QUIMICA Y EL LIDER: Marine Corrosion

Delaying the Inevitable: Marine Corrosion Prevention

Structures, ships and other equipment located on or near bodies of water face an inexhaustible enemy – corrosion. It is as unstoppable as the tides and has been a challenging factor in marine engineering throughout human history. Even bloodthirsty pirates would put plunder on hold so they could properly maintain their vessels and equipment to deter corrosion.An 18th century sailing ship is careened so shipmates can clean the hull and prevent corrosion by applying a waterproofer, typically pitch or tar.

An 18th century sailing ship is careened so shipmates can clean the hull and prevent corrosion by applying a waterproofer, typically pitch or tar.

Yet the materials of today’s ships and boats, waterside infrastructure such as buildings, breakwaters, bridges and piers, and offshore installations like wind turbines and oil rigs, are considerably different than in centuries past. That has led to multitude corrosion resistance strategies that are often deployed together for the optimal corrosion resistance.

Threat Factors

There are several mechanisms by which materials corrode in marine environments.

For metal materials, the most identifiable is oxidation, or rust. In continued presence of water, metal molecules will eventually combine with the oxygen of the water to form an oxide. This process degrades the surface appearance of the material and can ultimately lead to failure if unchecked. However, on some materials such as aluminum, copper and stainless steel, oxidation is practically negligible, as the oxidation layer can form a barrier that prevents the underlying material from further deterioration.

Glavanic potential of common metals. Source: Solar Professional

Metals are also subject to galvanic corrosion, prompted when the metals are in electrical contact, either directly or indirectly in an electrolyte. When this occurs the metal with the higher electrical potential will act as the cathode and accept displaced ions from the anode, ultimately resulting in blistering, pitting and material loss of the anode. Adjacent, submerged metals are essentially turned into battery.

Saltwater is a high quality electrolyte; distilled water is not. But freshwater becomes more conductive due to dissolved contaminants, such as minerals or pollution. Conductivity also improves in warmer temperatures, so galvanic corrosion will be more aggressive in a tropical climate than a cooler one. Metals that have corroded are at risk of stress-corrosion cracking, which accelerates mechanical weaknesses in corrosive environments.

Also worth mention is stray current corrosion, a type of galvanic corrosion that is more acute due to the presence of a live current in the electrolyte. This is a valuable mechanism in electrolysis, but occurs in marine environments usually due to a nearby electrical fault. Unchecked galvanic corrosion can sink a boat in months, but stray current corrosion can do so in days or weeks.

Wood’s primary vulnerability is its weathering. If left untreated or unmaintained, moisture seeps into the pores and crevices of the wood. If the moisture freezes, cracks, gauges and warping are likely. This harms the aesthetic of the wood and exposes internal cellulose that may not have received even initial preservation treatment. Wood that is submerged is at risk to several types of bacteria and shipworms. Wood rot can occur after fungal spores have invaded the wood and colonized the material, ultimately making it quite brittle. Invasive pests, such as termites, are another threat to the material.

Concrete, although incredibly resilient and long-lasting, is not impervious. Saltwater is a strong corrosive and does take a toll. It contains magnesium chloride, sulfate and hydrogen-carbon ions that will minimally corrode the concrete surface before producing layers of brucite and aragonite, although these layers can actually help prevent further corrosion. Further potential for corrosion typically comes from mechanical stress, such as erosion or fissures that grow where water has intruded. Should internal rebar come into contact with water, corrosion will be rapid, as the rebar will swell and create internal stress that could create cracks in the material.

In marine environments, the go-to plastic material tends to be fiberglass, or glass-reinforced plastic (GRP), which is stronger by weight than many metals and is quite versatile. It is composed of layers of woven glass fibers enveloped in layers of polyester resin or epoxy. This allows complex shapes to be manufactured in a single piece while also enabling engineers to control strength properties. It has zero corrosion concerns even when permanently immersed provided the resin withholds. However, it is susceptible to mechanical damage, which can affect the long-term stability of the glass fibers, or wood if it has been sandwiched between layers. Fiberglass resin will lose luster from UV exposure and isn’t as abrasion resistant as metal or concrete, but is otherwise quite weatherable provided the glass fibers remain unexposed. If the fibers are exposed, water will attack and degrade the glass fibers. In fact, archeologists measure the age of buried glass bottles by of amount of water absorption.

