While some coatings can be used as a one-coat application, coating systems usually consist of a primer, possibly a tie coat and one or more coats of the chosen product. All the coats have a role to play with the primer being the first barrier against corrosion with a tie coat, if necessary, bonding that to the topcoat that provides the protection the coating system is designed for. Products from different manufacturers may employ similar technologies but usually a manufacturer develops its own products to work best together. The paint store on a top-end ship will usually reflect this but further down the scale of good management this will not always be the case.
On any coating system, the base coat will be the foundation on which all subsequent coats are reliant. It must adhere to the steel substrate and will offer a degree of protection against corrosion. Destined never to be seen except when first applied, primers need to be matched to both the substrate and the tie or topcoat if the system is to function as intended. There are a small number of offerings in this category that are one-coat products, but most are two-part products. Most makers only offer guarantees on systems if the primer used is confirmed as compatible, which generally means that all layers must be from their own product ranges.
When coating contractors begin work on a newbuilding, the structure of the ship will almost certainly have some shop primer covering. Shop primers are used during fabrication and will need to be mechanically prepared or removed before the application of the system primer. They should not be confused with the main primer coat that forms the basis of the coating system.
Today it is common practice to use a universal primer in newbuilding coatings rather than the range of different products that were used until quite recently. A universal primer may seem to be a compromise solution but in fact it would seem to have many benefits. From a shipyard’s point of view, a universal primer may cost more but it does reduce waste and allows for economies of scale in bulk purchases.
For the paint manufacturers, product lines can be optimised and R&D more focused on whole systems rather than individual products. Shipowners might not appear to benefit directly but, assuming that yards’ cost savings are passed on, then there is a financial benefit and in all probability the coatings will be of a higher standard as workers will be more familiar with the product.
For newbuilding work, almost all primers will contain zinc because of its highly effective anti-corrosion properties. There are two basic types of zinc primers – zinc silicate and zinc epoxy. The former is considered better for initial coating at the shipyard while the epoxy is considered as the best option for repair and maintenance work because there is less need for a high standard of surface preparation.Epoxy and alkyd products
Epoxy and alkyd products
This group of products are widely used on ships and account for most coating systems other than those below the waterline for biofouling management. They come as either pure epoxies or modified epoxies. The modified epoxies will have added ingredients designed for a specific purpose such as reducing curing time or application on specific areas.
Epoxy coatings need to harden or cure to form the outer protective coating. This can take a considerable time but with the use of a hardening agent the time is much reduced. For this reason, most epoxy coatings are two-pack products; one being the resin and the other the hardener. The two parts have to be mixed in precise ratios for the declared properties of the final coating to be achieved. The hardening process is affected by a number of factors with temperature being particularly important. Coating thickness and humidity also affect drying times and quality of finish.
Even when a coating is applied in full accordance with the manufacturer’s instructions the curing process can take several days or weeks before the maximum protective effect is achieved. During this period, care should be taken to avoid damage to the coating. Attempting to speed up a repair job by adding more hardener or adding solvents to extend the working time of a properly prepared mix will also have detrimental effects on the final finish.
Epoxy coatings are used extensively for ballast tanks, cargo tanks and cargo holds. In some instances, it will be possible for a coating to react with particular cargoes.
Operators of chemical tankers should be well aware of the compatibility of coatings and cargoes and will doubtless have procedures in place to prevent loading cargoes into inappropriate tanks. Some regulatory authorities such as the EU and the FDA prohibit the carriage of some foodstuffs in tanks and holds with a coating that can react with or be absorbed into the cargo. Coatings makers will publicise the products that have been given approval by such bodies.
Several of the products used on the superstructure and side shell of the ship above the waterline will be chosen for their cosmetic effect as well as their durability. Often these will not be epoxy coatings but will be alkyds. Alkyds comprise a polyester modified by the addition of fatty acids and other components and alkyd coatings are typically one-component systems (no hardener added) and will be slower to fully dry. They use the oxygen in the air to complete a reaction that changes the coating from wet-flowing to hard and durable.Moisture cure urethane
Moisture cure urethane
Moisture-curing urethane or polyurethane coatings are single-component coatings in which the curing process is a chemical reaction with atmospheric water. They are an effective and economical coating and can be applied independent of the weather (temperature, humidity and dew point), dry quickly and are surface-tolerant. Because this type of coating does not have to be mixed in the same way as two-component coatings, there is no risk of error in mixing components. In addition, the coatings remain elastic and resistant to UV radiation to provide long term durability.
