Corrosion mass transfer effects

Ellison and Schmeal [19] explored a similar approach by using the corrosion of carbon steel in concentrated sulfuric acid as the mass-transpoit/corrosion probe. A model was proposed to reconcile corrosion rate data from pipe and rotating cylinder geometries based on the premise that the corrosion rate is controlled by the transport of Fe + from the interface. The data were subsequently used by Silverman [21] to construct a more precise model for correlating mass-transfer effects between pipe flow and a rotating cylinder. 

This chapter outlines the basic aspects of interfacial electrochemical polarization and their relevance to corrosion. A discussion of the theoretical aspects of electrode kinetics lays a foundation for the understanding of the electrochemical nature of corrosion. Topics include mixed potential theory, reversible electrode potential, exchange current density, corrosion potential, corrosion current, and Tafel slopes. The theoretical treatment of electrochemistry in this chapter is focused on electrode kinetics, polarization behavior, mass transfer effects, and their relevance to corrosion. Analysis and solved corrosion problems are designed to understand the mechanisms of corrosion processes, learn how to control corrosion rates, and evaluate the protection strategies at the metal-solution interface [1-7]. 

The effect of mass transfer on electrode kinetics is shown in Fig. 3.12. Many useful kinetic rate expressions based on Tafel conditions, mass transport limitations can be developed from Eq. (3.59). Prediction of mass transfer effects may be useful in corrosion systems depending on the system s corrosion conditions. The mass transport limitations in corrosion systems may alter the mixed potential of a corroding system. Under Tafel conditions (anodic or cathodic), Eq. (3.59) can be written as ... 

The velocity effects on CO2 corrosion have been studied in several projects dealing with multiphase flow. Mechanistic models based on electrochemistry, reaction kinetics, and mass transfer effects have been developed, e.g. by Nesic et al. [6.28]. A semi-empirical model was presented by de Waard et al. [6.29]. This model and the corresponding experimental results are valid for cases without carbonate scales. Inhibition can be accounted for by inserting an inhibitor efficiency factor. A model mainly based on the same data as the one developed by de Waard et al. is included in the standard NORSOK M-506 [6.30], The application of the NORSOK model is based on a computer program. Common to these models is that they are valid for cases with a bulk phase of water. For mist flow and dewing conditions the calculation basis is inferior, but such conditions give low corrosion rates. 

In general, fluid velocity has two effects on corrosion the mass transfer effect and the surface shear stress effect. 

However, as during corrosion no net current will pass the interface, the theory of electrochemical reaction kinetics will have to be applied in order to calculate the current density under free corrosion conditions. This current density is called the corrosion current density. For a corroding surface under simple electrochemical conditions (no mass transfer effect), the relation between the current density and its driving force, the potential drop across the interface (electrode potential), is given by the Butler-Volmer equation... 

Examples of some of these effects and the resulting mass transfer erosion corrosion behaviour are shown in Figure 1.92. 

Mass-transfer deposits can lead to blockages in non-isothermal circulating systems, cis in the case of liquid-metal corrosion. In fused salts, the effect can be reduced by keeping contamination of the melt by metal ions to a minimum e.g. by eliminating oxidising impurities or by maintaining reducing conditions over the melt . 

Sodium, potassium and sodium-potassium alloys Liquid sodium, potassium or alloys of these elements have little effect on niobium at temperatures up to 1 000°but oxygen contamination of sodium causes an increase in corrosionSodium does not alloy with niobium . In mass transfer tests, niobium exposed to sodium at 600°C exhibited a corrosion rate of approximately 1 mgcm d . However, in hot trapped sodium at 550°C no change of any kind was observed after 1 070 h . 

In general, it is fair to state that one of the major difficulties in interpreting, and consequently in establishing definitive tests of, corrosion phenomena in fused metal or salt environments is the large influence of very small, and therefore not easily controlled, variations in solubility, impurity concentration, temperature gradient, etc. . For example, the solubility of iron in liquid mercury is of the order of 5 x 10 at 649°C, and static tests show iron and steel to be practically unaltered by exposure to mercury. Nevertheless, in mercury boiler service, severe operating difficulties were encountered owing to the mass transfer of iron from the hot to the cold portions of the unit. Another minute variation was found substantially to alleviate the problem the presence of 10 ppm of titanium in the mercury reduced the rate of attack to an inappreciable value at 650°C as little as 1 ppm of titanium was similarly effective at 454°C . 

In practice, the gases exiting the fluidized bed reactor contain a certain amount of ash and have to be cleaned. Also, the combustion products of coal are sometimes corrosive, which means that in addition to air being fed into the reactor, various other chemicals are added to ensure "clean" combustion products that will not corrode turbine blades or violate environmental standards. Coal combustion is a very active field of research, and many exciting developments are occurring there. In this analysis, we make certain assumptions that illustrate the thermodynamic concepts as clearly as possible. Therefore, we do not examine the effect of hydrodynamics, heat, and mass transfer, which are very important in the combustion of the coal particle and the distribution of combustion products. We do not expect that this will have a significant impact on the analysis. 

The main advantages of a batch reactor are as follows. It is simple and allows rapid measurements. Many experiments can be performed in a short period of time. It is convenient when using pure, expensive, corrosive, or high boiling temperature chemicals. Its use is recommended if the catalyst is sensitive to traces of poisons since there is no accumulation effect. In principle, by varying the stirring conditions it is possible to investigate the influence of heat and mass transfer processes. 

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-... 

Based upon the electrodics, the exchange current density is related to the activation energy (16. pp.ll52). For an Arrhenius relation for the effect of temperature upon corrosion rate, AG and AG j are analogous to activation energy and equal 6.44 kcal/mol and 4.30 kcal/mol, respectively. The literature indicates activation energies for mass transfer limited processes range between 1 and 3 kcal/mol and for reaction limited between 10 and 20 kcal/mol (12). Based upon this criteria, corrosion of the 304 S.S. in pure water in the experimental system may lie between the mass transfer and reaction rate limited cases. 

Benedict, Pigford, and Levi have carried out mathematical analysis of the GS process. An exhaustive treatment of the process, including calculations for flow rates, dependence of composition on number of stages, effect of solubility and humidity on process analysis, temperature profile in cold towers, simultaneous heat and mass transfer in heat transfer section, concentration reversal in heat transfer section, corrosion, materials of construction, feed purification, and safety, etc. have been reviewed by Dave, Sadhukhan and Novaro. ... 

The approach developed by Newman for the treatment of both mass-transfer and electric-field effects in boundary-layer flows has had considerable success.L2 6 However, many flows of practical interest have separation and recirculation regions, features not amenable to a boundary-layer analysis. Fortunately, there has been significant progress in the heat-transfer and other communities in computational fluid dynamics (CFD), providing numerical methods applicable to problems important to electrochemistry. The pioneers in using CFD for electrochemical applications are Alkire and co-workers, who have been largely interested in flow effects in localized corrosion. The literature is briefly reviewed in the next section. 

Because of lithium s excellent physical properties as a heat transfer medium, more thorough studies need to be made of the corrosive properties of liquid lithium, with some attempt to evaluate the effect of known amoimts of impurities. The corrosion and mass transfer due to pure lithium should be determined as accurately as possible. The effect on corrosion due to a known quantity of impurities could then be determined. 

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