Images obtained from www.purafil.com
Purafil produces and analyzes metal “coupons” that are used to assess how corrosive a given environment is. Industries that include metal components and electronics, or other sensitive materials, need to ensure a mild environment for the stability of their products. Thus, corrosion classification coupons are used in spaces like data centers, server rooms, and museums.
The Purafil corrosion classification coupons are composed of two metal strips: one copper and one silver. The integrity of the metals is confirmed prior to their exposure to air in the intended location. The metal coupons are then exposed to air for 30 days. During this time, corrosive gases, including gases like hydrogen sulfide and sulfur dioxide, will react with copper and silver to form copper and silver sulfides and oxides. These oxides and sulfides form on the surface layers of the metal, increasing the coupon volume. Purafil is able to distinguish between oxide and sulfide formation through x-ray diffraction analysis at their laboratories, but the corrosivity of the environment is categorized based on the height of the accumulated corrosion (e.g. copper oxide + sulfide), measured in a unit of length, Angstroms.
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Corrosion classes have been standardized by the International Society for Automation (ISA). The table below, copied from Purafil’s Corrosion Classification Coupon brochure, lists ISA Standard 71.04-2013. Even though copper has a smaller ionic radius than silver, copper is slightly more reactive than silver, so there is a higher copper corrosion threshold for a “mild” environment. To be classified as a mild environment, which is required for rooms that industrially house electronics, over 30 days the copper cannot accumulate more than 299 Angstroms, nor the silver accumulate more than 199 Angstroms, of total corrosion.