Prediction of Corrosion Rates

The following table is a summary of units for the two corrosion penetration rate schemes  

Expression relating corrosion rate and current density 

Inasmuch as there is an electric current associated with electrochemical corrosion reactions, we can also express corrosion rate in terms of this current, or, more specifically, current density—that is, the current per unit surface area of material corroding—which is designated /. The rate r, in units of mol/m -s, is determined using the expression 

There are two types of polarization—activation and concentration. We now discuss their mechanisms because they control the rate of electrochemical reactions. 

All electrochemical reactions consist of a sequence of steps that occur in series at the interface between the metal electrode and the electrolyte solution. Activation polarization 

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Other Effects Stream concentration can have important effects on corrosion rates. Unfortunately, corrosion rates are seldom linear with concentration over wide ranges. In equipment such as distillation columns, reactors, and evaporators, concentration can change continuously, makiug prediction of corrosion rates rather difficult. Concentration is important during plant shutdown presence of moisture that collects during cooling can turn innocuous chemicals into dangerous corrosives. 

Corrosion likelihood describes the expected corrosion rates or the expected extent of corrosion effects over a planned useful life [14]. Accurate predictions of corrosion rates are not possible, due to the incomplete knowledge of the parameters of the system and, most of all, to the stochastic nature of local corrosion. Figure 4-3 gives schematic information on the different states of corrosion of extended objects (e.g., buried pipelines) according to the concepts in Ref. 15. The arrows represent the current densities of the anode and cathode partial reactions at a particular instant. It must be assumed that two narrowly separated arrows interchange with each other periodically in such a way that they exist at both fracture locations for the same amount of time. The result is a continuous corrosion attack along the surface. 

The term aggressive is often used to imply some approximately quantitative estimate of the likelihood of corrosion and depends on measuring factors such as soil water (resistivity), pH, redox potential, salt concentrations and bacterial populations in order to establish criteria for the prediction of corrosion rates . Similar measurements for predicting corrosion... 

Reliable pH data and activities of ions in strong electrolytes are not readily available. For this reason calculation of corrosion rate has been made using weight-loss data (of which a great deal is available in the literature) and concentration of the chemical in solution, expressed as a percentage on a weight of chemical/volume of solution basis. Because the concentration instead of the activity has been used, the equations are empirical nevertheless useful predictions of corrosion rate may be made using the equations. 

A critical but poorly understood factor related to atmospheric exposure is the connection between nominal environmental conditions (e.g., humidity, ultraviolet radiation, pollution, atmospheric particles, and so on) and the actual chemistry of the environment on the material s surface. Time-of-wetness is known to be an important parameter in outdoor exposure, given that water associated with precipitation or condensation is critical to the corrosion processes. However, accurate prediction of corrosion rates depends on knowing how the water on the surface affects the concentration of all the important corrosive species. 

During four years exposure at Cuxhaven, Germany, in a marine atmosphere the loss of material was up to 8(X) g m", a corrosion rate of s0.03 mm year". This example illustrates that marine and rural atmospheres vary globally and simple classification is not adequate for a serious prediction of corrosion rate. 

Early predictions of corrosion rates and estimates of adequate CP have traditionally been based on case studies and sample exposure tests. Applying these techniques to real structures usually involve extrapolations, use of large safety factors and ongoing corrections and maintenance of the system. In the late 1960s the finite element method was applied to the problem by discretization of the electrolytically conductive environment into a mesh and solving numerically with Laplace equations to define the intersection points, or nodes of this mesh [17]. 

The mixed potential diagram or vs / diagram allows accurate prediction of corrosion rate to be made in specific environments. It also allows the prediction of current based on current density higher than the corrosion current (icon) and utilization of advanced technique for corrosion measurement, such as galvanostatic and poten-tiostatic techniques. These techniques wifi be described at the end of the chapter. 

Y. Fukuda, Y. Kawahara, T. Shimada, M. Nakamura, and T. Hosoda, Prediction of Corrosion Rate for Superheater Tube Materials in Waste Incinerators by a Regression Analysis of Field Corrosion Test Data, Materials at High Temperatures, ROD, 118(s), pp 423 2... 

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