Here at Abfad Limited we have over 10 years’ knowledge and experience lining and repairing storage tanks of all different shapes and sizes, above ground, underground, some in good condition, some in bad condition, and many in very bad condition. One thing is clear from our time in this industry, corrosion is the main enemy of the storage tank and shortens its life span considerably.
So what is corrosion?
Corrosion is a natural phenomenon that is based around the laws of chemistry, metallurgy and electricity. Corrosion in its base form can be described as the gradual destruction of materials by chemical reaction with their environment. Corrosion degrades a materials useful properties, this includes the strength of the material, the way it looks, and its permeability to liquids and gasses.
During the process of producing steel, both for storage tanks and other uses, energy is added in the form of fire and heat, resulting in an imbalance of energy in the finished steel which nature wants to undo and return the material back to its natural elements. Nature works hard to release back this added energy in the form of electrons and reverse the imbalance that has been created, this causes the metal to corrode and return back to iron oxide, which appears as rust.
When water is added, either as a solid, liquid or vapour, the rate of corrosion is greatly accelerated as the stored water creates a strong electrolyte which creates a method for ion transfer and current flow within the micro structure of the steel.
Storage tanks and piping systems provide excellent conditions for the formation of corrosion, both the atmospheric corrosion described above, commonly called rusting, as well as providing both the necessary water and nutrients to support microbial growth, another main cause of corrosion.
Types of corrosion
Micro-biologically influenced corrosion, or MIC, refers to micro-organisms in the local environment accelerating corrosion due to their metabolic activity. One way these microbes are introduced into storage tanks, along with various dust particles and moisture, is through atmospheric tank venting systems. They require both water and nutrients to multiply and only negligible traces of water are needed for populations to thrive and multiply. The nutrients these populations can feed on include carbon, hydrogen, oxygen, nitrogen, sulphur, phosphorus, and lesser elements such as calcium, sodium, iron, magnesium and copper. This means fuel storage and piping systems will provide all the necessary water and nutrients to support substantial microbial growth.
Once inside of the storage tank these micro-organisms settle through the product and cling to the various internal surfaces of the tank. As the bottom of the storage tank provides the greatest area for water/fuel interface this is where most growth takes place and is where we see the greatest evidence of micro-biologically influenced corrosion found, these microbes produce organic acids which accelerate the corrosion process by chemically etching the metal surface. They are also a cause of the galvanic corrosion of the metal as the microbes create a biofilm on the surfaces they inhabit, this causes different electrical potentials between the surfaces that are covered with these biofilms and those that are not. Galvanic corrosion is when a metal corrodes in preference to another metal when an electrolyte is present and is known to cause a pattern of pinhole leaks in steel storage tanks.
There are many types of bacteria prevalent in water which can kick start micro-biologically influenced corrosion, these can be either aerobic (requiring oxygen) or anaerobic (existing without oxygen).
Sulphate reducing bacteria are one example of anaerobic bacteria that can cause problems in fuel storage tanks, they obtain energy by oxidising organic compounds, breathing sulphate rather than oxygen. These bacteria are some of the oldest forms of micro-organisms on the planet and can be traced back almost 3.5 billion years. Sulphate reducing bacteria produce hydrogen sulphide as a waste product, which will react with the metal ions in the water to produce various metal sulphides, these are mostly brown or black in colour and are the reason for the dark sludge at the bottom of most storage tanks.
The ullage area of a storage tank is another concern for accelerated corrosion as refilling causes this area to be replenished with hydrocarbons and water vapour, providing the required nutrients and water for microbial growth in this area of the tank. There can often be a substantial habitat for micro-organisms at the top and on the walls of the storage tank for these reasons alone.
Lining stops corrosion
Through the proper application of protective coatings to fully encapsulate the storage tank interior, corrosion can be effectively managed and controlled. These linings form a barrier between the stored product and the steel tank, interrupting the process that causes corrosion as the coatings interrupt the flow of electrons halting the corrosion process before it can begin. They also prevent microbes from directly attacking the steel, obstructing micro-biologically influenced corrosion.
Proper application of the coating is essential to prevent discontinuities (breaks or gaps in the coatings) which would provide a path back to the steel for corrosion. These discontinuities are more often than not the result of poor or improper surface preparation or poor coating application. Preparing the steel surface correctly alongside proper application of the coating system can help reduce the number of discontinuities, whilst also extending the coatings life. At Abfad Limited we use International Paints solvent free range of coatings as they can be applied at a thickness of 1mm or higher meaning less risk from discontinuities during the application process. Solvent free coatings also offer very long lasting protection and contain less harmful VOC’s meaning they are friendlier to the environment and to people.
Lining failures due to poor or incorrect application can be a disaster for the operator, not only costing thousands of pounds in lost revenue, but the expense of lining replacement and tank repairs, not to mention any possible fines which may occur from tank leakage. This is just one reason why it is vital to appoint an experienced and reputable company when it comes to lining your storage tanks.
A protective coating or lining to the interior of the tank not only assists with corrosion control, but also product purity. The slickness of the coatings used in tank lining can also help with the flow of the stored product to ensure that all product passes out of the tank, leaving minimal residue.
Environmental regulations are strict and tank storage facilities have a responsibility to guard against any tank leakage and ground or waterway contamination. The direct economic benefits of lining storage tanks are obvious, lower operating costs, fewer repairs and an extended storage tank design life, all of which lead to increased profits.