Inside the acid: how ships’ scrubbers beat corrosion with nickel alloys, FRP, and relentless inspections

The inside of a ship’s exhaust gas cleaning unit — a marine SOx scrubber — is a brutal place. Seawater absorbs SO₂ and turns into sulfuric and chloric acids, creating hot, acidic, chloride-rich liquids that scour towers and piping. As the Nickel Institute puts it, “the hot acidic chloride solutions require the use of highly corrosion-resistant nickel alloys” (nickelinstitute.org).

The penalty for missteps is swift. Field tests showed a duplex stainless alloy (254 SMO) losing ~0.7 mm/year under severe scrubber conditions while nickel‑rich Alloy 59 recorded almost zero measurable loss (marinelink.com). Extrapolated over a decade, 0.7 mm/year would exceed typical wall thickness, inviting leakage and cracking (marinelink.com). Sea‑trial reports have even logged pipework failures within months when inferior materials or coatings were used (lloydslist.com).

The stakes are big. By 2020, an estimated 3,000 ships — roughly 30% of the world merchant fleet — were fitted with scrubbers to hit the IMO’s 0.5% sulfur cap (lloydslist.com). DNV projects that by 2025 about 81% of scrubbers globally will be open‑loop systems (direct seawater wash) (id.scribd.com), which means vast volumes of untreated seawater — and chloride attack — in circulation. When retrofits surged, DNV also noted a 4–5‑month backlog due to shortages of glass‑reinforced pipe and welders in early 2020 (dnv.com).

Scrubber chemistry and operating envelope

Inside an operating scrubber, materials must resist pitting and crevice corrosion (localized attack modes that rapidly penetrate metal) in hot acidic chloride solutions, often above 50 °C with turbulent flow. This is one of the harshest industrial environments, and it dictates the metals and non‑metals that survive (nickelinstitute.org).

High‑nickel alloys for wet zones

Nickel‑molybdenum‑chromium families such as Hastelloy C‑276, Alloy 59 (UNS N06059), and Alloy 31 are recommended for towers and venturi sections directly exposed to the acidic wash (nickelinstitute.org). These alloys contain roughly 30–60% Ni with high Mo/Cr and have shown essentially zero corrosion under test conditions (marinelink.com), unlike stainless steels with ~20% Ni (e.g., 254 SMO), which corroded rapidly.

Within this group, Alloy 59 has been singled out: one study found near‑zero thickness loss in aggressive wash‑water, with its ~60% Ni, 29% Mo, 9% Cr composition delivering unusual resistance to sulfuric and chloric acids (marinelink.com).

Using duplex and advanced austenitics judiciously

Duplex grades such as 2205 (22% Cr, 5% Ni, 3% Mo) appear in moderately corrosive sections where temperatures and acid strength are lower, trading some corrosion margin for strength and cost (hynickelalloy.com). Field experience indicates even duplex steels will show detectable corrosion over time.

Superduplex (e.g., S32760; ~25% Cr, 3.5% Mo, 7% Ni plus N) can improve pitting resistance and may serve in upper tower sections or drain lines (no direct citations available, but common in industry). High‑grade austenitics such as AL‑6XN (24% Cr, 6% Mo, 22% Ni) often populate zones above the liquid level; at least one design pairs Alloy 59 in the hot bottom zone and AL‑6XN higher up (marinelink.com).

Lower‑grade stainless steels (304/316) are generally unsuitable inside the scrubber due to rapid pitting in chlorinated acid; even 316L can corrode quickly if any upset occurs. In practice, 316L might only appear in external fresh‑water plumbing — for example, in 316L stainless steel housings used in less aggressive service — not within the scrubber’s wet zone.

Welding quality and NDT expectations

Fabrication quality is decisive. Poor welds become attack points, so filler metals should be at least as corrosion‑resistant as the base alloy, and critical welds undergo 100% non‑destructive testing (NDT) at the factory, including UT (ultrasonic testing) and visual inspection, to avoid hidden crevices (marinelink.com). Industry voices note that truly corrosion‑resistant scrubbers combine premium materials and welds, while cheaper systems that cut corners are prone to pintail leaks (marinelink.com; lloydslist.com).

FRP and thermoplastics in corrosive duty

FRP (fiberglass‑reinforced plastic) resists chloride and acid attack because it has no metal to corrode. It is common in scrubber modules, stacks, or piping where temperatures permit, especially above the liquid level and in overhead ducts (torch-air.com; slyinc.com). The advantages are chemical resistance, light weight, and low maintenance; vendors describe FRP scrubbers as “minimal maintenance” with long service life, often greater than 10–15 years in corrosive duty (slyinc.com; torch-air.com).

