Naphthenic Acid Corrosion (NAP Acid)

Naphthenic (NAP) acid corrosion continues to be a major problem in high temperature refinery corrosion. Sulfur present in the oil leads to some additional corrosion but also to formation of iron sulfide scale which may protect the underlying steel from further corrosion. The content of naphthenic acids and sulfur species varies widely in crude oil fractions and leads to a complex pattern of attack which is still not well understood.

Laboratory studies are under way focusing on understanding the mechanism of combined NAP/sulfide corrosion under stagnant conditions (in autoclaves) and under flowing conditions using a high temperature rotating cylinder apparatus.

Aims

• Identify the role of sulfur containing species in NAP acid corrosion.
• Identify the kinetics of sulfide corrosion and scale formation under high temperature conditions.
• Evaluate factors affecting the protectiveness of iron sulfide scales formed on steel.
• Determine the kinetics of iron sulfide scale formation and removal under flowing conditions;
• Model the process of sulfide corrosion and protection in NAP acid corrosion.

Deliverables

A model of the NAP/sulfide corrosion simulating the attack found in refineries on a laboratory scale.

Timeline

The NAP Acid Project was initiated in September 2004 and is scheduled to end in August 2007.

Some Preliminary results

Test samples- mild and alloyed steel coupons.

Temperature – 300 – 400°C.

Pressure range: 0 – 100 psig.

Rotation range: up to 2500 rpm

Test oil – white oil with a given sulfur content.

Total Acid Number (TAN) achieved by spiking oil with commercial naphtenic acids.

• Corrosion rate and iron sulfide scale formation rates were determined by weight loss/gain measurements.
• Scanning electron microscope (SEM) was used for investigating corroded surface of coupons and iron sulfide scale.

Integral corrosion and scale formation rates for alloyed steel.

Cross section (SEM) of the coupons with sulfide scale (alloyed steel, 48 hours experiment)

Top views (SEM) of the coupon surface (mild steel, 48 hour experiment).

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