Cathodic Protection

CO2 corrosion has plagued the oil and gas industries for many years. CO2 gas becomes corrosive when dissolved in an aqueous environment. This is because it promotes an electrochemical reaction between the steel pipeline and the aqueous environment. The CO2 mixes with the water, creating carbonic acid and the acidic fluid then corrodes the pipe. This is often referred to as sweet corrosion. CO2 corrosion presents itself in these common forms:

• Pitting-type corrosion
• Mesa attack corrosion

When hydrogen sulfide (H2S) and moisture come in contact with metal, it causes corrosion known as sour corrosion. When hydrogen sulfide is dissolved in water, it becomes acid as well as a source of corrosive hydrogen atoms. These corrosive byproducts are iron sulfides (FeSx) and hydrogen. These are particularly damaging to drilling pipes and result in the following forms of corrosion:

• Uniform
• Putting
• Stepwise cracking

Oxygen reacts with metal extremely quickly. When it dissolves within drilling fluids, it becomes a major cause of drilling pipe corrosion. Due to the speed at which drilling fluid travels over a pipeline, even small concentrations of oxygen (as low as five parts per billion) will cause constant destruction to the pipeline itself. In fact, the presence of oxygen actually exacerbates the corrosive effects of other acidic gases, including the previously-mentioned CO2 and H2S. The forms of corrosion caused by oxygen include:

• Uniform corrosion
• Pitting-type corrosion

Microbiologically-induced corrosion (MIC) is a form of corrosion caused by bacteria. Bacterial waste often includes CO2, H2S, and other organic acids, increasing the toxicity of the fluids passing through an oil or gas pipeline. It can be identified by the presence of black waste material on the surface of pipelines and it can cause pitting underneath these waste deposits.