Coatings and Corrosion

Protecting the ship’s structure

Malcolm Latarche
Malcolm Latarche

18 December 2018

Protecting the ship’s structure

Whatever materials man has chosen to use for ship construction over the thousands of years that he has navigated the rivers and oceans of the world, none has been immune from damage and deterioration over time. Wood, which served as the prime material for most of maritime history, was eventually dropped because the need for larger vessels dictated use of a different material.

After a relatively short period when iron was the chosen alternative, steel has now become the dominant material. In the search for weight savings, aluminium has been adopted for some smaller vessel types and the future may see composites take a hold in the market. So far, no material has proved resistant to the inevitable decay caused by exposure to attack by organisms and the effect of corrosion.

Almost any form of paint will confer some delaying effect against corrosion but the diverse areas of a ship and the carrying conditions they are exposed to require specialist products for some areas. Developing coatings is a science and, since conditions experienced in the various locations can be very different, it is important that the coatings are matched to the expected conditions. The majority of coating manufacturers have a wide range of diverse products each intended for a particular purpose and care should be taken to ensure that when it is necessary to use products from different suppliers, they should be compatible.

All coatings except some very specific and niche products contain substances known as volatile organic compounds (VOCs). These are there for a variety of reasons such as reducing viscosity to aid application, controlling sag resistance, aiding flow and levelling, improving curing time and bonding with substrates and other coats.

VOCs evaporate into the atmosphere where they 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.

Layers of protection

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 top coat 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. The vast majority of coatings are applied when the ship is being constructed or during routine drydockings. Application by a ship’s crew is usually restricted to repair and maintenance work or for aesthetic reasons.

Except in a very few cases, the quality of work by the crew is unlikely to match that of coatings contractors. If insufficient attention is given to surface preparation, mixing and method of application, the result will inevitably be inferior. All leading makers supply the necessary information required for both health and safety reasons and for technical advice. For ships with internet access, most of this material can be accessed directly from the maker’s website.

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 top coat 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 of a requirement for a high standard of surface preparation.

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 (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 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.