Salt: IMSBC Code Schedule and Carriage

Salt (sodium chloride, NaCl) is classified as a Group C cargo under the IMSBC Code, meaning it is not liable to liquefy and presents no chemical hazard in its three commercial bulk grades: solar, rock, and vacuum salt. The practical concerns for shipowners and masters are the strong chloride-driven corrosivity to mild steel in holds and bilges, the hygroscopic tendency to cake hard in humid conditions, dust generation during loading and discharge, and the mandatory thorough hold washing required after every salt cargo to prevent accelerated corrosion damage.

Salt is one of the oldest traded commodities in maritime history, and it remains a significant dry bulk cargo in the twenty-first century. Global seaborne trade in sodium chloride runs at roughly 25 to 35 million tonnes per year, spanning three major end-use markets: road de-icing, which is the dominant volume use; chemical industry feedstock for chlorine and sodium hydroxide manufacture via the chlor-alkali process; and food processing and water treatment. The IMSBC Code, first adopted by IMO Resolution MSC.268(85) and mandatory under SOLAS Chapter VI since 1 January 2011, governs salt carriage in bulk. Its schedule assigns salt a Group C classification and identifies chloride corrosivity and hygroscopic caking as the cargo’s defining practical hazards.

Salt as a global commodity

Production and grades

Sodium chloride occurs in nature in three commercially recoverable forms, each of which produces a distinct bulk grade with different physical characteristics that affect shipping.

Solar salt is produced by evaporating seawater or natural brine in shallow solar pans over weeks or months. The crystals that form during slow solar evaporation are relatively coarse, typically 1 to 10 mm in diameter, off-white to light grey in color, and carry small residual amounts of calcium sulphate, magnesium chloride, and other minerals from the source brine. Solar salt’s purity ranges from approximately 96 to 99% NaCl depending on the degree of harvesting, washing, and stockpile management at the saltworks. It is the dominant production method globally by volume, accounting for roughly 35 to 40% of world salt output. The largest solar salt producers shipping by sea include Mexico (Guerrero Negro and Exportadora de Sal, which operates what is described as the world’s largest single saltworks on Baja California), Australia (Dampier Salt and others in Western Australia), India (Gujarat coastal saltworks), Chile, and several Mediterranean and Middle Eastern producers. Solar salt is the standard grade for road de-icing and for industrial chlor-alkali feedstock.

Rock salt is mined from underground halite deposits laid down by the evaporation of ancient seas. The mining method is conventional drill-and-blast or continuous mining, with the ore crushed and screened to a working particle size of roughly 5 to 50 mm for bulk shipment. Rock salt is characteristically grey, brown, or pink because of clay, shale, and mineral inclusions in the halite matrix. Its purity is lower than purified solar salt, typically 92 to 98% NaCl, and the impurities are distributed unevenly through the mass because the bedded geology concentrates clay and anhydrite in bands. Major underground rock salt operations shipping by sea include mines in Germany (Kali und Salz, with operations in the Zechstein basin), Poland, Canada (Sifto Canada in Goderich, Ontario, the largest underground salt mine in the world at approximately 2.5 km below surface), the United Kingdom (Winsford mine in Cheshire), and Chile (atacama brine and associated evaporites). Rock salt’s coarser particle size and lower moisture absorption rate make it somewhat more resistant to caking than solar salt, though the impurity minerals in some grades include hygroscopic magnesium chloride that can promote moisture uptake.

Vacuum salt is produced by dissolving mined rock salt or halite in water and pumping the resulting brine through a series of vacuum evaporators operating at progressively lower temperatures and pressures. The controlled crystallization in vacuum vessels produces very fine, very pure white cubic crystals, typically below 1 mm in diameter and 99.5 to 99.97% NaCl. Vacuum salt is the food-grade standard: table salt, food processing salt, pharmaceutical-grade saline, and water-softener salt are all vacuum salt products. Its extreme purity and fine crystal size make it highly hygroscopic and very prone to caking, which is why most vacuum salt moves in bags, big bags, or intermediate bulk containers rather than as free-flowing bulk in ship holds. Where vacuum salt is shipped in bulk (relatively rare), it commands special handling: completely weathertight holds, pre-loaded desiccant or ventilation runs to dry the hold atmosphere, and a prohibition on any residual moisture in the bilge wells.

