Stainless Steel Fasteners: Grades, Properties & How to Choose the Right One

Are you working in a caustic environment where corrosion is an important factor in material selection? Are you concerned about rust forming on structural parts and fasteners due to a wet or moist environment?

If so, then fasteners made of stainless steel may be the answer to your problem.

Many people ask us these questions:

• Which is better between 304 and 316 stainless steel fasteners?
• What's the difference between 304 and 316 stainless steel?
• What do A2-70 and A4-80 actually mean?
• Why does rust still form around stainless steel?
• Is 316 stainless steel suitable for seawater immersion?
• Is well-polished stainless steel maintenance-free?
• How do I stop stainless bolts from seizing when I tighten them?

We hope this article gives you a solid understanding of stainless steel and the best choice for your intended use, especially when it comes to fasteners. We've also incorporated some common misconceptions about stainless steel, courtesy of our friends at Hobson Engineering.

1. What Is Stainless Steel?

Stainless steel is a low-carbon alloy with at least 10.5% chromium, along with other alloying elements depending on the grade. The chromium reacts with oxygen in the air to form a thin, self-repairing chromium oxide layer on the surface — this is what gives stainless steel its corrosion resistance. If the surface is scratched, this layer rebuilds itself in the presence of oxygen.

But here's what a lot of people get wrong: using stainless steel does not guarantee that minor surface rust will not appear.

It is actually quite common, particularly in harsh environments, to see "tea staining" on the surface of stainless steel — a brownish discolouration from minor surface oxidation. This does not mean the fastener is failing. What matters is that rust does not penetrate the core material and compromise structural integrity.

According to Australian fastener manufacturer Hobson Engineering, if rust occurs within a few days or weeks of installation, it is almost certainly due to carbon steel contamination — either from fabrication tools, grinding residue, or the local environment. Longer initiation periods typically arise from surfaces that are too rough, aggressive environments (such as 304 stainless posts on a wharf), or lack of periodic washing (for example, drainpipes under eaves that never see rain).

For further reading on the metallurgy, The Atlas Steels Technical Handbook of Stainless Steels is an excellent reference.

2. The ISO Grade System: A1, A2 and A4

Stainless steel fasteners are classified under ISO 3506. The grade tells you the alloy composition; the property class (the number after the hyphen) tells you the strength. The "A" prefix stands for austenitic stainless steel — the most common family, non-magnetic, and the standard for almost all industrial fasteners.

A2 is the default for the majority of industrial applications. A4 costs roughly 20–30% more and is the correct choice where chloride exposure, salt air, or chemical splash is a real factor. The difference comes down to one element: molybdenum.

3. 304 vs 316 Stainless Steel

Grade 304 (A2) — Standard Stainless

Grade 304 (ISO designation A2) is sometimes called the standard "18/8" austenitic stainless steel — 18% chromium and 8% nickel. It is resistant to oxidation and corrosion and is best used in food preparation and processing environments, and wherever aesthetics matter. This is the grade you'll find in most general workshop and household stainless hardware.

Benefits: ease of fabrication, ease of cleaning, helps prevent product contamination, offered in a range of finishes.

Commonly used in: kitchen equipment, pressure vessels, piping, storage tanks, hose clamps, corrosion-resistant electrical enclosures.

Grade 316 (A4) — Marine Grade

Grade 316 (ISO designation A4) is also referred to, informally, as "marine grade stainless steel" — though this is not a recognised industry standard term. The key difference is the addition of 2–3% molybdenum, with chromium reduced to 16–18% and nickel raised to 10–14%.

The molybdenum significantly improves resistance to pitting corrosion, particularly in chloride environments. This is measured by the Pitting Resistance Equivalent Number (PREN): PREN = %Cr + 3.3 × %Mo + 16 × %N. Grade 304 has a PREN of approximately 18–20; Grade 316 sits at 24–27. That's a meaningful difference where chlorides are present.

Benefits: more resistant to chlorides, sulfuric acid, bromides, iodides and fatty acids at high temperatures; better protection in seawater and brine.

Best used in: marine construction, chemical processing, more corrosive environments where the material will be exposed to solvents and chemicals, coastal infrastructure.

Important caveat: while 316 is often called "marine grade", it simply means it will remain bright and shiny — provided it is properly maintained. It may still rust, especially in crevices, when submerged in seawater, which has approximately 20 times the chloride level that 316 is designed to withstand. For full seawater immersion in critical applications, consider duplex stainless (e.g. 2205) or titanium fasteners.

