Coatings for Marine Applications & Offshore Platforms
For optimum results, anticorrosive coatings used in the offshore structures and marine applications need to be carefully selected and applied after suitable surface preparation.
While offering new anticorrosion coatings for marine and offshore applications, manufacturers are under consistent pressure due to stricter environmental regulations, as well as the simultaneous demand to provide long-term, cost-effective protection of assets from corrosion.
These newly developed coatings, which meet the stringent regulatory norms, can also prove to be effective and economical for marine and offshore platform applications in the long run. These coatings should be applied under the prescribed conditions, after the necessary surface preparation, and then carefully monitored for continued effectiveness. These steps are necessary for protecting the value of many critical assets of national importance.
Our national and global economy is under constant threat due to corrosion attack on critical assets such as offshore oil and gas platforms and other marine assets. The aggravation of corrosion due to saltwater and the salt present in ambient air is an important factor in determining the type of corrosion protection to be incorporated. Designers provide a combination of barrier coatings along with cathodic protection (CP) to ensure optimum corrosion protection. Some aspects of anticorrosion coatings—suitable for marine vessels and the offshore oil and gas industry—are discussed in this article.
The anticorrosion coating industry has been under immense regulatory scrutiny for the last 25 years. As the increasing levels of water and air pollution continue to threaten sensitive ecosystems all over the world, the coating industry is trying its best to minimize any emissions that could impact the environment. It has been responding positively to the enormous challenge of formulating cost-effective products for the protection of critical assets, and to comply with stringent environmental regulations. The following are some of the critical regulatory issues being faced in the industry today.
1. Hull fouling problems and the antifouling paints
Minimizing the fouling on a vessel’s hull is an important element in ensuring efficient operation and controlling the damage to marine vessels. The most keenly scrutinized aspect of marine and offshore platform coatings is the antifouling biocide coatings. When the coating is applied on the surface of a hull, tiny particles of the chemical biocide are slowly released from the coating layer, thus preventing algae and other organisms from settling on the surface of the hull. However, biocide accumulation in harbors is harmful to ecosystems, and there are restrictions in place for the use of biocides.
In response, new antifouling coating formulations with biocides have been successfully developed. These release the bare minimum quantity (only around 5% to 10% of existing formulations) of biocide required for the antifouling protection, with minimal damage to ecosystems.
2. Solvent volatility regulations
The regulators have introduced a number of rules to control the emission of volatile organic compounds (VOCs) in respective countries. The VOC Solvent Emissions Directive of the European Union categorizes the shipyard facilities that emit volatile organic compounds into three tiers based on the amounts of solvent used. Boatyards that are using less than 5 tons of solvent per year are exempted from the regulation. This limit includes the solvent present in cleaning chemicals as well as adhesives, apart from the solvent directly present in the liquid coating.
In the second tier, for boatyards that use up to 15 tons of solvents annually, the total emission of VOCs has to be less than 37% of the net total weight of wet coating applied. In the case of third-tier yards, which use more than 15 tons annually, VOC emissions have to be reduced to a level of 27% of the total coating by weight. This means that coatings containing a lower percentage of solvent are to be developed.
These regulations have helped to popularize the use of adhesives, which are practically solvent-free and water-based cleaners. A lot of high solids coatings in the primer stages have also been developed, which are applied below the top coat, thus helping to meet the regulatory norms.
3. The REACH regulations on use of chemicals
REACH stands for Registration, Evaluation, Authorization and Restriction of Chemicals. The regulations were made effective in 2007 in the EU. The chemicals listed are components of all coatings and paints. These regulations mandate a plan to review worker health-safety, as well as the environmental impact of all chemicals. (Learn more about this topic in How to Enhance Safety When Working With Volatile Organic Compounds.)
Most hazardous chemicals must be reviewed within the first three years. Substances that are proven safe will be registered for use. Voluminous lab data must be produced by the manufacturer before registering a chemical substance for use. For the hazardous substances, continued use is permitted for some time, but only if less hazardous alternative chemicals don’t exist. Carcinogenic substances are highly restricted.
