304 often serves urban, low-chloride, rain-washed environments. In coastal salt, deicing salt, pool or damp-crevice service, molybdenum-bearing 316/316L generally offers better pitting/crevice resistance. Grade cannot compensate for water traps, iron contamination, rough welds or improper cleaning.
Values in this article support engineering screening during design review, RFQ and prototyping. They do not replace formal calculations, material data sheets, qualified procedures or contractual standards.
PREN AS A SCREEN
A common pitting-resistance equivalent is PREN=%Cr+3.3×%Mo+16×%N. Typical chemistry puts 304 around 18–20 and 316 around 23–28. PREN screens alloying tendency; it does not define a safe chloride concentration, temperature or service life. Use actual chemistry plus surface condition, temperature, chloride concentration, crevices and washing frequency.
DESIGN AND FABRICATION
1. Design drainage, ventilation and smooth continuous welds; minimize laps and deposits. 2. Separate carbon/stainless tools and abrasives; remove weld scale, heat tint and free iron. 3. ASTM A380/A380M-25 addresses cleaning/descaling/passivation practices; ASTM A967/A967M-25 covers chemical passivation. Passivation is not a painted “rustproof film”; cleaning and chemistry support a stable surface. 4. State product specification and grade on certificates, e.g. ASTM A240/A240M-26 for sheet/plate or the contracted EN/ISO equivalent.
PROJECT CHECKLIST
Chloride source; temperature/time of wetness; washing; crevices; roughness; post-weld treatment; dissimilar metals; certificate.