AISI 304 vs 316 vs 316L: Which Stainless Steel Survives at Sea?

AISI 304, 316 and 316L look identical on a new scale, but after a season of salt spray, fish blood and daily washdown they can behave very differently. The difference comes down to one alloying element, molybdenum, and one impurity limit, carbon. This article compares the three grades with published composition and pitting-resistance data, explains how chlorides attack stainless steel and why 316L matters for welded frames.

Composition: what separates 304, 316 and 316L

The key difference is molybdenum: 316 and 316L contain 2-3 % Mo, while 304 contains none. 316L is the low-carbon version of 316, with carbon limited to 0.03 %. The table gives ASTM-based limits as compiled by the International Molybdenum Association (IMOA, Practical Guidelines for the Fabrication of Austenitic Stainless Steels, Table 1).

Grade	UNS	EN	C max	Cr	Ni	Mo	N max	PREN
304	S30400	1.4301	0.07	17.5-19.5	8.0-10.5	-	0.10	18-20
304L	S30403	1.4307	0.03	17.5-19.5	8.0-12.0	-	0.10	18-21
316	S31600	1.4401	0.08	16.0-18.0	10.0-14.0	2.00-3.00	0.10	24-26
316L	S31603	1.4404	0.03	16.0-18.0	10.0-14.0	2.00-3.00	0.10	24-26
316L (higher Ni/Mo variant)	-	1.4435	0.03	17.0-19.0	12.5-15.0	2.50-3.00	0.11	27-29

All values are weight %; Mn is 2.00 % max for all grades. IMOA notes that EN chemistry does not correspond exactly to ASTM chemistry, so check the mill certificate for the standard actually ordered.

PREN: a number for pitting resistance

The Pitting Resistance Equivalent Number (PREN) is a corrosion-resistance index calculated from a steel's chromium, molybdenum and nitrogen content, plus tungsten where present, and it ranks stainless steels by resistance to localised corrosion in chloride environments. In the version IMOA uses for austenitic grades, chromium counts at face value, each percent of molybdenum counts more than three times as heavily, and nitrogen counts heavily too, although it is present only in small amounts.

Every percent of molybdenum adds a little over three points, so the 2-3 % Mo in 316 lifts its PREN from the 18-20 of 304 to 24-26 (IMOA steel grades and PREN). IMOA sets out three limits to using the number:

• it describes resistance under ideal conditions and does not account for intermetallic phases, poor heat treatment, surface condition or post-fabrication cleaning;
• it is an indicator, not a guarantee: if a grade fails by pitting in an environment, a grade with a higher PREN is needed, but how much higher is not certain;
• it should not be used where general corrosion rather than pitting is the likely mechanism.

316 and 316L have the same PREN range. For pitting and crevice resistance at sea they are equivalent; the difference between them is weldability.

How chlorides attack stainless steel

Stainless steel resists corrosion because of a very thin chromium oxide passive film; pitting and crevice corrosion start where chlorides break that film locally (IMOA, section 6.2.1). Factors that promote attack are high halide levels, especially chloride, oxidants, higher temperature and acidic conditions. On a fishing vessel all four can occur: seawater spray, evaporating salt deposits, chlorinated disinfectants and warm, acidic residues.

Crevice corrosion is the more dangerous form on equipment. IMOA lists threaded connections, lap joints, O-ring and gasket seals, connectors, dirt, grease, tape and deposits as common crevice formers. Inside a crevice the solution becomes higher in chloride and lower in pH, so corrosion is driven inside the crevice. A given alloy's critical crevice temperature is always lower than its critical pitting temperature, because crevice corrosion starts more easily.

Laboratory data put this in perspective: 316L has an ASTM G48 critical pitting temperature of 10 °C in ferric chloride and is described by IMOA as not suitable for steam condenser tubes in seawater, a service that highly alloyed 6 % Mo grades handle. 316 is therefore a sensible grade for spray, splash and washdown, not a material that is immune to seawater under all conditions.

Choosing a grade by environment

The European Hygienic Engineering and Design Group gives a practical threshold (EHEDG Document 8, 2nd edition, 2004, section 4.3):

Conditions of use	EHEDG guidance
pH about 6.5-8, chlorides up to about 50 mg/l, temperature up to about 25 °C	AISI 304 or 304L
Chlorides and temperature both above approximately double those values	Greater crevice and pitting resistance needed: AISI 316, or 316L for welded pipework and vessels
Temperatures approaching 150 °C with high chloride and high stress	Even 316 may suffer stress-corrosion cracking; other alloys may be required

Seawater, fish-hold brine and salt-laden spray are far above 50 mg/l chloride, which is why marine weighing equipment in exposed positions is normally built from 316 or 316L. 304 can still be appropriate for parts kept in dry, fresh-water-washed areas away from spray. Surface finish, crevice-free design and cleaning matter as much as grade; see cleaning and washdown of scales.

Welding, sensitisation and why 316L exists

In standard austenitic grades, chromium carbides can precipitate on grain boundaries when the steel is held in the 425-900 °C range, leaving chromium-depleted zones with lower corrosion resistance. This is sensitisation (IMOA, section 3.3). During welding the heat-affected zone passes through this range, and a band of sensitised metal can form on either side of the weld, known as weld decay (section 6.3.1).

IMOA gives three ways to prevent it: use a low-carbon L grade, use a stabilised grade, or solution anneal after welding. L grades typically contain less than 0.03 % carbon and resist sensitisation during normal fabrication welding; IMOA notes that it takes approximately one hour to sensitise a Type 304 with 0.042 % carbon at its fastest sensitisation temperature, far longer than a typical weld thermal cycle (section 12.1.7.7). The L grades do not protect against long service exposure within the sensitisation range, which is not a concern for scales.

Welding quality affects corrosion as much as grade:

• Heat tint and weld defects such as undercut, slag inclusions and stop-start defects are typical pitting sites; pickling after welding restores corrosion resistance.
• Incomplete joint penetration leaves a crevice that traps dirt, lowers corrosion resistance and is very difficult to sanitise (section 12.1.7.1).
• Iron contamination from carbon steel tools, grinding dust or shared fabrication areas causes rust spots on stainless surfaces; IMOA recommends separating stainless and carbon steel fabrication.

Mixed metals on deck

When stainless steel is electrically connected to less noble metals in seawater, the less noble metal corrodes faster. In the galvanic series in seawater published by the UK National Physical Laboratory, aluminium alloys, zinc and carbon steel are more electronegative than passive 316. A small area of the less noble metal coupled to a large stainless area is the worst case. Fastener and mounting choices are covered in preventing corrosion and extending scale life.

How WPL approaches this

The M2 Series and the M5 and M6 platform scales are built in AISI 316, and the M3 scientific scale in 316L. More on materials, sealing and hygiene is on the hygiene and materials hub.