Steam generation corrosive environments

Stress corrosion cracking, prevalent where boiling occurs, concentrates corrosion products and impurity chemicals, namely in the deep tubesheet crevices on the hot side of the steam generator and under deposits above the tubesheet. The cracking growth rates increase rapidly at both high and low pH. Either of these environments can exist depending on the type of chemical species present. 

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. 

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. 

D. Choi, G.S. Was, Pit growth in AUoy 600/690 steam generator tubes in simulated concentrated environments (cupric, chloride, and sulfate). Corrosion 146 (1990) 100-111. 

Modified 9 Cr-lMo (T91) has been chosen for steam generators because of its satisfactory strength at high temperature, freedom from stress corrosion cracking (problem with stainless steels both for chloride and caustic environment) and risk of decarburisaiton (problem with 2.25 Cr-lMo). 

Even with advances in alloy development technologies, it still took almost a quarter of a century to improve the nickel-containing alloys of the 600 family of stainless steels to the more corrosion-resistant versions (e.g., alloys 690,22,59, and 2000) that are enabling many applications in extremely aggressive environments found in the chemical industries, nuclear reactors, steam generators, and sour oil and gas production. 

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. 

Although nickel alloys on the whole offer a better resistance to stress corrosion cracking over the steels, their application in high temperature chloride or alkaline environment and hydrogen sulfide environment may put them to the risk of stress corrosion cracking. Cases of SCC have been observed in high temperature pressurized water in steam generating turbines. Incoloy 800... 

Another problem in the operation of steam generators in a radioactive environment is the generation of radiolytic oxygen in the boiler water. This oxygen is stripped very rapidly by the steam, which contains about 2 ppm of oxygen. Hydrogen is released at the same time. The corrosivity of this mixture is not yet known, but it can be controlled by the use of inhibitors and by proper selection of materials for use in thin metal sections where pitting attack is undesirable. 

R.N. Parkins, G.P Marsh, and J.T. Evans, Strain rate effects in environment sensitive fracture, in Proceedings of EPRI Conference on Predictive Methods for Assessing Corrosion Damage to B WR Piping and PWR Steam Generators, Mt. Fuji, Japan, May/June 1978, Eds. H. Okada and R. Staehle, Pub. NACE, 1982. 

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