Anodic polarisation

Fig. 1.39 Schematic anodic polarisation curve for a metal. Region AB describes active dissolution of the metal. BC is the active/passive transition, with passivation commencing at B. Passivation is complete only at potentials higher than C. The metal is passive over the range CD...

Fig. 1.40 Schematic anodic polarisation curve for a passivatable metal (solid line), shown together with three alternative cathodic reactions (broken line). Open-circuit corrosion potentials are determined by the intersection between the anodic and cathodic reaction rates. Cathode a intersects the anodic curve in the active region and the metal corrodes. Cathode b intersects at three possible points for which the metal may actively corrode or passivate, but passivity could be unstable. Only cathode c provides stable passivity. The lines a, b and c respectively could represent different cathodic reactions of increasing oxidizing power, or they could represent the same oxidizing agent at increasing concentration.

Fig. 1.41 Schematic anodic polarisation curves for a passivatable metal showing the effect of a passivating agent that has no specific cathodic action, but forms a sparingly soluble salt with the metal cation, a without the passivating agent, b with the passivating agent. The passive current density, the active/passive transition and the critical current density are all lowered in b. The effect of the cathodic reaction c, is to render the metal active in case a, and passive...

Suzuki, Yamake and Kitamura determined the pHs, chloride ion concentrations, metal ion concentrations and the potentials of artificial pits in Fe, Cr, Ni and Mo, and in three austenitic stainless steels during anodic polarisation in 0-5 N NaCl at 70°C. In the case of the pure metals the pH values were found to be lower than those calculated from the metal ion concentrations (Table 1.17), and the experimentally determined pHs were as follows ... 

Figure 1.53 shows diagrammatically various types of pits that can range from hemispherical with a polished surface, in which crystallographic etching has been completely suppressed, to crystallographic pits whose sides are composed of the crystal planes that corrode at the slowest rate. Pits formed on Ni during anodic polarisation in an acetic acid-acetate buffer of pH 4-6 are shown in Fig. 1.54. 

Fig. I.S4 Pits formed during anodic polarisation in an acetic acid-acetate buffer of pH 4-6 containing thiourra or NaCI. (a) General attack and formation of crystallographic pits on nickel in the buffer -t- 10 m thiourea (x 200), (ft) crystallograhic pits formed in the buffer -H0 M...

Fig. 1.55 Breakdown potentials of Fe-18Cr-8Ni stainless steel in 0-1 M NaCl plus various concentrations of Soj ions during potentiostatic anodic polarisation (after Leckie and...

Increase in velocity may increase the rate by bringing the cathode reactant more rapidly to the surface of the metal thus decreasing cathodic polarisation, and by removing metal ions thus decreasing anodic polarisation. 

Mieluch, J. and Smialowski, M., The Behaviour of Grain Boundaries in Iron During Anodic Polarisation in Ammonium Nitrate Solution , Corros. Sci., 4, 237 (1964)... 

Piggott, A. R., Leckie, H. and Shreir, L. L., Anodic Polarisation of Titanium in HCOOH— 1 Anodic Behaviour of Titanium in Relation to Anodising Conditions , Corros. Sci., S, 165... 

Bond, A. P. and Lizlovs, E. A., Anodic Polarisation of Some Ferritic Stainless Steels in Chloride Media , J. Electrochem. Soc., 114, 199c (1%7)... 

Figure 1.62b shows the result of raising the potential of a corroding metal. As the potential is raised above B, the current/potential relationship is defined by the line BD, the continuation of the local cell anodic polarisation curve, AB. The corrosion rate of an anodically polarised metal can very seldom be related quantitatively by Faraday s law to the external current flowing, Instead, the measured corrosion rate will usually exceed... 

It also follows that if the solution is stirred the rate of arrival of oxygen at the cathode will be increased. This will result in a corresponding increase in the rate of bimetallic corrosion as is shown in Fig. 1.63 for the aluminium-mild steel couple in stirred 1 - On NaCl solution . The increase in galvanic corrosion rate will be in the inverse relation to the slope of the anodic polarisation curve of the more negative metal, provided that the cathodic reaction is not totally diffusion controlled. 

If it is assumed that anodic polarisation of the more negative metal of the couple is insignificant, then it can be shown from the Tafel relationship for the hydrogen evolution reaction that... 

Note that Reference" draws attention to the possibility of an increase of anodic polarisation of the more negative member of a couple leading to a decrease in galvanic corrosion rate. There can also be a risk of increased corrosion of the more positive member of a couple. Both these features can arise as a result of active/passive transition effects on certain metals in certain environments. 

Fig. 2.3 Distribution of ions during anodic polarisation, showing the arbitrary value used for...

It is convenient to consider three stages of anode polarisation with regard to temperature effects, (a) under film-free conditions, (b) under film-forming conditions and (c) at the active-passive transition. 

These data have been obtained by anodic polarisation work and might therefore be more relevant when cathodic reduction of oxygen takes place that can increase the corrosion potential to high positive values. 

Fig. 2.7 Anodic polarisation of nickel at various temperatures, in 0-05 n H2SO4 + 0-05 N K2SO4, pH 1-3 (after Cowan and Staehle )...

Fig. 2.11 Influence of temperature on the anodic polarisation of copper in aerated 3% NaCI...

The general form of the anodic polarisation curve of the stainless steels in acid solutions as determined potentiostaticaiiy or potentiodynamically is shown in Fig. 3.14, curve ABCDE. If the cathodic curve of the system PQ intersects this curve at P between B and C only, the steel is passive and the film should heal even if damaged. This, then, represents a condition in which the steel can be used with safety. If, however, the cathodic curve P Q also intersects ED the passivity is unstable and any break in the film would lead to rapid metal solution, since the potential is now in the active region and the intersection at Q gives the stable corrosion potential and corrosion current. 

Fig. 3.14 Idealised form of a potentiostatic anodic polarisation curve ABCDE for stainless steels as determined in sulphuric acid solution. PQ and P Q arc two cathodic polarisation curves that lead to passivity and corrosion, respectively...

Table 3.19 Some critical values from anodic polarisation curves determined potentiodynamically in 20% sulphuric acid at 27°C (see Fig. 3.13)...

Polarisation from an external source may also affect the range of passivity. Cathodic polarisation may depress the potential from the passive to the active region (see Fig. 3.14) and thus care should be taken to avoid contact with any other corroding metal. Anodic polarisation, on the other hand, can stabilise passivity provided that the potential is not increased into the range of transpassivity (see Fig. 3.14) and anodic protection is quite feasible. 

Amorphous Fe-3Cr-13P-7C alloys containing 2 at% molybdenum, tungsten or other metallic elements are passivated by anodic polarisation in 1 N HCl at ambient temperature". Chromium addition is also effective in improving the corrosion resistance of amorphous cobalt-metalloid and nickel-metalloid alloys (Fig. 3.67). The combined addition of chromium and molybdenum is further effective. Some amorphous Fe-Cr-Mo-metalloid alloys passivate spontaneously even in 12 N HCl at 60° C. Critical concentrations of chromium and molybdenum necessary for spontaneous passivation of amorphous Fe-Cr-Mo-13P-7C and Fe-Cr-Mo-18C alloys in hydrochloric acids of various concentrations and different temperatures are shown in Fig. 3.68 ... 

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