SS 316 (SS 316)

SS 316 (AISI Type 316) is a molybdenum-alloyed austenitic stainless steel containing 16–18% chromium, 10–14% nickel, and 2–3% molybdenum. The molybdenum addition is the key compositional difference from SS 304 — it dramatically improves resistance to pitting and crevice corrosion in chloride-bearing environments and to attack from reducing acids such as sulfuric and phosphoric acid. SS 316L is the low-carbon variant (carbon below 0.03%) preferred for welded service because it eliminates carbide precipitation in the heat-affected zone, preventing intergranular corrosion.

In CBG, biogas, and recycling plants, SS 316 is specified wherever corrosion conditions exceed SS 304's capability. The most common applications are hydrogen sulfide removal systems (iron-oxide scrubber vessels, dosing skids, sample lines), raw biogas piping with sulfide-rich condensate, amine scrubber columns and reboilers in upgrading systems, vapour-phase pipework downstream of pyrolysis condensers where acidic condensate forms, liquid digestate handling where chloride levels exceed 200 ppm, and battery recycling acid leach circuits, lithium precipitation reactors, and electrolyte recovery equipment. SS 316 is also the standard for high-purity food and beverage applications where CBG plants sell biogenic CO₂ as a by-product — beverage-grade CO₂ specifications require austenitic 316L throughout.

The cost premium over SS 304 — typically 30–50% for fabricated equipment, with raw plate prices at ₹350–500 per kg versus ₹250–350 per kg — means that SS 316 specification must be justified on corrosion-life-cycle terms rather than across-the-board substitution. Above SS 316, even more aggressive environments require duplex stainless (SAF 2205), super-austenitics (SS 904L), nickel alloys (Hastelloy C-276), or titanium — each with another step in capex. Common mistakes in plant procurement include over-specifying SS 316 for non-corrosive structural applications (wasting capex), and under-specifying SS 304 for sulfide service (causing premature failure and process shutdowns within 12–24 months). A proper corrosion review during basic engineering — identifying chloride levels, temperatures, pH, and sulfide content for every process stream — is the only reliable way to allocate SS 304 versus SS 316 across a plant.