What seawater does to a bushing

Seawater attacks a bushing by four different routes, and a material that handles the first three well can still lose to the fourth.

Pitting and crevice corrosion

Chloride breaks down the protective oxide film locally, and a pit or a crevice then becomes self-sustaining because the solution inside it acidifies and deoxygenates. On a bushing, the crevice is almost always a geometry problem: the press-fit interface, the shoulder between bushing and housing, the underside of a flange face, the gap under a split line. Where the geometry can be changed, change it. Where it cannot, seal it.

Selective phase corrosion

Aluminium bronzes carry a dense, adherent, aluminium-rich oxide film that reforms in oxygenated seawater, which is why they hold up in immersion. The risk that comes with them is dealuminification: aluminium is selectively leached out, leaving a weakened, porous copper-rich layer. The iron- and nickel-rich kappa phases in the microstructure are what resist that attack, which is why nickel content and cooling rate both matter as much as composition. Published guidance within the nickel aluminium bronze family consistently favours the lower-aluminium composition for permanently immersed components and the higher-strength condition for wear surfaces — the two are not interchangeable, and asking for “NAB” without naming the grade is a real procurement risk. High-tensile brasses carry the parallel risk of dezincification and sit a step below the nickel aluminium bronzes for long-term seawater exposure.

Erosion-corrosion and cavitation

Where seawater moves fast, or where vapour bubbles collapse against a surface, the protective film is removed faster than it reforms. Pump impellers, valve trim, propeller-shaft water-lubricated bearings and any restriction that accelerates flow are the classic sites. In these locations a hard, adherent film and good cavitation resistance count for more than bulk tensile strength.

Boundary lubrication, abrasion and intermittent motion

Most hull and deck joints never build a fluid film. They oscillate through a small angle at low speed, under load, and the contact stays in the boundary regime its whole life. Suspended sand and silt become a third-body abrasive. Rain and spray remove whatever grease was applied, and the greasing round slips. Add polluted or stagnant harbour water, where sulphides attack copper alloys, and the fourth mechanism starts to dominate the other three.

Three service zones, three material answers

Dividing the vessel into immersion zones is more useful than dividing it by machinery type, because the corrosion and lubrication conditions change together. Most marine specifications collapse all three zones into one line item, and that is where the field failures come from.

Zone 1 — permanent immersion, below the waterline

Continuously submerged: stern tube bearings, strut and intermediate shaft bearings, rudder pintles and bottom bearings, sea chest and valve bushes, pump internals, jack-up leg guides. Oxygen supply is steady, so the oxide film is continuously repaired; temperature is stable; the dominant risks are selective phase corrosion, crevice attack at the press-fit, and abrasion from suspended solids. Material direction is nickel aluminium bronze — CuAl10Fe5Ni5-C or CuAl10Ni5Fe4 — with CuSn12-C tin bronze where sliding behaviour and resistance to seizure matter more than strength. Water-lubricated bearings in this zone run fast enough to carry a water film, which is a completely different design case from the joints described in Zone 3.

Zone 2 — splash and tidal

Alternately wet and dry: the region around the waterline, davit and gangway pivots, mooring equipment at deck edge, stern gear access hatches, anything in the tidal range. This is where corrosion rates peak. The film is mechanically damaged by wetting and drying, oxygen is at its most available, and salt concentrates by evaporation, so the surface never stabilises. No stable lubricant film exists here either. The material answer is still nickel aluminium bronze for the corrosion side, but the joint also needs a solid-lubricant system, because any grease put in at build will be gone within a season. Crevices at flanges and retaining shoulders must be sealed rather than left to drain.

Zone 3 — deck salt-spray atmosphere

Above the waterline, wetted by spray rather than immersed: deck crane slew and boom pivots, winch and windlass bearings, fairleads, hatch cover hinges and pivots on the weather deck, platform crane pedestals and handling links. The environment is an aerated salt film rather than bulk seawater, so bulk corrosion rates are lower — but the lubrication problem is at its worst. These joints oscillate through small angles at low speed, are washed by rain and spray, and are greased on a schedule that slips. This is where graphite-plugged bronze earns its place, and where leaded tin bronze is acceptable because temperatures stay low.

Service zones are a practical classification for specification, not a classification drawn from any published standard. The wetting condition and the lubrication regime should be recorded together on the drawing for every bushing, because they change independently of the machinery group the part belongs to.

Where the bushings sit on a ship or platform

Propulsion: propeller shaft and stern tube

Water-lubricated stern tube bearings are the one marine application where a genuine fluid film is present. Published practice for this duty puts shaft diameters commonly in the 50–500 mm range, bearing length at roughly two to four times shaft diameter, running clearance on the order of 0.1–0.3 mm, and a shaft surface finish around Ra 0.8–1.6 µm, with wear limits frequently set near 0.5 mm of radial wear before renewal. The bearing is asked to carry the shaft and propeller mass plus propeller-induced bending moments, at a surface speed high enough to form the water film and low enough that the film stays thin. Abrasion from suspended solids and occasional loss of the water supply are the two things that end its life. Oil-lubricated stern tubes are a different design, with the same geometry and a very different material and sealing requirement. Controllable-pitch propeller blade journals and trunnions add an oscillating, high-load, slow-speed contact inside the hub, which is a graphite-plugged duty rather than a water-film duty.

Steering: rudder stock, neck and pintle bearings

Rudder bearings are slow, heavily loaded, intermittent, and largely inaccessible between dry-dockings. The rudder stock carrier and neck bearing take radial load and keep the stock aligned; the pintle bearings at the bottom of the rudder take load in a location that is permanently immersed and, on most designs, effectively unmaintainable at sea. This combination — high load, small oscillation angle, salt water, no lubrication access — is the textbook case for graphite-plugged bronze on a nickel aluminium bronze or high-strength tin bronze matrix. It is also the case where the galvanic question has to be answered deliberately rather than by default.

Deck machinery

Deck cranes carry slew-support bushes, boom-foot pivots and sheave bushes, all oscillating and all exposed to spray. Winches and windlasses run at low speed under high line pull, with the same exposure. Fairleads and mooring equipment see line abrasion plus weather. Hatch cover hinges and pivots are the classic dry, slow, intermittently loaded joint: they move once or twice a day through a few degrees, and the grease applied at build is long gone. For the cylinders that drive this equipment — hatch covers, cranes, davits, ramps — see hydraulic cylinder bronze bushings for deck machinery.

Pumps and valves

Sea-water pump bushes and wear rings, ballast and fire pump bearings, and valve stem bushes and guides all sit in a flow path where velocity is high and the pressure drops. Erosion-corrosion and cavitation are the governing mechanisms, and material selection follows the flow velocity as much as the load. Valve stem bushes add a slow, oscillating, sometimes dry duty on top of the corrosion requirement, which is why a bronze with embedded solid lubricant is frequently specified there.

Offshore platforms

Jack-up leg guides and rack chock pivots, flare boom expansion and sliding pads, blowout preventer handling links, heave compensator pivots, riser tensioner pivots and crane pedestal bearings all combine very high static or slowly varying load with salt spray and long maintenance intervals. The sliding pads and expansion bearings are large flat contacts rather than journal bearings; bronze wear plates with graphite plugs are the usual form. Platform duty in the splash zone is the harshest combination on this page: high load, no grease, and a surface that alternates wet and dry.