Why Understanding Corrosion Is Important
(See footnote - corrosion control - steel mill, crude oil.)
Corrosion of the positive electrodes occurs in all lead-acid batteries. If it did not, the lead-acid concept would not work. The very earliest lead-acid battery cells were made by immersion of two lead sheets in dilute sulfuric acid. The cells were charged by applying a voltage sufficient to turn the lead on the surface of the positive electrodes into lead dioxide - and causing hydrogen gas to be released at the negatives.
One way to describe what happens is that the positives are being oxidized. Another way to describe what happens is that the positives are being corroded.
It is roughly analogous to iron being turned into rust. In the lead-acid battery, the "rusting" is what happens during charging and is necessary and beneficial because this is what brings life to the positive plate active material and is where, for all intents and purposes, 100% of the power is stored in a battery. When too much charging is forced into a battery, however, it affects the positive grids, which is a bad thing.
The early lead electrodes evolved into positive and negative plates, with grids containing active materials. Charged battery cells have lead dioxide positive active material and spongy lead negative active material, both of which are converted into lead sulfate upon discharge and back to lead dioxide and spongy lead upon recharging.
Oxidization/ corrosion of the positive plate grid surface is not a straightforward process. The formation of lead dioxide initially takes place rapidly, forming a layer that passivates the surfaces. This effect is very powerful on titanium, stainless steel and aluminum, less so on lead. After this layer of oxide has been established, the rate of corrosion abruptly falls almost to zero. This corresponds to a transition voltage or potential known as the Flade potential. The voltage applied to a lead electrode can be raised without any increase in the rate of corrosion - up to a point. Thereafter, increasing the voltage increases the rate of corrosion. The lead-acid battery is preferably operated across this voltage range of very low corrosion although, for practical reasons, this is not possible 100% of the time. This will now be explained.
A lead-acid battery can only be charged by applying a voltage sufficient to cause PbSO4 to be converted into PbO2 in the positives and PbSO4 to be converted into Pb in the negatives. If the charging voltage is marginally above the onset of these reactions, it can take many days to complete the task. Hence the applied voltage is deliberately raised. In the case of automobile batteries, 2.37 volts per cell, (equivalent to 14.22V for a 12V battery), can satisfactorily charge the battery. In the case of motive power batteries, 2.55 to 2.60 volts per cell applied briefly at the end of charge is deemed necessary to bring a battery to a full state of charge within a short enough period to make sure it is ready for the next shift.
Expressed in volts per cell, corrosion of the positive grids is for all intents and purposes zero below 2.15 volts. Corrosion proceeds extremely slowly at 2.25 volts. It becomes noticeable at 2.35 volts, more noticeable at 2.45 volts and significant at 2.55 volts. Corrosion becomes pronounced above 2.65 volts.
Lead-acid battery manufacturers found it is essential to slightly overcharge, in preference to risking undercharging batteries, to achieve maximum battery life. Repeated controlled overcharging results in very mild, almost ethereal corrosion. It is done intentionally and it is definitely beneficial. It keeps the positive plates in good condition and prevents the negative plates from becoming sulfated. It is a process that has withstood the test of time for 130 years. Excessive overcharging, however, can shorten battery life.
Sulfation is way over at the opposite end of the scale. Lead-acid batteries get sulfated when they are deprived of sufficient charge. Sulfation has nothing to do with batteries getting old. Sulfation is not an aging process. Boredom causes sulfation. Batteries that are left standing for long periods of time become sulfated. Batteries housed above exhaust manifolds in automobile engine compartments get sulfated. Light aircraft, leisure marine, military and golf-cart batteries that are laid over for months, even years, get sulfated. Truck batteries that get drained during overnight stops by their "hotel load" get sulfated. Motive power batteries that power rental forklift trucks that are not frequently used get sulfated. Batteries that are routinely brought to full state of charge do not get sulfated.
Battery industry sources are unanimous when they say that on average, 30% of automobile-type batteries end up sulfated and only 5% of motive power batteries end up sulfated. The vast majority of motive power batteries, they say, ultimately go down because of corrosion, not sulfation. Sulfation, they always point out, is caused by neglect. Desulfation product makers insist it is the other way around and insist that sulfation naturally builds up in batteries over time. Many can only vaguely discuss how batteries work. They describe corrosion as more of an incidental mechanical defect. Sulfation restorers usually treat batteries by charging the affected batteries, followed by pulsing in one form or another. This is simply modified, boosted equalization charging called by another name. Desulfation pulse units that use the battery's own power remain controversial because of the way in which they seem to defy the laws of electrochemistry.
The life expectancy of 95% of all motive power batteries is ultimately determined by corrosion, definitely not by a buildup of sulfation. The overwhelming majority of motive power batteries show no signs of sulfation at the end of their service lives but all will have become worn out, after many productive years in service, from the cumulative effects of mild corrosion and accompanying active material shedding. Corrosion control reduces this form of corrosion from a barely perceptible level to as close as possible to an imperceptible level. A very small change producing a huge, out of proportion beneficial effect. It has been helping to add between 40 and 60% extra service life to small, medium and large motive power batteries in a wide variety of applications.
Corrosion control has hitherto not been attracting the attention it deserves because battery experts have always insisted there is not much that could be done to meaningfully reduce corrosion. They were looking at it from the wrong direction. They were looking at corrosion of the positive electrodes via the positive electrodes. We achieved success by approaching it via the negative electrodes. Incredibly, some battery manufacturers have been using a very basic form of corrosion control technology for many decades without even being aware of it. Rubber separators! The use of natural rubber battery separators instead separators made of polyethylene or PVC reduces corrosion. The substance in the rubber that produces this effect has been synthesized. It dissolves in the acid, goes to work at the negatives and affects the positives. Corrosion control can provide far greater value and can be a vastly superior revenue generator, compared to desulfation. It is a new concept. New concepts can take some getting used to. That is why this website covers the finer technical aspects in detail.
- Corrosion control features prominently in steel milling and crude oil processing. The mechanism by which corrosion control chemicals work in these situations remains controversial. We worked out how corrosion control works in batteries and in doing so worked out how corrosion control works in pickling baths and oil pipes. Please contact us for details.