The superstructure, decks, hull plating above the waterline and deck machinery are, relative to other parts of the vessel, in less need of specialist products. In these areas the usual products will be a base primer, tie coat and one of the epoxy or alkyd cosmetic coatings. So long as any mechanical damage is kept under control and regular inspections made for more deep-rooted problems, the coating should remain intact. However, it should also not be forgotten that every surface has two sides and corrosion can begin in an area that may not be readily observed.
In the never-ending battle against corrosion, ships need to be protected inside as well as out. Some of the harshest conditions to be found in ballast tanks and in the cargo spaces of bulk and crude oil carriers. Protecting these areas against corrosion is an important consideration as they represent some of the largest areas of steel work and because they are integral parts of the structure of the ship.
Despite this fact, cargo holds are not subject to any regulations as regards coatings, but the condition of the steel work is regularly assessed by class and other inspectors. Similarly, the tanks on many tankers are not covered by regulations but in chemical and product tankers the tank coating may be important to the commercial operation of the vessel. Several manufacturers make products specifically aimed at chemical tankers.
Protecting the ballast tanks
While protection against corrosion in dry cargo holds and some cargo tanks is voluntary, protecting against corrosion and the coatings used in the ballast tanks of most vessels and crude oil tanks are now regulated under IMO performance standards for protective coatings (PSPC). The first of these affecting only ballast tanks and double-side skin spaces of bulk carriers was adopted in 2006 and came into effect in 2008 as Resolution MSC.215(82).
This PSPC applies only to dedicated seawater ballast tanks in all types of ships above 500gt and double side skin spaces in bulk carriers above 150m loa which are constructed of steel. It does not apply to cargo holds in bulk carriers that are sometimes used for ballast purposes. It is based on a detailed specification and requirements which intend to provide a target useful coating life of 15 years, which is considered to be the time period, from initial application, over which the coating system is intended to remain in ‘good’ condition.
‘Good’ is not a vague term but is defined as: “A condition with spot rusting on less than 3% of the area under consideration without visible failure of the coating. Rusting at edges or welds should be on less than 20% of edges or weld lines in the area under consideration”. The actual useful life will vary, depending on numerous variables including actual conditions encountered in service.
While there is a requirement for the owner to maintain the coatings to the standards during the life of the ship, the most onerous part of the regulation is directed at coating manufacturers, coatings contractors and inspectors. The PSPC is formulated around two-coat epoxy coatings but permits alternatives such as Nippon Paints’ NOA 60HS Self-Indicating one-coat epoxy coating system.
Two coats were stipulated for the simple reason that the second and lighter colour coat allows easy identification of areas where coverage has been missed. The one-coat epoxy system has a very different colour depending upon the coating thickness making it relatively easy to see both under and overcoated areas.
The PSPC requirements say that products used for ballast tank coatings must be type-approved but they go far beyond that simple statement with information on how the type-approval process should be carried out. A great deal of the PSPC is about preparation of the tank surfaces and structures. This is understandable given that even the best coatings will fail if surfaces have not been properly prepared. Inspection standards are also covered in the standard.
On delivery of the ship, the owner should also be given a Coatings Technical File (CTF) detailing the coatings used, shipyard work records, type-approval certificates, results of inspections during construction and guidance on repair and maintenance.
Once the ship is in service, the owner will be responsible for recording all repairs and recoating activities together with appropriate documents. The flag state should not issue a Safety Construction Certificate until the CTF is completed and its own inspection recorded.
One aspect that was not covered in the PSPC and which some consider is a serious omission is the potential for some ballast water treatments systems – particularly those that make use of active substances – to be incompatible with the particular coatings used on individual ships. There is a degree of dispute between coatings manufacturers and treatment system suppliers as to which of them should test for compatibility. Some treatment system makers have begun tests with different coatings and can give assurance but with so many systems and coatings on the market there are many permutations and owners should initiate discussions on compatibility at an early stage if later problems are to be avoided.
Most of the many paint products marketed a suitable for ballast tank coatings are epoxy systems with high abrasion resistance. The ability of the coating to be able to withstand abrasion is essential given that, even after filtering, most ballast water will contain fine grit and sand. With the constant moving of the vessel when at sea, the presence of this grit can easily strip less robust coatings in a very short period allowing corrosion to set in quite quickly.
Crude oil tank coatings
SOLAS Regulation II-1/3-11, which entered into force on 1 January 2012, on corrosion protection of cargo oil tanks of crude oil tankers, requires cargo oil tanks to be protected against corrosion and makes IMO Resolution MSC.288(87) Performance Standard for Protective Coatings for Cargo Oil Tanks of Crude Oil Tankers mandatory. These regulations are generally referred to as PSPC COT.
As with the PSPC for ballast tanks, the regulation is directed more to the shipyard and coating supplier than to the vessel owner although the obligation to maintain and repair does pass to the owner on delivery.
