Pitting corrosion parameters

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]. 

With small modifications, ASTM standard G48 can be used to determine a CPT. The test is used as a ranking parameter for the resistance to pitting of high-alloyed austenitic stainless steels. In this method, material coupons are typically exposed for 24 or 72 h to a 6% FeCl3 (=1.11 mole/liter) solution at fixed temperatures (typically with 2.5°C intervals). The CPT is defined as the lowest temperature at which the specimen is attacked by pitting corrosion. 

These tests focused on the determination of a materials resistance to localized (pitting) corrosion. To accomplish this goal, three types of electrochemical experiments were conducted (cyclic polarization, electrochemical scratch, and potenti-ostatic holds) to measure several key parameters associated with pitting corrosion. These parameters were the breakdown potential, EM, the repassivation potential, Etp, and the passive current density, tpass. 

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. 

E7.8. Electrokinetic parameters of an active-passive alloy are given in Table E7.5 and the anodic polarization curve of the active-passive alloy in the presence of chloride ions is shown in Fig. 7.23. Pitting corrosion initiation occurs at 0.045 V vs. SHE. 

Parameters for Pitting Corrosion of Austenitic Chromium-Nickel Steels... 

The model can be run using a range of the rates. An example might be to vary one or more of the parameters, e.g., using a maximum SS pitting corrosion value, and a SOO year Furfurol(F) lifetime, as opposed to the lASAP recommended lifetime of 100 years. To illustrate this, the model was run using three sets of corrosion rates and barrier lifetimes. The rates or lifetimes were chosen at the upper and lower ends of the published or advised values (see Table XVII), and compared to the lASAP recommended values. 

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... 

Selection of environmental conditions the electrol)fte should be selected in view of its known oxidative or reducing power. The test temperature can be ambient temperature or any temperature relevant for the field ap>plication it should reproduce. A decisive parameter in the selection of the electrolyte is its pH. The selection of the pH can be based on pH-potential diagrams (Pourbaix, 1974). In the case of a metallic alloy, a pH range should be selected by paeference where at least one of the constituents passivates. An electrolyte that may cause localized corrosion, like pitting corrosion in particular, should be avoided. 

Figure 7 Diagrammatic representation of the formation of microbial consortia and their influence on the corrosion processes, (a) When the film becomes sufficiently thick its inner part will be anaerobic with the possible development of SRB microcolonies (black cells), (b) The SRB attracts secondary colonizers by its metaboUc products and forms a consortium with thenr (c) The development of local areas with varying physicohemical parameters leads to pitting corrosion. (From Ref. 21.)...

Table 4.3 Weibull distribution parameters for the pit corrosion of Mg-Gd-Y alloy under various thin layer thicknesses...

Potentiodynamic polarization (intrusive). This method is best known for its fundamental role in electrochemistry in the measurement of Evans diagrams. A three-electrode corrosion probe is used to polarize the electrode of interest. The current response is measured as the potential is shifted away from the free corrosion potential. The basic difference from the LPR technique is that the apphed potentials for polarization are normally stepped up to levels of several hundred millivolts. These polarization levels facihtate the determination of kinetic parameters, such as the general corrosion rate and the Tafel constants. The formation of passive films and the onset of pitting corrosion can also be identified at characteristic potentials, which can assist in assessing the overall corrosion risk. 

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