Corrosion Inhibitors - Chemicals Services
Unlike typical iron corrosion, oilfield corrosion generally does not occur as a result of a metal’s reacting with oxygen produce rust. Produced multi-phase fluids are usually oxygen free reductive environments but due to the predominant use of mild steel in the construction of production pipework, acid gases dissolved in the produced water when in contact with the steel can lead to corrosion.
For corrosion to occur, a galvanic cell must be established. Small variations within the body of the pipework or across weld sections create an electric potential. The galvanic circuit can be completed if water touches the iron surface, which itself is reduced as the iron is oxidized. Under acidic conditions typical of oilfield production, the cathodic reaction leads to the addition of electrons to aqueous protons producing hydrogen atoms. At the anode, iron is oxidized to ferrous (II) ions, leading to iron dissolution; Two types of corrosion occur in the oilfield:
- Carbon dioxide (C02)-induced - called "sweet corrosion" - which is ubiquitous.
- Hydrogen sulfide-induced corrosion – called "sour corrosion" - which is less common but more damaging.
Sweet Corrosion
The severity of sweet corrosion will depend upon the conditions of production, but is usually worse at high pressures, due to the presence of higher concentrations of dissolved CO2 in the water (present as carbonic acid), and at higher temperatures (increased rate of reaction). Carbonic acid can continue to react directly with the iron surfaces, but under the right conditions can form a protective iron hydroxide film. However, if this is displaced, corrosion will continue. Sweet corrosion is characterized by the presence of closely grouped, smooth-edged pits. Rates of metal loss are usually lower than with sour corrosion.
Sour corrosion
Sour corrosion is more aggressive than sweet corrosion. Hydrogen sulphide (H2S) reacts directly with the iron surface. A protective film of ferrous sulphide (FeS) can form at the corrosion site, however, even low fluid flow rates are sufficient to abrade the surface, enabling severe corrosion to continue.
A further issue with sour corrosion is the poisoning of the hydrogen diatomization process. The hydrogen atoms diffuse into the metal where they can cause blistering, embrittlement and cracking in weak steels. Hydrogen sulphide can also be generated locally by sulphate-reducing bacteria (SRB). These SRBs are often most active under scale deposits in the production system, which can lead to severe localized pitting corrosion.
Various methods of corrosion control are employed in the field, but continuous-dose, film-forming corrosion inhibitors are one of the most commonly employed. The mechanism of action is disruption of the galvanic cell. The film-forming surfactants have a delta-positive charge that attracts them to the delta-negative pipe surface. The hydrophobic tails of the surfactants pack together to create a hydrophobic layer, minimizing contact between the water and pipe and reducing the corrosion potential. The schematic illustrates film formation and the protective nature of the film.
SAPESCO Chemical Services – SCS has developed a broad range of products that can be used to tackle oilfield corrosion. They can provide corrosion inhibitor in a variety forms, including oil soluble, oil-soluble/water dispersible and water-soluble. Guidance regarding formulation and inhibitor selection call be found in separate documentation.
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