Corrosion Flashcards
Metallic corrosion can be defined as…
The natural process of deterioration of metals (and alloys) as they interact with the surrounding environment, usually in the presence of water. As the solid metal oxidizes the metal transforms into positive ions, forming either dissolved species or solid corrosion products.
Many archaeological metal objects on display in museums have had corrosion layers stripped off or electrochemically reduced to reveal or create a metallic surface. Is this good or bad?
Corrosion layers contain various types of valuable information, which makes them biographical records (Schweizer 1994) of the object. They contain information about raw materials, process of manufacture, use, and deposition. Furthering our understanding of past human activities and ideologies.
Describe how metals are found in nature and why
some are found as metallic deposits, other in mineral states, as their conversion to a purely metallic form requires an increase in their energy levels, in the form of electrons. Metals tend to get rid of this extra energy by reacting in the environment to form more thermodynamically stable compounds (corrosion).
describe chemical corrosion
the local transfer of electrons between materials.
describe electrochemical corrosion
transfer of electrons from one surface to another through a conductor, an electrolyte. made up of the oxidation of a metal, at the anode and the reduction of an oxidation agent at the cathode.
can we predict the rate of corrosion?
Thermodynamics can predict the formation of corrosion products but not corrosion rate. The formation of insoluble green copper carbonate hydroxide (malachite) can be predicted on a copper‐alloy object buried in a soil containing carbonate ions. If conditions are such that a uniform layer of malachite is formed over the whole surface of the object corrosion rates would be low as the metal underneath is protected from further corrosion. However, numerous factors influence the corrosion of metal objects in the soil including access to oxygen, changes in water level and pollutants (Carvalho et al. 2022a). One factor influences many others with all factors feeding back into the corrosion environment. If conditions are such that a corrosion product does not precipitate on the metal surface, or if the corrosion product is soluble or unstable or lacks uniformity, corrosion rates would be faster.
how does copper typically corrode
layers of corrosion, the outer being blue green corrosion, copper carbonated sulphates and chlorides. under this is powdery red or dark compact cu(I) oxide, normally cuprite, close to the surface which retains the metal’s shape.
What are the most common Cu (II) compounds found on copper objects, and when are they formed
malachite and azurite (both copper II carbonate hydroxide) . malachite formed with contact with groundwater rich in carbon dioxide. azurite in drier conditions- see copper objects from Pompeii.
Tell me about bronze disease.
Happens when chloride ions are present, and the inner corrosion layer contains nantokite (grey, waxy green) which converts to hydrochloride acid and cuprite in presence of moisture and oxygen. this is a process that is self starting and the acid attacks the metal.
what’s cuprite
Cuprite is an oxide mineral composed of copper (I) oxide Cu 2 O. red and common
features of corrosion layers of copper-tin alloys
tin oxides are more stable, less soluble than copper carbonates and are present at higher concentrations in the outer corrosion layers, this is problematic for alloy composition analysis
features of corrosion of copper-zinc alloys
the high solubility of zinc corrosion products (white), leaves behind a surface enriched in copper
Corrosion of copper in a marine environment
- Cu (I) oxide is toxic to marine creatures, inhibiting them from colonising the surface, until this is breached or converted into Cu (II) products
- under anaerobic conditions, precipitation of copper sulphides over the surface
- marine growth is supported by oxygen supply
- if there’s a proximity of a less noble metal (iron), forms an electrochemical cell, increasing the PH, facilitates a calcium carbonate concretion over the surface.
copper corrosion in waterlogged soils and underwater
- sulcate-reducing bacteria produce hydrogen sulfide/ sulphur.
-one these objects, can identify copper sulphides (chalocite) and organic molecules produced by bacteria (polyaromatic compounds and medium chain organic acids)
‘tentacle like’ corrosion morphology
when might I find iron-containing copper sulphides on copper alloy objects.
objects recovered from anaerobic soils. chalcopyrite is golden yellow, and looks a bit like gilding, could either look deliberate or a result of a microbiological attack
