Assets in the marine industry face the risk of severe degradation due to the uncertain and harsh operational environments they are constantly exposed to. Surface preparation and coating application processes must be executed by trained and motivated technicians under expert guidance and controlled climate conditions to mitigate fouling and corrosion. In this article we'll discuss nine essential topics to keep in mind when applying a marine coating. Although targeted at vessels, most of the topics apply to stationary assets such as offshore oil platforms and similar infrastructure.
1. Appreciate the Key Challenges Faced by Marine Coatings
The marine coating application process is very challenging because of the aggressive nature of the marine environment. Low temperatures, extreme temperature variations, high relative humidity and harmful airborne chlorides are some of the factors that add to the environmental challenges. Due to the harsh environment a marine coating can only be applied after thorough surface preparation. These requirements are more stringent compared with non-marine applications.
Corrosion risk varies depending upon the location of the surfaces to be protected, such as submerged surfaces, inner hulls, tanks, outer hulls, underwater pipes and interiors. Regulatory requirements also add to the issues to be addressed by the marine coating industry.
Demands to minimize and optimize the time spent in dry dock for essential repair and maintenance activities forces coating contractors to improve time management, the efficiency of any surface preparation processes and the coating application process itself. Contractors need to train and motivate technicians to put in their best effort under expert guidance.
As the vessel's cruising speed increases, the antifouling coatings must become more effective at minimizing drag.
Restrictions on sand blasting for surface cleaning is also a limitation to cope with.
The coating's longevity must improve as intervals between dry dock maintenance coatings increases.
This ever-growing list of challenges is being met by the coating industry through innovative formulations and improved surface preparation and application techniques. Hence thorough planning is essential for a successful surface preparation and coating application.
2. Upfront Planning for Climate Control Systems
Ideally, surface preparation, inspection and coating application can be performed under controlled temperature and humidity conditions. (See Applying Protective Coatings to Ships in Cold Winter Conditions for less than ideal environments.) Moisture condensation on the coated surfaces must be prevented to avoid splitting, warping, thermal contraction or expansion, poor adhesion or other forms of premature coating failure. A dehumidifying facility can ensure that dry air is provided throughout the process. Climate control systems must be able to adjust and fine tune the temperature and the dew point to minimize condensation on the surfaces to be treated.
Time estimates for the entire procedure should include:
- Surface preparation time
- Surface checking time
- Surface treatment time specific to some coating processes
- Primer application time
- Coat mixing time
- Tie coat application time
- Top coat application time
- Curing time
3. Abrasive Cleaning Techniques Matter
Abrasive blast cleaning is the most popular surface preparation method in marine industry. In the case of new construction, steel plates are shot blasted through automated processes. In the case of ships in service, an open blast cleaning method using washed sand or mineral grit is used. Fine size and medium size grit can be mixed in equal proportion to achieve good results in open blasting.
The prepared surface is then checked and compared with the appearance of a standard finished surface. (Note: The Swedish Standards Institute has published reference photos of different standard grades of grit-blasted surfaces). The required surface grade is generally determined by the coating system selected for the project. Surfaces to be immersed in seawater require more stringent surface preparation.
Because grit blasting can lead to unacceptable level of contamination in confined spaces, other methods such as vacuum blasting are used in confined spaces. These slower methods use hand-operated equipment.
4. Surface Preparation in Dry Dock Conditions
Surface cleaning in dry dock conditions is a complicated task that includes removing fouling, rust and blisters.
Common surface cleaning includes the complete removal of grease and oil, sea salts, shell fouling and seaweed fouling.
Grease and oil is best removed by a brush application of a water-soluble oil remover fluid and then rinsing the resulting oil-grease contaminant solution with a water spray.
Seaweed fouling and salts are removed by a freshwater high pressure water sprayer. Hand-scrubbing is often required, particularly for removing dry weeds. The technician then uses his palms to check for any slime remaining on the surface.
Removing shell fouling organisms from a vessel's surface is much more complicated than removing seaweed fouling. Some organisms can be dislodged by high pressure water jet blasting while others will need to be scraped manually.
Loose or damaged coatings are removed by high pressure water washing.
Rust and blisters due to corrosion and other sticky contaminants can be removed by using hand-operated vacuum blasting equipment or by open grit blasting.
5. Special Precautions When Applying a Primer
Complexity arises due to the diversity of surfaces in different areas of a ship and the type of cargo being carried. Compatibility with the cargo and the other coating products being used must be considered.
Most of the coating application occurs during either the vessel's construction or while in dry dock. The vessel's maintenance crew may sometimes have to repair a coating when it becomes damaged or to improve the aesthetic appeal.
If the coating system consists of multiple layers, such as a primer or a base coating, a tie coating (middle layer) and a top coating, then proper bonding and compatibility between layers as well as with the substrate (in the case of the primer) must be ensured. Newly built ships sometimes have a shop primer applied to the steel surface during fabrication, which must be thoroughly removed and the surface prepared afresh for a primer application.