Since SOLAS did not actually contain a definition of a crude oil tanker it was necessary to address this omission as well. The new regulation came into force in January 2012 and applies to crude oil tankers of 5,000dwt and above for which the building contract is placed on or after 1 January 2013 or in the absence of a building contract, the keels of which are laid or which are at a similar stage of construction on or after 1 July 2013 or the delivery of which is on or after 1 January 2016.
It was already common practice to apply protective coatings in the upper and lower areas of cargo oil tanks but the IMO PSPC COT regulations set out more exact specifications. The following areas are the minimum areas that shall be protected according to the IMO text:
- Deckhead with complete internal structure, including brackets connecting to longitudinal and transverse bulkheads. In tanks with ring frame girder construction the underdeck transverse framing to be coated down to level of the first tripping bracket below the upper faceplate.
- Longitudinal and transverse bulkheads to be coated to the uppermost means of access level. The uppermost means of access and its supporting brackets to be fully coated.
- On cargo tank bulkheads without an uppermost means of access the coating to extend to 10% of the tanks height at centreline but need not extend more than 3m down from the deck.
- Flat inner bottom and all structure to height of 0.3m above inner bottom to be coated.
Coatings must be type-approved according to strict guidelines aimed at simulating exposure to a generic crude oil. The development of a testing protocol for new products was entrusted by the IMO to the International Paint and Printing Ink Council. To undertake this work, the council established a working group that was composed of representatives from class societies, shipowner groups, shipyards, testing laboratories and coating companies.
The PSPC COT approval process for the actual coating on a vessel follows closely the procedures used in the PSPC for ballast tanks including the requirements for surface preparation, for a CTF and a target useful life of 15 years in ‘good’ condition for the coating.
Most of the leading coatings manufacturers have developed type-approved coatings to meet the standard so compliance should not present any problems for owners. For owners looking for an alternative option to coatings for newbuildings, Japanese class society ClassNK has developed what it claims as the world’s first set of guidelines for the application of corrosion-resistant steels to the cargo oil tanks of oil tankers. They were accepted as an alternative to coatings as part of Resolution MSC.289(87).
As a result, crude oil tankers over 5,000dwt contracted for construction after 1 January 2013 are able to use corrosion-resistant steel for the inner surfaces of cargo oil tanks. As the use of corrosion-resistant steel eliminates the need for the expensive facilities, preparation and finishing work associated with coating application, while reducing the need for maintenance and coating reapplication, the demand for such steels is expected to grow in the future.
The Guidelines on Corrosion Resistant Steel for COT document was released in January 2012. It is too early to estimate how the cost of corrosion-resistant steel will compare with conventional material through the lifetime of a ship, but ClassNK is confident that any additional cost will be more than offset by avoiding the additional time, labour and resources needed to apply coatings.
Chemical tanker coatings
Although there is a PSPC for tank coatings on crude carriers, far more harsh environments can be encountered in the holds of chemical tankers. All of the major coatings manufacturers and several smaller niche specialists have developed coatings for use in chemical and product tankers.
Not all of the available products are suited to protect against all of the various chemical cargoes that may be carried so tankers will often have a range of different coatings applied to some of the ship’s tanks. In addition to being able to protect against highly acidic or alkaline chemicals, there is a need for coatings that are approved to the various national requirements that may apply to the carriage of liquid foodstuffs.
Coatings that are suited to food products must obviously not react with the product being carried but they must also not be prone to permeation of the product into the coating as this could later leach out and contaminate future cargoes. Matching the suitability of cargoes and different coatings is an essential skill that must be acquired by all involved in operating such specialist ships.
Dry cargo holds
There are no regulations affecting dry cargo holds in the same way as there are for ballast tanks and crude oil cargo tanks. However, holds and particularly those in bulk carriers are subject to corrosion and damage caused by the cargo handling methods and the cargo itself. Since the majority of bulk carriers are single-skin vessels, the inside of the hold is also the hull and the double bottom tank tops. Therefore, any corrosion is likely to affect the structural integrity of the vessel and as a consequence will be given special attention by PSC, P&I and class inspectors and surveyors.
Some typical bulk cargoes such as coal, sulphur and fertilisers can themselves be corrosive and under the appropriate conditions of temperature and humidity can cause severe corrosion wastage in unprotected parts of the structure. For these reasons cargo holds are often coated, usually with epoxy coatings.
The coatings used in cargo holds should be able to withstand physical damage such as experienced when ‘shooting’ hard cargoes such as coal and ores and abrasion and gouging caused by the movement of the cargo during the voyage. Mechanical damage can also be caused by cargo handling equipment such as grabs and buckets used during discharge.
Many leading manufacturers produce coatings specially designed for use in cargo holds. The coatings are notably robust and often of a hard coat type. Because some coatings can contain
substances harmful to human health, certificates proving that the coating on a ship is harmless may be required by some administrations. As with the specialist coatings for food products in chemical tankers, most manufacturers will have products that comply with the various national regulations that can be found around the globe.