Culture and heritage

Maritime heritage

Rusting of iron

Learn about conditions under which rusting of iron occurs and the process of rusting

The effect of rusting is very significant in the management of Historic Shipwreck sites. A vessel when it sinks in ocean salt waters, or inland fresh water bodies, begins the slow process of deterioration. This involves the natural breakdown of its component parts including organic materials (eg timber, sail cloth, rope), and metal components (anchors, fastenings, engines, machinery and hull). The process occurs naturally where there is an availability of Oxygen (H2O in water) and can lead to the total disintegration of a shipwreck over several hundreds or thousands of years.

While there is no over-riding need to address this process for most shipwreck remains, mechanical damage to sites can greatly increase the rate of corrosion. This action can be caused by SCUBA divers dragging equipment against the fragile archaeological structure, by handling elements, but more seriously by a vessel's anchor and chain pulling through a site, by the actions of commercial fishing nets entangling a wreck, or through the removal of artefacts.

Why does this lead to increased corrosion or "rusting"? This action causes the corrosion products that naturally form around deteriorating metals to crack open or for the underlying structure to be broken. Removal of this barrier between the metal of the shipwreck and the oxygenated water environment leads to rapid and accelerated corrosion occurring. This leads directly to a greater amount of metal loss over a shorter period of time and can lead to further weakening of the surviving hull, and therefore collapse of elements of the wreck.

The removal of marine organisms including corals and weeds that colonise a wreck site acts in a similar way to destabilise a site. Damage is immediately obvious to a diver by the tell-tale bright orange colour of the exposed bare metal as it actively corrodes. Normally, the barrier formed by marine growth and metal corrosion products acts as a barrier to rapid corrosion activity - the reaction generally achieving an equilibrium rate of deterioration if left undisturbed. While always resulting in the loss of original fabric and strength, the process is far slower than the aggravated deterioration witnessed by severe mechanical damage.

Destabilisation of these protective coatings can also be caused by natural processes, such as the action of extreme wave, storm or surge activity, that can result in the breaking down of exposed elements of a shipwreck, abrasion by mobile sand, or removal of the underlying sand or mud base supporting the shipwreck. Rapid removal of, eg, sand around a shipwreck can lead to the exposure of portions of a shipwreck not normally exposed, allowing fresh corrosion reactions to begin.

Page last updated: 31 August 2012