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Naphthenic Acid
Susceptible materials: Carbon steels Environmental conditions: Temperatures in the range 220C to 400C Expected failure mode: Leakage  
Naphthenic acid is the collective name for all organic acids present in crude oil. A large number of acids can exist spanning a wide boiling range. However, the majority is within the heavy gas oil range of boiling point (375 – 425C) and exist in sour rather than sweet crude oils. Naphthenic acid content is often defined by the amount of KOH required (in mg) to neutralise 1 gram of the acidic oil, otherwise known as the Total Acid Number (TAN) (refer to ASTM D664 or D974). For instance, corrosion is noted with crudes and sidecuts with TAN’s in the range 0.5 to 6 mg KOH/g.
Naphthenic acid corrosion occurs at high temperatures above 220C by the direct reaction of the acid with steel to produce iron naphthenates, which are oil soluble and non-protective, so that attack is made significantly worse under high liquid velocities and turbulence. Naphthenic acids are most corrosive around 280C, although the acids decompose above 400C, where corrosion becomes negligible. For a given temperature, corrosion rate is proportional to TAN, while, for a given TAN, corrosion rate triples for each 40C increase in temperature.
Carbon, low alloy, and stainless steels (with up to 12%Cr), all exhibit uniform attack (up to 20mm/yr) often in the form of sharp sided grooves/ripples. In contrast, austenitic stainless steels, e.g. 304, tend to pit in the presence of naphthenic acid. Molybdenum-additions (> 2.3%Mo) to stainless steels are recommended for corrosive naphthenic crudes. Material selection should consider attack by other mechanisms, e.g. hydrogen sulphide, and the products of cracked naphthenic acid, e.g. formic, acetic and carbonic acids. Susceptible process units and locations include:
Crude and vacuum distillation units;
Sometimes in thermal and catalytic cracking units;
In reformer units: top part of reactor and upstream of reactor, during hydrodesulphurization of gas oil and heavier feedstock;
Furnace coils, transfer lines, vacuum columns (overhead condensers), side stream coolers, pumps (especially where velocities exceed );
Under high velocity (> 30 m/s), turbulence, or impingement conditions;
Elbows, weld reinforcements, pump impellers, thermowells, steam injection nozzles; and
Towers where freshly formed acid runs down metal surfaces
Methods of avoidance
Blend oils to reduce TAN to <1.0;
Replace carbon steel with 5Cr 0.5Mo steel for temperatures above 230C;
Type 316/317 for most conditions (i.e. >2.3% Mo), ideally stabilised with Ti, e.g. 316Ti;
Weld with Type 309Mo, the molybdenum is required for corrosion protection
Aluminised steels, Alloys 600 and 800 are also resistant, but with limited experience
Further reading
Piehl RL: “Naphthenic acid corrosion in crude distillation units”, Materials Performance, January 1988, p37-43.
Slavcheva E, Shone B, Turnbull A: “Review of naphthenic acid corrosion in oil refining”, British Corrosion Journal, Vol.34, No.2, 1999, p125-131.

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