Here’s another one of my interview questions
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It’s still a question related to certain qualification requirements; the mechanism of damage caused by high-temperature hydrogen is the most common corrosion mechanism in hydrogenation reactors. The selection of materials can be guided by the Nelson curve (shown below). A higher chromium content enhances resistance to high-temperature hydrogen corrosion. The horizontal and vertical axes represent hydrogen partial pressure and temperature respectively, and we consider the area below a particular material curve to be safe. Austenitic stainless steels are not shown in the diagram because they have complete resistance to high-temperature hydrogen damage. 2.25Cr-1Mo steel is a base material for general hydrogenation reactors, with its inner wall clad or surfaced with 304L, 316L, or 347L. Here comes the question: Under the normal operating conditions of hydrogenation reactors, and referring to the Nelson curve, 2.25Cr-1Mo is completely safe. Then why do all hydrogenation reactors still require a stainless steel lining on their inner walls? It can be said that stainless steel is more resistant to high-temperature hydrogen corrosion than Cr-Mo steel, but such an answer seems a bit weak; could the examiner’s intent really be that simple?
I have no experience with hydrogenation reactors; could you give some advice?
The surfacing layer is added to resist hydrogen sulfide corrosion
Simply put: the outer wall provides strength, while the inner wall provides corrosion resistance.
Hydrogen corrosion. Under high temperatures (≥200°C), high pressures, and in the presence of hydrogen, steel can suffer from hydrogen corrosion (irreversible hydrogen embrittlement): Fe3(carbonide) + 4H → 3Fe + CH4 ； C (solid-solved carbon) + 4H --- CH4; this results in decarburization of the steel, microcracks at the grain boundaries, and bulging on the surface, thereby reducing the material’s plasticity and even leading to cracking and failure of the equipment. c) Hydrogen embrittlement. During operation, the reactor wall material absorbs hydrogen; when the reactor is shut down, especially when residual hydrogen accumulates in the welded areas, the remaining hydrogen in the wall material can cause the material to become brittle (reversible brittleness)
d) Creep embrittlement. It has been proven through experiments that in the hydrogenation reactor, the operating temperature reaches over 440°C, which is more than 40% of the material’s melting point (in absolute terms), resulting in creep embrittlement. e) The combined effect of temper embrittlement and hydrogen embrittlement; after temper embrittlement, the material’s resistance to hydrogen embrittlement decreases. f) The combined effect of hydrogen corrosion and creep embrittlement. Hydrogen corrosion causes decarburization of steel, thereby reducing its creep strength; the application of creep strain accelerates hydrogen corrosion and decarburization.
The principle of hydrogenation reactions is generally to reduce aldehydes to acids. Hydrogenation reactors contain a small amount of acid, so it is necessary to take into account not only the mechanisms of damage caused by high-temperature hydrogen but also the corrosion effect of this acid. 2Cr steel is likely not resistant to acid corrosion; therefore, cladding with 304 or 316L steel is used to prevent such corrosion. It is also advisable to carry out pickling and passivation processes. I’m not sure if my answer is correct; please feel free to provide feedback
Under the same high-temperature H2S/H2 conditions, the corrosion rates of lined and unlined surfaces can differ by an order of magnitude. CrMo steel is used primarily due to its high-temperature strength; therefore, in some hydrogenation reactors that are not exposed to H2S, lining with weld-on coatings is not used either, when the corrosivity of the medium is not very high.
Reply 1# realben: Personally, I think it’s a bit unnecessary to use 2Cr heat-resistant steel for the cylinder body, since a stainless steel lining is already in place. The shell material used for the hydrogenation reactors in our facility is currently Q245R (probably due to low pressure or some other factor)
It is resistance to polysulfate stress corrosion cracking.