Do those little felt washers you put on battery terminals actually stop corrosion, or are they a gimmick?

What works better, and it is cheaper than the pads: grease the terminals after cleaning.

The order matters and it is where people go wrong:

1. Clean the post and the clamp back to bright metal — a wire brush made for the job costs very little.
2. Fit the clamp and tighten it properly. This is the step that actually matters.
3. Then smear the outside of the finished joint with petroleum jelly or a proper terminal grease.

Grease before tightening is the classic mistake: it insulates the joint, and a poor connection produces heat, which drives more venting, which produces more corrosion. Grease after tightening seals the metal from the air without ever being in the current path.

Done that way, terminals stay clean for years, with or without the felt rings.

Worth knowing what heavy corrosion is telling you, because it is often a symptom rather than a maintenance failure.

A battery that vents enough to coat its terminals is usually being overcharged or is near the end of its life. If it keeps coming back within months of a proper clean, the interesting question is not what to put on the terminals — it is what the charging voltage is doing.

The quick check: measure across the battery with the engine running and warm. Somewhere around 13.8-14.6V is normal. Consistently above about 14.8 means the regulator is pushing too hard, and that will keep destroying terminals and cooking the battery whatever you smear on it.

Also: corrosion on the negative side more often points to undercharging, and on the positive to overcharging. Not a hard rule, but a useful hint about which way to look.

The corrosion is a reaction between escaping acid vapour and the metal of the terminal, so anything that works has to address the vapour, the metal, or the gap between them.

A lead-acid battery vents small amounts of hydrogen and sulphuric acid mist during charging, particularly when it is charged hard or is getting old. That mist settles on the terminals and reacts with the lead and with whatever the clamp is made of — usually producing the white-green powder you cleaned off.

The pads are felt soaked in an anti-corrosion compound. They sit between the clamp and the battery, so they occupy the gap where the vapour would otherwise collect and reach the metal, and the compound neutralises acid that lands on them.

So the honest answer to question 2: they do address the real cause, in a limited way. They are not a gimmick and they are not a solution either.

Small thing about the two colours, since nobody explains them: the red and green rings are usually identical apart from the dye, and the colours are there so you fit one to each post and can see at a glance that both are present.

They are not chemically different for positive and negative, despite the colour coding implying it. Fitting them the other way round makes no difference to anything except your ability to tell them apart later.