Rate of Pitting Corrosion

Experience has shown that in most cases, the rate of deepening of pits formed in natural environments such as freshwater, seawater, and rain water decreases with time. This explains the very long lifetime (several decades) of aluminium used in construction (roof sheet), in naval construction, etc. 

Studies performed at the beginning of the 1950s in 25 different waters in Canada [7] showed that the deepening rate of pits on 1100 follows the equation 

This equation can be explained by the fact that for an ideal, hemispherical pit with a radius r, the quantity of dissolved metal during a period of time t is constant and equals 

This equation was checked by measuring the depth of pits at regular intervals over 13 years in water conveyance installations comprising more than 100 km of tubes (each with a unit 

Unlike uniform corrosion, the intensity and rate of pitting corrosion can be assessed neither by determining the mass loss, nor by measuring released hydrogen. In fact, these measurements do not make sense because a very deep and isolated pit results only in a small mass loss, whereas a very large number of superficial pits can lead to a larger mass loss. 

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Chloride Chloride ions are strongly adsorbed by steel, making it difficult to passivate. High chlorides in steam-water circuits increases the risk of stress corrosion cracking of austenitic steels (type 300 stainless steels) and increase the rate of pitting corrosion under sludges and deposits. 

Because of the high penetration rate of pitting corrosion and the uncertainty due to structural consequences of locaHzed attack, the methods aimed at repassivation of steel should be preferred for chloride-contaminated structures. Only if the chloride content in concrete is low and the penetration of chloride is Hmited in extent, can other repair techniques be taken into consideration. 

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)]. 

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. 

Pitting is a form of localized corrosion in which part of a metal surface (perhaps 1 per cent of the exposed area) is attacked. Rates of pitting penetration can be very high type 316 stainless steel in warm seawater can suffer pit penetration rates of 10 mm per year. This is a natural... 

Local corrosion or pitting is more important for practical purposes than the rate of general corrosion, and may proceed 10 times or so more rapidly than this. Inasmuch as certain types of cast iron are liable to suffer graphitic corrosion, whereas steel does not, steel might theoretically be expected to show to some advantage when used for buried pipelines. In practice, however, a cast-iron pipe has to be of stouter wall than a steel pipe for equal strength, and it is doubtful whether any distinction between the rust resistance of the two materials in the soil is justified. 

Copper reduces the corrosion resistance of aluminum more than any other alloying element. It leads to a higher rate of general corrosion, a greater incidence of pitting, and, when added in small amounts (for example, 0.15% ), a lower rate of pitting penetration. 

The presence of high concentrations of salines such as the sodium salts of chlorides and sulfates may increase the rates of localized corrosion and produce deep pits (especially in the presence of boiler sludge). 

Nitrite-based programs require a relatively high application rate to ensure that all anodic areas within the system are fully protected from the risk of pitting corrosion. Undertreatment exposes anodic areas, which are subject to localized pitting as a result of the concentrating power from surrounding cathodic sites. 

NOTE Where MU rates are high, a supplementary oxygen scavenger may also be required to avoid the problem of pitting corrosion in the boiler and associated pipe work. 

That is, to determine the correct corrosion rates in pitting corrosion, as shown in Fig. 37, it is necessary to know the local corrosion currents on the electrode surface. The corrosion current observed is, however, obtained as the total current, which is collected by the lead wire of the electrode. From the usual electrochemical measurement, we can thus determine only an average corrosion current (i.e., the corrosion rate). Hence if we can find some way to relate such an average rate to each local corrosion rate, the local corrosion state can be determined even with the usual electrochemical method. 

Under these circumstances, appropriate water management objectives should probably include a requirement to take all measures to prevent or minimize localized corrosion processes occurring that could cause pitting, crevice attack, tuberculation, etc. This will undoubtedly require close attention to maintaining clean waterside metal surfaces, but may also require more tolerance of an acceptable rate of general corrosion, to, say, 4 mpy, or a little higher. 

As we saw in the foregoing section, pitting corrosion of passive metals occurs beyond the critical pitting potential, plt. In order to protect passive metals from pitting corrosion, therefore, it is advisable to hold the corrosion potential as far less positive from plt as possible in the passive potential range. The presence of p-type oxides, however, makes Econ more positive and hence enhances the breakout of pitting corrosion. In the same way, metals corroding in the active state will accelerate their corrosion rates when their electrode potential is made more positive (more anodic) by the presence of p-type oxides. 

The onset of pitting corrosion occurs suddenly If one performs electrochemical experiments with stainless steel, e. g. by applying a constant electrical potential to a sample immersed in dilute NaCl solution, the electrical current - which is an indicator for chemical activity (corrosion) on the metal surface - is low over a wide parameter range. But if critical parameters like temperature, potential, or electrolyte concentration exceed a certain critical value, the current rises abruptly and the metal surface is severely affected by pitting corrosion. The transition to high corrosion rates is preceded by the appearance of metastable corrosion pits. 

Initiation of pitting corrosion takes place when the chloride content at the surface of the reinforcement reaches a threshold value (or critical chloride content). A certain time is required from the breakdown of the passive film and the formation of the first pit, according to the mechanism of corrosion described above. From a practical point of view, the initiation time can be considered as the time when the reinforcement, in concrete that contains substantial moisture and oxygen, is characterized by an averaged sustained corrosion rate higher than 2 mA/m [8], The chloride threshold of a specific structure can be defined as the chloride content required to reach this condition of corrosion. 

Whether the total corrosion process is determined by the kinetics of anodic metal dissolution or the cathodic process depends on the size of the cathode and the kinetics of the partial electrode processes. The slowest reaction is the rate-determining step, as is usual in kinetics. In the case of a well-passivated valve metal, this is most probably the cathodic reaction, whereas for metals with semiconducting oxides, the rate-determining step win he anodic metal dissolution. In order to study the partial reactions of pitting corrosion separately, potentiostatic experiments are preferred. The cathodic process is replaced hy the electronic circuit of the potentiostat to investigate the anodic metal... 

EIS is a method that is not very well suited for the study of pitting corrosion because, as described earlier, it should be applied to electrodes at steady state, and pitting is a non-steady state condition. Nonetheless, EIS can be used to assess the low-frequency impedance at open circuit, which provides a measure of corrosion rate without the need to polarize away from open circuit. It is therefore possible to use EIS to determine if pitting is occurring at open circuit. 

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