room temperature with a variety of metals and alloys, including carbon steels. It becomes increasingly more corrosive with increasing temperature and concentration. The useful safe limit of carbon steel is approximately 150oF/65oC, both with regard to caustic stress corrosion cracking (CSCC) and corrosion. Stainless steels are more resistant to general corrosion compared with carbon steel; however, they can suffer CSCC at approximately 250oF/121oC.

As a general rule, the resistance to caustic solutions increases with increasing nickel content. Susceptibility to caustic SCC is dependent on several variables, including alloy content, caustic concentration, temperature and stress level. As with other cracking mechanisms, there is a threshold stress level where cracking will not occur; unfortunately, the threshold level for the high nickel alloys in high-temperature caustic has not been determined precisely. Much data has been obtained on alloy 600 in caustic environments because of its extensive use as steam generator tubing in pressure water reactors (PWR). Alloy 200 (pure nickel) is considered to be immune to all but the most severe caustic environments, including molten caustic. Literature corrosion data for other nickel-based alloys is more difficult to find. This is partly due to the fact that many nickel-based alloys, (i.e., alloys 625, C-276, B-2), contain significant quantities of molybdenum for resistance to aggressive acid solutions. As the molybdenum containing nickel-based alloys are more expensive than comparable nickel content alloys without molybdenum (alloy 600), and molybdenum does not significantly contribute to caustic resistance, they have not been studied much.Another difficulty with ranking alloys for caustic service based strictly on nickel content is the dual problem one has with caustic, i.e., it can cause general corrosion and well as SCC. Also, depending on caustic concentration, temperature and other environmental factors, including whether oxygen is present or not, ranking of alloys can change. This is true of alloys 800 and 600.

Caustic stress corrosion, or caustic embrittlement, is another form of intergranular corrosion cracking. The mechanism is similar to that of chloride stress corrosion. Mild steels (steels with low carbon and low alloy content) and stainless steels will crack if they are exposed to concentrated caustic (high pH) environments with the metal under a tensile stress. In stress cracking that is induced by a caustic environment, the presence of dissolved oxygen is not necessary for the cracking to occur.

Caustic stress corrosion cracking was first encountered in the operation of riveted steam boilers. These boilers were found to fail on occasion along riveted seams. Failure was attributed to caustic-induced cracking at the highly stressed regions near and under the rivets. Boiler water could easily flow into the crevices which existed under the rivets.

Radiative heating would cause the water in the crevices to boil. As steam was formed, it would escape from the crevice. More boiler water would then flow into the crevice, boil, and pass from the crevice as steam. The net result of this continuing process was concentration of caustic under the rivet. The combination of high stress and high caustic concentrations eventually led to destructive cracking of the boiler vessel.

Where the rate of steam generation (boiling) is high, it is more difficult to eliminate the problem of solute concentration in regions of the boiler. Caustic stress corrosion may concentrate in such regions as the water evaporates rapidly, but sufficient concentration of caustic by such a mechanism to induce stress cracking is considered unlikely.

Available data indicates that caustic concentrations greater than 10,000 ppm, and probably up to 50,000 ppm, are required to induce caustic stress cracking (40,000 ppm NaOH is equivalent to 40 grams per liter or 1 mole per liter). The pH of such a solution is on the order of 14. An alkaline environment is produced and controlled by use of a solution having some properties of a buffer, that is, one that tends to retard or slow a reaction or tends to force it in one direction or the other.

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