Bolt hydrogen embrittlement cracking dispute

A cracked bolt that Dr. Ballinger circulated in class. Tom and Ballinger are still debating whether the mechanism is hydrogen embrittlement (cathodic) or stress corrosion cracking (anodic). Used to introduce the anode/cathode distinction between the two mechanisms.

People have taken polished sections of steel and put them in a microscope in an aqueous solution, and they actually can look in the microscope and see bubbles of hydrogen forming right there at those crack tips. Over time the hydrogen accumulates at the tensile stress region, because there's more space, and if enough hydrogen gets there you get hydrogen embrittlement, and steel will crack. Dr. Ballinger sent around a piece of bolt that had cracked in service, and he and I are still debating about whether it's hydrogen embrittlement or stress corrosion cracking. The difference between hydrogen embrittlement and stress corrosion cracking — seventy percent of the time metallurgists make no distinction — but in fact one occurs at the anode and one occurs at the cathode. Hydrogen embrittlement occurs at the cathode; oxidation corrosion occurs at the anode. You can very simply change the polarity, and you'll either be cathodically protected — you won't get stress corrosion cracking if you're cathodically protected — but you might introduce hydrogen and get cracking from that.