Coatings and Corrosion

Keeping coatings in good order

Sarah Carter
Sarah Carter

18 December 2018

Keeping coatings in good order

Some may see coatings as an adornment to the structure or other parts of a ship’s equipment but in reality the various coatings on a ship are a system unto themselves and to function correctly they need regular attention and maintenance. This is essential because any damage to the coating will mean that it is the structure of the ship that is at risk, not just a small area of paint. Except for occasional in-voyage repair of coatings, application is performed either at the initial building stage or during regular drydockings. This means that the owner is reliant upon firstly classification societies and then on its own superintendents to ensure coatings are applied correctly so that later problems can be avoided. In both cases, the surveyors and superintendents will have many other tasks to undertake and constant supervision is neither feasible nor desirable from a cost perspective.

It should be understood that in almost all cases the coatings themselves are not at fault but the causes will be either inadequate preparation of the steel, the application of the coating or allowing insufficient time for the coating to cure. There will be comebacks upon contractors if coatings fail within a very short space of time but often an inherent problem may take some time to develop allowing claims to be avoided.

Not all coatings are able to be regularly monitored either because they are underwater or inside ballast or cargo tanks but for those that can be there are signs that those involved in the ship’s maintenance should be able to spot.

Orange peel effect, pinholing, cracks, wrinkles, sags and runs and many more imperfections are easily visible and are indications of incorrect application or preparation and almost always are precursors of trouble ahead even though there may be no signs of immediate corrosion. If the area is small, it should be within the ability of the crew to rectify it but if large then consideration should be given to pursuing a claim against the contractor.

Sometimes, the problems can be caused by an inappropriate choice of coating given the ambient weather conditions at the yards or the working practices within yards and dry docks. Coatings designed for application within certain temperature or humidity ranges will likely have a shorter life if the conditions prevailing during work do not coincide with the designed conditions.

Shift patterns too can have an effect on the proper application. Where a time between different coats has been specified by the maker, these should be taken into account before work begins. If it is likely that the working pattern would mean an extended time between two coats or perhaps the painters are likely to rush the job to complete it without sufficient time between different coats, then action needs to be taken to ensure that neither case will have a detrimental effect on the coating. If time is limited, then perhaps consideration should be given to opting for an alternative coating.

Almost all manufacturers will quote expected lifetimes for coatings systems but there are very good reasons why these claims need to be considered as guidelines or best case scenarios rather than absolute guarantees. No manufacturer can be held responsible for incorrect preparation, application or use in service but unfortunately these are most frequently the causes for poor coatings performance.

At the time when the concept of PSPC for ballast tanks was first mooted and included into IACS common structural rules, there was much debate over who would undertake inspections and whether there were sufficient qualified inspectors to meet demand.

There are industry standards for surface preparation and at the very least the owner should be able to expect that these will be met. Beyond that, the skill of the contractors, environmental conditions with regard to temperature and humidity and how the coatings are allowed to cure before use are all factors that will affect its performance and longevity.

Two fouling protection products that have recently been developed – Micanti and eSHaRk –are very different from conventional coatings and mark a distinct change of direction with regards to application. It remains to be seen if this type of ‘coating on a reel’ and the need to ‘wallpaper’ a ship is a technology that will gain mainstream acceptance but if it does then there will be a need for training shipyard personnel in the skills needed and also for class surveyors to learn the issues that may arise with regards to preparation and application.

Hull coatings are highly susceptible to damage when navigating in ice so the choice of an appropriate coating is very important. Hard coatings are claimed as being better suited to ice navigation than some of the less resilient products and some manufacturers produce coatings specifically developed with ice navigation in mind.

Operators of ships that regularly operate in the normal winter activity areas such as the Baltic Sea and the St Lawrence Seaway should be fully aware of the problems that ice can cause to coatings. While there has been a lot of talk about more use being made of high latitudes for commercial operations and oil and gas activity, the deteriorating geopolitical situation and falling crude oil prices are likely to see deferments in any such activity for the short term.