Crude oil tanks are generally made of steel. While steel offers the strength needed to hold and protect oil, it’s vulnerable to corrosion. During the manufacturing stage, modern steel tanks receive a protective surface coating to prevent corrosion. The coating, however, doesn’t last forever. It is necessary for the tanks to undergo maintenance every 5 to 10 years to reapply a surface coating to prevent corrosion, pitting and oil leaks.
Types of Corrosion in Crude Oil Tanks
- General: General corrosion occurs throughout a tank.
- Local: This type of corrosion appears in specific areas of a tank. Corrosion occurs faster in localized sites because of the dislodging of surface rust or scale when the tank flexes. As the surface thins due to stress, corrosion accelerates. Localized corrosion also occurs where water collects or flows within a tank.
- Pitting: Pitting is a type of local corrosion found along horizontal surfaces, the bottom of tanks and places where water accumulates. It tends to occur when seawater collects on a metal surface, causing microbial or electrolytic corrosion.
- Weld metal corrosion: When the metal used for welding causes interacts with the metal in the oil tank, an electrolytic action occurs.
Factors that Promote Corrosion on Crude Oil Tanks
- Lack of surface coating: A lack of protective coating leaves a tank exposed to destructive elements.
- Coating defects: The breakdown of a surface coating due to age, wear or improper application can cause localized corrosion. Accelerated pitting may occur if the breakdown causes an electrolytic action.
- High sulfur contents: General corrosion or pitting occurs when crude oil and high levels of sulfur mix with seawater because the combination forms acidic compounds and an active corrosion cell.
- Microorganisms: Crude oil tanks are exposed to a wide range of bacteria that produce corrosive acidic compounds.
- Water and humidity: Moisture from wash water, poor drainage, slop tanks, leaks and condensation causes oxidation, electrolytic corrosion and microbial-influenced corrosion.
- Hot temperatures: High temperatures within an oil tank increase corrosion rates, promote bacterial growth and encourage microbial-influenced corrosion.
- Accumulation of scale and sludge: Sludge or scale at the bottom of crude oil tanks may hide pitting and contain harmful microorganisms.
- Construction materials: Higher tensile steels manufactured using thermal mechanical control processes (TMCP) are more susceptible to accelerated corrosion than those using different types of steel.
- Excessive crude oil washing: Crude oil sometimes causes a waxy, protective layer to form inside tanks. Exposure to different temperatures of seawater can wash away the protective layer, exposing the surface to oxygen, moisture and salts.
- Quality of inert gas: Inert gas within a tank should have an oxygen content level of less than 8 percent. Levels below 1 percent are even better. When the oxygen combines with sulfurous compounds or flue gas soot, accelerated corrosion occurs where moisture condenses in the tanks.
- Poorly grounded electrical equipment: Currents flowing through steel because of the ineffective grounding of electrical equipment lead electrolytic actions and pitting corrosion.
- Structural flexing: When a tank is under tensile stress, flexing can cause protective scale on surfaces to fall, exposing the metal to the environment.
The Importance of Proper Preparation and Surface Coating
Preparing the surface of a tank and successfully applying a protective coating is crucial to preventing corrosion. When corrosion remains unaddressed, it can lead to oil leaks. Similarly, corrosion decreases a tank’s structural integrity. With time, corrosion leads to the formation of fault lines that are vulnerable to fatigue and failure.
Without proper preventive maintenance, corroded areas may ultimately necessitate extensive steel renewals, which increase maintenance costs. By using temporary climate control solutions, you can create the ideal environment for a tank’s surface preparation to ensure that it remains free of debris, salts and humidity. Temperature and humidity controls also allow you ensure that the tank’s surface is at the ideal temperature and the interior has a low relative humidity level when you apply the protective coatings.
Polygon offers a range of temporary climate control solutions to ensure the success of crude oil tank maintenance projects. When you create the optimal environment, drying times are faster, the overall project takes less time to complete and workers are more productive. Contact Polygon to learn more about the climate control solutions available for your application.
[Photo by Rick Hurdle via CC License 2.0]