Effect on corrosion rate

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. 

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. 

Variation of pH Between 4 and 8, variation of pH does not appear to have a very significant effect on corrosion rate in de-aerated water. 

Passivation of the metal and the associated sharp decline of its anodic dissolntion rate have a strong effect on corrosion rates (curve 5 and the point of intersection C in Fig. 22.2b). Passivation is encountered more often under the effect of oxidizing agents (e.g., in the presence of oxygen). 

If the definition of TOW established by ISO is used (TOW-ISO), a linear relationship between time and TOW is obtained, in spite of the different possible changes in corrosion rate caused by changes in the nature of TOW. It has to be remarked that it is not the same effect on corrosion rate caused by a heavy rain than dew, fog or water adsorption, so for the same interval of relative humidity (80-100%), notable changes in nature of TOW-ISO and consequently in corrosion rate could take place. 

Figure 2 illustrates corrosion behavior and oxygen solubility as a function of the salinity of the solution at a fixed temperature. It is to be noted that a small variation in salinity in the neighborhood of the normal 3.5% content of sea water has no effect on oxygen solubility and little effect on corrosion rate. 

Sulfide inclusions have been found to act as initiation sites for pitting corrosion of mild steels in neutral-pH solutions [45, 46]. On the other hand, sulfur content has been found to have no significant effect on corrosion rates in adds of steels containing more than 0.01% Cu [42]. 

The presence of 5% bovine serum in lactated Ringer s solution (pH=6.5) increases the corrosion rate of Ti-6A1-4V alloy, as shown in the last two columns of Table 9.13 (Lewis and Daigle, 1993b). This table gives data obtained by direct current (d.c.) and alternating current (a.c.) methods. The difference between d.c. and a.c. corrosion rates found in this system is not unusual. The same table also shows that decreasing the pH of lactated Ringer s solution to 1 has a dramatic effect on corrosion rate. 

Windspeed can have a dual effect on corrosion rates. With no rain, a high windspeed can have a drying effect and corrosion rates will decrease, and combined with rain it will have a large washing effect again lowering corrosion rates. On the other hand, increasing wind speeds could enhance the rate of supply of SO2 per unit area. 

Mass transport-controlled corrosion impHes that the rate of corrosion is dependent on the convective mass transfer processes at the metal/fluid interface. Mass transfer can have a significant effect on corrosion rates of metals and alloys depending on factors such as bulk solution chemistry, temperature, flow conditions, surface roughness, and geometry. 

Under immersed conditions in natural waters, in neutral or alkaline solutions, or when buried in soil, the chemical composition of carbon, HSLA, and alloy steels has little effect on corrosion rates. In acid solutions, however, the composition can have a pronounced effect on corrosion. Rates of corrosion in acid increase with increasing carbon and nitrogen content of the steel. Sulfur and phosphorus... 

Even corrosion tests carried out in controlled conditions often yield results with more scatter than many other types of materials test results because of a variety of factors, an important one being the effect on corrosion rates due to minor impurities in the materials... 

It has long been recognized that surface finish (e.g., grinding, polishing) has a strong effect on corrosion rate [50,51]. In SCW, preparations that give a strain-free... 

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