The chlor-alkali industry and its salt requirements

The chlor-alkali process is the electrolysis of sodium chloride brine to produce chlorine gas (Cl2), sodium hydroxide (caustic soda, NaOH), and hydrogen (H2). These three products are foundational to the chemical industry. Chlorine is used in PVC manufacture, disinfection, paper bleaching, and pharmaceutical synthesis. Sodium hydroxide is used in paper, alumina refining, soap, and chemical manufacture. The chlor-alkali process consumes sodium chloride in large quantities: a rough rule of thumb is that 1.8 tonnes of NaCl are consumed per tonne of chlorine produced. Global chlorine production was approximately 88 million tonnes in 2022 (IHS Markit / S&P Global Commodity Insights data, referenced by industry associations), implying an annual NaCl demand from chlor-alkali alone on the order of 55 to 60 million tonnes worldwide. Much of this demand is satisfied from local brine sources rather than bulk NaCl shipments, but where plants lack proximate brine wells, they import solar or rock salt by sea as their electrolyte feedstock. Japan, South Korea, and some European chemical plants operate on imported salt. The quality specification for chlor-alkali salt is stringent: NaCl content above 96.5%, low sulphate (below 0.5%), and low calcium and magnesium, which otherwise deposit on the membranes of membrane-cell electrolyzers and reduce current efficiency.

The road de-icing trade

Road de-icing is the largest single-season driver of seaborne salt demand. Salt dissolves on icy and snowy road surfaces, lowering the freezing point of water through the colligative property of ionic solutions. A 23.3% NaCl solution (the eutectic composition) freezes at minus 21.1 degrees Celsius. At operational spreader rates of 5 to 30 grams per square meter, road authorities in cold-climate countries consume salt in very large quantities per winter season. The United States is the largest single road de-icing salt market, with consumption estimated at 18 to 22 million tonnes per year in a heavy winter and 12 to 14 million tonnes in a mild one. Canada, the United Kingdom, Germany, Russia, and Japan are other large consumers. The seasonal pattern of this demand is sharp: North American buyers build stockpiles from late summer through early winter (September to November) in anticipation of the spreading season from December through March. Mexican solar salt, loaded at Guerrero Negro and shipped to terminals on the US East Coast, Gulf Coast, and Great Lakes, has historically been the dominant import flow into the US market. The Australian saltworks at Dampier and Port Hedland export primarily to Japanese and Korean chemical plants, not to road de-icing markets.

Food and water-treatment markets

Food-grade salt consumption is large in absolute terms (the global food and food-processing market for salt is estimated at 15 to 18 million tonnes per year) but most of it moves as packaged salt rather than bulk, because food-grade quality requires vacuum or highly refined salt that cannot be handled in the same way as bulk solar or rock salt. Where food-grade salt does move by sea in bulk, the consignment is typically vacuum salt in pneumatically discharged ships. Water treatment (for ion-exchange water softeners and municipal drinking-water softening) is a medium-volume market for high-purity block or granular salt, again mostly vacuum salt moved in package form. These markets do not drive the bulk carrier market to any significant degree.

The IMSBC Code schedule for salt

Group classification and regulatory history

The IMSBC Code Appendix 1 lists SALT under the individual schedule entry for sodium chloride and also lists ROCK SALT as a closely related separate entry. Both are Group C cargoes under the Code’s definitions. Group C covers solid bulk cargoes that are not liable to liquefy (Group A) and do not possess chemical hazards (Group B) warranting special carriage controls. Salt’s Group C assignment reflects its non-flammable, non-self-heating, and non-reactive character: it will not catch fire, will not generate toxic or flammable gases, and will not react with water in a hazardous way.

The Code was first adopted in 2008 under IMO Resolution MSC.268(85) and entered force on 1 January 2011. Amendment 06-21 (MSC.500(105)) and Amendment 07-23 (MSC.539(107), mandatory from 1 January 2025) have both been issued since, but neither altered the Group C classification for SALT or ROCK SALT. The schedule’s essential character has been stable since the Code’s adoption.

SALT CAKE, which is sodium sulphate (Na2SO4), appears as a distinct entry in the IMSBC Code under a completely different schedule. It is not sodium chloride, and the two cargoes must not be confused in cargo declarations. SALT CAKE has its own separate hazard profile and is not covered in this article.

Schedule particulars

The following table summarizes the physical and carriage properties as defined by the IMSBC Code schedule for SALT and ROCK SALT, combined with established shipping industry data for the major commercial grades:

Bulk density figures are at-rest, as-loaded values for freely poured material. Compaction during the voyage adds roughly 2 to 5% to the in-hold density over a 7- to 14-day deep-sea passage, reducing the cargo surface height and affecting quantity reconciliation in cargo draught surveys. Salt’s stowage factor of 0.77 to 0.91 m3/t (for solar salt) is lower than grain (approximately 1.2 m3/t) but higher than most mineral concentrates, meaning a standard Handysize bulk carrier of 28,000 DWT fully loads on salt without exhausting hold volume before reaching its loadline.

What Group C means in practice for salt

The Group C classification delivers three practical benefits for the commercial team. First, no Transportable Moisture Limit (TML) test is required before loading: salt is not liable to liquefy, so the IMSBC Code’s TML certification chain does not apply. Second, no chemical hazard provisions attach: no hold atmosphere monitoring, no emergency response for gas release, no inerting requirement. Third, the cargo does not trigger the Code’s Group B MHB (Materials Hazardous only in Bulk) obligations.

