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Marine Engineering

Seawater is a complex electrolyte containing approximately 3.5% dissolved chlorides, oxygen, living organisms, and suspended solids. It attacks metals through multiple simultaneous mechanisms: chloride pitting, crevice corrosion, galvanic coupling, microbiologically influenced corrosion (MIC), and flow-accelerated erosion. No single material resists all of these perfectly, but the correct alloy selection—matched to the specific service condition—can extend component life from years to decades.
 
At JN Alloy, we supply a full spectrum of marine-grade stainless steels, duplex and super duplex grades, super austenitic alloys, and nickel-based alloys engineered for seawater service. Our materials are produced to ASTM, ASME, and NACE standards, with full mill test certification and traceability. This page serves as your technical reference for understanding corrosion mechanisms, interpreting PREN values, and selecting the optimal alloy for each marine application.

Application Scenarios

Offshore Platforms

Offshore Platforms

Marine Infrastructure

Marine Infrastructure

Cruise Ship and Cargo Carriers

Cruise Ships and Cargo Carriers

How Seawater Attacks Metals: Corrosion Mechanisms Explained

Understanding corrosion mechanisms is the foundation of alloy selection. Seawater does not attack all metals in the same way—different mechanisms dominate depending on temperature, flow velocity, oxygen content, and geometric constraints. The five primary corrosion modes in marine environments are:

  • Chloride Pitting Corrosion
    Chloride ions (Cl-) penetrate the passive oxide film on stainless steel at microscopic weak points, creating localized pits that grow rapidly. Pitting is the #1 failure mode for under-spec'd stainless in seawater. It is stochastic—once a pit initiates, it autocatalyzes and can perforate a pipe wall in weeks. Resistance is quantified by the PREN value (see below). Alloys with PREN above 32 resist pitting in ambient seawater; PREN above 40 is needed for warm or chlorinated seawater.
  • Crevice Corrosion
    Crevice corrosion occurs in narrow gaps—under gaskets, bolt heads, barnacles, or sediment deposits—where oxygen is depleted and chlorides concentrate. It is more aggressive than pitting because the crevice geometry creates a differential aeration cell that drives sustained attack. Crevice corrosion initiates at a Critical Crevice Temperature (CCT), typically 10-15°C lower than the Critical Pitting Temperature (CPT) for the same alloy. Super duplex and nickel alloys offer the best crevice corrosion resistance.
  • Galvanic Corrosion
    When two dissimilar metals are electrically connected in seawater (an efficient electrolyte), the less noble metal becomes the anode and corrodes preferentially. For example, connecting 316 stainless steel to carbon steel in seawater will accelerate carbon steel corrosion dramatically. Marine engineering designs must carefully manage galvanic couples using compatible material pairs, dielectric isolation, or sacrificial anodes. Nickel alloys and copper alloys occupy different positions on the galvanic series, making material pairing a critical design decision.
  • Stress Corrosion Cracking (SCC)
    SCC is the sudden brittle cracking of a normally ductile metal under the combined action of tensile stress and a corrosive environment. In marine service, chloride SCC affects austenitic stainless steels (especially 304 and 316) at temperatures above 60°C. Duplex stainless steels have significantly better SCC resistance due to their mixed ferrite-austenite microstructure. Nickel alloys are virtually immune to chloride SCC.
  • Microbiologically Influenced Corrosion (MIC)
    Sulfate-reducing bacteria (SRB) and other microorganisms thrive in seawater and produce hydrogen sulfide and organic acids that aggressively attack metals. MIC is particularly insidious in stagnant or low-flow seawater systems, such as ballast tanks, closed cooling loops, and dead legs. It causes localized pitting that can penetrate several millimeters per year. Copper-nickel alloys (e.g., 90/10 CuNi) resist MIC due to natural biotoxicity, while stainless steels require careful flow management and biocide treatment.
  • Erosion-Corrosion & Cavitation
    High-velocity seawater (above 2-3 m/s for copper alloys, above 20 m/s for super duplex) mechanically removes the protective film faster than it can reform. This is erosion-corrosion. Cavitation occurs when collapsing vapor bubbles in pumps and propellers cause mechanical damage combined with corrosion. Nickel aluminum bronze and super duplex 2507 excel in high-flow marine applications due to their combination of hardness and corrosion resistance.

