SS 304 (SS 304)

SS 304 (AISI Type 304, also called 18/8 stainless steel for its 18% chromium and 8% nickel content) is the most widely used grade of austenitic stainless steel in industry. Its composition — with carbon below 0.08%, manganese below 2%, and chromium-nickel balance providing a stable austenitic (face-centred cubic) crystal structure — gives it excellent corrosion resistance in most non-chloride environments, good formability and weldability, and a tensile strength of approximately 515 MPa.

In CBG, biogas, and recycling plants, SS 304 is the standard material of construction for the majority of process equipment where mild corrosive conditions prevail. Common applications include digestate storage tanks, mixed-liquor return lines, feed-prep hoppers, separator housings, biogas pipework upstream of upgrading, water-scrubbing system internals, secondary process tanks, conveyors, and walkways/handrails. It is the default choice in food-grade installations such as silica gel beds, downstream biomethane piping at compressor discharge, CBG cascade frames, and dispenser cabinet panels. SS 304 also dominates non-corrosive structural roles — control panel enclosures, electrical conduits in plant areas, and outdoor equipment skid frames.

The principal limitation of SS 304 is chloride and acid resistance. Above approximately 50 ppm chloride concentration combined with temperatures over 60°C, SS 304 is susceptible to chloride stress corrosion cracking and pitting corrosion — invisible internal damage that can fail under pressure without warning. In acidic environments (pH below 5), SS 304 corrodes uniformly at rates that make it unsuitable for service. These limits drive substitution to SS 316 (molybdenum-bearing) for any application where chlorides, acidic condensate, sulfide-rich gas streams, or seawater spray are present — typically the H₂S removal section, raw-biogas piping with condensate, pyrolysis-oil tankage, and battery recycling acid-leach circuits. Pricing favours SS 304 — typically ₹250–350 per kg of fabricated equipment versus ₹350–500 per kg for SS 316 — making the trade-off between material cost and corrosion-driven life-cycle cost the central decision in equipment selection.