The vast majority of sources on the Internet broadcast to naive motorists that the terminals on the battery are oxidized due to the electrolyte that gets on them. At the same time, absolutely no one explains what the acid has to do with the oxidation process. Oxidation is not from the word acid, but from the word oxygen. Nevertheless, oxygen, and acid, and hydrogen, and even copper leave their traces on the terminals. Sometimes the flowing currents are “guilty” of this. And all together this is not called the oxidation of the terminals, but their corrosion.

Types of corrosion on battery terminals

It’s worth starting with a definition of terms, since it is in them that the confusion lies in relation to the oxidation of battery terminals.

Metal corrosion is a chemical reaction of metal destruction as a result of its interaction with the environment.

Metal oxidation is the chemical reaction of a metal with oxygen, resulting in the formation of oxides or oxides. Oxidation is nothing but corrosion. Rather, one of its types is oxygen corrosion.

Metal sulphation is a chemical reaction of a metal, for example, with an acid, in the presence of an electric current, as a result of which sulfates, that is, salts, are formed. This is also corrosion, but already electrochemical.

Oxidation of battery terminals

We have lead battery terminals and oxygen. It is in the air. The result of the interaction of lead and oxygen is an oxidation reaction, which results in the formation of lead oxide. Or rather, an oxide film. It can be seen if you clean the battery terminal to a shine, and then leave it for a while in the fresh air.

Normally oxidized terminal If the oxide film is only on top, it is better not to touch
Normally oxidized terminal If the oxide film is only on top, it is better not to touch

This process is called battery terminal oxidation. In this case, the ingress of electrolyte has nothing to do with it. Most often, no one pays attention to any oxide film. But the white coating around the terminals is already noticeable. But this is not oxidation.

Sulfation of battery terminals

A hill of white sulfate with an admixture of blue. These are lead and copper salts.
A hill of white sulfate with an admixture of blue. These are lead and copper salts.

To start such a reaction, lead, acid and an electric current are needed. Under the influence of an electric current, lead interacts with acid. The result of this reaction is lead sulfate, which is a white powdery substance that collects on the terminals.

But that's not all. Not all car owners oxidize the same terminal. In some, sulfates are formed on the positive terminal, in others - on the negative. Few people pay attention to this. But in vain. According to which terminal salt deposits form, one can say what the battery is “sick of”.

If sulfates accumulate on the positive terminal, the battery is regularly recharged. That is, the voltage regulator relay does not work correctly, as a result of which a voltage higher than the normalized 14.4 V goes from the generator to the battery. You can verify this by measuring the voltage at the terminals with the engine running.

If sulfates accumulate more at the negative terminal, the battery is often in a weakly charged state. This happens for several reasons. Firstly, because of the same voltage regulator relay. The charge voltage is much lower than the rated 14.4 V, and the battery is never fully charged. And if it also holds a charge poorly by itself, then sulfates on the negative terminal are guaranteed.

A similar process occurs in the battery itself. There is also lead (plates), acid (electrolyte) and electric current (discharge current). When the battery releases the accumulated energy, exactly the same lead sulfate is formed on its plates, which we observe on the terminals.

The only difference is that when the battery is charging, the salt deposits from the plates dissolve into the electrolyte. But on the terminals, he does not go anywhere by himself, because he has nothing to dissolve in. So with each period it accumulates, and in some neglected cases one can see a whole mountain of such a white powder.

On the plates inside the battery, sulfate can also irreversibly accumulate. This happens as a result of prolonged use of the battery in a poorly charged state. The sulfate on the plates, roughly speaking, “gets stronger”, and it can no longer be dissolved by ordinary charging. Such an “ailment” is called plate sulfation, and it leads to a loss of battery capacity and inrush current power.

It also happens when not a white coating accumulates on the battery terminals, but a bluish one. This is also sulfate (or salt). But the sulfate is not lead, but copper. It appears when the clamps for the battery terminals are made of copper. This metal similarly interacts with an acid under the influence of a flowing electric current. A bluish coating can be not only on the clamps, but also on the wires, if they are bare near the battery.

A good example of copper sulfate is beautiful, but...
A good example of copper sulfate is beautiful, but...

What threatens the oxidation of the terminals?

If the terminals are oxidized or sulfated, the electrical contact deteriorates. As a result, the car starts with difficulty, and in advanced cases, it does not start at all. The current required to spin the starter cannot normally pass from the battery through a layer of oxide film and salt deposits.

Paradox. Lead sulfate itself is a salt. And salts are good current conductors. However, in the case of battery terminals, as practice shows, sulfates do not improve the transmission of electric current. Why? We understand further.

The second problem is the destruction of the terminals. That the oxidation of a metal, that its sulfation is always such a reaction in which part of the terminal irrevocably turns into another substance - into an oxide or salt. If this happens all the time, then it is quite possible to wait until there is almost nothing left of the terminals. It is because of this that over time, poor contact is observed on the terminals, which was mentioned earlier.

Doesn't look safe at all
Doesn't look safe at all

The third problem is the self-discharge of the battery. A dirty battery is a bad thing anyway. Any acid-containing dirt (salt, dust - it doesn't matter) conducts current. If you do not follow the cleanliness of the battery, then over time current will flow through this plaque from one terminal to another. Of course, there will be no short circuit, since the electrical resistance of such a “conductor” is very large. However, this will be enough for the battery to slowly discharge. That is, its self-discharge increases and it holds the charge worse.

The terminal is oxidized - what to do?

