How to Choose Stainless Steel Fasteners: A2 vs A4, Property Class and Galling

In the ISO 3506 system, A2 is the austenitic stainless steel known as 304 or 18-8, with roughly 18 percent chromium and 8 percent nickel. A4 is the 316 steel, which has the same base but adds 2 to 3 percent molybdenum. That molybdenum is what resists chlorides, so A4 is the grade for marine, coastal, de-icing salt, swimming pool and chemical environments, while A2 is sufficient for most indoor, food, architectural and general outdoor use.

A2 will rust in salt water over time, usually as brown surface stains and pits, and in a chlorinated pool atmosphere it can fail within months. If you are unsure, use A4 for anything within a few kilometres of the sea, or in contact with chlorides, and A2 elsewhere. A3 and A5 are stabilised versions used for welded or high-temperature use and are less common. For a longer comparison read 304 vs 316 stainless steel fasteners.

Stainless fasteners carry a two-part marking, for example A2-70. The number is the strength class: 50 is soft (500 MPa tensile, normally for nuts and parts that are not highly loaded), 70 is cold-worked (700 MPa tensile) and is the normal class for bolts and screws, and 80 is more strongly cold-worked (800 MPa) for higher loads. Class 70 is the most common and the easiest to find in standard sizes.

Even class 80 is lower in strength than a carbon steel 8.8 bolt in yield terms, so a stainless bolt should not replace a high-tensile one in a load-bearing joint without recalculating. Stainless steel also creeps and relaxes more under load, so preload is lost sooner. The carbon-steel classes are in our hex bolts guide.

When a stainless bolt is tightened into a stainless nut, the surface oxide layer can tear and the two threads weld together under pressure and friction. This is called galling, and it ends with a bolt that cannot be tightened or removed, and often has to be cut. It happens most with fast power tools, dry threads and nuts and bolts of the same grade.

To avoid it, apply a nickel or copper anti-seize paste or a wax to the thread, tighten at low speed, avoid impact tools, and where possible use a nut of slightly different hardness or a coated nut. For critical joints, request nuts with a lubricant coating from the supplier.

Stainless steel is noble compared with aluminium, zinc and carbon steel. When a stainless bolt is used in aluminium or galvanized steel in the presence of water, the less noble metal around the bolt corrodes faster. The risk grows with a large stainless area against a small area of the other metal, and in salt water.

In wet or outdoor joints, place an insulating washer or sleeve (see our washers guide) between the stainless fastener and aluminium, use a sealant or paste, and avoid mixing carbon steel and stainless parts in the same assembly where moisture collects.

Stainless steel resists corrosion because of a thin, invisible chromium oxide film. Machining, forming and handling can leave iron particles on the surface that rust as small spots. Pickling and passivation remove these and restore the film, and a properly treated fastener has a clean, uniform surface. Ask whether the order is passivated, particularly for food, pharma and marine use.

Bright, polished bolts are not necessarily better. What matters is the steel grade and the cleaning, not the shine.

Poor-quality stainless is sold as 304 or 316 when it is a cheaper grade, such as 201 (low nickel, manganese) or even plated steel. A magnet is not a reliable test, because cold-worked austenitic steel can be slightly magnetic, though a strongly magnetic bolt is a warning sign. A better method is a spectrometer reading of nickel, chromium and molybdenum, or a chemical spot test for molybdenum that separates 316 from 304.

Ask for the mill certificate with the heat number and the chemical analysis. For A4, the molybdenum content should be around 2 to 3 percent, and nickel around 10 to 14 percent. Check that the head marking shows both the grade and the class (for example A4-80) together with the manufacturer's mark.

• Heat number and chemistry on the certificate, linked to your lot.
• Head marking: manufacturer, grade and class.
• Sample check with a spectrometer where the order value justifies it.
• ASTM equivalents: F593 for bolts and screws, F594 for nuts, in group 1 (304) and group 2 (316).

Global Fasteners Exports is a trading and export company: our stainless steel fasteners are made by partner factories and we handle inspection, packing, documents and shipping. Because we buy in volume from those factories, prices are typically up to 30% lower than the market price.

For stainless steel, name A2 or A4 and the class, and ask for the heat number and chemistry on the certificate; we pass the mill certificate of the lot to you.

Stainless range includes hex bolts, nuts, socket screws, machine screws, washers and rivets. Send the product list with grade (A2 or A4), class and quantity, and we quote the whole set together so it can ship in one consignment.

• Samples or detailed photos can be arranged before a bulk order; sample and courier costs are normally adjusted against the first order.
• Standard payment terms are 50% advance to confirm the order and the balance before dispatch.
• Documents: commercial invoice, packing list, certificate of origin and bill of lading, plus mill test certificates and inspection reports on request.
• Different products can be combined in one consolidated FCL or LCL shipment.