The drying rate of one-component coatings is dependent on the relative atmospheric humidity and the temperature. Low temperature and low atmospheric humidity may slow down the drying considerably.VOC Content
Virtually all coatings products contain substances known as volatile organic compounds (VOCs) which are released into the atmosphere during application and curing of the coating.
VOCs in the atmosphere combine with pollutants such as NOx and SOx to produce smog and low level ozone which is considered a health hazard. Because of these characteristics, VOC content in coatings has been the subject of regulation on a local scale. The US, EU, China and South Korea have been most active and as a consequence of local regulation, coatings manufacturers have been obliged to develop products that comply with them.
Low-VOC coatings, some of which are water-based, have different properties from older products and application methods may be different. Since the regulations were mostly made up to 10 years ago, most products now available can be considered as complying with them. However, in anticipation of further regulation, manufacturers continue to develop new products that can be considered as future-proof.
The level of VOCs is frequently controlled by local regulations particularly in the developed world and places such as shipyards may be subject to inspection to ensure rules are being adhered to. In most instances this will be a matter for the contractor and not the ship operator to concern themselves with. However, VOCs present both a health and fire risk and should be taken into consideration when crew are carrying out any repair and maintenance to coating systems. A prudent operator would include the risk assessment in its ISM procedures and should ensure that any other hazards associated with any particular product are identified from the maker’s material safety data sheets (MSDS) or other issued safety advice.Cargo hold and tank coatings
Cargo hold and tank coatings
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 are 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 steelwork 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 PSPC 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.Dry cargo holds
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 – 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.Chemical tanker 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 coating 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.Adhesive films
In 2016 a new anti-fouling product was developed that, although applied to the outer hull, is not a coating in the accepted sense. Micanti Antifouling film is a physical barrier against fouling and therefore movement of the vessel through the water is not needed for it to be effective. The product comes on a roll and is applied to the vessel in strips such as wallpaper is applied to walls.
It is described by its Netherlands-based developer as a self-adhesive foil consisting of backing paper, pressure sensitive modified acrylic adhesive, a 12μm polyester carrier foil and antifouling layer or cured acrylic adhesive with embedded nylon fibres. The product feels like coarse velvet but the maker says this texture prevents settlement of organisms and the total weight as-applied is comparable with a typical anti-fouling coating system. Micanti has been tested for more than 10 years in cooperation with institutions including MARIN, TNO and Delft University of Technology as well as in practice on a variety of vessel types.
Another new type product that is not yet available on the market but was announced as in development in early 2016, combines the concepts of traditional coatings and application from a roll. PPG, which makes Sigma products, is working on the eco-friendly Ship Hull film system with fouling release and fuel-saving properties (eSHaRk) project drawing on the work of a development group including PPG, MACtac, Meyer Werft/ND Coatings, VertiDrive and Hamburg Ship Model Basin HSVA.
The project, which secured EU funding through to November 2018, aimed to establish an automatic application process which enables a self-adhesive fouling-release film to be used on commercial ocean-going vessels. Although the project website has not been updated in many months and has no final report on the project, in April 2019 PPG and Avery Dennison Graphics Solutions announced an agreement to further commercialise self-adhesive fouling release foil for ship hulls. Avery Dennison will market the foil solution to the leisure sector under the brand name Mactac MacGlide. PPG will market the product as PPG SIGMAGLIDE Foil to the commercial marine sector, focusing primarily on smaller vessels that suit the foil’s manual application.
Early in the project, PPG, claimed the new film is superior to existing paint-based solutions in terms of eco friendliness, ease of application, robustness and drag reduction effects. eSHaRk is expected to have superior drag reduction properties compared with existing anti-fouling and fouling release technologies, up to 10% drag reduction as compared to the currently-available maximum 5%. The system incorporates a fine-tuned fouling release system, based on PPG’s premium 100% silicone binder technology, and a self-adhesive film specially designed by MACtac for underwater use.
As part of the eSHaRk project, a robot application technology was being developed by VertiDrive which will be used to apply the film automatically on large commercial vessels. The surface morphology of the film will be optimised to enhance drag reduction, increase fuel savings and reduce emissions to previously unattainable levels.