Marine Coatings: "Corrosion and Anti-Fouling"

Marine coatings are protective coatings that protect marine vessels from substances originating from salt or fresh water. A marine coating possesses specific functional properties, and therefore provides superior protection to the surfaces to which it is applied. Marine coatings protect marine vessels from corrosion, wear and various organisms living in water. The global marine coatings market is estimated to be approximately USD 12 billion by 2020. In the marine environment, pitting corrosion and bacterial corrosion can occur. Marine corrosion has a significant impact on marine vessels and their service lives. Billions of dollars are spent annually against marine corrosion. For this reason, effective corrosion control strategies must be selected through suitable coating choice for the marine environment. Marine coatings have a special function for protecting marine vessels and other carriers both above and below the waterline. Surfaces coated with marine coatings can be easily cleaned. In this article, we have attempted to explain the concept of corrosion and anti-corrosion coatings. In the final section of our article, we address anti-fouling coatings, another important topic in marine coatings – a type of coating that prevents organisms living in water from attaching to ships.

Corrosion

Corrosion poses a serious threat to all metals (including ships) that come into contact with earthly conditions, whether in soil, water or soil.

Causes of Metal Corrosion – Rust

Rust is far less aggressive than electrolytic corrosion. It exists only when there is 10 percent or more humidity in the air. After all electrolytic causes are eliminated, rust is easily prevented with paint or coating.

Galvanic

Galvanic corrosion is much less aggressive than electrolytic corrosion. Galvanic corrosion results from the natural voltage between two different metals in a conductive medium such as water or atmospheric moisture.

Electrolytic

Electrolysis is several times more aggressive than rust or galvanic corrosion. It is produced by all electrical and electronic equipment that has a different voltage from the ground, flowing as current from installation to ground, to other ships or installations. A ship causing electrolysis in a conductor will also corrode nearby other ships. Electrolytic corrosion can only be measured as current and is then eliminated.

What Does Corrosion Control Mean?

Corrosion control refers to measures applied in various fields to control corrosion in soil, metal, concrete, water, sand and concrete. It consists of various corrosion monitoring and control techniques that can be used by industries to solve corrosion problems according to their needs. With such measures, the harmful effects and negative consequences of corrosion can be prevented. Corrosion can lead to countless environmental problems. For example, ships, tankers and pipelines are often exposed to the dangerous effects of corrosion. Worn water systems can contaminate drinking water. These pose a threat to the environment and mankind, and therefore effective corrosion control methods should be applied to prevent the harmful effects of corrosion.

Corrosion Can Be Controlled in Various Ways

Cathodic protection (CP): This technology uses direct current to counteract corrosion in structures such as gas pipelines and storage tanks. This helps prevent corrosion formation and even prevents it from getting worse. Coatings: These serve as main tools for combating corrosion. They are usually applied in combination with CP to provide the highest level and most cost-effective corrosion protection. Corrosion inhibitors: These are substances that, when placed in a specific environment, reduce the corrosion rate of that environment for specific materials such as metal. These can extend equipment life, prevent failures and system shutdowns. Corrosion inhibitors can also prevent heat transfer loss, fouling and preserve aesthetic appearance. Material selection: This means selecting corrosion-resistant materials such as special alloys, plastics and stainless steel.

How Does a Corrosion Inhibitor Work?

A corrosion inhibitor, or preventive, is a substance that, when added to an environment in small concentration, effectively reduces the corrosion rate of a metal exposed to that environment. There are three types of corrosion inhibitors: anodic inhibitors, cathodic inhibitors, and mixed inhibitors. Corrosion prevention is usually achieved through one or more of three general mechanisms. The inhibitor molecule is adsorbed on the metal surface through chemical dissolution, forming a thin protective film by itself or together with metallic ions. The inhibitor causes a metal to form its own protective metal oxide films and thereby increases its resistance. When selecting a corrosion inhibitor for application, several things should be considered as follows: • Materials to be protected, • Application method (immersion, spray, brush, etc.), • Type of protection required (during process, storage or transport), • Desired type and thickness of coating residue, • Storage, packaging and/or transport conditions (temperature, humidity, seasonal conditions), • Interaction with subsequent processes, • Environmental, health and safety requirements, • Product type (oil/solvent or water-based).   Electrochemical methods are routinely used to evaluate corrosion inhibitor efficiency. The advantages of electrochemical methods are short measurement times and mechanical knowledge that assists in the design of corrosion protection strategies and the design of new inhibitors.

