THE COPPER STRIP TEST: A REVIEW OF EXISTING PROCEDURES FOR THE ASSESSMENT OF THE QUALITY OF PROPANE AND LNG

摘要

The objective of the work described in this paper was to determine the influence of possible interferences in the ASTM copper strip test as they might relate to impurities in the hydrocarbons under analysis and materials that could be inadvertently incorporated into the test system. A theoretical assessment of the surface chemistry involved in the copper strip test suggests that oxygen and carbon dioxide could tarnish a copper surface in the absence of any sulfur compound and produce an array of green – red – black colors not dissimilar to those shown in the standard reference corrosion copper strip set. These projections were borne out in the case of oxygen (air) which, in particular, gave a black tarnish within the timeframe and conditions of the ASTM test. Moreover, it was shown that oxygen enhanced the discoloration of copper when H2S was present, most likely, due to the conversion of H2S to elemental sulfur. The presence of oxygen in an evaluation could explain the “synergistic” phenomenon sometimes observed in the copper strip test as sulfur tends to give a greater tarnish than H2S. In general, the experimental studies confirmed that H2S and sulfur are the only materials that cause a “failing grade” directly with H2S giving rise to a rainbow of colors and sulfur producing a generally black tarnish.