Naphthenic Acid Corrosion Mechanisms

Introduction

Naphthenic acid corrosion (NAC) and high temperature crude corrosivity in general is a reliability issue in refinery distillation units. The presence of naphthenic acid and sulfur compounds considerably increases corrosion in the high temperature parts of the distillation units. The difference in process conditions, materials of construction and blend processed in each refinery and especially the frequent variation in crude diet increases the problem of correlating corrosion of a unit to a certain type of crude oil. In addition, crude oil composition from the same field can change with time. When steam flooding or other recovery methods begin in an oil field, specific gravity and the organic and sulfur content of the crude may change. For example, fire flooding, when used in some fields, tends to increase the naphthenic acid content.

There are at least three mechanisms of naphthenic acid corrosion. Each one is predominant in specific areas of the distillation unit.

Furnace Tubes and Transfer Lines

In furnace tubes and transfer lines, the influence of temperature, velocity and degree of vaporization is very large. Process conditions such as load and steam rate and especially turbulence affect corrosivity. The presence of any naphthenic acid most likely increases sulfidic corrosion. The corrosion mechanism at the furnace tubes, transfer lines, areas of high turbulence such as thermowells and pumps, is most likely an accelerated corrosion due to the velocity and the two-phase flow.

Vacuum Column

In the vacuum column, preferential vaporization and condensation of naphthenic acids increase TAN of condensates. The corrosion is similar to corrosion in very high TAN cuts and velocity has virtually no effect on the process. The naphthenic acids are most active at their boiling point but the most severe corrosion generally occurs on condensation. The corrosion mechanism is mainly a condensate corrosion and is directly related to content, molecular weight and boiling point of the naphthenic acid ². Corrosion is typically severe at the condensing point corresponding to high TAN and temperature.

Side Cut Piping

In side-cut piping, conditions of low vaporization and medium fluid velocity exist. In these conditions, an increase in velocity increases corrosion rates up to the point where impingement starts and corrosion is accelerated dramatically. Sulfur has been shown to inhibit somehow the corrosion.