Pitting corrosion characteristics

The last example presented in this section deals with the pitting corrosion of Fe in CIO solutions. Perchlorate is less known as an aggressive ion but reveals some unique and remarkable characteristics with regard to pitting corrosion. For example, the critical pitting potential (1.46 V against a standard hydrogen electrode (SHE) for Fe/1 M NaClO ) can be measured with an accuracy of less than 4 mV [61] which is very unexpected if compared to... 

Szkiarska-Smialowska, Z. and Janik-Czachor, M., The Analysis of Electrochemical Methods for the Determination of Characteristic Potentials of Pitting Corrosion , Corros. Set., U, 901... 

Broli, A. and Holtan, H., Use of Potentiokinetic Methods for the Determination of Characteristic Potentials for Pitting Corrosion of Aluminium in a Deaerated Solution of 3< o NaCl , Corros. Sci., 13, 237 (1973)... 

Condenser design usually calls for thin-walled tubing with high thermal conductivity characteristics plus a high degree of resistance to biofouling and the many forms of corrosion that may occur. (These forms include crevice and pitting corrosion, biocorrosion, and erosion-corrosion). 

The black scale was carefully removed to identify the morphology of pits (Figure 7.60). The pits were located at the bottom of a hole (Figure 7.61). The morphology of the pits is characteristic of microbiologically influenced corrosion (MIC). 

Studies on the effect of hydrodynamics on localized corrosion and electrochemical etching processes have been reviewed by West et al. Much of the work has been performed by Alkire and co-workers." They have used FIDAP, a commercial FEM code, to investigate the influence of fluid flow on geometries relevant to etching and to pitting corrosion. In most cases, Stokes flow was considered. The Stokes flow approximation is frequently valid inside the cavity because its characteristic dimension is small. However, the flow outside the cavity may not be in the Stokes flow regime. Since it is the external fluid motion that induces flow inside the cavity, under many (especially unsteady) situations, the use of the Stokes flow approximation may be problematic. Some of the work of Alkire and co-workers has been extended hy Shin and Economou, " who simulated the shape evolution of corrosion pits. Natural convection was also considered in their study. 

For materials like stainless steels, the mechanisms are quite different. Corrosion resistance in stainless steels is provided by a passive film that acts as a barrier between the alloy and the water. The passive film is a continuous, non-porous and insoluble film, which, if broken under normal conditions, is self-healing. Due to these characteristics, the uniform corrosion of stainless steels is usually very low and the major risk is pitting corrosion. The pitting corrosion risk of stainless steels is influenced not only ly the composition of the alloy and by water quality but also by service conditions, quality of the material and quality of the installation (fitting, soldering conditions, etc.). 

Figure 10.28 Sequence of events in erosion corrosion (schematic) (a) erosion of the film (b) corrosion of the metallic surface exposed to the flow and (c) formation of pits with characteristic elongated shape.

WUUams, D.E., Fleisehmann, M., Stewart, J. and Brooks, T. (1986) Some characteristics of the initiation phase of pitting corrosion of stainless steel. Materials Science Forum, 8, 151. 

These measurements lead, in particular, to an understanding that there is a critical solution composition that must be maintained within the pit, and if this does not occur then the pit will die. As a consequence, there is an important effect of the kinetics of active dissolution of the alloy within this critical solution. Measurements of the characteristics of metastable pits, coupled with studies of the dissolution kinetics of steel within the aggressive solutions that characterize the local pit environment, have provided an explanation as to why certain alloy additions (specifically Mo) act to inhibit pitting corrosion by making the maintenance of metastable pits more difficult. Other important effects include those of salt films, which... 

---