Friday, June 17, 2016
A paper given as part of the Aluminium Conference in Washington DC in 2014 (pasted as text without diagrams as a draft)
Metallic Heritage Consultant
Aluminium alloys have vastly different metallographic structures and material properties depending on composition and heat treatment regimens therefore they require specific preventive conservation approaches. One approach will not cover all alloys.
Reviews of current museum approaches to preventive conservation of aluminium alloys both published and unpublished are discussed along with industrial and military approaches. Methods considered include desert storage, dehumidification, the use of Volatile Corrosion Inhibitors (VCI) and Water Displacing Corrosion Preventatives (WDCP's).
Deterioration of objects in storage and on display are discussed with recommendations for best practice storage, environmental standards based on some highly successful and examples of catastrophic examples. Occupational health and safety hazards in relation to aluminium are addressed.
In terms of volume of production, aluminium is the world's leading nonferrous metal. Because of its lightness and unique physical and chemical properties, its suitability for the construction of aircraft was recognised early last century. As a construction material it was essential to have stable and resistant surface finishes. Surface finishing techniques developed rapidly in both Europe and the US. So much so that it is now thought of as an extremely stable material but the passivating oxide film can breakdown in a wide variety of circumstances leading to rapid deterioration.
The physical, chemical, electrochemical and mechanical properties of aluminium and its alloys all vary in accordance with composition, heat treatments and environmental exposure factors. With more than 100 variations in the composition and heat treatments there are a large number of preventive conservation options that need to be exercised.
An electronic search on the preventive conservation of aluminium in a museological context is dominated by the making of preventive conservation supports for historic works (Mazzolani,2009). The literature on the preventive conservation of Aluminium heritage objects is scant and covers mainly sculpture, built heritage and Large Technological objects (LTO'S).
Technology preservation -- has always followed engineers and technical practice. It is only really recently - in the last two to three decades that conservation practice has made an indent in the treatments and approaches of technological object conservation.
Back in the 1970's the preservation of aircraft stored out of doors was a concern of National Aeronautical and Space Museum (NASM) and curator Bob Mikesh (Robert C. Mikesh ,1971)(R. C. Mikesh, 1989)( R. Mikesh, 1997). By the 1980's the conservation of technology was well established as a growing theme in materials conservation methodologies were not well understood. MacLeod, Degrigny and Bailey (MacLeod 1983) (Degrigny 1993), (Bailey, 2004) all worked on the problems of aircraft recovered from the sea. Hallam, Adams (Adams and Hallam, 1993) and Ashton (Ashton and Hallam 1990) were looking at the original surfaces and approaches to technological conservation. Work on preventive approaches for aircraft continues today with the work of Schwarz (Schwarz and Fix, 2011) and Macleod (MacLeod, 2006). Michael Brunott, Ainslie Greiner, David Hallam, David Thurrowgood, worked on the Conservation Maintenance Programs for Functional Objects and the problems of corrosion in breaking system (Hedditch et al., 2011). Otieno-Alego, Hallam and Creagh did a lot of work on proving waxy and commercial coatings were useful for the conservation of technology items (Otieno-Alego et al., 1998) (Otieno-Alego et al., 1998) (Otieno-Alego et al., 1997).
With the development of aluminium and its alloys as a construction material, the need arose for stable and resistant surface finishes and techniques developed rapidly in both Europe and the US. These finishes can offer important historical and technical information. As intrinsic parts of the object, every effort should be made to devise conservation strategies to retain them. It is only through an understanding of the construction of an object and its deterioration that appropriate treatments can be developed. Aircraft are made up of a body that will be predominately one form of aluminium alloy. (e.g. in an US WW2 aircraft 2024 T3 alloy skin and 7075 alloy parts, in a Japanese aircraft of the same period it will be a SD and ESD alloy, ditto French or Australian). . For US WW2 aircraft they a thin laminate of pure aluminium called Alcad.
In the case of a WW2 binocular from the Japanese it will be copper plate then a "crinkle coat" nitrocellulose black paint. On a British seaplane it would be a synthetically grown aluminium oxide (anodising in chromic acid) and a good dose of zinc chromate paint. A German plane of the same period would have clad sheeting, anodised aluminium and also an alkaline chromate conversion coating called the MBV coating. Zinc chromate paint will also be present in a nitrocellulose or acrylic base.In all case paints and primers also will also cover the surface.
