Crevice surface films

Some nonhygroscopic materials such as metals, glass, and plastics, have the abiUty to capture water molecules within microscopic surface crevices, thus forming an invisible, noncontinuous surface film. The density of the film increases as the relative humidity increases. Thus, relative humidity must be held below the critical point at which metals may etch or at which the electrical resistance of insulating materials is significantly decreased. 

Macroscopic heterogeneities, e.g. crevices, discontinuities in surface films, bimetallic contacts etc. will have a pronounced effect on the location and the kinetics of the corrosion reaction and are considered in various sections throughout this work. Practical environments are shown schematically in Fig. 1.3, which also serves to emphasise the relationship between the detailed structure of the metal, the environment, and external factors such as stress, fatigue, velocity, impingement, etc. 

From the above discussion, it is clear that the stabilization or failure of graphite electrodes depends on a delicate balance between passivation phenomena (due to the formation of highly cohesive and adhesive surface films) and a buildup of internal pressure due to the reduction of solution species inside crevices in the graphite particles. This delicate balance can be attenuated by both solution composition (EC-DMC vs. EC-PC or PC, etc.) and the morphology of the graphite particles (i.e. the structure of the edge planes and the presence of crevices). 

Diffusion of a solute through immobile water to a reaction site also is affected by interstitial water velocity. If the diffusion rate is slow compared to the interstitial velocity, physical nonequilibrium occurs (5-7). The immobile water can be a layer on the grain surface (film diffusion), in dead-end pores between tightly packed grains (pore diffusion), or within crevices or pits on the grain surfaces (particle diffusion). Calcium and chloride breakthrough curves from column experiments done by James and Rubin (8) indicate that nonequilibrium transport occurs unless interstitial velocities are decreased so that the hydrodynamic-dispersion coefficient is of the same order of magnitude as the molecular-diffusion coefficient. 

III. When the environment has become suffieiently aggressive, the surfaee oxide film is attacked. The crevice surfaces are transferred to an active state and the corrosion rate increases. 

Avoid crevices between metals or between metals and nonmetals. Periodically remove contaminating surface films using alkaline cleaners with stainless-steel wool or the equivalent. 

The pH of the water inside the crevice of crevice couples was in the range S.2-5.8. This was about one pH unit lower than the pH of the bulk water. Water of reduced pH promotes dissolution of the aluminium oxide film on the crevice surfaces and increases the corrosion of aluminium. 

The improved performance of certain steels can be attributed to enrichment of the surface film by chromium. The quality of surface films affects the susceptibility of steels to crevice corrosion. The formation of passive films is dependent on oxygen and hence, its concentration affects the magnitude of crevice corrosion. It has been reported that as the seawater temperature is raised from ambient to 70°C, the resistance of steel types 304 and 317 is increased. This has been attributed to the enrichment of the passive film by chromium which increases its stability. 

Aluminum and its alloys have been extensively used for structural applications with success. Aluminum resists corrosion from the atmosphere if there is an absence of narrow crevices. Many statues erected, over a hundreds of years ago, have not deteriorated badly which is in contrast with aluminum cables used in seawater. The corrosion resistance of aluminum is due to its tendency to form a compact oxide layer over the surface. The oxide formed offers a high resistance to corrosion. The normal surface film present in air is about 1 nm thick. The film thickness increases at the elevated temperature. The film growth is more rapid in water than in oxygen. 

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