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Re: Recovery, Storage/Stabilisation and Cleaning (long)

Posted by Elliott on Fri Mar 21, 2003 09:55:16 AM

In reply top Recovery, Storage/Stabilisation and Cleaning (long) posted by Ross McNeill on Thu Mar 20, 2003 12:25:47 PM

Hi ross

Do you have any info on aluminium alloys? Sodium hydroxide should never be used on aluminium!

From my experience ammonium sulphate is the best thing to use on an alloy such as duralium.






: Hi Cees,
:
: The following detail is culled from some lecture notes for
: a Marine Conservation course.
:
: The process splits into recovery, storage/stabilisation an
: d 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 tran
: sported in a flooded condition. Any encrustation should be
: left in place until cleaning as this is an inhibitor of f
: urther corrosion.
:
: Storage/Stabilisation
: As the item has been exposed to oxygenated water it is imm
: ediately subject to anerobic corrosion from Sulphate reduc
: ing bacteria. These produce the greatest increase in degen
: eration of the item. Storage needs to be in a fluid that d
: iminishes or prevents the corrosion of metals. The two mos
: t commonly used fluids are Alkaline inhibitive solutions o
: r inhibitive solutions containing oxidizing agents.
:
: Alkaline Inhibitive Solutions (used for short term i.e. up
: to 6 months)
: The most common alkaline inhibitors used in conservation a
: re sodium hydroxide, sodium carbonate, and sodium sesquica
: rbonate. 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 ma
: kes the iron chemically inactive through the formation of
: an oxide film on the metal. In general, most ferrous metal
: s 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 oxy
: gen will increase the rate of deterioration; the corrosion
: will be localized and the attack will be even more intens
: e than if no inhibitor had been used. Best pH is between 1
: 0 and 13.
:
: A 5 percent sodium carbonate (pH 11.5) or a 5 percent sodi
: um sesquicarbonate (pH 9.7) storage solution is sufficient
: for most ferrous objects if chlorides are not abundant. A
: t high chloride concentrations, prolonged storage in eithe
: r of these two solutions is not advisable unless the solut
: ion is changed often.
:
: Oxidizing Solution (used for long term)
: The decision to use this method needs to be carefully cons
: idered with the cost of disposal of the solution.
:
: Various chromate compounds, such as potassium chromate, po
: tassium dichromate, and sodium chromate make effective sto
: rage mixtures. They are more reliable than alkaline inhibi
: tors as long as the concentration and pH are maintained at
: safe levels. Chromate solutions prevent corrosion by form
: ing a very thin passivating film of ferric oxide and chrom
: ic oxide on the surface of the metal. The natural alkalini
: ty of chromate (i.e., pH 9.1 to 9.3) is an important facto
: r in passivating iron. Dichromates (pH <7) are more acidic
: than chromates and will not passivate iron unless an alka
: li is added. The addition of alkali, NaOH, converts dichro
: mates to chromates and establishes the natural pH of chrom
: ate.
:
: 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 encrustati
: on but care should be taken to leave a small film on the s
: urface 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 hav
: e leached into the metal structure.
:
: Once split into the separate metals then the items need ve
: ry 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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