Etching Weld Cross-Sections

[Wheels17]

I took a couple of welding courses about 5 years ago, and finally purchased a MIG welder. I've been practicing, and think I'm doing OK on the weld geometry and penetration. I've seen the nice polished and etched cross sections showing the penetration of the weld bead into the parent metal. I have the polished cross sections, but have not etched the samples. What type of acid is normally used to etch the samples? I could steal a drop from my battery if sulfuric would work. Access to other acids is more of a problem.

[Harold_V]

Assuming sulfuric works, I don't recommend you remove any from your battery. It's readily available at auto parts stores.

If you discover that hydrochloric (muriatic) is used, it's readily available at hardware stores. It's used routinely for etching concrete and cleaning brick.

The only acid that you'd have trouble buying that you may wish to use is nitric. It's very difficult to buy now, and very expensive unless you buy in bulk.

Harold

Edit:
Thanks to the information provided by PeteH, it's clear that Hydrofluoric acid may be required for some procedures. It, too, will be difficult to source, and should be handled with the utmost respect.

Thanks for the links, Pete.

[PeteH]

This table may help: http://www.metallographic.com/Etchants/Etchants.htm

And for techniques: http://www.azom.com/article.aspx?ArticleID=3504

[Wheels17]

Exploring Pete's link is interesting. Nitric acid is usable with reasonable caution, but difficult to obtain due to concerns about explosives manufacture. Piciric acid is even worse, being an explosive itself. And hydrofluoric acid is a terrible health hazard. The initial contact doesn't immediately burn, but the tissue dies anyway, and it can chemically interfere with nerve conduction and cause cardiac arrest.

I've seen a site (that I can't immediately re-find) that had some home etchings of welds. If I find it, I'll post back here.

[Harold_V]

hydrofluoric acid is a terrible health hazard. The initial contact doesn't immediately burn, but the tissue dies anyway, and it can chemically interfere with nerve conduction and cause cardiac arrest.

According to the Merck Index, as little as 24 square inches of flesh exposed to hydrofluoric acid can lead to death.

Harold

[mechanicalmagic]

HF is truly nasty stuff. It can travel through the skin, once there is contact. It damages skin as it passes through, and continues until it hits bone. Then it dissolves the bone. One treatment is to inject large amounts of Calcium in front of the bone, to hinder the progress.

I have seen the results of HF contact, I would not even consider having it on my property, for any reason.

Dave J.

[PeteH]

Agreed -- HF is evil stuff. Not only does it go straight for the bone as though it were conscious, you don't even feel it until it's already done damage.

We had to use it (rarely) at work, and kept a jar of chalk powder in glycerin right at hand. If anyone got the acid on skin, the first step was a rinse, the second, a heavy plastering with chalk paste (provides calcium for the acid to react with) and the third, a trip to the ER.

Oddly enough, it used to be available as a rust remover. My mother worked in an industrial linen supply house back in the 50's, and brought a bottle of it home. IIRC (it was 60-odd years ago) it was 6% HF. I etched some bits of glass with it, and I'm amazed that I didn't do anything awful to myself. (I'm surprised that I survived childhood and adolescence -- let alone young-adulthood -- at all !!!)

More recently, I found a "glass-etching" kit at a local crafts shop. Since the only chemical that will etch glass at STP is hydrofluoric acid, I was curious. Turns out it contains sodium bifluoride, which will etch. The label was loaded with cautions, but none as serious as those you'd see on an MSDS for HF. I wonder if IT would work on a weld.

BTW - I missed the picric acid bit. I wonder why that's recommended. It's not a very strong acid; I wouldn't expect it to have much impact on steel/iron.

[AndrewMawson]

HF is truly nasty stuff. It can travel through the skin, once there is contact. It damages skin as it passes through, and continues until it hits bone. Then it dissolves the bone. One treatment is to inject large amounts of Calcium in front of the bone, to hinder the progress.

I have seen the results of HF contact, I would not even consider having it on my property, for any reason.

Dave J.

I started work in an electronics company in the late 1960's where they were etching silicon wafers using hydrofluric acid in glove boxes - the girls were instructed to air test the integrity of their gloves every two hours which they grumbled at. One day there was the most piercing scream I've ever heard, and still hear in my mind occasionally. A girl had removed her hand from the glove box and left a finger in the glove. Presumably a pin hole in the glove had let the HF in and it had penetrated to the bone. After that there were no more grumbles!

[JTiers]

Hydrofluoric acid may be STILL able to be purchased at your local grocery store......

It has been sold even quite recently (in a rather dilute form) under brand names such as "Whink" for removing rust stains. I have a bottle of it here, so I haven't looked for a few years to see if it is still out there.

I have not tried it for etching metallographic specimens, but it does a good job on rust stains in the sink.

Says right on the front that it contains hydrofluoric acid.

[refinery mike]

If you wish to see the quality of your weld you would be far better to cut a token of the weld and then break it forceably across the weld after notching it . look for fine grain good, pockets bad, is the weld attached well at the root and at the top Good, is there undercutting, bad. It may not be as pretty as an etched surface but it can tell you a whole lot about your weld. From my experience the thing most likely to go wrong with mig welding is insufficient penetration. Especialy check the root of the weld.

[Wheels17]

Thanks for the suggestion. There's not much that is more critical than refinery welding (I'm a ChemE).

In the class I took, I often had trouble with undercutting with stick, but I haven't seen that yet with MIG. At first, I had trouble with high beads, but I'm having much better luck with finding the right settings for wire speed and voltage. The chart on the machine truly is a guide, not an absolute.

I've been practicing on 1-2" wide coupons, and am to the point where I can't hammer them apart as the base metals bend rather than the weld failing. Anything I've made for real, I've done a prototype weld and taken the hammer to it. It's been very interesting to see the effect of inadequate cleaning (scale on hot rolled samples). Also, the need to start a little slow to ensure that the very beginning of the weld on cold steel has adequate penetration.

[SteveR]

In this month's Welding Journal, they talk about metallographic cross sections and they use 2% Nital for etching. Nital is available from http://www.reagents.com.

Steve