Iron and Alloys
Iron is the most useful and abundant metal and would probably be the most common metal type found in collections. It has been known from prehistoric times and in its various forms, such as cast iron, wrought iron and various steels, is the element upon which our present industrialised civilisation has been built.
In the presence of oxygen and moisture iron and steel (except some stainless steels) corrode to form rust. Rust is a term used to describe non-specific corrosion products which form on the surface of degraded iron (Figure 4). It is a mixture of ferrous and ferric hydroxides and hydrated ferric oxide. Iron chlorides usually form in the presence of aggressive salts such as sea water.
Unlike copper, the surface layers of iron corrosion products are not protective and tend to accelerate corrosion of the metal by forming localised corrosion cells.
When an object is first acquired, examine it to determine the extent of deterioration and to ascertain whether the corrosion is still active. If the surface is covered with brown droplets, active corrosion is still occurring and the presence of salt is indicated. This necessitates a specialised conservation treatment which involves the removal of chloride ions.
Although the general guidelines outlined in the introductory section of this chapter also apply to iron and its alloys, a few additional points need to be made. As iron is one of the most reactive of the commonly used metals, effective control of the storage environment is essential to ensure objects made from iron do not continue to deteriorate. To minimise corrosion the preferred relative humidity for storage and display of iron objects is less than 35 %. As noted earlier in this chapter however, the presence of particular corrosion products will affect recommended relative humidity levels for iron objects. Relative humidity levels of 15 % or less must be maintained to prevent corrosion of chloride-containing iron and levels of 12 % or less are necessary to prevent corrosion of iron that is in contact with mixtures of the corrosion products Akageneite (β-FeOOH) and hydrated ferrous chloride (FeCl2.4H2O, Watkinson and Lewis 2004). Details of methods used to control relative humidity are described elsewhere (see the chapter Preventive Conservation: Agents of Decay).
Once an object has been treated and coated, correct storage or display conditions and careful monitoring will maintain its stability.
If it is not possible to house large objects such as machinery and vehicles (horse-drawn and motor) in controlled environments at least provide some protection from the elements. This protection may be in the form of a shed, a veranda or even a lean-to. Unless some protection is provided moisture and dust accumulation will soon initiate deterioration processes.
If an object is displayed in the open, raise and support it above ground level, monitor it regularly for signs of deterioration and treat as necessary.
Clean and re-oil metal components frequently, particularly those of firearms (see the chapter Case Studies) and keep them in a protective environment if possible.
Coat highly polished metal surfaces that are not protected by a clear lacquer with a light machine oil or spray them periodically with a commercial product like CRC or similar. As long as they are stored in a dust-free environment this is a simple and effective means of preventing deterioration.
Although many objects may be covered with thick scales of rust there is often sound metal underneath. Do not clean an object that has very little metal remaining, but instead store it in a plastic bag containing silica gel (desiccant) to keep it dry. Note the following points:
- self-indicating silica gel is orange when active, changing to either colourless, pale yellow or green when no longer acting as a desiccant (colour change depends on the type of silica gel used);
- replace the desiccant when it changes colour;
- regenerate the activity of the silica gel by heating it in an oven (105 - 110 °C) until the orange colour returns; and
- before storing it, totally dry the object. If appropriate this may be done by placing it in an oven at 110 °C for three to four hours. Alternatively a brush down or soak in acetone or methylated spirits will assist moisture removal. The effectiveness of the silica gel is improved by removing moisture from fissures deep within the metal.
The future role of the object, either display or storage, will affect the choice of treatment method. If the purpose is to display an object in its working mode perhaps no treatment may be necessary other than maintenance, to keep it in a dry environment and/or coat it with an appropriate protective layer.
As with every metal type there is a range of treatment options available, with the final decision depending on the balance between aesthetics, economics and the proposed way in which the object will be used (for example, static display or as a functioning component).
Dirt, grease and loose or flaking rust must be removed before protective coatings can be applied to iron objects. Such deposits can be removed by chemical, mechanical and/or thermal techniques.
Chemical cleaning techniques include:
- using detergent solutions to dissolve grease and remove surface dirt;
- immersing the object in an aqueous alkaline solution (caustic soda) to remove grease and paint. Concentrations in the range of 20 - 40 g sodium hydroxide per litre of water are normally used for this purpose; and
- stripping corrosion products by immersion in a solution of citric acid (50 g) in water (1 L). Do not use this solution on spring steel however. Do not use acids such as hydrochloric and phosphoric acids as they attack the underlying metal.
As the effect of some mechanical and thermal cleaning techniques can be severe, be careful when using this option, especially with small or fragile objects. Techniques which may be applied include:
- wire brushing – either manual or rotary power driven, depending on the condition of the object;
- abrasives or sandblasting; and
- flame cleaning.
Wire brushing is often very effective in removing loose or flaking rust. As wire brushes are available in a range of bristle materials (steel or brass) and grades (coarse to fine), select the one appropriate for the condition of a particular object. Never use a brass brush on an iron object however as a thin layer of brass can build up on the surface being brushed and this, along with any entrapped shedding bristles, will increase the risk of galvanic corrosion.
