Tank Material and Coatings on Chemical Tanker

A vessel may have cargo tanks of solid or clad stainless steel, or mild steel coated with chemical resistant coating materials. Care is necessary with all of these materials to avoid filling tanks with cargoes, which may adversely affect them, and to minimize the bad affect by ballast water or mechanical damage.

STAINLESS STEEL

Stainless steel is probably the most widely used material for tank lining. Cargoes, which are highly corrosive to mild steel such as Sulphuric Acid or cargoes which require high standards of purity (wine or fruit juice) will require the use of stainless steel for the construction of tanks.

Chromium Oxide is one of the most important components of Stainless steel. It gives the Stainless Steel its corrosion resistance capability by the formation of a passive surface film. Although this film forms spontaneously when the metal is exposed to air or to water.

The Stainless Steel must be maintained at its best corrosion resistance levels. The surface of Stainless Steel usually re-passivates when the film is damaged and if the passivation procedures are carried out properly. This property of Stainless steel makes it a better choice over coated steel.

The thickness of oxide film is about one millionth mm on the surface of stainless steel. Despite the thinness of this film provided, it will provide good corrosion resistance.

Stainless Steel is a general name given to a whole family of high-alloyed corrosion resistant steels in which chromium is the main alloying element. It is widely accepted that under oxidizing conditions for steel with chromium contents of at least 12% are impervious. This film is an invisible oxide film that forms on the surface and is self-healing. Nickel is added to the steel as an alloy to improve the corrosion resistance. Molybdenum is added to reduce the possibility to pitting attack.

Austenitic stainless steel is generally used in the construction of chemical tankers, since they are compatible with a wide range of cargoes. The austenitic grades have good workability and comparatively good weld ability. Austenitic stainless steel is non-magnetic.

Good Austenitic Steel generally consists of:

  1. C : 0.02 to 0.10%
  2. Cr : 18 to 25%,
  3. Ni : 8 – 22.5
  4. Mo : 0 – 4%

Differences between SUS 316L and SUS 304

tank material

S

SUS 316 L is a low-carbon steel, containing less than 0.030 Carbon

Stainless Steel tanks on Chemical Tankers

  • Solid Stainless Steel

These tanks are built of full stainless steel plates. Unless the tank is an independent tank surrounded by a void space, the tanks adjacent to the stainless steel tanks cannot be used for the carriage of ballast or fuel or fresh water. The problem with using solid stainless steel is that the space outside the tanks contains mild steel, which is used for water ballast tanks. This ballast could harm the stainless steel, as the same is not compatible with the sea water.

Secondly, bimetallic corrosion takes place, causing the mild steel to corrode overall preferentially. Pitting of stainless steel due to the deposition of the mild steel may also take place. This corrosion could be avoided by coating both the steels.

  • Clad Steel

Clad steel is a mild steel base plate to which a thinner layer of a stainless steel is continuously and integrally bonded. This is economical and the mild steel side is used for the carriage of water ballast. Also bimetallic corrosion problems are done away with. The clad steel is formed by Roll cladding or Explosive bonding.

Many of the process used in tank construction affect the passivity of the surface. The metal surface near welds oxidizes during the welding operation and the oxide films produced are less protective than the normal film. This leads to corrosion of the areas close to the weld. These areas shall be checked thoroughly during tank inspection regularly.

Tank finishing

Many of the process used in tank construction affect the passivity of the surface. The metal surface near welds oxidizes during the welding operation and the oxide films produced are less protective than the normal film. This leads to corrosion of the areas close to the weld. These areas shall be checked thoroughly during tank inspection regularly.

Types of Surface Contamination

The common types of surface contamination which can lead to localized pitting are:

  • Embedding of matter such as iron and iron dust in to the surface.
  • Entrapments of slag weld metal.
  • Weld spatter-particles of slag and weld metal.
  • Coloured oxide films near welds – these results in a dechromatization of the surface beneath the film resulting in corrosion.
  • Splashes of paint etc that may stick to the surface with formation of crevices.

Stainless Steel Care

Although careful attention will minimize surface contamination, it is impossible to avoid this completely and some final surface treatment, such as acid pickling and washing is necessary.

This procedure is often referred to as ‘passivation’ implying that some form of protective film is developed on the metal surface. In reality, however, this treatment is to remove surface contamination, which would interfere with the passive film normally developed on the stainless steels.

