Cathodic Protection System - Essay Example

Section 1

The department I work produces above ground storage tank. The tanks are mainly used in plants. In any given plant, the tanks are used for the delivery and storage of several fluids and gases. In the storage tanks, corrosion is one of the problems that have been faced by plants that have used them for storage. One of the issues that I have provided guidance on is the corrosion on the bottom and top plates of the elevated tanks. Most of the tanks that our department produces are those used in the storage and the distribution of diesel, portable water, H.F.O, and petrol. I am providing guidance on the problem of corrosion that occurs in the bottom of the tanks; I carried out several experiments after noticing signs of pitting corrosion of varying degree and the roof also had holes of varying sizes. The corrosion was mainly in the areas where the roof was touching rafters. The magnetic flux leakage test was used to determine corrosion in the bottom of some tanks to have a full analysis of the extent of the corrosion on this part of the tank. Draining the tank by removing the slurry and sludge was important before the installation of the protection method. It was also hydro cleaned and the inside was inspected. The first observation was water-filled blisters in the paint used. In the construction of the tanks, I realized that the main sections that the engineers concentrate to prevent the corrosion are the bottom and the roof. I also learned that Cathodic protection is the standard method used in controlling corrosion but is not provided for these tanks.

First Example

I analyzed and presented the factors that could have led to the corrosion at the bottom of the tanks. I determined that the corrosion at the bottom of the plate depended on several factors including the material used, the design of the bottom plate, the medium of storage of the tanks and the corrosion protection system in the design. Since I needed to come up with the solution to corrosion in the tanks, I realized through the inspection of different tanks that design of the bottom plate. The bottom of most of the tanks is lap welded. The construction leaves small gaps on the inner side where the lap joints are used. The bottom constrictions also make them not sit well on soil, and there is a gap between the plate and the soil. This increases the chance of corrosion. The material used in making the bottom plates is a slow factor that leads to corrosion. The bottom plates of many of the tanks are made of structural steel material. There is the increased rate or a slower rate of corrosion depending on the material used to construct the bottom (Pedeferri, 2006). The medium stored in the tanks is another aspect of corrosion. There are mediums such as heavy fuel oil that does not lead to corrosion. In designing the tanks, I realized that the protection method against corrosion used in the tanks is an important aspect in corrosion in tank bottoms. Cathodic Protection is the main method for protecting the interior of the tanks, and it has not been applied in most tanks.

Second Example

The other research that leads to the achievement of the right recommendations for change was a failure of the roof plate of above the ground tanks. In the roofs, there were a large number of holes noticed. However, there were no signs of erosion at the external part of the plates. After research, I concluded that failure at the roof plate was mainly due to crevice and galvanic for which were caused by the gap between the roof plate and the rafter. There were no sealing welded areas that are inside and ventilation of hydrogen Sulphate.

Third Example

The last area that I looked at was the best practices and methods that could be used to repair the damaged bottom and top plates of the tanks. The current practices include replacing the existing plates and installation of new plates and installing new plates over the ones already existing. Through the research I carried out on the two methods that are currently being used, I recommended replacement of bottom plates since most of them are in poor condition. Addition of another plate on the bottom and top plates was recommended for the ease of provision and erection of cathodic protection.

Section 2

From the research and recommendation, I have also worked with the manufacturing department to ensure there is reduced corrosion in the bottom and top plates. Cathodic protection is the main protection method to be used in the manufacturing of the tanks. I recommended the introduction of cathodic protection for the new bottom plates. I also went ahead and investigated various cathodic protection systems that could be used for new tanks. Some of the methods that can be used include ICCP shallow ground bed, ICCP deep well-ground bed, ICCP ribbon type and CP system adhered (Pedeferri, 2006). ICCP shallow ground bed involves the installation of anode around the periphery. Investigation of the areas that the method has been applied led to the knowledge that CP system that has an anode at the edge is not applicable and suitable for the bottom plate since there is a non-uniform current distribution in the bottom plate. The other reason for unsuitability is shielding effect mainly on the ring wall concrete foundation that reduces the protection current which is in the bottom plate. The system when installed would lead to stray current on the adjacent structures such as piping, water tanks, and earthlings.

First Example

The other method that could be used is ICCP Deep well-grounded bed. The method involves the installation of an anode at the remote parts of the tank farm which provides a common cathodic protection for all the tanks that are in the farm (Pedeferri, 2006). In the cases, it may be challenging to achieve polarization due to the shielding effect as the tank in the farms may have several and complex structures in the underground structures. It may also be complex in the construction and the installation of road culverts, the foundation of buildings. The soil resistivity in these aspects is also low. Moreover, the water table may be another factor. ICCP ribbon type anode may be installed in the system in cases of individual tanks. When it is installed in a single tank, it covers all the areas of the bottom plate. Good supply of electrons would be necessary to ensure full protection of every part of the tank.

Second Example

The other method that can be used in the prevention of corrosion is CP system Adhered. In cases of repaired tanks, with bottom over bottom method, ICCP ribbon type anode is used, and it is the most suitable considering the factors of CP and corrosion. This will be placed directly between two of the bottoms. In the old bottom plate, it is electrically isolated from the new one. The installation will also be suitable for new tanks. The level of protection achieved by this method is great.

Third Example

I also decided to let the department carry out some repairs on the tanks that have been corroded. It was decided in the department that temporary repair work was needed in the tank for plate replacement. In making the repairs, the thinned and corroded areas were repaired through patch welding of the plates. The welded areas of the patches were checked using a vacuum box tester. It was however realized that 50mm area was close to the original lap welding of the plates which also had holes. It was also noticed that many of the MFL testing tools do not have the right techniques to inspect 100% of the surface. The patch welding was done with alloys to acquire full protection of the tanks.

Conclusion of findings

From the observations made on the tank, it can be concluded that the underside corrosion of the HFO tank was due to a combined action of pitting, stray current, soil consolidation, and soil corrosivity. The damage caused by the fluid is due to damages that are in the painting and the accumulation of water at the bottom that is in the holes in the roof that allows water to get into the tank. The corrosion that is on the roof may be due to the gap between the lap welded joints in the roof and the collection of hydrogen Sulphate and hence the creation of galvanic corrosion between the corroded area and the unpainted areas.

Recommendation

From the areas identified to cause corrosion in the tanks, several recommendations can be given to the manufacturers and those that have installed the tanks. For the tanks that already have corrosions, there is need to install new bottom plates over the existing bottom plates. The openings that are at the top plate and allows water and oxygen from entering the tank should also be reduced. The painting done at the bottom of the tanks should also include surface preparation which involves a continuity bond to avoid increased rates of corrosion. Before the installation and using an alternative CP system should be studied. The level of the liquid in the tank is also significant as all the bottom plates may not be in contact with the soil. Before installing the tanks, the CP method should be checked keenly to ensure its effectiveness.

The understanding of cathodic protection is based on the understanding of the cause of corrosion. The cause of corrosion in the cases mentioned above must first be understood before using the provided cathodic protection methods. The electrolyte and the metal for the protection are very important and must be chosen carefully to avoid causing more corrosion. Alloys are most frequently used. For instance, an alloy of steel and aluminum is an important way of protecting products made of steel. The cathodic protection is achieved through the conversion of the anodic sites on the metal surface to cathodic sites by supplying electrical current from another source. The reactions involve the transfer of electrons which help to achieve the protection of metals.

Reference

Pedeferri, P. (2006). Cathodic protection and cathodic prevention. Construction and Building Materials, 10(5), 391-402.