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 Caustic Stress Corrosion Cracking

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Caustic stress corrosion

 

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 corrosion 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.

 

 

this article is taken from: DOE FUNDAMENTALS HANDBOOK CHEMISTRY Volume 1 of 2 - DOE-HDBK-1015/1-93 JANUARY 1993

 

see also:

 

corrosion types

 

pitting crevice corrosion

 

preboiler and boiler corrosion (ext link)