Marine biofilms, often identified as boat sludge, often builds-up on the hulls of slow moving boats and stationary, immersed objects. The microbes and bacteria indusce corrosion and can produce chemicals that react with the material. A good example are the "rusticles" growing on the wreck of the Titanic, which are actually growths of bacteria. Anti-fouling and ablative biocide paints and barrier coatings are the most common prevention method.

Corrosion-Resistant Materials

A material that is inherently strong, workable and modular is key for marine installations. When used, each substrate requires additional consideration that might not be a concern when used inland.

A boardwalk with concrete pilings and a wood walkway employs several corrosion prevention methods to maintain structural integrity.

Wood is appreciated for its versatility and beauty, and it is the default framing material for many small- and medium-sized structures. Several types of wood are rot- and pest-resistant, but they are too expensive to be used as a common building material. Exposed wood weathers considerably from temperature fluctuations, moisture ingress and UV exposure, so it requires intensive maintenance to have an economical and sustainable service life. Lumber needs to have a primary preservative treatment, which can include chemical, oil and water-based preservatives that are soaked into the grain of the substrate. Piers and docks are likely to be treated with chromated copper arsenate or pentachlorophenol, both of which are highly toxic and high in VOCs. Lumber for residential and commercial structures will likely be treated with less toxic substances such as copper azole.

Masonry projects primarily use concrete, which is valued for extreme durability, although it carries some limitations in marine environments as it can be expensive and difficult to work or shape in challenging conditions. Concrete is the favored material for components such as bridge, pier and turbine footings or pilings. The mix of the concrete is key to longevity, preferably of type V cement with sulfate resistance. The concrete type should not absorb water from the environment and also contain an air entrainment agent to minimize the effect of freeze and thaw cycles. Cement aggregate, such as slag, should not be chemically affected by water. Granulated slag is favored for this as it offers higher compressive and bend strengths than Portland-type cement, protection against sulfates and chlorides and also strengthens the concrete over a long duration. Finally, the concrete should be well-packed and installed all at once, ensuring a dense, unified mass of material. This minimizes the chance of corrosion in rebar. Fiberglass or stainless steel rebar is a possible alternative.

Metals are a highly favorable material. Carbon steel is a common material for coastal building roofs and boat hulls, although it absolutely must be protected from direct exposure by a barrier coating or more, as it will corrode quickly. Stainless steel is not corrosion-proof, but exhibits excellent resistance in fresh and saltwater environments. Specifically, stainless steel 316 is the most corrosion resistant. It is more expensive than simple carbon steel. It is a common material for equipment and components, as well as architectural elements, but structural members are almost never stainless steel.

Aluminum is also highly valued for its inherent corrosion resistance. Aluminum is more corrosion resistant than steel in seawater environments – especially the 5000 series marine grade alloys. The 6000 series of aluminum alloys, like 6061, use copper as an alloying element, but copper rich phases in 6061 increase corrosion via galvanic action. The 5000 series aluminum alloys use magnesium. However, aluminum is not as corrosion resistant as stainless steel because its oxide film is porous. Anodizing builds an aluminum oxide trihydrate on the surface, which is hard, nonconductive and corrosion resistant. Dye and sealants are usually applied to the anodized surface to color and provide additional corrosion protection. In fact, aluminum would be the optimal material for boats and immersed structures, if not for the fact that is quite susceptible to galvanic corrosion when copper, brass, bronze or steel parts are in electrical contact. It is otherwise durable, lightweight, economical and repairable. It is a common boat-building and construction material.

Copper is exceptionally corrosion resistant due to the verdigris patina that oxidizes on the surface, such as on the Statue of Liberty. Copper roofs are incredibly durable, with some inland examples surviving for more than 500 years. It is also lightweight and low maintenance. The primary drawbacks are its initial expense and copper’s high galvanic number, which will drive corrosion in all metals other than stainless steel. Structural members are unlikely to be copper, in favor of copper cladding, flashing and other architectural and décor elements. It is also a natural biocide, inhibiting the adhesion of natural growths such as algae.