Coatings manufacturers rely on raw material suppliers to register their respective chemical substances. Unacceptable substances are now by and large being replaced by other efficient products that are fully compliant and sustainable.
Causes of Marine Corrosion
A characteristic feature of the marine environment is the high salt content that is universally present in the surrounding air and seawater. Other chemicals and pollutants present in seawater, in the ports, and in the surrounding air also influence corrosion damage. The other factors affecting the chemical and the electrochemical reactions leading to corrosion include:
- Dissolved gases, particularly dissolved oxygen, present in seawater
- Carbonate scales present on the metallic surfaces being corroded
- Carbon dioxide present in seawater, affecting the pH value of the water
- Electrical conductivity of seawater and the climatic conditions
- Marine life and bacteriological activity
- Coastal pollution
- Presence of sulfate-reducing bacteria, causing corrosion of steel in anaerobic situations
1) Lifetime cost including maintenance, monitoring or replacement
2) Mechanical characteristics
3) Easy availability and logistics
4) Fabrication requirements
5) Forming joints by soldering or welding
Preparing the surface of the metal prior to coating is an important requirement. Among the factors influencing the effectiveness of performance of coatings, surface cleanliness is probably the most significant. Any scale, grime, grease and rust must be completely removed before the application of a coating begins to ensure the complete and strong adhesion of the film of the coating with the substrate. Barrier protection is lost if the adhesion is inadequate.
The cleaning techniques used include acid pickling or shot blasting. Acid pickling is used for metal that is to be coated by a galvanizing process. (Pickling is discussed in the article Using Pickling and Passivation Chemical Treatments to Prevent Corrosion.) For offshore structures and ships, blast cleaning is the prescribed cleaning method. For structural metals like steels and alloy steels, an abrasive cleaning method is also used for surface preparation.
Selecting the Appropriate Protective Coating
For surfaces remaining in fully immersed conditions, a barrier coating system could be preferable, along with cathodic protection. Paint application is always considered as the first choice for corrosion protection, compared to a metal spray coating. Here, paint is used in multiple layers to enhance corrosion protection. (Discover polyurea coatings in Flexible Coatings for Protection of Marine Structures.)
Paint consists of a pigment, a binding medium and a solvent. The pigment, which forms the core of the coating, is made up of solid particles that are kept well dispersed by the solvent and the binder. Paints can be specifically formulated to ensure the essential properties necessary for the specific application, such as water resistance and scratch resistance. In order to minimize coating defects, multiple coats of paint are applied to the substrate. Before applying the first coat, the primer is applied to the substrate to improve adhesion.
Coatings for a Marine Environment
Impressed current systems fail to protect surfaces exposed to the atmosphere and waterline zone because splashing water causes severe corrosion. The thickest and best epoxy should be applied for waterline surfaces. Cathodic protection can protect the underwater hull, which is the area most prone to corrosive deterioration and fluid turbulence. The main characteristics of some marine coatings are discussed below.
A. Chlorinated rubber coatings
Chlorinated rubber coatings have:
- Superior intercoat adhesion
- Excellent drying ability even in sub-zero conditions
- Excellent water resistance and chemical resistance
The main disadvantage is inadequate heat resistance and a slight discoloring due to sunlight.
b. Amine - Epoxy system
Amine cured epoxy coatings create a hard and adherent layer with good chemical-electrochemical resistance and corrosion resistance. Amine as a curing agent enables hard-cured epoxy coatings to be used in marine and offshore applications and other severely corrosive environments. However, there are health issues related to its application and handling.
- Effective alkali and solvent resistance
- Good water resistance and acid resistance
- Very good hardness, mechanical strength and abrasion proofing
- Exceptional long-term corrosion resistance
- Excellent barrier to moisture and chemicals
- Amine epoxies are mildly toxic
- Shorter recoat duration (life)
- The film dries slowly
- Marine applications
- Chemical-resistant applications
- Offshore oil and gas platforms
c. Polyamide epoxy systems
Polyamide epoxies have strong weather resistance, higher resilience, and flexibility. They have lower solvent resistance and acid compatibility.