Salt’s Group C status does not, however, grant immunity from the Code’s general obligations. Section 4 of the Code requires the shipper to provide cargo information before loading, including the correct cargo name (SALT or ROCK SALT as appropriate), the IMSBC Code group, and the physical properties of the specific lot being loaded. A cargo declaration that understates moisture content, misidentifies the grade, or omits the corrosivity warning note does not eliminate the shipowner’s obligation to understand what he is carrying. The corrosivity of salt to steel is not a Group B hazard under the Code’s formal definitions, but it is a material property that any responsible master should know before opening the hatches.

Chloride corrosivity to ship structure

The electrochemical mechanism

Salt’s most significant practical risk to the ship is not to the cargo or the crew: it is to the vessel’s steel structure. Sodium chloride is ionic in solution, and the chloride ion (Cl-) is among the most aggressive chemical species encountered by ship’s steel in normal operations. The corrosion mechanism is electrochemical: chloride ions displace the passive iron oxide (magnetite, Fe3O4) layer that protects mild steel in air, attacking the underlying steel through anodic dissolution. This creates corrosion pits that deepen under cyclic wet-dry exposure far faster than general (uniform) corrosion would predict from weight-loss measurements alone.

Pitting corrosion under chloride attack is self-accelerating. The pit interior becomes an anodic zone at lower electrochemical potential than the surrounding cathodic steel surface. Chloride ions migrate into the pit to balance the positive charge from dissolving iron cations, building a highly aggressive ferrous chloride microenvironment inside the pit. Pit depth can reach structurally significant levels (0.5 to 2 mm penetration in unprotected mild steel) within a single season of salt carriage if the bilge is persistently wet with salt brine. Classification society thickness gauging campaigns consistently find the fastest corrosion rates in bulk carriers with a history of frequent salt cargoes concentrated in the bilge wells, the lower frame webs, and the tanktop plating adjacent to bilge strainer covers.

Areas of the vessel most at risk

Four structural zones accumulate the highest chloride exposure during a salt cargo and are where post-discharge inspection should concentrate:

Bilge wells and bilge piping. Salt brine is denser than fresh water and sinks to the lowest point of the hold. Bilge wells accumulate brine at higher salinity than the average hold moisture, because moisture absorbed from the cargo progressively concentrates as it drains. If bilge covers are missing or damaged, salt crystals pack directly into the bilge suction pipe openings. When the bilge is pumped at the end of discharge, remaining brine pools in low spots. Post-discharge, if the hold is not washed and the brine is not pumped out, the residual chloride concentration in the bilge well corrodes the uncoated or coating-damaged carbon steel of the well structure at rates that can perforate 8 mm plate within a few years of repeated exposure.

Tanktop plating and longitudinal framing. The tanktop is the inner bottom surface of the double-bottom tank, and it carries the cargo directly. In a well-painted hold the tanktop coating is the first defense against chloride attack. Where paint has been damaged by previous cargo operations (grab discharge of limestone or iron ore is particularly abrasive), bare steel is exposed to salt brine under the cargo. Class survey records show that tanktop wasting rates of 0.3 to 0.8 mm per year are achievable in inadequately maintained holds carrying salt.

Frame webs and brackets in the lower hold. The lower hold framing, particularly the lower brackets connecting the side frames to the tanktop margin plate, sits in the zone of highest brine pooling. Ballast water corrosion (external to the inner hull) already attacks this zone from the water-ballast side; salt cargo attacks it from the cargo side simultaneously.

Ballast piping and sounding pipes. Sounding pipes that penetrate the tanktop and pass through the cargo sometimes leak at their gland fittings or threaded connections, introducing salt brine into the double-bottom tanks. If the ballast water treatment system (BWTS) is absent or the ballast is not changed frequently, this combined attack can cause hidden corrosion in the double-bottom structure that is not detectable from the hold side.

The post-discharge washing requirement

The IMSBC Code schedule for SALT includes the instruction that cargo spaces shall be washed down after discharge to remove salt residues. This is not a suggestion: it is a schedule requirement that applies to every salt cargo, regardless of the next cargo type. The rationale is that residual sodium chloride left on hold surfaces continues to absorb atmospheric moisture and corrode steel after the main cargo has been discharged.

In practice, post-discharge washing means a high-pressure fresh-water wash of all hold surfaces, including deckhead, frames, side plating, tanktop, hopper slopes, and bilge wells. The wash must be thorough enough to dissolve and remove the film of NaCl crystal adhering to painted surfaces and deposited in frame crevices. A quick rinse with the fire hose is not sufficient. The recommended procedure, as applied by well-run operators, is:

1. Remove all visible cargo residue by mechanical sweeping and bulldozer pass.
2. Apply fresh-water high-pressure wash to all surfaces (pump pressure 100 to 150 bar at the nozzle; lower pressures fail to penetrate frame crevices).
3. Allow bilge drainage to collect in the bilge well; pump to bilge holding tank or overboard under MARPOL Annex V requirements (wash water from salt cargo does not contain oil and is generally permitted for overboard discharge but check port authority rules for enclosed waters).
4. Repeat the wash if the bilge pump discharge or cloth wipe on hold surfaces shows elevated salinity (a simple conductivity meter or silver nitrate test on a wipe sample will confirm chloride removal).
5. Open hatch covers and run hold ventilation fans for at least 24 hours to dry the hold before the next cargo is loaded.

