A corrosion inhibitor is not a single product you buy once; it is chosen for a specific line. The first thing that decides which inhibitor you need is whether the corrosion is sweet — driven by carbon dioxide — or sour, with hydrogen sulphide present. After that, temperature, flow velocity, and water cut decide the product and the dose. This page explains the difference, how the chemistry works, and the field variables that settle the selection.
Both describe internal corrosion of carbon-steel tubing and flowlines by the water produced with the oil and gas, and the difference is which acid gas is present. Sweet service is corrosion driven mainly by carbon dioxide: CO₂ dissolves in the produced water and forms carbonic acid, which attacks the steel. Sour service is corrosion where hydrogen sulphide is present as well. H₂S is not simply worse than CO₂ — a trace of it can form a thin iron-sulphide film that slows corrosion, while more of it drives localised pitting and, above roughly 0.05 psi partial pressure, the risk of sulphide stress cracking of the steel.
An inhibitor — and a steel metallurgy — chosen for one service is not automatically right for the other, which is why the first question about any line is whether it is sweet or sour. Many Niger-Delta and West-African production streams carry both CO₂ and some H₂S, so the practical answer is often a matter of degree, confirmed by a gas analysis rather than assumed.
The common oilfield corrosion inhibitors are film-forming molecules — imidazolines and quaternary ammonium salts are the workhorse chemistries. Each molecule has a water-liking (polar) head and an oil-liking (hydrocarbon) tail. The polar head adsorbs onto the steel surface and the molecules pack together with their tails outward, forming a thin protective film that reduces the access of the corrosive water and acid gas to the metal. The film slows the corrosion — it does not stop it entirely.
Because that film is maintained by a supply of fresh inhibitor, the product is normally dosed continuously at low concentration — parts per million — at an injection point upstream of the metal you are protecting. The film is not permanent: heat and fast flow wear it away, which is why the field conditions below, not the drum, decide how much you need.
Four field variables, beyond sweet-versus-sour, decide the product and the dose:
| Condition | Why it matters | Effect on selection |
|---|---|---|
| Sweet (CO₂) vs sour (H₂S) | Different acid-gas attack; H2S adds cracking risk | Choose an inhibitor rated for the service; confirm with a gas analysis |
| Temperature | Heat desorbs the inhibitor film faster (bare-steel CO₂ corrosion itself eases at high temperature as mineral scale forms) | A more thermally stable inhibitor to hold the film |
| Flow velocity / shear | Fast flow strips the film, but also carries more inhibitor to the wall | A persistent film-former, sustained by continuous injection |
| Water cut | As water cut rises the steel turns water-wet and corrosion climbs | Higher dosing once the line runs water-wet |
| Oxygen ingress | Oxygen is a strong, separate corrosion driver | A different treatment; seal the system or treat for O2 specifically |
A datasheet number means little until it is matched to these conditions — which is why the honest way to pick an inhibitor is to state the service and confirm the result by monitoring, not to compare label claims.
Continuous injection meters a steady low dose into the fluid at an injection point so a film is always being renewed; it is the usual method for producing wells and flowlines. Batch treatment places a larger slug of inhibitor periodically — pigging a pipeline, or filling the annulus of a well — to lay down a film that then depletes until the next treatment. Many operations use both: continuous injection on the main flow, periodic batch treatment where continuous coverage is hard to reach. Which method, and at what rate, is confirmed by field monitoring, not by the label.
Ironstone is building a production-chemicals line for this market, partner-led: the plan is that we provide the China-direct material supply, formulation support, and testing discipline behind a Nigerian company’s registrations and field relationships. If you buy corrosion inhibitors for an operation, or you are a Nigerian production-chemicals company, message the technical desk and we will talk through the service and the chemistry.