Key Takeaway
No single alloy solves all six corrosion mechanisms. Effective marine material selection requires identifying which mechanisms are active in your specific service condition and choosing an alloy that addresses the dominant threat. The PREN value is the first quantitative screening tool, but temperature, flow rate, and galvanic compatibility must also be evaluated.

Alloys for Marine Engineering

PREN: The Pitting Resistance Equivalent Number Explained

PREN (Pitting Resistance Equivalent Number) is the single most important metric for ranking stainless steels and nickel alloys by their resistance to chloride pitting corrosion in seawater. It is a calculated value based on the alloy's chemical composition—specifically the three elements that build and maintain the passive film: chromium, molybdenum, and nitrogen.
 
The PREN Formula

The standard formula for austenitic and duplex stainless steels is:
 
PREN = %Cr + 3.3 × %Mo + 16 × %N
 
For nickel alloys that also contain tungsten (W), the extended formula is used:
 
PREN = %Cr + 3.3 × (%Mo + 0.5 × %W) + 16 × %N
 
The coefficients reflect each element's relative contribution to pitting resistance: molybdenum is 3.3 times as effective as chromium by weight, and nitrogen is 16 times as effective. This is why small nitrogen additions (0.2-0.3%) in duplex and super austenitic grades dramatically improve seawater performance.
 

PREN Classification and Seawater Suitability

The table below classifies alloys by PREN and indicates their suitability for marine service:

 
PREN Range Corrosion Resistance Seawater Suitability Typical Alloys
< 25 Low Not recommended for immersion; acceptable for atmospheric/splash zone only 304 (PREN ~19), 316/316L (PREN ~24-26)
25 - 32 Moderate Limited seawater use; risk of pitting and crevice corrosion in warm conditions 317L (PREN ~30), 904L (PREN ~35-37)
32 - 40 High Suitable for ambient seawater; some risk at elevated temperatures or in crevices Duplex 2205 (PREN ~34-38), 254 SMO (PREN ~43)
> 40 Very High Recommended for continuous seawater immersion, warm seawater, and critical service Super Duplex 2507 (PREN ~42-46), Alloy 625 (PREN ~41-51), Zeron 100 (PREN ~41)
> 50 Extreme For the most severe conditions: hot chlorinated seawater, acid gas environments Alloy C-276 (PREN ~65), Alloy 686 (PREN ~71)

 

Application Scenarios: Alloy Selection by Marine Sector

Marine engineering encompasses three major sectors, each with distinct corrosion challenges. The following sections provide detailed alloy recommendations for each.

 

1. Shipbuilding & Marine Vessels

 

Ships and vessels face corrosion from hull immersion, cargo exposure, ballast water, and engine room heat. The key is matching alloy performance to each zone: submerged hull, splash zone, superstructure, piping systems, and machinery spaces.

 

Seawater Piping & Heat Exchangers

  • Super Duplex 2507: For high-pressure, high-flow seawater systems where copper alloys are inadequate. PREN ~42-46. Used in pump casings, valve bodies, and high-velocity piping.

Hull, Deck & Superstructure

  • 316L Stainless Steel: Used for deck fittings, railings, and architectural elements in the atmospheric marine zone. NOT suitable for continuous immersion. Requires regular cleaning to prevent pitting from salt deposits.
  • Monel 400: Used for boat shafting, propeller shafts, and pump shafts due to excellent combination of strength, corrosion resistance, and fatigue performance in seawater.
  • Monel K-500: Age-hardenable version of Monel 400 with higher strength for shafts, springs, and fasteners requiring mechanical performance plus corrosion resistance.

Exhaust Systems & Engine Components

  • Alloy 625: Nickel-chromium-molybdenum alloy for exhaust manifolds, turbocharger casings, and emission system components. Resists both high-temperature oxidation and chloride attack from salt-laden air.
  • 321 Stainless Steel: Titanium-stabilized grade for moderate-temperature exhaust components. Cost-effective for service below 550°C.
 

2. Offshore Platforms & Oil/Gas Rigs

 

Offshore platforms combine the severity of seawater immersion with high-pressure hydrocarbon service, sour gas (H2S) exposure, and fire risk. Material selection must satisfy NACE MR0175/ISO 15156 for sour service and API standards for structural integrity. The platform is divided into zones: splash zone (most corrosive), submerged zone, atmospheric zone, and process areas.

