For a project that will sit in seawater for 20 to 30 years, the choice of material is not a line item in a procurement spreadsheet; it is a decision that quietly shapes the maintenance schedule, the inspection budget, and the operating risk for the rest of the asset's life. Three material families dominate this conversation: copper nickel alloy, stainless steel (typically 316L or super-duplex grades), and commercially pure titanium. Each one solves the seawater problem in a different way, and the “right” answer depends on what the system actually has to do.

This guide walks through the metallurgical logic behind each choice, lays out a side-by-side comparison of cost, weight, fabrication, and service life, and ends with a decision framework that an engineer or buyer can actually apply to a real project datasheet. The examples and standards referenced reflect what EZ Steel Industrial manufactures and ships into shipbuilding, offshore, and process-plant projects every week from its mill in Changsha, China.

Seawater is roughly 3.5% dissolved salts, dominated by chloride ions, and it carries dissolved oxygen, biological organisms, and often sulfide pollution from harbor or estuarine environments. That combination attacks metals in three overlapping ways:

– Pitting and crevice corrosion from chloride breakdown of the passive film, especially under gaskets, deposits, or marine growth.

– Erosion-corrosion at high local velocities, where the protective film is mechanically stripped faster than it can heal.

– Galvanic corrosion when two dissimilar metals are coupled in the same electrolyte, with the more noble metal accelerating the loss of the less noble one.

Any one of these can end a piping system's life early. The trick is to pick a material that resists all three under the actual operating conditions of the project, not just the conditions on the original datasheet.

Copper nickel alloy (90/10 and 70/30)

Adding 10% to 30% nickel to copper produces an alloy that forms a thin, adherent, self-healing oxide film in contact with seawater. The film is what gives the alloy its corrosion resistance and, as a practical side effect, suppresses marine biofouling on the inner pipe wall. Copper nickel alloy is the historical default for shipboard seawater cooling, firefighting mains, and platform utility systems, and it is also widely used in refinery overhead condensers and desalination plant brine sections.

Stainless steel (316L, 904L, super-duplex 2507)

Stainless steel pipe is the workhorse of chemical, pharmaceutical, and hygienic process systems, and it shows up in seawater service wherever the project can justify the higher alloy cost. Standard 316L is acceptable for low-chloride, low-temperature water but generally underperforms in warm seawater because of pitting and crevice attack. Higher-nickel grades (904L) and super-duplex grades (UNS S32750, S32760) push the resistance boundary outward, but the cost of those grades rises fast, and the assembly is more sensitive to welding procedure and surface finish.

Commercially pure titanium (Grades 1 and 2)

Titanium is the most corrosion-resistant of the three in almost any chloride environment, and it is essentially immune to pitting, crevice corrosion, and stress-corrosion cracking in seawater. The trade-off is cost (typically several times the price of 90/10 copper nickel) and fabrication: titanium needs dedicated tooling, argon-purged welding, and a fabrication team that has done it before. Where it is specified, it is usually because the operating environment is too aggressive for either copper nickel or stainless, or because weight and lifecycle cost are dominating the design.

The table below summarizes the trade-offs that matter most to a buyer or engineer evaluating the three material families for the same service. Numbers are typical ranges; actual datasheets from the mill should always govern the final selection.

Looking at the actual projects the company has supplied, the choice between these three material families usually comes down to the operating environment rather than the budget alone. Three reference cases illustrate the pattern.

South-to-North Water Diversion and other large water schemes

Fresh-to-brackish water in large-diameter trunk lines, where pressure rating and weldability matter more than chloride resistance. Here, the company has supplied pressure tubes in carbon steel to ASTM A106, A53, and API 5L for the main line, and matched pipe fittings in the same material for the joint assemblies. Stainless and titanium are not justified at this scale, and copper nickel is reserved for the pumping-station auxiliary seawater systems.

Shipbuilding and offshore platforms

Shipboard seawater cooling, firefighting, ballast, and sanitary systems are the historical home of copper nickel alloy. The matching steel flanges and butt-weld fittings from the same procurement package ensure the joint metallurgy is consistent with the tube. For higher-temperature or higher-velocity service on offshore platforms, the project can be quoted in 70/30 copper nickel or, where the operator insists on titanium, in ASTM B338 Grade 2.

