Stainless Steel in Marine Engineering Material Grades for Seawater Cooling, Ballast, and Firefighting

Three systems on the same ship handle the same fluid - seawater - yet they demand three different stainless steels. Seawater cooling is a continuous-flow duty, so duplex 2205 handles most of it. Ballast is an intermittent, stagnant, sediment-loaded duty, so it needs higher molybdenum or a coating strategy. Firefighting is a normally-idle, suddenly-maximum-flow duty, and it is the hardest of the three: a wet fire main can sit still for a year and then run at full velocity in ninety seconds.

Stainless Steel in Marine Engineering

If you remember only one sentence, remember this one: in seawater, stainless steel fails by crevice corrosion and under-deposit pitting, not by uniform corrosion - so the grade you choose must be matched to how long the surface stays wet and still, not just to how salty the water is.

This guide is written for piping and equipment engineers, procurement teams, and classification-society surveyors who have to defend a material choice on paper. Every section starts with a question, gives the answer in one or two sentences, and then explains the reasoning and the numbers behind it.

You can read it end to end, or jump straight to the system you are specifying: seawater cooling, ballast, or firefighting. The comparison matrix in the middle consolidates all three, and the appendix lists the standards, assumptions, and publishing metadata.

Three variables decide the outcome - alloy content (expressed as PREN), service temperature, and how often the surface is wetted and flowing. Salinity alone tells you almost nothing.

Engineers often ask for "a seawater-resistant grade" as if resistance were a single property. It is not. Seawater attacks stainless steel through localised breakdown of the passive film, and that breakdown depends on a threshold that moves with temperature.

A 316L pipe that gives fifteen years in a North Sea cooling line at 12 °C may perforate in eighteen months in a Gulf condenser outlet at 45 °C. Same water chemistry, same alloy, different temperature. The alloy did not change; the threshold moved below the operating condition.

The practical consequence is that a material datasheet without a service temperature and a flow regime is not a specification. It is a wish. Any credible proposal for a seawater system should state the design temperature, the minimum and maximum velocity, the chlorination regime, and the expected stagnant periods - because each of those four inputs can move the answer by one or two grade steps.

Nothing formal. It is a marketing phrase with no definition in any standard, and it is most often applied to 316 - a grade that is genuinely suitable for deck hardware and handrails, and genuinely unsuitable for immersed seawater piping.

There is no "marine grade" classification in ASTM, EN, or ISO. What exists is a set of service-specific material selections published by classification societies, NORSOK, and owner specifications, and those documents are far more conservative than the marketing term suggests.

The confusion has a real cost. A purchaser who writes "marine grade stainless steel" on a requisition will receive 316L, because that is the cheapest grade the phrase can plausibly describe. That 316L then goes into a seawater cooling header, where it will be the first item on the failure list.

Write the UNS number instead. S31603, S32205, S32750, S31254 - these are unambiguous, they appear on the mill certificate, and they can be verified. "Marine grade" cannot be verified, which is exactly why it should never appear on a purchase order.

PREN = %Cr + 3.3 × %Mo + 16 × %N. For continuously flowing cold seawater you need roughly PREN 35; for warm, stagnant, or crevice-prone service you need PREN 40 or higher. But PREN is a screening tool, not a guarantee.

PREN, the pitting resistance equivalent number, is a weighted sum of the three elements that stabilise the passive film against chloride attack. Chromium forms the film; molybdenum and nitrogen repair it locally when chloride breaks through.

The formula is deliberately simple, which is both its strength and its weakness. It ignores microstructure, inclusion content, surface finish, and heat treatment - all of which matter. Two heats of 2205 with identical PREN can differ measurably in measured critical pitting temperature if one was improperly annealed.

That is why serious specifications do not stop at the calculated number. They require a measured result: a critical pitting temperature from ASTM G48 Method A, or a corrosion test to ASTM G150, reported on the actual heat. The table below shows typical values; treat the PREN column as a design aid and the CPT column as the acceptance criterion.

