Crevice Corrosion control methods

The forms of corrosion which can be controlled by cathodic protection include all forms of general corrosion, pitting corrosion, graphitic corrosion, crevice corrosion, stress-corrosion cracking, corrosion fatigue, cavitation corrosion, bacterial corrosion, etc. This section deals exclusively with the practical application of cathodic protection principally using the impressed-current method. The application of cathodic protection using sacrificial anodes is dealt with in Section 10.2. 

In 1980 Bemhardsson et introduced an automated electrochemical method for CPT determination. The specimen is mounted as described in Section IV.2 (ii) using a stream of argon to avoid crevice corrosion and 0.02-5% sodium chloride as electrolyte. The CPT is determined by a potentiostatic test method using an instrument called the Santron CDT 400 for potential control, temperature control, and current measurements. 

Lee and coworkers [49] explored crevice gaps on the size scale of practical crevices (<100 pm) both experimentally and computationally. In that work, microfabrication methods were used to produce crevice formers of rigorously controlled dimensions. These formers were then utilized in crevice corrosion experiments on Ni200 in 0.5 M H2SO4 in order to study the effect of crevice gap on the position of the critical distance for crevice corrosion (known as jCcnt or ATpass)- These results were... 

The tests can be classified into three general categories (i) non-electrochemical tests, (ii) electrochemical tests under open-circuit conditions, and (iii) electrochemical tests under controlled potential or current conditions. Brief descriptions of the more established methods for evaluating crevice corrosion appear in Table 1 along with comments concerning limits of applicability. Details of each method can be found in the literature cited. The reader is also advised to consider one or more of the excellent reviews on test techniques for crevice corrosion [40-45,76,114. ... 

The remote crevice assembly technique (see Chapter 19) is a research tool that allows one to separate the anode and cathode areas of a crevice corrosion test sample so that the current flowing between them can be measured with a zero-resistance ammeter. This technique is similar to the dual cell method, and it lends itself well to studies of microbial effects on crevice corrosion [7]. It allows direct measurement of microbial effects on both the initiation time and propagation rate for crevice attack, provided again that a suitable control experiment without the microbial influence can be done concurrently. The scime technique of separating the anode and cathode can be used to study the influence of microbes in biofilms on galvanic corrosion [li]. 

None of the control methods described above is ideal, and the one chosen for a given experiment will depend on the intended purpose and duration of the experiment. The utility of these control methods is not limited to studies of crevice corrosion. In general, they can also be used for the study of any type of corrosion in which the effect of a natural or cultured biofilm is being measured. 

Activation polarization is usually the controlling factor during corrosion in strong acids since both and iR are relatively small. Concentration polarization usually predominates when the concentration of the active species is low for example, in dilute acids or in aerated waters where the active component, dissolved oxygen, is only present at very low levels. The ohmic drop will become an extremely important factor when studying corrosion phenomena for which there is a clear separation of the anodic and cathodic corrosion sites, for example, crevice corrosion. The ohmic drop is also an important variable in the application of protective methods such as anodic and cathodic protection that forces a potential shift of the protected structure by passing a current in the environment. 

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