Unless protective measures are taken, above ground storage tanks(AST), piping, and other metallic components of fuel storage systems corrode (rust) and leak product into the environment. Corrosion can attack the carbon steel plates either over the entire surface of the metal (uniform corrosion) or in a small, localized area, creating a hole. Localized corrosion can perforate an unprotected tank in little as a few years and is the most common form of corrosion. Soil side corrosion of bottom plates of above grade storage tanks is a major corrosion challenge, especially when these tanks are constructed in high chloride corrosive environments. There are more than 500,000 AST and many of them exhibit accelerated corrosion and leaks
Monitoring and Mitigation of Corrosion in the Interstitial Space.
A typical system includes. Sealing any gaps between the tank floor and dead shell on double-bottom tanks, or gaps between the tank floor and concrete ring wall on single bottom tanks to prevent intrusion of fresh water and air into the interstitial spaces of these tank systems. Engineered application of the VCI into the interstitial space in such a way that effective distribution of the chemistry is ensured. A corrosion rate monitoring system utilizing electrical resistance probe technology or permanent reference electrodes is used to measure the real-time rate of corrosion within the interstitial space and near the tank floor.
In the following sections the pros and cons of different tank designs and adequate means of corrosion protection.
The section will focus on the advantages and issues for the different above grade storage tanks (AST) from a corrosion risk point of view.
This is the most often applied foundation construction because it is the cheapest and easiest to construct. It is made according to figure 1. This foundation is only used when the soil can bear the pressure of the upper steel construction and when anchorage is not necessary. In the event of a small leak moving out of the soil is very possible and can lead to the destruction of the tank. Earthen type foundations does not allow good leveling or drainage of the bottom of the tank. This can lead to uneven settlement which can cause negative effects for corrosion protection, do to voids and water pooling. Soil analysis is very use full to help determine the corrosivity of the soil and design of the most practical corrosion prevention system for the tank bottom. Determining the aggressive ions such as chlorides and sulfates along with measuring the pH and resistivity will help determine what corrosion control measures should be considered. This foundation type can have the highest current demand based on soil analysis and poor current distribution for cathodic protection do to voids in the soil to tank bottom interface. Because of this clean sand is normally used for the material beneath the AST bottom. This allows the designer to control the soil corrosivity but does not eliminate the need for cathodic protection since corrosions can still occur due to intrusion of water from rain or a shallow water table.
The concrete ring wall is the most widely used foundation for large diameter above grade storage tanks. It is made according to figure 2. The use of the rigid reinforced concrete ring allows for more stability and larger diameter tanks. It provides distribution of the concentrated load of the tank to produce a more nearly uniform soil loading under the tank shell. It provides a better means of leveling the tank grade and is capable of preserving its contour during construction. It retains the fill under the tank bottom and prevents loss of material do to erosion. A disadvantage of concrete ring walls is that they may not smoothly conform to differential settlements that could lead to voids in the soil to steel interface. Clean sand is the most common material used for backfill beneath the AST bottom. This allows the designer to control the soil corrosivity but does not eliminate the need for cathodic protection since corrosions can still occur due to intrusion of water from rain or a shallow water table. To minimize future corrosion problems and maximize the effect of corrosion prevention systems such as cathodic protection the material in contact with tank bottom should be fine and uniform. Large foreign objects or point contact by gravel or racks, wooden pegs, stubs of welding rods, mud or clay could cause corrosion cells that will cause pitting and premature tank bottom failure. The following material can be readily shaped to the bottom contour of the tank to provide maximum contact area and will protect the tank bottom from coming into contact with large particles and debris. Bitumen-sand (cold patch asphalt) mix 50 mm (2 in) thick laid on top of the foundation under the tank steel bottom has been proven as a corrosion prevention layer. Clean washed sand minimum of 75 mm (3 in) deep as a final layer on top of the foundation. If cathodic protection is to be utilized, the clean washed sand layer should be increased to 150 mm (6 in.). This type of foundation allows for Cathodic protection and leak detection materials/components to be placed in, or pass through, the sand pad for corrosion prevention and monitoring.