Operators carrying salt frequently should assess their vessel’s hold coating condition annually and schedule hold steel renewal or recoating more frequently than for cargoes like grain or limestone that do not produce a chloride-saturated hold environment.

Chloride attack on deck and deck equipment

Salt loading and discharge generates airborne salt dust and brine aerosol that settles on the vessel’s exposed deck surfaces, winch drums, wire ropes, capstans, navigation instruments, and mooring equipment. This is a secondary corrosion problem separate from the hold structure, but it accumulates over repeated salt voyages. Wire ropes carrying salt residue in their core strands corrode from the inside out and fail at lower loads than clean ropes of the same nominal breaking strength. Pre-departure fresh-water deck washing and lubrication of wire ropes after each salt cargo voyage is standard practice in well-run salt carrier operations.

Hygroscopic caking

The moisture absorption mechanism

Sodium chloride is hygroscopic: at relative humidity above approximately 75% (the deliquescence point of NaCl at 20 degrees Celsius), the crystal surface begins to absorb moisture from the surrounding air. This absorbed moisture partially dissolves the crystal faces, creating a thin saturated brine film. When the hold humidity then drops (as temperature decreases at night, or when the vessel moves to a drier climate), the brine film re-crystallizes, growing across adjacent crystal surfaces and cementing neighboring crystals together. Repeated wet-dry cycles over a voyage of several days build up solid cake masses that can achieve substantial mechanical strength.

The deliquescence threshold of 75% relative humidity means that caking is a genuine risk during loading operations in tropical or subtropical ports with high ambient humidity. Mexico’s Pacific coast saltworks at Guerrero Negro, Gujarat saltworks in India, and Australian saltworks at Dampier all operate in climates where ambient humidity during the loading season can regularly exceed 75% RH. At 30 degrees Celsius and 80% RH, freshly loaded solar salt on the top layer of the cargo pile will begin absorbing moisture within minutes of the hold being open. Rain ingress through insufficiently closed hatch covers during loading dramatically accelerates caking.

Effect on discharge operations

Caked salt does not flow under the grab. A shallow-caked surface (1 to 3 cm deep) breaks up under the first grab impact and causes only modest delays. A hard-caked mass extending 30 to 50 cm into the cargo pile, which is what can form if holds are exposed to persistent high humidity over a 10- to 14-day voyage, resists grab penetration entirely. Discharge of heavily caked cargo requires one of three interventions:

Pneumatic vibrators. Pneumatically powered vibrators (essentially jackhammers adapted for cargo work) are lowered into the hold by crane to break up cake masses. This is effective but slow and labor-intensive.

Water jets. Water is sprayed onto the caked surface at moderate pressure to redissolve the surface brine film and soften the cake. This reduces the mechanical strength of the cake but introduces additional moisture into the cargo, which the receiver may dispute on quality grounds.

Mechanical dozer. A rubber-tracked bulldozer lowered into the hold pushes and breaks up cake masses before the grab resumes. This is the fastest method but requires a hold large enough to maneuver the equipment and a structure strong enough to bear the dozer weight without tanktop damage.

The hold conditions that produce the worst caking are combinations of fine salt grade (vacuum salt or fine-washed solar salt below 1 mm crystal size), high ambient humidity at the loading port, rain ingress during loading or voyage, and a long voyage with frequent temperature cycling. Cargoes of coarse rock salt (5 to 50 mm) from a mine in a dry climate, loaded in cool weather and carried on a short coastal voyage, rarely cake to a degree that causes discharge difficulty.

Hatch cover weathertightness for salt cargoes

Unlike limestone, where hatch weathertightness is largely a quality-protection measure, for salt it is a caking-prevention measure with direct discharge-operations consequences. Leaking hatch covers admit rain and seawater spray that wet the top layer of the cargo. The IMSBC Code requires that a vessel’s hatch covers be weathertight before a salt cargo is loaded, and responsible operators carry out a hatch cover compression test or ultrasonic test before sailing. Class surveyor reports on Handysize bulk carriers engaged in the salt trade consistently identify deteriorated hatch cover rubber seals as a factor in cargo damage claims involving caking.

Impact on subsequent cargoes

Salt residues are difficult to remove completely from a hold that has carried a wet or caked cargo. Residual NaCl crystals in frame crevices, behind bilge strainer flanges, and in deck-plate seams resist sweeping and moderate washing. Subsequent cargoes that are contaminated by even small quantities of residual salt include agricultural products (elevated chloride can cause plant toxicity if the cargo is used as fertilizer or animal feed), food-grade commodities, and some chemical raw materials where chloride specification is tight. For this reason, operators transitioning from a salt cargo to a food-grade or quality-sensitive next cargo should inspect the hold after washing and run conductivity tests on swab samples from representative hold surfaces before accepting the next cargo.

