Crevice corrosion solution-conductivity

Artificial crevice electrodes have been used to study the effect of dichromate on active dissolution of aluminum. In these experiments, 50 pm thick commercially pure A1 foils were placed between thin plastic sheets and mounted in epoxy. This assembly was fixed against a square cell that accommodated counter and reference electrodes and a trap that allowed for H2 gas collection. A schematic illustration of this cell and electrode is shown in Fig. 9 (36). Crevice corrosion growth experiments were conducted in aerated 0.1 M NaCl solution with additions of either 0.01 or 0.1 M Na2Cr207. Artificial crevice growth experiments were conducted under potentiostatic polarization at potentials ranging from 0 to... 

Pickering and coworkers [31, 34, 35] have demonstrated both experimentally and computationally that for systems that meet the criteria of the IR theory, lA is predicted. The amount of potential drop increases as one moves into the crevice because of the current leaving the crevice. If the geometry, solution conductivity, and passive current density of the material in the environment conspire to create sufficient ohmic drop, then the potential of some portion of the material within the crevice falls to the primary passive potential. Under these circumstances, the passive film is not stable and active dissolution occurs. The potential difference between the applied potential and the primary passivation potential is referred to as IR. Deeper still into the crevice the ohmic drop leads to decreased dissolution as the overpotential for the anodic reaction decreases. Thus, ohmic drop is responsible for the initiation and stabihzation of crevice corrosion according to this model. 

The cathodic reaction kinetics increase on the free smfaces to balance the increased anodic dissolution and this induces a drop of their corrosion potential. Potential drops of several hundreds of mV can be observed on the free smfaces due to the initiation of crevice corrosion (see Fig. 5) Large cathodic areas, high solution conductivity, and cathodic reaction depolari2ers (such as carbon deposits) can increase significantly the amoimt of available cathodic current and thus the crevice propagation rate. 

The free surfaces outside the crevice gap constitute the cathode, which provides cathodic current to balance the anodic dissolution in the crevice gap. Thus, the larger the free surfaces and the higher the bulk solution conductivity, the higher the available cathodic current and the higher the corrosion potential in the crevice. Except in the cases where the IR drop is the driving factor for crevice corrosion (see earlier), this usually shortens the incubation period and increases the corrosion rate in the propagation stage. 

For reasons of ease of manufacture, the majority of solid electrodes have a circular or rectangular form. External links are through a conducting epoxy resin either to a wire or to a solid rod of a metal such as brass, and the whole assembly is introduced by mechanical pressure into an insulating plastic sheath (Kel-F, Teflon, Delrin, perspex, etc.) or covered with epoxy resin. It is very important to ensure that there are no crevices between electrode and sheath where solution can enter and cause corrosion. Examples of electrodes constructed by this process will be shown in Chapter 8. 

The corrosion mechanism entails the simple depletion of a passivating specie in the corrosive. This mechanism applies equally to nonconductive and electrically conductive ceramics. The passivating specie within the crevice is consumed by the corrosion reaction, making the crevice environment more corrosive than the bulk solution. Replacement of the passivating specie by diffusion is prevented by the occluded nature of the crevice therefore, the environment within the crevice remains more corrosive. The crevice grows as corrosion proceeds forming additional corrosion product, which further restricts diffusion. 

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