Stainless steels critical potential

The ease with which stainless steels can passivate then increases with the level of chromium within the alloy and so materials with higher chromium content are more passive (i.e. conduct a lower passive current density) and passivate more readily (i.e. the critical current density is lower and the active/passive transition is lower in potential). They are also passive in more aggressive solutions the pitting potential is higher. 

In the case of the stainless steels, or other readily passivated metals, the rapid reduction of dissolved oxygen on the freely exposed surface will be sufficient to exceed the critical current density so that the metal will become passive with a potential greater than whereas the metal within the crevice will be active with a potential less than. The passivation of the freely exposed surface will be facilitated by the rise in pH resulting from oxygen reduction, whilst passivation within the crevice will be impeded by the high concentration of Cl ions (which increases the critical current density for passivation) and by the H ions (which increases the passivation potential E, see Section 1.4). 

Table 1.21 Effect of molybdenum additions to Fe-15Cr-13Ni stainless steel on the critical pitting potential J S.H.E.)...

Suzuki, T. and Kitamura, Y., Critical Potential for Growth of Localised Corrosion of Stainless Steel in Chloride Media , Corrosion, 28, 1 (1972)... 

Potentiostatic tests " have been used and Wilde and Williams in potentiokinetic studies of the critical breakdown potential of stainless steels (Types 430 and 304) in 1 -0 mol dm" NaCl, showed that the nature of the gas used to purge the solution has a pronounced effect on the value of... 

More details of other factors that affect the critical pitting potential have been discussed by Uhlig and his co-workers" . They indicated that for stainless steel the critical pitting potential decreased with increasing concentration of chloride ion. At a fixed chloride level, passivating ions in solution, such as sulphate and nitrate, etc., cause the pitting potential to become more positive at a sufficient concentration these ions totally inhibited pitting, as shown in Fig. 19.40 for SO and CIO . 

Lizlovs and Bond reported a molar ratio of 5 1 (SO CI ) for inhibiting pitting in ferritic stainless steels. A plot of critical potential vs. 

The fact that scanning speed can affect polarisation behaviour has already been mentioned. In the case of stainless steel a plot of critical potential E, vs. rate shows how becomes more positive with potential change rate (Fig. 19.43) . When a specimen was held at a fixed passive potential while aggressive ions (Cl ) were added to determine the concentration required... 

Figure 20. Pit-dissolution current density pit radius and ion concentration buildup AC in the pit electrolyte corresponding to the critical condition for growing pits on 18Cr-8Ni stainless steel to passivate at different repassivation potentials, EK, in 0.5 kmol m 3 H2S04 + 0.5 kmol m-3 NaCl during cathodic potential sweep at different sweep rates.7 (From N. Sato, J. Electrochem. Soc. 129,261,1982, Fig. 1. Reproduced by permission of The Electrochemical Society, Inc.)...

The critical part of the valve consists of a synthetic sapphire ball resting on a seat. The seat may be of stainless steel, PTFE or, more usually, also of sapphire. When the flow is directed against the ball the ball moves forward allowing the liquid to flow past it. When the direction of pressure changes resulting in potential flow-back through the valve, the ball falls back on its seat and arrests the flow. 

The corrosion of stainless steel in 0.1 mol-1 NaCl solutions at open circuit potential was studied in detail by Bruesch et al. [106] using XPS in combination with a controlled sample transfer system [38]. It was verified by XPS analysis that the passivating film contains chromium oxide. The position and the height of the Cr concentration maximum depends critically on the bulk chromium content of the steel. Significant variations in the electrode passivation properties were observed at a Cr concentration of 12%, while the film behaviour was found to be rather independent of the other components like Mo, Ni, Cu. From the fact that the film structures and... 

The critical pitting temperature (CPT) is widely used as a measure of the resistance of stainless steel against pitting attack. Various methods for determination of the CPT are described here, special attention being given to the choice of test potential for the control of stainless steel quality. 

I0.6.8.I Cladding failure in oxide fuel pins of nuclear reactors. The long-term operational performance of nuclear fuel pins is critically governed by the reactions that occur in the gap between the fuel and its cladding. Ball et al. (1989) examined this for the cases of (1) Zircaloy-clad pellets of U02+, in a pressurised water reactor (PWR) and (2) stainless-steel-clad pellets of (U, P)02+, in a liquid-metal-cooled fast-breeder reactor (LMFBR). In particular they were interested in the influence of O potential on Cs, I, Te and Mo and the effects of irradiation on the gaseous species within the fuel-clad gaps. 