Grade (UNS)	Common name	Typical PREN	Typical CPT, °C (ASTM G48 A)	Practical seawater ceiling
S31603	316L	24–26	20–25	25–30 °C, flowing only
N08904	904L	34–36	40–50	35–40 °C
S32205	Duplex 2205	34–36	45–55	45–50 °C
S31254	6Mo / 254SMO	42–44	65–80	60–70 °C
S32750	Super duplex 2507	41–43	80–90	80–90 °C
N06625	Alloy 625	~50	>85	Severe, plus high strength

Note the ordering anomaly: 6Mo has a slightly higher PREN than super duplex 2507, yet 2507 performs better in many seawater duties. PREN does not capture the benefit of the duplex microstructure, which resists chloride stress corrosion cracking in a way that fully austenitic 6Mo cannot. This is the first place where a single number misleads.

Because its critical pitting temperature is only about 20–25 °C, and its critical crevice temperature is lower still. In most seawater systems the metal spends much of its life inside a crevice at a temperature above that threshold.

Why does 316L fail in seawater even

Look at where 316L actually fails. It is rarely a straight run of pipe. It is under a gasket at a flange face, beneath a pipe support saddle, under a barnacle cluster, inside a threaded joint, or in the dead leg behind a closed valve.

Each of those locations is a crevice: a geometry where seawater enters but cannot freely exchange. Oxygen is consumed faster than it can be replenished, the local chemistry shifts, chloride concentrates, and the pH inside the gap drops. The passive film cannot rebuild in that environment, and pitting starts.

This is why 316L seawater failures look the way they do - isolated deep pits with bright metal at the bottom, surrounded by untouched surface. The pipe did not corrode. One square centimetre of it did.

There is a counter-intuitive twist worth knowing. Natural seawater forms a biofilm on stainless steel that raises its corrosion potential by roughly 200–400 mV, and a higher potential means a greater driving force for crevice attack.

Chlorination prevents that biofilm, so chlorinated seawater is often less aggressive to stainless steel than clean, unchlorinated seawater - the opposite of what most people assume. Biofouling control is therefore a corrosion-control measure, not only a heat-transfer measure.

CPT is the temperature at which pits start on an open surface; CCT is the lower temperature at which they start inside a crevice. For austenitic grades CCT sits roughly 15–25 °C below CPT, and for duplex grades the gap is smaller but still real.

The distinction matters because almost every seawater stainless component contains crevices by design. A bolted flange with a spiral-wound gasket is a controlled crevice. A tube-to-tubesheet rolled joint is a controlled crevice. A valve seat is a controlled crevice.

If you select on CPT, you are selecting for the easiest location on the component. If you select on CCT, you are selecting for the location that will actually fail.

The engineering response is not only to upgrade the alloy. It is also to reduce the crevice. Weld instead of flange where maintenance access allows. Use full-face gaskets rather than raised-face. Specify a seal weld on tube-to-tubesheet joints. Keep supports clear of the pipe surface with a drainage gap, and never let a stainless line sit on a carbon steel saddle.

Design detail	What it creates	Mitigation
Bolted flange, raised face	Crevice between gasket and flange face	Full-face gasket, or weld the joint
Pipe support saddle	Permanent wet crevice plus galvanic couple	Clamp with drainage gap, isolate from steel
Threaded joint	Deep, uninspectable crevice	Replace with welded or flanged joint
Tube-to-tubesheet	Annular gap at the rolled joint	Seal weld, or controlled expansion
Dead leg behind a valve	Stagnant seawater, no oxygen renewal	Eliminate dead legs; add drain and flush point

Six grades cover almost everything: 316L for closed loops and non-immersed hardware, 904L for moderate chloride, duplex 2205 as the workhorse, 6Mo for small-bore and plate heat exchangers, super duplex 2507 for hot or high-chloride duty, and nickel alloys for the extreme cases.

The table below is the short list that appears in real specifications. Anything not on it is either a specialist choice or a mistake.

