Part 2
Electrolyte Degradation Over time
The electrolyte, as above mentioned, over time degrades, typically from electrolysis of water into H2 and O2, both of which are lost. So the fact that the electrolyte is gradually declining doesn't mean that the capacitor properly or consistently functions for the entirety of its lifespan, or that its lifespan is good. Remember, the purpose of the electrolyte is ion migration to permit plate charge. If the electrolyte's ability to move ions is impaired, as occurs with increased viscosity, the capacitor won't function or it will have frequency-dependent degradation. What really matters for the longevity of a wet electrolyte is that the liquid is over time lost, both through normal evaporation and being baked out by high temperatures.
Electrolytics, by virtue of their design, are limited-lifespan devices. The seals are imperfect and the inherently higher ESR creates internal heat. Poor-quality electrolytics can fail in a few years. Well-made ones can last a decade or two, sometimes longer if properly sealed. The issue is that the properties deteriorate with age, even if the capacitor does not fail. Old capacitors have no Magickal Mojo; what they have is a death sentence which will eventually be carried out. Electrolytics cans explode when they die, hurling metal around like an IED.
The main issue beyond evaporation is that the oxide layer, essentially being anodizing, is fragile and readily dissolves back into the electrolyte during disuse. The approximate values for deterioration of a modern electrolytic capacitor in storage are normally on the order of 2^Year, and the manufacturers set forth a variety of values:
After 1st Year: drop in capacitance is about 2%, leakage increases by about 2x
After 2nd Year: drop in capacitance is about 4%, leakage increases by about 4x
CDE or Nichicon application notes, for example, concur with these values. Rubycon states the changes are small after two years and then increase. TDK claims longer lifespan but doesn't fully specify the changes as it varies across the product line. CDE suggests only a few years. Much of these values are likely hedging to prevent the inevitable lawsuits — particularly after the spate of lawsuits for motherboard failures in the 1990s — and to protect the manufacturer's reputation.
Storage in this context normally means sitting in a distributor's warehouse at 35 or 40 degrees C. So in a home at a lower temperature it may deteriorate less. The reactions are temperature dependent.
While having implications for the corner frequencies of filters, the drop in capacitance with time, however, is not the main issue; that issue is the increase in leakage current which, in turn, causes ohmic heating, particularly from ripple current in a power supply. After three years the capacitor is supposed to be tested for leakage, and after four reforming is mandatory. The leakage current is very small in a modern electrolytic capacitor, at least one operated within design goals, so doubling or quadrupling it is likely still small. Reforming after a few years may just be CYA, but it would not surprise me that the oxide starts to rot after that.
The reforming current is normally severely limited, something like a series resistor of 100k Ω, with a reforming time is 30 to 60 minutes depending upon the voltage rating. Something like that. The goal is prevent ohmic heating while the oxide layer rebuilds. So it cannot be reformed in an actual circuit experiencing real current. The process requires disconnecting the capacitor, using a very high-value resistor as a load to limit current, run for a bit to reform, then reconnecting to the circuit.
The degradation of the electrolytic capacitor is a well-studied issue, with mountains of technical material on the subject trivially available on the interwebs. For example, see:
Physics Based Electrolytic Capacitor Degradation Models for Prognostic Studies under Thermal Overstress
Chetan S. Kulkarni, Jose R. Celaya, Kai Goebel, and Gautam Biswas
European Conference of the Prognostics and Health Management Society, 2012
https://ti.arc.nasa.gov/publications/5005/download/
2. ELECTROLYTIC CAPACITORS
Electrolytic capacitor performance is strongly affected by its operating conditions, such as voltage, current, frequency, and ambient temperatures. [...]
A primary reason for wear out in aluminum electrolytic capacitors is due to vaporization of electrolyte (Goodman et al., 2007) and degradation of electrolyte due to ion exchange during charging/discharging (Gomez-Aleixandre et al., 1986; Ikonopisov, 1977) , which, in turn leads to a drift in the two main electrical parameters of the capacitor: (1) the equivalent series resistance (ESR), and (2) the capacitance (C). The ESR of a capacitor is the sum of the resistance due to aluminum oxide, electrolyte, spacer, and electrodes (foil, tabbing, leads, and ohmic contacts) (Hayatee, 1975; Gasperi, 1996). The health of a capacitor is often indicated by the values of these two parameters. There are certain industry standard thresholds for these parameter values, upon crossing these threshold barrier the component is considered unhealthy to be used in a system, i.e., the component has reached its end of life, and should be immediately replaced before further operations (Lahyani et al., 1998; Eliasson, 2007; Imam et al., 2005).
