| FOM 1.6 |
Fleet Operations Manual General Tank Material and Coatings |
Doc No.: FOM 1.6
Revision: 01 Date: 15 Oct 2024 Issued by: DPA Approved by: MD |
1 APPLICATION
This document applies to all tankers managed by Company.
2 PURPOSE
This section specifies instructions for repairs and maintenance of cargo tanks and coatings.
3 TANK MATERIALS AND COATING
3.1 General
Take care to:
- avoid filling cargo tanks with cargoes which may adversely affect them, and
- minimize the detrimental effect which may be caused by:
- ballast water or
- mechanical damage
3.2 Prevention of Mechanical Damage to Tank Surface
Avoid scratching the surface of stainless-steel tanks. Scratches may seriously impair the resistance of the material due to corrosive attack by aggressive cargoes.
When entering stainless steel and coated tanks:
- Wear soft-soled shoes with a disposable shoe cover.
- Lower the required tools in a sack or bag to prevent damage due to falling off.
- Ensure equipment for mopping, etc., including buckets and scoops, is of rubber, plastic, or other soft materials.
- When positioning portable tank washing machines, take care to ensure that they do not knock against tank sides or structure.
Note:
Additional precautions will be necessary when tank cleaning in bad weather. If in doubt, contact the Marine Superintendent.
3.3 Stainless Steel Tanks
3.3.1 General
- Check the compatibility of the stainless steel with the cargo at the time of planning cargo stowage.
- Strictly control the use of salt water when tank cleaning. Fresh water, hot or cold, does not affect the stainless steel provided that it is relatively chloride-free.
- Ensure stainless steel tanks contain only fresh water except in emergencies.
Note:
It is acceptable to wash stainless steel tanks and lines with salt water; however, immediately afterward, they must be washed with fresh water and dried up.
3.3.2 Avoiding Corrosion of Stainless Steel by Sea Water
- Pitting is likely to occur if stainless steel is left in prolonged contact with static sea water. The attack is likely to occur particularly at crevices.
- The possibility of pitting is increased if the pH of seawater is reduced from the normal level of about 8.
- This is possible when acid cargoes have been loaded and cleaning of tanks that have had a balance residue is done.
Ensure:
- Such tanks are emptied of water as soon as tank cleaning is commenced.
- Any sea water mixed with acid does not stay in the tank for a long time.
Note:
In order to conserve the corrosion resistance throughout the lifetime of the vessel, it is necessary to retain the passive layer on the steel surface.
This may be done by applying the correct procedure for tank cleaning as described below:
- Continue the pumping out process without disruption.
- Minimize contact between stainless steel and acidic sea water.
- The quantity of seawater used must be the maximum possible to minimize the drop in pH from the acidic mixture value.
- Once this is done for a sufficient period, continue tank cleaning normally.
- Do not stop the washing under any circumstances until the tank is cleaned.
- Carry out final rinsing after seawater cleaning with fresh water immediately on completion of the sea water cleaning.
- Do not leave sea water for long periods static in contact with stainless steel.
- Any seawater kept for shorter periods must be kept at temperatures not higher than 50°C.
- The tank must be dried at the earliest.
For details on the care of Stainless Steel, see CTM 9.7 A3 Corrosion in Chemical Tanker and Care of Stainless Steel Tanks.
3.4 Passivation of Stainless Steel
3.4.1 General
- New stainless steel normally shines in appearance and is said to be active.
- In this state, it is susceptible to reaction with certain cargoes and seawater.
- Stainless steel, therefore, needs to be passivated before being put into use, which is done by treating the surface with Nitric acid.
- The ensuing reaction produces a thin film of chrome oxide on the surface and gives a dull appearance.
- Stainless steel can become active due to mechanical damage or, at times, due to certain aggressive cargoes such as Phosphoric acid or Sulphuric acid.
During tank inspection, examine stainless steel tanks and fittings for shiny areas, which indicate that the steel is active. Such areas need to be passivated.
3.4.2 Passivity Test
- An overall condition of the tank should be assessed every 12 months or after 3 voyages of aggressive acids like phosphoric and sulphuric.
- This inspection should ensure the presence of passive chromium oxide film on the stainless steel. When required, for example after carriage of aggressive cargoes, the surfaces of SS should be checked by Passivity meter or Palladium chloride test kit.
- Analysis of Passivity meter readings obtained shall be recorded in Form E25A Cargo Tank Inspection (Stainless Steel Tanks).
Analysis of Passivity Meter Readings Obtained
- Less than 30mV - shows that the surface is still in an active state.
- 30mV ~ 60mV - shows that the surface is not fully passive, or that the protective oxide "film" is incomplete.
- More than 60mV and stable or rising - indicates a passive surface.
Palladium Chloride Test
- If the above cannot be used, "Palladium chloride test" may be used as an alternative procedure.
- Use a palladium chloride solution of 0.5 g per 100 ml with 5 ml concentrated hydrochloric acid.
- Make sure that the surface to be tested is free of fatty material.
- Bring a drop of the palladium chloride solution on the surface, wait three minutes.
- Remove the drop of liquid with a tissue, and observe the colour of the spot:
- No change in colour: PASSIVE
- Discoloration to black: ACTIVE
- Discoloration to grey: the condition of the steel is doubtful. The darker the colour, the worse the condition of the steel.
In addition to regular testing as explained above, the need for passivation may arise after inspection of tanks. For details on the procedure of Passivation, see CTM 9.7 A3 Corrosion in Chemical Tanker and Care of Stainless Steel Tanks.
