A deep dive into internal corrosion

Corrosion can be described as the deterioration of materials, especially metals, by chemical interactions with their environment.¹ It can be classified into different categories based on the materials, environment, or morphology of the corrosion damage. Metals corrode due to the natural consequence of their temporary existence in metallic form. As shown in Figure 1, the more energy required to bring about the change of a metal from its natural state to its metallic form, the more reactive the metal will be to its environment or the more inclined it is to corrode. The most common form of corrosion is rusting, which occurs when iron combines with oxygen and water.

In oil and gas production, the flowline serves as the initial pipeline system that connects to the wellhead. As shown in Figure 2, the flowline carries the total produced fluids (e.g., oil, gas, and production water) from the well to the first piece of production equipment, typically a production separator. Gas flowlines transport both associated and non-associated gases to production equipment. Associated gas is produced from crude oil extraction. It contains methane, ethane, and hydrocarbons, while non-associated gas is a natural gas found in a natural reservoir and does not contain crude oil. Water, dissolved ions (e.g., Cl- and Ca2+), and acid gases (e.g., CO₂ and H₂S) are frequently found in the gases passing through flowlines. Severe corrosion of the inner wall of steel pipes happens when temperature, pressure, and stress are combined. The primary element affecting natural gas flowline integrity is internal corrosion. In addition to weakening pipes and thick walls, it might cause leaking incidents. When gas flowlines burst, it not only results in significant financial loss but also pollutes the environment.

Causes of corrosion in gas flowlines

Water, oxygen, and dissolved ions

Water, oxygen, and dissolved gases are major factors influencing corrosion, otherwise referred to as rusting. Rusting occurs in flowlines made of carbon steel, stainless steel, and other alloys containing iron. It is an electrochemical process that starts with the flow of electrons from iron to oxygen.² Iron acts as the reducing agent by donating electrons, whereas oxygen acts as the oxidising agent by accepting electrons. The primary reaction involves the reduction of oxygen.²

O₂ + 4e^– + 2H₂O → 4OH^–

Acidity strongly influences this process because it forms hydroxide ions. Indeed, at low pH, oxygen accelerates the corrosion of most metals. The oxidation of iron provides the electrons for the afore-mentioned reaction, which is described as follows:²

Fe → Fe^₂₊ + 2e^–

The following redox reaction, which is critical to rust formation, also occurs in water:

4Fe²⁺ + O₂ → 4Fe³⁺ + 2O^2–

In addition, the following multistep acid-base reactions affect the course of rust formation...

 

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