Modern examples are the MacBook computer surface made by chemical etching and abrasive blasting - to give a mat finish - then bright anodising - a technique it shares with the bright-work on a 1970 Volvo wagon.
Insert figure 1
Figure 1 Bright anodized aluminium a 1970 Volvo Wagon
These surfaces are complex and need to be understood before any preventive treatments are even thought about.
Surfaces are what makes up your object special and the nature of these oxide coatings has been previously reported (Adams and Hallam, 1993)
Aluminium was used for the dome at the San Gioacchino Church in Rome in 1897 The alloy 98.3% Al with Fe and Si has proved reliable with minimal corrosion over the last 100 years was the parent of the 1000 series alloys using 99.5% pure aluminium.But when aluminium fails it can be quite spectacular. I remember showing a visitor a Zero aircraft with ESD (extra Special Duralumin) spar caps. The visitor commented that it was amazing that they were still using wood in the structure having totally mistaken the corroded metal. The ESD had severe inter granular corrosion and looked just like bleached degraded wood. United States WW2 aircraft deteriorate in the same way as the figure (2) illustrates.
Lightning main spar showing spar cap delamination where it was immersed in a swamp
Aluminium alloys have failure mechanisms that include pitting, galvanic, filiform corrosion while for the 2000-7000 age-hardenable alloys exfoliation and stress corrosion are major factors. (Vargel, 2004);
Aluminium alloys have corrosion rates that are minimal till the RH% gets to be over 60% (Dean and Rhea E. C. 1982). Pollutants such as SO2, NH3, Cl increase the corrosion rate and promote pitting and crevice corrosion while reducing the threshold at which corrosion rates start to increase. When exposed to very high SO2 concentrations and relative humidity above 50%, most aluminium alloys will corrode rapidly, forming a hydrated aluminium sulphate.Dust deposits attract moisture and pollutants resulting in increased corrosion rates. In some cases this may lead to pitting of upper surfaces of objects in unprotected storage.
Insert figure 3
Internal Paint failure in a Dornier Seaplane
Water staining is also possible due to the thermal conductivity of aluminium promoting condensation on objects. Water build up in void spaces is also due to condensation with resulting high corrosion rates.
In aqueous solutions the stability domain of the oxide coatings that passivate aluminium surfaces with 8.4
4 (Pourbaix, 1966) (Deltombe and Pourbaix, 1958) for the metal
Insert figure 4
Inter-granular corrosion caused by chloride, high RH and the alloys type and heat treatment.
Mechanics and engineers, architects and artists, corrosion scientists and craftspeople that use aluminium will all approach it's preservation from differing perspectives. Their approaches will not be the same as a heritage preservation specialist or conservator. Having seen some of the failures in industry they will tend to take a non-preventative approach. Their "Corrosion is cancer" or the pragmatic "just blast clean it" approach will not win unified support with conservators. Neither will the "cotton bud" approach of conservators win them over.
A logical decision making approach is needed to the objects preservation needs.
A significance assessment, a document outlining what is important about the object, to whom, and why, in accordance with the methodology outlined in Significance 2 (Russell and Winkworth, 2010) is the key element of a preservation strategy. Such statements outline what about the object is significant and needs to be preserved and provide a rational basis for retention of objects in a collection in a vast array of conditions. It is a mechanism for reconciliation of materials..
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Figure 5 A Wankel engined generator after conservation treatment and inhibition note the surface tells the story of the use in the red sands of the Australian outback not the technological mastery of the Wankel engine
Any minimal gain in stability and preservation of original materials would be at the expense of the objects significance as a focal point of a community and continuing cultural tradition ( Wain, 2012). Aluminium objects are no different.
Insert figure 6
Figure 6 close up of the adhered red dirt on the fuel tank paint. Note the weld in the tank too adding to the surface information.
As we can see from above decisions about treating museum objects are not based purely only on their physical state or the science of preservation, many other factors come into play in the decision making process. Each object has a life and things that have happened to it over that life can be read from the object. There may be connections or evidence of important historic events, cultural or religious practices or ownership or use by an important national figure.
Insert figure 7
Figure 7 After Humphreys - conservation decision-making zone.
Conservation treatments have to be carefully crafted so that the evidence of the objectâ€™s “life” is not damaged. This requires an understanding of the values that make the object significant and a decision about what stage in the object’s life will be represented after work is complete. Significance and value are the primary determinates of the treatment approaches to be taken with an object or collection of objects.
