as these will corrode the metal at an alarming rate. A pH in the range 4–8 is
required to form a passive film on aluminium.
Sodium hydroxide is a very common solution used for ferrous artefacts. A
concentration of 0.5 M will give a pH of approximately 10.5 that is well in the
passivity region for this class of materials. The problem is that if any ferrous or
ferric chloride type compounds are present within the corrosion products, these
may react with the hydroxide to produce solid sodium chloride within the pores
in the rust film (Equation (12)).
(12)The iron oxide precipitates within the rust film and traps the sodium chloride in
the corrosion products. This is a serious problem when using a combination of
aqueous and thermal methods of conservation as will be discussed later.
Sodium carbonate is another widely used solution for the conservation of
ferrous artefacts. This maintains the pH in the zone of passivity (pH9.5) and
is also said to act as an anodic inhibitor. A concentration of 3 g dm^3 (0.2 M) was
used in the conservation of Holland 1in Gosport Submarine Museum. It is
advisable to use the sesquicarbonate form as it is far easier to dissolve than the
simple carbonate. The use of this solution is said to avoid formation of solid
sodium chloride in the rust.
The treatment of small ferrous artefacts in a 0.05 M lithium hydroxide dis-
solved in methanol or ethanol has its advocates, particularly in France. The
chlorides present in the rust layers react with lithium hydroxide to form
lithium chloride that dissolves in the alcohol phase. Any of the hydroxide left on
the metal surface combines with any carbon dioxide to form a solution with pH
above 9.5, which maintains any exposed metal in the passive region. Hence, this
solution is claimed to cause no corrosion of the underlying metal. The real
disadvantage of this solution is that any lithium chloride left on the surface of
the artefact is very hygroscopic. Water will form on the surface at a relative
humidity above 15% RH and corrosion of the metal will take place. Humidity
levels below 15% RH are very difficult to maintain in display cabinets or in
storage and is one of the main reasons why this solution has not been more
widely employed.
Alkaline sulfite reduction was developed in an attempt to reduce the time to
fully conserve the artefact. The pH of the solution is maintained in the passive
region by the use of sodium hydroxide (0.5 M) while at the same time, the
sodium sulfite (0.5 M) in the mixture will slowly reduce the red rust (FeOOH)
to magnetite (Fe 3 O 4 ).
32 FeO OH⋅ He Fe O 34 H 2 O (13)FeCl 33 33 NaOH Fe OH() NaClMetals 145