Regions close to critical potential for localized corrosion. This is observed for austenitic stainless steel in near-neutral solutions.95 145... 

The processes that we invoked in the past [65] to explain passivity breakdown that is induced by an aggressive anion (e.g., Cl ) are shown in Fig. 33. However, passivity breakdown also occurs in environments that are free of aggressive anions, with notable examples being passivity breakdown on steels and stainless steels in high-temperature pure water. Experimentally, it is found that breakdown occurs only when the corrosion potential is more positive than some critical value. Accordingly, it is reasonable to invoke a charge-transfer reaction as the process that generates cation... 

Corrosion of Iron Alloys. Other recent work Q) presents experimentally measured values of the open circuit potentials of iron alloys in water at ambient and supercritical conditions. For iron and its alloys (1080 carbon steel, 304 stainless steel and 316 stainless steel), the open circuit potentials varied from -0.112 to +0.055 volt in water at its critical point, and varied from -0.138 to -0.060 volt in water at ambient conditions. These values were... 

Electrochemical potentlostat measurements have been performed for the corrosion of iron, carbon steel, and stainless steel alloys in supercritical water. The open circuit potential, the exchange or corrosion current density, and the transfer coefficients were determined for pressures and temperatures from ambient to supercritical water conditions. Corrosion current densities increased exponentially with temperature up to the critical point and then decreased with temperature above the critical point. A semi-empirical model is proposed for describing this phenomenon. Although the current density of iron exceeded that of 304 stainless steel by a factor of three at ambient conditions, the two were comparable at supercritical water conditions. The transfer coefficients did not vary with temperature and pressure while the open circuit potential relative to a silver-silver chloride electrode exhibited complicated behavior. 

The most promising is the application of the Taylor-vortex column with Taylor-Couette flow. Sorption of hydrophobic organic compounds can lead to under estimation of actual removal efficiency of wastewater treatment, as weU as cross-contamination of the batches of the effluent due to release ofthe sorbed solute molecules. This is supported by the results ofthe laboratory studies, along with the ease ofthe potential scale-up. The critical part in the context ofthe removal of organic compounds from wastewater(s) is the material of the inner cylinder. The surface of the inner cylinder provides a potential sorption surface for hydrophobic organic molecules. Since the PTFE price could be prohibitive for the scale-up, stainless steel, or other materials should be explored as replacements for potential industrial applications. 

It is also an accepted fact that the crevice corrosion ceases to grow at potentials less positive than a certain critical potential resulting in crevice protection as shown for austenitic stainless steel in Figure 22.30 [59,61]. The critical potential, Ecrev, is called crevice protection potential or the critical crevice corrosion potential. It was found for a cylindrical crevice in austenitic stainless steel that the crevice protection potential shifts in the less positive direction as a logarithmic function of solution chloride concentration [61] ... 

FIGURE 22.30 Schematic polarization curves of a cyhndrical crevice in an anode of stainless steel in neutral solutions of three different chloride concentrations [59] h = crevice depth, Icrev — anodic crevice dissolution current, ccr = chloride concentration in the solution bulk, crcv = crevice protection potential, and /Crev— minimum crevice dissolution current at the critical crevice (protection) potential crcv. 

FIGURE 22.31 Schematic potential-dimension diagrams for localized corrosion of stainless steel in aqueous solution [63] Epit — pitting potential, ER = pit repassivation potential, Ep = passivation potential in the critical pit solution, Emv — crevice protection potential, rj]13 = critical pit radius for pit repassivation, a = pit repassivation, and b = transition from the polishing mode to the active mode of localized corrosion. 

M.J. Johnson, Relative Critical Potentials for Pitting Corrosion of Some Stainless Steels, Localized Corrosion, STP 516, ASTM, 1972, p 262-272... 

Interface Potential and Pit Initiation. It is generally accepted that pit initiation occurs when the corrosion potential or potentiostatically imposed potential is above a critical value that depends on the alloy and environment. However, there is incomplete understanding as to how these factors (potential, material, and environment) relate to a mechanism, or more probably, several mechanisms, of pit initiation and, in particular, how preexisting flaws of the type previously described in the passive film on aluminum may become activated and/or when potential-driven transport processes may bring aggressive species in the environment to the flaw where they initiate local penetration. In the former case, the time for pit initiation tends to be very short compared with the initiation time on alloys such as stainless steels. Pit initiation is immediately associated with a localized anodic current passing from the metal to the environment driven by a potential difference between the metal/pit environment interface and sites supporting cathodic reactions. The latter may be either the external passive surface if it is a reasonable electron conductor or cathodic sites within the pit. 

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