Cleaning holds after sulphur: washing, limewashing and stopping the corrosion

Why sulphur is dangerous to hold coatings and steel

Solid sulphur on its own is fairly inert. The problem starts when it meets water. Wet sulphur dust and residue form acidic compounds, and The Swedish Club, quoting the IMSBC Code, puts it bluntly: wet dust or residues may form highly corrosive sulphurous acid, which is extremely dangerous to personnel and corrosive to steel.

Add salt to the mix and it gets worse. Chlorides accelerate corrosion, which is why the code is specific about the type of water.

The other hazards worth knowing:

• Dust explosion and fire. Sulphur dust is flammable and can explode in a dense cloud with an ignition source. Sulphur fires produce toxic sulphur dioxide.
• Toxic gases. Hydrogen sulphide can be present around some sulphur cargoes. Treat holds as enclosed spaces and test the atmosphere.
• Health. The dust irritates eyes, skin and lungs.

What the IMSBC Code says

The IMSBC Code's entry for SULPHUR (UN 1350, crushed lump and coarse grained) is unusually specific. As quoted by The Swedish Club, it says:

• Cargo spaces and other structures that may have been in contact with the cargo or its dust shall not be swept.
• After discharge, the cargo spaces shall be washed out with fresh water to remove all residues.
• The cargo spaces shall then be thoroughly dried.
• People involved in cleaning shall have protective clothing, goggles and face masks.

Britannia's guidance also notes that the IMSBC entry for formed, solid sulphur states that the hold should not be washed with seawater. Always check the current edition of the code for the exact cargo name you carried. There are separate entries for different forms.

Should sulphur holds be washed with seawater?

Not according to the code. Fresh water is the requirement. We understand why crews reach for seawater: it's unlimited, and fresh water isn't. But washing sulphur residue with seawater leaves chlorides behind with an acid-forming residue, which is exactly the combination you're trying to avoid.

The practical answer is to plan fresh water before you load sulphur. Know how much you'll need for the wash-out, make sure it's on board, and if possible arrange shore water at the discharge port.

How to limewash cargo holds before loading sulphur

Limewashing is the traditional protection. It coats the steel with an alkaline layer of hydrated lime (calcium hydroxide), which neutralises acid as it forms and acts as a barrier between the cargo and the steel. Britannia's guidance describes it as a traditional barrier coating for corrosive cargoes like sulphur and salt.

A typical limewashing procedure looks like this. Always follow the specific instructions of the lime or barrier coating supplier, since quantities and mixes vary.

1. Start with a clean hold. Limewash goes onto a hold that's been cleaned of the previous cargo, with loose rust removed. It won't stick properly to dirt or flaking scale.
2. Mix hydrated lime with fresh water to the supplier's ratio, in a drum or tank with continuous stirring so it doesn't settle. Britannia notes that adding granulated sugar to the mix can make the limewash easier to remove later.
3. Apply by spray. An airless or diaphragm pump with a suitable nozzle works well. Cover the bulkheads, frames, hopper sides and other surfaces in contact with the cargo, plus the tank top. Pay attention to areas where coating has already broken down.
4. Let it dry fully before loading. Wet limewash is not a barrier.
5. Protect the bilge wells as required, and make sure lime slurry doesn't block the bilge system.

Intact paint coating reduces how much lime you need. Some operators now use proprietary barrier coatings instead of traditional limewash; they do the same job and are designed to wash off more easily.

Cleaning holds after a sulphur cargo

Here's the post-discharge sequence:

1. Remove as much cargo as possible during discharge. Get stevedores to clear residue from ledges and hoppers with their equipment.
2. Don't sweep. Remove remaining residue by other means, such as shovelling the bulk of it, and then washing, avoiding raising dust clouds.
3. Test the atmosphere and ventilate before entry. Wear proper PPE: goggles, face masks or respirators, protective clothing.
4. Wash out with fresh water, top down, thoroughly. You're removing residue and limewash together.
5. Remove the limewash completely. This is the stage that takes time. Limewash is designed to stick, and any left on the steel becomes contamination for the next cargo. High-pressure fresh water does most of it. The sugar additive helps. Some suppliers sell a mild acidic remover for stubborn areas, but use it carefully and rinse thoroughly, because you don't want to replace one residue with another.
6. Dry thoroughly. Damp sulphur residue is how the corrosion starts.
7. Inspect for damage. Look for fresh corrosion, pitting and coating breakdown, especially on the tank top and lower frames.

How do you prepare sulphur holds for the next cargo?

Once the sulphur and limewash are gone and the hold is dry, clean to the standard the next cargo requires. For grain, that means the full grain-clean routine, with extra attention to:

• White traces of lime in corners and along welds. Surveyors notice them.
• Odour. Sulphur has a distinctive smell; ventilate until it's gone.
• Loose rust that formed during the sulphur voyage. Remove it.

How do you prevent corrosion after sulphur cargo?

• Limewash or barrier-coat before loading, every time.
• Wash out with fresh water promptly after discharge. Don't leave residue to sit.
• Dry the hold completely.
• Keep the hold coating in good condition; sulphur finds every breakdown.
• Inspect after every sulphur voyage, and repair coating damage before the next one.

Watch the timing as well. Limewash doesn't last forever. Industry commentary suggests limewash protection is generally considered good for a limited period, around three weeks, after which corrosion can accelerate. Long waits at anchorage with sulphur on board are a known risk.

If you've got a sulphur discharge coming and need the wash-out and limewash removal done fast, our shore gangs bring the PPE and equipment for it. Contact us with the port and dates.