The subject of stress corrosion cracking (SCC) may be an old one, but remains one of the most important considerations today for designers, materials engineers, operators and scientists in industry since it’s discovery in the late 1800’s.
SCC is a critical consideration in relation to the reliability of many applications during the design stages. However, it is in the longer term over an asset’s life cycle that the interaction between the materials, stresses and environments combine to potentially cause this phenomenon. It is safe to say though that not all materials sustain SCC in every environment, but many common engineering materials are subjected to SCC in many environments common in industry, thus it does not require extreme conditions to occur just their individual ‘sweet spot’ between these factors.
Predominantly in materials of high toughness and high strength, it can occur in both brittle and ductile materials. SCC occurs almost always due to residual stresses, not applied stresses from the design calculations of the equipment. SCC will generally not display any gross degradation or deformation on the surface, the material often appears visually still shiny or metallic. Although SCC will propagate more readily at areas of existing defects or pits, it often initiates at surfaces that appear smooth and defect free. It is therefore imperative that effective methods are continually developed to monitor, predict and manage SCC to ensure long term reliable asset service.
To understand how we can deal with SCC in industry it is essential that we understand the mechanisms, materials, environments and testing methods to predict and effectively manage the many common forms of SCC we see such as Ammonia, Caustic, Chloride, Polythionic Acid and Sulphide. These different forms of SCC vary slightly in practice but they are all environment assisted damage mechanisms.
One of the most common we see is in 300 series stainless steels displaying Chloride SCC (CSCC). The synergy of tensile stress, temperature, low pH and an aqueous chloride environment can lead to cracks that are multi-branched, often transgranular but can be intergranular in sensitised material (material that has seen operating temperatures between ~425°C and 815°C and/or e.g. weld HAZ). Sensitisation results in chromium depletion along grain boundaries leading to a chromium reduction from 18/20% to ~13% (Figure.1). It is also a myth that the CSCC threshold is 60°C, CSCC can and does occur at ambient temperature, though the rate is usually low ~0.6 mm per year. It can be common under wet insulation and non-stress relieved welds.
Cracks in the early stages can be difficult to detect using conventional NDT methods, many types of SCC will be found on the process surface of the asset as very fine branched cracking (Figure.2). Although challenging, they can sometimes be detected by eddy-current and phased array ultrasonic testing. External testing is best done visually or with dye penetrant testing, but even still it’s likely to only show ~10% of the issue if found as it will likely be filled with oxides.
In summary there are vast SCC databases regarding susceptible material and environmental combinations, however we still see too often examples of SCC failures (Figure. 3). Ageing assets containing older SCC susceptible materials that are non-economical to replace, unpredicted detrimental environmental conditions over time, applied and residual stresses underestimated, environment and load variables during start up and shut down procedures and unplanned operational changes can all contribute to SCC. It is difficult to detect in the early stages and ultimately mitigation is the best form of defence. SCC unfortunately often occurs when no one expected it would, but sometimes doesn’t occur when conditions indicate it should.
For more information about AXIOM Materials Services, contact Adam Lawther, Materials Engineering Lead at adam.lawther@axiom-ltd.com
Published in Chemicals Northwest Elements Magazine