Dust from salt loading and discharge

Generation mechanisms

Salt dust forms whenever crystals are fractured or transported through air. The main generation points during a salt operation are:

Conveyor transfers. Each time the cargo stream changes direction or drops height on a conveyor system, crystal impact shatters fines from the crystal surface. At the loading spout where the shore conveyor delivers cargo to the ship’s hold, the free-fall of a stream of solar salt crystals from several meters’ height generates a visible dust plume within the hold. The plume’s intensity depends on the fall height (greater fall = more dust), the wind speed and direction on deck (cross-winds entrain dust from the open hatch), and the crystal size of the cargo (finer grades generate more fines per tonne handled).

Grab impact at discharge. A grab crane operating in a hold of solar salt impacts the cargo surface at roughly 2 to 4 m/s at the close of each descent. This impact shatters the surface crystals and produces a burst of fine airborne salt dust visible from the vessel’s deck. The shore discharge hopper, where the grab drops its load, is another major dust source if the hopper is open-topped.

Residual cargo disturbance. At the end of grab discharge, the remaining cargo in corners and under the tanktop frame ledges is disturbed by bulldozer and shovel work. This is often the dustiest phase of the discharge because the cargo at this stage is composed largely of the fine fraction that the grab could not pick up cleanly.

Health and safety considerations for crew and stevedores

Salt dust at exposure concentrations arising during normal loading and discharge is classified as a nuisance dust by occupational health authorities: it is not acutely toxic, does not cause respiratory sensitization, and does not contain established carcinogens unless the salt contains elevated silica from mineral impurities in rock salt. The practical effect of salt dust exposure during operations is primarily irritation of the mucous membranes (eyes, nose, throat) and respiratory tract at high concentrations, and corrosion of exposed metal surfaces as described above.

Half-face respirators with P2/N95 filters are appropriate personal protective equipment for deck crew working in the vicinity of loading spouts or open hatches during discharge. Deck crew should also wear eye protection (safety glasses or goggles) during discharge by grab crane, where coarse dust can cause eye injury from large salt crystal fragments entrained in the plume.

Dust control at loading terminals

Loading terminal operators at major saltworks have generally implemented dust control as a regulatory compliance matter, required by national environmental and occupational health authorities. The most effective measures are enclosed shiploader chutes (a telescoping chute that follows the cargo pile surface, reducing free-fall to near zero), dust suppression water sprays at the loading spout and hatch coaming, and partial hatch cover closure on holds not currently loading.

Where terminal dust controls are inadequate, the vessel’s mate should document the situation in the ship’s log and the Statement of Facts (SOF), request that the terminal provide spraying, and if refused, note the refusal formally. This documentation is the shipowner’s protection if the receiving port’s environment authority later raises a complaint about dust discharge.

Hold preparation for salt cargoes

Cleanliness requirements

Salt’s Group C classification does not impose strict chemical cleanliness requirements on the hold, in contrast to food-grain cargoes where pesticide or chemical contamination at parts-per-million levels can render cargo unfit for human consumption. For de-icing road salt, the principal cleanliness requirement is freedom from oil or fuel contamination (a diesel-contaminated salt cargo will leave oily residue on roads and drainage systems) and freedom from large inert debris. For chemical-grade chlor-alkali feedstock, the specification is more stringent: chloride purity must meet the buyer’s chemistry, and contamination with sulphate (from a prior fertilizer cargo), phosphate (from a prior phosphate cargo), or organic matter (from a grain cargo) will fail quality testing.

The required hold cleaning standard before loading salt therefore depends on the previous cargo. A common practical matrix is:

Previous cargo was coal or petroleum coke. Coal leaves combustible carbon dust in all frame crevices and produces oil-contaminated bilge water. A high-pressure wash followed by a hot-water or steam clean is required, with inspection of the hold for oily residue before salt loading is accepted. A surveyor’s inspection certificate is standard on this transition.

Previous cargo was fertilizer (urea, DAP, or ammonium nitrate). Fertilizer residues contaminate chemical-grade salt with nitrogen and phosphate. Diammonium phosphate or urea residue in even small quantities will register in a quality analysis. High-pressure wash and multiple rinse cycles, followed by bilge inspection and pump-out, are required.

Previous cargo was grain. Grain residue introduces organic nitrogen, fungi, and insect debris. Chemical-grade or food-grade salt buyers will typically reject a cargo with visible grain residue. Standard high-pressure wash is required. Fumigation is sometimes demanded by receivers if the grain cargo was carried under a phytosanitary regime.

