Concentration effect on corrosion

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. 

Reducing pH usually has a beneficial effect on corrosion caused by alkaline substances. However, this seemingly obvious solution has a number of drawbacks. Chemical treatment programs work most effectively in certain pH ranges. Decidedly acidic waters can cause corrosion problems as bad or worse, albeit different, than those caused by alkaline waters. Finally, if concentration mechanisms such as evaporation or condensation are present, merely decreasing pH may prove ineffective in controlling attack. 

Variations in solution composition throughout a test should be monitored and, if appropriate, corrected. Variations may occur as a result of reactions of one or more of the constituents of the solution with the test specimen, the atmosphere or the test vessel. Thus, it is important that the composition of the testing solution is what it is supposed to be. Carefully made-up solutions of pure chemicals may not act in the same way as nominally similar solutions encountered in practice, which may, and usually do, contain other compounds or impurities that may have major effects on corrosion. This applies particularly to artificial sea-water, which is usually less corrosive than natural sea-water. This subject is discussed in detail in a Special Technical Publication of ASTM, and tests with natural, transported and artificial sea-water have been described . Suspected impurities may be added to the pure solutions in appropriate concentrations or, better still, the testing solutions may be taken directly from plant processes whenever this is practical. 

Figure 7.16 is the polarization curves of the pyrite electrode in dithiocarbamate solution at different concentration for dipping for 48 hours. Electrochemistry parameters determined by the computer PARcal are listed in Table 7.3. It can be seen from Fig. 7.16 and Table 7.3 that the corrosive potential of pyrite electrode decreases gradually from 187 to 160 mV and the corrosive current decreases from 10.78 to 6.01 xA/cm without or with the DDTC addition of 5 x 10 mol/L, while polarization resistance increases from 6.2 to 10.1 kfl with the increase of dithiocarbamate concentration. It indicates the formation of surface oxidation products. Comparing with xanthate, DDTC has less effect on corrosive potential, current and polarization resistance. It indicates that collector function of DDTC on pyrite is less than that of xanthate. 

Production of differential aeration cell. A scatter of individual barnacles on a stainless steel surface creates oxygen concentration cells. The formation of biofilm generates several critical conditions for corrosion initiation. Uncovered areas will have free access to oxygen and act as cathodes, while the covered zones act as anodes. Underdeposit corrosion (crevice corrosion) or pitting can occur. Depending on the oxidizing capacity of the bacteria and the chloride ion concentration, the corrosion rate can be accelerated. However, the presence of a biofilm does not necessarily mean that there will always be a significant effect on corrosion. (Dexter)5... 

Summary of RSP data. Published data relating to R additions indicate both positive or negative effects on corrosion resistance. Therefore, it is difficult to draw conclusions as to what conditions produce these effects simply because many of the studies have lacked a systematic approach to the variation of important variables such as type and concentration of R addition. Furthermore, in most studies control data for pure or commercially pure Mg were never determined. In some cases commercial alloys with no R content were used for comparison, but these alloys were usually complex ternary or quaternary alloys with very different basic concentrations and methods of production. It is therefore difficult to know what constitutes a low or base corrosion rate. 

Figure 4.14 Effects of pH (left) and dissolved oxygen concentration (right) on corrosion in SCW.

Chlorides have probably received the most study in relation to their effect on corrosion. Like other ions, they increase the electrical conductivity of the water, so that the flow of corrosion currents will be facilitated. They also reduce the effectiveness of natural protective films, which may be permeable to small ions. Nitrate is very similar to chloride in its effects but is usually present in much smaller concentrations. Sulfate in general appears to behave very similarly, at least on carbon steel materials. In practice, high-sulfate waters may attack concrete, and the performance of some inhibitors appears to be adversely affected by the presence of sulfate. Sulfates have also a special role in bacterial corrosion under anaerobic conditions. 

As illustrated by Eq. (80), the pressure effects on corrosion reaction rate can be attributed to the impact of pressure on the activation process and on volume concentration of the aggressive species. " " The volumetric concentrations (mol/1 of the solution) of the aggressive species, Ch+, in Eq. (80) is density-dependent and can be expressed in terms of the molal (mol/kg of solvent) concentration by... 

Effect of concentration. In general, corrosion-erosion by slurries has been observed to increase with concentration [115-116], and roughly is indicated to be directly proportional to concentration, in concentration ranges of reactor interest. As concentration is increased, the effect of rheological properties on flow characteristics becomes more pronounced, and the effect on corrosion would become altered. 

Ammonia is a strong local irritant which also has a corrosive effect on the eyes and the membranes of the pulmonary system. Vapor concentrations of 10,000 ppm are mildly irritating to the skin, whereas 30,000 ppm may cause bums. The physiological effects from inhalation are described in Table 16. Prolonged, intentional exposure to high levels of ammonia is unlikely because its characteristic odor can be detected at levels as low as 1 —5 ppm (94). The real danger occurs when escape is impossible, or the exposure victim has lost consciousness. 

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