[A]n aluminum electrolytic capacitor, consists of a cathode aluminum foil, electrolytic paper, electrolyte, and an aluminum oxide layer on the anode foil surface, which acts as the dielectric. When in contact with the electrolyte, the oxide layer possesses an excellent forward direction insulation property (Gasperi, 1996). Together with magnified effective surface area attained by etching the foil, a high capacitance value is obtained in a small volume (Fife, 2006). Since the oxide layer has rectifying properties, a capacitor has polarity. If both the anode and cathode foils have an oxide layer, the capacitors would be bipolar. In this work, we analyze “non-solid” aluminum electrolytic capacitors in which the electrolytic paper is impregnated with liquid electrolyte. [...]
2.1. Overview of Degradation Mechanisms
The flow of current during the charge/ discharge cycle of the capacitor causes the internal temperature to rise. The heat generated is transmitted from the core to the surface of the capacitor body, but not all the heat generated can escape. The excess heat results in a rise in the internal temperature of the capacitors which causes the electrolyte to evaporate, and gradually deplete (Kulkarni, Biswas, et al., 2011b; Kulkarni, Celaya, et al., 2011). Similarly in situations where the capacitor is operating under high temperature conditions, the capacitor body is at a higher temperature than its core, the heat travels in the opposite directions from the body surface to the core of the capacitor again increasing the internal temperature causing the electrolyte to evaporate. This is explained using a first principles thermal model of heat conduction (Kulkarni, Biswas, et al., 2011b; Kulkarni, Celaya, et al., 2011).
Degradation in the oxide layer can be attributed to crystal defects that occur because of the periodic heating and cooling during the capacitor’s duty cycle, as well as stress, cracks, and installation-related damage. High electrical stress is known to accentuate the degradation of the oxide layer due to localized dielectric breakdowns on the oxide layer (Ikonopisov, 1977; Wit & Crevecoeur, 1974). These breakdowns, which accelerate the degradation, have been attributed to the duty cycle, i.e., the charge/discharge cycle during operation (Ikonopisov, 1977). Further another simultaneous phenomenon is the increase in the internal pressure (Gomez-Aleixandre et al., 1986) due to an increased rate of chemical reactions, which can again be attributed to the internal temperature increase in the capacitor. This pressure increase can ultimately lead to the capacitor popping.
All the failure/degradation phenomenon mentioned may act simultaneously based on the operating conditions of the capacitors. We first study the phenomenon qualitatively, and then discuss the steps to derive the first principles analytic degradation models for the different thermal stress condition. Electrolyte evaporations is caused either due to increase in internal core temperature or external surrounding temperature. Both phenomenon lead to the same degradation mode,caused either by the high electrical stress or thermal stress, respectively.
Non-Polar Electrolytic Construction
A non-polarized electrolytic capacitor is not magickal, merely being
two ordinary electrolytic capacitors placed back-to-back, such that each capacitor is in series with the other but the polarities are facing in opposite directions. Charge moving in the reverse direction, from the perspective of each device, consequently travels
through the electrolyte and is thus rectified, as it would in an improperly configured uni-directional electrolytic capacitor.
So as the oxide reduction from reverse current flow begins, it is far from uniform because the thinnest spots, of necessity, pass more current, and hence preferentially reduce. Such reduction eventually creates a thin spot, but it will not necessarily create uniformly greater leakage across the entire surface, and as long as the oxide is thick enough to minimize leakage at the applied voltage the layer still functions. This is key. So the capacitor will not necessarily explode from the application reverse-current , which is why a non-polar electrolytic can survive in an AC application, it merely poorly functions from a linear perspective.
The dielectric layer will, however, of necessity be damaged and have a dramatically reduce lifespan as compared to a properly configured, i.e. unidirectional current flow, electrolytic capacitor. This is why capacitors intended for long-lifespan or mission critical use are typically formed at 125% of rated voltage for general purpose use, and 200% and above. The oxide layer, of course, will over time dissolve into the electrolyte, so it is not clear how significant the benefit is.
Because the reverse-current has a lower duty cycle, in the case of symmetrical AC it would be 50%, the capacitor does not explode and has enough time to regenerate the degraded oxide. A thicker oxide layer is more robust for AC signals which gradually damage the oxide layer. The de-forming therefore only serves to shorten lifespan, not make the rectification effects more pronounced. Absent other design changes, NPEs, either in a single package or constructed from two devices, will always have shorter lifespans than electrolytics not subjected to AC currents.
Summary
To summarize, a conventional electrolytic therefore has substantial
distortion because the capacitor has (a) non-linear behavior, i.e. its resistance (ESR), capacitance, and group delay all vary with frequency, (b) rectification effects, and (c) dielectric absorption which performs signal averaging. Such properties, again, are well known to analog engineers. Electrolytic DA causes odd-order harmonics and significant IM. Cyril Bateman measured harmonic distortion and IM in non-polar electrolytics and reported that good electrolytic capacitors did not match the performance of poor film capacitors, and Walt Jung also did some work on the subject.
Aside from situations requiring higher values at lower price points, where NPE is really the only practical option, no good reason exists to ever use an electrolytic capacitor in a crossover.
Barbie says, "
Electrochemistry is Hard!"
Proud owner of a mint pair of Boston Acoustics A150