3.5 Coated Tanks
3.5.1 General
- Ensure coated tanks are compatible with the cargo to be carried.
Note:
For detailed information on compatibility of cargoes and coating materials, refer to coating compatibility lists supplied by the paint manufacturers.
- After discharging certain aggressive cargoes, the tank coating is cured for a period designed by the paint manufacturer before loading the next cargo.
- Tank cleaning chemicals are not used if they have a detrimental effect on the tank coatings.
- Dirty slops or washing are not stowed in coated tanks unless the cargo contaminant in the slops or washings is compatible with the coating.
3.5.2 Zinc Silicate Coatings
- Zinc silicate coatings withstand intermittent exposure to fresh and saltwater, but continuous immersion will greatly reduce the life of the coating.
- Do not ballast zinc silicate tanks unless absolutely necessary.
As a general rule, zinc silicate coatings are not affected by and do not affect cargoes in the following chemical families or groups:
- Alcohols
- Amines (if free of moisture, and tanks are dry)
- Aldehydes
- Animal oils and fats (free fatty acid under 2.5%)
- Cyanohydrins
- Esters
- Glycols
- Halocarbons
- Hydrocarbons
- Aromatic Hydrocarbons
- Lubricating Oils
- Clean petroleum Products
- Ketones
- Vegetable oil (free fatty acid under 2.5%)
Zinc silicate coatings are unsuitable for:
- Acids, alkalis, vegetable and animal oils and fats with a free fatty acid (FFA) content of more than 2.5%, unless otherwise advised by the manufacturer.
Also, see CTM 9.7 A4 Care & Maintenance of Zinc Silicate Coatings.
3.5.3 Epoxy Coatings
Epoxy coatings are suitable for fresh and salt-water ballast. In general, epoxy coatings are not affected by, and do not affect cargoes in the following list:
- Alkalis (Low grade) Amines
- Glycols
- Animal oils and fats (FFA below 5%)
- Hydrocarbons
- Vegetable oil (FFA below 5%)
- Alcohols (with certain restrictions) Except methanol
Generally, epoxy coatings are unsuitable for:
- Acid
- Aldehydes
- Animal oils and fats (FFA 5%)
- Cyanohydrins
- Esters
- Halocarbons (some)
- Ketones
- Vegetable oils (FFA over 5%)
Also, see CTM 9.7 A5 Aggressive Cargoes in Epoxy Coated Cargo Tanks.
3.5.4 Polyurethane Coatings
These coatings have compatibility similar to that of epoxy coatings plus some of the solvents compatible with zinc silicate coatings. However, it may contain isocyanides.
Obtain information from the coating compatibility list for the specific cargo in question.
3.5.5 Phenolic Epoxy Coatings
These coatings have a similar resistance as epoxy coatings with a wider range of chemicals, and also have fewer restrictions than either straight epoxy or polyurethane.
Obtain information from the coating compatibility list for the specific cargo in question.
3.6 Inspection and Maintenance of Tank Coatings
- Chief Officer shall take every opportunity possible to enter coated tanks for thorough close visual inspection.
- Any abnormality or sudden change of condition found through the inspection must be reported to the Master who shall inform the Marine Superintendent immediately.
- Chief Officer shall:
- Carry out inspection at least every 12 months. For Oil tankers (VLCC) and Gas tankers refer to FOM 1.5 Section 1.2, 1.4, and 1.5 respectively.
- Report status and any defects to the Marine Superintendent.
- Record of inspection shall be made in E25A Cargo Tank Inspection (Stainless Steel Tanks).
3.7 Repair of Tank Coatings
- Carry out Maintenance and possible repair of the coating at every possible opportunity.
- In coated tanks, follow the coating manufacturer’s instructions with regard to surface preparation, application of coatings, and curing time.
- In general, give due consideration to the following issues while performing maintenance and repair:
- De-rust the surface thoroughly. (If possible to an SA 2.5 standard.)
- Smoothen the edges by using a disc grinder.
- Wipe bare metal with methanol / thinner prior to the application of the first coat.
- Apply coating by a hand brush and not by roller.
- Keep overlap of the surrounding area to be bare minimum.
- Control humidity in the tank to be in accordance with the manufacturer’s instructions.
- Epoxy coatings require low relative humidity (dry air).
- Consider running ventilation fans in conjunction with steam to heating coils crack open, which is a good way to raise the air temperature and reduce relative humidity.
- Zinc Silicate coatings may require high humidity at the time of application and during curing – a steam hose lowered into the tank with the valve crack open raises the relative humidity in the tank.
- Ensure sufficient coats are given to obtain the necessary dry film thickness.
- 100 micron for zinc.
- 300 micron for epoxies.
Note:
Overcoating interval must be in accordance with the manufacturer’s instructions.
Note:
While the temperature of the air in the tank may be high, the substrate temperature may be quite low in the case of tanks exposed to seawater on the outboard side. In this case, the seawater temperature should be used when referring to the curing time table.
4 REFERENCED FORMS AND CHECKLISTS
- E25 – Cargo and Ballast tank inspection record.
- E25A – Cargo tanker inspection Stainless Steel tanks.
- CTM 9.7 A3 Corrosion in chemical tanker and care of stainless-steel tanks.
- CTM 9.7 A4 Care & maintenance of zinc silicate coatings.
- CTM 9.7 A5 Aggressive cargoes in epoxy coated cargo tanks.