Stainless is chosen for corrosion resistance, but the grade must match the chloride exposure or the part corrodes anyway — sometimes faster than a galvanized carbon steel part would have.
The grades in construction
A2 (304): the general-purpose austenitic grade. Good in rural, urban and most indoor environments, and in fresh water. It pits in chloride-rich conditions — coastal atmospheres, de-icing salts, marine spray.
A4 (316): adds molybdenum, which substantially improves pitting and crevice corrosion resistance in chloride environments. The standard choice for coastal and marine exposure and for chemical or water treatment plants.
DUPLEX AND SUPER DUPLEX: higher strength and better chloride resistance again, used for high-load marine applications and where stress corrosion cracking is a risk. Higher cost and less available in small fasteners.
Why stainless still corrodes
THREE conditions defeat any stainless grade: chloride concentration above what the grade tolerates, oxygen starvation in crevices (under a washer or in a thread root), and elevated temperature combined with chlorides.
CONTAMINATION is the fourth and most common: carbon steel particles embedded in a stainless surface — from a wire brush, a grinding wheel, or a carbon steel workbench — create local galvanic cells. The stainless stays intact and the embedded particle rusts, staining the surface and sometimes initiating pitting. Site rule: dedicated stainless tools, and passivation after any mechanical work.
The galling problem
Austenitic stainless threads gall: under pressure, the oxide layer breaks and the two surfaces cold-weld, seizing the joint. It happens most on new, clean threads and at high tightening speed. It is the single most common complaint about stainless fasteners.
Practical mitigations: use a lubricant or anti-seize formulated for stainless, slow the tightening, avoid power tools on stainless threads, and consider using a stainless nut on a stainless bolt only where necessary — a coated carbon steel nut is sometimes acceptable if the nut is not exposed.
Selection guidance
Indoor, dry, non-chloride: A2 (304) is sufficient and cheaper.
Coastal, marine, de-icing salt, swimming pool atmosphere: A4 (316) as a minimum.
Permanent embedment in chloride environment, or high load with chlorides: duplex or a non-metallic alternative, and specify the surface condition (pickled and passivated).
Galvanized carbon steel remains the economic choice for large volumes of permanent embedment in non-chloride environments.
Specification points
State the grade by standard designation (A2-70, A4-80 and the equivalent ISO class), the surface condition, and whether passivation is required. For safety-related applications, state the strength class explicitly: stainless fasteners are not automatically stronger than carbon steel, and a common error is replacing an 8.8 carbon steel part with an A2-70 part of lower strength.