Welding Tips and Tricks

Have any of you had success using acids for stripping mill scale and light red rust? I've been playing with phosphoric acid and hydrochloric acid and they both have their merits but I'm not entirely satisfied with either. Are there any other acids or mixes of acids that are worth trying?

-Sandow

Muriatic Acid removes mill scale , although i have not tried it myself

Muriatic acid is the 16th century name for hydrochloric acid. Thats ok though, I still prefer to think of my electrodes as being Wolfram

-Sandow

Muriatic acid and hydrochloric acid are the same kind of acid but they still name them diferent because one of them is watered down and not as strong. Any acid strong enough to remove mill flake will likely be strong enough to eat away at the base metal leaving the risk of chemical stress induced into whatever you make out of it.

Poland308 wrote:Muriatic acid and hydrochloric acid are the same kind of acid but they still name them diferent because one of them is watered down and not as strong. Any acid strong enough to remove mill flake will likely be strong enough to eat away at the base metal leaving the risk of chemical stress induced into whatever you make out of it.

I got curious about the archaic naming convention and looked around a bit. Muriatic vs hydrochloric has nothing to do with acid strength as it turns out. Muriatic refers to impure industrial crap and hydrochloric is usually reserved for chemistry grades. The acid at my lab is clear and comes as 6 molar and the home depot acid is yellow/green and 31.45% or 10.35 molar.

Well, vinegar and phosphoric acids are both weak acids (in terms of proton donation rates) but will reduce scale. Takes a few hours though. Neither will really reduce actual iron past the top layer though. If anything I'd have thought that acid wouldn't induce surface hardening or stress at all.

I googled around looking for stress effects from acids etching and mostly came back with articles talking about the stress induction from grinding... The one paper I found that did show cracking as a function of prolonged exposure to boiling hydrochloric acid was looking at ball bearings where it was the removal of surface material that allowed internal stresses to split the bearing.

That article also mentions the use of alcohol/acid mixes to speed things up. I think I'll try a Phosphoric acid/ isopropanol mix next and see of that pans out any better.

I could see there being a possibility of hydrogen embrittlement in acid. Was that your concern?

-Sandow

Yes in a general sense. There is a wide variety of steels that can have mill flake on them so unless someone is armed with chemical and metallurgical knowledge then results can vary greatly. Not to mention fumes and caustic dangers in the average shop. Grinding may be dirtier but it's defiantly faster, cheaper, and more concistant.

I thought I'd revisit this now that I've had a chance to play with big buckets of acid a bit more. When it comes to quickly stripping scale or galvanization, 1M hydrochloric acid is fast but dilute enough that it isn't violent in reactions or volatile in emissions. The addition of some isopropanol helps lift off surface oils and speeds things up a bit. The acid I have is about 10M so my final solution is 1:1:8 HCl:isopropanol:water.

This will strip the galvanization off EMT conduit to bare metal in about 2 minutes but doesn't seem to touch the iron.
The downside is that it will still rust pretty quickly afterwards since it is totally unprotected. A quick dip in phosphoric acid will add a monomolecular layer of iron phosphate that will keep rust from forming after rinsing. The phosphate is too thin to have a meaningful influence on weldability and just slightly greys the iron if left to soak.

As for the risk of hydrogen embrittlement, I spoke with a friend the deals with metallurgy and structural engineering and the conclusions were mixed. For metals over about 30 HRC, acid stripping can cause hydrogen embrittlement. Since plating also tends to cause hydrogen embrittlement, this can be a double whammy. The risk is proportionate to hardness though and mild steel averages 31.4 HRC so it is mostly ok but still worth consideration for critical applications.

-Sandow

How do y'all get rid of these harsh chemicals?

You can just put a lid on the buckets and reuse them until most of the acid is used up or it has gotten funky. Once you want to toss it, you just add baking soda until it stops reacting and then dump it down the drain if it is small volumes. For larger scale disposal, there is a liquid hazardous waste area at most local dumps that you can pour it into.

That said, hydrochloric acid gets used as a hardcore drain cleaner...

-Sandow