The quick summary: Muriatic acid seems to be a bad idea to clean metal, but phosphoric acid might have potential.
I recently used 5% muriatic acid to clean the soot build-up out of my exhaust
and mixing elbows. A 20 minute bath certainly cleaned out the build-up, but it also removed the outside paint
and it took a very short time after cleaning the elbows in water and baking soda (to neutralize the acid) for a surprising amount of rust to appear. The post copied below (from a different website) re: trying to clean metal with muriatic acid provides some interesting insight into why this is not a good idea.
At this point, I've doused the elbows in WD40, but the rust remains and there does not seem to be a rust paint
with a high enough temp rating to repaint them. I don't know what will happen if I try phosphoric acid now, but it might be worth a shot. Any other ideas would be greatly appreciated...
Copied Post: "The problem is that although it (using muriatic acid on metal
)will remove the rust you see (actually what you see is NOT rust, it is the already oxidized byproduct of the rusting process which you can not see) it will set in motion a process that will lead to more damage than you have now. Once the acid contaminates the iron (ESPECIALLY iron castings which tend to be somewhat porous) the chemical reaction that starts will be nearly impossible to stop. Some will say you simply need to neutralize the acid, much easier said than done, and while steps to do this will slow the damage it hardly ever stops it completely. The rust solutions (this appears to refer to phosphoric acid based solutions
) both dissolve the oxide on the surface and convert it to a phosphate that tends to stop the rusting process, the Hydrochloric (Muriatic) acid on the other hand will dissolve the oxides but as far as stopping the rusting process it will be like throwing gasoline on a fire! I have battled this for years on mining equipment that was exposed to Sulfuric acid and I can tell you that once the metal is contaminated it can really make a mess. It is not as simple as removing the iron oxides that you can see you must take into account the chemical process that you are about to start, and this is one that you don't want any part of! Do yourself a favor and use Phosphoric acid because it will remove the oxides also but it will start the right chemical reaction that will leave the phosphate coating that will inhibit rust instead of accelerating it. The Phosphoric acid may take a little longer but it is definitely the better way to go, usually the fastest way out of something just leads right back in!"