Abrasives or sandblasting, which uses a high speed jet of particles, may be applied to either small or large iron and steel objects. It is especially suited to large objects, such as agricultural machinery. This method is quick and produces an excellent surface for long-life coatings. Owing to the associated airborne dust problem sandblasting usually requires approval from local authorities. The use of an enclosed abrasive blasting cabinet for smaller objects will avoid this issue.
An alternative form of sandblasting, wet sandblasting, uses a suspension of sand in water combined with a corrosion inhibitor. This method causes less pollution and is more acceptable to local authorities. In both cases the work should be done by commercial operators with conservators close at hand to monitor the process and ensure damage is avoided. On drying and to prevent flash rusting, apply a protective coating immediately following wet sandblasting.
Flame cleaning involves the use of a blowtorch or an oxyacetylene flame to remove paint and rust quickly and effectively. Apply this technique very carefully as thin section metal is likely to distort rapidly and spring steel will lose its ‘temper’ and ‘spring’ if overheated. Wear eye protection as there is a risk of injury as rust particles fly off rapidly with this method.
All of the techniques described above can be used in conjunction with each other.
A typical cleaning scenario involving a steel object may proceed as follows:
- use a steel wire brush to remove as much loose rust or flaking paint as possible;
- immerse the object in a caustic soda solution to remove grease, old paint and dirt. This may require immersion for some hours. Occasionally remove the object, wash it with water, scrub it and replace it in the caustic solution;
- rinse the object under running water;
- transfer the object to a tub containing citric acid. Dissolution of rust may take several hours. Periodically remove, inspect and brush the object;
- fully immerse the object in the citric solution to prevent the development of ‘tide lines’ on the object as these are difficult to remove, often requiring extensive polishing;
- when the rust has dissolved, place the object in a caustic soda solution for a few minutes. This neutralises excess citric acid which would otherwise cause further rusting;
- thoroughly rinse the object with water and then immediately rinse it with acetone, methylated spirits or dewatering fluid to prevent further rusting. Brush these liquids onto the surface and allow the object to dry; and
- apply a protective surface coat to the object.
While citric acid is relatively safe to use on most objects, do not leave cast iron, cast steel and spring steel, or combinations of these metals unattended for long periods in a citric acid bath as they will corrode. Bubbles rising to the surface of the bath indicate that the surfaces of these iron materials are corroding. In the case of harder alloys, this can lead to hydrogen embrittlement and result in pitted, weakened or destroyed objects. Iron objects that are sprung can spontaneously break as they weaken during this corrosion process.
If in doubt regarding the type of iron or steel or the duration of acid treatment required, spend more time removing corrosion products by mechanical means. Once the worst deposits are removed, a short treatment in citric acid should clean the object with reduced risk of damage. Some spots of rust may remain on the object despite citric acid treatment. These can be picked off mechanically.
Large Steel Objects
In many cases it is impossible to find containers or tubs large enough to immerse an object for caustic or citric treatment. In such situations the acid or alkaline stripping solution can be applied to the surface by using a bentonite paste. Higher concentrations of chemicals are needed when a paste is used.
Bentonite paste is prepared and applied in the following way:
- prepare solutions of caustic soda (80 g/L of water) and citric acid (100 g/L of water);
- sprinkle and mix enough bentonite powder into each of the prepared solutions to make spreadable pastes;
- apply the pastes directly to the area to be treated in the order described immediately above;
- if the surface is not smooth, place a water-dampened tissue over the treatment area first and then apply the paste. This will minimise ‘clogging’ of surface indentations;
- if possible cover the paste layer with plastic film to prevent it drying out. Repeated applications of the paste may be required;
- remove the paste by hosing the surface with water and scrubbing with a bristle brush; and
- dry the object as described previously.
Avoid breathing caustic vapours and wear eye and skin protection when preparing and using caustic soda solutions. If large quantities of caustic solutions are used they must be disposed of according to government regulations.
Consult a conservator for advice before treatment as composite objects made up of different metals or metal and organic components require specialised treatments. For instance, if an object is made up of iron and aluminium or iron and zinc, do not clean it in a caustic solution as that solution will react with the aluminium and the zinc.
Likewise treatment of a composite of iron and brass in a citric acid solution will result in the copper from the brass corrosion products ‘plating’ out on the iron and accelerating the iron corrosion. Suspension and partial immersion of the iron portion only in the citric bath, or the localised use of bentonite paste (see above), may allow such a problem to be overcome. The selective application of a protective layer of wax may also be used to isolate and protect the surfaces that would otherwise be damaged by the citric acid solution.
Bentonite paste treatment is recommended if solder joints or related fastenings are present as these are also attacked readily by citric acid.
Iron may be plated with other metals including zinc (galvanised iron), tin, copper, chromium or nickel. These coatings protect the base iron sheet from corroding and also provide a bright surface finish. Corrosion usually occurs when the surface plate breaks down, exposing the iron which in turn begins to rust.
