Corrosion Inhibitors and Their Selection (With PDF)

Which is a corrosion inhibitor?

Corrosion inhibitors are chemical compounds that are added in very low concentrations to the flowing fluids (additives to the fluids) to significantly reduce the material corrosion rate. These corrosion inhibitors form a passive layer on the metal surface so the metal does not directly come in contact with the corrosive fluid. Depending on fluid composition, the quantity of water, and the fluid flow regime, the effectiveness of a corrosion inhibitor varies. To reduce the risk of corrosion in metals, corrosion inhibitors are widely used in the industry. Inhibitors are organic or inorganic compounds and are usually dissolved in aqueous environments. They are available in spray form in combination with a lubricant and sometimes a penetrating oil. However, they can be solids, liquids, or gases. Concentrations of corrosion inhibitors in the corrosive fluid vary from 1 to 15,000 ppm (0.0001 to 1.5 wt %).

In oil and gas exploration and production, chemical & petrochemical industries, petroleum refineries, water treatment industries, heavy manufacturing, and product additive industries, corrosion inhibitors are considered the first line of defense. Corrosion Inhibitors do not react with the fluid and can be applied in situ without causing any significant disruption to the process. To decrease the corrosion rate, Corrosion inhibitors affect any one of the four components of a corrosion cell (anode, cathode, electrolyte, and electronic conductor). The main mechanism on which most corrosion inhibitors work is adsorption.

Types Of Corrosion Inhibitors

Corrosion inhibitors can be classified based on the working mechanism, environment, and operating mode.

A. Depending on the mechanism or electrode process, corrosion inhibitors are classified as follows:

Anodic Inhibitor
Cathodic Inhibitors, and

Mixed Inhibitors.

B. Based on the environment, corrosion inhibitors are of the following types:

Acidic environment inhibitors

Inorganic inhibitors (Example: As₂O₃, Sb₂O₃)
Organic inhibitors (Example: amines, aldehydes, alkaloids, nitro, and nitroso compounds)
Alkaline inhibitors (Examples: thiourea, substituted phenols, naphthol, β-diketone, etc.)

Neutral inhibitors

C. Depending on the protection mode, the following types of corrosion inhibitors are found:

Chemical passivation (Nitrites, Chromates, Zinc molybdate, etc)

Adsorption inhibitors (quinolines, sulfur atoms in these compounds, nitrogen atoms in amines, and oxygen atoms in aldehydes)
Film-forming inhibitors (Zinc and calcium salts, Benzoate, etc)
Vapor phase corrosion inhibitors (Dicyclohexylamine chromate, benzotriazole, Phenyl thiourea, cyclohexylamine, dicyclohexylamine nitrite, etc.)

Volatile corrosion inhibitors (Molybdenum oxide, salts of dicyclohexylamine, cyclohexylamine, and hexamethylene amine)
Synergistic inhibitors (chromate-phosphates, polyphosphate-silicate, zinc-tannins, and zinc-phosphates)
Precipitation inhibitors (Sodium silicate)

Green corrosion inhibitors (Amino acids, alkaloids, pigments, and tannins)

In the following paragraphs, we will explore the Corrosion Inhibitor Selection for Oil & Gas Industry.

Sources of Corrosion for Oil & Gas Industries

Normally corrosion occurs in the water phase. The major sources of corrosion in the oil and gas industries are:

Hydrogen sulfide: sulfide stress corrosion cracking (SSCC)
Chloride: chloride stress corrosion cracking (CSCC)
Carbon dioxide: Sweet corrosion.

Oxygen: pits
Bacteria: Microbiologically-influenced corrosion (MIC)
Water Cut

Strong Acids: hydrofluoric acid and hydrochloric or acetic acids for sandstone and carbonates, respectively, which can cause corrosion of production tubing, downhole tools, and casing
Brines: In presence of dissolved oxygen, act as corrosive.

Corrosion Inhibitor Selection Consideration

The use of Corrosion Inhibitors (CI) as an alternative to CRA shall be decided in the earliest design stage based on ILSS, fluid corrosivity, total CAPEX & OPEX, and assumed efficiency & availability.

ILSS based on BP, Intertek, and DNV research, values below 2.5 indicate CI likely successful with below 50 ppm, and a value above 5.5 is usually not feasible, CI likely requires a very high dosage and high-reliability injection package.
Organic film-forming corrosion inhibitors’ efficiency is reduced in the presence of bacteria, oxygen, and scales. Hence, Compatibility with other chemical treatment and injection strategies shall be studied.
Continuous, batch, or squeeze treatments are predicted to contain high concentrations of water. Batch treatment tends to be used in non-severe applications when continuous treatment is technically difficult or when additional protection is required for severe conditions.

The chemical fluid analysis shall consist of temperature, pressure, flow rate, sand, CO₂, H₂S, Chloride, O₂, pH, cation & anion, organic acids, bacteria, and viscosity.
Failure risk and environmental policy & disposal must be addressed.
The possible impact on downstream processes shall be studied.

Corrosion inhibitors shall not be used as a primary option to mitigate cracking.
High water cuts, velocity, temperature, presence of solids, and pre-existing corrosion have a negative impact on the effectiveness of corrosion inhibitors and require higher concentration.
To comply with the environmental standard, rigorous test qualification, high-temperature stability, and long-term degradation, the use of green corrosion inhibitors from organic matter are increasing.

Probable issues with Corrosion Inhibitor Consideration

Corrosion Inhibitor is used in the oil & gas industry by continuous treatment via annulus or small tubing, batch treatment, tubing displacement, and squeeze treatment by adsorbing. In gas lift wells CI is introduced with the gas.

Possible issues for continuous treatment via annulus: flashing & gunking, slugging, valve plugging, CI degradation, etc.
Possible issues for continuous treatment via tubing: damage risk to injection line during installation, additional cost for accessories & wellhead modification, etc.

Possible issues for batch treatment: Corrosion can occur between treatments, wells are taken off production for several hours, many wasted chemicals on restarting the well, and stable emulsions may form.
Possible issues for squeeze treatment: expensive, may damage formation, wasted chemical, etc.
Special corrosion inhibitor types and injection methods are required for acidizing or fracturing treatments.

The following images provide more guidelines about the corrosion inhibitors in the oil & gas industry.

Thanks to Mr. Andry Soetiawan (Material & Corrosion Engineer) for preparing part of the article.

The following articles will serve more insights regarding corrosion:

Forms of Corrosion: Corrosion Types
Overview of Corrosion Under Insulation (CUI)
Guide for Coating Selection for External Bolting to Reduce Corrosion
Corrosion Monitoring Techniques & Surveys: A short Presentation
Corrosion Protection for Offshore Pipelines