Corrosion on the High Seas: How Shipowners Combat Corrosion With Protective Coating

Seawater is corrosive to steel and contains large amounts of dissolved salts. This makes infrastructure and assets near seawater particularly vulnerable to corrosion. Industries continue to develop and implement solutions that prevent asset degradation in an effort to reduce corrosion in marine environments.

The shipping industry faces constant corrosion problems from seawater and marine environments. The U.S. Maritime Administration reports that 41,674 oceangoing merchant ships (1,000-plus tons) 1 had been registered with an International Maritime Organization number on January 1, 2015. (The most recent report). The worldwide fleet includes container, dry bulk and passenger vessels as well as liquefied petroleum gases (LNG), liquefied natural gas, (LPG) and tanker vessels. All of these vessels are susceptible to corrosion-related degradation of cargo cargo tanks, ballast tanks and decks. This can result in higher maintenance costs, decreased service life and possible failures.

NACE International member Johnny Eliasson, structural and corrosion engineer at Chevron Shipping Co., San Ramon, California, USA) and Massimo Rubesa, coating and materials specialist at Stolt Tankers, Rotterdam, The Netherlands), shared their thoughts on the corrosion problems that can affect cargo ships and tankers and offered solutions.

What kinds of corrosion problems could be found on tankers?

The most common forms of corrosion found on chemical tankers include uniform corrosion, pitting, galvanic corrosion, and microbiologically influenced corrosion. Every type of corrosion can be found on a ship as it would in a city. You can think of anything that could corrode in a city, but it can also corrode onboard a ship. It's just as fast and worse.

Are certain tankers more susceptible than others to corrosion due to their cargoes or service environments?

Yes, some tankers are more vulnerable due to cargoes or service environments. There are many factors that can cause corrosion to tankers. There are approximately 1,000 different cargoes that tankers can carry, and some are more aggressive than others. For example, chemical tankers can carry very corrosive cargoes like sulfuric acid (H).2SO4), hydrochloric (HCl), nitric (HNO)3Other cargoes such as crude oil are less corrosive. Some cargoes can also be heated, which can lead to various types of corrosion.

Complex piping can be found on the main decks of chemical tankers. Stolt Tankers.

Is there any area of the tankers that could be more corrosive than other areas?

Cargo and ballast tanks are especially susceptible to corrosion because they contain materials that can cause corrosion. The materials that are transported in cargo tanks can lead to corrosion, as we have already mentioned. The vessel's life expectancy is dependent on the integrity of its ballast tanks. They are constantly being filled with seawater and the rate of corrosion will rise. Temperature changes can also affect the rate of corrosion. Ships' sewage tanks are also susceptible to corrosion due to the hostile environment that includes bacteria. The acid produced by microbes can cause damage to the tanks' walls.

Although the main deck is not as vulnerable to corrosion as cargo tanks and ballast, it is still susceptible to corrosion. Because of the salty air and seawater, the environment is extremely corrosive. The entire main deck, including all piping and supports, is covered with salt-laden water that contains corrosion-causing chlorine ions. Add to that the daytime heat, which increases the temperature, and you can corrode.

What's your strategy for addressing corrosion?

It is important to consider the expected life expectancy of the vessel. A vessel is usually designed to last 25 years at the lowest possible cost. At the design stage, it is determined whether it is more profitable to invest in high-cost materials at the beginning to lower maintenance costs. Or to use less expensive materials later on and plan for higher maintenance fees as the vessel is used. This strategy considers the entire life cycle of the vessel. After determining the initial costs of construction, it is possible to plot over time the expected consequences and costs. Next, the most cost-effective approach to a 25-year long life is chosen. This requires sound economic policy as well as a projection of the likely consequences over time.

