Crude oil, as a mixture of various hydrocarbons, is not inherently corrosive. However, certain impurities and components often found in crude oil can cause corrosion in pipelines, vessels, and refinery equipment such as atmospheric columns, overhead lines, exchangers, and condensers. In some cases, the corrosivity of crude oil can be so high that extracting and refining it profitably becomes impossible. Here we will look at some of the corrosive substances found in crude oil and what can be done to mitigate their effects.

One way to mitigate the effects of hydrochloric acid is by adding ammonia (NH₃) as a basic material to neutralize the HCl. Ammonia can react with HCl to form ammonium chloride (NH₄Cl), which is highly hygroscopic and can even react with water vapor. Water containing NH₄Cl is very corrosive to copper-based alloys such as brass and bronze. Another technique used to reduce this type of corrosion involves washing the crude oil with water and sending it to a desalter to remove the brackish water. Despite these measures, a small concentration of residual chloride salts in the crude oil can still cause breakdowns in upstream units.

CO₂ corrosion, also known as "sweet corrosion," is a common issue in oil and gas production and transportation facilities. This is because CO₂ is one of the main corrosive agents in oil and gas production systems. Dry CO₂ gas is not corrosive at the temperatures found in oil and gas production systems, but it becomes corrosive when dissolved in an aqueous phase, promoting an electrochemical reaction between the steel and the aqueous phase in contact. When mixed with water, CO₂ forms carbonic acid (H₂CO₃), which acidifies the fluid. The rate of this reaction depends on the temperature and the partial pressure of CO₂. Generally, when the partial pressure of CO₂ is above 0.5 bar (7 psi), sweet corrosion is expected. It's worth noting that in some cases, the partial pressure of CO₂ in crude oil significantly exceeds 400 bars.

To eliminate or mitigate sweet corrosion, two main approaches are used: adding inhibitors to the crude oil or replacing steel with stainless steel, which is becoming more common due to the higher cost of adding inhibitors.

Organic chlorides, sometimes referred to as "phantom chlorides," are difficult to remove during the desalting process. They decompose into HCl during preheating and cause severe corrosion in upper or downstream units. To avoid corrosion, the concentration of organic chlorides in crude oil must be below 1 mg/L. However, their concentration in most crudes tends to range between 3 and 3,000 mg/L.

Naphthenic acids are a type of organic acid that can be present in crude oil and cause significant corrosion under certain circumstances, known as naphthenic acid corrosion (NAC). NAC typically occurs at temperatures between 446°F and 752°F (230°C and 400°C) and in the presence of sufficient naphthenic acids in the crude oil. This type of corrosion occurs in refinery distillation units, such as furnace tubes, transfer lines, vacuum columns, and side-cut piping. The presence of sulfides in crude oil can reduce the rate of NAC, especially at lower temperatures. However, reproduced naphthenic acid continues the corrosion process. NAC is considered localized corrosion and is observed in areas where fluid velocity is high and organic acid vapors are present.

One common method to reduce NAC in crude oil refining systems is to mix high TAN crude with low TAN crude, reducing the overall TAN to an immunological range (less than 0.3 mg/g). Another method is injecting corrosion inhibitors into the crude oil stream. Traditional amine film inhibitors are not suitable for NAC since it occurs at high temperatures without forming iron sulfide deposits on the surface. Phosphorus-containing inhibitors and non-phosphorus inhibitors are very effective for mitigating NAC, although the latter can have economic and downstream catalyst poisoning concerns.

Sulfidation corrosion occurs at high temperatures in the presence of sulfides in crude oil, known as "sulfidation." The amount of total sulfur in crude oil depends on the type of oilfield and ranges from 0.05% to 14%. However, values as low as 0.2% are sufficient to create sulfidation corrosion in simple steels and low-alloy steels used in various refinery units.

To control high-temperature sulfidation, selecting a suitable material resistant to sulfidation is essential. McConomy curves are useful for selecting the appropriate steel, showing variations in sulfidation corrosion rates based on temperature and total sulfur content. However, only active sulfides (like H₂S) cause sulfidation, and McConomy curves overestimate corrosion rates. Couper-Gorman curves, based on experimental studies, consider factors like fluid velocity and H₂ gas presence.

Microbiologically influenced corrosion (MIC) is widespread in oil and gas storage and transportation facilities. Sulfate-reducing bacteria (SRB) are the most significant microbes causing over 75% of corrosion failures in U.S. oil wells. These anaerobic bacteria use sulfate as an acceptor to create sulfide, increasing corrosion rates. The best method to reduce MIC is adding biocides.

Effective corrosion management in the oil and gas industry can reduce costs and ensure safety, health, and environmental compliance. Protective coatings and paints are standard methods to prevent oxidation corrosion, with regular maintenance required. Some materials like galvanized steel, stainless steel, and nickel-plated pipes do not need painting. Oil can protect the interior of pipes from corrosion if it is non-corrosive. Additionally, coatings inside pipes and tanks can prevent some corrosion types. Insulating flanges and sacrificial anodes are used to prevent electrochemical corrosion, while suitable materials and technologies must be highly reliable due to high replacement costs in case of failure.