Hangbo Alloy Group
Shanghai · Est. 2012
Exhaust gas cleaning exposes ships to hot, acidic seawater rich in chlorides. See why 254 SMO, Alloy 926, Inconel 625 and Cu-Ni are specified for marine EGCS.
The IMO 2020 sulphur cap pushed ship operators toward either low-sulphur fuel or exhaust gas cleaning systems that wash combustion gases with seawater or freshwater. Both open-loop and closed-loop scrubbers create one of the most aggressive corrosion environments afloat: a hot, acidic, chloride- and fluoride-laden brine charged with sulphur oxides and soot particles, plus heavy droplet and mist impingement on internal surfaces. Carbon steel and standard stainless simply do not survive, and even the 300-series austenitics fail by pitting and chloride cracking within a few seasons. Nickel-containing alloys are the default specification for the wetted path, from the tower shell to the overboard discharge. As a dedicated nickel alloy manufacturer, Hangbo Alloy Group supplies the plate, pipe and fittings that go into newbuild and retrofit scrubber systems, with the chemistry control and welding procedure support that marine class societies expect.
Scrubber duty combines several attack modes at once. The wash water is acidic (pH can fall below 3 in closed-loop slurry) and rich in chlorides and fluorides from the seawater, while the gas is hot and carries abrasive soot. Surfaces see general corrosion, pitting, crevice attack under deposits, and chloride stress corrosion cracking in fabricated and welded zones. Mist eliminators and spray headers also suffer erosion-corrosion from high-velocity two-phase flow. Material selection therefore keys off pitting resistance equivalent (PREN), chloride tolerance and resistance to acidic chloride - not just a single property.
Scrubber tower, demister and internals are typically 6Mo super-austenitics such as 254 SMO, or higher alloys like Alloy 31 / 59 and Hastelloy C-276 where chloride and acidity are most severe. 254 SMO, Alloy 926 and Inconel 625 compared for seawater shows how PREN climbs across this range; Alloy 926 is a common mid-step above 254 SMO for the most exposed welds and corners.
Seawater and wash-water piping leans on copper-nickel: Cu-Ni 70/30 marine piping and Cu-Ni 90/10 seawater tubing resist biofilm fouling and chloride attack better than most stainless in flowing seawater, which is why they remain the naval and offshore standard.
Liners, spray headers, fasteners and transition pieces often use Inconel 625 for its combination of high PREN, fabricability and strength across the temperature swing from hot gas to cooled wash water.
Open-loop systems pass untreated seawater straight through, so the whole wetted path sees raw seawater chlorides plus captured SOx acidity. Closed-loop systems add alkaline dosing and recirculate, concentrating chlorides and slurry abrasion in the tower while discharging a treated bleed stream. Closed-loop and hybrid designs are generally the harsher case and push specifiers toward the higher end of the alloy range - frequently Alloy 59 / C-276 in the tower rather than 254 SMO. The discharge and overboard lines still follow seawater-piping practice with Cu-Ni or high-alloy stainless depending on velocity and treatment.
Specifiers usually start from a corrosion allowance and PREN target, then verify against the vessel's operating profile - loop type, seawater salinity and temperature, and gas inlet temperature. Our offshore and subsea alloy selection guide covers the same chloride-driven logic used in topside piping, and PREN and pitting resistance of nickel alloys gives the ranking method behind grade choices. For retrofits where space and weight are fixed, fabrication weldability and the ability to qualify procedures on thick plate often decide the final grade as much as the corrosion number does. Class approvals, weld filler matching and post-weld heat treatment limits should be fixed at the design stage, because re-qualifying a welded joint on an installed tower is far more expensive than choosing the right alloy up front. Where budget allows, a small step up in PREN buys margin against the variability of real seawater duty.
The wash water is acidic and heavily chlorinated, so ordinary steel pits and the 316 family is vulnerable to pitting, crevice attack and chloride stress corrosion cracking in welds. Super-austenitic and nickel alloys are specified because they resist this combined acidic-chloride attack.
254 SMO (PREN about 43) is widely used for open-loop towers, but closed-loop and hybrid systems concentrate chlorides and slurry abrasion and often call for Alloy 926, Alloy 59 or Hastelloy C-276. The trade-off is laid out in the 254 SMO vs Alloy 926 vs Inconel 625 comparison.
Flowing seawater lines are commonly copper-nickel: Cu-Ni 70/30 for larger bore and Cu-Ni 90/10 for tubing, because both resist fouling and chloride attack better than stainless in moving seawater.
They use the PREN (pitting resistance equivalent number), which weights chromium, molybdenum and nitrogen. Higher PREN means better resistance to pitting and crevice corrosion in chloride service; the ranking method is explained in our note on PREN and pitting resistance of nickel alloys.
As a dedicated nickel alloy manufacturer serving marine sectors, Hangbo Alloy Group supplies the grades discussed here across all standard product forms with full ASTM/ASME documentation and EN 10204 3.1 certification. Our material is specified for applications where corrosion resistance, high-temperature strength, and traceable quality are non-negotiable. See our production & testing facilities for more on how every heat is verified.