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Zinc-plated or stainless steel screws: a cost, corrosion and application comparison

2026-09-26

Compiled by JIALI Industrial (164.hk) — a Hong Kong fastener supplier specialising in screws, bolts, nuts and selection know-how.

Why the same M6x30 bolt differs threefold in price — and what choosing wrong costs

Zinc-plated screws or stainless steel screws is the most frequent question in fastener purchasing: the same M6x30 hex bolt differs 2-3x in unit price between zinc-plated grade 8.8 and 304, and choosing the wrong direction costs far more than the difference. Going cheap: a standard electro-plated bolt left outdoors in the rain starts showing red rust within 3-6 months, fails acceptance and costs ten times its own price in labour to replace. Going premium: switching the whole order to stainless looks safe, but A2-70 (ISO 3506) has a yield strength of only about 450 MPa — roughly seventy percent of grade 8.8 (640 MPa per GB/T 3098.1) — so a joint tightened to the original preload can work loose or leak. There is also a common conceptual mix-up: thickening the coating is not the same as changing the material. Our earlier surface-treatment article covered coatings on carbon steel — zinc plating, Dacromet and hot-dip galvanizing. This article answers the upstream question: which route should the screw take at all, coated carbon steel or stainless steel? Get the route wrong and no coating choice will rescue it; get it right and the coating and grade decisions that follow actually mean something.

Four dimensions compared: cost, corrosion, strength and temperature

Cost: unit prices for the same size sit roughly at 1 : 2.5-3.5 : 4-5 for zinc-plated 8.8, 304 and 316. Compare over the life cycle, though — outdoors, plated parts are replaced every few years while stainless is fitted once, so with labour included the gap closes within about two years. Corrosion: electro-zinc per GB/T 5267.1 carries only 5-8 micrometres of coating and shows white then red rust within 48-96 hours of neutral salt spray (ISO 9227); hot-dip galvanizing per GB/T 13912 carries 45-85 micrometres, passes 480 hours and serves 10-20 years in atmospheric duty; 304/316 resist corrosion through the material itself (ISO 3506) and easily last 10-plus years in normal atmospheres, but coastal chlorides and chlorine-based cleaners pit 304, calling for 316. Strength: grade 8.8 carbon steel reaches 800 MPa tensile / 640 MPa yield, and 10.9 reaches 1040 / 940; stainless runs a class softer — A2-70 is 700 / about 450 MPa and only A4-80 reaches 800 / 600 — so high-preload flanges and shear-loaded structures still belong to high-strength carbon steel. Temperature and appearance: stainless handles heat, resists oxidation and keeps its finish; zinc coatings surrender above about 200 degC (Dacromet holds to roughly 300 degC), so visible decorative faces favour stainless. Run the four dimensions and the route usually decides itself: tight budget, benign environment and high load point to plated carbon steel; harsh or hard-to-reach environments point to stainless.

Application tiers, incoming inspection and three pitfalls

Map the application to four tiers. Dry indoor use, furniture, appliances and the inside of cabinets: zinc-plated screws with white or colour passivation per GB/T 5267.1 give the best value. Outdoor rain, garden structures and railings: hot-dip galvanized or Dacromet carbon steel, or 304 when the budget allows. Coastal sites, docks, pools and regularly washed equipment: start at 304 and move straight to 316 where chlorides are present. Chemical, marine and food equipment: 316 or 2205 with EN 10204 3.1 certificates. Regardless of environment, keep 8.8 / 10.9 carbon steel with locking measures for high-load vibrating structural joints (wind turbines, bridges, tooling), adding Dacromet where corrosion protection is also needed. Inspect incoming goods along two lines: measure coating thickness on plated parts against the standard minimum and check the salt-spray report; on stainless screws confirm the head marking A2-70 / A4-70 matches the material certificate. Avoid three pitfalls. Slight magnetism on 304 after cold work is normal, not evidence of fake material — our material-basics article explains why. White rust on plated parts is usually storage damp on the oxide layer and the base metal is still sound; red rust is the rejection criterion. And pairing carbon-steel plated screws with aluminium structures or stainless plates sets up galvanic corrosion that rings the hole with rust — match materials or isolate with insulating washers. Unsure which route fits? Send JIALI Industrial (164.hk) your environment, load and budget for a material-plus-coating recommendation and quotation.

Related Reading

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• JIALI launches Stainless Steel zone with full 304 / 316 range

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