Electrolytic Corrosion of Metallic Resistors

Metallic resistors are encapsulated mainly for two reasons. Firstly to avoid mechanical damage, secondly to provide protection against moisture. If moisture penetration is not prevented and if at the same time ions are present capable of moving through or along the insulating materials, a resistance change will occur with time, whenever the resistor is subjected to a high relative humidity and a d.c. load. Under these conditions there will be an electrolytic current flowing through or along the insulating materials, which will cause damage to the resistance element at the anodic side of the resistor. Typical quantities expressing the behaviour are: the resistance value of the insulation resistance of the covering material under various humidity conditions and the caloric resistance of the whole component. Calculated are formulae, which sate put to the test and which show that: the electrolytic current flowing through or along the insulating materials as a function of applied load has a maximum values the load at which this maximum occurs depends on the resistor construction and the materials used$ the moisture resistance of a metallic resistor has to be tested at several loads, otherwise it will be impossible to draw con- clusions.