Corrosion of transversely heated graphite tubes by iron and lanthanum matrices

An in-depth study of corrosion phenomena in transversely heated graphite atomizers (THGA), caused by iron and lanthanum matrices, was performed by combining a variety of techniques. Analytical performance was determined by lifetime measurements under recommended conditions for vanadium determination. Average mass loss per atomization cycle and change of electrical resistivity were measured and compared to respective data for mineral acid matrices. Morphological changes of inner tube surfaces related to the corrosive attack were studied by scanning electron microscopy and compared to literature data of respective changes found for longitudinally heated graphite atomizer systems. Qualitative and quantitative depth profiles through the platform wall and through the tube wall were determined by wavelength-dispersive X-ray analysis in the electron microprobe. They were found to be dramatically different for iron and lanthanum. A quantitative evaluation of the remaining matrix elements after total tube system lifetime also revealed an essential difference between iron and lanthanum. Carbide formation and high temperature diffusion were discussed in detail for iron and lanthanum and were related to the observed phenomena. A combination of all these results, with data of semi-quantitative particle analysis and data from speciation studies by precision measurements of the line shifts for the C Kα X-ray emission line, allowed a profound description of phenomena occurring during one atomization cycle related to corrosion.