COMPLEXING AGENTS AND ELECTROLYSIS
RESTORE SOME ARTEFACTS FROM THE TITANIC, PROVIDING INSIGHTS
FOR RESEARCH IN DEEP-SEA AREAS
chemistry has played a central role in restoring artefacts
on the ocean bed at the wreck site, there is a complete absence of light, a temperature of
around 0 C, and very little oxygen
metallic objects undergo corrosion, the effect of an electrochemical process
aspects of the geochemistry of iron and other metals had been accidentally introduced
into the deep-sea environment
The Effect of Bacteria on the Titanic
bacteria plays a major role in promoting corrosion, resulting in fast-growing structures
such as rust flows and rusticles on the Titanic
bacteria on the seabed feed on metals, especially iron, producing black sulfides that degrade and stain many artefacts
a variety of bacteria are present in rusticles and these are predominantly sulfate-reducing species that multiply quickly in anaerobic conditions
rusticles have a brittle hydrous iron oxide shell with a dark red outer surface and an orange inner surface. The core of the rusticle and the inner surface of the shell consist of
spherical aggregates of needle-like crystals of goethite [αFeO(OH)]. The outer surface of the shell has the crystal structure of lepidocrocite [γFeO(OH)]
rust flakes contain a mixture of goethite, lepidocrocite and other minerals. They consist of a well-crystallised iron mineral, possibly hematite (Fe2O3), associated with a silicon-rich iron mineral. Black patches of siderite (FeCO3) and iron-rich cubes, possibly magnetite, are also present. They have a thin coat of PbCO3 and small cubes of galena (PbS) are also present
it is suggested that the source of the lead is the paint on the hull of the Titanic
The Effect of Exposing Artefacts to Air
objects from the wreck absorb chlorides and sulfates from the seawater, becoming
weakened, stained and encrusted so, when they are raised to the surface, they must be
kept wet
ceramic and glass items can lose surface layers if salts are allowed to crystallise
leather can harden, crack and shrink as it dries our if salts in the pores are not removed
before drying
metal objects undergo accelerated corrosion if exposed to air
Treatment of Artefacts at the Surface
as soon as the objects are brought to the surface, the acidic silt is washed off
artefacts are packed in foam according to material type. Foam is used instead of water to
reduce damage during transportation
at the laboratory, the objects are washed in fresh water and, if they are not too fragile, are
brushed to remove corrosion objects
artefacts are stored in water with biocide to remove troublesome fungi and bacterial
growth
salts trapped in an object are removed by electrolysis or treatment with complexingreducing
agents
corrosion of iron objects is reduced using an alkaline dithionite bath
sesquicarbonate (hydrated Na2CO3 and NaHCO3) is used for the desalination of copper
objects
removal of chloride is essential since it promotes corrosion and depolymerises organics.
For example, copper-based materials corrode to form copper hydroxychlorides
hydrolysis of metal ions formed by the oxidation of iron
Fe → Fe2+ + 2e-
Fe2+ + 2H2O → Fe(OH)2 + 2H+
can lower the pH of water around the artefact to as low as 4.2, resulting in chloride ions from the surrounding seawater diffusing onto the metal surface
electrolysis is used to clean and stabilise strong metal artefacts, including most bronze objects. It stops further corrosion and it removes the chloride from within the objects. The cell consists of the object itself (cathode), a stainless steel anode, and an alkaline electrolyte such as dilute NaOH(aq) or Na2CO3(aq). The passage of a weak electrical current draws out the chloride ions from the metal. When a stronger potential is applied, the hydrogen bubbles produced at the cathode loosed corrosion products and any calcareous deposits at the surface
ceramics and organic materials are cleaned using electrophoresis. The object is placed
between a positive and a negative electrode, and a potential is applied. Salts, dirt and other particles located within the electric field break up and their charged components migrate through the solution to the electrodes
an electric pen is also used to remove stains from thick ceramic objects. The pen is a miniature electrolytic cell. The object to be cleaned is either dipped into an electrolyte or the stained section is placed under an electrolyte drip. The pen is moved around on the object to clean areas
a complexing agent in neutral solution (eg. a neutral citrate solution containing sodium dithionate) is used to remove iron stains from delicate textiles or newspapers by reducing the oxidation state of the rust lattice or iron corrosion product from iron(III) to iron (II), destabilising the lattices so that it falls apart. The neutral solution buffers the citrate ion present, resulting in the formation of a hydrous iron oxide citrate complex which is very
stable
oxalic acid is used as a complexing agent to remove corrosion products by complexing with the iron oxide to form iron oxalate, which is removed by rinsing in water
the main problem with glass is surface iridescence caused by hydrolysis of the silica network
deep-penetrating iron corrosion stains are removed by electrolysis or a neutral solution of
complexing agent
leather and wood need a consolidant to replace the material consumed by microorganisms, minimising cell collapse
storage of bronze- and silver-plated objects in damp environments can cause corrosion to recur
the study of shipwrecks in a range of sites and conditions allows greater understanding
about long-term deterioration rates of materials. This is useful for the monitoring and maintenance of underwater constructions
