Corrosion probes, coupons and holders are corrosion-sensing devices that interface to instruments or monitors. This category also includes holders or access units for placing the probe or coupon within the process stream or corrosion system. Corrosion-sensing devices are necessary for detecting and measuring corrosion in manufacturing machinery, batteries and fuel cells, and field equipment. Corrosion is a chemical or electrochemical reaction between a metal and its environment.