APPLICATION OF AUTOMATED CORROSION SENSORS FOR REAL-TIME MONITORING IN ATMOSPHERES POLLUTED WITH ORGANIC ACIDS

摘要

Loggers for continuous measurement of the corrosion rate of metals under atmospheric conditions were developed. The electronic unit measures and records changes in the electrical resistance of a thin metal track applied on an insulating substrate. If the metal corrodes, the effective cross-sectional area of the track decreases and the electrical resistance increases. Part of the metal track is protected by an organic coating and, thus, serves as a reference to compensate for resistivity changes due to varying temperature. Sensors made of silver, copper, iron / steel, zinc, bronze and lead with sensitivities tailored to different environments are available. Thin film metal sensors with the metal track thickness from 50 to 800 nm were made by physical vapour deposition (PVD). Examples of the logger application for continuous measurements of metal corrosion rates in air are given. Due to the sensitivity of the measurement as high as 0.1 nm, in terms of corrosion depth, changes in air corrosivity were registered within tens of minutes or hours even in low-corrosive environments. At relative humidities from 15 to 80 % and at the temperature of 20 °C, it was used for monitoring in the air polluted with controlled amounts of formic and acetic acids at concentrations from 0-1590 and 0-870 ppb, respectively. Gases in these concentrations were reported to be found in museums and other cultural heritage institutions and are expected to cause accelerated deterioration of metal objects. Threshold limits of the formic acid concentration in air at 80 % RH and at 20 °C causing changes in the classification of indoor air corrosivity according to ISO 11844-1 are given.