Stress corrosion cracking, consequences

S = Safety what are the consequences of failure If they are serious, a more resistant material than usual may be justified. For example, on a plant where leaking v. ater would react violently with process materials, the water lines were made from a grade of steel resistant to stress corrosion cracking (from the chloride in the cooling water) as well as rust. 

The 18% Ni maraging steels do not display passivity and normally undergo uniform surface attack in the common environments. Of more serious consequence, however, for all high strength steels, is the degree of susceptibility to stress corrosion cracking (s.c.c.). 

Localised corrosion The various forms of localised corrosion are a greater source of concern to the plant designer (and operator) since it is usually difficult to predict an accurate rate of penetration, difficult to monitor, and consequently can be (especially in the case of stress-corrosion cracking) catastrophically rapid and dangerous. 

When a 60 MW turbine at Hinkley A power station disintegrated in 1969 from stress corrosion cracking of a low pressure turbine disc (consequences shown in Plate 1) it was considered that Na H solutions were most probably involved (84) and it was soon found that if NaOH were the sole electrolyte present its maximum concentration (based on vapour pressure depression) was sufficient to have caused the cracking. However, it was also found that in mixtures it was only the free NaOH which led to rapid stress corrosion cracking. Considerations of acid gas solubility and solution thermodynamics showed that at the CO2 and acetate levels present it was most unlikely that free NaOH was present in sufficient quantity to be responsible for the Hinkley failure (85). 

Originally, the reactor failed due to stress corrosion cracking from nitrates. The source of the nitrates was water sprayed from an external hose used for supplemental cooling. The inadequate cooling capacity was resolved with a less than adequate technical solution that caused unexpected and unwanted consequences. Management of change was not properly applied. 

Stainless steels exhibit the best resistance to corrosion when the surface is oxidized to a passive state. This condition can be obtained, at least temporarily, by a so-called passivation operation in which the surface is treated with nitric acid and then rinsed with water. Localized corrosion can occur at places where foreign material collects, such as in scratches, crevices, or comers. Consequently, mars or scratches should be avoided, and the equipment design should specify a minimum of sharp comers, seams, and joints. Stainless steels show great susceptibility to stress corrosion cracking. As one example, stress plus contact with small concentrations of halides can result in failure of the metal wall. 

In metal alloys the combination of stress and environment can also lead to premature failures, indicated as Stress Corrosion Cracking, SCC [1]. The influence of the environment on SCC is generally of a chemical nature a chemical reaction occurs between the metal and the environment. Most of the research published on the ESC of polymers focuses on ESC in which the environment influences the material only physically [2-8]. In such cases the mechanism of ESC is studied and models are established for ESC prediction [9]. These models for physical ESC are based predominantly on the solubility parameters of the considered polymer/environment combination. In other words, ESC is mainly a consequence of polymer softening, i.e. it is a reduction of the interaction between the polymer chains that lowers the yield stress. 

Failures due to stress corrosion cracking, while not common, may occur without warning, with catastrophic consequences. This cracking occurs only if both of the following conditions occur simultaneously (1) a susceptible material and (2) an appropriate combination of stress (tensile stress is required), temperature, crack-inducing agent, pH, aeration, etc. For example, chloride... 

In accordance with these definitions, stress-corrosion cracking has been a familiar problem in the petroleum and chemical industries for decades. Consequently, measures seem to be rather well established and generally known for preventing stress-corrosion cracking or for keeping it in check. Where conditions are such that this type of hydrogen attack can be expected, appropriate supplemental requirements should be included in the specification. A vessel built only to code requirements could be rendered unserviceable in a matter of hours by stress-corrosion cracking. 

Stress corrosion refers to cracking (or breakdown) of a structure due to the simultaneous presence of a tensile stress and a specific corrosive medium. During stress corrosion, the metal/alloy is virtually unattacked over most of the surface while fine cracks propagate through it. Failures due to stress corrosion cracking (SCC) occur with extreme suddenness and can have serious consequences. 

---