Refinery overhead condensers and petrochemical process

Refinery overhead systems combine chloride exposure with elevated temperature, and the alloy choice is sensitive to both. 90/10 or 70/30 copper nickel is the default for the cooler sections, with stainless steel and titanium specified only where the temperature or chloride concentration exceeds the copper nickel envelope. EZ Steel Industrial supplies the whole package from one mill: copper nickel tubes, copper nickel flanges, industrial valves in matched material, and the gaskets and stud bolts that complete the joint.

Use this five-step sequence when the datasheet lands on your desk and the material is not yet fixed. It mirrors the questions the EZ Steel Industrial technical team asks before they put numbers on a quotation.

1. Define the fluid and the operating envelope. Seawater vs brackish vs process chloride; maximum sustained velocity; maximum temperature; presence of sulfides or pollutants.
2. Check the velocity cap. If sustained velocity stays under 3.5 m/s and temperatures under about 80 °C, 90/10 copper nickel is usually the lowest-risk choice.
3. Decide on 90/10 vs 70/30. Default to 90/10 unless velocity, temperature, or chemistry pushes the project above its envelope. 70/30 earns its premium in higher-velocity condensers, hotter brines, and more polluted harbor water.
4. Promote to stainless only if the design rules out copper nickel. Specify 316L for mildly corrosive process service; promote to 904L or super-duplex only when the operating conditions genuinely require it and the budget can carry the alloy cost.
5. Promote to titanium only if both of the above fail. Titanium is the right call for hot, chloride-rich, oxygen-saturated service, or for weight-sensitive offshore applications where lifecycle cost dominates capital cost. It is rarely the right call on a short pipe run in temperate seawater.

In the majority of shipbuilding, offshore, and process-plant projects the company sees, the answer is copper nickel 90/10 with matched copper nickel flanges, or copper nickel 70/30 where the envelope demands it. Stainless and titanium each have their place, but they sit further down the selection tree than most specifications acknowledge.

Whichever material the project lands on, the datasheet should give the buyer enough information to compare two suppliers on equal terms. A serious seawater-material datasheet will include:

– Standard and grade quoted against the right ASTM / EN / EEMUA / JIS reference, not just a generic “seawater grade” label.

– Chemical composition for the actual heat, including iron and manganese for copper nickel, molybdenum for stainless, and residuals for titanium.

– Mechanical properties in the supplied condition, including yield strength, tensile strength, and elongation.

– Dimensional tolerances for outside diameter, wall thickness, and length, cross-checked against the standard's table.

– Testing and inspection regime — hydrostatic test, eddy current or ultrasonic NDT, flattening test, and grain size where the standard calls for them.

– Mill certificate to EN 10204 3.1 (or 3.2 where the project requires third-party inspection), traceable to the heat number marked on each tube.

If any of these are missing, the quotation is incomplete. A seawater system is the wrong place to discover that the certificate does not match the material.

A copper nickel tube welded to a carbon steel flange will fail at the joint long before the tube reaches end-of-life, because of galvanic mismatch. A stainless steel valve screwed into a copper nickel line creates the same problem in miniature. The procurement lesson is that the seawater system is not a collection of separate line items; it is a single metallurgical package that needs to ship together, with mill certificates that line up heat for heat.

EZ Steel Industrial's seawater material package covers the full scope: copper nickel tubes and pipes to ASTM B466, B111, EN 12451, EEMUA 144/234, JIS H 3300, and GB/T 8890; matching copper nickel flanges in 90/10 and 70/30; butt-weld, socket-weld, and threaded fittings from the broader fittings range; industrial valves in matched material for isolation and control; and the gasket, stud bolt, and nut selection that keeps the bolted joint from becoming the weak link.

There is no universally “best” material for seawater service; there is only the best material for the specific combination of fluid chemistry, velocity, temperature, fabrication constraints, and lifecycle cost that the project defines. For most marine, offshore, and process-plant duties, that answer is copper nickel alloy — 90/10 as the default, 70/30 where the envelope demands it. Stainless and titanium each win in their own corner, but they are answers to a more specific question.

What matters more than the material itself is that the system ships as a coordinated package, with matched tube, flange, valve, fitting, and joint components from a single source that can document the whole chain. That is the procurement pattern that reliably delivers a 25-to-30-year service life in seawater, and it is the one that EZ Steel Industrial's project team is set up to support.