Marine Scale Construction: Stainless Steel, IP Ratings and Hygiene

A marine scale has to meet two demands that pull in different directions: it must be sealed and robust enough for salt spray, green water and daily washdown, and it must be open and smooth enough to be cleaned properly after contact with fish. Construction decides whether it manages both for years or starts to corrode, leak and drift within a season. This hub page brings together materials, ingress protection, hygienic design, cleaning and maintenance, and links to four detailed guides.

Why construction matters more at sea

Weighing equipment on a fishing vessel faces a combination of stresses that land-based food equipment rarely meets all at once.

Stress	Effect on a scale	Construction answer	Detailed guide
Seawater and salt deposits	Pitting and crevice corrosion of stainless steel; galvanic corrosion of other metals	Molybdenum-bearing stainless steel, crevice-free design, insulated mountings	AISI 304 vs 316 vs 316L
Hose, high-pressure and hot-water cleaning	Water ingress into electronics and load cells	Enclosures tested for jets, immersion and high-pressure hot water	IP67, IP68 and IP69K explained
Fish residues, blood, slime	Food safety risk; soil trapped in joints and gaps	Hygienic design and a validated cleaning routine	Cleaning and washdown of scales
Detergents and disinfectants	Attack on seals, plastics and, with the wrong chemicals, stainless steel	Compatible materials; correct chemicals and rinsing	Cleaning and washdown of scales
Vibration, impacts, cable wear	Loose glands, damaged cables, debris under platforms	Robust cable routing and a maintenance schedule	Preventing corrosion and extending scale life

Vessel motion adds a further dimension that affects accuracy rather than durability; it is covered on the weighing at sea hub.

What EU hygiene law requires from equipment on vessels

EU food hygiene law sets outcome requirements rather than naming materials or IP codes.

• Contact surfaces on all vessels must be of suitable corrosion-resistant material that is smooth and easy to clean, and surface coatings must be durable and non-toxic (Regulation (EC) No 853/2004, Annex III, Section VIII, Chapter I, Part I.A, point 2).
• Equipment and material used for working on fishery products must be corrosion-resistant and easy to clean and disinfect (point 3).
• Landing equipment in contact with fishery products must be easy to clean and disinfect and kept in good repair (Chapter II, point 1(a)).
• Cleaning duty: operators in primary production keep equipment, containers and vessels clean and, where necessary, disinfected (Regulation (EC) No 852/2004, Annex I, Part A, point 4(b)); in establishments, food contact equipment is cleaned at a frequency sufficient to avoid any risk of contamination and installed so that it and its surroundings can be cleaned (Annex II, Chapter V).

These requirements apply alongside the fisheries control and metrology rules for weighing described on the legal metrology hub. A scale that corrodes or cannot be cleaned is a hygiene problem and, sooner or later, an accuracy problem too.

Materials: stainless steel grades and plastics

Austenitic stainless steel is the standard material for marine scales, and the grade decides how it copes with chlorides. Composition and pitting resistance data from IMOA:

Grade	Mo %	C max %	PREN	Typical role in marine weighing
304 / 304L	-	0.07 / 0.03	18-20 / 18-21	Parts away from salt spray; mild, fresh-water conditions
316	2.00-3.00	0.08	24-26	Frames, platforms and housings exposed to spray and washdown
316L	2.00-3.00	0.03	24-26	As 316, preferred for welded constructions

PREN, the Pitting Resistance Equivalent Number, is %Cr + 3.3(%Mo + 0.5%W) + 16%N. EHEDG guidance considers 304 suitable for near-neutral conditions with chlorides up to about 50 mg/l at up to about 25 °C, and recommends 316-type steels where both chloride and temperature are roughly double those values (EHEDG Document 8, 2nd edition, section 4.3). Seawater exposure is far beyond that threshold.

Non-metallic parts, such as conveyor belts, seals, keypads and cable sheaths, need equal care. EHEDG lists criteria for polymers that include regulatory compliance, resistance to cleaning agents and disinfectants, temperature resistance, stress-crack resistance, cleanability and abrasion resistance (section 4.4). The same parameters apply to elastomers, with traceability of finished parts as an additional issue (section 4.5). High-density polyethylene, polypropylene and acetal are among the polymers EHEDG lists as frequently used in hygienically designed equipment.

Ingress protection: reading IP codes correctly

The IP code of IEC 60529 describes tested protection against dust and water; the "K" codes such as IP69K come from ISO 20653. Three points matter most for washdown scales:

1. The codes are not cumulative. An enclosure coded only IPX7 or IPX8 is considered unsuitable for water jets, and one coded only IPX9 is considered unsuitable for jets and immersion, unless it is multiple-coded (IEC 60529, clause 6 as amended in 2013).
2. Each code is a specific test. IPX6 uses 100 l/min from 2.5-3 m; IPX7 is 30 minutes at 1 m depth; IPX9 uses water at 80 °C, 15 l/min from 175 mm.
3. IP tests use fresh water and exclude corrosion and condensation, which are left to product standards (Amendment 2 and clause 1).

For equipment that is hosed down, occasionally flooded and cleaned with hot high-pressure water, look for a combined rating such as IP66/IP67/IP69K and ask which parts were tested.

