Introduction

In anticorrosive protection of pipeline interiors, corrosion inhibitors have undoubtedly acquired a leading role. These are chemical products added in small quantities to systems that can slow down the kinetics of anodic/cathodic reactions or adsorb to metal walls, isolating them from contact with the corrosive electrolyte (transported product).

Due to the operational diversity in which they must be applied, selecting the right product for each line or installation is not simple. One might think that since operating conditions are very difficult to reproduce in the laboratory, it doesn’t make much sense to invest time and money in this stage prior to field testing. However, the opposite has been proven. Laboratory tests allow for interesting economic savings as they avoid complex operational maneuvers to adapt systems for field testing, reducing testing timeframes in the process of selecting the most suitable product. In this regard, standards have been developed detailing different accepted methodologies for evaluating corrosion inhibitors under well-defined flow conditions.

The Process

Corrosion Susceptibility Study

Prior to the inhibitor selection process, a review of corrosion potential is conducted in the facilities where they will be applied. Corrosion rate predictions will allow categorizing wells or lines based on their criticality and establish conditions for different tests. There are different models to estimate corrosion rates (for example, NORSOK, which allows estimation when CO2 is the predominant corrosive agent). Generally, these models provide a conservative estimate of corrosion rate, and the result should be interpreted as the maximum pitting rate that can exist in the pipeline.

Laboratory Evaluation of Inhibitors

The selection of a chemical product is governed by the criteria of determining which product can reduce and/or control corrosion rate at the lowest possible cost. Before evaluating a candidate chemical treatment in the field, laboratory tests provide a simple, compact, quick, and economical means to predict the behavior of a candidate inhibitor regarding relevant field operations.

The selection process includes efficiency tests, where the inhibitor’s ability to minimize corrosion is evaluated. Additionally, other properties that become important when applying to a specific system are analyzed, such as thermal stability, compatibility with other materials, and effects on the physical characteristics of process fluids.

During laboratory evaluation, certain particularities often manifest that make the final ranking more conclusive. It’s important to understand that in this process, there are no good or bad products, but rather products suitable for each of the conditions of the systems under study, which are infinitely diverse.

The entire process is executed under the guidelines of ASTM G170-06 (2020) “Standard Guide for Evaluating and Qualifying Oilfield and Refinery Corrosion Inhibitors in the Laboratory.”

Corrosion inhibitors are characterized by their solubility and dispersibility. They can be water-soluble, oil-soluble, or water-dispersible and oil-soluble. Although a corrosion inhibitor is soluble in one medium, a part of it may still be present (or distributed) in the other phase, known as inhibitor partitioning.

The effectiveness of a corrosion inhibitor depends on the pipeline material, inhibitor composition, corrosion mechanisms, fluid composition, and flow type. Furthermore, the performance of a chemical treatment can be affected by temperature, flow rate, solubility, compatibility with other chemicals, and chemical availability.

Temperature increases can cause chemical product decomposition and increased corrosion. While temperature decreases can affect product administration capability. High flow velocities’ shear stress can have an adverse effect on performance through mechanical removal of films formed by corrosion products or corrosion inhibitors, causing exposure of the metal surface to the corrosive medium, while low velocities can prevent effective chemical delivery.

To evaluate inhibitor efficiency, corrosion rate is determined using linear polarization resistance and Tafel polarization techniques. Kettle cell testing is widely used for different concentrations near the commercial optimum and at different water/hydrocarbon partition conditions. The standards recommend determining corrosion rate and efficiency using the rotating cylinder electrode technique at different inhibitor concentrations and flow rates.

In our next installment, we will present the evaluation of four commercial products, two water-soluble and two hydrocarbon-soluble, for implementation in a system with high water cut.