Grade	Where it earns its place	Where it does not
316L (S31603)	Closed cooling loops, instrument air, deck hardware, splash zone above the wet line	Any continuously immersed seawater piping
904L (N08904)	Moderate-chloride process lines, some scrubber circuits	Warm seawater; cost is hard to justify against 2205
Duplex 2205 (S32205)	Seawater cooling piping above DN50, plate heat exchanger plates, pump casings, valves	Stagnant ballast lines, hot seawater above 50 °C, high-stress welded joints without controls
6Mo (S31254)	Small-bore seawater piping, instrument lines, plate heat exchangers, firewater deluge nozzles	Large-bore piping on cost grounds
Super duplex 2507 (S32750)	Hot seawater, high-chloride brines, electrochlorination circuits, high-pressure firewater	Low-temperature low-chloride duty where 2205 is sufficient
Nickel alloys (625, C276)	Flare tips, severe sour plus chloride, critical unreplaceable components	General seawater piping; cost is an order of magnitude higher

A useful rule of thumb for procurement: the alloy step from 316L to 2205 roughly doubles the corrosion allowance in temperature terms, and the step from 2205 to 2507 roughly doubles it again. Each step costs far less than the step before it in maintenance terms, which is why the middle grades dominate real specifications.

Duplex 2205 for piping and plate packs, super duplex 2507 or 6Mo where the outlet temperature exceeds about 50 °C, and titanium for tube bundles in critical coolers. 316L belongs only in the closed loop.

which grade for piping plate heat exchangers and tube bundles

Seawater cooling is the most forgiving of the three systems because the flow is continuous. Continuous flow keeps oxygen supplied, keeps deposits from settling, and keeps the surface above the velocity threshold where biofouling cannot attach. That is precisely the regime where 2205 performs well.

The design details that matter most are the ones at the edges of the continuous-flow assumption. The standby pump branch, the idle spare cooler, the bypass line, and the drain connection are all stagnant. These are the locations where a 2205 system will eventually show its first pit, and they are the locations most often omitted from a material selection review.

Cooling system element	Recommended	Reason
Main seawater piping, DN50 and above	Duplex 2205	Continuous flow, moderate temperature, good CCT
Small-bore and instrument lines	6Mo	Thin wall, low flow, crevice at every fitting
Plate heat exchanger plates	2205, or 2507 above 50 °C	High turbulence, tight crevices at gaskets
Shell-and-tube tube bundle	Titanium Gr2 or super duplex 2507	Thin wall, highest temperature, least accessible
Pump casing and impeller	Duplex 2205, cast equivalent	High velocity, erosion plus corrosion
Strainer basket and housing	6Mo or 2507	Low flow, heavy deposit, frequent opening
Standby and bypass branches	Same grade as the main line, plus drain	Stagnant - the real failure point

One further note on plate heat exchangers: the gasket groove is a designed crevice, and the plate material is thin. A plate pack that is opened and re-gasketed repeatedly accumulates scratches that become pit initiation sites. Specify 2205 as a minimum, and inspect plate edges at every service.

Below about 1 m/s, biofouling and sediment settle and create under-deposit corrosion. Above roughly 3.5 m/s for copper alloys and 6 m/s for stainless steels, erosion-corrosion removes the passive film mechanically. The design window is narrow, and it is part of the material choice.

Velocity is usually treated as a hydraulic parameter, decided by the piping engineer and handed to the materials engineer afterwards. That sequence is backwards. Velocity determines whether the passive film is replenished or destroyed, so it belongs in the material selection discussion.

At low velocity the failure mode is biological. Larvae attach, mussels grow, sediment accumulates in low points, and the metal beneath the deposit becomes an anaerobic crevice. At high velocity the failure mode is mechanical: turbulence at elbows, reducers, and valve bodies strips the film faster than it can reform, and the metal erodes.

