Uniform corrosion characteristics

The Karr column is particularly well suited for systems which tend to emulsify since its uniform shear characteristics tend to minimize emulsion formation. It is also particularly well suited for corrosive systems (since the plates can be constructed of non-metals) or for systems containing significant solids (due to its large open area and hole size on the plates). Slurries containing up to 30 percent solids have been successfully processed in Karr columns. 

Corrosion effect of forming Elongation X gauge length Standard hydropress specimen test True stress-strain curve Uniformity of characteristics... 

Protective passive films similar to that of stainless steel, for example, result in uniform corrosion because of the mobility of active sites that passivate readily. Corrosion products and/or passive films are characteristic of numerous electrochemical corrosion of alloys. A film is protective depending on coverage capacity, conductivity,... 

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.). 

These conditions will cause various deviations in the geometry and appearance of the attack compared with uniform corrosion, and it is convenient to classify corrosion just after the appearance of the corroded surface, as done by Fontana and Greene [7.1] (possibly with supplementary information about important service conditions). The advantage of such a classification is that a corrosion failure can be identified as a certain corrosion form by visual inspection, either by the naked eye or possibly by a magnifying glass or microscope. Sinee each form of corrosion has its characteristic... 

The characteristics of individual forms of corrosion are taken into consideration by providing appropriate corrosion specimens. Welded coupons having the surface quality of the material used later in practice are sufficient for determining uniform corrosion rates and acquiring general information on the type of local corrosion. Resistance to crevice corrosion can be determined by using specimens as described in ASTM G 78-83. Conditions of heat transfer can be simulated by using hot-wall/cool-wall specimens under temperature-controlled conditions. 

The selection of materials to be used in design dictates a basic understanding of the behavior of materials and the principles that govern such behavior. If proper design of suitable materials of construction is incorporated, the eqiiipment should deteriorate at a uniform and anticipated gradual rate, which will allow scheduled maintenance or replacement at regular inteivals. If localized forms of corrosion are characteristic of the combination of materials and environment, the materials engineer should still be able to predict the probable life of equipment, or devise an appropriate inspection schedule to preclude unexpected failures. The concepts of predictive, or at least preventive, maintenance are minimum requirements to proper materials selection. This approach to maintenance is certainly intended to minimize the possibility of unscheduled production shutdowns because of corrosion failures, with their attendant possible financial losses, hazard to personnel and equipment, and resultant environmental pollution. 

Most of this is because in a abort cylindrical tank, the entering fluid does not move through the tank with a uniform interface. The poor flow characteristics also are due partly to the deposition of solids, and scale, to corrosion of spreader pipes, etc. 

Both anodic and cathodic reactions occur uniformly over the surface in simple examples of corrosion, but the individual sites will depend very much on the surface characteristics such as grain boundaries, crevices and, of course, discontinuities in any protective oxide layer, as the protective film acts as an electrical barrier. The protection realized will depend on the uniformity of the protective layer and the intimacy of its contact with the underlying metal. The potential for corrosion will depend on the pH of the liquid in contact with the metal. 

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