Polarisation curve

Polarisation curves can be used to determine the intensity of possible galvanic corrosion. However, for practical reasons, these measurements are carried out in conductive 

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Fig. 1.31 Shape of cathodic polarisation curve when transport overpotential is rate controlling, (a) Effect of velocity on ( l and corrosion rate, (b) effect of concentration on tY and corrosion rate and (c) effect of position and slope of anodic curve (after Stern...

Passivity of a metal lies in contrast to its activity, in which the metal corrodes freely under an anodic driving force. The passive state is well illustrated by reference to a classical polarisation curve prepared poten-tiostatically or potentiodynamically (Figure 1.39). As the potential is raised... 

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...

Graphic estimation of the corrosion rate and corrosion potential of a metal immersed in a corrosive high-conductivity electrolyte, from the intersection of the polarisation curves for the appropriate anodic and cathodic reactions, has been proposed and explained by several authorities. These polarisation curves can be further used to illustrate the effect of imposing additional anodic or cathodic potentials on to a corroding metal (see also Sections 1.4 and 10.1). 

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. 

Fig. 1.64 Polarisation curves for zinc anodes coupled to aluminium and copper cathodes in I.On NaCI at 25°C (potentials vs. S.H.E.) (after Pryor and Kier - )...

Polarisation-curve procedures The Denison method is to measure the current at various degrees of polarisation of metal in soil in a special cell. While this test is considered quite accurate, it has the disadvantage that the measurements are made in the laboratory and cannot be made in the field. 

Electrochemical corrosion of metals Since the aggressiveness of salt melts is governed by redox equilibria, and is often controlled by composition of the external atmosphere, effects analogous to electrochemical or oxygen-concentration corrosion in aqueous systems can occur in salt melts. Tomashov and Tugarinov determined cathodic polarisation curves in fused chlorides and concluded that the cathodic reactions of impurities could be represented as ... 

In the case of CaCl2 and NaCl, the order corresponds with the corrosion behaviour expected from cathodic polarisation curves . The order of aggressiveness of chlorides can also be explained on the basis of redox potentials of the melts, calculated on thermodynamic grounds from the free energies of formation of the appropriate oxides and chlorides . The order of aggressiveness of nitrates is complicated by passivity effects , while that of alkalis in contact with air is... 

The literature contains a number of studies on the susceptibility of the cobalt-based alloys to pitting corrosion. In-vitro studies conducted by Mueller and Greener , involving static conditions, revealed no evidence of pitting having occurred. Syrett and Wing ", utilising cyclic polarisation analyses, observed that neither as-cast nor annealed Co-Cr-Mo alloy demonstrated hysteresis loops in their cyclic polarisation curves. They... 

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 tests indicate that maraging steel does not exhibit passive behaviour in 3% NaCl, and that the polarisation curves are unaffected by changes in heat treatment. 

Fig. 3.56 Potentiodynamic polarisation curves for high-chromium white irons in nitrogen-saturated solution containing 800mg/l Cl , pH 3.5. 25°C...

Fig. 3.71 Change in polarisation curve of amorphous Fe-10Cr-l3P-7C alloy in 1 N HCI with the time of heat treatment at 723 K. The time of heat treatment is expressed in the figure in...

Turning now to the acidic situation, a report on the electrochemical behaviour of platinum exposed to 0-1m sodium bicarbonate containing oxygen up to 3970 kPa and at temperatures of 162 and 238°C is available. Anodic and cathodic polarisation curves and Tafel slopes are presented whilst limiting current densities, exchange current densities and reversible electrode potentials are tabulated. In weak acid and neutral solutions containing chloride ions, the passivity of platinum is always associated with the presence of adsorbed oxygen or oxide layer on the surface In concentrated hydrochloric acid solutions, the possible retardation of dissolution is more likely because of an adsorbed layer of atomic chlorine ... 

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