One strategy to combat corrosion is materials selection. You can design corrosion control into your ship. A tanker's deck can be equipped with approximately 3 km (2 mi) worth of piping. Typically, this piping will be made of stainless steel (SS). However, SS is anodic to carbon steel (CS), which means that CS acts as an ode for the SS piping and will corrode preferentially in order to protect it. The ideal piping material should reduce the current density between the cathodes and the anodes (CS), which will prevent the deck from corroding. Stolt Tankers ships have SS coated to lower the current density. Coatings are excellent resistors. To protect the CS structure and supports from the corrosive environment, they are also coated.

Another strategy to combat corrosion is to use high-quality coatings. Protecting the main deck with a combination composite coating system is a good choice. It starts with zinc primer and continues with an epoxy coating with aluminum pigment, then a second coat of polyurethane (PUR). Zinc primer is applied for adhesion, which allows the coating system to adhere on the metal surface. The epoxy acts as an adhesive to protect the metal surface. It also reduces water, chlorides and other contaminants that could reach the surface and cause corrosion problems. The PUR topcoat protects epoxy from UV light and acts as a sunscreen. The epoxy could chalk and become brittle. The deck's PUR color is tinted to give it a more attractive finish.

Ballast tanks are also protected by an epoxy coating with aluminum pigmentation. The International Maritime Organization's Performance Standard for Protective Coatings 2 sets a 15-year target for seawater ballast tank coatings. Stolt Tankers aims for a longer service life and a 25 year service life for ballast tanks. Stolt Tankers applies two coats of epoxy to the CS ballast tank coatings.

After 20 years of service, a seawater ballast tank was installed on a Stolt Tankers ship. Stolt Tankers.

Most cargo tanks are made of SS and are therefore uncoated. Some cargo tanks are made with CS. Stolt Tankers covers CS tanks with a three-coat epoxy phenolic coating system. This includes a primer, undercoat and finish coat. Each coat is applied with a DFT (dry film thickness) of 90 um (3.25 mils). A three-coat epoxy system with a DFT (90 um) for each coat. A zinc silicate coating with a DFT (80 um) for each coat. Or a two-coat cyclosilicon epoxy with a DFT (150 um) for every coat

Are there particular industry standards you follow for corrosion mitigation? How do these standards help with corrosion mitigation?

These industry standards for corrosion offer guidelines for vessel planning and construction, as well protocols for maintaining corrosion protection systems over the lifetime of the vessel. There are many standards. There are many standards that cover maintenance of coatings. They include areas like surface preparation, coating application and inspection. It also covers how to detect coating deterioration. These standards provide clarity and allow all involved in ship construction and maintenance to understand the requirements. Four parts of ISO 8501 are most commonly used. They include "Preparation Steel Substrates before Application of Paints and Related Products-Visual Assessment Surface Cleanliness" and multiple parts ISO 4628, “Paints and varnishes-Evaluation Of Degradation Coatings-Determination of Quantity and Intensity Uniform Changes of Appearance.” Without these standards, it would be impossible to build and maintain ships.

What are your expectations from a coating system? In terms of performance, application, service life and return on investment? 

JE/MR The expected or desired coating life will differ depending on where the coating is placed. The requirements for coating conditions are set out in maintenance guidelines and regulations. The requirements for the coating's condition may vary depending on where the ship is located. Some areas may require more work to maintain the desired coating condition than others.

It is expensive to reblast the steel and change the coating on the main deck. We want a coating that is durable, long-lasting, and will not need to be replaced. Ballast tank coatings have a 25-year life expectancy. The coatings must also meet specific conditions. Ballast tank coatings, for example, are inspected at specific intervals and rated "good", "fair", or "poor" during the ship’s lifetime. This is a high standard. This rating is not required if the coating's condition drops below its intended service life. However, the goal is for every ballast tank coating not to deteriorate before its 25-year anniversary.

There are many aspects of ship design and maintenance to consider when fighting corrosion at sea. However, there are many standards, guidelines and corrosion protection technologies that can be used to help the maritime industry achieve the desired service life. Stolt Tankers' strategy is to maintain its ship's coatings aggressively, using the philosophy of "see and fix rust" rather than waiting for coating degradation to become an issue.

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