Why tank base corrosion is an issue
Corrosion at the base of a heavy structure is obviously bad news for the asset owner or operator. Regulations have been set in place designed to prevent the escape of hazardous media into the environment. And whilst it would take years for any significant loss of integrity to occur, those years, would be remembered for frequent, costly maintenance and inspection.
When and why tank base corrosion occurs
The industry has learned to protect the base of the tank from corrosion using cathodic protection and rectifying existing problems by installing a new floor if needed. With the base protected, the rim could at times be neglected.
The crevice at the interface between a steel tank and its base is where we see corrosion set in, at the chime angle. The main cause of corrosion is water ingress, sometimes exacerbated by poor drainage and sloping of the foundation towards the tank.
A sealant would prevent this water penetration. However, it is difficult to seal a steel tank to its foundation, typically concrete, as these dissimilar materials are subject to movement, expansion and contraction at differing rates.
“Solutions” or more problems?
Tank cement, bitumen, asphalt, caulks and other sealants have been tried for many years, typically proving to be temporary solutions due to poor adhesion and rigidity while the tank is in operation. In some cases, this tank base “protection” can do more harm than good. Localised failures can effectively seal the moisture in, thus exacerbating the corrosion. As well as this, inspection of these systems can prove difficult as devices cannot “see” the remaining steel thickness through the sealant.
Microporous membranes: more than just roofing systems
Originally developed for water- and weatherproofing roofs, tank maintenance engineers started noticing some features of these microporous membranes, which could prove useful on tank bases:
- Application of the membrane system is straightforward, without the need for hot work or any specialist tools. Conditioner/primer is applied first to enhance adhesion, followed by the membrane with a reinforcement sheet, applied by brush. A two-coat system ensures there are no misses.
- Unlike sealants, membranes prevent water ingress but do not trap moisture. Similar to human skin, their microporous nature allows the vapour to escape, leaving the underlying substrate dry and firm.
- Membranes adhere very well to different substrates as they have been used on many roofing materials with great success for many years. Most carry a globally recognised approvals, such as BBA, which tells the specifiers that they can expect a 25-year service life from their membrane system.
- We know that most tank base protection systems fail due to their rigidity, however, membranes can accommodate the natural movement of the substrate and move in sympathy. For example, Belzona 3111 (Flexible Membrane) has been tested for elongation against BS 2782. After 7 day cure at 20C/68F, the elongation of a reinforced system will be 20% lengthwise and 100% crosswise. The same system will exhibit tear strength tested in accordance with ASTM D624 (7 days cure at 20C/68F) of 188 pli/ 33 N/mm.
Membrane in action
Tanks at a refinery in France were suffering from water ingress. They had a polyester tank base sealing system in place, which was failing due to its rigidity. The failed system had resulted in water ingress, causing corrosion at the base of the tank.
Belzona 3111 was used to waterproof the tank base in 2004. The annular ring was subsequently inspected in 2004 and 2017, using non-destructive methods, and found to be in excellent condition.
The future of tank corrosion protection
These days we have a combination of ageing structures that need preserving and new tank farms, where we are focusing on corrosion prevention. The industry needs to keep looking for alternatives to common maintenance techniques, such as the use of membranes over sealants. What aids progress in corrosion management is conversation. Material manufacturers, contractors, asset owners and operators, engineering design houses, testing houses and classification societies among others can jointly facilitate continuous progress.
Author: Marina Silva