Inorganic and hybrid transparent protective coatings for clean and corroded steel surfaces : a conservation approach alignment

Abstract

Corrosion degradation of metal artefacts is a major concern in museums where the indoor atmospheric conditions are uncontrolled. A common approach to mitigate corrosion on museum objects is to apply protective organic coatings. This thesis explores the possibilities of using inorganic and hybrid organic-inorganic coatings as candidates for the protection of historical metal surfaces: if successful, such coatings would represent a complete breakaway from traditional measures insofa1 as steel artefacts are concerned. Inorganic protective films would offer advantages over organic coatings in being less permeable to oxidative and other corrosive gases and electrolytes and far more resistant to photochemical degradation. Hence, less maintenance would be necessary than with organic coatings. The Palace Armoury collection (mid-16 to 17 century) situated in the Grandmaster's Palace in Valletta, Malta was chosen as case study for this work The collection was chosen on grounds of it being almost entirely composed of steel objects and which displayed visible signs of corrosion. A comprehensive study of the Palace armoury indoor atmosphere was undertaken as a first part of this work which included an 18- month-long measurement campaign for relative humidity, 03, S02 and N02• The results showed that the internal atmospheric environment is not conducive to the safe storage and optimal display of general metals collections, pointing to the need for more effective protection of the displayed and stored artefacts. In a second part of this study, an in-depth metallurgical examination of a representative set of authentic plate-armour objects from the same collection was carried out yielding significant information on both the manufacturing technology of these artefacts and their state of preservation. X-ray diffraction studies performed on the corrosion layers indicated that the iron oxy-hydroxide phases present ubiquitously on the armour surfaces, including the er-containing akageneite, were indeed very similar to what is observed on steels exposed to a marine coastal environment. In a next phase of the study, the potential application of Ti02 and Si02 films as protective measures was investigated. Transparent films of these oxides were deposited as coatings using a physical vapour deposition technique starting off with titanium (III/IV) oxide and fused silica sources respectively. The Ti02 films were found to be heavily tinted by interference colours and poorly protected the underlying steel substrate. Further work on the use of this material as a 'conservation-friendly' coating was eventually abandoned on grounds of poor aesthetical qualities. Slightly better results were achieved with Si02 films in that interference colours were subdued but costly alterations to the deposition chamber were required in order to deposit coatings with improved anti-corrosion properties at lower temperatures. Attention was thus turned to the novel use of four groups of organic-inorganic hybrid coatings. The first group was based on a 60:40 mol% methyltriethoxysilane (MTES): tetraethylorthosilicate (TEOS) precursor mixture. The latter was altered by additions of 3-methacrylopropyltrimethoxysilane (y-MPS) in exchange for TEOS to furnish 60:30:10 mol% and 60:20:20 mol% MTES:TEOS:MPS sols. The second group was based on a 50:50mol% mixture of y-MPS and TEOS. Other member within this group were 40:40:20 mol% TEOS:MPS:MTES and 40:40:20 mol% TEOS:MPS: octyltriethoxysilane (OCTES). In the latter two sols, alkyltriethoxysilane precursors were added in an attempt to improve the water repellence of the final coatings. The third and fourth group of coatings were based on binary mixtures of either MTESTEOS or phenyltriethoxysilane (PTES) and TEOS where the phenyl- or methyltriethoxysilane precursor was varied from 20 to 80mol%. All coating lacquers were prepared by an acid catalysed hydrolysis-condensation procedure and the neutralized sols deposited onto mild steel substrate and glass slides by a dip-coating technique. Prepared sols for groups 1 and 2 were characterized by viscosity measurements and particle sizing and their temporal stability during the coating process was ascertained. Coatings were deposited by a dip-coating process on glass slides and steel substrate which was either cleaned uncorroded or pre-corroded to simulate the surfaces of historical steel. [...]