The Chemistry of Failure of Aluminum Electrolytic Capacitors

SOME of the most common failure modes of low voltage aluminum electrolytic capacitors are the result of slow chemical and electrochemical reactions on the electrode surfaces. These reactions are an inherent property of the A1/A12O3/ electrolyte system. In this paper the relationship between particular failure modes and these electrode reactions is demonstrated and a detailed description of these reactions is provided. Analysis of 5V and 10V capacitors that failed because of excessive changes in capacitance, dissipation factor or leakage current on test showed that in many cases at least one of the following phenomena had taken place: (1) increase in anode foil capacitance accompanied by decreased ability to withstand voltage stress; (2) increase in cathode foil capacitance; (3) production of gas, probably hydrogen. In all cases the capacitor contained a glycol-borate electrolyte. These same phe'nomena were observed by heating samples of commercial etched foil in a jar of glycol-borate electrolyte at 85°C. Accompanying the capacitance increase of anode foil was a weight-loss due primarily to dissolution of the aluminum oxide dielectric. A much smaller weight loss of metal substrate also occurred. Cathode foil treated in a similar fashion suffered a capacitance increase and a weight loss of metal substrate. No gas was evolved from the anode foil during this reaction but a gas, probably hydrogen, was collected over the cathode foil. Electron micrographs showed that non-uniform attack of the anode foil took place at grain boundaries and at random sites within a grain.