Corrosion problems in practice

So far the discussion has been Umited to a consideration of homogeneous metals in a uniform environment where corrosion will occur at the same rate over the whole surface. This situation seldom constitutes a major corrosion hazard. The corrosion engineer is generally much more concerned with situations where corrosion is accelerated because it is limited to particular sites. There are several mechanisms whereby this can occur. 

Many hazardous corrosion situations arise because of differential aeration. To understand differential aeration one should first consider two pieces of the same metal placed in media identical except for their oxygen concentration. For example, as in Fig. 9.8, one is placed in unstirred solution below an air atmosphere and the other is placed in a solution saturated with oxygen by a stream of gas. The metal samples will take up different potentials and corrode at different rates that in the oxygen-saturated solution must be more positive and the corrosion current will be 

Lx)calized corrosion can also occur when two different metals are in contact. 

For example, for different applications iron is protected by a layer of zinc or tin. The former is the better choice although it is potentially more prone to corrosion than tin. If a scratch or hole is formed in a tin layer, the corrosion of the underlying iron is rapid again oxygen reduction can occur over the whole tin surface while the anodic oxidation is limited to an exposed area of iron. On the other hand, zinc is more electropositive than iron so even if the top layer is damaged, the iron does not 

All situations when two different metals are in contact are potential corrosion hazards. It is always likely that one of the metals will dissolve more rapidly and a corrosion cell will be set up. This metal will become anodic while the more stable metal will become cathodic as a result all the metal loss will occur from the more susceptible metal. 

These data are estimated readily from a knowledge of the averaged conosion current density Icorr = icoaa/A in amperes and the application of Faraday s laws of electrolysis. The weight loss of metal Aw at the constant current i is given by  

Assuming 100% current efficiency for the anodic dissolution and inserting  

The averaged penetration rate Axjt may be calculated from a knowledge of the density of the metal p, again with the assumption of uniform corrosion  

The most common units are mm year (mpy). As an example, consider the corrosion of an iron pipeline by an acid liquor. If the steady corrosion current density is 0.1 mAcm  

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Corrosion problems in practice 509 10.4 CORROSION PROBLEMS IN PRACTICE... 

The complementarity of electrochemical and surface analysis has been described. In principle, both should be accessed regularly in assessing any new corrosion problem in practice, electrochemists and surface scientists are often ignorant of the power of their respective techniques. It is hoped that new microscopic and spectroscopic methods will provide added inducement by their ability to examine the fragile water-filled films that are believed to be so important to pa.ssivation. This could open up a greater dialogue than has been possible in the past. 

It is not always practical or convenient to investigate corrosion problems in the laboratory. In many instances, it is difficult to discover just what the conditions of service are and to reproduce them exactly. This is especially true with processes involving changes in the composition and other characteristics of the solutions as the process is carried out, as, for example, in evaporation, distihation, polymerization, sulfona-tion, or synthesis. 

Most of the corrosion problems in power transmission can be reliably detected and assessed using conventional and well-established corrosion testing instruments, practices, and methods. The data, from field or laboratory, are essential information for the implementation of ameliorative measures. The corrosion control technology available today can, in a cost-effective way, protect reliability, performance, and safety of transmission hnes. 

On the other hand, the electrical conductivity information is essential for several practical applications of the hydro-thermal systems. It is well known, for instance, the use of electrical conductivity measurements on Une to prevent corrosion problems in steam generators and other applications of the steam/water cycle. Tracer diffusion coefficients of electrolytes can be calculated from limiting electrical conductivity and used to predict mass transfer in many geochemical processes and hydrothermal reactions. 

The example offered here as an illustration of the coordinated approach to the understanding of a corrosion process, is, of course, much more extended than is normally practical for 90% of industrial corrosion problems. In particu-... 

In many practical situations, carbonation- and chloride-induced corrosion can occur in tandem. Research studies have shown that corrosion caused by carbonation was intensified with increasing chloride ion concentration, provided that the carbonation rate itself was not retarded by the presence of chlorides. According to these studies, chloride attack and carbonation can act synergistically (the combined damage being more severe than the sum of its parts) and have been responsible for major corrosion problems in hot coastal areas. 

Uniform corrosion can usually be predicted and monitored. Therefore, even though the ultimate consequence associated with this type of failure can be high, it is unlikely to be a serious problem in practice because corrective action can be taken in plenty of time. Localized corrosion is more difficult to monitor and predict—thus it can often present a higher risk. 

Because corrosion is known to result from acid attack on metals, the removal or neutrahzation of acids is an obvious solution to the corrosion problem. In theory, any material sufficiently basic to neutralize the acid and raise the pH to the desired level should be satisfactory. In practice, the situation is complicated by other factors. 

Although many problems still remain to be overcome to make the process practical (not the least of which is the question of the corrosive nature of aqueous HBr and the minimization of formation of any higher brominated methanes), the selective conversion of methane to methyl alcohol without going through syn-gas has promise. Furthermore, the process could be operated in relatively low-capital-demand-ing plants (in contrast to syn-gas production) and in practically any location, making transportation of natural gas from less accessible locations in the form of convenient liquid methyl alcohol possible. 

The process is similar to the catalytic liquid-phase oxidation of ethylene to acetaldehyde. The difference hetween the two processes is the presence of acetic acid. In practice, acetaldehyde is a major coproduct. The mole ratio of acetaldehyde to vinyl acetate can he varied from 0.3 1 to 2.5 1. The liquid-phase process is not used extensively due to corrosion problems and the formation of a fairly wide variety of by-products. 

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