Material	Minimum velocity	Maximum velocity	Failure mode outside the window
316L	1.0 m/s	~4 m/s	Crevice and under-deposit pitting at low flow
Duplex 2205	1.0 m/s	~6 m/s	Biofouling at low flow, erosion at high flow
Super duplex 2507	1.0 m/s	~6–7 m/s	Same, with wider temperature margin
90/10 CuNi	1.0 m/s	~3.5 m/s	Erosion-corrosion above 3.5 m/s
Titanium Gr2	1.0 m/s	>10 m/s	Essentially velocity-unlimited in clean seawater

The practical design target for stainless seawater lines is 1.5–3.5 m/s. That range keeps the surface scoured without approaching the erosion threshold, and it leaves margin for the fouling that will inevitably occur between dockings.

Because ballast duty is intermittent by nature. A ballast line may be full for a week, empty for a month, then half-full with sediment for a year. That cycle produces the worst possible environment for stainless steel: anaerobic, sulfide-bearing, and loaded with deposits.

A ballast system is not a cooling system with a different name. It is a completely different duty cycle, and the duty cycle drives the metallurgy.

During a ballast voyage the tanks and lines are full and quiet. Oxygen is consumed by respiring bacteria and by the corrosion reaction itself. Sulfate-reducing bacteria produce hydrogen sulphide, which is aggressive to both stainless steel and copper alloys. Sediment settles in tank bottoms and low points, creating deep deposits where the metal is fully shielded from any oxygen renewal.

This is the environment where microbially influenced corrosion, or MIC, becomes the dominant mechanism. MIC is not a separate kind of corrosion; it is crevice corrosion accelerated by a biologically modified local chemistry. The metallurgical response is the same as for any crevice: more molybdenum, more nitrogen, or a barrier coating.

Ballast system element	Typical material	Key consideration
Ballast tank, coated	Carbon steel with epoxy to IMO PSPC	Coating is the barrier; stainless is not the primary defence
Ballast tank, uncoated	316L or duplex 2205	Chemical tankers commonly use 316L; higher chloride favours 2205
Ballast piping, main	Duplex 2205 or GRP	Drainability matters more than grade
Ballast piping, small bore	6Mo or 2507	Thin wall plus stagnant equals early failure
Ballast pump casing	Duplex 2205, cast	Erosion at impeller, corrosion at cutwater
Tank cleaning machines	316L or 2205	Intermittent, high velocity, often overlooked

Two design decisions outweigh the grade choice. The first is drainability: a line that cannot be fully drained will hold seawater in its low points indefinitely, and no alloy survives that indefinitely. The second is coating integrity in tanks. A coated carbon steel tank with a maintained coating outperforms an uncoated stainless tank with a failed coating strategy, at a fraction of the cost.

The most aggressive stream on the ship. Electrochlorination generates hypochlorite at high chloride concentration, and the treatment unit, its pipework, and the neutralisation circuit all see oxidiser plus chloride plus occasional hydrogen. This is super duplex, 6Mo, or titanium territory - never 316L.

What changes when a ballast water management system is installed

The Ballast Water Management Convention required all ships to meet the D-2 discharge standard by 8 September 2024, so essentially every vessel afloat now carries a treatment system. That retrofit changed the material selection problem, and many specifications have not caught up.

Treatment technologies split into two families with different material consequences. Physical systems - filtration plus UV - are comparatively benign: the wetted surfaces see filtered seawater and ultraviolet light, and duplex 2205 is generally adequate. Chemical and electrochemical systems are not benign.

Electrochlorination is the demanding case. The cell generates sodium hypochlorite from seawater, so the local environment combines high chloride, high oxidiser concentration, a low pH at the electrode, and a hydrogen by-product. Titanium is the standard electrode material, and the cell housing and immediate downstream piping are normally super duplex, 6Mo, or a non-metallic.

BWMS element	Recommended material	Why
Filter housing	Duplex 2205	Continuous seawater, moderate temperature
UV reactor chamber	316L or 2205	No oxidiser, but crevices at seals
Electrochlorination cell body	Super duplex 2507, 6Mo, or PVDF	High chloride plus high oxidiser
Hypochlorite injection line	2507, 6Mo, or PTFE-lined	Highest oxidiser concentration on the ship
Neutralisation circuit	2507 or non-metallic	Reducing agent plus seawater
Sample and monitoring lines	6Mo	Small bore, stagnant, frequent shutdown

The common retrofit error is to reuse the original 316L ballast piping downstream of the injection point. That section now carries chlorinated seawater at a concentration the original design never anticipated, and it typically fails within the first two years.