Previous cargo was gypsum or limestone. These chemically inert cargoes are the most benign prior cargoes for a salt hold. Residual calcium sulphate or calcium carbonate dust in the hold will not affect de-icing grade salt and only needs to meet the buyer’s specification for chemical-grade salt. A standard sweep and rinse is normally adequate.

Previous cargo was salt. The hold already carries residual NaCl from the last voyage. If the previous cargo grade and the incoming grade are compatible (same end-use), a sweep with bilge pump-out and a fresh-water rinse to remove brine pooling in bilge wells is typically sufficient, subject to the cargo surveyor’s hold inspection.

Bilge system preparation

Bilge strainer covers must be installed, intact, and correctly seated before any salt is loaded. Salt fines can be as small as 0.1 mm and will pass through even slightly damaged strainer screens into the bilge suction pipe. Once salt paste accumulates in the suction line, it dries and hardens into a crystalline plug that requires mechanical cleaning. Unlike the calcium carbonate cement produced by wet limestone paste, NaCl blockages dissolve readily in fresh water, but access to the suction pipe for flushing is often impractical without taking the hold out of service.

Bilge wells must be dry and clean before loading. Any residual brine from the previous cargo represents a corrosion source for the bilge structure during the upcoming voyage. A post-loading check of the bilge well water level (by sounding or bilge alarm) should be part of the daily routine on a salt voyage; brine level rising faster than expected indicates either seawater ingress or moisture absorption from the cargo.

Paint coating and hold condition

Salt is more aggressive to poorly maintained hold coatings than almost any other Group C cargo. A hold carrying limestone or gypsum with minor paint blistering or small bare-steel spots will show modest corrosion after the voyage. The same hold carrying salt for a week at tropical humidity will show active pitting in those bare-steel areas visible to the naked eye at the post-discharge inspection. Responsible operators assess hold coating condition before accepting a salt cargo and, where the coating has significant breakdown (more than 5 to 10% of any surface with paint failure), either repair the coating before loading or accept the consequent corrosion risk with full awareness of what a class gauging survey may reveal.

Draft survey for salt

The cargo draught survey is the standard quantity-determination method for most bulk salt shipments, particularly at loading ports without calibrated conveyor belt scales. Salt’s physical properties affect the draft survey in three specific ways that surveyors note in practice.

Density variation with grade. Solar salt and rock salt have slightly different densities reflecting their impurity content. Rock salt with clay and anhydrite inclusions is denser than pure solar salt. If the draft survey’s density figure is taken from the cargo declaration without laboratory measurement of the specific lot, the tonnage calculation may have a systematic error of 2 to 4% depending on the variation between the declared and actual density. For large cargoes (30,000 to 60,000 tonnes on a Supramax), this is a meaningful quantity. Survey practice is to collect density samples from the loading stream or the cargo pile and measure bulk density independently where the survey is of commercial significance.

Voyage compaction. Salt compacts under vibration by roughly 2 to 5% in void volume over a 7-day voyage. This reduces the hold freeboard and alters the correlation between the loaded draft and the discharged draft when compared with a theoretical constant-density model. The difference is small but real, and it is systematically biased toward showing a shortage at discharge compared with the loading draft survey if the survey methodology does not account for compaction.

Brine loss from bilge pumping. During the voyage, moisture absorbed by the cargo and moisture from any rain ingress drains to the bilge. Bilge pumping removes this brine from the vessel. The mass of NaCl removed from the cargo by bilge pumping (as dissolved salt in the brine) is a genuine physical loss from the cargo. It is rarely significant enough to account for survey disputes of more than 0.1 to 0.2%, but in a precisely measured survey on a high-value food-grade salt cargo, it should be considered.

Freshwater allowance. Loading in a river port or sheltered estuary where dock water density is below 1.025 t/m3 requires the standard freshwater allowance correction to draft readings. Several major salt loading ports, including some Indian Gujarat ports and certain Chilean ports, are in water of variable density depending on seasonal freshwater runoff. The surveyor must measure dock water density at the time of the survey, not assume it.

Loading and discharge operations

Loading: shiploaders and conveyor systems

Established saltworks loading terminals at Guerrero Negro (Mexico), Dampier (Australia), and Kandla (India) operate dedicated shiploaders feeding Handysize and Supramax bulk carriers. Typical shiploader rates are 1,500 to 3,500 t/h for solar salt, allowing a 35,000-tonne cargo to be loaded in 10 to 23 hours. The operational sequence at a modern saltworks terminal is:

1. Hold preparation certificate accepted from surveyor.
2. Loading spout positioned over hatch No. 1 (or the sequence designated in the loading plan).
3. Shore conveyor starts at reduced rate; shiploader boom adjusts height to minimize free-fall.
4. Water spray activated at loading spout.
5. Loading proceeds to the planned tonnage per hold; holds are sealed between loading stages.
6. Draught survey (first mark survey) taken at the start of loading to establish departure condition.
7. After loading of each hold, hatch covers are closed and sealed.
8. Final draft survey taken on completion; bill of lading tonnage agreed with shipper.

