Assessing the influence of agrochemicals on the rate of copper corrosion in the vadose zone of arable land – Part 2: laboratory simulations

Abstract

This is the second in a series of three papers from a project that has attempted to answer the question ‘does the application of agrochemicals accelerate the corrosion of archaeological metals in the top 50cm of the soil?’. We have approached it through a combination of field-based experiments, by establishing laboratory microcosms and by using geochemical modelling techniques to understand the processes involved. This paper reports on two different experimental approaches in the laboratory - a microcosm designed to mimic one of the burial sites (the ‘Lab Bin’ experiments), and a simpler one to understand the reaction between metal samples and concentrated aqueous solutions of the fertilizers and laboratory reagents used (the ‘Lab Beaker’ experiments). The bins were monitored for in situ corrosion and aqueous effluent collected for13 weeks, after which they were excavated and the metal coupons examined. The Lab Beakers were monitored for in situ corrosion for seven weeks, and then the coupons examined. We focus here on a sub-set of the data relating to the behaviour of the thinnest samples of copper in each case. As with the field data previously reported, the results are sometimes contradictory, but on balance this project has demonstrated that applied agricultural chemicals are likely to accelerate the rate of corrosion of metal objects within 50cm of the surface. In particular, it is likely that any fertilizers containing KCI will be particularly aggressive.