Soil Side Bottom (SSB) corrosion of aboveground storage tanks (ASTs) is a major cause of tank bottom failures and a constant problem for tank owners in the oil and gas industry.
The use of Vapor Corrosion Inhibitors (VCIs) and Cathodic Protection (CP) are common practices for protecting ASTs from corrosion. Below I will briefly review these practices and when it is most effective to use them independently or in combination for the most effective corrosion protection, specifically for ASTs.
What are Vapor Corrosion Inhibitors (VCIs)?
VCIs are a chemistry which slows the corrosion process in enclosed spaces of virtually any space, size or shape. An enclosure can also be created and then the VCIs can be added to that environment to provide corrosion protection. VCIs volatize from a solid or liquid into a gas and equalize with the atmosphere in the enclosure. Those molecules then adsorb to the steel surface and block other contaminant molecules from getting to the surface by taking precedence.
VCIs will remain intact within the enclosed area and provide effective corrosion protection if there is a proper seal, in case the enclosed area is not properly sealed the VCI effectiveness will be reduced. VCI replenishment will be required at a later stage and replenishment time will vary from project to project and based on the client’s requirements.
VCIs are generally being used to protect the soil side bottom plates of Aboveground Storage Tanks (AST) by themselves but they can be used in combination with CP systems as well to protect the bottom plates of AST.VCI applications and monitoring techniques are outlined in section 12.5 of the API 2610 Standard as a method for mitigating corrosion of metallic structures or components such as tank bottoms, cased pipelines, electrical enclosures, etc.
For more information on how VCIs work please visit https://www.zerust-oilgas.com/technology/ to view an animation.
Cathodic Protection (CP)
CP is a well-known technique commonly used for AST bottom plate soil side corrosion protection. With past experiences due to poorly designed CP systems or premature failure of CP systems, continued tank integrity is not ensured. Field performance of tank bottoms show that several other factors such as Microbiologically Influenced Corrosion (MIC), improper current distribution from the anodes and increased resistance of the tank pad material leads to ineffective CP systems.
CP and VCI in Combination
When the liquid stored in the tank is transported, the fill levels of the storage tank change and the bottom plate of the empty tank flexes, potentially causing the plate to lose contact with the sand electrolyte. This flexing of bottom plates is a commonly known phenomenon and is widely observed during routine inspection cycles. CP is ineffective in areas where the tank bottom loses contact with the sand electrolyte. Due to these limitations, CP systems may not always be enough in providing effective corrosion protection.
On other hand, VCIs provide corrosion protection of metals by vapors adsorbing to the metal surface; the corrosion protection mechanism is through formation of a chemical barrier that reduces oxidation of the metals. VCIs form an invisible layer on the metal surface and act as a coating. An example of this is shown in the animation image above. The key advantage of using VCIs is that inhibitors can reach metal surfaces in small confined spaces that could be difficult to reach using CP.
VCIs, when used in the presence of a CP system, could complement the CP system by providing protection in vapor spaces and to the rest of the bottom plate in the event of a CP system failure or when CP is either ineffective or partially effective.
How to Determine When to Use VCIs, CP, or Combination of Both?
When determining which techniques to use, VCIs or CP, the first thing you want to consider is the status of the tank.
Either it is a new tank construction or a complete floor replacement, and/or it is a tank that is out-of-service and the floor will remain in place or the tank is in-service.
In each of these scenarios, what are your corrosion protection options? These answers are found in the yellow rows in the table below.
Once the status of the tank has been determined the tank bottom foundation needs to be considered when choosing which corrosion protection option will work best.
In the gray rows in the table below, the answers may change if the tank is in-service or out-of-service. For example, with a Sand Base w/liner, if the tank is in-service, it will be virtually impossible to install or repair any type of CP system whereas if it’s out of service and the floor remains in place, there may be some VCI options. In the column of using Powder VCI with the tank in-service, the powder may only be applied at the edge of the tank, therefore the protection area is limited to the vapor carry distance of the VCI molecules.
Common VCI applications include the following:
Soil Side Bottoms (SSBs) of Aboveground Storage Tanks (ASTs)
Pipe Casing Annulus VCI Gel Fill
Flanges & Valves
Large Equipment Preservation
Protection of Voids/Enclosures
If you are considering the use of VCIs, CP, or a combination of the two for corrosion protection, please do not hesitate to contact us to discuss your concerns and allow me to help provide a unique solution that works for you.
Dr. Sujay Math is a Sr. Technical Services Engineer at Zerust Oil and Gas and is based in Cleveland, Ohio. He received his Ph.D. in Civil Engineering from Clemson University, Clemson, SC. He is an active member of NACE and has achieved NACE CP Technologist certification.
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