Over the years we have gained vast 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 lifespan considerably.
So what is corrosion?
Corrosion is a natural phenomenon that is based on the laws of chemistry, metallurgy and electricity. In its base form, corrosion can be described as the gradual destruction of materials by chemical reaction with their environment.
Corrosion degrades a materials useful properties. Including 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. This results in an imbalance of energy in the finished steel. Nature wants to undo this and return the material back to its natural elements.
Nature works hard to release this added energy in the form of electrons, reversing the imbalance that has been created; causing the metal to corrode and return back to iron oxide. This appears to us as rust.
When water is added, either as a solid, liquid or vapour, the rate of corrosion is greatly accelerated. The stored water creates a strong electrolyte. Creating a method for ion transfer and current flow within the microstructure of the steel.
Storage tanks and piping systems provide excellent conditions for the formation of corrosion. As well as the atmospheric corrosion described above, they provide the necessary water and nutrients to support microbial growth.
Types of corrosion
Micro-biologically influenced corrosion (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.
Fuel storage and piping systems 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 microbiologically influenced corrosion.
These microbes produce organic acids which accelerate the corrosion process by chemically etching the metal surface. They’re also a cause of galvanic corrosion to 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 one metal corrodes in preference to another 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. The hydrogen sulphide reacts 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. 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, 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. The coatings interrupt the flow of electrons halting the corrosion process before it can begin.
The internal coating also prevents microbes from directly attacking the steel, obstructing micro-biologically influenced corrosion.
Good preparation and good coating application
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. These solvent-free coatings can be applied at a thickness of 1mm or higher. At these thicknesses, there is less risk from discontinuities during the application process.
Solvent-free coatings also offer very long lasting protection and contain less harmful VOC’s. Making them friendlier to the environment and to people.
Lining failures due to a poor or incorrect application can be a disaster for the operator. These not only cost thousands of pounds in lost revenue but the expense of lining replacement and tank repairs. Plus the cost of any possible fines which may occur from tank leakage.
It’s 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. Ensuring all product passes out of the tank and leaving minimal residue.
Environmental regulations are strict. 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.