Feasibility of using fringe projection system for corrosion monitoring in metals of interest in cultural heritage

Cultural heritage is affected by degradation processes related to intrinsic factors, environmental or human activities. Corrosion is one the most deteriorative phenomena and it can introduce huge damages. Patina or thick corrosion crust formed on artifacts, may display complex products and structures. The identification of the corrosion products or characterization of the patina on artifacts is an essential task to acquire a better knowledge about the condition of ancient objects, corrosion processes and conservation treatment or preventive procedures for long-term, stable preservation. These considerations show that corrosion monitoring is a very important aspect for the evaluation of the degradation of cultural heritage. It is important to have a system that could be used for easy monitoring of the surface condition to put in evidence the beginning of dangerous localized corrosion phenomena. Nowadays optical methods allow detecting and representing, with remarkable accuracy, three-dimensional objects such as sculptures and archaeological finds, as well as large items such as architectural structures. One of the most powerful approaches is the Fringe Projection System (FP) based upon the projection of a complex pattern of light on the object to be reconstructed. In the following work, Fringe Projection is proposed for monitoring and to make quantitative evaluations of the corrosion process occurring on surfaces. The corrosion tests have been performed. Before starting the test, a fringe pattern is projected and the frequency spectrum of the recorded image is analyzed. Successively the samples are submerged in a solution of synthetic acid rain. At different time intervals the specimens were extracted from the solution and analyzed. Fringe pattern is projected again on the surface and new analysis of the frequency spectrum is run. Moreover surface analyses by microscopy and weight loss measurements were performed. At the end of the test, the specimens extracted from the solution were studied by surface analyses and weight loss measurement. The corrosion behavior of the bronze sample has been checked by fringe projection and optical microscopy observation during the exposure to synthetic rain. The average weight loss values were calculated using weight of sample measured before and after exposure to synthetic rain to quantify the corrosion rate of the sample in the corrosive solution.