Some materials which are contaminated react rapidly in the atmosphere. They are subject to active corrosion. Copper and iron based artefacts permeated by chlorides are particularly sensitive to this form of corrosion; as are lead objects exposed to corrosive acid vapours (e.g. acetic and formic acids).

Dechlorination of copper based artefacts

This treatment is carried out in an aqueous solution of sodium sesquicarbonate (1% by weight) (pH=10). Due to the complex nature of the formed corrosion products, the monitoring of the dechlorination process using plots of Ecorr versus time is difficult to interpret. As indicated in the picture below (right), the treatment solution turns blue.

The chloride compounds (atacamite, paratacamite and nantokite) one aims to remove have reduction potentials which are much less negative than for cuprite and copper sulphates and sulphides. The cathodic linear voltammetries (below) carried out in sodium sesquicarbonate solution show how the presence of nantokite is changing with time during immersion. It is eliminated after a few days.

Dechlorination of iron based artefacts

As indicated in the section “Application in Conservation-restauration: cleaning treatments”, the stabilization of chloride contaminated iron artefacts is carried out at -800mV/SHE in an electrolyte of aqueous KOH (1% by weight). This treatment was applied to the series of ingots below.

Reductive consolidation of lead artefacts

Lead artefacts which were exposed to corrosive vapours (mainly acetic acid) emitted by oak extensively used for storage in the 19th century are subject to an active form of corrosion. It provokes the formation of thick and powdery carbonates of lead which displace the original surface of the metal. Handling of these artefacts causes disintegration of the layers of corrosion products (see image below).

These materials can be stabilized through the reduction of the layers of lead carbonates. This consolidative reduction prevents the disintegration of the metal surface. The treatment is carried out in 0.5M aqueous sodium sulphate (Na2SO4) at -660mV/SHE with a  lead plate counter-electrode. It is monitored by chrono-amperometry; not versus time but the log of time. Indeed, following a sharp decrease the current decreases slowly (as shown on the plot below). The progress of this slow decrease can only be monitored using the log function (Degrigny et Le Gall 1999).

Cathodic chrono-amperometry at -660mV/SHE versus the log of time. After a sharp decrease the reduction current decreases very slowly and becomes constant when the whole layer of carbonates of lead is reduced.

Stabilisation of aluminium based artefacts

Aluminium based objects recovered from underwater environments are also subject to active corrosion. The materials can be polarized cathodically to extract chlorides, but cathodic corrosion might develop. This specific form of corrosion is due to the local alkalization of the metal surface during polarization which initially provokes the dissolution of the protective aluminium hydroxide film. The removal of metal inclusions occurs and eventually pits evolve. The polarization is carried out in a neutral electrolyte (buffered sodium citrate, pH=5.4) at -860mV/SHE with a mesh stainless steel 316 counter-electrode (Degrigny 1995). If the aluminium elements are attached to encrusted iron based elements, stabilization is preceded by a cathodic polarization in 0.04M sodium metasilicate  (pH=12.4); a corrosion inhibitor for aluminium alloys. Therefore the crust covering the iron parts can be safely removed using hydrogen bubbling without damaging the aluminium parts.