Because it combines the worst features of the other two systems. A wet fire main is stagnant for months, accumulates sediment and marine growth, and then must deliver full flow at full velocity the instant a pump starts. Crevice corrosion, under-deposit pitting, and erosion all occur in the same component.

Firefighting is the system people think about least and should think about most. It has no redundancy in operation - it either works or the ship is in trouble - and it spends 99.9% of its life in the condition that stainless steel likes least.

Consider the duty cycle. The main is pressurised and static. Seawater sits in it, oxygen is consumed, biofilm forms, sediment settles at low points and behind closed valves. Then a fire occurs, the pump starts, and the line goes from zero to design velocity in seconds, carrying that sediment load through every elbow and strainer.

The failures that follow are predictable. Pits under the sediment at low points. Crevice attack at hydrant branch connections. Erosion-corrosion at the first elbow after the pump. Blocked strainers and nozzles from dislodged growth. None of these is caused by the water being salty; all of them are caused by the water being still.

Firefighting element	Recommended material	Reason
Fire main, wet, large bore	6Mo, super duplex 2507, or 90/10 CuNi	Long stagnation plus sudden high flow
Fire main, dry	Carbon steel with internal coating, or 2205	Dry condition removes the crevice driver
Hydrant branches and valves	Same grade as main, welded not threaded	Branch connection is the classic crevice
Deluge and sprinkler nozzles	6Mo or 2507	Small orifice, stagnant, easily blocked by corrosion product
Foam concentrate piping	316L	Concentrate chemistry, not seawater, governs
Fire pump casing	Duplex 2205, cast	High velocity at start-up

Two design measures reduce the material burden more than any upgrade. First, arrange the main so it can be fully drained, and drain it during maintenance. Second, flush the main with fresh water after every flow test if the ship has the capacity. A drained main is a main that is not corroding.

There is also a regulatory dimension. SOLAS Chapter II-2 requires the fire main to deliver two simultaneous jets at a minimum hydrant pressure, and classification societies require the material to be suitable for the service. A main that has lost half its bore to corrosion product still passes a visual survey and still fails the flow test.

The three systems diverge on duty cycle, not on water chemistry. Cooling is continuous, ballast is intermittent and anaerobic, firefighting is idle-then-extreme. That single difference explains almost every material recommendation in this guide.

Criterion	Seawater cooling	Ballast	Firefighting
Flow regime	Continuous, controlled	Intermittent, long stagnant periods	Idle for months, then maximum
Dominant failure mode	Crevice corrosion at flanges and supports	MIC and under-deposit pitting	Under-deposit pitting plus erosion at start-up
Temperature	10–50 °C, highest at HX outlet	Ambient, roughly 0–35 °C	Ambient
Oxygen	Replenished by flow	Consumed, anaerobic	Consumed during stagnation
Chlorination	Continuous, 0.2–0.5 mg/L residual	Intermittent, from BWMS	None, unless flushed
Default grade	Duplex 2205	2205 with drainage, or coated steel	6Mo, 2507, or 90/10 CuNi
Grade that fails first	316L	316L, especially small bore	316L and galvanised steel
Cost of failure	Production loss, tube rupture	Regulatory non-compliance, tank damage	Loss of firefighting capability

Read the table down the "default grade" column and the logic is clear. As the duty cycle becomes less continuous, the required alloy content rises. The water did not change; the wetting pattern did.

Copper-nickel wins where biofouling resistance is the dominant requirement, because copper ions suppress marine growth. Titanium wins where velocity, thin walls, and heat transfer dominate, because it is effectively immune to seawater at any velocity. Stainless steel wins on strength, weight, and the absence of copper discharge.

When do copper-nickel and titanium beat stainless steel

The 90/10 and 70/30 copper-nickel alloys were the default seawater piping material for most of the twentieth century, and they remain a legitimate choice. Their advantage is biological: copper released from the surface is toxic to marine organisms, so the pipe stays clean without chlorination.

