Which metals are inert w/ respect to which acids?

OK, anyone out there got a good chart or reference as to which common metals are reactive/inert with, say, sulfuric, hydrochloric, acetic, nitric, and phosphoric acids?

Thanks, all!

Take a look at the Corrosion Tables on page 1 and pages 13-26 of this document:


Note that most of these corrosion-resistant alloys are very expensive. Type 316L stainless steel is reasonable, though.

Thanks, but what I’m looking for is a bit more general. I’d like to know, for instance, what acids might attack steel but leave aluminum alone.

Practical use-getting stuck items unstuck from within a dissimilar metal.

Maybe there’s a better way. What exactly is the problem?

Perhaps you could try heat/and or cold? Dissimilar metals expand/contract at different rates.

Short story long- I am rebuilding a motorcycle engine. The one hurdle I have not been able to get past is a stuck compression ring on a piston; been working on it nearly 3 weeks. I’ve soaked it in kerosene, PB blaster, Marvel Mystery Oil, and 2 kinds of carburator cleaner. I have also applied heat in the form of inverting the piston, filling it with alcohol, and lighting it (this last 3 times). The ring gap is too small (.040" or so) to fit a tool into.

So, an acid which would destroy the steel ring and yet leave the piston untouched seems like the next likely thing to try, and a friend has suggested that sulfuric acid fits that bill. Naturally, I am nervous about trying it without a little more basic knowledge.

And while I’m at it, maybe it would be useful to know about other combinations, thus my request for info in a chart form. Oh, and if you have another idea I haven’t tried, I’m still open to suggestions…

That’s a tricky one. The problem with using acid is that steel is a noble metal compared to aluminium. Aluminium should in theory be attacked very rapidly by plain water, but it is protected by the rapid formation of a sturdy, self-repairing aluminium oxide film.

The oxide film on aluminium is stable at neutral pH, but breaks down rapidly in acids and alkalis. So any acid is likely to attack your aluminium faster than the steel (in fact, because of their galvanic relationship, the aluminium will protect the steel.)

Your best bet chemically is therefore clean, pure water, which will hopefully rust the steel off and leave the aluminium alone. This would be SLOW (you’re talking weeks or even months) and not guaranteed to work - if there is any salt contamination, you will again risk corroding the aluminium. Probably more trouble than it’s worth.
Theoretically, you could do it electrochemically. Get a big plastic bucket of water. Clean water - use deionised, battery-top-up water in a clean bucket. Add a few grams of a non-chloride salt for conduction. Epsom salts (magnesium sulphate) would do. (NOT edible salt, rock salt, lo-salt or anything like that.) Degrease the piston (detergent/alcohol/acetone), hook it up to the positive of a 12V battery, stick it in the bucket. Hook up any piece of clean steel scrap to the negative of the 12V battery and stick that in the bucket too, but not touching the piston.

What should happen is, the aluminium, being a “valve metal”, will anodise without ill effect. Whereas the ring will corrode like crazy. You’ll also get a lot of oxygen bubbling off the ring and hydrogen off the steel scrap, so do it somewhere ventilated, just like charging a lead-acid battery. There are some possible problems - keep your leads and crocodile clips or whatever out of the liquid for a start. You might lose conduction between the aluminium and the ring as the aluminium anodises, which will kill the process. This is very much a last resort - I can think of too many things that might go wrong, and I worry about the things I haven’t thought of.

I can’t help thinking that a mechanical route would be more practical. A Dremel tool’s an idea - some of the cutting discs are really thin, and the cheapest Dremel imitations are practically disposable. (I’m still trying to burn out my $16 model.) Maybe you could cut along the ring? Very fine drills might also help - drill the ring, tap in an oversized pin e.g. a thin nail and now you have something to grip. Tapping in an oversized pin may even crack the ring. Or you could use a grinder to turn a small nail point into a very fine wedge and have a go at the ring gap.

I’d like to modify matt’s method a little.

I would use sulfuric acid (battery acid) as an electrolyte, an aluminum electrode, and a battery charger rather than a battery. Garage anodizing.
Mix the acid 10-20 percent by volume with distilled water (ALWAYS add acid to water NEVER water to acid). Add a little at a time (it gets hot). And have at least 10lbs of baking soda ready (just in case).
I’ve also experienced some minor “erosion” of parts in a fresh electrolyte, so you might want to anodize some other aluminum part(s) first to “season” the electrolyte.

That said, IMHO the hassle really outweighs the benifit in this case. If the ring is seized that tight, the piston is probably toast anyway. and you’re going to have to strip off the anodize before putting that piston back in the block. Is a new piston really that expensive?

I feel like I should be wearing a hazmat suit in this thread.

Yes, I was afraid the two metals together would behave as a battery.

As to the mechanical solutios you mention, I’m afraid the ring is too thin for that (thinner than the dremel cutting disks or pretty close to it). I might be able to drill the hole to perforate the ring and get more solvent in it, but I definitely couldn’t thread the hole. I don’t think the piston’s trashed, as it is dimensionally OK, but if I can’t get this ring out, I’ll have to get a new one. Drat.

Thanks to everyone for the replies.


Freeing Stuck Piston Rings

Sorry for the confusion, folks. This post was me logged in as my daughter.

I looked at that and saw a few ideas I hadn’t thought of. If my lathe chuck is big enough to hold the piston, I’ll try the toolpost wheel method. Thanks, Astro!