Gas chromatographic determination of carbonates in the corrosion products of metals

Abstract

Abstract A method was developed for the gas chromatographic determination of carbonates and oxycarbonates to be used for the analysis of corrosion products formed on the surface of Cu, Zn, Fe, Al and their alloys immersed in sea water. The samples were treated for 15 min at room temperature with 30% glycine solution, which dissolved the compounds of bivalent metals but left unchanged other corrosion products and the metallic matrix. The carbon dioxide evolved was stripped with a flow of purified inert gas and trapped at liquid nitrogen temperature in the modified loop of a gas sampling valve. It was then thermally desorbed and injected into a gas chromatograph equipped with a Porapak Q column and a thermal conductivity detector. The sensitivity and linearity of the method were tested in the range 1–5000 ppm of carbon dioxide by using an exponential dilution flask and calibration was effected by dissolving known amounts of carbonates in the stripping unit. The minimum detectable amount was 0.5 μg of carbon dioxide (with an accuracy and precision of about ± 5%). The method is about 20 times more sensitive than other methods currently used, such as trapping with pyridine and titration with sodium methoxide, or conductimetric measurements. The procedure, with some modifications, can also be used for the determination of carbonates in other types of samples. The sensitivity limit of the method depends on the area of the corroded specimen, on the trapping efficiency of the sampling loop, on the thermal conductivity detector filament current and, mainly, on the purity of the stripping gas. This is because its impurities are concentrated in the cooled loop and the blank carbon dioxide value is therefore much greater that the actual concentration in the stripping stream.