Their disadvantages are equally real. The maximum velocity is about 3.5 m/s, above which erosion-corrosion becomes rapid. Sulphide-polluted or ammoniacal water attacks them severely. And copper discharge is increasingly restricted in some ports and inland waterways, which is pushing owners toward stainless steel and non-metallics.

Titanium is the technical answer for the hardest locations - tube bundles, high-velocity seawater, thin walls where a pitting perforation is unacceptable. It is not the economic answer for general piping, at roughly five times the material cost of 316L.

Material	Relative material cost (316L = 1.0)	Biofouling resistance	Velocity limit	Best application
316L	1.0	Poor without chlorination	~4 m/s	Closed loops only
Duplex 2205	1.3–1.5	Poor without chlorination	~6 m/s	General seawater piping
6Mo	2.4–2.8	Poor without chlorination	~6 m/s	Small bore, plate HX
Super duplex 2507	2.3–2.7	Poor without chlorination	~6–7 m/s	Hot seawater, BWMS
90/10 CuNi	2.2–2.6	Excellent	~3.5 m/s	Biofouling-critical piping
Titanium Gr2	5.0–6.0	Good	>10 m/s	Tube bundles, high velocity

Costs are indicative and move with nickel and molybdenum prices. Use the ratios for structure, not for budgeting.

Duplex and super duplex are less forgiving than austenitic grades. They need controlled heat input, nitrogen purging, no preheat, limited interpass temperature, and verification of ferrite balance. Get these wrong and the corrosion resistance you paid for disappears in the heat-affected zone.

The metallurgy is straightforward. Duplex steels are roughly half ferrite and half austenite, and that balance gives them their strength and their chloride resistance. Welding disturbs the balance: too fast a cooling rate leaves excess ferrite, too slow a rate precipitates sigma phase and nitrides.

Both errors reduce corrosion resistance, and neither is visible on a finished weld. That is why duplex welding is a procedure-qualification problem, not a skill problem.

Parameter	Duplex 2205	Super duplex 2507
Preheat	None	None
Heat input	0.5–2.0 kJ/mm	0.5–1.5 kJ/mm
Interpass temperature	≤150 °C	≤100–150 °C
Root purge oxygen	<100 ppm	<50 ppm
Filler metal	ER2209	ER2594
Ferrite in weld metal	35–65%	35–65%
Post-weld heat treatment	Not permitted	Not permitted

Two further fabrication points apply to all the grades in this guide. First, iron contamination: carbon steel tooling, grinding dust, and fabrication-yard debris embed iron particles in the stainless surface, and those particles rust and initiate pitting. Dedicated stainless tooling and a final pickling or passivation step are not optional.

Second, surface finish matters more than most specifications admit. A rough weld cap or a ground surface with embedded contamination will pit before the parent metal, regardless of alloy content. Specify the finish, inspect it, and do not accept "as welded" on a seawater line.

On a 25-year basis, duplex 2205 is usually the lowest-cost option for seawater piping, and 316L is usually the most expensive - more expensive than carbon steel. The cheapest material per kilogram is rarely the cheapest material per service life.

The table below models 400 metres of DN200 seawater cooling line. Installed cost includes material, fabrication, and installation. Replacement cost includes removal, access, reinstallation, and lost availability, which is why it is higher than the original installation.

Option	Installed cost, USD	Expected life	Replacements in 25 years	25-year total, USD
Carbon steel, epoxy lined	76,000	7 years	3	442,000
316L	132,000	9 years	2	554,000
Duplex 2205	192,000	25 years	0	192,000
Super duplex 2507	300,000	25+ years	0	300,000
90/10 CuNi	312,000	25 years	0	312,000
GRP / GRE	96,000	20 years	1 partial	250,000

The result that surprises most project teams is the 316L line. It costs 74% more than 2205 over the life of the system, because it is replaced twice and each replacement costs more than the original installation. The second surprise is that carbon steel with a maintained coating beats 316L outright.

