Mention a cruise and most people’s minds turn to exotic ports, exciting excursions, and fabulous food. But for a ship’s engineer, a cruise ship also means battling corrosion from briny seawater, battering winds, and incessant sun.
The harsh marine climate makes ships prone to corrosion because of high salt and moisture content, and corrosion can weaken the ship’s construction and lead to expensive repairs, writes Anish Wankhede, a marine engineer officer and co-founder of Marine Insight (marineinsight.com), in “How Ships Fight Corrosion at Sea” (March 2023).
Types of Corrosion in a Marine Environment
Finnish engineer Liisa Ojaniittu set out to study marine corrosion and to determine the causes of corrosion on ships from Finnish shipbuilder Meyer Turku. Her master’s thesis, “Corrosion of Cruise Vessel Outfitting Parts,” focused on corrosion on such cruise ship staples as pool decks and lifeboat decks.
The rate of cruise ship corrosion is influenced by location, atmospheric conditions, time of day or year, altitude, and wind velocity. Deposited salts decrease the protection properties of a protective oxide layer and increase the water vapor condensation on the metal surface. Among Ojaniittu’s findings:
- The marine environment is highly corrosive due to its high chlorine content. Marine atmospheric corrosion is affected by many factors such as high temperatures, sunshine, high relative humidity, and high NaCl (sodium chloride). Factors affecting atmospheric corrosion in the marine environment are time of wetness, thickness of the electrolyte layer on metal surfaces, chloride deposition, and temperature.
- Increased salt content increases the conductivity of seawater. High conductivity leads to more aggressive corrosion. In higher temperatures, the conductivity is even higher. Temperature affects atmospheric corrosion in marine environments in two ways: influencing electrolyte film formation and directly affecting corrosion reaction time.
- Precipitation can either increase or decrease the corrosion rate. Rain affects corrosion by flushing harmful salts and pollutants from metal surfaces. Fog and dew, however, wet the metal surfaces without the beneficial flushing effect of rain.
- Tropical marine environments are generally more corrosive than Arctic marine environments. Temperature is a major factor, but other corrosion factors also vary with geographical location.
Costs of Corrosion
Ojaniittu, who attained her master’s in materials science and engineering at Finland’s Tampere University, found that the annual repair and maintenance cost for cruise vessels totals $337 million a year.
In the U.K. shipbuilding industry, 21% of capital outlay is spent on corrosion and corrosion protection, she found, and better design and better protective coatings could increase savings by one-fifth.
“The costs of corrosion include indirect costs such as increased workload in design and building, decreased performance, and cost of repairs,” she says. “These costs should be evaluated when deciding the level of corrosion prevention and the price for it. Many factors such as corrosion resistance, cost, mechanical properties, and availability will affect material selection.”
Forms of Corrosion
The most common forms of corrosion in a marine environment are galvanic corrosion, pitting, and crevice corrosion, Ojaniittu says.
Pitting is a form of localized corrosion where small areas corrode and forms cavities or pits. Pitting occurs when a small surface area undergoes a rapid attack while the surrounding surface area remains unaffected.
Reasons for crevice corrosion are narrow gaps between two metals or between metal and non-metal, presence of cracks and other defects on the metal surface, deposition of a biofouling organism and bacteria, or deposition of dirt. The crevice becomes a permanent anode while the surrounding area becomes a permanent cathode. The crevice becomes oxygen-starved compared to the surrounding area, which leads to the formation of differential oxygen cells and to the initiation of corrosion in the crevice.
Galvanic corrosion occurs when two metals or alloys are coupled in the same electrolyte. Electron flow between the dissimilar metal or alloy occurs due to the potential difference. A more active metal or alloy becomes an anode, and a more noble metal or alloy becomes a cathode.
Ship Design Battles Corrosion
The right coatings are essential for corrosion protection on cruise ships.
Wankhede writes in Marine Insight that properly placing scuppers and drains is essential to aiding the draining of water from decks, wells, and bilge areas. These drains eliminate a direct cause of corrosive activity. He also recommends setting up insulation in locations where different metals are placed close to each other, which will reduce galvanic corrosion. Insulation will also stop thermal fatigue in locations with extreme temperature shifts.
Other design considerations include anti-vibration practices, such as fitting turbine machinery with sliding feet to minimize metal fatigue, as well as sacrificial anodes made of magnesium, aluminum, or zinc.
Building with corrosion-resistant alloy steel or stainless steel can also decrease corrosion, as will installing rubbing strakes—designed to prevent chafing of dock, fender, and anchor lines—and doubling plates for strengthening to take in extra wear and tear.
Wankhede also recommends a user-friendly ship design to allow for maintenance and coating applications once a vessel is in operation.
Ojaniittu, who today works as a material engineer in the transformer industry, also encouraged the use of steels, stainless steels, aluminum, and their alloys for ship building along with titanium, zinc, and nickel.
In her research, she found that the three main reasons for corrosion in Meyer Turku ships were coating failures, materials selection, and flying rust.
“The main principle of anticorrosive coatings is to act as a physical barrier by preventing the electrolyte from touching the surface of the metal,” Ojaniittu says. “Corrosion prevention paints should have intrinsic durability, adequate flexibility, good adhesion to the surface, and adequate toughness to withstand impacts and cracking.”
Cruise ship paints should maintain their appearance when subjected to weathering, stress, mechanical abuse, and swell, she says. Other important properties are resistance to UV-radiation, scratching, impact, abrasion, seawater, and cleaning agents. They also should be non-slip and non-toxic.
Cruise Ship Challenges
While the battle against corrosion in maritime settings is largely similar, cruise ships do pose some extra challenges. Outfittings such as swimming pools and even lifeboats need to be treated to prevent corrosion, as even the aesthetics of rust and wear will have an impact on the impression the boat’s condition makes on passengers.
“Corrosion in lifeboats and swimming pools can be prevented with the right material selection, with suitable paint, with careful painting, with sufficient maintenance, and with good design,” Ojaniittu says.
Once a ship is underway, contaminants on the surface such as particles of iron, rust, or salt deposits can cause contamination. Surfaces need regular washing to flush away these impurities, she says.
Ojaniittu said her main takeaway after extensive research is how many types of corrosion can occur on a cruise ship.
“There are so many things to consider when designing a corrosion prevention program for cruise ships,” she says, “but also many simple ways to prevent corrosion, such as better maintenance, better design, and better material selection.”
Editor’s note: This article first appeared in the November 2023 print issue of Materials Performance (MP) Magazine. Reprinted with permission.