Steel Manufacturer Case Study: 2205 Duplex in a Seawater Cooling Loop

A coastal process plant was replacing its seawater cooling lines for the third time, and each replacement had come sooner than the last. Carbon steel had pitted through in the warm return sections. We supplied 220 tonnes of duplex 2205 plate, tube and fittings, cut and bevelled to the fabricator's nest, marked by spool number and delivered in four staged calls. The loop went back into service on the original outage window, and the replacement cycle stopped repeating.

Project Snapshot

The Problem: Warm Seawater and Two Years of Service

Seawater is not simply corrosive. It is corrosive in a specific, local way, and the damage shows up exactly where the water slows down.

The original carbon steel loop had failed at the point where chloride-rich water sits still: behind valves, in the low points of horizontal runs, and in the warm return leg. Pitting starts as a surface mark, then perforates the wall, and once the water reaches the insulation the leak is invisible until it appears on the floor.

A previous replacement in a molybdenum-bearing austenitic stainless steel had fared better but still showed attack in the warm, stagnant sections, and the plant had lost confidence in that option for the return leg. The corrosion engineer's brief was blunt: the next material had to hold in warm chloride service with no protective coating, and it had to be available in cut lengths quickly, because the outage was fixed and the fabricator's programme left no room for a late delivery.

Why We Chose 2205 Duplex Over Carbon Steel, 316L and Titanium

Carbon steel. Excluded. It is the cheapest per tonne and the most expensive over a replacement cycle, and the plant had already proved that on this line three times.

316L. Excluded. Its molybdenum content is enough for many chloride duties but not for a warm, partly stagnant seawater return. This was the option the plant had already tried.

Titanium. Excluded. It performs well in seawater, but the price and the mill lead time put it outside the approved budget and the outage window.

2205 duplex. Selected. Duplex carries far more molybdenum and chromium than the austenitic grades, which raises its resistance to pitting in chloride service, and it is stocked rather than made to order, so cut material was available inside the outage programme.

The 2205 Package We Supplied

The order was specified as a nested cutting job rather than a set of full plates, because the fabricator wanted to weld spools, not trim skeletons. Our yard near Wuxi holds duplex among the grades it keeps on the floor, so the material was drawn from standing stock instead of a mill rolling slot.

Every plate was cut on the laser to the fabricator's nest plan, bevelled where the weld preparation sat on the cut edge, and marked with the spool number from the isometric. Tube and fittings were cut to the same schedule so that a spool could be assembled without matching parts on the ground.

Installation: How the Work Ran

The material went to the fabricator's yard rather than to the site, so our job ended at delivery. That made the sequencing the important part.

Four deliveries were scheduled against the fabricator's welding programme for the four loops, with the return leg first because it was the longest lead item in their shop. Each call contained the spools for one loop, which kept the yard footprint small and stopped cut material from sitting outdoors in coastal air.

Testing and Quality Control

Each heat came with its mill certificate, and we checked the certificate against the grade and the dimensions before the plate was cut. Cut pieces were measured against the nest tolerance, bevelled edges were inspected, and every bundle was checked against the spool list before it was wrapped.

Third-party inspection was arranged for the duplex heats at the fabricator's request, with test certificates issued to the project's quality file. Because the material came from standing stock, the certificates were available at quotation stage, which let the plant's corrosion engineer approve the grade before the order was placed.

Results: What the Numbers Show

The four loops were fabricated inside the outage programme with no delivery held up a spool, and the plant restarted on the planned date. No coating was applied to any part of the loop, which removed a maintenance item from the site's inspection routine.

Because the fabricator received nested cut material with the hard marking already done, shop hours on preparation dropped against its estimate, and the cut-edge tolerance meant weld fit-up needed no on-site dressing. The line has now run through two cooling seasons with no pitting reported in the warm return sections, which is where the earlier materials failed first. Duplex is held in our stainless steel stock alongside the austenitic grades.

What This Case Means for Your Project

Material choice on a seawater line is a life-cycle question, not a price-per-tonne question. The additional alloy content costs more at the mill gate and stops the loop from being replaced every few years, which is where the money actually sits.

If you are specifying for seawater, brine or another chloride duty, send us the medium, the temperature and the wall thickness, and tell us whether you are cutting yourself or want it nested. You can also compare seamless and welded tube for the parts you would rather buy finished. Our team will confirm the grade from standing stock, cut and mark to your spool list, and stage the deliveries. You can also compare it against the marine grades we hold, or send your specification for a quotation with mill certificates attached.