 

Splash Zone & Submerged Structures

  • Super Duplex 2507 (UNS S32750): The premier structural alloy for splash zone brackets, riser clamps, and fasteners. PREN >40 provides reliable resistance in the most corrosive marine zone where alternating wet/dry cycles concentrate chlorides.
  • Alloy 625 (UNS N06625): Cladding on carbon steel structural members in the splash zone. Also used for subsea connectors, riser joints, and flowline termination assemblies where sour service resistance is required.
  • 254 SMO (UNS S31254): Super austenitic stainless for seawater handling systems, fire water systems, and ballast water treatment units. PREN ~43. Excellent weldability for complex structural fabrications.

Process Piping & Pressure Vessels

  • Duplex 2205 (UNS S32205): For process piping handling corrosive fluids with moderate chloride content. Higher strength than 316L allows thinner wall sections. PREN ~34-38.
  • Alloy 825 (UNS N08825): Nickel-iron-chromium alloy for sour gas service, downhole tubing, and process vessels handling H2S and CO2. NACE MR0175 compliant.
  • Alloy 718 (UNS N07718): Age-hardenable nickel alloy for high-strength fasteners, valve stems, and wellhead components. Combines 1034 MPa yield strength with excellent seawater and sour gas resistance.

Seawater Injection & Cooling Systems

  • 904L (UNS N08904): High-alloy austenitic stainless for seawater injection piping and cooling water systems where 316L is inadequate but super duplex cost is not justified. PREN ~35-37.
  • Super Duplex 2507: For seawater injection pumps, valves, and high-pressure piping where mechanical strength and corrosion resistance are both critical.
 

3. Desalination Plants

 

Desalination is the fastest-growing marine-related industry, with both thermal (MSF, MED) and membrane (SWRO) processes demanding corrosion-resistant materials. The challenge: concentrated brine, high temperatures (up to 120°C in MSF), chlorination, and high operating pressures (up to 80 bar in SWRO). Each process stage has different corrosion drivers and requires different alloys.

 

Seawater Reverse Osmosis (SWRO) Plants

  • Intake & Pre-treatment (ambient seawater): 316L for low-stress non-critical components; 904L or 254 SMO for chlorinated intake screens and filter housings where chlorine dosing raises the oxidation potential.
  • High-Pressure Piping (60-80 bar): Super Duplex 2507 is the industry standard. Its high yield strength (550 MPa min) allows thinner pipe walls than austenitic grades, reducing weight and cost despite higher per-kg price. PREN >40 ensures pitting resistance in concentrated brine.
  • Energy Recovery Devices & Pumps: Super Duplex 2507 for rotor and casing components. Alloy 718 for high-stress shaft components.
  • Brine Discharge: 316L or Duplex 2205, as the brine is at ambient temperature and the corrosion severity is moderate.

Multi-Stage Flash (MSF) & Multi-Effect Distillation (MED)

  • Low-Temperature Stages (<70°C): 316L is widely used and generally adequate, though 904L provides greater margin against pitting in high-salinity brine.
  • High-Temperature Stages (70-120°C): 904L or 254 SMO for heat exchanger tubes and flash chamber linings. At these temperatures, 316L suffers rapid pitting and crevice attack.
  • Brine Heater & Top Brine Temperature Zone: Alloy 625 or titanium for the most critical high-temperature heat transfer surfaces. These alloys resist both pitting and stress corrosion cracking at 110-120°C in concentrated brine.
  • Vent & Ejector Systems: Alloy 825 for non-condensable gas handling, where CO2 and O2 are concentrated.
 

Alloys for Marine Engineering

JN Alloy supplies the following marine-grade alloys in all product forms—plate, sheet, bar, pipe, tube, fittings, and forgings. Click each alloy for detailed specifications and properties:
 

