Hangbo Alloy Group
Shanghai · Est. 2012
Naphthenic acid corrosion attacks refinery crude units at high TAN and temperature. Learn which nickel alloys resist NAC and how to specify them safely.
Naphthenic acid corrosion (NAC) is one of the most aggressive and least visible degradation mechanisms in petroleum refining. It strikes in crude distillation units, vacuum columns and atmospheric pipestills whenever the feedstock carries a high Total Acid Number (TAN). Above roughly 220°C the naphthenic acids vaporize and then re-condense on steel surfaces, stripping away any protective sulfide film and eating into the metal directly. The outcome is deep pitting, rapid wall loss and sudden leaks that trigger unplanned shutdowns — often with no warning from routine inspection.
Naphthenic acid corrosion is governed by three interacting variables: acid content, temperature and fluid velocity. The TAN value of the crude is the first warning sign — once it climbs above about 0.5 mg KOH/g the risk is real, and above 1.5 it becomes severe in the hot sections. The damage window sits between roughly 220°C and 400°C, with a peak around 280–350°C where the acids are most active.
Velocity matters because NAC is a flow-dependent mechanism. At low velocity a passive sulfide film can survive and even protect the steel; once velocities rise in transfer lines, furnace outlets and tower draw-offs, the film erodes away and the bare metal is exposed to fresh acid. That is why the worst thinning shows up at elbows, tees, pump suctions and reboiler returns rather than in straight runs.
Sulfur complicates the picture. In some units dissolved sulfides help form a protective iron sulfide layer that slows NAC; in others, high sulfur and high TAN act together to accelerate attack. This is why simplistic rules like "high sulfur is safe" fail, and why each crude blend has to be evaluated on its own.
Carbon steel and even standard 300-series stainless steels lose their protective films in aggressive NAC service and corrode at rates measured in millimetres per year. Low-alloy chromium steels (5Cr, 9Cr) extend life but still fall short once TAN and temperature both climb. The alloys that survive are those with substantial nickel content — nickel resists the organic acid attack directly and keeps the matrix stable.
For refinery hardware the field-proven choices include Monel 400 (Alloy 400) for lower-temperature, high-TAN overhead and desalter systems, Alloy 825 and Alloy 20 for mixed acid-chloride duty, and the molybdenum-rich Hastelloy C-22 or C-276 where chlorides and acids coexist. Alloy 625 is the workhorse for weld overlay and cladding on carbon-steel bodies, rebuilding a corrosion-resistant surface without replacing the whole component.
The practical rule refiners use: when TAN exceeds about 1.5 and metal temperatures pass 260°C, specify a high-nickel alloy or a nickel-clad overlay rather than betting on chromium steel. As a dedicated nickel alloy manufacturer, Hangbo Alloy Group supplies these grades across bar, tube, plate and weld wire so the same heat chemistry can be used for both the base part and its overlay.
Material selection is only half the battle. Refineries that run heavy, high-TAN crudes keep NAC under control with a combination of operating discipline and inspection. Efficient desalting removes chlorides and buffers the overhead, while blending high-TAN crudes with sweet feedstock dilutes the acid load before it ever reaches the furnace.
Velocity management is equally important: oversized lines, smooth transitions and avoided dead legs keep local velocities below the threshold where films wash away. On the inspection side, risk-based inspection (RBI) with frequent ultrasonic thickness mapping of the known hot spots — crude tower bottoms, transfer lines and vacuum column washes — catches wall loss before it becomes a leak. Where carbon steel must be retained for cost, a nickel-alloy weld overlay of Alloy 625 restores years of service life.
Risk appears around a TAN of 0.5 mg KOH/g and becomes severe above roughly 1.5, especially when metal temperatures are in the 260–350°C window. Below 220°C the acids are not active enough to attack steel directly.
Sometimes, and sometimes not. Sulfides can form a protective iron sulfide film that slows NAC, but in many high-TAN, high-sulfur crudes the two mechanisms act together and accelerate damage. Each crude blend must be assessed on its own rather than relying on a blanket rule.
For most hot, high-TAN sections the practical choices are Alloy 825, Alloy 20, or molybdenum-rich grades such as Hastelloy C-22 and C-276 where chlorides are present. Carbon-steel equipment is often protected with an Alloy 625 weld overlay instead of being fully replaced.
Only in cold, low-velocity, low-TAN locations such as the desalted crude inlet and certain overhead lines below 220°C. Any hot, high-velocity or high-TAN zone should use a high-nickel alloy or a nickel-clad overlay to avoid rapid wall loss.
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.