Protection of cultural heritage by real-time corrosion monitoring

Testing in Real Environments

Introduction. In 2011 and 2012, large internal and end-user testing programmes were conducted in order to assess the performance of the AirCorr monitoring system in real environments and to obtain feedback for the last stage of development. Over 25 loggers with multiple sensors were distributed to conservators and other cultural heritage professionals around the world. The following institutions participated in the testing programme: Swiss National Museum; Centre for Research and Restoration of the Museums of France; National Museum of Denmark; English Heritage, UK; Swiss National Library; Museum of Art History, Austria; Australian War Memorial; St. Fagans: National History Museum, Wales; The Royal Library, Denmark; Czech National Archive; and The Mariners' Museum, USA. The loggers were used to assess old and new storage facilities, to study the effects of supposedly corrosive factors on air corrosivity, in comparative studies of showcases, for ranking of locations and institutions, for problem solving and in fundamental studies on optimal conservation techniques. Several examples of the applications are given below.

Figure 1. Corrosion depth, temperature and relative humidity during transport
and exhibition of a tapestry loaned out by Louvre museum.

Loan of a tapestry. A unique historical tapestry with copper threads from collection of the Louvre Museum was loaned to two galleries in Japan. A logger equipped with a Cu-500nm sensor accompanied it on its journey to monitor the air quality surrounding the tapestry. The recording plotted in Figure 1 reveals that although the relative humidity and temperature were kept close to target levels, the air corrosivity varied with time. Packing, transport to Japan in a crate, exhibition in Gallery 1 and transport to Gallery 2 were associated with only minor corrosion with the cumulative corrosion depth in 3 months reaching 1.8 nm. However, upon opening the transport box and exhibiting the tapestry in Gallery 2, the corrosivity increased significantly. During the 3-month exhibition in Gallery 2, 8.5 nm of copper corroded and the corrosion rate in the first month was 7.6 nm/30 days. Although this still corresponds to IC 2 – Low corrosivity class according to ISO 11844-1, the air quality control was not ideal in Gallery 2. The record shows that aside from higher fluctuations in the relative humidity, a small amount of some pollutant had to be present in the air to accelerate the corrosion rate of copper. In particular, the increase in the corrosion depth starting on 30/06 indicates deterioration of the air quality.

Such information is indeed very valuable for future decisions on loans and the conservation measures required from partner institutions.

Figure 2. Record of silver and copper corrosion depth in an archive building.

Assessment of a new archive building. The corrosion aggressiveness of the atmosphere within a multi-story building made of reinforced concrete which was to be used as an archive for paper documents was determined using silver and copper sensors. The building’s concrete was poured during winter. The upper floors were completed at below-freezing temperatures, so an antifreeze agent containing urea was added to the concrete. Urea decomposes slowly in concrete releasing ammonia to the surrounding environment. Indeed, the air in the top floors has a typical ammonia odour. It is believed that ammonia in the air is not harmful to the documents but might be dangerous for the metallic parts of stored objects made of copper or brass. A stable temperature of 15°C and relative humidity of 52% is maintained in the archive.

An AirCorr I Plus logger equipped with two highly sensitive sensors, silver 50nm and copper 50nm, was installed in the archive. Initially, the logger was placed on a floor built from urea-free concrete with no ammonia odour. Then, it was moved to a floor with a strong ammonia odour. The aim was to test the effect of ammonia-type air pollutants on the air corrosivity. During the exposure in the odour-free area, the measured corrosion depth of 2.1 nm corresponds to the corrosion rate of 27 nm/year for the silver sensor, see Figure 2. The testing period was not long enough to classify the corrosivity according to the ISO 11844-1 standard but extrapolation of the data yields the IC 2 – Low corrosivity class. The corrosion rate identified at the same location for copper was 5 nm/year (Δh of 0.4 nm) corresponding to the IC 1 – Very low classification. After moving the logger onto the floor with the ammonia odour, the corrosion rate of silver dropped to 16 nm/year and remained constant during whole exposure period. In the case of copper, the corrosion rate was 2 nm/year and it slowly decreased.

Surprisingly, the corrosivity on the floor with the ammonia odour was lower than in non-polluted areas. The surface of copper was analysed by XPS after the exposure. The film of corrosion products contained a significant amount (app. 50%) of copper (II) and nitrogen bound up in an organic compound. Since urea is known for its corrosion inhibition toward copper, adsorption of urea from the atmosphere onto the metallic surface might be the reason for the reduction in the corrosion rates of copper and silver during exposure on the upper floor. The study showed that the locations made with the urea-modified concrete were safe for documents with copper and bronze parts, at least in the short term. The effect of this specific environment on other types of stored materials and its long-term effect on copper and copper-alloy materials is under study.

Figure 3. Corrosion depth recorded on a Ag-50nm sensor at three
locations in The Royal Library of Denmark. (ND: Not detected)

Regular air quality monitoring of indoor premises. The Danish Royal Library considered AirCorr loggers as a possible replacement for passive sampling, which is carried out regularly at different locations within the institution. The high cost and delay of at least 2 months between the samplers’ deployment and return of the results are considered drawbacks of the passive sampling method.

Silver and copper sensors at low thicknesses, Ag-50nm and Cu-100nm, were used in an AirCorr I Plus logger. The monitoring was carried out at three locations over a total of 5 months. It started in a small room containing manuscripts with low air exchange, Location 1. Passive sampling revealed elevated concentrations of acetic and formic acids. The logger was then moved to storage with air filtration and climate control, Location 2. Acetic acid was not detected and the concentration of formic acid was very low in the second location. Finally, the monitoring was carried out in a visitor centre with low levels of organic acids but non-negligible outdoor pollution in form of SO2, NO2 and O3, Location 3. Cumulative data for the silver sensor along with the results of the accompanying passive samplers and the relative humidity record are shown in Figure 3. The corrosion depth of silver measured in the first 30 days at each of the locations reached 2.0, 0.2 and 4.8 nm, respectively. The air quality classifications according to the ISO 11844-1 standard were IC 2 – Low, IC 1 – Very low and IC 2 – Low. Following the Sacchi and Muller recommendation, S1, Extremely pure class would apply to the first two premises and S2, Pure to the last one.

Location 2 with tight air quality control was one order of magnitude less corrosive than Location 1 with elevated levels of organic acids. The highest corrosivity was found in the visitor centre (Location 3), which was open to outdoor air containing typical urban pollutants, including sulphur dioxide, nitrogen dioxide and ozone. Although silver is not particularly sensitive to these compounds, the last two have some effect on its corrosion stability. The corrosion rate of copper was negligible in all three locations and was to be 0.3, 0.1 and 0.1 nm/year, respectively.