Sometimes, terminals or posts of lead acid batteries corrode due to which connection between batteries and appliance cuts off completely or weakens. In vehicles, corrosion restricts ignition of engines because enough current is not drawn from batteries. What causes this corrosion, what compounds are formed on terminals, and how to prevent this corrosion from happening again, complete details are given below. Equations of chemical reactions are also mentioned to help understand the process of corrosion.
Cause And Compounds Formation:
In case of sealed lead acid battery (also known as SLA battery or dry battery), corrosion happens when electrolyte makes its way out to battery terminals through any leaks or joints. In case of flooded lead acid batteries (wet batteries), electrolyte can jump out while carelessly pouring water into cells. Also, fumes of sulphuric acid which is a part of electrolyte and actually responsible for corrosion keeps arising out of vents of flooded lead acid batteries when battery is charging or hot. Overcharging heats up the battery and heat increases the volume of electrolyte filled inside it. This electrolyte can leak out of vents of overcharged flooded lead acid battery if it is topped up with too much water and also SLA batteries through loose joints.
Posts of lead acid batteries are mostly made from lead. Lead itself is a stable metal. Its reaction with sulphuric acid is very slow. If contact happens at battery posts, the compound which is formed as a result of reaction is lead sulphate ( PbSO4 ). The chemical reaction between lead and sulphuric acid is accordingly:
Pb (s) + H2SO4 (aq) → PbSO4 (aq) + H2 (g)
Sometimes, it also happens that copper clamps or ringed copper terminals which are used to connect battery with wires corrode. Copper itself is not reactive even if dilute sulphuric acid is poured on it. But when electricity passes, it reacts with sulphuric acid and produce copper sulphate ( CuSO4 ) along with water and sulphur dioxide gas. The equation of this chemical reaction is following:
Cu (s) + 2 H2SO4 (ℓ) → CuSO4 (aq) + 2 H2O (ℓ) + SO2 (g)
The white substance which you see around battery terminals is either lead sulphate made from the reaction explained one paragraph above or anhydrous copper sulphate made from the reaction explained in above paragraph. Anhydrous copper sulphate changes to blue colour when water is added to it. The bluish substance which you see around corroded copper terminals or copper clamps is hydrated copper sulphate.
Cleaning Corroded Battery Terminals:
To clean lead sulphate or copper sulphate from terminals, first disconnect terminals from battery. Make sure, you are wearing gloves as these chemicals can affect the skin. Now, wash terminals with clean water. If rust washes away, then no more hassles. Otherwise, wash them with the solution of any of these bases caustic soda, washing soda or baking soda made by dissolving base into water. Simply dipping battery terminals or clamps for few minutes into solutions of these bases also works. After cleaning with base solution, wash terminals again with clean water to clear away the remnants of base.
It is advised not to pour the solution of any of these bases over battery posts to clean them as it might gain access to battery interiors through vents, joints or leaks. If entered, it can badly affect the performance of battery. Instead, use a cloth dipped in base solution to clean them or use a brush to rub the rust off. Take extra precautions while cleaning hydrated copper sulphate which is bluish in colour because it is poisonous.
Prepared solutions are also available in markets to clean the corrosion. You can also use them.
Prevention from Corrosion:
1- Corrosion can occur in dry environment but it is boosted by moisture and salts present in water. Therefore, keep the batteries away from moisture and damp places.
2- Do not wash interior of car engine with water. Water increases speed of rusting metallic parts of engine which are not covered with paint and also joints of power cables and battery terminals.
3- Always keep the battery top dry and free from dust and other pollutants. After pouring water into flooded lead acid battery, never forget to dry the surface of battery. Close the caps of individual cells tightly.
4- Apply Petroleum jelly or grease to battery terminals to protect them from corrosion.
5- Use clamps and battery terminals made from good quality copper which are also alloy plated. Layer of alloy prevents terminals from corroding.