Pipe Fittings for Wet CO₂ and Carbon Capture Service: Corrosion Mechanism and Grade Selection
Carbon capture and storage (CCS) projects, CO₂ injection for enhanced oil recovery (EOR), and wet CO₂ transportation pipelines share a common material challenge: dry CO₂ is non-corrosive to carbon steel, but the presence of even small quantities of free water (liquid water, not just water vapour) creates carbonic acid (H₂CO₃) that corrodes carbon steel at rates of 1–10 mm/year. In supercritical CO₂ transport (above 7.4 MPa), the CO₂ remains single-phase and dry conditions can be maintained with adequate dehydration — but in subcritical systems, liquid water dropout is common and the material selection must account for it. This article covers the CO₂ corrosion mechanism, the dehydration specification required to keep carbon steel acceptable, and the grade upgrades required when dehydration cannot be guaranteed.
The CO₂ Corrosion Mechanism
CO₂ dissolves in water to form carbonic acid: CO₂ + H₂O → H��CO₃. Carbonic acid dissociates to provide H⁺ ions and bicarbonate — the H⁺ ions depolarise the cathodic surface and drive iron dissolution at rates far higher than simple oxidation. The corrosion rate depends on CO₂ partial pressure (higher partial pressure = more H₂CO₃ = faster corrosion), temperature (peak rate around 60–80°C, then decreasing as FeCO₃ scale forms and becomes protective above 120°C), and flow velocity (high velocity strips protective FeCO₃ scale — a flow-accelerated version of the mechanism). The de Waard-Milliams equation and NORSOK M-506 are the standard prediction models.
Dehydration Specification for Carbon Steel
For carbon steel to be acceptable in CO₂ service, free water must be absent. The typical dehydration specification for supercritical CO₂ pipeline transport is ≤50 ppm water content (by volume) — this keeps the CO₂ above the water dew point throughout the operating pressure and temperature range. This specification must be maintained continuously — any upset that allows water ingress will cause rapid localised corrosion. The PO for carbon steel fittings in CO₂ service should note the dehydration specification and design water content: "CO₂ service — design water content ≤50 ppm v/v. Dry CO₂ service. If free water excursion is possible, upgrade to [duplex/316L]."
Grade Upgrades When Dehydration Cannot Be Guaranteed
| Service | Condition | Grade |
|---|---|---|
| Supercritical CO₂ pipeline | Dry (≤50 ppm H₂O), no H₂S | Carbon steel WPB (X65 for pipelines) |
| CO₂ capture absorber outlet | Wet CO₂, high water content | 316L or duplex 2205 |
| Wet CO₂ + H₂S (sour CCS) | Wet, sour, high partial pressure | Duplex 2205 or 2507 (NACE qualified) |
| CO₂ injection wellhead (EOR) | Supercritical CO₂ with produced water | Duplex 2205 or Inconel 625 for highest risk |
| CO₂ dehydration unit inlet | Wet CO₂, ambient temperature | 316L or duplex as upset protection |