What Is Corrosion Resistance?

Corrosion resistance refers to the resistance that a material provides against reaction to adverse elements that could cause the material to corrode. Various properties carry this property inherently depending on corrosion resistance rates. Alternatively, some methods or processes can be used to counteract corrosion through painting, coating, hot-dip galvanizing or a combination of these methods. Corrosion is a process in which a material is oxidized by substances that cause the material to lose electrons in the environment. Corrosion resistance is a metal's ability to retain bonding energy and withstand deterioration and chemical degradation that may occur if the material is exposed to such an environment. Resistance to corrosion is typically expressed in terms of corrosion rate and measured in millimeters per year or "mils". These measurements of corrosion resistance are taken under defined working conditions in a specific environment, pressure, temperature and fluid velocity. In addition to intrinsic corrosion resistance, a metal's resistance to corrosion can be increased by applying different methods such as cathodic protection, coating, painting and corrosion inhibitors. There is no corrosion-resistant material that covers all environments. Materials must be suitable for the environment in which they will live. For metals, corrosion resistances are predicted and data are used to monitor a metal's durability in a specific environment. Protective coatings are the most commonly used corrosion control technique. Essentially, protective coating materials serve as a tool to separate surfaces that are sensitive to corrosion. Another method, cathodic protection, interferes with the natural movement of electrochemical cells responsible for corrosion. Cathodic protection can be effectively used against corrosion of surfaces submerged in water or exposed to soil. Corrosion resistance is an important factor to be considered when selecting materials for corrosion control. Materials most resistant to corrosion are those for which corrosion is thermodynamically unfavorable. Some metals naturally have slow reaction kinetics despite their corrosion being thermodynamically favorable. These include metals such as zinc, magnesium and cadmium. Therefore, a material's corrosion protection methods fall within the corrosion resistance category. In addition to corrosion control, another critical issue in marine coatings is organisms that attach to marine vessels. At this point, anti-fouling coatings come into play.

What Does Anti-Fouling Paint Mean?

Anti-fouling paint is a special coating applied to the hull and propellers of a marine vessel. The coating is used to slow the growth of marine organisms such as mussels, sludge, algae and seaweed. In addition to anti-fouling, the coating prevents corrosion on metal hulls and propellers. It also improves and accelerates water flow through the hull of the vessel. Anti-fouling paint is also known as bottom paint. When anti-fouling paint is applied to metal hulls, it serves two purposes: in addition to corrosion, it prevents the settlement and growth of microscopic marine animals and plants on the hull. The concepts of corrosion and fouling are related because poor corrosion resistance causes paint film deterioration and thus loss of anti-fouling properties. Typical anti-fouling paint contains a biocide or a toxin in the paint; this is released to the area surrounding the hull to poison any added organisms and prevent others from adhering to the paint. The four basic ingredients of anti-fouling paints are biocide, resin, solvent and pigment. Three main categories of anti-fouling paints: • Soft bottom paint – A moderately effective copper-based coating used in colder waters where growth potential is lower compared to warmer climates. It is the most inexpensive coating. • Hard bottom paint – Hard anti-fouling paint contains more biocide (copper oxide), making it more effective but more expensive. It offers a hard surface and provides good surfaces for racing boat bottoms. • Ablative anti-fouling paint – Designed to maintain the amount of biocide in the remaining layers despite wear on the outer surface. It is suitable for many environments and is available in single season or multiple season options. At the end of our article, let us briefly summarize the factors to be considered when selecting anti-fouling paint. These can be listed as: whether the ship is used in fresh, salt, cold or warm water, whether the hull material is steel, wood or fiberglass, whether the vessel is fast or slow and compatibility with paint on the hull. Prevention of fouling and corrosion extends the ship's service life, increases efficiency, reduces operating costs and prevents marine accidents and costly repairs.

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