GRP is competitive on cost but limited by temperature, pressure class, and fire performance, which is why it dominates ballast and seawater lines on some vessel types and is excluded from others.

These figures are a model, not a quotation. The assumptions are listed in the appendix, and the ranking is robust to reasonable changes in discount rate and downtime cost. What is not robust is any comparison that counts only first cost.

Product standards define the material; test standards prove the properties; project standards define the acceptance criteria. A purchase order that cites only a product standard is incomplete, because it does not say what evidence the mill must supply.

Three layers of documentation govern a marine stainless purchase. Product standards such as ASTM A240 or A790 tell the mill what to make. Test standards such as ASTM G48 or A923 tell the laboratory how to verify it. Project specifications such as NORSOK M-630 tell the supplier what acceptance criteria apply.

Layer	Standard	What it covers
Plate and sheet	ASTM A240 / A480	Composition, mechanical properties, finish
Seamless and welded pipe	ASTM A312, A790, A928	Austenitic and duplex pipe
Heat exchanger tube	ASTM A249, A789	Welded tube for exchangers
Fittings	ASTM A403, A815	Austenitic and duplex fittings
Flanges and forgings	ASTM A182	F304L, F316L, F51, F53, F55, F44
Bar	ASTM A276, A479	Bar and forgings for valves and pumps
Pitting test	ASTM G48 Method A	Critical pitting temperature
Intermetallic phase	ASTM A923	Sigma phase in duplex
Ferrite content	ASTM E562	Ferrite count in duplex welds
Material data sheets	NORSOK M-630, MDS D41 / D51 / D55 / D57	Duplex, super duplex, 6Mo acceptance
Ballast water	IMO BWM Convention, BWMS Code	Treatment standard D-2
Ballast tank coating	IMO PSPC, MSC.215(82)	Coating specification
Fire main	SOLAS Chapter II-2, Reg 10	Capacity and arrangement

For a seawater purchase, the minimum acceptable document package is: mill certificate to EN 10204 3.1, heat-specific chemical analysis including nitrogen, ASTM G48 Method A result at the specified test temperature, and for duplex, an ASTM A923 or ferrite check. If the supplier cannot produce the G48 result for the actual heat, the grade claim is unverified.

Most seawater stainless failures trace back to a specification error, not a material defect. These eight account for the majority of them.

One: specifying "marine grade" instead of a UNS number. The phrase has no definition, so it defaults to the cheapest interpretation. Write S32205, not "marine grade".

Two: ignoring crevices. A grade selected on CPT will fail at the gasket, because CCT is 15–25 °C lower. Design the crevice out, or select for it.

Three: designing for stagnation. Dead legs, undrainable low points, and idle standby branches are where the first pit appears. They are also the easiest problems to fix at the drawing stage.

Four: treating PREN as a guarantee. Two heats with the same PREN can differ in measured CPT. Require the test result, not the calculation.

Five: mixing grades without galvanic isolation. A stainless line connected to carbon steel becomes a large cathode driving a small anode. Use isolating kits at transitions, and never use graphite gaskets.

Six: specifying duplex without fabrication controls. Heat input, purge oxygen, interpass temperature, and ferrite balance must be in the specification, not left to the shop.

Seven: forgetting the biofouling strategy. Without chlorination or mechanical cleaning, stainless seawater lines foul and then corrode under the deposit. Material selection and fouling control are one decision.

Eight: buying on cost per kilogram. The 25-year comparison shows 316L costing more than 2205. First cost is not a proxy for life-cycle cost.

Is 316 stainless steel suitable for seawater?

No, not for continuously immersed service. 316L has a critical pitting temperature of only about 20–25 °C and a lower critical crevice temperature, so it pits at gaskets, supports, and under deposits in most seawater systems. It is suitable for closed cooling loops, deck hardware, and splash-zone items that stay dry.

What is the best stainless steel for seawater cooling?

Duplex 2205 is the best general choice for seawater cooling piping and plate heat exchangers. Use super duplex 2507 or 6Mo where the outlet temperature exceeds about 50 °C, and titanium for critical tube bundles.

