Engineering Ships for Better Coating Performance
When examining why marine coatings fail, the industry commonly assumes the causes lie in substandard surface preparation or paint application mistakes. However, the structural design of ships can play a significant role in coating failures, but the industry lacks meaningful guides and standards.
When asked why a coating has failed to deliver some expected performance, most people immediately respond that there was a failure in the surface preparation or the application process that caused an application defect. (These topics are covered in the articles Substrate Surface Preparation for Corrosion Prevention and Increasing Coating Performance through Application Process Control.)
Yet the processes we use for surface preparation, such as abrasive blasting / mechanical treatment, are well understood and their capabilities well defined. The same can be said for the airless spray process, which dates back to the 1960s.
Thus the key processes we use are mature technologies whose capabilities are well understood. Surely they cannot be the root cause of coating failures?
Perhaps then, it is not the process but the skill of the worker that has declined. Perhaps the fact that there are fewer people who are willing to take on these dirty, poorly paid jobs and make a lifelong career out of them. Better to use them as a stepping stone to other skilled jobs. But hasn’t this always been true? Is turnover of personnel such an important factor in coating failures?
In discussions with a major shipbuilding yard in the Far East, I was advised that they could hold onto a painter for about 18–24 months before they move on. That implies six months' learning and six months' good work before they start looking at alternative options.
Some Alternative Root Causes of Coating Failures
In my experience, these are honest workers who set out to do their work to the best of their ability. I think the coating failure problem lies elsewhere, in two key issues:
- Structure design
- Access afforded to carry out the painting work
Designers very rarely think about the impact of the complex geometry on the coating process. For example, in storage tanks it would be useful to have fewer but larger access holes to enable workers easier access for themselves and their equipment, such as hoses, ventilation, etc.
We know that coatings are far more likely to fail on edges than on flat plate, but very little consideration is given at the design stage to minimize edges, or at least use radius-edge stiffening wherever possible.
Design also involves material selection. I remain surprised at how often materials are used in such a way as to create a galvanic cell. (For more on this topic, see 5 Ways to Avoid Galvanic Corrosion.)
The key to achieving coatings performance is to have coating-friendly design. Research by Broderick carried out at the University of Newcastle upon Tyne for the first time looked at how design changes can improve the performance of coatings by assessing coating performance against factors that measure the complexity of a space to be coated and taking into account factors such as access.
The research (supported by ABS, Jotun Paints A/S and IHC Merwede; and managed by Safinah Ltd.) has shown that better design can considerably improve the quality of surface preparation and coating performance, while at the same time reducing the overall costs of the project.
Cases in Point: Ship Hull & Ballast Tank
A number of case studies come to mind. The first has to do with access to a relatively simple structure: the outside hull of a ship. This was a large flat area, one where it should have been relatively easy to control application quality. Yet the dry film thickness (DFT) applied gave the following results:
- Specified DFT: 610 µm
- Average DFT: 990 µm
- Standard deviation: 170 µm
- Process capability to 3 s: 480–1500 µm
Thus, even on a relatively simple structure, the ability to apply paint to a narrow range is limited. However, this should not come as a surprise. In this case, the applicator was faced with poor access and poor prevailing wind conditions. Given those criteria, the applicator probably performed to his best capability.
As surface design becomes more complex, the ability to meet the specification drops off. Consider the equivalent data for a ballast tank:
- Specified DFT: 320 µm
- Average DFT: 602 µm
- Standard deviation: 162 µm
- Process capability to 3 s: 116–1088 µm
This results in increased time to apply the maintenance paint, increased paint consumption, and hence longer drying/curing times and overcoating intervals. All this can add considerable cost to a project. Of course, as the coating gets thicker, the ability to repair it is reduced and the performance of the coating may be compromised in one way or another (if not in the short term, then in the longer term).
The industry lacks meaningful guides and standards for designers to follow. Those that do exist, such as ISO 12944 part 3, are rarely specified or used. The Broderick research showed that some simple guidelines could be produced to help designers engineer more coating-friendly ship designs.