ABOVE GROUND STORAGE TANK CORROSION RISK ASSESSMENT
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.
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.
Our approach for corrosion risk assessment and corrosion mitigation of ASTs are :
- Review of drawings and previous maintenance/repair reports
- Review environmental conditions and soil corrosivity data
- On-site inspection of the tanks by NACE Certified Corrosion/Materials Selection/Design/Coating/Cathodic Protection Specialist .
- Evaluation of cathodic protection system if active
- Recommendation for corrosion mitigation: Cathodic Protection and VCI inhibitors
- Design and Installation of cathodic Protection System
- Selection of VCI and Application of VCI
- Review cost and maintains of CP system and VCI.
- Maintenance and Monitoring of Corrosion Control System: Cathodic Protection and VCI
- Analysis and Interpretation of Legislative rules and Regulations or Aboveground Storage Tanks
Corrosion challenge
Soil side corrosion of bottom plates of above grade storage tanks is a major corrosion challenge
Foundation Types
The section will focus on the advantages and issues for the different above grade storage tanks (AST) from a corrosion risk point of view.
EARTH FOUNDATION
Advantages:
- The cheapest and easiest to construct
- Only used when the soil can bear the pressure of the upper steel construction and when anchorage is not necessary
Disadvantages:
- Earthen type foundations do not allow good leveling or drainage of the bottom of the tank, which can lead to uneven settlement which can cause negative effects for corrosion protection, do to voids and water pooling
- 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
EARTH FOUNDATION WITH A CONCRETE RING WALL
Advantages:
- Most widely used foundation for large diameter above grade storage tanks
- Use of the rigid reinforced concrete ring allows for more stability and larger diameter tanks
- Provides distribution of the concentrated load of the tank to produce a more nearly uniform soil loading under the tank shell
- Retains the fill under the tank bottom and prevents loss of material do to erosion.
Disadvantages:
- May not smoothly conform to differential settlements that could lead to voids in the soil to steel interface
EARTH FOUNDATION WITH CRUSHED STONE RING WALL
Advantages:
- Provides better distribution of the concentrated load of the shell to produce a more nearly uniform soil loading under the tank
- Provides a means of leveling the tank grade, and it is capable of preserving its contour during construction
- Retains the fill under the tank bottom and prevents loss of material as a result of erosion
- Can more smoothly accommodate differential settlement because of its flexibility
Disadvantages:
- 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.
CONCRETE SLAB FOUNDATION
Advantages:
- The do not allow uneven settlement of the tank and is recommended when the level of the underground water is high
- May be effective in eliminating intrusion of groundwater, soil-side corrosion, and the need for cathodic protection
Disadvantages:
- Very expensive
- 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
- Corrosion may occur due to this moisture accumulation. Cathodic protection is generally not considered an effective way to combat this corrosion
CONCRETE SLAB FOUNDATION WITH PILES
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.
Plethora of storage tanks
There are more than 500,000 AST and many of them exhibit accelerated corrosion and leaks
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 the table below.
| Resistivity Range (ohm-cm) | Potential Corrosion |
|---|---|
| <500 | Very Corrosive |
| 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.
