The ASTM Copper Strip Corrosion Test:  Application to Propane with Carbonyl Sulfide and Hydrogen Sulfide

Carbonyl sulfide (COS), which occurs as an impurity in commercial sources of propane, can hydrolyze in the presence of water to form hydrogen sulfide (H2S) and carbon dioxide (CO2). All commercial and HD-5 grade propane is required to pass the copper strip corrosion test, ASTM Test Method D 1838. While it is known that hydrogen sulfide will cause the failure of the copper strip test, it is widely believed that the hydrolysis of carbonyl sulfide can also cause the failure of the corrosion test. We have tested gravimetrically prepared mixtures of COS and H2S in pure and commercial grade propane with a variant of the ASTM copper strip corrosion test. Minor changes to the published ASTM corrosion test were implemented for diagnostic or monitoring purposes in making the measurements. Surprisingly, mixtures containing as much as 1000 ppm (mass/mass) COS did not cause a failure of the test, even when the copper strip was in contact with the COS mixture for 2 h (double the normal contact time). Mixtures containing H2S caused the failure of the test; however, concentrations as high as 3.5 ppm (mass/mass) H2S passed the test. Moreover, we were not able to produce the colors and patterns shown on the ASTM copper strip corrosion standards lithograph.