To remove rust, a citric acid solution containing an inhibitor can be used. The inhibitor is included to prevent any attack on the plating metal. Before the solution described below is used, test it on an inconspicuous area of the object or on a scrap piece of the same material. Prepare and apply the solution as follows:
- dissolve citric acid (50 g) and thiourea (10 g) in water (1 L);
- immerse the object in the solution, periodically remove it from the solution and brush it gently with a soft bristled brush;
- when the rust has been removed rinse the object thoroughly with fresh water followed by immediate rinses with either acetone, methylated spirits or dewatering fluid; and
- clean the plating metal with methylated spirits or, if there are rust stains present, with a mild abrasive such as pumice powder in methylated spirits.
After a final cleaning, if a bright surface finish is required, apply a proprietary metal cleaner as a once-only polish. The artefact may then be coated with a clear lacquer.
There are many methods available to give the object the ‘right’ colour and protective coating. The type of finish chosen will depend on the intended role of the finished object, with the final decision being a balance between aesthetic, practical and personal considerations. The most commonly used techniques include:
- tannic acid-based rust converters;
- fish oil;
- oil quenching;
- clear lacquers;
- microcrystalline wax;
- flame colouring; and
- natural patina.
Tannic acid-based rust converters are commercial products which can be applied to an object that has been cleaned either chemically, mechanically or which still has light rusting present on its surface. This product initially turns the object a brown-black colour. Subsequent applications darken it further until it is totally black. This coating system results in the formation of stable iron tannates which passivate the metal and protect it from further corrosion. Do not use this treatment on iron objects which will be displayed outdoors unless an additional protective coating is applied. To maintain the black appearance, tannic acid-based rust converters can be sealed with clear acrylic lacquers (see below for details).
A mixture of 80 parts white spirit to 20 parts fish oil can be applied to freshly cleaned iron objects with great effect. Thinning of the mixture with white spirit allows it to soak into micro-fissures and any light rust on the steel. It usually dries within minutes. Several coats can be applied and when dry the object can be painted if required. This mixture does not change the metal colour and gives effective protection.
Oil quenching is an old blacksmith’s method which provides effective protection from rust. The end result is a deep blue-black object. This method works best on low carbon steel as flaking occurs in small patches on high carbon or alloy steels. Wear protective clothing and eye protection to avoid injury. Steps in this method are as follows:
- heat the iron object to a dull red colour using either an oxyacetylene torch or a forge and then plunge it into old, dirty engine oil (the dirtier the oil, the blacker the final colour);
- agitate the object for 30 - 60 seconds (depending on size), remove it from the oil and then wipe it with a rag; and
- repeated applications of this method will darken the object further.
‘Blueing’ is a method that has been applied to many types of firearms, especially their barrels, to produce a lustrous dark blue finish. Although this is usually done by commercial gunsmiths, a ‘blueing’ paste is available from gunsmiths and is easily applied. Take care when handling ‘blued’ objects as acids from skin contact etch these surfaces.
There are numerous paints available, both enamel and water-based, that protect and beautify metal surfaces. A range of primers, undercoats and topcoats may be used. If applied correctly to properly prepared and cleaned surfaces adequate protection should be maintained for many years. For iron objects displayed outdoors, particularly in aggressive marine environments, use an inorganic zinc primer, a high build epoxy top coat and a final clear polyurethane coating with a UV-absorbing agent in the paint.
Clear lacquers are available in spray cans or can be applied by brush. Obtain the desired surface finish and colour before applying the lacquer according to the instructions on the product. Provided the entire object is coated, this finish gives a lasting protection against oxide build-up. If air or moisture penetrate beneath this layer however, lifting will occur and oxidation will recommence.
Vinyl co-polymer based Senson Ferroguard FS (Full Spectrum) with vapour phase corrosion inhibitor is also an appropriate sealer for bare metals but being a less durable coating is only suitable for objects that are infrequently handled such as those in museum collections.
Microcrystalline wax provides both thorough protection and an attractive finish to iron objects. Its preparation and application are described below:
- mix the wax with white spirit until it forms a smooth paste;
- apply using a soft rag; and
- allow the wax to harden and then gently buff the surface to the desired finish.
Microcrystalline waxes are available commercially for a range of metals and timbers, each formulated for a particular use. Use white spirit to remove the wax at any time.
Flame colouring involves alteration of the colour of iron and steel by applying direct heat from a forge or an oxyacetylene flame. After cleaning and de-rusting, apply a gentle flame to the object. The colour will change from light straw through to deep blue. When the object has attained the desired colour, plunge it into water. Note that this method can change the molecular structure of the steel depending on the grade and its carbon content. As the temper or spring will be lost if flame coloured, do not use this method on spring steel.
If an iron object is in a stable condition, with only a lightly rusted surface, it may be that this natural patina is the type of finish you require to demonstrate the history and past use of the object. Such finishes can be maintained if the storage and display conditions are controlled to prevent further corrosion.