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Iron corrosion & Concretion formation

Iron that has been excavated from a marine environment

Unlike iron excavated from terrestrial sites, iron that has been excavated from a marine environment poses a unique problem and solution to the conservator.

Iron can be classified into two distinct categories:

  1. wrought iron
  2. Cast iron
  • Cast iron corrosion
    Cast iron is known for quickly disintegrating if not kept in a wet environment after excavation from the sea. This Cast iron can be corroded by graphitization which leaves behind a metal core but has an outer shell of graphite that contains iron corrosion products. Iron that has reached this level of decay is considered highly unstable and will corrode rapidly at the graphite/metal core boundary once it is exposed to the atmosphere. This rapid corrosion will force the flaking of the outer layers of iron object. This type of corrosion is considered exothermic, which produces heat that speeds up the rate of decay for an artifact. There have been vast examples of iron objects that once recovered from the sea are hot to touch . These exothermic reactions can actually explode the surface layer of an iron object (anchor chain, cannon, cannonball, etc.) before it reaches its conservation receptacle. Sometimes this process may only take a few hours in warm to tropical conditions.
  • Wrought iron corrosion
    Wrought iron tends to corrode in a different manner than cast iron. This iron contains only trace amounts of carbon therefore it does not have a graphitized surface layer like cast iron exhibits Wrought iron corrodes along the slag lines which tend to look like long rope-like strands (Figure 1). The presence of chlorides in the metals, especially those metals which made of iron, accelerates the corrosion process. Brown tear drops also know as ‘sweating’ form on the surface of iron and are an indicator of the presence of chlorides. One cannot emphasize the importance that iron be placed in wet storage immediately upon excavation since the main agents involved in the rapid decay of an object are water, oxygen and chloride ions. It is these factors that must be controlled and stabilized.

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Concretion formation
Formation of concretions is one of the most interesting features of iron corrosion. Concretions occur on iron when it is either buried or exposed to marine seawater conditions. Concretions occur due the iron interacting with its surrounding matrix. Since iron is not toxic any iron that has been suspended in seawater is rapidly colonized by marine organisms and in time builds up layers of calcium (CaCO3). As North states , “The subsequent diffusion of Fe2+ and Fe3+ from the underlying corroding iron produces chemical changes in this biological material”. These forms of concretions are prolific in the Caribbean and Mediterranean Seas.

Corrosion of iron in almost all of marine environments is usually covered with a layer of concretion. These concretions encase the iron artifact and form a cocoon type barrier against the under sea environment causing an almost micro-environment upon the surface of the iron object. This micro-environment creates a different type of corrosion upon the artifact than if it was laid bare in typical maritime conditions.

From: http://www.artifactlab.com/