But what acid? Some swear by muriactic (hydrochloric) acid because it is fast and cheap. Some prefer white vinegar (acetic acid) because it is low-hazard to the body. In fact, neither are all-around optimal for most things.
First, toxicity. Hydrochloric acid breaks down into chloride (sea salt) when spent, so it is harmless to the environment. You can get burned, but there is no long-term toxicity (stomach acid is mostly HCl). Vinegar is non-toxic and biodegradable. But so is lactic acid (the stuff that makes milk sour), the primary ingredient in CLR.
Which is best? It depends on the materials involved. I did some tests, descaling joker valves, with the results below. I also placed metal samples in with the acid, to measure corrosion rates.
The bottom line? Look at the green lines for metals loss during the cleaning period. CLR as clearly safest for aluminum, HCl is safest for steel (with CLR a good second), and other acids proved slow and quite damaging.
- Outboards. If aluminum is a concern, CLR will work 20 times faster than vinegar and cause 4 times less damage than vinegar. I also use CLR in the bathroom at home, where it removes scale without risk to aluminum shower doors.
- Steel. If only steel parts or for plastics, let it rip with HCl.
- Vinegar. Save it for salads.
- Oxcalic acid. Other than brightening teak, keep it away from the boat. In the case of teak, it removes a layer so use it with extreme caution. I suggest keeping it way from the boat.