Objects with lubrication, hydraulic and cooling systems corrode and deteriorate if they are stored without maintenance and monitoring program that circulates and changes inhibitors. This was exemplified by the studies carried out by Heath et al on a Lancaster Bombers coolant system (G. A. Heath et al., 2002). Objects with void spaces and lap joints faire a little better in static storage but will still corrode where moisture and debris can build up. The risk of corrosion detrimentally affecting the significance of the objects is very high.
Looking at the objects under your care are the best methods of accessing the risks of deterioration.
Monitoring methods for tracking corrosion and deterioration objects can be divided into the following types;Sensual, Visual, Physical analysis,and Chemical analysis
The interpretation of these variables is often subjective! Typically an older traditionally trained mechanic or engineer will use all of these to access the state of an object and its ability to perform its original function. Smell, taste, feel, sound and look are all critical in the engineers' decision-making. Rather than measure, the pH of a fluid the traditionalist will access the acidity much as a traditional winemaker would – smell and taste.
Conservators do not necessarily have the skills (or desire) to emulate this so have to rely on a quantitative approach to assessment of condition or change in condition.
Fluid analysis has been used as a tool in the monitoring and failure analysis of operating LTO's for decades (Myshkin et al., 2003). This has mainly been with high value installations or with LTO that will cause a high loss of life if they should fail (e.g. aircraft). Detection of changes in corrosion or wear before its effects can be seen or heard means that remedial action can avert tragedy and the high value asset can be repaired and put back into service. XRF using a portable XRF unit is useful; for tracking alloying elements in oils , brake fluids and coolants but as it is a bulk monitoring it only give an indication of what is happening in the thin films and crevasses and often results are visible only after substantial damage has already happened.
Measurement of changes in structural dimensions are one way of monitoring the state of large externally displayed objects and are currently being used with success in Hong Kong (Tse et al. ,2011) and Fremantle on (dry docked) ship and a submarine.
Risk assessment in industry is a well managed process (Fujiyama et al., 2004). Getting conservators to place formal risk assessement into a practical framework is challenging however, by multiplying the risk by a fractional consequence factor a hierarchy of needs is rapidly developed (Hallam et al, 2010)
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Figure 8 In this example storage and limited use is the best preventive outcome.
The effect of environment on the materials of construction is critical to the long term preservation of objects. Having effective methods for assessing the environment of storage and display would mean this data could be used to predict the maintenance requirements of the objects and predict rates of decay and lifetimes in a meaningful way (Ryhl-Svendsen, 2008). This kind of approach has been used in the Defence forces to predict maintenance requirements of the fleet and predict budgets.
Cleanliness is critical with aluminium since defects in the protective oxide film will respond to differential aeration microenvironments created by grease, dirt and dust. In addition the different porosity of coatings and their variable permittivity to oxygen migration will seriously affect the long term performance of the underlying metal It is essential to avoid alkaline solutions since these will dissolve significant parts of the oxide film which contains all the historical information about the object. Similarly it is vital not to use strong mineral acids to clean the metal. Use of non-ionic detergents such as Lissipol The metals finishing and inhibitor companies like Turco, Cortec or Ardrox will have a product that can do exactly what you want in the manner you wish to.
Removal of dust and light grim with microfiber cloths is very effective.
Insert figure 9
Figure 9 Reducing time of wetness by reducing pooling of water.
Reducing water accumulation and time of wetness on objects will reduce corrosion rates by several orders of magnitude (Vernon, 1927) In the simplest case it could mean making sure water does not pool in, on or around the object . (Fig 10) or moving the object into a covered area or placing it in a appropriate storage area.
Insert figure 10
Figure 10 Moving objects indoors significantly reduces corrosion rates.
Waxes and acrylics are not effective as a "maintenance” coating. For large objects in the less than ideal storage conditions, such as commercial aircraft, the best approach is to use Water Displacing Corrosion Preventatives (WDCP) (Wilson and Devereux 1984). These coatings can be used in museums as part of a cyclical maintenance program (Hallam et al. 2004). Likewise they could also be used on internal void spaces of sculptures and other objects where condensation is a risk. With lacquers we need to be aware that aluminium will undergo filiform corrosion under them if no inhibitor is present and pollution residues are present on the metal surface. Insufficent film thickness is a critical factor in promoting this form of corrosion.
