Uncovering Battery Corrosion

Open the hood on most vehicles that have had a battery in them for some time and you’re bound to see at least some corrosion forming on the tops of one of the two posts of the battery. When you’re faced with purchasing a new battery, you may be asked about different forms of protection for the terminals. To some, this may seem like crazy “snake oil” or an unnecessary item, but it’s not. To understand it’s function as a protector against corrosion, it’s important to understand exactly what’s happening to cause it.

What Causes Battery Corrosion?

Battery corrosion around the posts of the battery is caused by the sulfuric acid vapor that can escape from the battery. Sometimes this vapor is vented out of the top vent blocks on the battery, but other times, small amounts of this vapor leak out in the area between the posts and where they seal to the plastic battery casing. This vapor mixes with other gas under the hood, the heat of the engine, the differences in lead composition between the posts and the terminals, the copper of the battery wires, and creates corrosion. Under the hood of a car, temperatures can exceed 250 degrees, and this heat causes the metal of the posts and the plastic of the casing of the battery to expand at different rates. This is the reason why you rarely see corrosion on the posts of a battery that’s under a seat, or in the trunk of a car. These batteries aren’t exposed to the extreme temperatures you find under the hood.

How to Protect Car Batteries from Corrosion

One of the easiest and best solutions to help protect against corrosion is felt battery washers and a protective grease. Battery washers are chemically treated to help absorb that vapor at the source, right at the base of the post, before it gets to the terminal. Battery grease is a silicone-based grease. Unlike petroleum jelly which is, like its name says, petroleum based, silicone grease will last longer when exposed to under-hood heat. Coat the top of the washer and the terminal in protective grease before installation, and that’s it! Over a long period of time, these washers lose their ability to absorb that vapor. They don’t last forever, but they help keep the terminals clean for several years.

On a side post battery, the post is actually BELOW the acid level, so a felt washer, even though it could be used, is not going to provide the level of protection as it does on a top post battery. This is where buying terminal grease alone will help protect. When a side post battery is subjected to heat and has the same separation between the battery and the post, it can actually leak acid. This heavily corrodes the terminal, sometimes so badly that the bolt seizes in the bolt hole. By applying terminal protectant into the bolt hole and on the terminal, you are actually creating a barrier that helps work like an anti-seize.

Battery terminal grease also provides great protection for non-automotive batteries. Marine applications, ATV, lawn and other small engines that operate on a battery are subjected many times to extreme heat. These batteries don’t offer the ability to use a washer on them, but do allow you to coat the terminal lugs, or bolts, with protectant.

Advice, how-to guides, and car care information featured on and AutoZone Advice & How-To’s are presented as helpful resources for general maintenance and automotive repairs from a general perspective only and should be used at your own risk. Information is accurate and true to the best of AutoZone’s knowledge, however, there may be omissions, errors or mistakes.

Be sure to consult your owner’s manual, a repair guide, an AutoZoner at a store near you, or a licensed, professional mechanic for vehicle-specific repair information. Refer to the service manual for specific diagnostic, repair and tool information for your particular vehicle. Always chock your wheels prior to lifting a vehicle. Always disconnect the negative battery cable before servicing an electrical application on the vehicle to protect its electrical circuits in the event that a wire is accidentally pierced or grounded. Use caution when working with automotive batteries. Sulfuric acid is caustic and can burn clothing and skin or cause blindness. Always wear gloves and safety glasses and other personal protection equipment, and work in a well-ventilated area. Should electrolyte get on your body or clothing, neutralize it immediately with a solution of baking soda and water. Do not wear ties or loose clothing when working on your vehicle.

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