Primers made of zinc silicate are commonly selected for both new ship building projects and maintenance coating applications. (Related reading: Surface Preparation for Inorganic Zinc Silicate Coatings.) However, the zinc epoxies are more suitable as a primer for dry dock maintenance coatings due to their surface tolerance. These zinc-based primers have exceptional corrosion prevention properties.
6. Heed the Precautions When Coating Above the Waterline
For surfaces above the waterline (e.g., ballast tanks), epoxies are more popular because of their curing property. The formulator selects the ratio of hardener to be mixed with the resin based on the intended film thickness, ambient temperature and humidity. These factors also affect the appearance of the top coating. If the epoxies are applied without a hardener then the curing will take too long and the surface could become damaged in the meantime.
Polyurea, urethane and polyurethane can be also used as marine coatings. Polyurethane is a single component coating that does not require a hardener for curing; it is cured by atmospheric moisture. Polyurea requires a perfectly prepared surface and accurate formulation. Though expensive, it provides excellent long-term protection against impact, skidding, chemical attack and abrasion, thus bringing down the lifecycle cost. (Read The History and Industry Adoption of Polyurea Coating Systems for more about this type of coating.)
Keeping the aesthetics in mind, outer surfaces above the waterline are often coated with alkyds. These are cured slowly by the oxygen present in the air as hardeners are not used.
In case of chemical cargo ships, the coating chosen for the cargo tanks must be compatible with the chemical being transported.
7. Choose the Correct Coating Application Technique
The best coating technique for the job depends upon the climatic conditions and the surface type.
In dry dock, the hull surface is at the temperature of the water while surfaces above the waterline are near the ambient temperature. Water in the ballast tank can be removed to avoid condensation on surfaces above the waterline.
If the surface temperature is below 3°C (37°F) some types of coatings fail to cure at all. Epoxies are particularly affected by low temperatures. Rain and snow can also affect the surface. Suitably formulated vinyl coatings may cure near 3°C. (Read The Use of Vinyl Coatings for Tools and Small Metal Parts for more about vinyl coatings.)
Chlorinated rubber coatings are also often chosen if the coating must be applied at low temperatures. Instead of spray coating, a brush application or roller application can be used in these low temperature situations because these techniques disturb any moisture that may have condensed on the surface to ensure better adhesion.
On the other hand, if the surface temperatures are above 30°C (86°F) then the spray could dry before it even reaches the surface. In such cases the coating will fail to adhere to the surface. A thinner can be added within limits, and the spray nozzle should be held as close to surface as possible and oriented at a right angle to the surface.
8. Take the Necessary Precautions When Coating the Hull Surfaces
Maintaining a smooth hull surface is crucial for maximizing the operational speed and fuel economy of the vessel. Hull resistance, which impedes the speed of the vessel, is a result of shell fouling, seaweed fouling and corrosion coupled with mechanical damage. Biological fouling alone can result in a loss of speed of around 15%, which increases the fuel cost substantially.
When selecting the antifouling coatings we must meet any regulatory requirements as well. Copper-based antifouling coatings are now widely used after the tributyltin (TBT) ban. Two layers of an anti-corrosion coating material comprised of a polyurethane and coal tar combination or an epoxy and coal tar combination are applied below the antifouling layers. Underwater hull and top areas require a greater coating thickness and greater care in primer application and surface preparation because the risk of delamination is higher. Coating thickness for underwater surfaces could be as high as 250 microns to 400 microns.
9. Special Precautions for Cargo Tanks and Ballast Tanks
The interior surfaces of cargo tanks should be coated with anticorrosive substances that can also resist the:
- Cargo being transported
- Cleaners used to periodically clean the tanks
- Ballast water
For food grade cargo, coating systems must be approved by food regulatory authorities. The compatibility of the coating system with the tank cleaning methods and cleaners must be considered.
Ballast tanks have very large surface areas. Their condition determines the resale value and expected service life of the vessel because they are corrosion-prone and costly to maintain. Coatings should be able to resist highly polluted seawater, be free from porosity, and free from disbonding due to the cathodic protection (CP) that is now commonly provided to vessels.
While coating ballast tank surfaces, if possible the shop primer should be removed by blast cleaning. Based on the life expectancy of the coating, the thickness could be a single coat of 200 microns, two coats of 200 microns each or three coats of 130 microns each. Light color modified epoxies are generally preferred for ballast tanks. Thicker coatings may have a higher longevity.
The surface preparation and application of marine coatings are very critical processes because of climate conditions, the risk of biological growth (fouling) and severe corrosive conditions due to polluted seawater. However, when the challenges are met with due diligence, the coating can succeed in preserving marine assets from damage.