- Excellent water resistance and alkali resistance
- Moderate acid resistance
- Convenient to coat and cure, as it has good adhesive capacity and layer flexibility
- Moderate weathering ability
Limitations of usage:
- More viscous than other epoxies
- Coating strength is dependent on temperature
- Vessel parts immersed in seawater
- Marine and offshore platforms
- Oil and gas tankers
d. Siloxane epoxy systems
Siloxane epoxy systems are effective in some marine applications, where in addition to corrosion protection, color stability and high gloss are also required. It is one of the fastest curing systems.
- Low heat capacity and solvent resistance
- Marine coatings such as yachts, boats and passenger ships
- Coating for architectural needs in moist environment
e. Coal-tar-epoxy system
Coal-tar-epoxies consist of a blend of coal tar and epoxy resin with polyamides and amines acting as curing agents. This unique blend produces excellent saltwater resistance combined with a unique protection from the cathodic disbondment of coating layers.
- High saltwater and freshwater resistance
- Strong film strength
- Good protection against cathodic disbondment
- Fairly cost-effective
- Not suitable for potable water; certain regulatory restrictions on usage
- Dark color makes it unacceptable in many applications
- Takes too long to recoat and repair
- Coating the offshore oil rigs and structures
- Oil and gas pipe coating
- Refineries and chemical factories
Typical Applications of Marine Coatings
Coating selection for ballast tanks should be based on seawater quality and the contamination present in the harbors.
A. Coats for ballast tanks
Epoxy-based hard coating; single coat of 300-micron dry thickness provides 5-year protection. Double coat can provide 10-year protection and triple coat can give up to a 14–15 year protection.
B. Slope tanks and oil cargo tanks
For slope tanks: Suggested coating epoxy, 300-micron thick
For product tanks: Phenolic epoxy, 300-micron thick, double coats, and zinc silicate layer 100 micron
Freshwater tanks: Epoxy coating, 200-micron thick, double coat
Main engine room-accommodation rooms: Alkyd coating, 150-micron thickness, double coat
For corrosion under insulation, on tank tops: Epoxy coating, 300-micron thick, double coat
Major hull coating systems:
1. For external hull, between loaded ballast waterline as well as submerged surface: Chlorinated rubber or epoxy-coal tar or vinyl-tar coating; 300-micron triple layer as well as antifouling coating.
2. For external hull (above water level) and superstructure: Triple coats of chlorinated rubber or epoxy or vinyl coating, 250-micron thick.
3. External hulls in ice environment: Solvent-free epoxy, applied hot, 700–1400 micron thick.
Metallic Coating for Marine Application
The following techniques are used for coating different metal layers on steel for corrosion protection:
- Galvanizing including hot dipping
- Hot spray coating
- Thermal spraying, as well as diffusion coating
New thermal spray technologies are being adopted for producing stronger abrasion-resistant coatings, (HVAF and HFPD for example). However, the frictional wear versus corrosion resistance needs to be optimized.
For marine applications, a metallic coating of steel is very useful. The protection provided by such a metallic layer depends on the coating material and the thickness. The coating materials that are used to protect steel in aggressive marine environments are nickel, zinc, cadmium and aluminum. Under immersion in saltwater, an aluminum coating provides higher sacrificial protection, which is gradually lost over the course of time. For surfaces that are not immersed, zinc provides greater protection of an anodic coating. Under the duplex system of protection, the metallic coating is further strengthened by additional polymer coating layers.
Marine Maintenance Coatings
The original corrosion protection, design and application are the basis for the longevity of offshore and marine assets. Again, the vital element in maintenance and repair is the preparation of the surface. In case of painting defects, some amount of rust can accumulate on the surface. Metallic salts such as chlorides can form on the unprotected portions of the surface over a period of time. Properly monitoring the coatings and applying repair coatings after removing rust and salts accumulated on the surface can help lengthen the life of the assets. Certain advanced primers are exclusively available for the coating maintenance market.
