Blame it on super-rust, a virulent form of corrosion that has destroyed hundreds of ships and could sink the oil industry.
By Richard Martin, June 2002. Wired Magazine.
Key points (see the full article at http://archive.wired.com/wired/archive/10.06/superrust.html):
Ships that cost hundreds of millions of dollars to build are rusting and falling apart, spilling millions of gallons of oil every year, many of them oil tankers. From 1995 and 2001, 2856 oil tankers broke apart at sea or barely escaped that fate, according to the International Association of Independent Tanker Owners. The main cause was collision, but nearly as many suffered from excessive corrosion.
The latest generation of oil tankers are more vulnerable to rust due to the mandate that all tankers operating in the US have double hulls by 2015. This innovation has inadvertently propelled corrosion to unheard-of levels. A 2000 Intertanko report concluded that excessive rust is afflicting double hulls within two years of launch. In double hulls, accelerated corrosion is engineered right into the ships themselves. The extra layer of steel gives rust many more square feet of surface area to attack, much of it hidden in cramped, inaccessible crawl spaces. What’s more, these crawl spaces form an insulating layer that keeps the internal temperature much higher than it would be in a single-hull tanker. Corrosion rates tend to double with each 20-degree Fahrenheit increase.
Manufacturing efficiencies have reduced the thickness of hulls and decks so now many shipbuilders trade corrosion-resistance for lower cost. Every ounce of steel saved in the construction of a ship translates into greater profits for the builder and reduced fuel bills for the owner. Between 1970 and 1990, the amount of steel used to construct a tanker declined by almost one-fifth. Modern tanker walls are only 14 to 16 millimeters thick, compared with 25 millimeters a generation ago. Assuming a microbial corrosion rate of 1.5 millimeters a year, rusted-out pits would reach halfway through those hulls in five years.
Lack of Maintenance
Rust attacks steel from the moment the metal encounters moisture. To keep that from happening, shipowners paint steel surfaces with corrosion-resistant coatings and are supposed to reapply them, but first-class ship maintenance has become increasingly rare in recent decades, as ships trade hands several times and new owners care more about maximizing their investment than maintenance. When a ship is cited for corrosion, maintenance can be avoided by shifting to another flag.
How Rusting Happens
Rust arises from an intricate subatomic dance in which water’s oxygen and hydrogen atoms snatch electrons from atoms of iron. Because saltwater conducts electricity better than freshwater, the iron in steel oxidizes more quickly in seawater – up to 0.10 millimeter per year.
The way corrosion attacks the interior of a tanker, however, is more insidious. It can be seen most vividly in the cargo tanks, which line up along the ship’s backbone beneath the deck, and in the ballast tanks that cushion the cargo tanks along their outer edges. In these areas, steel deteriorates at five, ten, even thirty times the nominal rate. In the ballast tanks, which are normally filled with seawater when the cargo tanks are empty, water conducts electrons between plates on either side, and between separate areas of a single plate – that is, the tanks become huge, if weak, batteries. The increased electrical activity hastens the metal’s degradation.
At the top of the cargo tanks, the vapor space between the oil’s surface and the underside of the deck traps highly acidic gases – products of the reaction between petroleum, oxygen, and water – that condense against the metal. The deck flexes at sea, causing degraded steel to flake off the ceilings of the tanks, exposing more bare steel for the acid to attack.
At the bottoms of the tanks, in the water that settles under the oil, corrosive bacteria thrive. Consuming hydrocarbons, microbes like Desulfovibrio desulferican produce acids that dissolve the tanks’ floors and lower sides at rates as high as 2 millimeters per year. Some microorganisms even feed on the coatings that protect the tanks from rust. Essentially, a tanker is a gigantic floating petri dish for a peculiarly vicious sort of steel-eating sludge – the ultimate metallivore.
Enforcement is hard
In addition to switching to another nation, the tanker industry is overrun with so many holding companies, limited-liability partnerships, and owners-of-record that even determining who bears ultimate responsibility for a ship can be difficult.