Preventing Corrosion and Extending Scale Life at Sea

Stainless steel scales rarely fail because the steel dissolves across its whole surface. They fail at details: a carbon steel bolt, a gasket that traps salt water, a cable gland that was never tightened after service, a connector left uncapped. This article explains the corrosion mechanisms that matter at sea, how to prevent them at the mounting, cable and connector level, and gives a maintenance schedule you can adapt for your vessel.

The forms of corrosion that affect marine scales

Grade selection and welding are covered in AISI 304 vs 316 vs 316L; this article focuses on what happens after installation. Mounting location and cabling are covered in installing marine scales.

Galvanic corrosion: stainless steel meets the rest of the ship

Bimetallic corrosion needs four conditions at the same time, according to the UK National Physical Laboratory guide on bimetallic corrosion:

1. an electrolyte bridging the two metals, which can be a bulk liquid, a condensed film, salt deposits or corrosion products;
2. an electrical connection, through direct contact or indirectly through structures, conductors or earthing;
3. a sufficient difference in potential between the two metals;
4. a sustained cathodic reaction on the more noble metal, in practice usually reduction of dissolved oxygen.

Seawater is an effective electrolyte: NPL gives a typical conductivity of 40 000 µS/cm, against 50-1 500 µS/cm for supply water. In NPL's simplified galvanic series in seawater, aluminium alloys, zinc, carbon steel and cast iron sit on the electronegative side of passive stainless steels such as 316, so they are the metals that corrode when coupled to a stainless scale. 316 itself appears twice in the series, because its potential is very different when its passive film has broken down in a crevice.

Area ratio decides severity. The larger the cathode compared with the anode, the greater the corrosion of the anode (NPL, section 3.5). A carbon steel or aluminium bolt holding a large stainless frame is the worst arrangement; a stainless bolt in a large aluminium deck plate is far less severe, though not risk-free.

Preventing galvanic corrosion at the mounting

• Use fasteners of the same or a more noble material than the parts they join (NPL).
• Insulate dissimilar metals with plastic bushes and washers such as nylon or PTFE, or impervious gaskets.
• Remember that jointing compounds do not usually insulate electrically, but they do exclude water from the joint and so prevent bimetallic and crevice corrosion within it.
• If only one metal can be painted, paint the more noble (cathodic) one; painting only the anode concentrates attack at coating defects.
• Check insulation after installation: NPL warns that earthing, which is often a safety requirement on offshore installations, can completely bypass insulating washers. Agree the earthing and bonding arrangement with the vessel's electrician rather than removing an earth.

Crevices, cable glands and connectors

IMOA lists threaded connections, O-ring seals, gasket seals and connectors among common crevice formers, together with dirt, grease and tape. On a scale these are exactly the places where the enclosure is sealed, so maintenance of seals serves two purposes: keeping water out of electronics and keeping salt water from standing in crevices.

• Cable glands. The IP rating of an enclosure assumes that it is correctly assembled; a loose gland or a cable of the wrong diameter breaks that assumption. Re-tighten glands to the manufacturer's instructions after any cable work, and replace sealing inserts that have hardened or cracked.
• Cable routing. Route cables so that water runs away from the entry, with the cable dropping below the gland before it rises, and avoid entries on top surfaces.
• Connectors. Fit protective caps on every unused connector, keep caps on their tethers, and inspect pins for green or white deposits. Dry connectors before mating them.
• Cable sheaths. Look for cuts, crushing under platforms and abrasion at edges; water can travel inside a damaged cable into the load cell or indicator.
• Tape and temporary repairs. Tape wrapped around cables or fittings creates a crevice on stainless surfaces; replace it with a permanent solution.

IEC 60529 explicitly leaves corrosion and condensation outside the scope of IP testing, so an IP rating does not replace inspection; see IP67, IP68 and IP69K explained.

Maintenance schedule

This schedule is an example based on the mechanisms above; adapt it to the equipment manual, operating hours and exposure.

Opening sealed parts can affect legal verification; plan such work as described in calibration, verification and re-verification.

Dealing with rust spots and stains

Most brown spots on stainless steel scales are surface contamination rather than failure of the steel. The Nickel Institute handbook and IMOA point to three practical rules:

1. Clean mildly first. Warm water with detergent, followed by mild non-scratching cleaners, removes most deposits; always rinse thoroughly with fresh water and dry.
2. Use the right tools. Use soft cloths, fibre brushes, plastic or stainless steel pads; carbon steel brushes and steel wool leave particles that rust.
3. Look for the source. Recurring spots in one place usually mean a carbon steel tool, grinding work nearby or a crevice holding salt water. Deep pits, cracking near welds or corrosion at load cell mounts need assessment by a service engineer.

How WPL approaches this

WPL's M5 and M6 platform scales use four load cells and AISI 316 construction, and the R10 Configuration Panel supports diagnostics, maintenance and backup and recovery of scale data when a scale is serviced. The hygiene and materials hub connects all construction topics.