A crushed stone or gravel ring wall will provide adequate support for high loads imposed by a shell. A foundation with a crushed stone or gravel ring wall has the following advantages. It provides better distribution of the concentrated load of the shell to produce a more nearly uniform soil loading under the tank. It provides a means of leveling the tank grade, and it is capable of preserving its contour during construction. It retains the fill under the tank bottom and prevents loss of material as a result of erosion. It can more smoothly accommodate differential settlement because of its flexibility. A disadvantage of the crushed stone or gravel ring wall is that it is more difficult to construct it to close tolerances and achieve a flat, level plane for construction of the tank shell.
For crushed stone or gravel ring walls, careful selection of design details is necessary to ensure satisfactory performance. This type of foundation is significant details include the following. The 0.9 m (3 ft.) shoulder and berm shall be protected from erosion by being constructed of crushed stone or covered with a permanent paving material. Care shall be taken during construction to prepare and maintain a smooth, level surface for the tank bottom plates. The tank grade shall be constructed to provide adequate drainage away from the tank foundation. The use of cathodic protection on this type of foundation has produced mixed result. The tank pad should be fine and uniform, since differential aeration corrosion cells will cause pitting at contact areas between the large particles and the metal. The intrusion of water from rain or groundwater makes the environment under the tank alkaline, which may reduce corrosion. If contaminants are present in the pad, or with time infiltrate the pad, corrosion may accelerate. Thus, the use of crushed limestone or clam shells does not clearly eliminate the need for cathodic protection. This type of foundation allows for Cathodic protection and leak detection materials/components to be placed in, or pass through, the sand pad for corrosion prevention and monitoring.
The slab foundation although very expensive is favorable for large steel tanks. They do not allow uneven settlement of the tank. The concrete slab is also recommended when the level of the underground water is high. A properly designed concrete tank pad constructed on stable, properly prepared subsoil may be effective in eliminating intrusion of groundwater, soil-side corrosion, and the need for cathodic protection. Preparation of a stable soil to support the concrete slab is very important to ensure the continued integrity of the pad. Unstable soil may induce cracks in the slab through which water and contaminants can permeate to the steel tank bottom and provide a corrosive environment. Although corrosion from the soil may be prevented by a concrete pad, there may still be a collection of moisture between the tank bottom and the pad due to condensation, blowing rain or snow, or flooding due to inadequate drainage, rain water from the roof of the tank flowing down the tank walls, for example. Corrosion may occur due to this moisture accumulation. Cathodic protection is generally not considered an effective way to combat this corrosion. A free-draining concrete pad or ring wall and a seal around the periphery of the tank may be effective in eliminating the accumulation of moisture between the pad and the tank bottom where flooding in the dike area above the tank bottom does not occur. In situations where water may condense on the tank bottom or water is retained above the concrete pad, accelerated corrosion may occur. Due to numerous complex factors that can affect the corrosion of a tank bottom underside in the presence of concrete, prediction of the propensity of corrosion in this case is extremely difficult. Thus, care should be observed with tanks on concrete pads since cathodic protection most likely will not help reduce any corrosion that might occur may be ineffective in this case. Consideration should be given to keep the surface of the concrete tank pad and steel bottom plate free of contaminants during construction.
Piles beneath the concrete slab may be required for proper tank support. This type of foundation has the same advantages and disadvantages as the concrete slab. It is also noted that additional reinforcing steel is added to the foundation design that could increase the current demand.
