Corrosion prevention pitting potential

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... 

If the passive film cannot be reestablished and active corrosion occurs, a potential drop is established in the occluded region equal to IR where R is the electrical resistance of the electrolyte and any salt film in the restricted region. The IR drop lowers the electrochemical potential at the metal interface in the pit relative to that of the passivated surface. Fluctuations in corrosion current and corrosion potential (electrochemical noise) prior to stable pit initiation indicates that critical local conditions determine whether a flaw in the film will propagate as a pit or repassivate. For stable pit propagation, conditions must be established at the local environment/metal interface that prevents passive film formation. That is, the potential at the metal interface must be forced lower than the passivating potential for the metal in the environment within the pit. Mechanisms of pit initiation and propagation based on these concepts are developed in more detail in the following section. 

The electrochemical behaviour of aluminium is strongly influenced by the permanent presence of a natural oxide film on its surface. Therefore, a mixed potential corresponding to the pitting potential is measured on aluminium (see Section B.1.7) this potential represents a threshold below which pitting corrosion can be prevented. 

A comparable effect to the sacrificial electrodes is provided by the direct supply of a cathodic current to the dissolving metal. A proper cathodic current applied to the metal structure sets the potential to a value where corrosion is prevented. A disadvantage is the requirement of a permanent cormection to a current supply (or even a potentiostat). However, with this approach, the metal construction may be tuned in for complicated situations. Metallic structures often consist of several metals with different corrosion properties. The environmental conditions may be very difficult. For example, passivation should be maintained but the potential should not become more positive than the critical pitting potential. In these complicated cases, a potentiostat with a CE and a RE is useful. The WE of this circuit is the metal construction. Protection by a current source is applicable to chemical reaction vessels or constructions with permanent location but not always to mobile devices like cars, ships, etc. Therefore both methods are useful, and the choice depends on the specific requirements for the construction in service conditions. In well-conducting electrolytes, one has to take care of the equilibrium potentials of the involved electrodes (metal/metal ion and redox electrode). If the environment has a low conductivity (wet soil), ohmic drops have to be taken into account in order to establish an appropriate protecting... 

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 ... 

A somewhat alternative analysis of pitting attributes pit initiation to the activation of defects in the passive film, defects such as those induced during film growth or those induced mechanically due to scratching or stress. The pit behavior is analyzed in terms of the product, xi, a parameter in which x is the pit or crevice depth (cm), and i is the corrosion current density (A/cm2) at the bottom of the pit (Ref 21). Experimental measurements confirm that, for many metal/environment systems, the active corrosion current density in a pit is of the order of 1 A/cm2. Therefore, numerical values for xi may be visualized as a pit depth in centimeters. A defect becomes a pit if the pH in the pit becomes sufficiently low to prevent maintaining the protective oxide film. Establishing the critical pH, for a specific oxide, will depend on the depth (metal ions trapped by diffiisional constraints), the current density (rate of generation of metal ions) and the external pH. In turn, the current density will be determined by the local electrochemical potential established by corrosion currents to the passive external cathodic surface or by a potentiostat. Once the critical condition for dissolution of the oxide has been reached, the pit becomes deeper and develops a still lower pH by further hydrolysis. 

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