316 vs. 18-8 Stainless Steel Hex Nuts: Which Do You Need?

Choose a 316 stainless steel hex nut when the joint will see chlorides: salt spray on the coast, pool water and pool chemicals, or road de-icing salt. Choose an 18-8 hex nut for everything else, indoors or out. 316 has molybdenum added, which is what helps it hold up where chlorides would make 18-8 pit. Our 18-8 vs. other stainless grades guide covers the grades themselves. This post is about the nut.

You can thread a 316 nut onto an 18-8 bolt, and the two won't attack each other. But the joint only resists corrosion as well as its weaker part, so a 316 nut on an 18-8 bolt in salt spray still gives you an 18-8 joint. If the environment calls for 316, use 316 for the bolt, the nut and the washer. The size and thread rules are the same for both grades, so a 3/8-16 nut fits a 3/8-16 bolt either way.

When does a hex nut need to be 316 stainless steel?

The deciding question is whether chlorides reach the nut and stay there. Fastenio's hex nut catalog sums up the split: 18-8 is "the standard stainless for outdoor and wet environments" but is "not suited to salt water or harsh chemicals," and 316 is "chosen for salt water, pool and chemical exposure, where 18-8 can pit over time."

Nuts are a worse case than flat surfaces. The Australian Stainless Steel Development Association (ASSDA) points out that corrosion in salt water is worse where there is a crevice, and names a nut and bolt as an example.

Is 316 stainless steel suitable for seawater?

Not for full immersion. In its article "Common misconceptions about stainless steel", ASSDA states that seawater has about 20 times the chloride level 316 can withstand. "Marine grade" means 316 stays bright when it is rinsed by rain or kept clean. It does not mean it can sit in the sea. ASSDA also notes that 316 can visibly corrode in severe coastal spots where salt spray builds up and isn't washed off.

For a buyer, that means two things:

• Above the waterline, 316 is the right call, and ASSDA ties its good looks to being rain-washed or cleaned, so don't let salt deposits sit on it.
• Below the waterline, neither 18-8 nor 316 is a seawater immersion grade. ASSDA names super austenitic and super duplex grades for that service. Fastenio doesn't sell those, so treat any submerged joint as one to spec with the equipment maker.

Can you put a 316 nut on an 18-8 bolt?

Yes, as far as galvanic corrosion goes. Both are austenitic stainless steels, and ASSDA's example is direct: 304 bolts in a 316 panel immersed in tap water "will not show galvanic effects." 304 is the main grade in the 18-8 family, so a 316 nut on an 18-8 bolt (or the reverse) won't set up the kind of attack you get when stainless meets carbon steel or aluminum.

What mixing does not do is upgrade the joint. If a coastal job calls for 316, an 18-8 bolt under a 316 nut is the part that pits first, and the threads are a crevice in their own right. Match the grade to the harshest exposure and use it for every part of the joint: bolt, nut and washers.

The threads still have to match, regardless of grade. Our guide on how to match nuts and bolts walks through diameter, threads per inch and length.

Does a 316 nut on an 18-8 bolt prevent galling?

No. Some people pair a 316 nut with an 18-8 bolt in the hope that two different grades won't seize, but ASSDA lists "using a 316 nut on a 304 bolt stops galling" as a misconception. ASSDA describes galling as the cold welding of clean stainless surfaces under load, made worse by fine threads, tight clearances, poor thread profiles, missing lubrication, dirt and over-tightening. Controlling it takes lubrication, clean parts and care in design and installation, not a change of grade on one part.

That applies to 316 on 316 as much as 18-8 on 18-8. Our post on preventing stainless steel galling covers the steps that do help, including anti-seize and slower installation.

Finished, heavy or machine screw nut: which 316 nut fits the job?

Fastenio carries 316 stainless steel in three hex nut styles, and each one is a different shape, not just a different size.

• 316 finished hex nuts are the standard hex nut, made to ASME B18.2.2 and listed as grade 316 (A4-70). Coarse and fine threads run from 1/4-20 to 2-4.5.
• 316 heavy hex nuts are also ASME B18.2.2, but the heavy pattern is wider across the flats and thicker than a finished nut on the same thread. Use them for structural, anchor-bolt and flange joints, or wherever the drawing calls for heavy hex. Coarse threads run from 1/4-20 to 2-4.5.
• 316 machine screw hex nuts are made to ASME B18.6.3 for machine screws, with a lower profile than a finished hex nut. Sizes run from #6-32 to #12-24.

The heavy pattern changes your wrench size. Here are the widths across the flats for both inch patterns:

Source: ASME B18.2.2. Finished hex values are the maximum width across flats as listed in the B18.2.2-2010 table reproduced by AmesWeb; heavy hex values are the basic width across flats from the B18.2.2 table reproduced by AFT Fasteners.

Thickness grows too. At 1/2 inch, a heavy hex nut is 31/64 inch thick (basic), while a finished hex nut runs 0.427 to 0.448 inch (same sources). The heavy hex nut catalog and machine screw nut catalog list both grades side by side.

In metric, 18-8 is sold as A2 and 316 as A4. The property class after the dash, as in A4-70, is the ISO 3506 strength class.

How do you tell a 316 nut from an 18-8 nut?

Once they're out of the bag, they look the same. Both are bright, both are austenitic, and you can't sort them by eye.

Check the marking. Inch stainless steel nuts from 1/4 inch up are covered by ASTM F594, which sorts the alloys into groups. Group 1 covers the 18-8 family (303, 304, 304L, 305, 384, XM1, 18-9LW, 302HQ and 303Se). Group 2 is 316 and 316L. The F594 marking letter tells you the group and the condition the nut was supplied in:

Source: ASTM F594 alloy groups and condition markings, as reproduced by Portland Bolt. Codes C, D, G and H are cold-worked conditions that apply to limited diameter ranges.

If a nut carries no mark, you can't confirm the grade by looking at it. Keep parts in their labeled packs, and keep 316 stock separate from 18-8 on the shelf.

Don't rely on a magnet. The catalog describes 18-8 as non-magnetic in the annealed state. ASSDA notes that heavily cold-worked 304 can show a weak magnetic effect, while cold-worked 316 shows a negligible one. A slight pull from a magnet doesn't tell you the nut is the wrong material, and no pull doesn't prove it is 316. ASSDA also stresses that magnetism isn't related to corrosion resistance.

The bottom line

Use 316 stainless steel hex nuts wherever chlorides reach the joint (coastal spray, pools, chemical wash-down, de-icing salt), and use 316 for the bolt and washer too. Use 18-8 everywhere else. Mixing the grades won't cause galvanic trouble, but it won't stop galling, and the joint is only as corrosion-resistant as its 18-8 part. Neither grade is meant for seawater immersion.

Shop by style: hex nuts for 18-8 and 316 finished nuts, heavy hex nuts for structural and flange work, and machine screw nuts for #6 through #12 screws. The 316 finished hex nut page lists every coarse and fine thread size in one table.