Welding Tips and Tricks

How does one acid etch a cross section of a weld

Damn good question.

I've never studied what happens to make acid-etch show a weld cross-section so clearly.

Steve S

JDIGGS82 wrote:How does acid etch a cross section of a weld

Acid etch is sort of a precision corrosion. Acid attacks different constituents of a metal in different ways.
there is a whole science to it and I although I am def not a metallurgist, I did spend over 5 years working in a metallurgical lab.

There are all kinds of etches formulated to reveal different features in metal.
One etch will reveal carbides, while another will show grain boundaries at microscopic levels like 100x , 250x, 1000x. Different chemicals are used to examine different features and microstructures. but that kind of etching not what you do when checking penetration on a tee joint.

Most of us here are probly more interested in a quick way to check penetration rather than microstructure...
Weld testing to reveal the weld nugget is referred to as macro etching because magnification is usually 20x or less because the main goal is just seeing the weld vs base metal.

Carbon steels etch easily using a 5% solution of nitric acid and water. Other acids can work also and some are easier to get like phosphoric for example...someone recently posted an ebay listing for phosphoric acid in fact. Who knew you could get a small bottle of phosphoric on ebay?

Anyway... Since weld metal is a cast structure while the base metal is usually wrought, the etch attacks them differently and if the cross section is sanded to a good enough finish, it clearly shows where weld metal is vs base metal. this is how you can see penetration in the cross section of a tee or lap joint easily.

Some metals like stainless and nickel alloys are hard to etch and require an electric current in addition to acid. Same as the kits that remove heat tint from stainless steel.

Also, just a note...some acids used for etching are very dangerous so please be careful out there.
for example , avoid hydrofluoric acid completely. its just too dangerous.


hope this helps ,

jody

Thanks for all the useful info! I agree can never be safe enough when using that stuff

When sanding back, use 300 to 400 grit wet & dry,with plenty of water or even under a dripping tap to get a good shiny polished finish. The better the polish, the better the reveal. Also the straighter the initial saw cut the less sanding you have to do. Dam I hate that sanding.

It basically changes ferric oxide to ferro-ferric oxide. Cold bluing and Ospho do the same thing.No need for hard core acid dips.Cut and dab a little cold blue. You'll see what you need to see.

More info on this topic here http://forum.weldingtipsandtricks.com/v ... 652#p33652

Here is my first attempt showing the technique in video one of my overhead stringers