Rensselaer Hydrogen Diffusion Experiments (1950s–1960s)

RPI experiment from "back in the 1970s" — polished, stressed, cracked steel specimen immersed in water under microscope; bubbles observed at the crack tip increasing with applied stress. Used to motivate the energy argument for why hydrogen migrates to crack tips.

I was looking online this morning — there are a couple of nice videos that my secretary will send you a link to. I didn't find the one I was interested in, but I did find two others, one older and one younger. Sometime back in the 1970s, some people at Rensselaer Polytechnic took a piece of steel which they polished, and they had it so they could stress it, and they had a crack in it, and they immersed it in water and put it in a microscope, and they looked at just the crack tip, and they could see bubbles coming out. As they increased the stress, they would see the hydrogen bubbles come out with greater frequency right at the crack tip. Why is that? Well, hydrogen is an interstitial atom. It likes to strain the lattice, and if you strain the lattice, the hydrogen will go to the lowest energy state where the lattice is already expanded. That's at the crack tip. I'm pulling on the lattice at the crack tip.

Notched-tensile specimens showing hydrogen bubbles diffusing to the advancing crack tip under microscopy. Tom recalls a researcher (rendered by captioner as "Apollo Tiffany" — identity unconfirmed). ## Figures and framing statistics referenced (not cases)

You always have flaws in the steel — microscopic, smaller than a human hair. Seriously, microscopic flaws. The hydrogen will diffuse to those. If you go on YouTube you can probably find somebody from the 1950s or 1960s at Rensselaer, Apollo Tiffany [?]. They took a piece of steel that they welded and polished — or they may have introduced the hydrogen not by welding but electrochemically, through a rapid corrosion process. They put a notch in it, just like a torn piece of paper, and they put tension on it. They put some blistering on top, and under a microscope they could see the bubbles coming right out of the crack tip, because the crack tip is larger in volume — you're stressing in tension. The hydrogen wants to diffuse there, and as it concentrates at the crack tip, it allows the crack to progress. You can see in the microscope the crack growing, and you can see the bubbles coming up right ahead of it, because the hydrogen wants to diffuse to that crack tip — unless you can diffuse it out to the air faster. It will start even at these little inclusions.