In this month’s failure column, Tim J Carter talks about hydrogen and the importance of paying attention to the possible effects atomic hydrogen may have in embrittling steels and other materials, which can cause cracking and catastrophic failures of structures under stress. Carter is a Consulting Physical Metallurgist, previously in private practice and now with ImpLabs in Benoni.
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When I mention hydrogen to almost anyone, the first pictures they imagine are either the Hindenburg crashing in flames at Lakefield, New Jersey, or a large mushroom cloud over some now non-existent coral atoll somewhere in the pacific. Hydrogen has received much publicity in recent years as the ‘Fuel of the Future’ – and perhaps it is. It is, after all, the most common element in the universe.
But when mixed with metals, hydrogen does present potentially serious problems. There are two main ones, hydrogen embrittlement and hydrogen attack or damage. The two are quite different, but the eventual effects are the same. The first can, and the second one will, reduce the component you are working with to scrap. The first can be fixed, the second cannot. There are also other, much rarer problems, which we’ll deal with later.
Hydrogen embrittlement
By far the most common problem with hydrogen is that it will embrittle steels, and one or two other metals as well. Molecular hydrogen, H2, the gas which comes from your gas supplier in a bright red cylinder with a left-hand thread on the valve isn’t the problem. That’s why we can keep it under pressure in steel cylinders.
The problem arises with atomic hydrogen, H. Not available in cylinders, it arises during operations where hydrogen is generated at the metal surface, such as acid pickling and electroplating. It can also be generated during various corrosion processes.
Atomic hydrogen is the smallest atom known, it consists of one proton and one electron. Iron, in its outermost ‘shell’ of electrons has only seven, but would like eight. The iron atom gets around this problem by ‘sharing’ an electron with its nearest neighbour. When a single hydrogen atom is in contact with an iron surface, it can ‘donate’ its electron to the nearest iron atom, which then appears to have a full outer shell of eight electrons. The hydrogen atom becomes an hydrogen ion, H+, effectively a single proton. Very small and thus highly mobile within the iron lattice.
When it encounters a lattice defect such as a dislocation, however, it can get far enough away from the nearest iron atom to reclaim its electron. If another hydrogen atom is present, they will combine to form a hydrogen molecule (H2), which is too large to move through the lattice and which pins the defect in place. Since dislocation movement is an essential part of plastic deformation, this robs the material of plasticity and brittle fracture usually follows.
Getting rid of hydrogen is relatively easy, a low temperature ‘de-embrittlement’ treatment, usually at around 150 to 180 °C for a few hours is quite sufficient. The hydrogen, even molecular hydrogen, diffuses out and the problem goes away. But it’s surprising how often that treatment is left out of the process – and broken bits, usually threaded fasteners (again!) end up on my desk. Hydrogen embrittlement fractures are easy to identify under a scanning electron microscope, having very characteristic features.
One hydrogen embrittlement failure I encountered was in the compressor disc of a jet engine. When the engine surged and stopped and didn’t want to re-light, the aircraft crash-landed and the airfield fire service put out the ensuing fire with foam. The problem was that the burning magnesium of the turbine compressor casing could, and did, strip the oxygen out of the water in the foam releasing atomic hydrogen, which first embrittled and then caused cracking in the high strength steel disc, which was being thermally shocked due to the contact with the same foam.
The disc went on cracking long after it reached my desk. The pilot survived and had a grand-stand view of the proceedings from the relative safety of his parachute.
Another failure was in the rollers of a rock crusher, which lasted just ten hours in service. When impact tested, the material gave an absorbed energy of just 4.0 J. After de-embrittling at 200 °C for 24 hours, this improved to 29 J. Still not great, but acceptably better.
Hydrogen attack
Another hydrogen-caused problem is hydrogen attack or damage. Very different and much less common, it happens at high temperatures when molecular hydrogen, usually at high pressure, dissociates into the atomic form on contact with iron at high temperatures. The hydrogen atoms enter the steel surface and react with the carbon present to form methane, which de-carburises the material and forms bubbles or micro-fissures. Both the presence of micro-fissures and the decarburisation cause a significant loss of strength, resulting in major problems in high temperature, high pressure vessels containing hydrogen, usually in the petrochemical industries.
Unlike hydrogen embrittlement, hydrogen attack is irreversible, the only cure is replacement. It can be reduced, but not eliminated, by adding powerful carbide-forming alloying elements such as chromium or molybdenum.
Hydrogen attack can also occur in copper alloys. And in silver, too. I have seen this in copper conductors on an electric arc furnace. If the materials, copper or silver, contain oxygen as oxides, the hydrogen will strip the oxygen out of the oxide, forming water. At elevated temperatures, this is steam, and forms voids in the same way that methane does in steels, with the same effect on properties. The cure? Using oxygen-free copper alloys.
There is a form of hydrogen problem I have never seen, but I’ve never worked in the nuclear fuel industry. It’s called ‘tritium bubbling’, where the hydrogen forms a fairly rare isotope called tritium (3H), which happens when the hydrogen in the material gains two extra neutrons in the nucleus when the material is exposed to a high neutron flux. The consequences are similar to those caused by hydrogen attack but it’s also radioactive. I hope you never see this problem either.
Please note that the opinions expressed in this column are mine and mine alone.
timjcarterconsulting@gmail.com
