IMAGE: A C38 oil/chemical tanker. Photo courtesy of Stolt Tankers.
Because seawater contains a significant concentration of dissolved salts and is very corrosive to steel, infrastructure and assets in or near marine environments are particularly susceptible to corrosion. Efforts to mitigate corrosion in marine environments continue as industries develop and implement solutions, including the use of high-quality coating systems, to prevent asset degradation.
NACE International member Johnny Eliasson (JE), structural and corrosion engineer with Chevron Shipping Co. (San Ramon, California, USA), and Massimo Rubesa (MR), coating and materials specialist with Stolt Tankers (Rotterdam, The Netherlands), shared their insights on the corrosion challenges that can affect cargo ships, particularly tankers, and the solutions.
What types of corrosion issues could be encountered on tankers?
JE/MR: The most frequent forms of corrosion we find on chemical tankers are uniform corrosion, pitting corrosion, crevice corrosion, galvanic corrosion, and microbiologically influenced corrosion. We have every type of corrosion on board a ship that you would have in a city. Anything you can imagine that would corrode in the city can corrode on board a ship—just much faster and much worse.
Are some tankers more susceptible to corrosion than others because of service environments or cargoes?
JE/MR: Yes, many factors can cause corrosion on tankers. There are about 1,000 types of cargoes that are common for tankers and some are much more aggressive than others. Chemical tankers, for example, carry some very corrosive cargoes, such as sulfuric acid (H2SO4), hydrochloric acid (HCl), and nitric acid (HNO3), while other cargoes, such as crude oil, are not as corrosive. Additionally, some cargoes are heated, which also can contribute to many types of corrosion.
Are there areas of the tankers where corrosion could be more severe than in other areas on the ship?
JE/MR: Because they have materials in them that cause corrosion, the cargo and ballast tanks are particularly prone to corrosion. As mentioned previously, the materials being transported in the cargo tanks can cause corrosion. Ballast tanks, which are critical to the life of the vessel, are constantly emptying and refilling with seawater, and this will increase the rate of corrosion. Changes in temperature will affect the corrosion rate as well. The sewage tanks on ships also have a propensity toward corrosion because of the aggressive environment, which includes bacteria. The microbes create acids that attack the walls of the tanks.
The main deck, although not as susceptible to corrosion as the ballast and cargo tanks, is also subjected to corrosion. The general environment is very corrosive because of the combination of salty air and seawater. Additionally, the entire main deck—plus all the piping and supports installed on it—is covered nightly with salt-laden dew that contains corrosion-causing chloride ions. Then add the daytime heat that increases the temperature, and the capability to corrode is aggravated.
What is your strategy to address corrosion?
JE/MR: The overarching strategy is to keep the expected service life of the vessel in mind. Usually a vessel is designed to meet a 25-year life at the lowest cost. At the design stage, an analysis determines whether it is most profitable to initially invest in higher-cost materials that will reduce maintenance costs over time, or to use more economical materials up front and plan for higher maintenance costs as the vessel is maintained throughout its service life. This strategy addresses the life of the vessel from the cradle to the grave. Once initial construction costs are determined, expected consequences and their costs are plotted over time. Then the most economical approach for a successful 25-year life is selected. This approach requires sound economic policy and an appropriate projection of probable consequences over time.
Materials selection is one strategy to address corrosion. Corrosion control can be designed into the ship itself. For example, the deck on a tanker can be fitted with ~3 km (2 mi) of piping. Typically, this piping is constructed of stainless steel (SS); however, SS is cathodic to the carbon steel (CS) used on the supports and structure of the ship, which means the CS acts as an anode for the SS piping, and it will corrode preferentially to protect the SS piping. Ideally, the piping material should be designed so the current density from the cathodes (SS) to the anodes (CS) is reduced, which will the prevent the deck of the ship from corroding. Coatings are very good resistors, and on Stolt Tankers ships, the SS is coated to reduce the current density. The CS supports and structure are also coated to protect them from the corrosive environment.
Using high-quality coating systems is another strategy for battling corrosion on the ship. The main deck is protected with a high-grade, combined composite coating system that starts with a zinc primer, followed by an epoxy coating with aluminum pigment as the second coat, and then a topcoat of polyurethane (PUR) coating. Zinc primer is used for adhesion purposes, enabling the coating system to adhere to the metal surface. The epoxy acts as a barrier to protect the metal surface, and reduces the amount of water, chlorides, and other contaminants that can access the surface and cause corrosion problems. The PUR topcoat acts as a sunscreen and protects the epoxy from ultraviolet light. Otherwise, the epoxy would chalk and deteriorate. The PUR is tinted and provides an aesthetically appealing finish to the deck as well.
The epoxy coating with aluminum pigmentation is also used to protect the ballast tanks. Although the International Maritime Organization (IMO) Performance Standard for Protective Coatings has a specific objective of achieving a target service life of 15 years for the seawater ballast tank coatings, Stolt Tankers aims to go beyond the standard and meet a 25-year service life of the ballast tanks. To do this, Stolt Tankers protects the CS ballast tanks by applying two coats of this epoxy coating system.
Since most of the cargo tanks are constructed of SS, they are typically uncoated. Some cargo tanks, though, are built with CS. In this case, Stolt Tankers lines the CS tanks with either a three-coat epoxy phenolic system comprised of a primer, undercoat, and finish coat, with each coat applied with a dry film thickness (DFT) of 100 µm (4 mils); a three-coat epoxy isocyanate system with a DFT of 90 µm (3.5 mils) for each coat; a zinc silicate coating with a DFT of 80 µm (3 mils); or a two-coat cyclosilicon epoxy system with a DFT 150 µm (6 mils) for each coat.
Are there particular industry standards you follow for corrosion mitigation? How do these standards help with corrosion mitigation?
JE/MR: The industry standards available for corrosion provide guidelines for anticorrosion design during a vessel’s planning and construction stages, as well as protocols to follow for maintaining corrosion protection systems during the life of the ship. There are a number of standards used. For example, maintenance standards for coatings cover areas such as surface preparation, coating application, coating inspection, and how to determine coating deterioration. The standards clarify what is required and make it possible for all parties involved with ship construction and maintenance to have a comparable understanding of the requirements. The most common standards used are the four parts of ISO 8501, “Preparation of Steel Substrates Before Application of Paints and Related Products—Visual Assessment of Surface Cleanliness,” and multiple parts of ISO 4628, “Paints and Varnishes—Evaluation of Degradation of Coatings—Designation of Quantity and Size of Defects, and of Intensity of Uniform Changes in Appearance.” Without standards, we wouldn’t be able to build or maintain a ship.
What are your expectations for a coating system in terms of application, performance, service life, and return on investment?
JE/MR: The expected or desired coating service life will vary depending on the area of the ship where the coating is installed. We follow maintenance regulations and guidelines that define the requirements for the condition of the coating. And, depending on the area of the ship, the regulations and guidelines that apply to the coating’s condition in terms of what is required to meet a specified condition can be different. In some areas, more work may need to be done to maintain the specified coating condition than in other areas.
Although there are many facets of ship design and maintenance that need to be considered when battling corrosion while at sea, there are many tools and technologies to assist the maritime industry in economically achieving the desired service life of its vessels. The Stolt Tankers strategy is to aggressively maintain its ships’ coatings with a philosophy of “see rust equals fix rust,” and not wait until coating degradation becomes an issue.