Vapor Corrosion Inhibitor (VCI)

VCI is an innovative method for preventing corrosion on the external surfaces of storage tank bottom plates. It can be applied to both new and existing tanks. VCIs evaporate or sublime under ambient conditions, react with moisture in the air beneath the tank bottom to raise pH, form a protective film, and inhibit corrosion. Simply put, they create a barrier film on the tank bottom metal against aggressive ions and moisture.

Standardized in:

• API 655 (Vapor Corrosion Inhibitors for Storage Tanks)
• NACE AMPP SP21474 (External Corrosion Control of On-Grade Carbon Steel Storage Tank Bottoms)

Temporary protection of tank bottoms before CP commissioning is a key challenge. When transformer rectifiers are not yet energized and tank filling is delayed for up to two years, corrosion can continue. VCI provides effective interim protection during this period.

Moisture, atmospheric contaminants, and corrosive species such as chloride ions can penetrate beneath unprotected tank bottoms, causing severe corrosion at the annular area and bottom plates. The consequences can include leakage, environmental and safety risks, high replacement costs, and downtime.

For existing tanks, holes are drilled according to the design documents, nozzles of the specified length are installed beneath the bottom plate, and VCI is injected by hand pump. This method is especially useful for tanks without CP, tanks with inaccessible undersides, and large-diameter tanks where geomembrane or geotextile layers limit discrete CP installation.

For tanks under construction, the injection arrangement is tailored to the foundation and site conditions, with nozzles installed beneath the plate. Construction and welding delays of one to two years may prevent timely energizing of CP transformer rectifiers; VCI helps control the tank bottom corrosion rate during this period.

Monitoring uses electrical resistance (ER) coupons. The bare metal tip corrodes in the environment, increasing electrical resistance. ER probes are matched to the tank bottom plate material; corrosion changes their cross-section and resistance, while stable readings indicate effective protection.