Placing collections into long term storage has been practiced by the Military and their techniques of outdoor storage can be emulated with success (Mikesh 1989) Techniques involving dehumidification have been well documented (Munters ,1984) (Cargoaire, 1988)(Turner, 2005)
Wrapping with plastic shrink wrap to create sealed environments as well as the use of , inhibitors coating and washing (Gelner, 1998) (Miksic, Johnson, and Martin, 1984)
None of these strategies is a panacea as they all need to be accessed for periodic inspections and this will often seriously compromise the protective systems.
Products and Procedures
Finding a successful preventive practice model or suite of options involves a critical assessment of the relevance of commercial products and trade practices that have been rigorously assessed in conservation laboratories for their relevance and effectiveness (figure11)
Insert figure 11
Figure 11 Produce a collaborative workspace.
Maintenance of operating functional objects in safe condition is essential to avoid the serious consequences of incorrect storage. Works by Weil and Hallam et. al. should be consulted for details of practice and procedures that need to be followed, particularly for LTO’s. Maintenance Framework
Experience at the National Museum of Australia (NMA) has developed new procedures that assist museum collection managers to develop scheduled monitored maintenance and maintain full functionality preservation as cost effective approaches to care of technology collections. This approach targets conservation for the most important, most used and most vulnerable and demonstrates the consequences of no response. Details of the methodology can be found on the NMA website and in recent publications in the ICOM CC Metals Working Group conferences (Hallam in Metal 2010).
Insert figure 12
Figure 12 David Monthan Airforce base
Dinsmore and Lund (1962), ,Erickson.(1954) and Killingray (1986) discuss the problems of the storage of functional objects in a military context for periods of non-operation. Dehumidification and periodic exercise are necessary to maintain functional objects in operable condition in long term storage. David Monthan AFB (Airforce Base) (fig 12) uses natural dehumidification and yearly maintenance on its flying aircraft. Some aircraft have been stored for 20 years prior to being flown out. The Prototype 707 owned by NASM SI was stored under this regime before being flown to Seattle for a rebuild in 1990 and finally to The Dash 80's to Dulles International Airport near Washington, D.C. on August 27, 2003.
Dehumidification has been used in museums for objects such as the optical equipment at the Australian War Memorial in Canberra or tanks in a maintenance storage cycle. A similar approach using the natural dry environments of Canberra has been used to store vehicles (fig 13).
Insert figure 13
Figure 13 Vehicles kept dust free and dry in storage maintenance cycle.
When dealing with materials that were produced in the last century one has to assume the primers used were the best available, hence we can assume that zinc chromates were used extensively in the construction of objects. Chromates will also have been used in the anodizing processes. Cadmium will have been used in any associated ironwork. If we are dealing with aircraft we can assume that radioactive luminous paints will have been part of the instruments and may be redeposited through lower sections of the aircraft structure.
Since PCB’s will exist in the electronics it is essential to take a cautious approach to ensure we understand the hazards and how to deal with them within appropriate legislative backgrounds of your state or country.
We need to be aware of what you are conserving and why, and for whom you are doing this job. It is important to understand what is significant and what are the risks of losing that significance due to degradation? We need to choose our preventive processes to minimize that degradation with minimal intervention. We need to choose and execute our preventive processes in a collaborative manner and unbiased manner. Aluminium alloy artefacts need to be kept clean, dry and maintained. During these processes it is vital to be alert to a wide range of hazards to health associated with the use of past technologies where the implications of long-term exposure to toxic and radioactive materials was poorly understood.
I would like to thank Ian MacLeod for reviewing the paper in its draft form and giving some valuable guidance.
David Hallam has been involved in the conservation of cultural heritage since the mid 1970’s and have specialized in metallic and technological objects conservation. For more than 20 years headed the metallic objects conservation program at the Australian War Memorial and has a practical understanding of World War Two objects and their deterioration.
Since then he was Head of Conservation at Queensland Museum and in 1999 established the objects conservation program at the National Museum of Australia (NMA). In 2012 David Hallam left the public service in 2012 to become a consultant, undertake further study and rebuild a shop in Tasmania.
During his career he has always had a practical hands on approach to preventive conservation programs, treatment and treatment development programs. This approach lead to his appointment as a Woodrow Wilson Fellow of the National Air and Space Museum of the Smithsonian Institution (NASM SI) in 1989 and to several appointments as a visitor or Visiting Fellow at the Research School of Chemistry at ANU.
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