Pickling is done by:

  •  Applying pickling paste. These pastes are based on nitric acid or nitric/ hydrofluoric acid mixtures and are applied to the areas of the tank where surface contamination is evident, e.g. coloured oxide films near welds.
  • To wash down the tank surfaces with a mixture of nitric acid (6 to 15% by volume) and hydrofluoric acid 0.5 - 1.5% by volume) or stronger solution of nitric acid (10-20% by volume). Although nitric/hydrofluoric acid is faster that nitric acid care must be taken to ensure that it does not over etch the steel surface

Continuous exposure to pickling solutions for more than 30 minutes is not recommended. The item should be drained and rinsed after 30 minutes and examined to check the effectiveness of the treatment. Acid cleaning is not generally effective for removal of oils, greases and waxes. If any repairs or welding have to be carried out on the tank after it has gone in to service, the cleaning and passivation procedure should be repeated, although this need only be over a local area.

Related Post:  Important Items to be checked by Duty Officer during Rounds

Cleaning and Surface Treatment

Fresh water is the preferred cleaning medium for stainless steel and where possible it should always be used.

  • Due to unlimited availability of seawater, it is often used for initial tank cleaning operations. Where this is done a final tank wash with fresh water must be given to avoid prolonged contact of pools of stagnant seawater with stainless steel surfaces.
  • Piping systems require particular attention to avoid leaving inadequately drained areas full of seawater.

It is not necessary to repeat the acid cleaning and passivation process unless the tank has been repaired or it has carried a particularly corrosive cargo like phosphoric acid or Sulphuric acid or on that contains high levels of chloride and fluoride impurities. The acid could leave steel surface in an active condition with pits containing corrosion products. In such circumstance nitric acid treatment after washing would be necessary.

In short the tank has to be made passive again, whereby it will not support corrosion.

Corrosion Resistance of Austenitic Stainless Steel

Certain chemicals are known to cause a local breakdown of the passive film on the steel surface. This breakdown is followed by an electrochemical action, which results in pitting, which rapidly propagates into the steel. Chlorides are particularly harmful and may cause such attacks even when present in small quantities.

Solid sludge on the surface of stainless steel may also result in pitting. When sludge is known to be present in certain type of cargoes such as Phosphoric Acid, circulation of the cargo should be done to keep the sludge suspended in the cargo. The sludge particles are thus discharged into the shore facilities with the cargo.

Corrosion of Stainless Steel by Sea Water

The passive film on Austenitic steels is prone to local breakdown when chloride ions are present. Hence pitting is likely to occur if stainless steels are left in prolonged contact with static sea water. The attack is likely to occur particularly at crevices. The addition of molybdenum to stainless steels improves their resistance to pitting and crevice corrosion. The possibility of pitting of stainless steel is increased if the PH of the seawater is reduced from normal level of about 8. This is possible when acid cargoes have been loaded and cleaning of tanks which have had a balance residue is done. Such tanks must be emptied of water as soon as tank cleaning is commenced. The sea water mixed in acid must not be allowed to stay in the tank for a long time.

In order to obtain the optimum corrosion resistance from stainless steel cargo tanks it is necessary to avoid damage to the tank surface. This can be achieved by care in design and fabrication and by treating the steel surface after fabrication in such a way to remove surface contamination.

Care in service should aim at retaining the passive layer on the steel surface, which will provide corrosion resistance throughout the life of the ship. Great care and right procedure should be used for tank cleaning especially after discharging corrosive cargoes like the various acids and using seawater as initial cleaning medium.

In short the pumping out process must continue without disruption to minimize contact between stainless steel and acidic sea water. The quantity of seawater used must be the maximum possible so as to minimize the drop in PH from the acidic mixture value.

Once this is done for a sufficient period, tank cleaning must continue normally. One must bear in mind that for such cargoes, sea water cleaning once started, the washing should not be stopped under any circumstances, till the tank is cleaned.

The final rinsing after seawater cleaning should be carried out with fresh water immediately on completion the sea water cleaning. There should be no possibility of leaving sea water for long periods static in contact with stainless steel. The seawater temperature should not be kept higher than 50°C.

The tank must be dried at the earliest.

Passivation of Stainless Steel

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. This 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, stainless steel tanks and fittings should be examined for shiny areas, which indicate that the steel is active. Such areas need to be passivated by using nitric acid in liquid or paste form.

If there are only small isolated areas to be passivated, the manual application of nitric acid is carried out. In cases where large areas are to be treated, such as after the carriage of certain cargoes, then tank washing with nitric acid is carried out to ensure that the passivation is properly carried out. The acid must be re-circulated.

Test for checking Tank

All vessels with Stainless steel tanks or Stainless steel claded tanks will have an Electronic Passivation Test Meter and the passivation check at least every 3months may be carried out using this meter.

