304 vs 316 Stainless Steel: Fittings and Fasteners Selection Guide

Short answer: If there are no chlorides around (salt, sea air, indoor pool air, chlorinated chemicals), AISI 304 is sufficient and more economical for most water, heating and cooling, compressed air and food applications. In chloride-bearing environments, on the coast, around pools and in chemical lines, choose AISI 316, which contains 2–2.5 % molybdenum by mass. The same rule applies to bolts and nuts: A2 corresponds to 304 and A4 to 316.

This guide explains the difference in composition, which grade suits which environment, how to choose A2 or A4 fasteners, galvanic corrosion with brass and galvanised steel, and thread types. Examples are taken from the NVS AISI 304 and AISI 316 stainless range, Novus's own label.

304 vs 316: chromium, nickel and molybdenum

Both are austenitic chromium-nickel stainless steels. Their European designations are 1.4301 (X5CrNi18-10) for 304 and 1.4401 (X5CrNiMo17-12-2) for 316, and this is how the material number appears on NVS product pages. The decisive difference is molybdenum: 316 contains molybdenum and slightly more nickel, while 304 has none. Molybdenum markedly improves the steel's resistance to chloride environments.

Values are taken from Euro Inox tables based on EN standards. The “L” grades are low-carbon (max. 0.030 %) and are preferred for welded fabrication; in terms of corrosion behaviour 304L belongs to the 304 family and 316L to the 316 family.

Which grade for which environment?

Drinking and domestic water, heating and cooling

According to Euro Inox, chloride in drinking water is limited to 250 mg/L; grade 1.4301 (304) has many successful applications in such water, and 316-type grades are corrosion resistant in most cases if properly fabricated. The Nickel Institute guideline quoted by the Australian Stainless Steel Development Association (ASSDA) allows roughly 200 ppm chloride for 304 and roughly 1,000 ppm for 316 under steady exposure at neutral pH and room temperature. The limits drop as temperature rises; above 60 °C both grades risk sudden chloride stress corrosion cracking. Gasketed faces, thread roots and narrow crevices with deposits are the most vulnerable spots.

Swimming pools

The most aggressive environment in a pool is not the water but the air above an indoor pool. Chloramines from disinfection accumulate on surfaces and can initiate stress corrosion cracking even at 26–32 °C. The Euro Inox guide to EN 13451-1 makes the following distinction:

• Outdoor pools: standard grades including 1.4301 (304) are suitable, as rain washes the surfaces.
• Indoor pools, regularly cleaned parts (ladders, grab handles, railings): 1.4401/1.4404 (316/316L) are suitable.
• Indoor pools, load-bearing parts that cannot be cleaned (suspended-ceiling hangers, light and sign fixings): neither 304 nor 316; highly alloyed grades such as 1.4529, 1.4547 or 1.4565 are required.
• Parts fully immersed in pool water are outside this atmospheric cracking risk.

Coastal sites and seawater

For coastal atmospheres and splash zones, 316 is the preferred grade. Parts permanently immersed in seawater are a different matter: for seawater with about 16,000 mg/L chloride, Euro Inox recommends higher-alloyed grades such as 1.4462 (duplex 2205), 1.4439, 1.4539 or 1.4565.

Food and chemicals

304 is the general-purpose grade for food plants and commercial kitchen equipment. Where parts touch salty or acidic products or chlorinated cleaning agents, in pharmaceutical equipment and in chemical process lines, choose 316. Chemical resistance depends on the chemical, its concentration and temperature, so check an aggressive medium against a chemical resistance table.

Fasteners: A2 or A4?

Stainless bolts and nuts are marked with a letter-number code under ISO 3506-1. The letter A denotes the austenitic steel group and the digit the alloy group: on NVS products A2 is given as AISI 304 (1.4301) and A4 as AISI 316 (1.4401). The 70 after the hyphen is the property class, meaning a minimum tensile strength of 700 N/mm² (and a minimum 0.2 % proof stress of 450 N/mm²). The size ranges below refer to NVS products in our stainless steel fasteners category.