SUBSURFACE CONDITION FOR TANK SITES
Tanks should be built on an elevated berm to allow adequate drainage away from the tank bottom. The use of fine particles will provide a denser pad to help reduce the influx and outflow of oxygen from the perimeter of the tank as it is emptied and filled. If large particle sizes are used, differential aeration corrosion may result at points where the large particles or debris contact the steel tank bottom. In this case, cathodic protection current will be shielded and may not be effective in eliminating corrosion the pitting. There are a wide variety of pad materials available, some of which may actually prevent the beneficial effects of cathodic protection. Conversely, there are situations where some of these materials, when properly selected and installed, can be beneficial in reducing corrosion to the extent that cathodic protection may not be needed, as described later in this section. There are several different materials that can be used for the grade or surface on which the tank bottom will rest. To minimize future corrosion problems and maximize the effect of corrosion prevention systems such as cathodic protection, the material in contact with the tank bottom should be fine and uniform. Large foreign objects or point contact by gravel or rocks, wooden pegs, stubs of welding rods, mud, or clay could cause corrosion cells that will cause pitting and premature tank bottom failure. The following material can be readily shaped to the bottom contour of the tank to provide maximum contact area and will protect the tank bottom from coming into contact with large particles and debris. Bitumen-sand (cold patch asphalt) mix 50 mm (2 in) thick laid on top of the foundation under the tank steel bottom has been proven as a corrosion prevention layer. Clean washed sand minimum of 75 mm (3 in) deep as a final layer on top of the foundation. If cathodic protection is to be utilized, the clean washed sand layer should be increased to 150 mm (6 in.)
In coastal areas, salt spray on tank surfaces will be washed down the sides of the tank by rain and may flow beneath the tank to contaminate the tank pad for all foundation types. This also can occur in areas where fertilizers or chemicals may be in the atmosphere either from spraying or industrial operations. The tank pad also can become contaminated by wicking action that can draw contaminates such as chlorides up from the water table. Cathodic protection is usually necessary for corrosion prevention in these situations.
If a leak occurs in a tank bottom, the leaking material can also influence corrosion on the external side. If water leaks from the tank, the environment under the tank may become more corrosive. If product leaks from the tank, it could create corrosion cells that did not previously exist or adversely affect the effectiveness of cathodic protection. A leak may wash away part of the pad material and eliminate the contact of the tank bottom with the ground in some areas. Cathodic protection will not be effective in such areas. Additionally, the drainage properties of the pad material may be deteriorated by a leak and allow water and contaminates to remain in contact with the tank bottom. The condition of the tank foundation should be evaluated after a leak to access change in corrosion properties of the foundation.
SOIL RESISTIVITY AND SOIL CORROSIVITY TANK SITE
Soil resistivity may provide valuable information about the corrosivity of the material used under and around a tank. A general resistivity classification is given in Table 1.
Table 1—General Classification of Resistivity
|Potential Corrosion Activity|
|500 – 1,000||Corrosive|
|1,000 – 2,000||Moderately corrosive|
|2,000 – 10,000||Mildly corrosive|
|>10,000||Progressively less corrosive|
There are several techniques for measuring soil resistivity. A common method is described in ASTM G 57. It should be noted that soil resistivity alone should not be used to determine soil corrosivity. The resistivity of the pad material may be higher than the existing surrounding soil. Corrosive soil beneath the higher resistivity pad material may contaminate the pad fill by capillary action and should be a consideration when determining sand pad thickness. Thus, resistivity of surrounding soil may be used to help determine the probability of corrosion on the tank bottom. The results of soil resistivity surveys should be considered and used to help determine the need for cathodic protection. However, other properties such as chlorides sulfides and sulfates of the soil should also be considered.
We provide corrosion risk assessment and corrosion mitigation of bottom plates on aboveground tanks used for the storage. The standard method of determining the corrosiveness or the effectiveness of cathodic protection on a tank bottom is the tank-to-soil potential measurement. One of the problems associated with monitoring cathodic protection systems on tank bottoms is the inability to assess empty space under the tank or to place a reference electrode in close proximity to the underside of the tank resulting in measurements that may not represent the tank-to-soil potential at specific areas or at the center of the tank bottom. When utilizing earthen foundations, soil analysis is useful to help determine whether the potential corrosion activity will be high enough to make cathodic protection necessary and whether cathodic protection will be a practical application to prevent corrosion. The advantages and disadvantages of each will be discussed. Determination of aggressive ions such as chlorides and sulfates along with measurement of moisture, pH and resistivity at shallow and deep burial are helpful for further corrosion analysis. Predictive modeling based on soil corrosivity data will provide life expectancy or remaining life with moderate to high confidence layer. Corrosion mitigation techniques such as cathodic protection, concrete foundations and VCI will discussed briefly.
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