Gas chromatographic determination of carbonates and sulphides in the corrosion products of metals

Abstract

A method has been developed for the gas chromatographic (GC) determination of sulphides in the corrosion products formed on the surface of metals immersed in sea water. The samples are treated at room temperature with 30% glycine solution, which dissolves the compounds of bivalent metals, but leaves unchanged other corrosion products and the metallic matrix. The evolved gases (carbon dioxide and hydrogen sulphide) are stripped with a flow of purified inert gas and trapped at liquid nitrogen temperature in the modified loop of a gas sampling valve. They are then thermally desorbed and injected into a gas chromatograph equipped with a Porapak Q column and a thermal conductivity detector.

The method permits the simultaneous determination of CO₂ and H₂S and the measurement of the amounts of carbonates, oxycarbonates and sulphides in the corrosion layers. The detection limit is about 1 µg of H₂S (with a precision of ±15%). The GC method is therefore about ten times more sensitive than the spectrophotometric method using methylene blue.

The performance of different methods was evaluated by analysis of the corrosion products of copper and iron specimens subjected to corrosion in surface and deep (200 m) sea water.