Mass transport corrosion-rates

Dissolved oxygen reduction process Corrosion processes governed by this cathode reaction might be expected to be wholly controlled by concentration polarisation because of the low solubility of oxygen, especially in concentrated salt solution. The effect of temperature increase is complex in that the diffusivity of oxygen molecules increases, but solubility decreases. Data are scarce for these effects but the net mass transport of oxygen should increase with temperature until a maximum is reached (estimated at about 80°C) when the concentration falls as the boiling point is approached. Thus the corrosion rate should attain a maximum at 80°C and then decrease with further increase in temperature. 

In acidic solutions, the corrosion rate is relatively high. Studies on cadmium monocrystals and polycrystals in acidic chloride solutions revealed anodic dissolution independent of the crystallographic orientation the dissolution rate was controlled by the mass transport of CdCl" ions [331]. The inhibitive influence of adsorbed organic substances, for example, alcohols [332], phenotiazine [333], and some polymers (e.g. poly (vinyl alcohol), poly(acrylic acid), sodium polyacrylate. 

The information required to predict electrochemical reaction rates (i.e., experimentally determined by Evans diagrams, electrochemical impedance, etc.) depends upon whether the reaction is controlled by the rate of charge transfer or by mass transport. Charge transfer controlled processes are usually not affected by solution velocity or agitation. On the other hand, mass transport controlled processes are strongly influenced by the solution velocity and agitation. The influence of fluid velocity on corrosion rates and/or the rates of electrochemical reactions is complex. To understand these effects requires an understanding of mixed potential theory in combination with hydrodynamic concepts. 

In this chapter you will learn that proper assessment of mass transport controlled corrosion reactions requires knowledge of the concentration distribution of the reacting species in solution, certain properties of the electrolyte, and the geometry of the system. A rigorous calculation of mass transport controlled reaction rates requires detailed information concerning these parameters. Fortunately, many of the governing equations have been solved for several well-defined geometries. 

Figure 2 Evans diagram illustrating the influence of solution velocity on corrosion rate for a cathodic reaction under mixed charge transfer-mass transport control. The anodic reaction shown is charge transfer controlled.

As z app approaches iL, the cathodic overpotential, r c, becomes very large and the cathodic reaction rate becomes independent of overpotential. For a completely mass transport limited cathodic reaction, the concentration of the reacting specie in solution, Cb, approached zero at the electrode interface and z COff = iL. This is shown at Vi and V2 in Fig. 2. The limiting current density is increased by increasing solution stirring or rotation rate, co, in the case of a rotating cylinder or disk electrode. The corrosion rate would be increased. 

Either anodic or cathodic mass transport limited corrosion may be observed in numerous corrosion systems. Such phenomena may be simulated and investigated in the laboratory by establishing experimental conditions that match those in the field application. This is accomplished by equating z L or 8d in the laboratory to the same values present in the field. In this way the effect of fluid velocity or mass flow rate on the corrosion rate may be investigated. Similarly, the hydrodynamic conditions in the field must be matched by those in the laboratory. Procedures for establishing such correlations between field and laboratory measurements are described below. 

Data is shown in Figs. 7a and 7b for oxygen reduction on carbon steel in room temperature 0.6M NaCl. iL increases with co0 5 as predicted. Hence if the corrosion rate is determined by the mass transport of oxygen to the disk surface to support oxygen reduction, then the corrosion rate will increase as a function of the rotation rate, co, raised to the 0.5 power and linearly with dissolved oxygen concentration. The diffusion boundary layer thickness, 8d, may be calculated from Fick s first law after iL is determined. Recall that 8 = nFDCJiL for one dimensional diffusion at the steady state. This leads to the following expression for the diffusional boundary layer thickness ... 

Hence, if the corrosion rate is determined by the mass transport of cathodic reactant to the pipe surface, then the corrosion rate will increase as a function of the solution velocity raised to the 0.8 power and linearly with the dissolved reactant concentration. Note that at the same fluid velocity and reactant concentration, the limiting c.d. and hence the corrosion rate will be greater for pipes of smaller diameter. A similar relationship was proposed by Harriot and Hamilton (14) and applied by various investigators concerned with anodic mass transport controlled corrosion of ferrous piping materials (5,15). 

It was shown above that the limiting c.d. increases with velocity raised to the 0.8 power and the pipe diameter raised to the -0.2 power for piping corrosion rates that are controlled by mass transport. In contrast, it is evident that the shear stress increases with the fluid velocity raised to the 1.75 power and the pipe diameter raised to the -0.2 power. Thus equality of shear stress does not give equality of mass transfer rates. In both cases corrosion is enhanced in pipes of smaller diameter for the same solution velocity. Such a relationship can be rationalized based on the effect of pipe diameter on the thickness of the mass transport and hydrodynamic boundary layers for a given fixed geometry. Cameron and Chiu (19) have derived similar expressions for defining the rotating cylinder rotation rate required to match the shear stress in a pipe for the case of velocity-... 

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