Materials Performance Supplements

Corrosion Prevention and Control for Tanks 2017

Materials Performance is the world's most widely circulated magazine dedicated to corrosion prevention and control. MP provides information about the latest corrosion control technologies and practical applications for every industry and environment.

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20 MAY 2017 MATERIALS PERFORMANCE TANK CORROSION CONTROL SUPPLEMENT TO MP Vaporized Corrosion Inhibitors Protect Underside of Aboveground Storage Tank Ab o v eg roun d stora ge t ank s (A STs) that contain oil and hazardous liquids are subject to certain federal, state, and local regulations, which can require facilities to develop and implement spill prevention and response plans with processes and countermeasures in place for preventing the discharge of oil into navigable waters and adjoining shorelines. Under-tank con- tainment such as high-density polyethylene (HDPE) liners or bentonite materials can be installed directly under the tank bottom of ASTs. For corrosion protection, cathodic protection (CP) is the most commonly used method for protecting tank bottoms and within the past decade most new tanks were constructed with CP systems. CP installation after the tank is put in service may not be possible, particularly if the tank has a liner. Also, for tanks with CP systems that are inadequate or have failed, replace- ment of these systems requires access to the tank bottom, which is very costly and often impractical. According to Tunde Kingsley Adelakin with Kinder Morgan Liquids Terminals, LLC, and Sujay Math and Dale Lindemuth with Corrpro Corp., vapor corrosion inhib- itors (VCIs) available in liquid or powder form are a proven technology to inhibit cor- Tank Protection Articles rosion. The vapor phase or volatile nature of the inhibitors makes it possible to for them to diffuse into hard-to-reach spaces underneath the tank bottom . Adelakin , Math, and Lindemuth investigated the use of VCIs as an alternative for underside cor- rosion control of a tank bottom with a liner to evaluate the methodology 's effective- ness. In CORROSION 2017 paper no. 9544, "External Corrosion Protection of Under- side Bottom of Above Ground Storage Tank Using Vaporized Corrosion Inhibitors," they report the details of their study. A 67-ft (20-m) diameter AST at a Kinder Morgan bulk liquids terminal was selected for this research project. The tank has a double steel bottom with a HDPE liner and rests on a concrete ring wall. Conduits were installed in the sand base beneath the tank's outer bottom shell to install electric resistance (ER) probes and inject a liquid VCI. The ER probes, used to determine real- time corrosion rates at the underside of the tank, were installed close to the underside of the outer tank floor at 10 different loca- tions. Their test leads were run into junc- tion boxes where real-time information was collected and data were uploaded into a computer. The corrosion rate was moni- tored on all the 10 probes. Any active corro- sion occurring on the probes would cause a reduction in the probe's thickness, and the rate of corrosion could be calculated through changes in thickness measure- ments. Corrosion occurring at any particular location underneath the tank bottom was associated with the local corrosive envi- ronment and the availability of oxygen to promote electrochemical reactions. Base- line probe thickness measurements were collected on all the ER probes before the VCI liquid was injected. During this "no mitigation" period, which lasted ~4 months, the probes were left to corrode freely in the sand environment. At a later stage, VCI liq- uid was injected through the conduits to f lood the sand base. The applied quantity saturated the spaces in the sand directly between the plate and the soil interface. The VCI corrosion protection was achieved by VCI molecules adsorbing into the metal surface and forming a protective barrier on the tank bottom, which neutralized all the anodic and cathodic sites. This barrier interrupted the electrochemical reactions by blocking oxygen and other reactants that promote corrosion. After the VCI was intro- duced, periodic thickness measurements and corrosion rates were collected over 24 months. The mitigation effectiveness was measured as the reduction in corrosion rates on the probes. The authors report that the formation of the protective barrier is a function of VCI concentration, and complete system effectiveness was achieved when the VCI traveled to all the locations underneath the tank bottom. Initial corrosion rates at different locations differed and tended to decrease with time. The lowest corro- sion rate was observed in one of the loca- tions closest to the VCI supply spot and the highest rate was observed at one of the locations farther away from the VCI supply spot. Their conclusion is that application of the VCI reduced the corrosion rates of the underside of the AST bottom, and they note it may take over 12 months and as long as 24 months for the VCI treatment to achieve the required effectiveness. Additionally, system results may not be uniform due to local environmental or physical conditions under the tank.

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