Hangbo Alloy Group
Shanghai · Est. 2012
Hydrotreating and hydrocracking reactors see wet H2S, hydrogen and high heat. See how nickel-alloy cladding, overlay and NACE MR0175 limits protect parts.
Hydrotreating and hydrocracking sit at the heart of every modern refinery, cleaning sulfur and nitrogen out of feedstocks and cracking heavy oil into transport fuels. Inside those reactors the environment is brutally combined: high hydrogen partial pressure, temperatures of 300 to 450 degrees C, and sour, wet process streams. Carbon and low-alloy steels can carry the pressure but not the corrosion, so the wetted surface is almost always a nickel alloy applied as clad plate or weld overlay. Getting the grade, the overlay procedure, and the material qualification right is what keeps a reactor on-stream for its full turnaround cycle.
The dominant threat is hydrogen sulfide in the presence of water. Above about 230 degrees C, hydrogen attacks steel directly through high-temperature hydrogen attack, while wet H2S promotes sulfide stress cracking and blistering on the low-alloy base. Hydrochloric acid from cat-cracked feed, ammonium chloride salt, and polythionic acid formed during shutdowns add more mechanisms. No single bare steel resists all of these, which is why the internal surface is protected by a corrosion-resistant alloy layer.
Temperature, hydrogen partial pressure, and H2S concentration together set the severity. Licensors use these variables to pick both the base material and the overlay thickness, and small changes in operating envelope can move the required grade up a step. Designers who under-specify to save cost usually pay for it at the first internal inspection.
The standard solution is a corrosion-resistant alloy applied to the bore of the reactor shell and internal parts. Two routes dominate: explosion-bonded or roll-bonded clad plate, where a thin nickel-alloy layer is metallurgically joined to a low-alloy backing; and weld overlay, where the alloy is deposited by submerged-arc or strip-cladding directly onto the machined shell. Overlay is common because it gives a fully homogeneous, pore-free layer and lets the fabricator choose the exact chemistry.
For sour hydrotreating, Type 309L plus Type 347, or a straight 347 austenitic stainless overlay, is widely used. Where the service is more aggressive, or where the reactor also sees naphthenic and hydrochloric acid, a nickel alloy such as Alloy 625 (ERNiCrMo-3) overlay is specified for its higher molybdenum and better resistance to chloride and sulfur attack. The overlay is not decorative; it is the only thing between the process and the pressure boundary.
Different zones of the reactor see different duty, so the alloy choice is zoned. The shell bore and inlet hardware take the overlay described above. Internals such as an trays, quench-pipe liners, and thermowell guards that see the hottest, most corrosive flow are often made from solid Alloy 625 or, for extreme service, a C-family grade. Catalyst-support baskets and screen meshes use corrosion-resistant alloys chosen for both strength at temperature and freedom from embrittlement.
Downstream of the reactor, air-cooler tubes and fractionator overheads can be the real corrosion bottleneck, often running solid 300-series stainless or duplex where chloride and ammonia chloride appear. The lesson for specifiers is to treat the reactor and its downstream train as one system rather than optimizing the vessel alone.
Buying a hydroprocessing reactor is less about the base steel and more about proving the overlay. The clad or deposited layer must meet ferrite, dilution, and step-coupon corrosion tests, and the chemistry must fall inside the window that gives the rated resistance. A frequent failure is accepting an overlay with too much dilution from the backing steel, which quietly lowers the chromium and molybdenum at the surface where they are needed most.
Sour service also pulls in materials qualification rules. The equipment must satisfy the hardness and heat-treatment limits in NACE MR0175 / ISO 15156 so that no component can sulfide-stress-crack. Traceability from melt to final overlay, plus independent inspection at the hold points, is what lets a refiner trust the vessel for the next decade of operation.
A solid nickel-alloy reactor would be prohibitively expensive and the base still needs low-alloy strength for pressure. A thin overlay or clad layer delivers the corrosion resistance where it matters while the cheap backing carries the load. The nickel alloy cladding and overlay welding guide explains how the layer is applied and tested for refineries.
NACE MR0175 / ISO 15156 sets hardness and heat-treatment limits so that components in wet H2S service cannot sulfide-stress-crack. Every wetted part, including the overlay and internals, must comply. Our NACE MR0175 guide for nickel alloys in sour service breaks down the limits that apply to hydroprocessing equipment.
Hydrogen attack is blocked by keeping hydrogen and steel apart through the corrosion-resistant overlay, and by choosing a base steel with enough chromium and molybdenum to resist attack at the design temperature and pressure. The high-temperature hydrogen attack notes cover the Nelson-curve thinking and where nickel alloys enter the picture.
As a dedicated nickel alloy manufacturer serving oil & gas sectors, Hangbo Alloy Group supplies the grades discussed here across all standard product forms with full ASTM/ASME documentation and EN 10204 3.1 certification. Our material is specified for applications where corrosion resistance, high-temperature strength, and traceable quality are non-negotiable. See our production & testing facilities for more on how every heat is verified.