Corrosion Solutions for Oxygen Concentration Corrosion and Pitting Corrosion Under Your Aboveground Storage Tank Bottoms
Corrosion Solutions for Dealing with Oxygen Concentration and Pitting Corrosion
Aboveground Storage Tank (AST) bottoms are usually protected using a cathodic protection (CP) system. Secondary containments are installed to prevent any leaks from spilling into the environment. Typically, high resistive sand is used as tank pad and is placed in-between the secondary containment and the bottom plate, and anodes are installed in the interstitial space. Typically, mixed metal oxide (MMO) anode ribbons are placed in the sand pad either in a grid pattern or as concentric rings with MMO wires encased in coke-breeze filled socks to deliver CP current. Use of high resistive sand has led CP designers to use a constant voltage rectifier to be able to provide adequate CP current. With the rectifier set at a constant voltage, the CP anodes generate more current than the designed value when the sand pad resistivity decreases.
High current outputs often lead to increased oxidation process at the MMO anode surface and oxygen gas is evolved. Oxygen is a cathode depolarizer and in the case of close coupled CP systems, the oxygen evolution from MMO anodes depolarizes the cathode (tank bottom) polarized potentials as shown in Figure 1.
When the tank bottom plate flexes, a void is created between the sand pad and the tank bottom plate. CP current cannot reach areas of the bottom plate through the void and the oxygen gas gets accumulated in the void space, which accelerates pitting corrosion on the bottom plate. For AST bottoms, pitting corrosion is the main hazard as pitting causes leaks and operational downtime.
Mitigation of Pitting Corrosion Due to Oxygen Concentration
Pitting corrosion due to oxygen concentration in void space and the mitigation using Zerust FVS Vapor Corrosion Inhibitor (VCI) was investigated using ER Probes and corrosion coupons. As seen in Figure 2, the ER probe metal loss (mils) in void space increased over time at atmospheric oxygen and at high oxygen concentrations, whereas the ER probe in VCI test setup did not corrode even in the presence of high oxygen concentrations. Pitting corrosion was observed on the coupon surfaces and investigated by measuring pit depths on ER probes and weight loss coupons.
The pitting corrosion data is presented in Figure 2 in the form of colored maps of the profiled coupons’ surfaces. The deep blue areas of the colored maps represent loss of material, and the red colored areas show coupons’ initial surface that was exposed to the environment but did not corrode. The deepest pit information from the scanned data is listed below each scanned coupon.
As seen in the figure for both corrosion coupon and ER probe, the high oxygen condition exhibits most loss of material, followed by atmospheric oxygen condition, and no corrosion of the high oxygen plus VCI condition.
VCIs Can Help Prevent Pitting Corrosion Due to Oxygen and Moisture in the Void Space
In summary, MMO anodes for CP systems generates oxygen due to water hydrolysis when operated at higher current outputs. The presence of oxygen and moisture in the void spaces accelerates the pitting corrosion of the AST bottom plate. The pitting corrosion could lead to tank bottom plate failure and cause leaks and operational downtime for the tank owners. VCIs can help prevent pitting corrosion due to oxygen and moisture in the void space.
VCIs, when used in combination with CP systems, lower the resistivity of the sand pad. The low resistive environment decreases the anode ground bed resistance to the tank bottom and CP system can be operated at lower anode polarized potentials yet achieving effective polarization. The use of VCI and CP in combination is advantageous as both technologies complement each, and thus increase overall life of an AST bottom plate from corrosion.
*Source: Materials Performance Magazine February 2021 Edition, “Tank Bottom Corrosion Induced by Anode-Generated Oxygen” by Sujay Math, Ph.D. and Terry Natale of Zerust Oil & Gas, and Pavan K. Shukla of Savannah River National Laboratory. Click here to view full feature.
Zerust Corrosion Solutions for Tank Bottoms
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