Corrosion Morphologies

By Sarah Bagnall, CEng, MRes, BEng, MIMMM, Director R-Tech Consultancy Services 

Material: 
A106 Grade B.

Environment: 
Nozzle contained both steam and process water used for both heating and cooling.

Operating Pressure: 
10 bar.

Operating Temperature: 
185^oC.

Corrosion Mechanism:  
Stereomicroscopy revealed localised ‘Mesa’ type attack of the weld area along the internal surface. Analysis of the corrosion deposit revealed the presence of iron carbonate. Mechanism was deduced to 
be CO₂ Corrosion.

Editor’s Notes

The economic impact of CO₂ corrosion is well-documented, prompting the industry to implement various mitigation strategies over the years. However, as exploration expands into new fields with higher temperatures, pressures, and diverse fluid compositions, it is crucial to continuously assess and improve the corrosion resistance of both existing and new materials.

In the oil and gas industry, the presence of carbon dioxide (CO₂) in fields poses a significant challenge due to its potential to cause severe corrosion, especially when water is present. While dry CO₂ in either gas phase or supercritical fluid form is not corrosive to metals and alloys, the introduction of water-containing fluids leads to the formation of carbonic acid, which can rapidly corrode infrastructure.

The extent of CO₂-induced corrosion is influenced by several key factors: Concentration of CO₂ (and other components like H2S), 
Water Chemistry, Operating Conditions, Material Type.

Picture 1: Nozzle Extracted from a Heat Exchanger Vessel Due to Pinhole.

Picture2: Internal Surface of Pinhole Location.

Picture 3: Internal Surface of Circumferential Weld, Detailing Localised Attack

Picture 4. Internal Surface of Circumferential Weld, Detailing Localised Attack