NACE International Standard TM0208 defines volatile corrosion inhibitor (VCI) as a chemical substance that acts to reduce corrosion by a combination of volatilization from a VCI material, vapor transport in the atmosphere of an enclosed environment, and condensation onto surface in the space, including absorption, dissolution, and hydrophobic effects on metal surfaces, where the rate of corrosion of metal surfaces is thereby inhibited.
VCIs come in various formulations that are dependent on the type of system they will be used in; for example, films, oils, coatings, cleaners, etc.
[1] Because they are volatile at ambient temperature, VCI compounds can reach inaccessible crevices in metallic structures.
[citation needed] The first widescale use of VCIs can be traced to Shell's patent for dicyclohexylammonium nitrite (DICHAN), which was eventually commercialized as VPI 260.
[4] At present, commercial VCI compounds are typically salts of moderately strong bases and weak volatile acids.
First the molecule may adsorb onto the metal surface thereby forming a barrier to aggressive ions and displacing any condensed water.
[6][7] The second path involves the condensed water layer that has been shown to exist on the metallic surface.
Large Equipment/Assets are wrapped in VCI heat shrinkable film for long term outdoor storage.
In the last few years, however, with growing environmental pressure to reduce the use of traditional inhibitors containing heavy metals, they have gained in popularity.
This application has recently been of wider interest as it has been approved by PHMSA as a means to address a shorted casing in a CP protected pipeline.
[9] Top-of-the-line TOL corrosion typically occurs in wet gas pipelines that have a stratified flow regime and poor thermal insulation.
[15] Tests showed that the best potential for providing corrosion protection for TOL came from azoles, certain acetylene alcohols, and a "green" volatile aldehyde.
Historical data has shown that significant corrosion issues can arise as a result of residual hydrotest water.
[14] For a non-piggable pipeline, the low sections where residual hydrotest water may collect after draining are identified and an aqueous VCI solution is added at nearby high points such that the inhibitor solution will flow into the low sections, thereby treating the residual water with inhibitor.
For tanks with a concrete ringwall, a sand bed and a liner, the VCI is typically installed as an aqueous solution.
For a double bottom with a liner and sand bed, the VCI is supplied as an aqueous solution which is injected through the leak detection ports.
For an El Segundo bottom that is in service, the VCI is again supplied as an aqueous solution that is injected through the leak detection ports.
For an El Segundo bottom that is out of service, perforated pipes are installed into the grooves in the concrete that have leak detection ports.
Mesh sleeves containing inhibitor powder is inserted into the perforated pipes and the leak detection ports are closed.
Corrosion protection is supplied via a system of dispensers that have been attached to ports that have been installed on the tank roof.
The VCI has a high vapor pressure such that the inhibitor will saturate the airspace within the dispenser and then will diffuse through the open port into the storage tank headspace.
[22] Water treatment – Aqueous VCI solutions have been used to flush/rinse pipelines, pumps, manifolds, enclosed pits, heat exchangers, etc.