Protecting ballast tanks against corrosion is particularly important as they represent integral parts of the structure of the ship.

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Focus on ballast tank coatings

Ballast tanks are an essential element of ships and yet are often overlooked until they are found to be causing a problem. Jotun Maritime Insider takes a closer look at the issues and how the use of suitable solvent-free coatings can bring environmental and cost benefits for yards and ship owners.

It probably seems to most ship owners that the main thrust of International maritime Organisation (IMO) regulation over the last 25 years has been environmental protection rather than safety. Aside from rules around emissions, there has been the ballast water treatment convention of 2004 and prior to that the anti-fouling systems convention of 2001. Just on the horizon there is a new challenge looming in the way of biofouling management. These changes have meant either large additional expenses for ship owners or changes to operating practices and, in some cases, both at the same time. 

However, safety and other matters have not completely been overlooked by regulators even if sometimes the result is less obvious and immediate. At the turn of the century, a number of ships – not always of great age – were found to be suffering from significant degrees of corrosion, leading to structural damage and worse. In 2001, the case of the product tanker Castor , which the classification society ABS said was suffering from ‘super rust’, was a highly prominent case but there were many more.

Corrosion in ballast tanks

Even before Castor , the IMO had become concerned about the corrosion in ship structures and had begun to address the issue. One area of ships which was of particular concern was the ballast tanks and void spaces. Ballast tanks are an essential element of ships and yet were – and still are – overlooked for most of the time until they are found to be causing a problem. Unless discovered in a routine drydocking, corrosion in the ballast tanks was perhaps first noticeable if there were leaks of ballast water into the cargo holds or if maintaining an even trim was difficult because ballast water was leaking from one tank into another or to a void space. Ballast and void spaces also account for a high percentage of the structural steel used in a vessel, which is why corrosion is of such concern. 

The environment in a typical ballast tank is extremely aggressive. While many modern ballast treatment systems rely on quite fine filtration to prevent organisms entering the ballast tanks, this was not always the case, and often the only filtration was aimed at preventing large pieces of debris from entering the system. Coarse sand and grit and other sediment quite often built up to significant levels in ballast tanks and the constant movement of the vessel agitated the sediment, which had a severe abrasive effect on the tank structures. Coatings used in the tanks could be short-lived, and the abrasion reduced steel thickness to critical levels quite quickly. 

Also, the cyclic environment in the ballast tanks where temperature, condensation, and immersion to seawater routinely change due to ballast operations, results in regular expansion and contraction as well as adsorption and desorption of water in the ballast tank coatings. This adds to the high stress build up in the coatings where hairline cracks can occur, resulting in corrosion.

Guidelines for corrosion prevention systems

At the time, the coating of ballast tanks was not universal and when it was done, the effectiveness was not subject to any rules or standards. In 1995, the IMO adopted Resolution A.798(19) which contained guidelines for corrosion prevention systems for seawater ballast tanks in advance of changes to SOLAS due to enter  into force in 1998 requiring corrosion prevention systems to be fitted in dedicated seawater ballast tanks of new bulk carriers and oil tankers.

It is worth noting that in 2002, the TSCF ( an industry body comprised of class societies, tanker operators and major oil companies) had formulated its own guidelines for ballast tank coatings and surface preparation intended for minimum coatings life of 10, 15 and 25 years.

Performance standard for protective coatings

By 2006, the IMO guidelines had evolved into a performance standard that would be compulsory after MSC 82 adopted amendments to SOLAS requiring all new ships of 500gt and above ordered after 1 July 2008 or delivered after 1 July 2012  to have protective coatings applied to dedicated seawater ballast tanks. The performance standard for protective coatings (PSPC) rules are contained in IMO Resolution MSC.215(82).

With a standard to be met, it was for coatings manufacturers, class societies and shipyards between them to establish methods by which the coating of ballast tanks would be achieved. There was much debate at the time over where the responsibility lay and also concern that the PSPC rules required the coating system to be effective for 15 years. 

While some specific products have been developed by coatings manufacturers, it has been the application methods that have been the main issue. Coatings have until recently needed to be two coat systems with a darker and lighter coat so as to ensure that difficult areas such as joints have been properly covered to meet the requirements of the PSPC. Most of the coatings used are pure epoxy products that may be low in VOCs but otherwise formulated to meet the PSPC rather than specifically formulated for environmental purposes.


