Black stain spreading on keris during vinegar cleaning

[Jean]

Originally Posted by jagabuwana

A word to the wise:

A great regret of mine is opting to use citric acid as opposed to regular white cooking vinegar, on one of my old blades.

It was the first time for me cleaning any blade.

Firstly I put too much citric acid in and it flaked some of the steel off the blade.
It was already in quite a fragile state due to its purported age, and the concentration of citric acid was far too harsh.

For this reason I will always opt to use cooking vinegar and father time.

My personal experience is quite different from yours Jaga, probably because your blade was in too poor condition for being treated.
I tested citric acid at 10% concentration (100 g of pure citric acid powder diluted in one liter of water) in controlled conditions (regular checking) and it worked quite well without attacking the metal itself, but not better than pure vinegar. I also tested oxalic acid (used for de-rusting bolts) in similar conditions and it also worked correctly. Vinegar (acetic aid), citric acid, and oxalic acid are weak organic acids with a PH of about 3 in solution so they can safely be used with ferric metals.
By the way and as a ex-chemical engineer, I would not use citric acid and sodium bicarbonate together as the bicarbonate neutralizes the acidity of the citric acid!