The cargo plan for salt must distribute the cargo across holds to maintain longitudinal bending moment and shear force within the vessel’s permissible envelope at each stage of loading. Salt at 1,100 to 1,400 kg/m3 bulk density is light enough that a 28,000 DWT Handysize vessel will fill its holds to volume capacity before reaching its deadweight limit on some grades, or vice versa, depending on the specific salt and the vessel’s design. The mate should verify which constraint governs for the specific shipment and trim the loading plan accordingly.

Self-trimming and mechanical trimming

Solar salt in the 1 to 10 mm crystal size range self-trims reasonably well: the cargo flows outward from the loading point under its own weight, filling the hold to a roughly conical surface. The IMSBC Code Section 5 specifies that the cargo must be trimmed reasonably level when loading is complete, to prevent cargo shifting at sea. For solar salt this is normally achieved by the loading pattern alone with modest adjustment of the shiploader boom position toward the end of each hold fill. Rock salt in the 5 to 50 mm range is less self-trimming and may require a bulldozer pass at the cargo shoulders to push material from the central pile toward the bulkheads and prevent an unstable peaked surface that could shift in a seaway.

Vacuum salt, where loaded in bulk, requires careful attention to dust during the bulldozer trimming pass. The very fine crystals become heavily airborne under dozer disturbance. Trimming of vacuum salt bulk cargoes is done in short passes with hatches closed on adjacent holds.

Discharge: grab crane and pneumatic options

Discharge of solar and rock salt at the receiving terminal is by shore-based grab crane in most ports. Grab discharge rates of 500 to 2,000 t/h are typical for Handysize to Supramax-sized vessels. The operational considerations at discharge are:

Caked cargo. If the surface layer is caked as described above, the grab operator will need to spend time breaking up cake masses before normal grab cycling resumes. The master should notify the terminal in advance of any caking observed during the voyage, so the terminal can have vibrators or a dozer ready on arrival.

Residue and hold cleaning. After grab discharge reaches approximately 95 to 98% cargo removal, a dozer pass is required to consolidate residue to the hold center for final grab pickup. The final 1 to 3% of the cargo is typically removed by shovel and manual labor to a central collection point, then grabbed out. Salt residue is stickier and harder to shovel cleanly than limestone or coal because the fine crystals adhere to the hold surface and to each other.

Post-discharge washing, again. The mandatory post-discharge wash described in the corrosivity section applies here. It is not optional. An operator who completes discharge and immediately requests a hold inspection certificate for the next cargo without washing is exposing the vessel to corrosion and the surveyor to a justifiable rejection of the hold as unsuitable for sensitive subsequent cargoes.

For fine food-grade or chemical-grade salt, some terminals use pneumatic suction discharge (similar to the systems used for grain or cement), which avoids grab contamination and achieves lower residue. Pneumatic discharge rates are 100 to 400 t/h for salt, making it impractical for large parcels but viable for specialty food-grade parcels in the range of 5,000 to 15,000 tonnes.

Major trade routes and vessel types

Dominant flows

The seaborne salt trade is characterized by a relatively small number of high-volume dedicated routes from surplus-production saltworks to large industrial or municipal consumers:

Mexico to North American East and Gulf Coast. Exportadora de Sal’s operations at Guerrero Negro and Isla de San Marcos on the Baja California peninsula produce roughly 6 to 7 million tonnes per year. The majority is exported to US East Coast de-icing stockpile terminals (Baltimore, Norfolk, Providence, Portland, and Boston are historically major receiving ports) and to US and Canadian chemical plants. Vessel sizes are predominantly Handysize (25,000 to 35,000 DWT) and Handymax (35,000 to 55,000 DWT), constrained by the terminal drafts at Guerrero Negro and by the draft restrictions at some receiving terminals.

Australia to Northeast Asia. Dampier Salt (owned by Rio Tinto) and Shark Bay Salt in Western Australia produce solar salt from some of the world’s largest solar salt operations. The production is almost entirely exported to Japan and South Korea for chlor-alkali and other chemical uses. Vessel sizes range from Handymax to Panamax (60,000 to 80,000 DWT). This is one of the more stable long-haul salt trades, driven by Japan’s structural dependency on imported salt for its chemical industry (Japan has no significant domestic salt production).

India to Northeast Asia. Gujarat’s coastal saltworks supply solar salt to Japan and South Korea, as well as to Southeast Asian markets. Indian salt production is large but fragmented across thousands of small producers, with aggregation occurring at port. Vessel sizes are predominantly Handysize and Handymax.

Mediterranean and European coastal trade. Tunisia, Egypt, and France export salt to Northern European de-icing markets. This trade is largely short-sea, using coasters of 3,000 to 15,000 DWT. Some longer voyages from Tunisian and Egyptian producers to UK and Scandinavian ports use Handysize vessels.

Chile to North America. Chilean solar salt from Atacama region operations exports to North American de-icing and chemical markets, competing with Mexican salt on the West Coast. Smaller volumes than the Mexican trade.