FRP’s limits are real: maximum continuous temperature is typically ~80–100 °C, so it is reserved for cooler sections. Structural support is also critical; DNV cautions that GRE (a common FRP) pipes up to 1 m diameter need supporting struts to avoid damage when heavy washwater flushes through them (dnv.com). As an example of FRP components suited to aggressive media, lightweight composite housings are chemical and seawater resistant.

Thermoplastics such as polypropylene (PP) and PVDF appear in acid scrubber designs alongside stainless steel and FRP, typically in smaller parts like pump housings or valve bodies. Their sulfuric‑acid resistance is good up to about 80 °C, beyond which PP softens (torch-air.com). FRP housings — for instance, FRP filter housings designed to resist chlorine and acids — mirror the same corrosion logic in ancillary equipment.

Rubber‑lined steel and coatings experience

Rubber‑lined carbon steel is a proven Flue Gas Desulfurization (FGD) approach now adopted in marine scrubbers. Natural rubber, neoprene, and chlorobutyl linings offer resistance to sulfuric acid and abrasion, combining the strength of steel with a chemical barrier (rubberlining.org). Guidance notes that “Rubber has good resistance to sulfuric acid” and, with proper specification, one can expect 15–20 years’ service life; quality rubber linings can outlast many welded metallic systems at far lower initial cost (rubberlining.org).

By contrast, non‑metallic coatings (epoxy, ceramic) inside scrubbers rarely survive the hot, erosive spray environment, and ceramics are brittle. These remain more a research topic than a widespread solution (marinelink.com).

Class rules and inspection routines

Regulators now bake corrosion awareness into the rulebook. After 2017, DNV GL explicitly required that “exhaust gas cleaning units and exhaust piping exposed to cleaning water or treated exhaust shall be suitable for the corrosive properties of the two media,” and in practice this means routine thickness measurements and structural surveys (dnv.com). Leakage and corrosion frequently show up in scrubber overboard pipes, with DNV recommending annual ultrasonic thickness (UT) gauging; instrumentation suppliers echo the need to regularly check for signs of corrosion (dnv.com; cygnus-instruments.com).

Ultrasonic gauging and weld checks

UT gauging quantifies wall loss on tower walls, spray floors, discharge lines, outlets, and overboard pipes before leaks occur; modern gauges can even measure through paint and coatings (cygnus-instruments.com; cygnus-instruments.com). Visual inspection of welds and joints (including stainless‑to‑nickel transitions) looks for cracks, pitting, or crevice traps; the build standard targets 100% NDT of critical welds (marinelink.com).

Cleaning, neutralizing, and sensor discipline

Scheduled flushing removes sludge or gypsum and keeps the system within design parameters; MARPOL mandates ≥6.5 pH at discharge, so operators ensure pH and chloride stay within limits during operations. Accurate chemical dosing supports these cleaning and neutralizing routines, which is why equipment such as an industrial dosing pump is aligned with the control task.

Sensors for pH, salinity, and flow drift or foul quickly in this environment. DNV warns that sensor failure is a leading risk; a bad sensor can misconfirm compliance or force shutdown. Schedules for cleaning, calibration, replacement, and cross‑checking with sister sensors reduce that risk (dnv.com).

Repair thresholds and downtime impact

Any detected coating damage or thinning is repaired immediately; rubber‑lined vessels follow manufacturer rules for re‑lining once elastomer thickness falls below spec (rubberlining.org). The payoff is reliability. One case reported 0.0012% scrubber downtime over 12 months — 60 minutes in a year — attributed to proactive care (lloydslist.com). The flip side is costly pipework replacement in less than six months when corrosion is allowed to get ahead (lloydslist.com; marinelink.com).

Material choices and maintenance together

The pattern is consistent across fleets and yards: use high‑Ni, high‑Mo alloys (Alloy 59, C‑276, Alloy 31) in the most aggressive wet zones (nickelinstitute.org; marinelink.com), AL‑6XN or superduplex where conditions are less severe, and avoid standard stainless inside the scrubber. For non‑metals, FRP and plastics are compelling in cooler sections, while rubber‑lined steel excels in abrasive acidic slurries (slyinc.com; torch-air.com; rubberlining.org; dnv.com). Pair that with annual UT gauging (especially outlets and overboard pipes) and disciplined sensor maintenance, which have become standard practice (dnv.com; cygnus-instruments.com).

Sources: Authoritative industry and research publications have been cited throughout (see references) to provide data‑driven guidance (nickelinstitute.org; marinelink.com; torch-air.com; rubberlining.org; dnv.com; cygnus-instruments.com; lloydslist.com; id.scribd.com).