Capacitor Plague - Evidence of Defective Electrolyte

Evidence of Defective Electrolyte

The situation of unimpeded formation of hydroxide (hydration) and associated hydrogen gas production occurred during "capacitor plague" or "bad capacitors" incidents involving the failure of large numbers of aluminum electrolytic capacitors. This has been demonstrated by two researchers at the University of Maryland who analyzed the failed capacitors. These two scientists initially determined by ion chromatography and mass spectrometry, that there really is hydrogen gas present in failing capacitors, which is what leads to bulging of the capacitor’s case or bursting the vent. Thus it was proved that the oxidation takes place according to the first step of the formation of aluminum oxide.

Because it has been customary in electrolytic capacitors to bind the excess hydrogen with the help of reducing or depolarizing compounds to reduce the resulting pressure, the researchers then searched for compounds of this type. Usually aromatic nitrogen compounds or amines are used for this purpose. Although the above-mentioned analysis methods are very sensitive to detecting such pressure-relieving compounds, no traces of such agents were found within the failed capacitors.

With capacitors in which the internal pressure build-up was so great that the capacitor case was already bulging but the vent had not opened yet, then the pH value of the electrolyte could be measured. The electrolyte of the faulty Taiwanese capacitors was alkaline with pH (7 < pH < 8). Comparable Japanese capacitors on the other hand had an electrolyte with a pH in the acidic range (pH ≈ 4). As it is known that aluminum can dissolve in alkaline liquids, but not in mildly acidic media, then with the electrolyte of the faulty capacitors an energy dispersive X-ray spectroscopy (EDX or EDS) fingerprint analysis was made, which actually detected dissolved aluminum.

To protect the metallic aluminum against the aggressiveness of the water, some phosphate compounds known as inhibitors or passivators can be used to produce long-term stable capacitors with high-aqueous electrolytes. Phosphate compounds are mentioned in patents regarding to electrolytic capacitors with aqueous electrolytic systems. Since in the investigated Taiwanese electrolytes, phosphate ions were missing and the electrolyte was also alkaline, they evidently lacked any protection against water, and the formation of more-stable alumina oxides was inhibited. Therefore, only aluminum hydroxide was generated.

The results of chemical analysis were reinforced by the measurement of electrical capacitance and leakage current in a long-term test lasting 56 days. Due to the chemical attack, the oxide layer of these capacitors had been weakened, so that after a short time the capacitance and the leakage current increased briefly, before both parameters dropped abruptly upon opening of the vent. The report of Hillman and Helmold proved that the cause of the failed capacitors was a faulty electrolyte mixture used by the Taiwanese manufacturers, which lacked the necessary chemical ingredients to ensure the correct pH of the electrolyte over time, for long-term stability of the electrolytic capacitors. The further conclusion that the electrolyte with its alkaline pH value then had the fatal flaw of continual growth of hydroxide wíthout conversion into the stable oxide, was verified on the surface of the anode foil both photographically and with an EDX-fingerprint analysis of the chemical components.

Even in a microscopic image with only 10-fold magnification, as shown in the pictures above, a significant change in the structure of the anode surface is visible. On the surface of the "fresh" anode from an unused electrolytic capacitor the parallel scratches from manufacturing processing of the anode are clearly visible. However, the enlargement is not sufficient to show the openings of the pores in the anode, which are visible using a scanning electron microscope (SEM). In the picture of the used anode, which comes from a failed capacitor out of Taiwanese production, the surface is overgrown with a plaque-like substance transverse to the running direction of the scratches. An EDX-fingerprint analysis showed the chemical difference in the surface oxide. The surface of the "fresh" electrolytic capacitor was covered with stable aluminum oxide. The surface of the failed capacitor was covered with unstable aluminum hydroxide; the EDX scan shows a significantly higher oxygen peak.