Marine


RUST IS A COMMON SIGHT ON SEA-GOING VESSELS, WHICH APART FROM SMALL PLEASURE BOATS, ARE MADE OF STEEL. THE PREVAILING HUMID, SALTY ATMOSPHERE AGGRESSIVELY DOES ITS WORK.

Corrosion also impacts safety. Statistics for ship hulls show that 90% of ship failures are attributed to corrosion (MELCHERS RE. Corrosion uncertainty modelling for steel structures. Journal of Constructional Steel Research 1999;52:3).

PRINCIPAL MECHANISMS OF SHIP CORROSION

Ships are primarily exposed to atmospheric corrosion, caused by a combination of high moisture and salt-laden sea spray, both of which directly attack the steel through the smallest deficiencies of the pain layer.

Ships also suffer from fretting corrosion, caused by the repeated relative surface motion between loaded metal surfaces, typically induced by vibration (caused by machinery) and structural flexing (caused by sea currents and wind).

Like all metal structures containing different metals, at the contact points between different metals, galvanic corrosion takes place – by the two metals forming a parasitic galvanic cell with the sea water acting as an electrolyte. This cell’s action causes one of the metals, typically the steel to oxidise.

Weak or absent earth connections between a docked ship’s hull and its pier-side power supply causes leakage earth currents to flow out of the ship’s hull (typically through a hull protrusion or sharp edge) into the water, and thereafter to the seabed, which is the harbour power-supply’s earth. The point at which current leaves the ship’s hull is prone to stray current corrosion.

The abundance of sea spray causes water to become trapped and accumulate in crevices, whether formed by silt, sand, marine organisms, structural recesses, or sealing material. This forms a stagnant always humid area on the metal, which gives rise to a hidden-from-the-eye corrosion patch, which gradually erodes the metal and allows water to ingress inner structures. This is known as crevice corrosion.

Microbial corrosion is caused by certain types of marine bacteria, which stick to the hull or accumulate in crevices. Their biological activity modifies local chemistry by acid production, thus accelerating corrosion.

Internal corrosion takes place in tanks, piping and pumping equipment in oil tankers.

 

CORROSION VICIOUS CIRCLES

Structural weakening – Since stress-bearing components of ships are made of steel, any serious corrosion causes structural weakness, compromising safety – In stormy seas, ships’ hulls are subjected to much torsional stress, and normal-duty payload (e.g. ship cargo) also stresses the structure. A weakened structure will flex more, increasing fretting corrosion, and in turn further weakening the structure, as well as allowing the ingress of water and dirt through weakened seals or through welded/rivetted joints becoming porous, as explained below. And of course, a sufficiently weakened structure can catastrophically fail.

Ingress of water and dirt causing more corrosion – Corrosion around openings, often caused by salt water trapped under rubber seals, weakens the effectiveness of these seals and allows water and dirt to enter enclosed areas. Those enclosed areas may have an intricate internal structure with many places where water and dirt can accumulate – usually invisible from the outside. The accumulated dirt forms a sponge which retains any water ingress, forming stagnant pools of salty water, causing further corrosion.

Ingress of water causing electrical faults – The weakening of seals mentioned above can also cause water to enter electrical connection boxes and equipment, causing short circuits and the corrosion of electrical connections.

Ingress of water causing flooding – Corrosion-weakened seals may cause ingress of enough water to destabilise the ship. Corrosion-weakened rivetted or welded joints (which may be below the water line) may become porous, allowing massive flooding.

 

WHICH PARTS OF A SHIP ARE VULNERABLE TO CORROSION?

HULL & SUPERSTRUCTURE

The principal part of the ship vulnerable to corrosion is the hull & superstructure, which are exposed to the elements 24 hours a day.

ENGINE HEAT EXCHANGERS

Marine engines are typically cooled by a coolant/antifreeze/anticorrosion mixture circulating in a closed circuit, similarly to modern motor vehicles. After passing through the engine, this mixture is cooled by heat-exchangers in which sea-water flows in open circuit (analogous to the motor-vehicle radiator). The sea-water side of the heat exchangers are prone to corrosion due to the salinity of the water and the accumulation of sea-borne debris which act as sponges causing stagnant salty humidity and thence crevice corrosion.

BALLAST TANKS

Some ships, especially large cargo ships and oil tankers have ballast tanks, which are used to change the weight distribution of the ship, or to change its draft, in order to optimise stability. Ballast tanks being filled with water, often sea water, are very prone to corrosion.

Ballast tanks do not corrode uniformly: The upper part, which forms an empty headspace, even when the tank is full, undergoes thermal cycling (due to ambient temperature changes), contains much oxygen and is affected by vibration – corrosion easily takes a foothold. The bottom part, which is often immersed in water is prone to the accumulation of marine microorganisms coming from the seawater pumped into the tanks. Also, when emptied, a thin and salty electro-conducting moisture film remains on the surface. Both of these phenomena cause corrosion.

Modern double hull tankers, with fully segregated ballast tanks suffer from an additional vulnerability: An empty tank acts as insulation from the cold sea, allowing the warm cargo areas to retain their heat longer. Since corrosion rate increases with temperature differences, the cargo side of the ballast tank corrodes more quickly than it would in a conventional configuration.

 

PARTICULAR VULNERABILITIES OF OIL TANKERS

Oil storage tanks in tankers are vulnerable to “sweet” and “sour” corrosion. Sweet corrosion takes place in systems containing only carbon dioxide, with a low level of hydrogen sulphide (H2S partial pressure < 0.05 psi). Sour corrosion takes place where the hydrogen sulphide’s partial pressure is above 0.05 psi. H2S itself is not corrosive, but a water-containing gas environment causes reactions which lead to corrosion of the inner surface of the steel pipeline.

Sweet corrosion results in internal pitting of the pipeline, whereas sour corrosion is much more severe, and can compromise the structural soundness of the tank.

In addition, moisture ingress, and its accumulation in the tank bottom aggravates corrosion. This moisture may also evaporate in warm weather, and with a fall of external temperature, re-condensate all over the internal surface of the tank.

 

THE COST OF CORROSION

The annual U.S. marine shipping industry corrosion-related costs are estimated at $2.7 billion. This is made up of costs associated with new construction ($1.12 billion), maintenance & repairs ($810 million), and corrosion-related downtime ($785 million) (JOHNSON J.; Cost of corrosion in ships, report. Dublin, Ohio: CC Technologies Laboratories, Inc.; 2001; KOCH GH, BRONGERS MPH, THOMPSON NG, VIMANI YP, PAYER JH.; Corrosion costs and preventive strategies in the United States. US Federal Highway Administration; 2002. Report FHWA-RD-01-156; DE BAERE K, ET AL. In situ study of the parameters quantifying the corrosion in ballast tanks and an evaluation of improving alternatives, NACE Conference Papers, Houston; 2011).

“A ship is composed of 90% steel. We estimate that approximately 25% of the world’s steel production is destroyed by corrosion, in other words, 5 tons per second. The costs arising from corrosion are calculated to be several tens of millions of euro per year for leading navies such as the French or American navies.”