Corrosion environments, generation

Mechanical property testing. HT/HP vessels have been designed to conduct a variety of mechanical tests, such as slow strain rate (SSR), fracture, or fatigue testing. The main problem is always one of selecting fixtures that can withstand the corrosive environments generated in HT/HP tests. 

Some investigatorshave advocated a type of accelerated test in which the specimens are coupled in turn to a noble metal such as platinum in the corrosive environment and the currents generated in these galvanic couples are used as a measure of the relative corrosion resistance of the metals studied. This method has the defects of other electrolytic means of stimulating anodic corrosion, and, in addition, there is a further distortion of the normal corrosion reactions and processes by reason of the differences between the cathodic polarisation characteristics of the noble metal used as an artificial cathode and those of the cathodic surfaces of the metal in question when it is corroding normally. 

Generation of Corrosive Environments. The materials of construction of a water circuit are, of course, selected to be... 

The presence of specific chemical species in the corrosive environment poisons or retards the rate of the Hads atom combination reaction, thereby permitting a higher fraction of the H atoms generated by corrosion to become absorbed by (enter into) the steel. Bisulfide ions (HS ), formed when H2S molecules are dissolved in water, are very effective H atom combination poisons. Other effective H atom combination poisons are cyanide ions (CN ) and arsenic ions (As3+). 

It is concluded that elimination of the separate catalyst complex phase in the AICI3 alkylation process adds significantly to its attractiveness. In addition to the ease with which a liquid phase catalyst can overcome any poisoning and get back on stream, the new homogeneous process can operate at higher temperatures and recover the heat of reaction to generate steam. It can also operate in a less corrosive environment while producing an ethylbenzene product of exceptional purity and can reduce the amount of aluminum chloride required several fold. 

Commercial success of sulfur-iodine hydrogen production depends largely on the capacity to identify materials of construction that can handle the corrosive environments and on the ability to manufacture process components with these materials economically. This chapter has reviewed a cross-section of materials data generated by testing in the various process settings within the cycle. Ta alloys and SiC have been shown to have good corrosion characteristics in all the liquid environments, but individual candidates have also been suggested for use in particular conditions. Both of these materials have unique mechanical properties, and much effort is needed in order to use them for component fabrication. 

In order to raise the temperature from 450° to about 850°C and to supply the endothermic heat of reduction — 40 kcal/mole), considerable heat must be supplied. Because of materials limitations at this high temperature, the most feasible way to supply the heat of reaction is to generate the heat internally by the combustion of carbon with air. This eliminates the need for heat transfer surfaces operating at high temperatures in a corrosive environment. 

An example is the SCC of stainless steel at 200 °C in a caustic solution or in aerated chloride solution where no traces of dissolution are visible on the crack face. The three conditions, namely, tensile stress, susceptible sample material, and a corrosive environment are the conditions necessary for stress corrosion to take place (73, 90). For instance, SCC of metals has been by far the most prevalent cause of failure of steam generator components in pressurized water reactors (PWRs) to an extent of 69% of all cases, piping in boiling water reactors (59.7%) and PWRs (23.7%). More than 60% of inspected steam turbines in nuclear power plants have disks with stress corrosion cracks (91). 

Because of the complex and often corrosive environments in which the power plants operate, corrosion has been a serious problem. Corrosion continues to be a problem with electrical generators and turbines. Specifically, SCC in steam generators in PWR plants and boiler tube failures in fossil fuel plants continue to be problems. 

The moisture resistance of cyanoacrylate adhesives, like the heat resistance, is poor, and the problem is particularly apparent on metals. A cyanoacrylate bond responds in several ways to a moist environment. Polyalkyl cyanoacrylates are very susceptible to alkaline hydrolysis, which degrades the molecular weight rapidly (see Section II.F.). Corrosion products generated at the interface may catalyze this hydrolysis. Loss of adhesion due to displacement of the adhesive from the adherend by moisture is a common occurrence with many adhesives. Evidence that this mechanism... 

Some specific corrosion environments, in the presence of applied tensile stress on the metal siuface (above some threshold vtilue), can cause stress-corrosion cracking (SCC). The somce of stresses can be extemtil, but residual stresses can tilso cause SCC failures. Specific corrosive pollutants, which may contribute to the SCC of carbon steels, are for example, the carbonates in water. Sulfide stress cracking (SSC) commonly occms on the outside of a pipe where sulfate-reducing bacteria me present at a soil pH of 3-7. Because of the produced iron sulfide (FeS), which adsorbs readily on the steel surface, hydrogen atoms generated... 

The large variety of corrosive environments is unique for the steam generation industry. This is because the pressure around the steam cycle chtmges from vacuum to hundreds of atmospheres (thousands of psi) and the temperature from ambient to over 600°C (1100°F). This changes the aqueous environment from low-temperature water to high-temperature pressurized water, to wet, saturated, and superheated steam. Concentrations of impurities in these... 

The most severe corrosive environments are encountered in the areas where corrosive impurities concentrate. These impurities can concentrate on heat transfer surfaces such as in boiler and steam generator tubes where the surface temperature is above the saturation temperature of the boding water, and on the surfaces in superheated and saturated steam where the impurities concentrate by precipitation from a steam solution and deposition [4,5,11,14]. Figure 6 shows the gradient of concentration of sodium hydroxide at the surface of a boiler tube. 

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