4. Property Classes: A2-70, A4-70 and A4-80

The number after the hyphen in a marking like A2-70 is the property class — it indicates minimum tensile strength in units of 10 MPa. So A2-70 = 700 MPa minimum tensile strength. This marking should be stamped or embossed on the bolt head or socket. If there's no marking, treat it as unknown grade.

A2-70 is what most suppliers stock and what most engineers default to. If you're not sure what you need, you almost certainly need A2-70. A4-80 is specified for applications where both superior corrosion resistance and higher clamping force are required together — offshore structures, marine fittings, chemical plant flanges. Achieving the -80 property class requires cold working the steel after forming, which is why it carries a price premium.

5. Types of Corrosion — What Stainless Actually Resists

Stainless steel's chromium oxide passive layer is effective against many forms of corrosion — but not all. Here are the five types most relevant to fastener selection:

General (Uniform) Corrosion

Even material removal across the surface. Both A2 and A4 resist this well under normal industrial conditions. Failure occurs with strong acids or prolonged exposure to strong oxidising agents at elevated temperatures.

Pitting Corrosion

Localised attack at the passive layer, forming small pits that penetrate deeply. The primary driver is chloride ions — found in seawater, pool water, de-icing salts, and some cleaning chemicals. A2/304 is susceptible in chloride environments. A4/316 with its molybdenum content resists pitting significantly better. This is the most common reason to upgrade from A2 to A4.

Crevice Corrosion

Occurs in tight gaps — under bolt heads, between flanges, in threaded joints — where oxygen is depleted. The passive layer cannot self-repair without oxygen, creating aggressive local chemistry. Both grades are susceptible; A4 is more resistant than A2. Mitigate by using washers to reduce crevice depth, ensuring joints can drain and dry, and using appropriate sealant where joints are permanently sealed.

Intergranular Corrosion

Chromium carbide precipitation at grain boundaries during heat exposure (450–850°C). Standard A2 and A4 fasteners at normal service temperatures are not affected. This is primarily a welded structure concern, not a bolted joint concern.

Stress Corrosion Cracking (SCC)

Cracking driven by the combination of tensile stress and a corrosive environment. Austenitic stainless is specifically susceptible to SCC in chloride solutions at temperatures above approximately 60°C — swimming pools, steam, heated seawater. This is a genuine failure mode with no visible warning. If your environment is hot and chloride-bearing, stainless may not be the right material regardless of grade. Consider duplex stainless (2205), titanium, or Inconel for such conditions.

6. Thread Galling: The Silent Killer of Stainless Fasteners

Thread galling — also called cold welding — is the failure mode most unique to stainless steel fasteners, and it catches people off guard because it happens during installation, not in service. If you've ever had a stainless bolt seize solid partway through tightening, this is what happened.

What happens

As stainless threads make contact under load, local pressure destroys the thin passive oxide layer. The exposed metal surfaces — both austenitic stainless — weld together microscopically. The fastener becomes impossible to turn in either direction. The result is a seized joint that often requires drilling out the bolt or cutting the parent material.

Why stainless is prone to it

Three factors combine: the passive oxide layer is thin and easily disrupted; austenitic stainless has a relatively low hardness (typically 200 HV); and the material work-hardens as thread surfaces shear against each other — friction actually increases as you tighten. High rotational speed (power tools) significantly increases galling risk.

How to prevent it

• Use anti-seize compound. This is the primary fix. Apply a small amount of copper-based, nickel-based, or PTFE-based anti-seize to the thread before installation. Copper-based is the most common; use nickel-based where copper contamination is a concern (food, pharmaceutical). Reduce your target torque by 15–25% when anti-seize is applied — it reduces friction, meaning a given torque produces more clamping force than it would dry.
• Tighten by hand to start. Ensure threads are aligned (not cross-threaded) before applying torque. Cross-threading is a major galling initiator.
• Control speed. Use a torque wrench rather than a power driver for final tightening. Slower installation means less heat and surface disruption.
• Consider mixed materials. Pairing a stainless bolt with a brass or non-stainless nut reduces galling risk — same-material contact is the highest-risk combination.
• If it starts to feel tight early, stop. Back it off carefully, inspect the threads, and apply anti-seize before continuing. Forcing a galling fastener makes the situation significantly worse.

7. Galvanic Compatibility

Stainless steel is highly noble — it sits near the top of the galvanic series. When stainless contacts a less noble metal in the presence of an electrolyte (water, humidity, salt solution), the less noble metal corrodes preferentially and at an accelerated rate.

