Pitting potential critical temperatures

It is apparent that the critical pitting potential for a given alloy depends on the concentration of chloride ions, on the concentration of inhibiting anions in the solution and on the temperature of the solution. Unfortunately, the situation is complicated further by the fact that there is an induction period for the onset of pitting, which means that the pitting propensity... 

Critical Pitting Potential the most negative potential required to initiate pits in the surface of a metal held within the passive region of potentials (it varies with the nature of solution, temperature, time, etc.). 

In research laboratories, potential is often used as the critical value for comparison purposes. A pitting potential is determined at a given suitable constant temperature, recording the anodic polarization curve while keeping other factors constant (Fig. 11). 

Figure II. Schematic anodic polarization curves at a fixed temperature. Determination of either transpassive potential ( ,) or pitting potential ( p and repassivation potential ( ,) at the critical current density (/ ). t rcvis the current density at which the scan is reversed. ...

Figure 12. The limits of pitting as a function of potential and temperature according to the experimental results of Brigham and Tozer. CPT, critical pitting temperature.

Fig. 7.22 Correlation between the critical pitting temperature and critical pitting potential of 1 7 high-performance alloys. The alloys are ...

N.J. Laycock, R.C. Newman, Temperature dependence of pitting potentials for austenitic stainless steels above their critical pitting temperature, Corros. Sci. 40 (1998) 887—902. 

Pitting tendency increases with increasing temperature, with the result that the critical pitting potential decreases as temperature increases. The critical pitting temperature (CPT) is defined as the temperature below which an alloy does not... 

In addition to the critical pitting potential (CPP) (noble to which pits initiate), critical pitting temperatures (CPT) have been measured. Stable pitting occurs at temperatures above the CPT [42]. Below this temperature, stable pitting does not take place at any potential. The CPT has been explained as the... 

The alloys resist pitting in FeCls at room temperature, except Alloy 6B because of its high carbon content of 1.2%. At a lower carbon content—for example, 0.4%—the alloy is resistant. Vitallium, having a very noble critical pitting potential is also resistant to pitting in dilute NaCl solutions, a property that extends to Alloy 25 and MP35N as well. 

Initiation of pitting in titanium is more pronounced in Br and T than in CT solutions. The critical pitting potential in 1M Br and 1M T solutions at room temperature are 0.9 and 1.8 V, respectively [23]. Mansfeld [24], following earlier Russian investigations, showed that titanium loses passivity in anhydrous 1N HCl in CH3OH, but that passive behavior as indicated by polarization curves is restored when >0.6% H2O is present, and that the pitting potential becomes increasingly noble as more H2O is added. 

Figure 25.3. Critical pitting potentials of commercial titanium and 1% Mo-Ti alloy in 1 M NaCI as a function of temperature [22].

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... 

Critical pitting temperature can be rapidly obtained using electrochemical equipment to maintain a preset potential, increase temperature and detect the onset of corrosion by monitoring corrosion current [24], In ASTM G 150, Test Method for Electrochemical Critical Pitting Temperature Testing of Stainless Steels, a pre-set potential provides consistent conditions and a potential-independent critical pitting temperature. Sensitive detection of attack using the current allows an accelerated evaluation. 

The materials Monit and Sea-Cure are characterised by good resistance to pitting, crevice and stress corrosion cracking in seawater. The critical pitting corrosion temperature in the FeCls test is 328 K (55 °C) and the critical crevice corrosion temperature is 318 K (45 °C). In Table 36, the pitting potentials of the two superferrites and the austenitic steels 1.4539 (SAE 904 L, XlNiCrMoCu25-20-5) and X3CrNiMol7-13-3 (SAE 316,1.4436) measured in 5% NaCl solution are presented. 

In a superferritic steel (ELI steel) with the composition 0.022% C, 24.9% Cr, 3.63% Ni, 3.52% Mo and 0.50% Ti a critical pitting corrosion temperature of over 333 K (60 °C) was determined in a seawater-like chloride sulphate solution. A chlorine addition to the solution shifts the corrosion potential in the positive direction, reducing the pitting resistance equivalent in the critical temperature range [116]. 

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