The Home Machinist!

I have noticed that metal tends to rust more in one area of my garage. My rotary table is getting light rust through a coat of oil. Last night, I realized there was a gallon jug of muriatic acid in that area. Question: is this a possible cause? The jug is capped, but you can still smell the acid.

Yes, I moved the acid elsewhere.

Yes. Move it to an outside shed or something, I would never store that in my shop.

Annoying.

I'd say yes. Muriatic is nasty stuff. I had a bottle under the wash sink and it rotted the floor tiles under and around it. I used it on my steel welding table and everything in the path of the cloud of fumes ended up with a light surface rust like you described. I just use phosphoric acid, no more muriatic, at least not in the shop.

Yes, Muriatic stays in the pool pump area outside the shop.

think barometric pressure when it goes up it inhales and when it drops it exhales. volia rust.

I don't understand how it manages to do it, but it even gets through the plastic jugs they sell it in. I had some under my sink for a couple of months (I'm not the best housekeeper) and when I opened the cabinet (A) there was a smell of HCl and (B) the tin cabinet was rusted.

I won't even keep it in the garage any more -- it lives out in the yard shed.

The dilute stuff -- like toilet-bowl cleaner -- doesn't seem to give off as much gas. Maybe the hardware store stuff is close to saturation, so you get some serious vapor pressure ? Dunno. When I worked (I was a chemist) we just kept the stuff in a cabinet connected to a fume-hood, so any vapors just went up the pipe.

I'm glad I figured it out. It was really aggravating.

When I was at the JD shop, the old steam cleaner got the coils scaled up to the point it would barely work, so one of the owners decided to "fix" it...so he got a jug of muriatic acid & poured the coils full of it. After bringing them into the shop of course...he left it open all weekend, and Monday morning it stunk like heck in there & every unpainted metal surface in the shop had that fine coat of rust. ..tools, vises, parts, everything...on top of that, the coils were still plugged.

Down here everyone has a pool. Also, if you have acid handy, you can buy cheap galvanized steel for welding projects. The acid takes the zinc off fast.

Polyethylene is "waterproof" mainly to the extent that it won't allow fluids to pass through. But it and other plastics do allow gas molecules to diffuse through them. The rate is generally very small, and depends on many factors, such as the thickness of the plastic, temperature, the "vapor pressure" of the molecule trying to go through, and the concentration inside the bottle [very dilute HCl will diffuse out much more slowly than concentrated!] But it's definitely NOT zero.

Water vapor can diffuse either inward or outward through most plastics, as can oxygen or other gases - including hydrogen chloride (HCl, the "active ingredient" in muriatic acid). Once free, the HCl molecules will "fly around," often bouncing off of inert surfaces like paint. But if they EVER get to unprotected steel, they find a happy home and cause corrosion (making use of any H2O molecules already on the surface). Because they stay there and keep working, their effect is much greater than just a 1:1 ratio of HCl molecules to iron oxide molecules.

Acids like phosphoric are much less likely to diffuse through plastics because they have much lower vapor pressure. They like to stay sirupy. I'm pretty sure this is true of sulfuric acid, too. Sodium hydroxide (lye) has near zero vapor pressure. It's a solid when pure. Ammonium hydroxide ("ammonia"), on the other hand, just loves to blow off its vapor.

Your nose is pretty sensitive, and (USING CAUTION), you can pretty well predict which acids or bases have high vapor pressure, just by the fact that they do or don't smell when you open a bottle a couple feet away. The smell of acid fumes in a cabinet containing a closed plastic bottle is, of course, a dead giveaway. The TSA's and other agencies' bomb sniffing equipment, and drug dogs' noses make use of the fact that, even if completely sealed, plastic bags and containers allow diffusion of the "substances of interest."

DIffusion of gases through membranes is whole field of study for some chemists. No way to go into it all here, but I hope some of these (simplified) ideas are helpful.

Hman, interesting first post.

At one time I kept a variety of acids and compounds used for weld etching in a small OSHA required metal locker. All in glass, with I imagine plastic or bakelite type screw tops. Everything inside rusted. There were open tubs of baking soda on each shelf to neutralize the sample or emergencies.
Commercial codes prevented outdoor storage.