Difference between ballast water tanks and freshwater tanks  

In contrast with ballast water tanks (BWT), the much smaller freshwater tanks (FWT) that are also essential for any ship have no anti-corrosion standard like PSPC for the coating used and are of much less concern. Ismail Tan, Jotun’s global marketing manager for newbuilds explains that there is a need to understand the difference between BWT compared with FWT.  

“When it comes to the safety or the load line of the vessel itself, the FWT do not contribute to the stability of the vessel, while the BWT need to perform, and have no corrosion to ensure the safety of the vessel. So there’s different risk levels. Another important point is when it comes to testing. It’s a requirement to carry out performance testing for corrosion in WBT, but there’s no requirement in FWT. 

“Of course, you have to do internal testing to check corrosion levels but there’s no performance standard when it comes to corrosion for FWT. The only thing you have for FWT is potable water testing and this is to make sure that the FWT coatings do not leach any materials that may contaminate the potable water. BS 6920-1 and ANSI/AWWA are commonly used material testing schemes. Also, FWT are not located near any heating elements. BWT are and this can lead to stress in the coatings due to the constant expansion and contractions caused by the temperature variations,” added Tan.  

Need for suitable coatings for the demanding BWT use  

Arguably, the most important factor for a FWT coating is that it should ensure zero toxicity which is why coatings manufacturers have traditionally used solvent-free coatings that do not contaminate potable water. Most manufacturers now include a solvent-free product that is specified for FWT use but which has not been formulated for the more demanding BWT use. Some may contain benzyl alcohol which will leach out slowly, and possibly cause the coating itself to deteriorate as a result. 

Solvent-free coatings are relatively new as ballast tank coatings although manufacturers such as Jotun have been actively developing them for several years because of their environmental and cost benefits.

“Solvent-free coatings could be particularly relevant as some types do have better corrosion protection characteristics than others and that these could be extremely useful in areas beyond current usage including in ballast water tanks” - Ismail Tan, global marketing manager newbuilds, Jotun Marine Coatings.

“There are existing solvent-free coatings in the market for ballast tanks but some of these have been found to have limitations, and they’re also not suitable from an application point of view due to high viscosity. With even more stringent regulations now being formulated around the globe, there’s a huge amount of work being undertaken to ensure that suitable products are available to meet the new requirements,” says Tan.  

Drive for more sustainable shipping and environmental protection   

As part of its drive for more sustainable shipping and environmental protection, Jotun has developed a single coat PSPC approved product which Tan feels could be particularly relevant because it will help yards meet the new environmental regulations and improve productivity. It also has better corrosion protection characteristics, and these could be extremely useful in areas beyond current usage including BWT.  

“In the never-ending battle against corrosion, ships need to be protected inside as well as out. Protecting ballast tanks against corrosion is particularly important as they represent integral parts of the structure of the ship. While a typical two coat PSPC approved product would need to go through the standard testing regime, single coat PSPC products like the one from Jotun are subject to tougher, more stringent testing regime for non-typical coating as stipulated by the PSPC guidelines,” explains Tan.   

According to Tan, between 60-80% of coatings used in marine newbuildings are made up of epoxy primers. “The majority of this volume is typically applied in critical areas such as ballast water tanks. These epoxy primers are also used in almost all areas in a vessel, resulting in a high volume of use so a solution addressing such a large volume product can solve environmental related issues on a massive scale,” believes Tan.   

Further details of the product will be released later this year.  
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Volatile Organic Compounds (VOCs) have been regulated across many industries in the US and Europe for over ten years. Even more stringent regulation is now being formulated around the globe and in particular in South Korea and China with the EU expected to follow suit. Some European countries, notably Germany, have already implemented strict VOC requirements. This means a huge amount of work is being undertaken to ensure that a new generation of suitable products are made available to meet the new requirements.  
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Solvent-free coatings have been in existence for several decades, and in the maritime industry these have been targeted at freshwater, potable tanks, ensuring no contamination of drinking water which can happen with solvent-borne coatings. But ballast tanks are subjected to temperature variation and wear and tear from regular ballast operations, and so the requirements are much more demanding. 

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