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Recovery, Storage/Stabilisation and Cleaning (long)

Posted by Ross McNeill on Thu Mar 20, 2003 12:25:47 PM

In reply top Ross, salt water preservation again!!!!! posted by Cees Broere on Thu Mar 20, 2003 11:47:17 AM

Hi Cees,

The following detail is culled from some lecture notes for a Marine Conservation course.

The process splits into recovery, storage/stabilisation and finally cleaning.

Recovery
Here the item needs limitation of the length of time it is exposed to air. Ideally it should be wrapped in moisture retaining cloth before it breaks the surface and then transported in a flooded condition. Any encrustation should be left in place until cleaning as this is an inhibitor of further corrosion.

Storage/Stabilisation
As the item has been exposed to oxygenated water it is immediately subject to anerobic corrosion from Sulphate reducing bacteria. These produce the greatest increase in degeneration of the item. Storage needs to be in a fluid that diminishes or prevents the corrosion of metals. The two most commonly used fluids are Alkaline inhibitive solutions or inhibitive solutions containing oxidizing agents.

Alkaline Inhibitive Solutions (used for short term i.e. upto 6 months)
The most common alkaline inhibitors used in conservation are sodium hydroxide, sodium carbonate, and sodium sesquicarbonate. Solutions containing these alkalies will prevent the corrosion of iron in oxygenated water as long as they are in concentrations sufficient to maintain a pH which makes the iron chemically inactive through the formation of an oxide film on the metal. In general, most ferrous metals can be passivated in a chloride-free solution with a pH above 8.
In solutions with a pH of less than 8, the presence of oxygen will increase the rate of deterioration; the corrosion will be localized and the attack will be even more intense than if no inhibitor had been used. Best pH is between 10 and 13.

A 5 percent sodium carbonate (pH 11.5) or a 5 percent sodium sesquicarbonate (pH 9.7) storage solution is sufficient for most ferrous objects if chlorides are not abundant. At high chloride concentrations, prolonged storage in either of these two solutions is not advisable unless the solution is changed often.

Oxidizing Solution (used for long term)
The decision to use this method needs to be carefully considered with the cost of disposal of the solution.

Various chromate compounds, such as potassium chromate, potassium dichromate, and sodium chromate make effective storage mixtures. They are more reliable than alkaline inhibitors as long as the concentration and pH are maintained at safe levels. Chromate solutions prevent corrosion by forming a very thin passivating film of ferric oxide and chromic oxide on the surface of the metal. The natural alkalinity of chromate (i.e., pH 9.1 to 9.3) is an important factor in passivating iron. Dichromates (pH <7) are more acidic than chromates and will not passivate iron unless an alkali is added. The addition of alkali, NaOH, converts dichromates to chromates and establishes the natural pH of chromate.

Cleaning
After a few months of storage/stabilisation the items are now ready for cleaning. This is where the cost is incurred.

Mechanical means are used to remove most of the encrustation but care should be taken to leave a small film on the surface of the metal. This final film will easily come off when either Galvanic or Electrolytic cleaning methods are used. Both of these methods will remove the salts that have leached into the metal structure.

Once split into the separate metals then the items need very specific methods to deal with their conservation.

Enough to be going on with for now, I'll post more detail on the cleaning methods later.

Regards
Ross

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