• Use the same grade for bolt, nut and washer. A joint is only as corrosion resistant as its weakest part; bolt a 316 flange with 304 bolts in a chloride environment and the bolts will rust first.
• Do not over-tighten. Over-tightening stainless threads can cause galling (cold welding).
• A magnet test does not identify the grade. According to ASSDA, annealed 304 and 316 are essentially non-magnetic, while cold-worked parts may be slightly attracted. Magnetic response alone says nothing about grade or corrosion resistance.

Galvanic corrosion: brass, galvanised and carbon steel

When two different metals touch in an electrolyte such as water or condensation, the less noble one corrodes faster. Euro Inox's findings can be summarised as follows:

• Brass and copper: in drinking water, combinations of stainless steel with copper, copper alloys and gunmetal have been used for many years in cold and hot water installations without contact corrosion; the risk is moderate.
• Galvanised steel and aluminium: coupled with stainless steel in water, it is the zinc coating or the aluminium that corrodes.
• Carbon steel: a small carbon steel part on a large stainless surface is the worst combination. In a one-year seawater test reported by Euro Inox, raising the stainless/carbon steel area ratio from 1:1 to 10:1 increased the carbon steel corrosion rate from 0.31 to 2.10 mm/year, roughly sevenfold.
• The reverse is safer: small stainless fasteners on large galvanised or aluminium parts do not normally cause problems.

Practical rule: do not connect stainless fittings directly to galvanised or carbon steel pipe; fit an insulating (dielectric) union in between.

Threads: BSP or NPT?

NVS stainless steel fittings and stainless steel valves have BSP (Whitworth) threads, and each product page states the standard, for example DIN 2999 for the 90° threaded elbow, DIN 4144 for the 316 threaded tee and ISO 228 for ball valves. BSP threads have a 55° profile, American NPT threads 60°, and the pitch also differs in some sizes, so the two systems must not be mixed. Seal threads with PTFE tape or a liquid sealant compatible with stainless steel.

Summary: AISI 304 vs AISI 316

Size, weight and stock-code tables for NVS fittings, valves and fasteners are on each product page; the 316 female-thread ball valve, for instance, is rated PN16–PN63 with an operating temperature of -10/+230 °C. The current price is shown on the product card, with the VAT-inclusive price first and the VAT-exclusive price below it.

Frequently Asked Questions

Is 316 always better than 304?

In corrosion resistance, yes. But in chloride-free indoor environments 304 is sufficient and 316 only adds cost. The extra cost pays off on the coast, in indoor pools and wherever salty products or chlorinated chemicals are involved.

Can a magnet tell 304 from 316?

No. Both grades are essentially non-magnetic when annealed, and cold-worked parts may be slightly attracted. The grade is shown by the product marking, the material number (1.4301 / 1.4401) or a mill certificate.

Can stainless fittings be connected to galvanised pipe?

Not directly. In water the galvanised coating corrodes where it touches stainless steel; fit an insulating (dielectric) union between the two metals.

What does A4-70 mean?

A4 is the molybdenum-alloyed austenitic group in ISO 3506-1 (316 type), and 70 means a minimum tensile strength of 700 N/mm². A2-70 is the 304-type equivalent with the same strength.

Which grade for pool ladders and railings?

In indoor pools, regularly cleaned ladders and railings can be 316 (1.4401/1.4404); in outdoor pools 304 is also adequate. Load-bearing parts that cannot be cleaned, such as suspended-ceiling hangers, require highly alloyed grades like 1.4529.

Can NVS threaded fittings be used on NPT pipework?

No. NVS threaded parts have BSP (Whitworth) threads; because of the different thread profile they will not seal on NPT threads.

Prepared by the Novus Mühendislik ve Enerji Sistemleri technical team · Updated: November 2026