Hold cleaning chemicals: alkaline vs acid, dilution, dwell time and when to use them

When do you need chemicals at all?

Water, used properly, removes loose dust and soluble residue. It doesn't reliably remove:

• Oily films from green petcoke and some coals
• Coal and mineral staining bonded into the coating
• Set or hardened residue like cement, clinker and some mineral scales
• Fatty or greasy residues from some agricultural products

If the next cargo only needs normal clean, you can often skip chemicals. If you're going to grain clean or better after a staining cargo, you'll almost certainly need them.

Alkaline vs acid cleaners

Almost every hold-cleaning chemical falls into one of two families.

Alkaline cleaners (degreasers) break down oily and organic residue by emulsifying it, so water can carry it away. They're the workhorse for:

• Petcoke (especially green)
• Coal and coal stains
• General grime and soot
• Oily or fatty residue

Acid cleaners dissolve mineral deposits. They're used for:

• Cement and clinker residue
• Lime scale and limewash remnants
• Some mineral stains and rust bloom

So, should you use an alkaline cleaner? For carbon-based residue, yes. When should you use acid? Only for mineral residue that alkaline products won't touch, and with real care, because acids attack bare steel and can damage some coatings. Never mix the two families, and never apply one straight after the other without a thorough rinse in between.

Which chemical removes what?

Brands differ, so check the product data sheet for what it's designed to remove.

Can chemicals damage hold paint?

Yes, if they're the wrong product, too concentrated, or left too long. Strong alkalis can soften some coatings, and acids can attack both paint and bare steel. To avoid trouble:

• Check the product data sheet against your hold coating specification.
• Use the weakest dilution that does the job.
• Try a test patch if you're unsure.
• Don't exceed the stated dwell time.
• Rinse thoroughly.

How much hold-cleaning chemical do you need?

It depends on the surface area, the residue and the product. Suppliers quote coverage in square metres per litre of concentrate at a given dilution. As a rough planning exercise, estimate the hold's internal surface area (a Supramax hold has several thousand square metres once you count frames, hoppers, bulkheads and tank top), apply the supplier's coverage rate, then add a margin for second passes on stubborn areas. Running out halfway through the fourth hold is a classic and avoidable problem, since the right product often isn't available locally.

How should hold-cleaning chemicals be diluted?

Exactly as the manufacturer says, for the residue you're tackling. Most products have a range: weaker for light soiling, stronger for heavy. Use fresh water for mixing unless the instructions say otherwise. Hardness and salt can reduce effectiveness. Mix in a clean drum, and if you're injecting chemical through a combi-gun or foam unit, set the ratio and check the output.

How long should chemicals remain on the surface?

This is dwell (or contact) time, and it's where most chemical cleaning goes wrong. Too short and the chemical hasn't had time to break down the residue. Too long and it dries on the steel, leaving its own residue and potentially damaging the coating. Typical dwell times are measured in minutes, so the manufacturer's figure for your product is what counts. In hot weather, the surface dries faster, so work in smaller sections.

Should chemicals be applied before or after HP washing?

The usual sequence is:

1. Sweep and remove loose residue
2. Rough wash with water to remove loose dust (optional for some products)
3. Let the surface drain to the condition the product needs. Many work best on a dry or slightly damp surface
4. Apply chemical
5. Dwell
6. High-pressure wash to remove the chemical and the residue it has loosened
7. Fresh water rinse

So chemicals go on before the main HP wash, not after it. The HP wash is what removes them.

Also note the direction. Water washing is done top down. Some chemical products are best applied bottom up, so the chemical doesn't run down over dry surfaces and streak. Follow the product instructions.

Do you need to rinse after chemical cleaning?

Always. Chemical residue left on the steel can contaminate the next cargo, and with food cargoes that's a serious problem. Finish with a thorough fresh water rinse, top to bottom.

Can chemical residue cause cargo contamination?

Yes. That's why the rinse matters, and why some charterers ask for details of any chemicals used. Keep the product data sheets and records of what was used, where and when.

MARPOL and chemical wash water

Under MARPOL Annex V, wash water containing cleaning agents can only be discharged at sea if the agents aren't harmful to the marine environment, and the other discharge conditions are met. Skuld's guidance sets out the test: a cleaning agent is considered not harmful if it isn't a harmful substance under MARPOL Annex III and contains no components known to be carcinogenic, mutagenic or reprotoxic. Ask the supplier for a statement confirming this and keep it on board. If the agent is harmful, the wash water must go to a reception facility.

What PPE is required for hold-cleaning chemicals?

At a minimum, follow the safety data sheet. Typically:

• Chemical-resistant gloves and boots
• Full-face protection or goggles plus a face shield
• Chemical-resistant overalls or rain suit
• Respiratory protection where mist or vapour is generated, such as when spraying in an enclosed hold
• An eyewash station and fresh water available at the work site

Atomised chemical mist in an enclosed hold is easy to inhale. Ventilate, and keep people out of the spray zone.

Getting help

If you're not sure which product suits your residue and coating, or the timing's too tight to experiment, our shore gangs arrive with matched chemicals and equipment. Ask us for a recommendation for your next cargo change.