Aboveground and underground tanks are essential components in almost all industrial facilities and tank farms. They are responsible for collecting and storing various fluids including water, wastewater, petroleum products, chemicals, food products, etc. Unfortunately, steel storage tanks, like all metal components, are susceptible to corrosion damage. In addition to being unsightly, corrosion damage can:
- Significantly compromise the tank's structural integrity
- Result in leaks and spills that pollute the environment
- Contaminate the stored fluids
- Lead to fire and explosions (in the case of petrochemicals and other hazardous liquids)
All of these corrosion-related issues present hazards that can threaten the environment and public health and safety. Additionally, tank corrosion can lead to billions of dollars in direct and indirect costs. The ramifications of tank failure due to corrosion are so dire that the U.S. government has implemented federal regulations that require individuals and companies that own or operate underground storage tanks (USTs) to have adequate corrosion protection systems in place.
Why Do Tanks Corrode?
Most storage tanks are constructed from steel, a material that is susceptible to corrosion attack. Corrosion is a deteriorative process that results from chemical or electrochemical reactions between the steel and its environment. Without the proper protection mechanisms in place, steel tanks will naturally corrode in the most vulnerable areas.
The rate and severity of tank corrosion are highly influenced by several factors including environmental conditions, the tank's construction (joints, insulation, etc.), the length of exposure of unprotected steel to moisture, chemical properties of the moisture and undesirable temperature ranges.
Some of the most common causes of tank failure include:
- Voids between the tank and the concrete base
- Failure and cracking of sealants
- Saturated or inappropriate insulation resulting in corrosion under insulation (CUI)
- Naturally aggressive and corrosive stored material
- Aggressive soil chemistry affecting underground tanks (Read An Introduction to Soil Corrosion for more on this topic.)
If left untreated, corrosion damage will gradually reduce the thickness of the wall panels, which can weaken or destroy tank components resulting in holes, decreased structural capacity and tank failure.
Strategies to Prevent Storage Tank Corrosion
Storage tank corrosion can be prevented or controlled using several methods ranging from state-of-the-art technology to simple, historically proven methods. While protection methods may be used individually, it is typical for them to be used in combination with one another to increase the effectiveness of the overall corrosion protection system. Common tank corrosion prevention strategies include the following:
Corrosion Resistant Materials
One of the first line of defenses against tank corrosion is selecting corrosion resistant materials for the tank's construction during the initial design phase. While steel is the preferred material for tank construction, other materials that are less prone to corrosion can be used.
For example, significant advancements have been made in fiberglass technology, allowing them to be used for storage tank construction. Wastewater disposal and hazardous chemical storage are just some of the uses of fiberglass tanks. Polyethylene (PE) and polyvinyl chloride (PVC) wall panels have also been used in many industries to much success. If a metal tank is necessary, stainless steel panels, fittings and connections can be used. Nickel plated fittings are also commonly used in highly corrosive environments.
Coatings and Linings
Paints and coatings are one of the most widely used storage tank protection methods. Their primary function is to provide barrier protection, that is, the coating acts as a shield that prevents air and moisture from coming into contact with the metal substrate. Eliminating these two elements prevents further corrosion from occurring.
On the tank's interior, in addition to barrier protection, paints and coatings may also contain corrosion inhibiting compounds that suppress the chemical reactions that contribute to the corrosion process. Specialized linings may also be used to shield the metal substrate from the electrolyte. (See The 5 Most Important Considerations when Selecting Internal Tank Linings for more information on this method.)
Cathodic protection, also known as galvanic cathodic protection, is a corrosion protection technique in which the metallic surface to be protected is intentionally made the cathode in a galvanic cell. This method involves electrically connecting the tank's metal components (either directly or indirectly) to another easily corroded metal. In this case, the metal that corrodes more easily becomes the anode.
There are two main types of cathodic protection:.
- Passive cathodic protection – In passive cathodic protection systems, a sacrificial metal is connected (directly or indirectly) to the tank. The potential difference between the two metals is enough to generate sufficient current to form a galvanic cell.
- Impressed current cathodic protection (ICCP) – ICCP systems are used when the passive current between the two metals is not enough to provide adequate protection. In this case, an external DC power source is used to facilitate the flow of electricity and drive the electrochemical reaction.
Cathodic protection systems are mostly used in underground storage tanks. (Learn more about cathodic protection in the article Cathodic Protection of Earthed Tank Farms.)
Corrosion inhibitors are compounds that consist of chemical substances that dampen the corrosion process. Inhibitors may be applied to the tank's interior or exterior and are available in various forms. Depending on the stored product, liquid inhibitors may be applied to the electrolyte directly. Other inhibitors can be applied near vulnerable external areas, such as the tank chime, chime rings, sleeve systems and perforated PVC pipes.
Corrosion inhibitors work in several ways, some involving complex chemical reactions. However, most inhibitors work by removing dissolved oxygen from the electrolyte, which prevents oxidation and reduction reactions from occurring.
While inspections cannot stop corrosion from occurring, they can assist in early corrosion detection. Early identification of corrosion and deterioration is useful because remedial measures can be taken before the onset of serious issues such as leaks, spills and tank failure. For tanks with external insulation, inspection ports can be provided to allow personnel to inspect the steel wall panels at various points around the tank.
Lack of adequate corrosion prevention measures in aboveground and belowground tanks can result in several environmental and public safety hazards including fluid contamination, leaks, spills, and in extreme cases, fires and explosions. While the effects of corrosion can be devastating, they can be avoided by taking the necessary precautions.
Several factors, including the composition of the stored fluid, the tank's construction material and the surrounding environment, should be carefully assessed to determine the most effective corrosion protection measure.