During routine quality control testing, small circuit breakers exhibited high contact resistance and, in some cases, insulation of the contacts by a surface film. The contacts were made of silver-refractory (tungsten or molybdenum) alloys. Infrared analysis revealed the film to be a corrosion layer that resulted from exposure to ammonia in a humid atmosphere. Simulation tests confirmed that ammonia was the corrodent. The ammonia originated from the phenolic molding area of the plant. It was recommended that fumes from molding areas be vented outside the plant and that assembly, storage, and calibration areas be isolated from molding areas.