Hygienic design principles

Hygienic design means the scale can be cleaned to a defined standard in a reasonable time. EHEDG Document 8 (2nd edition, 2004) gives criteria that translate directly into features to check:

• No crevices: no metal-to-metal joints other than welds in product areas, and seals designed so that no crevice traps soil (section 6.2).
• Rounded corners: internal corners preferably 6 mm radius or more, minimum 3 mm (6.2).
• Smooth surfaces: large product contact areas Ra 0.8 µm or better, free from pits, folds and crevices; cold-rolled steel of Ra 0.2-0.5 µm usually needs no polishing (6.3).
• No exposed threads in contact with product (6.2).
• Self-draining: no horizontal surfaces where liquid can pool (6.4).

EHEDG published the 4th edition of Guideline 8 Hygienic Design Principles in December 2025 as a free download; it is the reference for EHEDG equipment certification.

Cleaning essentials

A reliable cleaning routine for fish-contact equipment follows the sequence described by FAO for seafood plants: remove products and gross soil, pre-rinse with cold water, clean with a moderately alkaline detergent for about 10-15 minutes, rinse, disinfect, and rinse and dry. Stainless-specific precautions come from the Nickel Institute:

• avoid hydrochloric acid cleaners, which generate ferric chloride, a strong pitting agent;
• never use carbon steel brushes or steel wool, which leave particles that rust;
• rinse thoroughly with fresh water after every cleaning operation and wipe dry;
• use chlorine disinfectants at the supplier's dilution and rinse them off; FAO describes about 200 ppm free chlorine at ambient temperature as non-corrosive to high-quality stainless steel.

Protect electrical installations against water and chemicals before cleaning, and keep pressure jets off glands, connector caps and seals. After cleaning, check that the platform moves freely and that the zero and a check weight are correct before regulated weighing resumes.

Corrosion and maintenance essentials

Most corrosion on stainless scales starts at details. The National Physical Laboratory explains that bimetallic corrosion needs an electrolyte, an electrical connection, a potential difference and a cathodic reaction; seawater, with a typical conductivity of 40 000 µS/cm, supplies the first condition very effectively. Four habits prevent most problems:

1. Insulate dissimilar metals at mountings with nylon or PTFE bushes and washers, and avoid small less-noble parts such as carbon steel or aluminium bolts against large stainless areas.
2. Treat seals as crevices. Gaskets, O-rings, washers, connectors and tape are common crevice formers where chloride concentrates and pH drops.
3. Keep glands and connectors in tested condition: correctly tightened glands, intact inserts, caps on unused connectors.
4. Inspect on a schedule and after heavy weather, impacts or repairs, and record what was found.

Cold decks, ice and freezer areas

Construction must also cover the temperatures a scale will see, because materials and electronics are only qualified within a tested range. Three reference points are useful:

• Default range. Where a manufacturer does not specify a range, a non-automatic weighing instrument must meet the requirements from -10 °C to +40 °C; class III and IIII instruments must cover a span of at least 30 °C (Directive 2014/31/EU, Annex I, point 7.2).
• No extrapolation. WELMEC Guide 2:2023 (section 2.2.7.5) states that an instrument must be tested over its complete temperature range; a range of -10 °C to +40 °C cannot be extended to -20 °C on the basis of a partial test. Weighing equipment must not be used outside its approved range, and heaters for load cells with the indicator in a heated enclosure are an accepted alternative for cold sites (section 2.2.7.3).
• Freezer vessels. EU hygiene law requires freezer vessels to keep fishery products in storage holds at no more than -18 °C (Regulation 853/2004, Annex III, Section VIII, Chapter I, Part I.C), so equipment used near or in those areas needs a stated low-temperature rating.

Ice build-up is also a weighing issue: ice on or under a platform adds weight or blocks free movement, so drainage and cleaning access matter as much in cold conditions as in washdown areas.

A specification checklist for buyers

Ask about	Good answer includes	Why it matters
Stainless steel grade of frame, platform and housings	AISI 316 or 316L for exposed parts, with EN number	Chloride pitting and crevice resistance
Welds	Continuous, full-penetration welds; heat tint removed by pickling	Weld defects and heat tint are pitting sites; partial penetration leaves crevices
IP ratings	Combined codes, stating which parts were tested	Codes are not cumulative
Cleaning access	Liftable or removable platform, open underside, rounded corners	Soil under platforms affects hygiene and accuracy
Plastics and seals	Food-contact compliance statements and chemical resistance data	Polymers must match cleaning chemicals and temperatures
Fasteners and mounting kit	Stainless fasteners, insulating parts for aluminium or steel decks	Prevents galvanic corrosion
Cleaning instructions	Permitted chemicals, temperatures and pressure washer limits	Protects seals and electronics from avoidable damage

For wider buying criteria, including capacity and accuracy, see the buying guides hub; for processing lines, see the on-board processing hub and weight grading of fish.

Matching construction to the application

Application	Main exposure	Construction priorities
Bench scale on a sorting deck	Spray, fish residues, frequent hosing	316 housing, rounded edges, jet and immersion ratings, sealed indicator
Platform scale on a working deck	Green water, impacts, cleaning underneath	316 platform, access for cleaning under the platform, insulated deck mounting
Grader in a processing area	Daily high-pressure hot-water cleaning, detergents	Hygienic belts and drives, IPX9 or IPX9K-rated motors, open frame design
Scientific scale in a wet lab on a research vessel	Seawater samples, small residues, precise readings	316L construction, smooth platform, protection for the weighing chamber

How WPL approaches this

WPL builds the M2 Series in AISI 316 with smooth, rounded edges for washdown, and the M3 scientific scale in 316L. The M5 platform flips up for cleaning and deck space, while the M6 has a fully sealed AISI 316 construction for daily washdown and saltwater exposure. Browse the fishing and offshore collection for bench scales.