Relative sensitivity factor corrosion

The relative sensitivities of the two instruments are discussed in the section on corrosion rate measurement. These measurements are affected by temperature and R.H., so the conditions of measurement will affect the interpretation of the limits defined earlier. The measurements themselves should be considered accurate to within a factor of two (Feilu et al., 1995). 

The suitabiHty and economics of a distillation separation depend on such factors as favorable vapor—Hquid equiHbria, feed composition, number of components to be separated, product purity requirements, the absolute pressure of the distillation, heat sensitivity, corrosivity, and continuous vs batch requirements. Distillation is somewhat energy-inefficient because in the usual case heat added at the base of the column is largely rejected overhead to an ambient sink. However, the source of energy for distillations is often low pressure steam which characteristically is in long supply and thus relatively inexpensive. Also, schemes have been devised for lowering the energy requirements of distillation and are described in many pubHcations (87). 

In developing the arguments that are presented later in this review, it is necessary to keep in mind the relative scales (dimensions) at which each phase occurs. This is important because the effect of flow on localized corrosion is largely (though not totally) a question of the relative dimensions of the nucleus and the velocity profile in the fluid close to the surface. However, the velocity profile is a sensitive function of the kinematic viscosity, which in turn depends on the density and the dynamic viscosity. Because the kinematic viscosity of water drops by a factor of more than 100 on increasing the temperature from 25 °C to 300 °C, the conclusions drawn from ambient temperature studies of the effect of flow on localized corrosion must be used with great care when describing flow effects at elevated temperatures. 

The initial exposure conditions have a very marked influence on the subsequent corrosion rate. During the first days of exposure, wet conditions (caused by high relative humidity or rainfall) cause higher corrosion rates than dry conditions. These effects will vary from one material to another. For example, zinc is more sensitive than steel. Differences are explained by the fact that different materials form different corrosion products with different protective properties. A wide variety of structurally related corrosion products can be formed on zinc, the nature of which depends on initial exposure conditions. The seasonal dependence on the concentrations of peroxide and ozone in the atmosphere might also be a contributing factor. 

The reviews made by F. L. LaQue on this subject indicate that the salt spray test cannot realistically be used, for example, for parts with complicated shapes. This deficiency is principally due to the fact that the salt spray particles fall in vertical patterns, creating a strong orientation dependency. Another major inadequacy of the test is the variable sensitivity of different metallic materials to the ions present in various service environments. Since different metals also are affected differently by changes in the concentrations of salt solutions, the salt spray test is not really appropriate for ranking different materials in an order of relative resistance to salt water or salt air. The variability of the environments, even for seagoing equipment, is another factor that is extremely difficult to reproduce in a laboratory. Before attempting to simulate such natural environments, it is thus recommended that the chemistry of the environment and all other parameters controlling the corrosion mechanisms be monitored over time, in a serious attempt to characterize the worst exposure conditions. 

---