What PREN value do I need for seawater?

PREN 35 is the practical minimum for continuously flowing cold seawater, and PREN 40 or higher is needed for warm, stagnant, or crevice-prone service. PREN is a screening number; confirm the actual resistance with an ASTM G48 test on the heat.

Can duplex 2205 be used for ballast water piping?

Yes, provided the line is drainable and the chlorination regime is understood. 2205 is widely used for main ballast piping. Small-bore ballast lines and any section downstream of a chlorination injection point should be upgraded to 6Mo or 2507.

Why does a fire main need a higher grade than a cooling line?

Because it is stagnant for months and then flows at maximum velocity. That combination produces under-deposit pitting during idle periods and erosion at start-up. 6Mo, super duplex 2507, or 90/10 CuNi are the usual choices; 316L and galvanised steel are the usual failures.

What is the difference between CPT and CCT?

CPT is the temperature at which pitting starts on an open, freely exposed surface. CCT is the lower temperature at which pitting starts inside a crevice. Because real components contain crevices, CCT is the number that governs service performance.

Is super duplex 2507 worth the extra cost?

It is worth it where the service temperature exceeds about 50 °C, where chloride concentration is elevated, or where the component cannot be replaced. Below those conditions, 2205 delivers the same service life at roughly half the material cost.

Can stainless steel be used in ballast tanks?

Yes. 316L is common in chemical tanker ballast tanks and duplex 2205 is used where chloride or temperature is higher. Coated carbon steel remains the default for dedicated ballast tanks because the coating, not the steel, is the corrosion barrier.

How does chlorination affect material selection?

Chlorination prevents biofilm formation, and biofilm raises the corrosion potential of stainless steel by 200–400 mV. Chlorinated seawater is therefore often less aggressive than clean unchlorinated seawater. However, the concentrated hypochlorite at an electrochlorination injection point is highly aggressive and requires 2507, 6Mo, or titanium.

Which filler metal should be used for duplex welding?

ER2209 for duplex 2205 and ER2594 for super duplex 2507. Both require nitrogen purging, controlled heat input, and interpass temperature limits, and neither permits post-weld heat treatment.

Does stainless steel suffer MIC in seawater?

Yes. Sulfate-reducing bacteria produce sulphide in anaerobic, sediment-loaded conditions, which is exactly the environment inside a stagnant ballast line or an idle fire main. The result is accelerated under-deposit pitting. Higher molybdenum grades and drainable design are the mitigations.

What certification should I require from the mill?

EN 10204 3.1 mill certificates with heat-specific chemistry including nitrogen, ASTM G48 Method A results at the specified temperature, and for duplex, ASTM A923 or a ferrite content check. Third-party witness of the corrosion test is advisable for critical systems.

Specify by UNS number, state the duty cycle explicitly, require the corrosion test on the heat, and define the fabrication controls. A specification that does any one of these without the other three is incomplete.

The checklist below is the minimum content of a purchase specification for a seawater stainless system. It is deliberately short, because a specification that nobody reads is worse than a short one that everybody does.

Item	What to write	Why it matters
Grade	UNS number plus common name	Removes ambiguity from "marine grade"
Product standard	ASTM A240, A790, A928, or A815 as applicable	Defines what the mill must make
Duty	Design temperature, velocity range, stagnant periods	These three inputs drive the grade choice
Corrosion test	ASTM G48 Method A at a stated temperature	Converts a claim into evidence
Microstructure	Ferrite content for duplex, A923 where specified	Detects harmful phases
Surface finish	Pickled and passivated, or specified Ra	Finish controls initiation sites
Fabrication	Heat input, purge oxygen, interpass limit	Protects the HAZ corrosion resistance
Documentation	EN 10204 3.1, test reports, heat traceability	Makes the whole chain auditable

Two commercial notes complete the picture. First, lead time for duplex and super duplex is longer than for 316L, and mill rolling schedules for 2507 plate can extend a project by weeks. Order early or accept the substitute. Second, price volatility in molybdenum and nickel means a quotation older than about a month is indicative only.