Pitting corrosion described

Corrosion can be suppressed when there is total water saturation due to oxygen starvation, but if some oxygen gets in, then the pitting corrosion described in Chapter 2 can occur. 

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. 

More often the passive layer is broken down locally and then the steel is said to be attacked by localized corrosion, the most important forms being pitting, crevice corrosion, and corrosion cracking. Most often the localized corrosion is caused by halogen ions such as chloride, bromide, and iodide. Pitting or pitting corrosion is seen as small pinholes on the surface of the steel. This section describes electrochemical instrumental methods to investigate and measure this form of corrosion attack. 

Corrosion described as pitting is usually localized and can be in the form of small, deep pits as well as large, shallow pits. It can occur in areas of stagnation where an anode site can develop. 

The term uniform corrosion describes the more or less uniform wastage of material by corrosion, with no pitting or other forms of local attack. If the corrosion of a material can be considered to be uniform, the life of the material in service can be predicted from experimentally determined corrosion rates. 

In this sub-section, we present the available theoretical descriptions for the effect of convection on pitting corrosion and metal salt deposition. Three models described in the literature are relevant these are discussed in the next three sections. 

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. 

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. 

E7.9. Pitting inhibitor effectiveness depends strongly on type and anion concentration. The thermodynamic (critical) activity, ai, required to inhibit pitting corrosion for a given activity of the aggressive anion is described by [13] ... 

Figure 3. Mixed potential diagram illustrating controls on the kinetics of corrosion at a pitted, oxide-covered metal. The potential range is from -700 to +300 mV/NHE. Arrows (B) corrosion current at the bottom of the pit, controlled by Fe Fe + (acid) and 2H - H2 (M) corrosion current at the mouth of the pit, controlled by the partial currents for Fe -> Fe2+ (passivated) and RX RH (Pit) corrosion current for the short-circuited pit, controlled by Fe Fe + (acid) and RX - RH. The three solid curves are generated using the Tafel equation and exchange current densities and Tafel slopes from reference (9). The dashed curve was measured at 5 mV s in pH 8.4 borate buffer, using methods described in reference (9).

Below the CPP, CT cannot displace adsorbed oxygen so long as the passive film remains intact hence, pitting is predicted not to occur. Should passivity break down because of factors other than those described [e.g., reduced oxygen or depolarizer concentration at a crevice (crevice corrosion), or cathodic polarization of local shielded areas], pitting could then initiate independent of whether the overall prevailing potential is above or below the critical value. But under conditions of uniform passivity for the entire metal surface, application of cathodic protection to avoid pitting corrosion need only shift the potential of the metal below the critical value. This is in contrast to the usual procedure of cathodic protection, which requires polarization of a metal to the much more active open-circuit anode potential. 

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