Techniques for Evaluating Localized Corrosion

As described in Chapter 4.2 on localized corrosion, alloys that are protected by a thin passive film tend to be susceptible to locaKzed corrosion resulting from a breakdown of that film and aggressive dissolution at the breakdown sites. The tendency for breakdown increases as the potential increases. The electrochemical techniques described in Sect. 7.3 can be used to address localized corrosion. Other techniques specific to localized corrosion 

If the potential scan direction is reversed after some period of localized corrosion growth, a hysteresis is observed. Once 

A correlation has been between pitting potential and pitting in the field the tendency to form pits naturally at open circuit increases as the experimentally determined pitting potential decreases [51]. The difference between E-p and E, which is related to the extent of hysteresis in a cyclic potentiodynamic polarization curve, has also been considered to be a measure of the susceptibility to localized corrosion [3,52]. Issues associated with this interpretation have been reviewed [53]. 

Cyclic polarization measurements to determine Ep and E are simple to perform. However, care must be taken to avoid artifacts or misinterpretation. The biggest problem is the possibility for crevice corrosion, which is large for 

Temperature can also be used as an acceleration factor in a fashion similar to potential. Many materials wiU not pit at a temperature below a critical value that is often extremely sharp and reproducible [56-62]. At low temperatures, extremely high breakdown potentials are observed, corresponding to transpassive dissolution, not localized corrosion. Just above the critical pitting temperature (CPT), pitting corrosion occurs at a potential that is far below the transpassive breakdown potential. This value of CPT is independent of environmental parameters and applied potential over a wide range, and is a measure of the resistance 

---

The most commonly-used steady state techniques are potentiodynamic tests to determine the corrosion rate in systems that experience a uniform corrosion process. This type of attack can also be studied by measuring resistance to polarization. Cyclical polarization ciurves are also used to study localized corrosion and potentiokinetic reactivation is the most suitable study technique for evaluating intergranular corrosion produced by a sensitization phenomenon following ASTM G108 standard test. 

For detection of more localized corrosion, such as crevice corrosion or SCC, other ultrasonic inspection techniques may be useful. Baseline data generated at the time of installation will also be helpful in evaluating results. One benefit derived from this type of inspection technique is that it can often be conducted with little or no interference with production. Periodic planned visual inspection of equipment utilized under conditions likely to cause stress cracking is also an effective technique, especially when combined with non-destructive inspection techniques such as dye penetrant inspection. It may be necessary to remove coatings or insulation from the equipment surface to facilitate inspection. 

Again, EN is a veiy useful technique for the study of inhibition of localized corrosion, and tremendous progress has been made in the evaluation of the data using mosdy proprietary computer algorithms [60]. It is a methodology that certainly lends itself for monitoring purposes. However, the results are strictly qualitative. 

Electrochemical tests are rapid techniques to determine mechanisms, determine the effect of various parameters on corrosion rate, and screen out a large number of materials [43]. They usually involve measurement of corrosion potentials, corrosion currents, polarization curves, and electrochemical impedance. They are used to evaluate metals and alloys and the behavior of metallic, inorganic, and oiganic coatings. The simplest test involves the measurement of the corrosion potential and its use in conjunction with other measurements. A zero resistance ammeter (ZRA) is commonly used to measure corrosion currents between dissimilar metals and alloys. Controlled potentitd tests and anodic and cathodic polarization curves using potentiostats are the most commonly used electrochemical tests. These are powerful tools for investigating the effect of various parameters on corrosion behavior. These incorporate the use of cycUc polarization and polarization resistance for localized corrosion and corrosion rate measurements. Table 4 lists electrochemical tests that can be used for corrosion tests in the automobile industry. 

Electrochemical tests provide a means to understand the corrosion process, simulate service conditions, or accelerate evaluation of a material [27]. ASTM G 3, Practice for Conventions Applicable to Electrochemical Measurements in Corrosion Testing ASTM G 5, Standard Reference Test Method for Making Potentiostatic and Potentiodynamic Polarization Measurements and ASTM G 61, Standard Test Method for Conducting Cyclic Potentiodynamic Polarization Measurements for Localized Corrosion Susceptibility of Iron-, Nickel-, or Cobalt-Based Alloys provide background in some of these techniques. 

Second, we will focus briefly on electrochemical techniques in general. This will be used as a starting point for a brief description of the more recently developed local techniques. In the part also the information on corrosion mechanisms as presented earlier in this chapter will be used to evaluate the different methods. 

---