Hello World
Let me preface the start of the first blog by saying I’m still getting used to the world of additive manufacturing, etching, electrochemical etching, and optical microscopy. Most of my metallography experience is with metal matrix composites and electron microscopy which did not require extensive amounts of etching. Hope you don’t mind the half science half stream of consciousness below.
I’ve been primarily focused on inconel 718 the past few months, but this week I had some trouble etching big 316 stainless steel samples. In my (still learning) eyes, inconel and stainless steels should react similarly to various chemical attacks. After all, they are both corrosion resistant alloys with chromium oxide passive layers you have to eat through. Sure, inconel is a nickel alloy and steel is ferrous, but iron and nickel aren’t that different!
Wrong. Or at least misguided. My favorite inconel etching recipes and plans for greatness were squashed. I’ve basically got two options that I’m fairly comfortable with:
Immersion etching with Kalling’s II
Electrochemical etching with 14% Phosphoric acid
I’ve got a decent dataset and working knowledge of inconel etching times in the above procedures. About 30 seconds works best for Kalling’s and gives great grain structure images and 5 volts for 5 seconds in 14% H3PO4 gives well defined melt pool boundaries.
For stainless, I’ve found Kalling’s 2 to be a bit aggressive in the past but gave it a shot. I really did not like the results and did not want to go down the road of dilution and doing a whole new etching time study. I tried the electrochemical procedure of 5 volts 5 seconds. Nothing. 5 volts 10 seconds. nothing. 10 volts 10 seconds. nothing. 25 volts. oops that did something. it’s burnt.. damn. Well at least I got it to etch. Maybe there’s goldilocks zone in there somewhere. But that’s a problem for another day because I decided to try a new etchant.
Modified Murakami’s reagent. A basic etchant - not something I’ve done before but have been keeping in my back pocket. Found a recipe online that says this is good for stainless so I’ll give it a shot. The recipe says immerse for 1 second to several minutes. Wait, hold on, several minutes? That’s a cool two orders of magnitude swing. I took it easy and tried 1 second, nothing, 5 seconds, nothing, 10, nada, 5 minutes, nothing. Ok I give up, if the needle hasn’t moved in 5 minutes I’m done.
See, the thing you have to know about random etching recipes you find online is that they’re about as good as great grandma’s cookie recipe. You know, the one that tells you to add a few dashes of salt and a pinch of baking soda that leaves you wondering WTF is a dash. Baking is chemistry grandma! We want exact measurements!
I later found some recipes that recommended using Murakami’s reagent at elevated temperatures. I was etching at 20C, so that might’ve been the problem. I’ll swing back around to this etchant and use a hot plate at some point.
I tried 5% Nital, which is used for low carbon steels. I didn’t expect this one to work but I needed to mix up stock solution anyway. It didn’t work. Obviously. It’s important to take a step back and not get frustrated in moments like these. I have to remember I am working with sketchy acids and keep focus.
After all this failure to find successful recipes that other people recommended, I was emboldened to do the dumbest thing possible and make my own recipe. I have never seen anyone recommend a kalling’s type solution in electrochemical etching, and that made me curious enough to see why. It was already too strong for immersion etching in my opinion, so I diluted 40 grams of Kalling’s in 160 grams of water, cranked the DC source to the tried and true 5 volts 5 seconds and generated the image posted above.
The overall sample was not uniformly etched and had mixed results but most areas looked quite pretty like this one. I find this region particularly curious because the top left right corner is where the electrical contact was made with the sample surface. There is a gradient from the contact point to the sample edge going from under etched to over etched. I have a theory that if I get all the settings just right I can target any of those colors on the gradient.
