Corrosion pitting

The pitting corrosion of copper has been inhibited by injection of ferrous salt solution. Ferrous ion injection probably results in the formation of lepidrocrocite, FeO.OH, and provides protection. 

Measurement of the average corrosion rate, per square centimeter of the sample, yields only part of the pertinent information, often a small part Consider a piece of metal corroding in a given environment at a rate of 0.1 mmy . This is a rather low rate, which may not worry the designer too much, if corrosion were uniform. Adding 1 mm to the wall thickness of a pipe, for example, would provide an additional service life of 10 years. On the other hand, if corrosion occurs on 1% of the surface, the same average corrosion rate would correspond to a penetration of 10 mm y and one could not very well increase the wall thickness to 100 mm to provide a service life of 10 years. 

Two important forms of localized corrosion are pitting and crevice corrosion. Although the causes of these phenomena may be quite different, the chemistry involved is similar and the following discussion is relevant to both. 

Consider a pit formed in a piece of aluminum that is in contact with seawater. As we shall show, the pH of the solution inside a pit can become quite low, leading to an increased rate of corrosion, which further lowers the pH, and so on. Thus, pitting corrosion can be considered to be an autocatalytic process, with its rate increasing with time. 

The important point to remember in the context of pitting corrosion is that the volume of the solution inside the pit is very small. More accurately stated, the volume 

10) Remember that at the open circuit corrosion potential, the total anodic and cathodic currents must be equal, but the current densities may be quite different. 

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 and transpassive 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]. 

Pitting can also occur under atmospheric conditions. The corrosion can start at the break and continue to undercut the coating, forming a rather heavy tubercle of hard rust or scale with the pit underneath the original metal. The corrosion products help to isolate the aggressive medium in the pit. This type of corrosion is common in marine environments as well as in other industrial environments where strong corrosive conditions exist (15). Also, pits with open mouths (uncovered) exist and are responsible for loss of thickness and can also act as stress raisers. 

The mechanism of pitting is self-initiating and self-propagating. Pit initiation can result from a discontinuity in the film, an impurity, different phase, or a scratch on the surface. The active metal immersed in aerated sodium chloride solution dissolves in the pit, and the oxygen moves toward the pit. The positively charged iron cations attract the chloride anions in the pit. The resulting iron chloride hydrolyses and the sequence of the reactions in the pit are as follows  

There are two distinct processes before the occurrence of stable pit formation (i) pit nucleation and growth of metastable pit and (ii) enabling the pitting potential - metastable pits cannot grow otherwise (18). There are many examples of pitting in practice as follows  

For characterizing the corrosion damage due to pitting one measures the depth distribution of pits. From such data the evolution of maximum pit depth with time or the time to perforation can be estimated using statistical methods [23,24]. Because pitting is a random process the observed pit depth depends on the surface area taken into account. The depth L has been found to vary according to a power law (7.25) or a logarithmic law (7.26) where A is the surface area and K (i = 1,2. .) are constants. 

These relationships can be interpreted in terms of extreme value statistics by assuming different types of distribution laws [23], 

For thermodynamic reasons, the corrosion potential is always lower than the reversible potential of the oxidizing agent (Ecor rev.oxidant)- Dissolved oxygen is the 

There are two ways to study the resistance of a metal to pitting corrosion  

In electrochemical experiments for the determination of the pitting potential one either controls the potential (potentiodynamic method) or, more rarely, the current (galvanostatic method). The composition and temperature of the electrolyte are selected such as to represent the real environment to which the metal will be exposed, but without the oxidant present, whose effect is simulated by the anodic polarization. 

Probably the most common type of localized corrosion is pitting, in which small volumes of metal are removed by corrosion from certain areas on the surface to produce craters or pits that may culminate in complete perforation of a pipe or vessel wall (Fig. 6.8). Pitting corrosion may occur on a metal surface in a stagnant or slow-moving liquid. It may also be the first step in crevice corrosion, poultice corrosion, and many of the corrosion cells described in Chap. 7. 

Pitting is considered to be more dangerous than uniform corrosion damage because it is more difficult to detect, predict, and design against. A small, narrow pit with minimal overall metal loss can lead to the failure of an entire engineering system. Only a small amount of metal is corroded, but perforations can lead to costly repair of expensive equipment. 

