Major Causes:
– Leaking electrolyte from cracks or loose joints.
– Fumes of acid escaping battery through loose joints or vents.
– Bad alternator/generator causing battery to overheat and evaporate its electrolyte
Details
Often old lead acid batteries including maintenance free (MF), valve regulated lead acid (VRLA) and flooded lead acid batteries cause corrosion of car battery terminals. Over time, car battery go through wear and tear, joints become loose and battery electrolyte may leak out and cause terminals to corrode. If acid itself does not seep out, the acid fumes escaping bad joints can easily corrode copper terminals. Copper is one of those metals that can easily get damaged with corrosion. Once corrosion starts eating it, it won’t stop until the whole terminal is gone.
The overcharging battery is one of the major reasons for corroding battery terminals. Overcharging causes overheating of battery and acid fumes may start escaping through vents, loose joints or cracks developed over time. Once these fumes of sulfuric acid come in contact with copper or lead copper sulpfate (CuSO4) or lead sulfate (PbSO4) are formed. Passing current through these terminals stimulate the corrosion and even new terminals get damaged in no time.
If you see white or blue powdery substance formed around copper battery terminals, that could be either anhydrous copper sulfate (CuSO4) or hydrated copper sulfate (CuSO4·5H2O pentahydrate). Anhydrous copper sulfate is whitish in color which turns to blue color when water is added to it. White powdery substance can also be lead sulfate (PbSO4) formed due to the contact between sulfuric acid and lead battery posts.
• To learn about chemical reactions between sulfuric acid (battery electrolyte) and copper or lead, please refer to this post: Corrosion of Battery Terminals: Explanation, Cleaning & Prevention
• To learn about how to prevent battery terminals from corroding and wasting your money on repeated replacement of damaged terminals, please see this post: How to Stop and Protect Battery Terminals from Corrosion
Both positive and negative battery terminals can get corroded. We have personally observed it with the use of flooded lead acid batteries connected with power inverter. With sealed lead acid batteries installed in car we have observed that very minor crack around terminal in plastic container of battery let some acid fumes escape the battery or some electrolyte itself seeps out due to jumps and vibrations caused by driving on rough roads. That causes battery terminal to corrode and bluish white substance forms around the terminal.
To test at home how copper corrodes with acid, you may use very diluted sulfuric acid and a copper wire. When copper comes in contact with acid it will start corroding but the speed of corrosion will be very slow. To speed up the corrosion process, pass electricity through that copper wire and you will see that copper wire will get eaten away quickly. Still this quick process will take some days with diluted acid until you observe same level of corrosion you see on your battery terminals. Using concentrated sulfuric acid is not recommended but that will further speed up the corrosion process.