Theory and Method
The copper strip corrosion test detects reactive sulfur compounds that survive refining and remain in the finished product. Elemental sulfur, hydrogen sulfide, mercaptans, and similar species react with metallic copper to form copper sulfide and copper oxide films on the strip surface. These films discolor the metal, and the color and pattern of the tarnish track the type and concentration of the corrosive species present. Total sulfur content alone does not predict this behavior, because the chemical form of the sulfur governs how aggressively it attacks copper.
During a test, the operator polishes a copper strip to a bright finish, immerses it in a measured volume of sample, and holds it at a set temperature for a fixed period. Low vapor pressure products sit in a test tube, while volatile fuels such as aviation gasoline require a sealed pressure vessel, or test bomb, to keep the sample from boiling off. The QualiCCT™ D130 heats both configurations in the same stainless steel bath and stirs the medium so every position reaches the target temperature. Stable temperature and even circulation keep the reaction conditions uniform, which the standards require for valid results.
After the exposure period, the operator removes the strip, washes it, and compares it against the ASTM Copper Strip Corrosion Standard, a printed color scale. The scale runs from 1a, a slight tarnish, through class 4, which shows heavy corrosion, pitting, or blackening. The classification tells the buyer whether the product will stay compatible with copper and copper alloy hardware in service. Because the reading relies on visual comparison, the tester's tight temperature control removes one of the main sources of variation between runs.
