Polarization curves active metal electrode, corrosion potential

As described in Sec. 11.3, the spontaneous corrosion potential of a corroding metal is represented by the intersection of the anodic polarization curve of metal dissolution with the cathodic polarization curve of oxidant reduction (Figs. 11—5 and 11-6). Then, whether a metal electrode is in the active or in the passive state is determined by the intersection of the anodic and cathodic polarization curves. 

Linear (resistance) polarization, hi the realization of a polarization curve, the working electrode reaches high potential values causing a strong irreversible dissolution of the material. Thus, when attempts are made to evaluate the change in the corrosion rate over time of a metal under a uniform corrosion process, under control for activation, another type of non-destructive experiment is employed, namely the measurement of resistance to polarization. This is also a steady state technique and it is based on the application of a low amplitude signal of direct current around the corrosion potential, ensuring that the material continues in a situation of equilibrium. 

Corrosion protection of metals can take many fonns, one of which is passivation. As mentioned above, passivation is the fonnation of a thin protective film (most commonly oxide or hydrated oxide) on a metallic surface. Certain metals that are prone to passivation will fonn a thin oxide film that displaces the electrode potential of the metal by +0.5-2.0 V. The film severely hinders the difflision rate of metal ions from the electrode to tire solid-gas or solid-liquid interface, thus providing corrosion resistance. This decreased corrosion rate is best illustrated by anodic polarization curves, which are constructed by measuring the net current from an electrode into solution (the corrosion current) under an applied voltage. For passivable metals, the current will increase steadily with increasing voltage in the so-called active region until the passivating film fonns, at which point the current will rapidly decrease. This behaviour is characteristic of metals that are susceptible to passivation. 

The concepts in Chapters 2 and 3 are used in Chapter 4 to discuss the corrosion of so-called active metals. Chapter 5 continues with application to active/passive type alloys. Initial emphasis in Chapter 4 is placed on how the coupling of cathodic and anodic reactions establishes a mixed electrode or surface of corrosion cells. Emphasis is placed on how the corrosion rate is established by the kinetic parameters associated with both the anodic and cathodic reactions and by the physical variables such as anode/cathode area ratios, surface films, and fluid velocity. Polarization curves are used extensively to show how these variables determine the corrosion current density and corrosion potential and, conversely, to show how electrochemical measurements can provide information on the nature of a given corroding system. Polarization curves are also used to illustrate how corrosion rates are influenced by inhibitors, galvanic coupling, and external currents. 

Transition metals, such as Fe, Cr, Ni and Ti, demonstrate an active-passive behavior in aqueous solutions. Such metals are called active-passive metals. The above metals exhibit S-shaped polarization curves which are characteristic of such metals. Consider, for instance, the case of 18-8 stainless steel placed in an aqueous solution of H2SO4. If the electrode potential is increased then the current density rises to a maximum, with the accompanying dissolution of the metal taking place in the active state. The current density associated with the dissolution process indicates the magnitude of corrosion. At a certain potential, the current density is drastically reduced as the metal becomes passivated because of the formation of a thick protective film. Iron shows passivity... 

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