If due to some reason the Electronic Passivation Test Meter cannot be used to carry out the check, below mentioned procedure may be adopted.

Related Post:  Information provided in MSDS

“Palladium chloride test” is used to determine the passivity of the stainless steel. 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 worse is the condition of the steel.

COATED TANKS

Tanks of most ships are made of mild steel. Though mild steel has a good resistance to many cargoes, many other cargoes attack it. Various coatings as preferred by the ship owner are used, such as natural and synthetic rubbers, synthetic resin based paints and paints based on inorganic compounds.

The most commonly used coatings are epoxy-based (organic) paints and zinc silicate (inorganic) paints.

The wide use of tank coatings must be considered when loading chemical cargoes and when chemical cleaning is to be carried out because coating resistance to chemicals varies by type and manufacturer. The most useful criterion is the PH range to which a coating is resistant. The coating compatibility guide of the paint manufacturer must be referred to confirm that the cargo to be carried in the tank is compatible with the coating. If not compatible, another tank with a compatible coating must be selected.

The tank coating guide mentions if the coating is suitable for a particular or it specifies the range of PH values to which a particular coating is resistant.

Coating of mild steel tanks is carried out to:

  • Protect the mild steel from a cargo attack,
  • Provide a smooth surface making it easier to clean tanks between grades and
  • Minimize contamination from previous grades or by rust of tank structure.

If tank coatings are scratched or flaked the resulting damage may cause large areas of coating to break down with a possible deterioration in the cargo quality. It is extremely difficult to repair these areas effectively without shot blasting the damaged area dehumidifying the tank.

After discharging certain aggressive cargoes, the tank coating must be cured for a period designed by the paint manufacturer, before loading next cargo. Tank’s paint coatings manufacturers’ Resistant List and Reference Notes should always be referred. Tank cleaning chemicals must not be used if they have a detrimental effect on the tank coatings. Dirty slops or washing must not be stowed in coated tanks unless the content in the slops or washings is compatible with the coating.

Zinc silicate

Generally, 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% in all circumstances specific guidance on the compatibility of a certain with the coating is to be obtained from the cargo information and the coating compatibility guides.

Zinc silicate coatings withstand intermittent exposure to fresh and salt water, but continuous immersion will greatly reduce the life of coating. For this reason zinc silicate tanks are not to be ballasted unless absolutely necessary.

Epoxy coatings

Specific guidance must be obtained from cargo information and coating compatibility lists for the cargo in question. Epoxy coatings are suitable for fresh and salt water ballast. Vessels coated with bi-modal epoxy coatings such as Interline, Marine line etc. requires greater caution during their inspection. These coatings are designed to deliver greater efficiency and flexibility in the operation of chemical tankers, can switch from one cargo to the next easily, with minimal downtime and tank cleaning.

Phenolic epoxy coatings

These coatings have a similar resistance to epoxy coatings with a wider range of chemicals and also fewer restrictions than either straight epoxy or polyurethane. However information of the coating compatibility list must be obtained for the specific cargo in question.

Polyurethane coatings

These coatings have compatibility similar to that of epoxy coating plus some of the solvents compatible with zinc silicate coatings. However, information from the coating compatibility list must be obtained for the specific cargo in question.

Repair of tank coatings

Repair of tank coatings is a very important issue. The maintenance of the coating is to be carried out at every possible opportunity.

In coated tanks coating manufacturers instructions are to be followed with regard to surface preparation, application of coatings and curing time.

In general, following shall be given due consideration while doing such maintenance:

  1. Surface is to be thoroughly de-rusted.
  2. Edges should be smoothened by using disc grinder.
  3. Wipe bare metal with methanol / thinner prior application of the first coat.
  4. Coating should be applied by a hand brush and not by roller. Overlap of the surrounding area should be kept to bare minimum.
  5. Humidity in the tank is to be controlled in accordance with manufacturer’s instructions.
  6. Epoxy coatings require low relative humidity (dry air) - Running ventilation fans in conjunction with steam to heating coils crack open 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.
  7. Ensure sufficient coats are given to obtain the necessary dry film thickness.
  8. Over coating interval should be in accordance with the manufacturer’s instructions.
  9. Sufficient curing time is to be allowed. 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 sea water temperature should be used when referring to the curing time table.

10 Replies to “Tank Material and Coatings on Chemical Tanker”

  1. Ι know thіs web рage presents գսality dependent posts
    and extra information, is there any other website which
    gives such things in quality?

Leave a Reply

Your email address will not be published. Required fields are marked *

Privacy Preference Center