If the battery terminals are oxidized, and not sulfated, then this is not so bad. If the contact between the terminal and the clamp is good, oxygen gets there with difficulty, or does not penetrate at all. In such cases, only the open part of the terminals is oxidized, which is not critical.

Fighting this is almost useless, and pointless. No matter how much you remove this grayish coating by cleaning the lead to a shine, it will still appear. But since it does not affect the operation of the battery, it is better not to touch it. If you constantly clean off the oxide, then over time the terminal can be “grinded off” to the very clamp, which is also not good.

As for the oxide film directly between the terminal and the clamp, it is necessary to deal with it periodically, since it does not transmit electric current well. But there are several nuances here, or rather, mistakes made by motorists after reading articles on the Internet in which sulfation is confused with oxidation.

Firstly, it is not necessary to “skin” lead with coarse-grained sandpaper. As a result of such processing, very deep grooves are formed on the terminals, along which oxygen will later seep even with the clamp on and well tightened. If possible, remove the oxide film with sandpaper, then only fine-grained. The smaller the furrows, the more difficult it will be for oxygen to get to the lead in order to react with it and oxidize it.

Do not use coarse sanding paper!
Do not use coarse sanding paper!
Better like this: toothbrush + water with soda
Better like this: toothbrush + water with soda
Then wipe dry with microfiber. The entire battery is cleaned in the same way.
Then wipe dry with microfiber. The entire battery is cleaned in the same way.

Secondly, do not apply various technical lubricants to the junction of the terminal with the clamp - engine oils, grease, lithol, grease, petroleum jelly, and so on. All these compounds are absolute dielectrics. They protect the metal from oxygen and moisture, slightly slow down corrosion. But they conduct current only when the terminals are cleaned with coarse sandpaper. After tightening the clamp, the excess of such a lubricant is squeezed out, and current flows only through these grooves and tubercles. Most of the contact patch is isolated.

This is not advertising - buy any tool - they are all plus or minus effective
This is not advertising - buy any tool - they are all plus or minus effective

If you already use all of the above lubricants, then only after the clamp is put on the terminal and tightened well. And so, for this kind of protection, special compounds are sold in stores today, which protect the metal from oxidation and do not prevent the flow of current.

What to do with sulfation of battery terminals?

It's more difficult here. First of all, because we have already found out that if there are sulfates, then there is acid. Without it, this reaction will not start. And this, in turn, means that the electrolyte or its vapors somehow penetrate out of the battery. In fact, he has several paths.

The most common case is the banal carelessness of a motorist. I twisted the battery plugs, measured the density, recharged it, added water, and so on. Meanwhile, electrolyte drops settle on the battery case and, as a rule, on the terminals. Everything. The necessary components of the sulfation reaction are available.

(hydrometer vs refractometer)

The second way of acid to the terminals is the holes, which are designed for emergency discharge of electrolyte vapors when the battery is boiling. This happens for only one reason - overcharging. When boiling, even if there is no visible splash of acid, there are fumes. They necessarily settle on the terminals and condense. Again, all the components for a harmful chemical reaction are available.

The last way the electrolyte is out is microcracks between the case and the terminals. They appear when the battery often “twitches” for charging, that is, the terminals are constantly, and not always carefully, removed. Naturally, they become loose, and gaps form between them and the body.

The terminal is oxidized - what to do:

  1. Keep the battery case clean. Accidentally splashed during maintenance, the electrolyte can be safely removed with soda dissolved in water (after soda, be sure to rinse with plain water). A simple rag. The acid will be neutralized by the alkali.
  2. Do not wash the battery with plain water from a well or faucet. There are salts there. The water will evaporate from the case, but the salts will remain. Sooner or later, current will flow through them, and the self-discharge of the battery will increase. Use distilled water.
  3. If the battery is recharged from the generator - replace the relay-regulator. The battery will stop constantly boiling, and the electrolyte vapor will not fall on the terminals. Plaque on the positive terminal will no longer appear.
  4. Keep the battery more than 60% charged . If the battery is old, you drive a little, the relay-regulator is acting up - use stationary chargers.
  5. If there is a suspicion that microscopic gaps have formed between the terminals and the case, felt or felt pads help. You can either cut these yourself or buy them ready-made at an auto shop. Gaskets are slightly impregnated with oil or a special agent for treating battery terminals, and put on before installing the clamps. Their role is to absorb the electrolyte seeping out of the battery. Therefore, you should not hope that they will save you for an infinitely long time.
Gaskets for battery terminals, which can be made from felt or felt
Gaskets for battery terminals, which can be made from felt or felt

If the battery is worn out or damaged, buy a new one as soon as possible. How to choose - read here. Oxidation and sulfation of terminals is not yet a deadly problem. But if the body metal is constantly covered with electrolyte, you can see not only corrosion, but also whole through holes in the engine compartment.

Table-cheat sheet for terminal oxidation




Gray film on the open area of the terminal

Terminal oxidation

It is possible (and better) not to touch

Gray film on the covered area of the terminal

Clean with fine-grained sandpaper, put on the terminal and treat with a protective agent

Plaque or accumulation of white powder on the positive or negative terminal

Terminal sulphation

Clean, keep the battery clean, recharge the battery regularly,

The positive terminal of the battery is oxidized

Eliminate the cause of the systematic overcharging of the battery

The negative terminal of the battery is oxidized

Do not allow systematic battery discharges lower than 60%

Bluish coating on terminals

Sulfation of copper clamps

Eliminate the cause of acid contact with copper parts

Terminals are destroyed


Replace the battery with a new one, having previously identified and eliminated the cause of corrosion development


VIDEO: how to remove battery corrosion