Hydrogen chloride. Used for dissolving mortar stains from masonry. And cleaning my spirit levels and tools. And exhaust pipes it seems.
I’m a bricklayer. We use it to wash down facing bricks after building especially after being built during wet damp weather.
Hydrogen chloride is actually a gas which when dissolved in water becomes hydrochloric acid. Found in many cleaning solutions in different strengths such as drain cleaner, brick cleaner, spirits of salts etc. Can cause burns to skin and eyes. Also gives off very pungent fumes. Stains or dissolves chrome and zinc plate. A great cleaning agent but use it carefully and research what you are planning to do.
When using corrosive liquids for cleaning make sure that you're meticulous about flushing it, especially from tight spaces, cavities, etc. Crevice corrosion is something that can cause future problems. Some info: https://en.wikipedia.org/wiki/Crevice_corrosion
Yes it is hydrochoric acid. Though looking in the spec sheet on my bottle it calls it something like hydrogen chloride suspension or solution. Something to do with the legalities of selling "acid" to Joe Public I expect. Even on masonry you always test an area before applying. And it's always a diluted solution like 5:1. If you get the dilute on your skin it stings like nettle rash and you'd want to wash it off straight away but it won't burn you to the bone. The neat stuff straight out of the bottle is powerful and needs to be treated with care. I also thoroughly wet the masonry with water before applying otherwise it gets sucked into the masonry and mortar immediately which can attack the substrate while leaving the staining that you want to remove high and dry. And as @Keith_P says above, even on brickwork, flushing is important to avoid ongoing discolouration and damage.
