Beware of highly stressed components working in a corrosive environment
The combined influences of tensile stress and a corrosive environment can lead to catastrophic failure of susceptible materials by stress corrosion cracking mechanisms (SCC). Often these failures occur after relatively short periods in operation without warning, but with proper understanding and care, SCC can be avoided.
Stress Corrosion Cracking is a material failure mechanism that causes cracking in susceptible material under the combined influence of tensile stress and a corrosive environment. Besides stress, the environment and material susceptibility variables that effect susceptibility include temperature, solution composition, metal composition (exact composition including tramp elements) and microstructure. SCC manifests in many metals but is typically highly dependent on the particular environment (i.e. some metals show resistance in certain environments that cause cracking in others). For example stainless steels are susceptible to SCC in chloride environments but not in ammonia containing environments, while brasses are not susceptible to SCC in chloride environments but are susceptible in ammonia containing environments. Although SCC is generally limited to metals, it has been known to occur in components manufactured from both ceramics and polymers.
Failure by SCC is frequently encountered in seemingly mild chemical environments and at tensile stress levels well below the yield strength of the metal. The failures often take the form of fine cracks that penetrate a significant distance into the metal, with little or no evidence of corrosion on the nearby surface. The specific environment in which SCC occurs is of crucial importance, where only very small concentrations of the material specific chemicals are needed to produce catastrophic cracking, often leading to devastating failure.
The site of initiation of SCC may be sub microscopic and determined by local variances in metal composition, thickness of the protective film, concentration of corrodent, stress concentration and/or residual stress. The role of the tensile stress is important in that it causes the rupture of protective films during both initiation and propagation of cracks. Specific high concentration environments can occur in crevices where ions such as chlorides can concentrate sufficiently to cause SCC.
SCC crack paths can be either transgranular or intergranular (following grain boundaries), with crack paths dependent on both the microstructure of the metal and the environment. Typically, stress-corrosion cracks exhibit extensive crack branching and proceed in a general direction perpendicular to the stresses contributing to their initiation. However this branching is not always observed, with relatively straight crack propagation paths also being possible, even within the same component.
The mechanism causing stress corrosion cracking is not fully understood and many theories to explain SCC have been proposed. One of the more dominant theories is that SCC is caused by specific species in the environment, being absorbed into, and interacting with, the strained (and hence stressed) material at the tip of a crack or corrosion pit and reducing bond strength at the tip which promotes further crack advance and ultimately failure.
The removal of, or change to any of the factors which drive SCC (i.e. the presence of a local tensile stress, isolation of the material from the environment, changes in material selection to avoid SCC susceptible materials, a reduction in temperature) typically reduces the potential for SCC and are therefore an obvious way of controlling SCC in practice. Material selection and the specification of appropriate materials at the design stage is generally the first line of defence. Lowering of the applied stresses and elimination of tensile residual stresses (or the superposition of a compressive stress field on tensile service stresses) can be used to prevent SCC. Unfortunately the stress threshold at which SCC occurs is dependent on both the material and the environment in question and can vary from as low as 10% of yield to up to 70% of yield stress. Exclusion of the environment (e.g. by coating) or minor changes/chemical additions to the environment can help. The application of cathodic protection has also proven effective in preventing SCC.
Stress-corrosion cracking is a severe and highly undesirable failure mechanism, but with proper identification and understanding failures can be avoided. Unfortunately, misunderstanding of potential effects resulting from sub-standard operation often causes very costly failure and consequent losses.
Published in Technical Tips by Origen Engineering Solutions on 1 May 2017