The hidden effects of coatings

Whilst the internal paint coating of a vessel’s ballast tanks or cargo oil tank might seem the least of a shipbuilder or owner’s concerns in the current industry climate, paying close attention to this fundamental aspect of vessel construction and maintenance can reap dividends in the long run. Although international regulation can weigh heavily on the shipping community, the International Maritime Organisation (IMO) is working hard to ensure ship builders and operators receive value for their investment in this particular area.

In December 2008, the IMO’s Performance Standard for Protective Coatings (PSPC) for ballast water tanks was adopted to measure and mark the anticipated performance lifetime of a protective coating formulated for seawater ballast tanks and double side-skin spaces of bulk carriers. This standard ensures that protective coatings are evaluated to maintain performance standards for at least 15 years - subject to regular upkeep - without succumbing to the corrosive and abrasive qualities of ballast contents. From 1 January 2013, similar PSPC regulations for the coating of cargo oil tanks on newly constructed crude oil tankers above 5,000 dwt (dead weight tonnage) have also become effective.

According to new IMO statistics, the average life-span of a ship is estimated at 30 years; choosing a coating that has an expected performance lifetime of 15 years therefore ensures that a ship’s tanks and holds endure as little recoating as possible.

Integral to the durability of a coating is its ability to withstand the effects of corrosion over an extended period of time. For heavy duty marine applications, guarding against the dual effects of water and salt corrosion are major factors when considering the type of coating. In structures such as ballast water tanks, some areas are particularly prone to corrosive forces, such as the bell housing at the bottom of the pipe, which can be subject to vibration from the pump activity and the possible consequences of cavitation.

Epoxies and MCU

Epoxy coatings have been the industry standard for repelling corrosion for several decades. An epoxy is a chemical cure formed over a period of five to seven days by two separate chemical components reacting with one another.

In recent years, a rival to epoxy coating technology has emerged. Moisture Cure Urethane (MCU) coatings are single component pure urethane products, which achieve their cure from exposure to moisture to create a polyurea.

According to Morten Sørensen, director of MCU coatings, it is the single component aspect that prevents the coating from becoming brittle and cracking over time. “The dual component reaction that forms an epoxy cure in the days after application can continue to react even after the full chemical cure, however at a much reduced slower rate. This can cause the epoxy to become brittle over a few months. Then when you have expansion and contraction or excessive vibration, it can be subject to cracking and peeling.”

Carl Barnes, Business Manager, International Paint, agrees that the issues of cracking and peeling can affect some epoxy-based coatings when applied to non-conducive and complex areas such as welds and corners, which are often coated beyond the recommended DFT (dry film thickness). “Excessive DFTs can give rise to stress within the coating, which can cause cracking, detachment and subsequent corrosion.”

However, Barnes states that International Paint’s aluminium pure epoxy coating, Intershield300, has undergone extensive testing to prove that the coating can withstand such negative side effects when applied to challenging components. With 14,000 vessel applications over a 23 year period and a proven in-service performance standard of 15 years, it would seem that the testing is rigorous.

Barnes says: “We have applied thickness to 4 x standard in our internal tests and have field data with similar thickness that has not shown any cracking issues. We test and formulate all coatings for water ballast tanks to have resistance to cracking at higher DFT as we know in these hard to coat areas that DFT will be higher than standard. Intershield 300 has shown not only with in house tests but also in field on vessels that it has excellent resistance to cracking.”

Chemical component

Incorporating zinc into a coating has long been a common method for manufacturers to give their products its anti-corrosive quality. According to Sorensen, however, systems with inorganic or epoxy zinc-based coatings aren’t without problems in relation to their practicality, reliability and durability. “Epoxy zincs and inorganic zincs have a very small application DFT tolerance and will form mud cracks if over applied by 10-20 microns. In addition, they form zinc salts - sometimes within hours depending on temperature and humidity - and this causes the overcoat time limitation window to be extremely short, whilst subsequent layers are prone to peel off.”

Despite such difficulties, Sorensen admits that MCU’s typical specification is to apply its zinc-based primer, Miozinc, to the internal surfaces of structures such as ballast tanks before an overcoating. However, the problems typically associated with zinc-based coatings are not applicable to Miozinc, according to Sorensen. “The Miozinc primer will not form zinc salts and has an unlimited overcoating time period. It also has a wide dft tolerance of about 200 microns.”

In the case of International Paint, the company preferred an epoxy with an aluminium base to one with a zinc base due to the enhanced anticorrosive qualities that are offered by the lamellar shape of an aluminium-based pigment in situations of liquid immersion.

Barnes says: “For use underwater and in permanent immersed situations experience has shown that the use of lamellar pigments is proven to give better long term anticorrosive performance than using sacrificial pigments such as zinc dust. The lamellar pigment shape (of Aluminium) improves the coating barrier properties and decreases the rate of transport of water and oxygen through the coating.”

Barnes states that the advantage of using an aluminium flake pigmented coating over a glass or mica flake pigment becomes most apparent when the coating is damaged. “The aluminium pigment reacts with the hydroxide ions produced at the cathode in the corrosion cell. This reduces the pH at the coating-steel interface and decreases the rate of cathodic disbonding.”

The fact that International Paint believes aluminium to be most effective for coatings that come into contact with liquids is not to downplay the anti-corrosive benefits that a zinc coating can provide in other instances. “The use of coatings containing metallic zinc finds favour in many non-immersed situations where the sacrificial aspect of the zinc can enhance corrosion protection,” Barnes says.

Flexibility

Whilst coatings must be formulated to counter the effects of corrosion, it is important to recognise that they are susceptible to the effect of other forces and environmental factors. For example, abrasion can become a significant issue in the case of ballast tanks that take in water in shallow regions of the ocean, where silt and sand are liable to exist in the water that is pumped in. Both MCU coating and Intershield300 have been formulated to counter abrasion.

Sorensen also highlights the significance of an MCU coating’s elongation range of 22 to 40 per cent, which he says is crucial for being able to mitigate the movements and flexes that are typical of the structures it can cover. “All steel will expand and contract especially with temperature variations. Furthermore, in the case of ballasting and unballasting, there will be potentially many hundreds and possibly thousands of tons of water moving in or out in a short time and the steel will tend to flex in this operation. Also there will be impact from hull flex due to contact from encounters with docks, jetties and tug boats.”

Other considerations

A perennial issue associated with any paint coating is its compliance with health, safety and environmental legislature. The emissions that emanate from the Volatile Organic Compounds (VOCs) present in paints ensure that regulation is on hand. The problem is that such regulation is not standardised across the international community, presenting coatings suppliers with a headache.

Furthermore, the prospect of ballast water treatment systems being installed en masse across the world’s fleet leaves scope for another dimension to be added to the coating procurement process in the future. Barnes, however, does not foresee many of the current treatment procedures impacting on coatings. “With regards to their effect on ballast tank coatings, it is unlikely that ballast water treatment systems relying primarily on mechanical or physical means will have a detrimental effect.

“However those systems that primarily use active substances, particularly if not designed, operated and maintained properly, could detrimentally affect coatings resulting in increased corrosion. Ballast water treatment system suppliers have the responsibility to ensure no detrimental effect on coatings and this is part of their type approval process.”

So, whilst a lick of paint might seem to be one of the more basic and mundane aspects of readying a ship for action in an age when complex software and electronic systems are overtaking the industry, choosing a tank coating is clearly not an issue that should be brushed over without consideration.