Answer
In a spiral ring, the multiple turns are in close contact, creating a capillary space. In chemical processing, fluids can be drawn into this space and become trapped. If the fluid is corrosive, it leads to 'crevice corrosion,' where the lack of oxygen prevents the reformation of the passivating oxide layer on stainless steel. This causes rapid pitting between the turns. Over time, the pits reduce the effective cross-sectional area of the ring, leading to a sudden shear failure under a normal thrust load. Diagnosis shows deep pits or 'tunneling' between the turns while the outer surfaces may look clean. Mitigation requires using highly corrosion-resistant alloys like Hastelloy or ensuring the rings are thoroughly passivated and perhaps coated with a PTFE-corrosion-inhibiting polymer.