One spectacular catastrophe resulting from a single pit has been described in the television series called Seconds from Disaster. The sewer explosion that killed 215 people in Guadalajara, Mexico, in 

Pitting cavities may fill with corrosion products and form caps over the pit cavities sometimes creating nodules or tubercles (Fig. 6.10). While the shapes of pits vary widely (Fig.6.11) they are usually roughly saucer-shaped, conical, or hemispherical for steel and many associated alloys. The following are some factors contributing to initiation and propagation of pitting corrosion  

The complex interactions between these factors may cause major differences on how pitting corrosion will initiate and develop in real situations. Copper, for example, a relatively simple material in terms of its metallurgy, can suffer three well-documented types of pitting corrosion depending on specific conditions in the water it carries  

Different types of corrosion, more or less visible to the naked eye, can occur on aluminium, such as uniform (generalised) corrosion, pitting corrosion, stress corrosion, etc. The predominant type of corrosion wiU depend on a certain number of factors that are intrinsic to the metal, the medium and the conditions of use. 

There is no form of corrosion that is specific to aluminium and its alloys. 

This type of corrosion develops as pits of very small diameter, in the order of a micrometer, and results in a uniform and continuous decrease in thickness over the entire surface area of the metal. 

The dissolution rate can vary from a few micrometers per year up to a few micrometers per hour, depending on the nature of the acid or base (see Chapters E.4 and E.5). Appropriate inhibitors can reduce it. As an example, sodium silicate greatly reduces the dissolution rate of aluminium in alkaline media. 

The rate of uniform corrosion can be easily determined by measuring the mass loss, or the quantity of released hydrogen (see Section B.4.3.2) [2]. This is a useful parameter to evaluate the dissolution rate of aluminium in a pickling bath. 

Pits begin somewhat hemispherical in shape, but the corrosion follows the grain morphology. Therefore pitting in aluminum is not necessarily a smooth attack and pits can be significant stress risers. 

Pits can initiate relatively quickly and grow to a limiting depth, at which mass transport no longer provides sufficient oxygen and the corroding species. At this point, further penetration of that pit is stifled. A few isolated deep pits have a small effect on the reduction in cross-section, so that the initial reduction in strength and load-canying ability is less pronounced than is the depth of penetration. However, new corrosion pits initiate at other sites and corrosion continues, but at a reduced rate. Eventually, a 

Compatibility of Aluminum Alloys with Selected Corrodents 

Fundamentals of Metallic Corrosion Atmospheric and Media Corrosion of Metals 

The chemicals listed are in the pure state or in a saturated solution unless otherwise indicated. Compatibility is shown to the maximum allowable temperature for which data are available. Incompatibility is shown by an X. When compatible, the corrosion rate is 20 mpy. 

Figmre 1.13 a) Localized corrosion on 2195 Al-Li allc after a potentiodynamic polarization run in 3.5% NaCl. Th ealloy was aged at 190C for 0.5h. SEM photomicrograph talmn at a magnification of 2,00Qx. b) Pitting mechanism. 

In a water-base electrolyte containing chlorine Cl ions and orq gen molecules (02)1 the Cl ions migrate towards the bottom of the pits and O2 molecules react with water molecules on the metal surface [5]. Therefore, metal 

The metal hydroxide 2M OH)2 compound is unstable and therefore, it reacts with oxygen and water to form the final corrosion product Hence, 

Typical examples of specific formation of M OH)3 type corrosion product are given below 

AI 0H)3 for aluminum hydroxide pits Cr(OH)3 far chromium hydroxide pits 

Pits occur as small areas of localized corrosion and vary in size, frequency of occurrence, and depth. Rapid penetration of the metal may occur, leading to metal perforation. Pits are often initiated because of inhomogeneity of the metal surface, deposits on the surface, or breaks in a passive film. The intensity of attack is related to the ratio of cathode area to anode area (pit site), as well as the effect of the environment. Halide ions such as chlorides often stimulate pitting corrosion. Once a pit starts, a concentration-cell is developed since the base of the pit is less accessible to oxygen. 

Pits often occur beneath adhering substances where the oxidizing capacity is not replenished sufficiently within the pores or cavities to maintain passivity there. Once the pit is activated, the surface surrounding the point becomes cathodic and penetration within the pore is rapid. 

Pitting Environments. In ordinary sea water the dissolved oxygen in the water is sufficient to maintain passivity, whereas beneath a barnacle or other adhering substance, metal becomes active since the rate of oxygen replenishment is too slow to maintain passivity, activation and pitting result. 

Exposure in a solution that is passivating and yet not far removed from the passive-activity boundary may lead to corrosion if the exposure involves a strong abrasive condition as well. Pitting of pump shafts under packing handling sea water is an example of this abrasive effect. 

The 18-8 stainless steels pit severely in fatty acids, salt brines, and salt solutions. Often the solution for such chronic behavior is to switch to plastics or glass fibers that do not pit because they are made of more inert material. 

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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 ... 

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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