At Tank Connection Affiliate Group, we are the only tank manufacturers worldwide that design, manufacture and install all four types of steel storage tanks including bolted RTP, field-weld, shop-weld and hybrid tank construction. We are the experts in the industry on storage applications and we understand cathodic protection requirements. We also prescribe CP for specific applications and storage products as noted below. This document is specific to BOLTED RTP (rolled, tapered panel) steel tank construction only, which utilizes a state-of-the-art powder coat system. Storage tanks that utilize this system do not require cathodic protection, nor is it recommended by TC or our coating supplier, Akzo Nobel, the largest powder coat supplier worldwide. The following information is intended to inform our clients on what types of storage products and applications require cathodic protection.
Definition of Terms (as noted per AWWA Manual M42):
- Principles of Cathodic Protection: CP systems are used to prevent or retard the corrosion that would naturally occur in a steel water tank. These systems prevent or slow corrosion by altering the electrochemical environment so that the submerged tank shell becomes the cathode of a corrosion cell. Since the cathode of a cell does not corrode, the submerged metallic tank shell is protected.
- Nature of Corrosion: The corrosion of steel in aqueous solutions is an electrochemical process in which a current flows and chemical reaction occurs. A corrosion cell has four basic elements: anode, cathode, electrolyte, and closure path. The anode is the metal that will corrode, that is, metal ions leave its surface and enter the electrolyte solution. The cathode is a metal from which no metal ions enter the solutions. The electrolyte may be any solution, such as drinking water that is capable of conducting electricity. The closure path, also called the return current path, is the electrical conductor, usually metal, that connects the anode and cathode together. If any one of these elements is missing, corrosion does not occur. For example, coating stops corrosion from occurring by providing a barrier to the current flowing between the metal and the electrolyte.
Two common types of CP include:
- Galvanic or sacrificial anodes
- Impressed current system
- Galvanic Cathodic Protection (CP)
- Galvanic systems are also known as "sacrificial anode systems" because an anode (usually zinc or magnesium) corrodes instead of the exposed tank steel. In essence, the anode provides a protective current by reversing the electron flow. The anodes are designed and selected to have a more "active" voltage (technically, a more negative electrochemical potential) than the exposed tank steel. The galvanic anode continues to corrode, consuming the anode materials until eventually they require replacement.
- Impressed Current Cathodic Protection (ICCP)
- A cathodic protection system in which the protective current is supplied by an external source (rectifier). The level of protective current that is delivered to the structure is adjustable and is much higher than that associated with galvanic anodes. For this reasons, impressed current system are utilized on steel tanks that are uncoated or require a high amount of protective current.
In Water Storage Containment, TC recommends:
- Field-welded tanks — Recommended CP due to uncoated, exposed steel.
- Underground Storage Tanks (UST) — Recommended CP due to uncoated, exposed steel.
- Glass-lined Tanks — Recommended CP due to internal/external exposed steel edges of bolted panels, uncoated bolt holes and cracks/micro fractures in glass coating that will expose steel.
- Fusion Bonded Epoxy Tanks — No CP required due to no internal or external exposed steel.
- LIQ Fusion 7000 FBE™ Coating System
- Tank Connection's LIQ Fusion 7000 FBE (fusion bonded epoxy) powder coat system is robotically applied to bolted steel panels and is fused to the steel surface. Advancing through a corridor of high temperature ovens, the epoxy powder melts and undergoes a molecular fusion bond to the steel. The powder system is electrostatic charged during application and the bolted panels are negatively charged. The amount of the charge helps insure that every area of "exposed steel" on a panel is coated per specification. The process guarantees powder deposited on all edges, in bolt holes and across the panel uniformly. As the panel moves through the ovens, the electromechanical bond holds the powder to the steel while it is fused to the steel substrate surface. The end result is a molecular bonded layer of TC's proprietary fusion powder coat system that forms a non-conductive dielectric layer across the surface of the steel panel.
- Improper CP
- Under the best conditions, CP will never extend the coating/lining life. A side effect of an improperly functioning CP system is the production of molecular hydrogen (over-voltage of hydrogen bubbles) at the metal surface, which can lead to blistering and disbondment of the coating/lining. Field surveys show that a vast number of CP systems are not properly maintained or functioning in the field as intended. Improper maintained CP systems will shorten the life of a tank lining. (What is meant to be supplemental in tank designs that maintain "uncoated exposed steel areas", can be detrimental to superior designed "powder coat systems" that fusion-bond to a steel surface. Fusion bonding encapsulates the entire steel panel surface, leaving no exposed, uncoated areas. It is formulated to provide maximum protection to liquid tank interiors without cathodic protection.
The underlying purpose of cathodic protection is to protect the steel substrate surface. Field-weld tanks, steel UST's (underground storage tanks) and glass-lined tanks all have exposed steel surfaces. These products will require cathodic protection in water storage applications. Tank Connection's FBE (fusion bond epoxy) system does not allow exposed steel surfaces and subsequently does not require cathodic protection. For the reasons as outlined, Tank Connection does not recommend CP installed in FBE bolted liquid storage tanks.
LIQ Fusion 7000 FBE™ is the #1 bolted tank coating/lining for water and wastewater storage available worldwide. Tank Connection is the exclusive provider of this proprietary system, maintaining the only powder coat line certified to apply this system for bolted storage tanks. LIQ Fusion 7000 FBE™ is a stronger system than glass lined coatings and is 100% factory holiday tested. Additionally, it is superior to ALL other powder and liquid coating/lining systems available today for water storage applications.
When you have questions about CP requirements in liquid storage applications, contact Tank Connection for the answers.