Reliability of steels for long term Hg storage

Abstract

Due to the high toxicity of Hg for humans and their harmful effect on ecosystems and wildlife, the EU has designed a strategy for the future industrial use of Hg. This strategy considers the control and phase out export of mercury from the European Community by 2011. Chlor-alkali industry is the largest present user of the mercury in the EU since their technology is based on mercury-cell process. As result of the conversion of this industry to a mercury-free technology will create a large amount of surplus mercury in the near future - about 12000 Tm-. The immediate final fate of this mercury is their storage for an unspecified period of time until a strategy for re-use and/or safe disposal is available. Based on a coherent strategy EU through LIFE program has funded the MERSADE project which aims are the design and construction of a safe storage installation prototype for mercury surplus but also to study the stabilization of liquid Hg and mercury-wastes into more stable species. Metal corrosion can be caused by different process. Under storage conditions, the main corroding process is the dissolution. If an element of the metal alloy used to build the container is soluble in the liquid metal, dissolution will occur until the liquid metal is saturated. At this point, dissolution would cease under isothermal conditions. However, the presence of impurities, thermal cycles, flowing Hg may increase the corrosion rate. The present paper studies the corrosion damage promoted on different metallic containers used for transport, storage, trading installations which have been in contact with this metal for different periods of time ranging from 6 to 25 years. Scanning electron microscopy, EDS analysis and chemical composition profile have been performed on steels took from different containers, pipes, scales which have been exposed to Hg under static or flowing conditions. Depending on the type of steel and working conditions the specimens show different degrees of damage. The selective depletion of some of the main alloying elements of the steel and the loss of material of the outer surface depends on the type of steel, time of exposure and storage conditions. Despite these differences all the steels have showed a good performance and there were not significantly affected from the mechanical viewpoint and corrosion resistance.