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.

Summary

The important information of this chapter is summarized below.

Specialized Corrosion Summary

Pitting corrosion occurs where the anodic site becomes fixed in a small area and the formation of holes in an otherwise unaffected area takes place.

Crevice corrosion is a type of pitting corrosion that occurs specifically within the low flow region of a crevice.

Stress corrosion cracking (SCC) is a type of intergranular attack corrosion that occurs at the grain boundaries under tensile stress.

Pitting corrosion requires two conditions to occur, low flow and areas of both high and low oxygen concentration. When these conditions are met a differential aeration cell is established which produces an electron flow from the point of low oxygen concentration to the area of high oxygen concentration. The difference in oxygen concentration is usually due to a low flow condition.

Pitting corrosion is a hazard due to the possible rapid penetration of the metal with little overall loss of mass. Pitting corrosion is minimized by:

Avoiding stagnant conditions

Using the correct metals and alloys that are less susceptible to the corrosion

Avoiding agents in the medium that cause pitting

Designing the system and components such that no crevices are present

Stress corrosion cracking occurs when three conditions are met; the alloy is susceptible to stress corrosion cracking, the alloy is exposed to specific environment, and the alloy is in a stressed condition.

Chemisorption is the formation of a monomolecular layer of a compound between the metal's surface atoms. This layer separates the metal's atoms thereby weakening the metal and allowing any existing defects to propagate when a stress is applied.

Stress corrosion cracking (SCC) is a great concern due to the hazard that it can readily crack metal of appreciable thickness. The crack can lead to a serious failure of the component, or system, and all the attendant results such as:

Contamination Loss of coolant Loss of pressure

SCC is prevented in reactor system by:

Proper design

Reducing stress levels

Avoiding stagnant areas and crevices in heat exchangers, where chlorides and hydroxides might become concentrated.

Chloride stress corrosion

The three conditions that must be present for chloride stress corrosion to occur are:

Chloride ions present Dissolved oxygen present Metal under tensile stress

Chloride stress corrosion is controlled by maintaining low chloride ion and oxygen content in the environment and the use of low carbon steels.

Caustic stress corrosion

Caustic stress corrosion is similar to chloride stress corrosion. The metals involved are mild steels (steel with low carbon and low alloy content). The metals are exposed to concentrated caustic environments and under tensile stress. The presence of oxygen is not required for cracking to initiate. Concentration of greater than 10,000 ppm is required for cracking to initiate. The level may be higher for different caustic environments.