Marine Coatings for Offshore Platforms
The problems faced by offshore structures include:
- Higher costs of monitoring and maintenance
- Severe hostile environment
- Fouling of underwater structure due to marine life
- Down-hole corrosion
- Severe stress corrosion
- Fatigue related to corrosion
For epoxy systems such as coal-tar epoxy, polyamide epoxy and amine epoxy, coats of 300 to 400 micron thickness are generally used. For waterline and submerged zones of structures, an antifouling biocide layer is applied on offshore structures as well, after the epoxy layers are dried. For mechanical equipment working in offshore conditions, advanced coatings are being developed by harnessing nanoparticle technology.
Nano Coatings and Other Developments
Many different groups of researchers have patented different nanoparticle coatings suitable for advanced offshore and marine applications. Durable coatings needed for the use by the Navy for ships and submarines are being tried out. The nano-size particles of alumina and titania crystals, and other ceramics (almost a thousandth of normal particles) have been used. These particles in the coatings ensure a very long service life combined with exceptional adhesive properties, abrasion resistance, mechanical toughness, impact strength and fatigue protection. These nano-particle structures of titania-alumina coating are used for coating moving components of pumps and drives in naval ships. A plasma spray method is used for applying the coatings on the surfaces to be protected.
Some of the advanced coatings use carbon-based nanotechnology for higher durability. This is an epoxy resin containing carbon nanotubes that are well-dispersed in the liquid resin. The nanotubes help reduce resistance to flow, and work to provide a tough core for the coating system.
Most surfaces on ships and offshore facilities are exposed to severe work conditions such as extreme temperatures, humidity, vibration and severe winds. Surfaces of ballast tanks continuously face saltwater, abrasion of other surfaces, impulse shocks and fluctuating temperatures, all of which can affect adhesion of the coating. (These conditions also present challenges to inspectors, described in detail in the article Marine Structures Create Unique Challenges for Third-Party Inspectors.) Nano-particle additions in coating structure help improve adhesion and other mechanical properties.
New epoxy coatings, which are suitable for crude oil tanks at 80°C (176°F), are being developed. All season epoxy primers suitable for extreme high and low temperatures are also in the market. On the antifouling front, biocide-free antifouling slime release formulations are under trial for meeting the regulator requirements for eco-friendly coatings. Low-friction antifouling products are also being developed side by side. New products also provide hull protection during the voyage, as well as layups ensuring fuel-saving through higher speeds. Certain fouling release agents are silicone-based for ensuring non-sticky surfaces. New eco-friendly coatings developed for the smooth surface of rudders enable good hydrodynamic conditions that minimize cavitation damage.
Wind Turbine System Coatings (Offshore)
The corrosion protection of wind turbine towers is planned according to certain zones. The surfaces of the moving parts in the turbine must have coatings able to withstand cavitation failure and pitting corrosion. A thermal metal spray coating combined with an epoxy barrier coating is used as a multi-function coating for this application. A zinc-based duplex epoxy coating is used for the atmospheric zone as well as splash zones; however, a thicker coating is needed for the splash zone. The immersion zone is basically protected by impressed current cathodic protection (ICCP), and the organic coating can reduce the impressed current demand.
Wind turbine blades can suffer due to erosion caused by moisture droplets, affecting aerodynamic efficiency at the high speed range. Outer surfaces will face the brunt of maximum erosion, causing vibration and balancing problems too. Erosion resistance can be increased by using elastomeric tapes on the leading edges of blades. Different coatings of nano composite layers with elastomers and ceramics are being tested to improve blade life and aerodynamic performance. Self-healing coatings are also being formulated. New techniques such as pulse plasma nitriding are used to increase the surface hardness of steel, at times at the cost of lower corrosion protection.
For optimum results, anticorrosive coatings used for offshore structures and marine applications need to be carefully selected and applied after suitable surface preparation. New eco-friendly coatings are being constantly developed to improve the performance and the economic life of the assets. Nano coatings are being increasingly deployed for critical applications.
Novel coatings with tailor-made properties need to be developed to improve the aerodynamic and wear properties of the turbine blades. Nanotechnology-based coatings show a high potential to reach these goals.
Different wear-resistant coatings are commercially available, but further studies are still needed in order to ensure their reliability in harsh offshore conditions.