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Stress Corrosion Cracking

Stress-corrosion-cracking (SCC) is a well-defined failure mechanism in metals. In order to have SCC, three conditions must be present: susceptible material, environment, and enough tensile stress. When an alloy is stressed in tension below the yield point and simultaneously exposed to a corrosive environment to which the metal is sensitive, SCC failures occur over a period of time. SCC failures are brittle in nature and are referenced by the type of environment and susceptible material. Some examples are cracking of high-strength low-alloy steels in hydrogen-enhanced environment known as hydrogen embrittlement (hydrogen damage is not to be confused with hydrogen embrittlement); cracking of brass in an ammonia environment, known as season cracking; cracking of austenitic stainless steels in a chloride environment, known as chloride stress-corrosion cracking; and cracking of carbon steels in a strong alkali environment, known as caustic cracking or embrittlement. SCC can be either intergranular (along the grain boundaries) or transgranular (across the grains) or both depending on the conditions. If a crack is induced by the brittle film in the ductile material then the crack will be arrested by ductile blunting. The film reinitiates by the corrosive species and propagates by the residual or applied stresses and this process repeats, resulting in transgranular cracking. The austenitic stainless steels and austenitic materials containing a large amount of Ni are resistant to hydrogen embrittlement resulting from cathodic reactions due to a lower diffusion coefficient and higher ductility. But austenitic stainless steels are susceptible to chloride SCC at room temperature and severe at elevated temperatures. Ferritic carbon steels are susceptible to caustic SCC when exposed to hydroxides. 

Whether these SCC cracks are intergranular or transgranular with branching, they always exhibit brittle and thick-edged failures without any significant plastic deformation. Crack orientation can be circumferential or longitudinal depending on the direction of the tensile stress.  Most cracks initiate on the internal surface, but some cracks have been initiated from the outside diameter due to exposure to chlorides, nitrates, sulfates, or hydroxides. It is common for grains to be missing at the surface where the SCC initiated.  

Probable cause
For stress corrosion cracking to occur a synergistic interaction of a tensile stress and a specific corroding species to which the metal is sensitive must be present. The root cause of stress corrosion cracking can be verified by determining the source of the corrosive materials and the reason for the threshold tensile stress.  Internal surface corrosive materials are usually chlorides or hydroxides. The stress level in the tube can be estimated by stress analysis, while the susceptibility of the tube material can be established by testing the material according to ASTM Standard practices.  Residual stresses in bends introduced during the forming operation are sometimes involved.

Corrective Action
As mentioned earlier, the factors affecting SCC are susceptible material, the right environment and tensile stresses. Controlling SCC may start with the design process when material is selected for the operating environment. Selection of the right material which is not susceptible to SCC, and the fabrication process plays a very vital role in controlling SCC. Also, we have to monitor the processes and components (desuperheaters/attemperator sprays) to avoid catastrophic failures. The other requirement for SCC is the presence of tensile stresses. By eliminating or reducing the tensile stresses, we could eventually reduce the risk of SCC; but this is not always feasible. SCC failures resulting from residual stresses (cold working or welding) can be eliminated by the appropriate annealing process. However, stress relieving in 300-series austenitic stainless steels may not provide satisfactory results because of the formation of undesirable changes in the steel such as sensitization and sigma phase. A very high annealing temperature is required to counteract those undesirables. Careful attention is required when stress-relief annealing is performed on large structures. This may induce very high residual stresses in the newer regions if not done properly. Controlling the environment, including adding corrosion inhibitors or isolating the susceptible material with coatings, may give satisfactory results. Chlorides or hydroxides are the primary susceptible species for SCC in boiler tubes. Austenitic stainless steels are susceptible to chloride and NaOH and ferritic steels to NaOH. These can introduce from the carryover of volatile chemicals from water walls to the high temperature superheaters/reheaters or desuperheater/attemperator sprays or contamination by chemical cleaning. Close monitoring of feedwater treatment is required to reduce the risk of SCC in boilers. 
Common Locations
Stress corrosion cracking can be found in areas such as Superheaters, Reheaters, and water tubes.  Often times stress corrosion cracking is hard to identify with the naked eye unless a failure has occurred. Failures typically occur in austenitic stainless; however, ferritic steels are also susceptible to SCC. 

             Convection Pass
             Water walls