Corrosion rate continued

For some materials in some environments, it is not possible to form passive films for corrosion protection. In this case, the corrosion rate continues to increase with increasing oxidizing conditions, and satisfactory use of materials of this type depends upon maintaining acceptably low oxidizing conditions and, therefore, acceptably low corrosion... 

Refining and Isomerization. Whatever chlorination process is used, the cmde product is separated by distillation. In successive steps, residual butadiene is stripped for recycle, impurities boiling between butadiene (—5° C) and 3,4-dichloto-l-butene [760-23-6] (123°C) are separated and discarded, the 3,4 isomer is produced, and 1,4 isomers (140—150°C) are separated from higher boiling by-products. Distillation is typically carried out continuously at reduced pressure in corrosion-resistant columns. Ferrous materials are avoided because of catalytic effects of dissolved metal as well as unacceptable corrosion rates. Nickel is satisfactory as long as the process streams are kept extremely dry. 

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 measurements can be made quickly—in a few hours or days, or continuously. This enables sudden increases in corrosion rate to be detected. In some cases, it will be possible then to modify the process to decrease the corrosion. 

Measurements are recorded intermittently or continuously. Changes in the slope of the curve obtained thus yields the corrosion rate. 

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. 

If the amount of metal removal by erosion is significant the surface will probably be continually active. Metal loss will be the additive effect of erosion and active corrosion. Sometimes the erosion rate is higher than that of active corrosion. The material selection judgment can then disregard coirosion and proceed on the basis of erosion resistance provided the corrosion rates of aetive surfaces of the alloys considered are not much different. As an example of magnitudes, a good high-chromium iron may lose metal from erosion only a tenth as fast as do the usual stainless steels. 

Figure 4-469 shows the effect on corrosion rates of 1020 steel in different water systems with dissolved hydrogen sulfide. The difference in corrosion rates is due to different corrosion products formed in different solutions. In solution I, kansite forms. Kansite is widely protective as the pyrrhotite coats the surface giving slightly more protection until a very protective pyrite scale is formed. In solution II, only kansite scale forms, resulting in continued increase in the corrosion rate. Finally, in solution 111, pyrite scale is formed as in solution I however, continued corrosion may be due to the presence of carbon dioxide. 

The limitation of these instruments is that they only indicate overall corrosion rate. Their sensitivity is affected by deposition of corrosion products, mineral scales or accumulation of hydrocarbons. Corrosivity of a system can be measured only if the continuous component of the system is an electrolyte. 

Figure 1.62b shows the result of raising the potential of a corroding metal. As the potential is raised above B, the current/potential relationship is defined by the line BD, the continuation of the local cell anodic polarisation curve, AB. The corrosion rate of an anodically polarised metal can very seldom be related quantitatively by Faraday s law to the external current flowing, Instead, the measured corrosion rate will usually exceed... 

A comprehensive table of corrosion rates in sea water has been compiled by LaQue . This appears to show no obvious dependence of corrosion rates on the geographical location of the testing site, and few of the rates depart widely from an average of 0-11 mm/y. It is suggested that a figure of 0-13 mm/y may be taken as a reasonable estimate of the expected rate of corrosion of steel or iron continuously immersed in sea water under natural conditions, in any part of the world. 

Except for zones below the level of permanent ground water where the environment is water-saturated, and for zones of dry surface sand, continual variation may be expected to occur in the water content of soils. This is usually dependent on rainfall, snow, flooding and such climatic influences, though irrigation practices in many agricultural areas influence water content and hence the corrosion rates. 

From what has been said already, it is clear that determinations of corrosion rates from small-scale experiments must be treated with great caution. If the metal cannot ptissivate, it will corrode until it becomes immune, at which point the corrosion rate will fall to zero between initial exposure and the attainment of immunity the corrosion rate will be continually changing. If, on the other hand, it is impossible for the metal to come to equilibrium... 

If millscale was perfectly adherent, continuous and impermeable, it would form a good protective coating, but in practice millscale soon cracks and flakes off in places. In air, the presence of millscale on the steel may reduce the corrosion rate over comparatively short periods, but over longer periods the rate tends to rise. In water, severe pitting of the steel may occur if large amounts of millscale are present on the surface. 

Fig. 3.50 Corrosion rate versus temperature in de-aerated sea-water continuous test exposure...

Most of the published evidence suggests that marine fouling cover— particularly where it is continuous and well established — reduces corrosion rates of steels . Indeed, 35%o seawater is by no means the most corrosive of saline environments towards steel. Brackish water, as found in estuarine or certain other coastal areas, is considerably more aggressive towards steel, and careful design measures should be taken to ensure that effective corrosion control is achieved in such circumstances. 

Tin is anodic to steel in alkaline solutions, the corrosion rate for a continuous coating being similar to that of pure tin, and tinned articles that are washed in aerated alkaline detergents slowly lose their coating. 

Rowlands and Bentley have provided an account of the possibilities for continuously monitoring corrosion rates by polarisation resistance measurements, and they also describe the development of a commercial instrument, which uses low-frequency square-wave current to polarise the test specimens. 

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