for this reason, these materials are susceptible to pitting corrosion and to crevice corrosion. For example, type 304 stainless has very good resistance to moving seawater but does pit in stagnant seawater. 

The second class of anodic inhibitors contains ions which need oxygen to passivate a metal. Tungstate and molybdate, for example, requke the presence of oxygen to passivate a steel. The concentration of the anodic inhibitor is critical for corrosion protection. Insufficient concentrations can lead to pitting corrosion or an increase in the corrosion rate. The use of anodic inhibitors is more difficult at higher salt concentrations, higher temperatures, lower pH values, and in some cases, at lower oxygen concentrations (37). 

Z. Szklarska-Smialowska, Pitting Corrosion ofMetalSs National Association of Corrosion Engineers, Houston, Tex., 1986. 

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)]. 

Slides Pitting corrosion on a marine turbine blade [4] corroded tie bars, etc., in furnaces, heat exchangers, etc. oxidised cermets. 

Slides Corroded automobiles, fences, roofs stress-corrosion cracks, corrosion-fatigue cracks, pitting corrosion. 

Ancient iron structures sometimes show no sign of corrosion or at most, very little. The clean atmosphere of past centuries may be responsible in that it allowed a very thin adherent layer of oxide to develop on the surface [22], This layer very often protects against even today s increasingly aggressive industrial pollutants Very often the conditions of the initial corrosion are the ones that determine the lifespan of metals [23], A well-known example is the sacred pillar of Kutub in Delhi, which was hand forged from large iron blooms in 410 a.d. In the pure dry air, the pillar remains free of rust traces but shows pitting corrosion of the iron... 

Electrochemical corrosion is understood to include all corrosion processes that can be influenced electrically. This is the case for all the types of corrosion described in this handbook and means that data on corrosion velocities (e.g., removal rate, penetration rate in pitting corrosion, or rate of pit formation, time to failure of stressed specimens in stress corrosion) are dependent on the potential U [5]. Potential can be altered by chemical action (influence of a redox system) or by electrical factors (electric currents), thereby reducing or enhancing the corrosion. Thus exact knowledge of the dependence of corrosion on potential is the basic hypothesis for the concept of electrochemical corrosion protection processes. 

Figure 2-11 shows weight loss rate-potential curves for aluminum in neutral saline solution under cathodic protection [36,39]. Aluminum and its alloys are passive in neutral waters but can suffer pitting corrosion in the presence of chloride ions which can be prevented by cathodic protection [10, 40-42]. In alkaline media which arise by cathodic polarization according to Eq. (2-19), the passivating oxide films are soluble ... 

Generally, pitting corrosion only occurs on passivated metals when the passive film is destroyed locally. In most cases chloride ions cause this local attack at potentials U > U q. Bromide ions also act in the same way [51], The critical potential for pitting corrosion UpQ is called the pitting potential. It has the same significance as in Eqs. (2-39) and (2-48). 

Nitrate ions have a special influence by inhibiting pitting corrosion in neutral and acid waters atU> [Eq. (2-50)] [48,52], corresponds to a second pitting potential and is designated the inhibition potential. The system belongs to group IV, with pitting corrosion at U U... 

Other passivating materials suffer pitting corrosion by chloride ions [62] in a way similar to stainless steels (e.g., Ti [63] and Cu [64]). The pitting potential for aluminum and its alloys lies between = -0.6 and -0.3 V, depending on the material and concentration of chloride ions [10,40-42]. 

AI, A1 alloys Cold water Protection against weight loss corrosion and pitting corrosion [36,39,42]... 

Stainless steels in soil can only be attacked by pitting corrosion if the pitting potential is exceeded (see Fig. 2-16). Contact with nonalloyed steel affords considerable cathodic protection at f/jj < 0.2 V. Copper materials are also very resistant and only suffer corrosion in very acid or polluted soils. Details of the behavior of these materials can be found in Refs. 3 and 14. 

Enamel coatings are used for the internal protection of storage tanks that in most cases have built-in components (e.g., fittings with exits, probes, temperature detectors) that usually exhibit cathodic effectivity. These constitute a considerable danger of pitting corrosion at small pores in the enamel. Corrosion protection is achieved by additional cathodic protection which neutralizes the effectiveness of the cathodic objects. 

In applying electrolytic protection, galvanized tubes can be installed downstream from copper components in water boilers without danger of Cu " -induced pitting corrosion. The protection process extends the application range for galvanized tubes with respect to water parameters, temperature and material quality beyond that in the technical regulations [16, 17]. 

On about 25(X) km of pipeline laid since 1970, overline surveys showed 84 places totalling 5 km in length where the protection criterion had not been reached. In 21 exploratory excavations, 7 cases of pitting corrosion with penetration depths > 1 mm were found. At three places the pipe had to be replaced or repaired with split sleeves. Seven hundred sixty-five places with a total length of 95 km in 25(X) km of pipeline laid between 1928 and 1970 were found to have failed to reach the protection criterion. Thirty-two examples of pitting corrosion with > 1 mm were... 

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