Greetings...

A dilute (~10% solution) of formic acid is what I used to dislodge deposits on chromed bronze parts exposed to seawater growing a greenish something on them, with excellent results. Would ~5% vinegar perform similarly, or, suggest a commonly found magic compound ?

-Image borrowed from the web-

Don't use vinegar, it will react with both metals. Try citric acid.

6 hours ago, StringJunky said:

Don't use vinegar, it will react with both metals. Try citric acid.

I'm curious as to why that would be. In terms purely of acid strength, citric has a (1st) pKa of 3.13 whereas acetic acid is 4.76 (formic is 3.7). So citric acid is nominally the strongest of the the three and acetic the weakest. So if what you say is true there must be some other effect going on.

8 hours ago, Externet said:

suggest a commonly found magic compound ?

For best results, you need a mildly abrasive component. Anybody's kitchen solution is usually given as a paste made from baking soda mixed with a few drops of lemon juice.

Personally, I'd reach for the Duraglit as I always have a tin of it somewhere.

When clean and dry, a dab of Vaseline on all bearing surfaces (eg threads etc)

9 hours ago, Externet said:

Image borrowed from the web-

The 'blue' corrosion products appear to be coming from the 'thing' on the bottom right that appears to be made out of something else (phosphor bronze filter maybe?).

The 'white' bits look more zincy or aluminiumy to me. So they're probable a scale coming from somewhere else too.

8 hours ago, exchemist said:

I'm curious as to why that would be. In terms purely of acid strength, citric has a (1st) pKa of 3.13 whereas acetic acid is 4.76 (formic is 3.7). So citric acid is nominally the strongest of the the three and acetic the weakest. So if what you say is true there must be some other effect going on.

I just did some casual searching, but I am perfectly willing defer to your experience. Perhaps there is some other effect going on.

20 minutes ago, StringJunky said:

I just did some casual searching, but I am perfectly willing defer to your experience. Perhaps there is some other effect going on.

Did the searching not give any reasons?

By the way, it came as a slight surprise to me initially that citric acid is a slightly stronger acid than formic. Though it makes sense when one thinks about it a bit. But they are all similar, at the stronger end of the weak organic acids. I use vinegar around the house for limescale removal: pots and pans, shower heads, that sort of thing, as it is so cheap: 45p for a bottle of spirit vinegar in the supermarket.

Edited March 19Mar 19 by exchemist

Have you considered ammonia solution?

You could also try (if you can obtain it) ethylenediaminetetraacetic acid (EDTA).

You can use Sodium Thiosulfate to clean Silver.

Paul

On 3/19/2026 at 9:12 AM, exchemist said:

I'm curious as to why that would be. In terms purely of acid strength, citric has a (1st) pKa of 3.13 whereas acetic acid is 4.76 (formic is 3.7). So citric acid is nominally the strongest of the the three and acetic the weakest. So if what you say is true there must be some other effect going on.

What happens if you dilute acetic acid ? So 100ml measuring cylinder, put 10ml of acetic acid and 90ml of water (probably distilled) you then get a 10% solution. Would that make a difference, same would go for Citric acid, which as that can be bought in powder solution you could perhaps make known molar concentrations.

Paul

57 minutes ago, paulsutton said:

You can use Sodium Thiosulfate to clean Silver.

Paul

What happens if you dilute acetic acid ? So 100ml measuring cylinder, put 10ml of acetic acid and 90ml of water (probably distilled) you then get a 10% solution. Would that make a difference, same would go for Citric acid, which as that can be bought in powder solution you could perhaps make known molar concentrations.

Paul

True enough. But at a given molarity, the amount of dissociation into H⁺ would be in the order of decreasing pKa, i.e. lower pKa more H⁺. I suppose there could be other effects, though, from the conjugate base of the acid. Citrate for instance can in principle chelate with the metal ions in the corrosion. I don't know whether in practice this helps remove the corrosion, but would not be surprised if it did.

12 minutes ago, exchemist said:

I don't know whether in practice this helps remove the corrosion, but would not be surprised if it did.

It does, though the heavy lifting should be done by a mild abrasive rather than chemical action.

In some formulations, Brasso uses a bit of oxalic acid (pKa 1.25) as its main chelant, though the pH is balanced by the addition of ammonia. One does not want acid pitting on the Bauerfeind bronze valve casing of a custom Inderbinen trumpet. Does serious damage to the resale value. (Actually wouldn't even use Brasso on mine!)