Monel, a copper-nickel alloy, is corrosion resistant and the copper creates an anti-fouling surface. Monels are stronger than pure copper. Titanium is essentially corrosion proof in seawater – less than the thickness of a sheet of paper will corrode off a titanium surface over 4,000 years, based on corrosion rates. Unfortunately, copper, monel and titanium are costly so use is limited.

Doubling Down on Corrosion Prevention

Offshore wind turbines such as these often employ footings or caissons, steel structures and fiberglass turbine blades. Source: Senu Sirnivas/NREL

In addition to a material’s innate corrosion resistance, UV-stable barrier layers can be applied over the material to repel water, oil or foulants and provide sealing, abrasion resistance, deterioration control or other useful properties.

Paint is an excellent beginning as a barrier layer and it can be laden with various fillers A prime example is anti-fouling paint, which is standard on all ships and boats and many waterborne components. Growth of algae and other organisms can stimulate corrosion, so anti-fouling paint contains ablative biocides to prevent their adhesion.

Depending on the pretreatment method, lumber will be paintable, which is highly recommended for secondary UV, abrasion, anti-fouling and ingress protection. Wood can also have a watersealer or woodstain barrier instead. In each case, the barrier must be reapplied regularly.

Paints and coatings are not limited to wood materials; metals, concrete and fiberglass also benefit. Paints may be specially formatted depending on the application and workpiece material. Zinc dust is a common additive to inhibit steel corrosion. Fiberglass materials carry virtually no concern of chemical corrosion in common marine environments, provided the polyester or epoxy resin remains intact. It is possible that in a highly polluted environment environmental stress cracking may occur that plasticizes the polymer of fiberglass, but this typically requires prior physical damage in the nature of a craze or crack. Painting is the favored means to prevent fouling of fiberglass substrates.

One of the most common barrier coatings in industrial off-shore uses are types of epoxy coatings that are filled with coal tar or bituminous pitch, as they exhibit excellent corrosion and chemical resistance while being easy to apply and flexible in use. However, they are high in VOCs and are slowly being phased out for more environmentally friendly solutions. One such type is high build polyamine epoxy, designed for aggressive splash and moderate immersion applications. Other barrier coatings are based on silicone resin and vinyl ester polymer technologies.

There remains much research activity in anti-corrosion control, so new developments occasionally arise. There have been advances in a super high build epoxies that are maintenance-free and can last for decades in the most challenging offshore environments. Hexagonal boron nitride is an industrial lubricant, but recent research has yielded an ultra-thin, 2D coating of the material that is added to metal via vapor deposition and is exceptionally corrosion resistant. Also consider a recently unveiled thermally sprayed aluminum for offshore steel structures such as wind turbines. The compound is filled with an antifouling substance and as it decays an insoluble calcium deposit is revealed that further protects the underlying material.Sacrificial anodes placed around the propeller housing and rudder of a large ship. Source: WRS marine

Sacrificial anodes placed around the propeller housing and rudder of a large ship. Source: WRS marine

Metal materials have other opportunities to keep corrosion at bay. Paint alone is rarely enough and not all metal materials are suitable for it. Cathodic protection is an important consideration when two dissimilar metals will be submerged nearby. This method employs a sacrificial anode, which is a material with galvanic potential lower than the metals that need protection. Once immersed in an electrolyte, the anode corrodes as expected, but prevents it from occurring on the other metals. The sacrificial anode needs to be replaced periodically and will not offer much protection from stray current corrosion. This is the same mechanism that protects galvanized steel.

When the sacrificial anode is not enough alone, an impressed current cathodic protection system can be used. This system monitors the electrochemical environmental of a metal material and applies a current to the metal to be protected, thereby reducing its galvanic potential. This system can outright eliminate galvanic corrosion on large structures such as bridges, oil rigs and ships. The anodes, which are typically metal oxides, are strategically positioned and must be periodically replaced.

Conclusion

Corrosion is ubiquitous and omnipresent. Meanwhile, thousands of corrosion engineers worldwide devote entire careers to fighting it. And in marine environments, corrosion risks are especially prominent.

Corrosion is like a medical condition — it must be assessed, managed, treated and monitored. Failing to do so results in death of the patient; in this case the failure of buildings, vessels and infrastructure