Why is my 316L mesh pitting in brine or chloride service?

The symptom

Small perforations and rust-brown spots appear on stainless mesh or filter elements. In warm chloride streams this can occur within months. Attack is often concentrated at crevices, clamped edges and welds while the surrounding material still looks bright.

Root causes

1. 01 The chloride exposure may exceed the resistance of 316L. Higher temperatures can make pitting more likely.
2. 02 Crevice corrosion can develop at overlaps, gaskets and clamped edges. Chlorides may concentrate and oxygen may become depleted in the gap.
3. 03 Weld zones without appropriate post-treatment may be vulnerable due to heat tint and local sensitisation.
4. 04 Chloride-bearing liquid left in the system during shutdown can promote local attack.

Suitable filter media

• ▸ Assess alternative alloys against the full operating conditions. Options include Duplex for strength and chloride resistance, Monel™ for brine and seawater environments, and Hastelloy™ for more aggressive chemical media.
• ▸ Reduce crevices with continuous welds, suitable edge finishing and gaskets that avoid stagnant gaps.
• ▸ Include weld treatment, such as pickling and passivation, in the fabrication specification. Record material and treatment requirements in the required documentation.
• ▸ If changing alloy across the whole plant is too costly, assess targeted replacement of wetted, hot or crevice-prone components.

Prevention

• ⊕ Include chloride level, temperature and pH in the enquiry. These data are needed to assess the material.
• ⊕ Flush and drain chloride-bearing systems before shutdown to limit retained liquid.
• ⊕ Inspect crevices and welds first at each maintenance stop, as local attack may begin there.