Where stainless must be used with less noble metals, use isolating nylon washers and sleeves to break the electrical contact, apply a compatible sealant at the joint to exclude moisture, and maintain any protective coating on the active metal substrate.

8. Stainless vs High-Tensile — Strength Comparison

A common assumption is that stainless steel is a strong fastener material. It has moderate strength — adequate for most applications, but well below high-tensile grades. If you're selecting fasteners primarily for clamping force or structural load, and corrosion is not a factor, high-tensile is the better engineering choice. For a related article discussing high tensile bolts, see our bolt grade guide.

The stainless steel properties that may or may not be to your advantage (depending on application) include: better aesthetic appeal, better corrosion resistance, better resistance to extreme temperatures and oxidation, harder to weld, higher work-hardening rate, higher cryogenic toughness, higher tensile strength than mild steel — but lower electrical conductivity and lower thermal conductivity than carbon steel.

9. Torque Reference Guide (M6–M20)

Torque values for stainless fasteners are lower than equivalent-sized high-tensile steel bolts. The values below are for lubricated conditions (anti-seize or equivalent lubricant applied to threads and under the bolt head) — which, as noted above, you should always be using with stainless fasteners anyway. For dry installation, increase these values by approximately 20–25%, and accept the higher galling risk that comes with it.

These are indicative values based on standard friction coefficients for lubricated stainless. Always verify against the fastener manufacturer's data sheet for critical or structural joints. Values assume full thread engagement (nut on bolt). Do not use for self-tapping or thread-forming screws — those require separate data based on the parent material.

10. Application Selection Guide

11. When NOT to Use Stainless Steel Fasteners

Stainless is a good choice for many environments — but it's not always the right one, and specifying it where it's not needed adds cost. There are also situations where stainless is actively the wrong material:

• High-strength structural joints. If Grade 10.9 or 12.9 strength is required and corrosion is not a factor, zinc-plated or geomet-coated high-tensile fasteners deliver more strength at lower cost.
• Hot chloride environments. Above approximately 60°C in a chloride-bearing environment, austenitic stainless is susceptible to stress corrosion cracking — a brittle failure mode with no visible warning. Specify duplex stainless, titanium, or Hastelloy C276.
• Contact with hydrochloric acid. Neither A2 nor A4 resists HCl. Attack is rapid. Use Hastelloy, Inconel, or PTFE-coated fasteners as appropriate.
• Aluminium structures in wet or coastal environments. Without proper isolation, stainless bolts will accelerate corrosion at the bore through galvanic action. Evaluate whether aluminium or titanium fasteners would be more appropriate.
• Very low-cost, high-volume assemblies with zero corrosion exposure. If the assembly is dry, indoor, and non-food-related, zinc-plated mild steel fasteners deliver adequate service life at a fraction of the cost. Specifying stainless where it adds no value is a materials cost problem.

12. What to Look for When Buying Stainless Fasteners

With the technical decisions made, here's what to check when selecting a supplier and placing an order:

• Safety compliance. Make sure the product meets relevant Australian safety standards and regulations. If work health and safety (WHS) laws apply to your application, look for relevant certifications and markings. ISO 3506 certification should be specified for any structural or safety-critical use.
• Supplier reliability. Choose reputable brands with a proven track record for delivering quality product. Unbranded or uncertified stainless fasteners from unknown sources may not meet the stated grade — the A2-70 marking on an unmarked import is meaningless without traceability.
• Warranty and support. Check warranty terms and after-sales support options. For high-volume or critical applications, ask for material test certificates (MTCs).
• Lead time and availability. Confirm product availability and estimated delivery times to avoid project delays. Common sizes in A2-70 are typically ex-stock; A4-80 in larger sizes may require lead time.
• Returns. Familiarise yourself with the supplier's returns and exchange policy in case you receive incorrect or damaged items.
• Delivery. Clarify delivery terms, including estimated delivery times, shipping costs and who handles insurance during transit where applicable.

Need help with a purchase, or looking for multiple categories of fasteners for a project? You can set up business accounts with AIMS Industrial for ongoing supply, or check our FAQs for common questions about ordering, shipping and returns.

Frequently Asked Questions

AIMS Industrial stocks A2-70 and A4-80 stainless fasteners across the full metric range — hex bolts, socket cap screws, set screws, nuts, washers, and studding. Browse our fasteners range or contact our team for grade selection advice on your specific application.