Canada domestic (Great Lakes). Compass Minerals’ Goderich rock salt mine ships through the Great Lakes system using lakers (Great Lakes bulk carriers) to terminals in Ontario, Michigan, Ohio, and other Great Lakes states. This is not an ocean trade but it is one of the world’s largest single-origin salt shipments by volume.

Seasonal pattern and fleet implications

The road de-icing market creates a pronounced seasonal charter market for salt tonnage. Charterers building pre-season stockpiles in August to November pay a rate premium over the summer doldrums for Handysize and Handymax tonnage for loading in Mexico, Australia, and the Mediterranean for delivery to US East Coast, UK, and Scandinavian terminals. Vessels available on short notice in the autumn can achieve charter rates 15 to 30% above the annual average for this cargo type. The flip side is that vessels completing a salt cargo in December or January, after the stockpiling rush, sometimes struggle to find a backhaul if the Atlantic market is soft.

Related cargoes in the IMSBC Code

Several other cargoes appear in close proximity to salt in seaborne trade flows or share properties that create practical comparisons:

Soda ash (sodium carbonate, Na2CO3) is another major sodium compound shipped in bulk for the chemical and glass industries. It is also Group C, but unlike salt it is not corrosive to steel in the same way (carbonate solutions are mildly alkaline and can passivate steel, unlike chloride solutions that attack it). Soda ash is more strongly hygroscopic than solar salt and cakes more severely.

Potash (potassium chloride, KCl) is chemically close to sodium chloride: another alkali metal chloride, also Group C (in its standard bulk grades), also hygroscopic, also corrosive to steel by the same chloride ion mechanism. Potash and salt share the same chloride corrosivity risk for ship structure, and the same post-discharge washing requirement applies. The distinction in trade terms is that potash is a fertilizer feedstock while salt is not.

Gypsum (calcium sulphate dihydrate) is also Group C and also hygroscopic, but it is not chloride-corrosive to ship steel in the same way. Gypsum shares salt’s dust generation and hold preparation considerations but does not impose the same mandatory post-discharge chloride wash requirement.

Diammonium phosphate and other ammonium-based fertilizers are chemically incompatible with salt as prior or subsequent cargoes in food-grade or pure-chemical contexts, but are otherwise structurally similar Group C or Group B cargoes. DAP is moderately hygroscopic and cakes in humid conditions, similar to salt.

Steel scrap, while not chemically similar, is often carried as a prior or subsequent cargo to salt on spot-market bulk carriers. Steel scrap residue in a hold that then carries food-grade salt introduces iron contamination. Thorough sweeping and inspection is required on the steel scrap to salt transition.

Limitations of this article

This article describes the IMSBC Code schedule for SALT and ROCK SALT as defined through Amendment 07-23 (MSC.539(107), mandatory from 1 January 2025). The Code is revised on an approximately two-year amendment cycle; readers should verify the current schedule text against the edition published by the IMO, not against secondary reproductions in commercial databases. The IMSBC Code is available from IMO Publications.

Physical property ranges given here (bulk density, stowage factor, angle of repose, hygroscopic behavior) represent the range across commercial grades and origins. Actual values for a specific shipment depend on the origin, production method, crystal size specification, moisture content at loading, and the specific lot’s chemical analysis. The shipper’s cargo declaration is the contractual statement of properties for each consignment.

The corrosion discussion is based on established electrochemical principles for chloride attack on mild steel and on classification society survey data. It does not substitute for a class-surveyor thickness gauging inspection on a vessel with a known history of salt carriage. Actual corrosion rates in any given hold depend on coating condition, bilge management, ventilation practices, and the specific ambient conditions during each voyage.

Trade route data and volume estimates are drawn from public industry sources and are approximate. Salt trade statistics are not collected with the same precision as major dry bulk commodities like iron ore or coal, because salt is produced from a large number of small and medium operations, and not all trade is captured in single-commodity statistics.

The post-discharge washing requirement described in this article reflects the IMSBC Code schedule text and standard industry practice as documented in P&I club loss-prevention publications. Specific washing procedures must be adapted to the vessel’s hold configuration, available water supply, and the requirements of the next cargo surveyor.

See also

• IMSBC Code
• IMSBC Code Group C Cargoes
• Limestone: IMSBC Code Schedule and Carriage
• Soda Ash: IMSBC Code Schedule and Carriage
• Potash: IMSBC Code Schedule and Carriage
• Gypsum: IMSBC Code Schedule and Carriage
• Diammonium Phosphate: IMSBC Code Schedule and Carriage
• Steel Scrap: IMSBC Code Schedule and Carriage
• Cargo Draught Survey for Bulk Carriers
• Cargo Hold Preparation Standards
• Marine Cargo Hold Ventilation
• Bulk Carrier

Related calculators

• IMSBC - Salt
• IMSBC - Rock Salt
• IMSBC - Salt Cake