Corrosion rate factors controlling

Corrosivity. Corrosivity is an important factor in the economics of distillation. Corrosion rates increase rapidly with temperature, and in distillation the separation is made at boiling temperatures. The boiling temperatures may require distillation equipment of expensive materials of constmction however, some of these corrosion-resistant materials are difficult to fabricate. For some materials, eg, ceramics (qv), random packings may be specified, and this has been a classical appHcation of packings for highly corrosive services. On the other hand, the extensive surface areas of metal packings may make these more susceptible to corrosion than plates. Again, cost may be the final arbiter (see Corrosion and corrosion control). 

Corrosion Theory 1259. Forms of Corrosion Attack 1268. Factors Influencing Corrosion Rate 1292. Corrodents in Drilling Fluids 1300. Corrosion Monitoring and Equipment Inspections 1312. Corrosion Control 1323. Recommended Practices 1340. 

Figures 1.27a to d show how the Evans diagram can be used to illustrate how the rate may be controlled by either the polarisation of one or both of the partial reactions (cathodic, anodic or mixed control) constituting corrosion reaction, or by the resistivity of the solution or films on the metal surface (resistance control). Figures 1. lie and/illustrate how kinetic factors may be more significant than the thermodynamic tendency ( , u) and how provides no information on the corrosion rate.

Combination electrical methods Tomashov and Mikhailovsky describe a method developed in the Soviet Union. This test is essentially a combination of resistivity measurement and polarisation rates on iron electrodes in soil in situ. The usefulness and value of this procedure has not as yet been determined by practical application by corrosion engineers. The development of this combination test does, however, represent an attempt to integrate some of the complex factors controlling corrosion rates in soil. Much more research on these factors and methods of measurement should in the future enable the corrosion engineer to evaluate soil properties with respect to application of corrosion-alleviating operations. 

It has been concluded from data reported in these studies that the skin temperature is the major controlling factor in corrosion, not the rate of heat flow through the metal . It has also been concluded, however, that corrosion rates at a given mid-specimen temperature do depend on the presence or absence of thermal flux . The difference between temperatures at skin and mid-specimen positions may account for this discrepancy. 

The conductivity and the composition of the medium are controlling factors in the extent of galvanic corrosion, as evidenced by the corrosion rates in 3% NaCI and tapwater (see Table 4.71). Cadmium-plated steel gives a lower corrosion rate than steel. 

Fig. 4.23 Schematic representation of polarization curves for the analysis of galvanic coupling when diffusion control of the oxygen reduction reaction is the dominant factor governing the corrosion rate...

The moisture content in concrete is the main factor controlling the corrosion rate. When concrete is in equiUbrium with the atmosphere, the absorbed water can be correlated to the relative humidity of the environment (Section 2.1.2). Actually, in real structures this condition normally occurs only at the concrete surface. 

The solution velocity in this case is one of the major factors that control the corrosion potential and the corrosion rate in the active state of the alloy. The effect of mass transfer on electrode kinetics is discussed in detail in Chapter 3. Figure 4.11 correlates the... 

Some of the factors that control corrosion rate in galvanic couples are the corrosion potential difference between dissimilar metals, hydrogen evolution exchange current... 

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