FAQs

  • What is the best stainless steel for seawater applications?
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    Super duplex stainless steel 2507 (UNS S32750) and super austenitic 254 SMO (UNS S31254) are among the best stainless steels for continuous seawater immersion, both with PREN values above 40. For the most severe conditions—hot chlorinated seawater, sour service, or high-temperature brine—nickel alloys such as Alloy 625 (UNS N06625) provide superior and more reliable resistance. For shipboard seawater piping with moderate flow rates, 90/10 copper-nickel remains the most cost-effective choice due to its natural biotoxicity and proven 20+ year service life.
  • What is PREN and how is it calculated?
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    PREN (Pitting Resistance Equivalent Number) is a calculated value that ranks stainless steels and nickel alloys by their resistance to chloride pitting corrosion. The standard formula is: PREN = %Cr + 3.3 × %Mo + 16 × %N. For nickel alloys containing tungsten, the extended formula adds 0.5 × %W to the molybdenum term. A PREN above 32 is generally required for ambient seawater service, and above 40 for warm seawater or critical submerged applications. PREN is a useful screening tool but should be validated with ASTM G48 (pitting) and G78 (crevice) testing for critical applications.
  • Can 316 stainless steel be used in seawater?
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    316 stainless steel has a PREN of approximately 24-26, which is below the threshold (32) for reliable seawater immersion service. It may be used in splash zones, atmospheric marine exposure, and low-temperature desalination stages (below 40°C) where it is not continuously immersed. In continuous seawater immersion, 316 will develop pitting and crevice corrosion within months to a few years, depending on temperature and flow conditions. For submerged service, upgrade to 904L (PREN ~35), 2205 duplex (PREN ~36), or preferably 2507 super duplex (PREN >42).
  • What alloys are recommended for desalination plants?
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    For SWRO desalination: Super Duplex 2507 for high-pressure piping (60-80 bar), 904L or 254 SMO for chlorinated intake and pre-treatment, and 316L for low-pressure brine discharge. For MSF/MED thermal desalination: 316L for low-temperature stages (<70°C), 904L for intermediate temperatures (70-90°C), 254 SMO for high-temperature stages (90-110°C), and Alloy 625 or titanium for the brine heater at top brine temperature (110-120°C). The general rule: when in doubt, upgrade—the cost of a plant shutdown for corrosion repair far exceeds the material premium.
  • What is the difference between duplex 2205 and super duplex 2507?
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    The main differences are in alloy content and resulting corrosion resistance: 2507 has higher chromium (25% vs 22%), higher molybdenum (4% vs 3%), and higher nitrogen (0.27% vs 0.17%), giving it a PREN of ~42-46 versus ~34-38 for 2205. This means 2507 provides significantly better pitting and crevice corrosion resistance in seawater. 2507 also has higher yield strength (550 MPa vs 450 MPa). Choose 2205 for moderate chloride environments and cost-sensitive projects; choose 2507 for continuous seawater immersion, warm seawater, and critical service where failure is not an option.
  • Why is Monel 400 used for marine shafts?
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    Monel 400 (UNS N04400) is a nickel-copper alloy that offers an exceptional combination of properties for marine shafting: (1) excellent resistance to seawater corrosion across all flow velocities, (2) high fatigue strength essential for rotating shafts, (3) natural biotoxicity that prevents biofouling and MIC, and (4) good machinability for precision shaft manufacturing. For applications requiring even higher strength, the age-hardenable Monel K-500 variant provides yield strength up to 760 MPa while maintaining the same corrosion resistance, making it ideal for pump shafts, valve stems, and high-load fasteners.
  • How does temperature affect seawater corrosion of stainless steel?
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    Temperature is a critical factor in seawater corrosion. As temperature increases, the rate of all corrosion mechanisms accelerates: pitting initiation becomes more probable, crevice corrosion propagates faster, and stress corrosion cracking risk rises sharply above 60°C for standard austenitic grades. Each alloy has a Critical Pitting Temperature (CPT) and Critical Crevice Temperature (CCT) determined by ASTM G150 and G48 tests. For example, 316L has a CPT of approximately 15-20°C in seawater, while 2507 super duplex has a CPT of 70-80°C. This is why MSF desalination plants require progressively higher-grade alloys as brine temperature increases from ambient to 120°C.
  • What standards govern marine-grade stainless steel and nickel alloys?
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    Key standards include: ASTM A240/A240M (stainless steel plate/sheet/strip), ASTM A312/A312M (welded and seamless pipe), ASTM A479/A479M (bars for pressure vessels), ASTM B625 (nickel alloy plate), ASTM B622 (nickel alloy seamless pipe), NACE MR0175/ISO 15156 (sour service qualification), ASME Section II-D (pressure vessel allowable stresses), and ASTM G48/G150 (pitting corrosion test methods). For desalination specifically, ASME Section VIII and NACE standards often apply. JN Alloy supplies all materials with full mill test reports (EN 10204 3.1/3.2) and can provide NACE compliance certification on request.

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