Oxygen Evolution from the MMO Anode Cathodic Protection System and its Effect on the Corrosion of the Soil-Side Bottom Plate of an Above Ground Storage Tank
Sujay Math Zerust Oil and Gas
Beachwood, Ohio, 44122 USA
Pavan K. Shukla Savannah River National Laboratory
Aiken, SC, 29808 USA
Terry Natale Zerust Oil and Gas
Beachwood, Ohio, 44122 USA
ABSTRACT
A close coupled mixed metal oxide (MMO) anode, grid or concentric ring, cathodic protection (CP) system is a state-of-the-art technique for corrosion prevention of the soil-side bottom of an aboveground storage tank (AST) resting on sand pads. The regulatory requirement of secondary containment for ASTs, especially with the use of polyethylene liners, has eliminated the use of traditional shallow anodes around the periphery or the deep well anode systems. Recent experiences with close coupled CP systems have shown that adequate corrosion protection is not achieved for ASTs and are subject to leaks, before their scheduled out-of-service inspection interval. Literature information on failed tank bottoms with CP systems have identified several issues such as improper current distribution due to anode spacing, shorting of the anode to tank bottom and acid generation at anode resulting in low pH. One of the issues not adequately discussed with close coupled MMO anodes include oxygen generation at the anode surface due to electrolysis of water at polarized potentials above a threshold. For close coupled CP systems such as ASTs, the oxygen evolution from the MMO anodes can depolarize the cathode (tank bottom) polarized potentials. When the tank bottom plate loses contact with the sand pad due to flexing, oxygen can accumulate in the void space and accelerate corrosion. CP cannot provide protection to the areas of bottom plate that loses contact with the electrolyte. The floor scan data of the tank bottom plates indicate patches of corrosion even with good clean sand and an active CP system.
In this paper, experimental investigations are carried out to understand the evolution of oxygen and its concentration in the void spaces. The effect of oxygen concentration in void space versus corrosion growth rate with increased oxygen concentration is studied. This paper provides information on preventive measures that can be adopted to prevent oxygen concentration corrosion in the void space of AST soil-side bottom.
INTRODUCTION Aboveground Storage Tanks (ASTs) soil-side bottoms are usually protected with a cathodic protection (CP) system. There is regulatory requirement to use secondary containment for ASTs to prevent any leaks from spilling into the environment. The commonly used secondary containment method is the use of high density polyethylene (HDPE) liner placed at a certain depth below the tank bottom plates. This method has eliminated the use of traditional shallow anodes around the tank periphery or the deep well anode systems. Figure 1 shows a schematic of an AST bottom resting on a concrete ring wall with secondary containment and CP system anodes. Sand as a tank pad is placed in between the secondary containment and the bottom plate. Secondary containment made of materials such as HDPE liners potentially act as a dielectric shield, hence anodes are installed in the sand base in between the membrane and the bottom plate. For larger diameter tanks, an impressed current cathodic protection (ICCP) system using mixed metal oxide (MMO) anode is preferred because of the current output needed to protect the bottom. Typically, MMO anode ribbons are placed in the sand pad either in a grid pattern or as concentric rings underneath the tank bottom to deliver CP current throughout the tank bottom surface.
Figure 1: Schematic of AST bottom with Secondary Containment and CP System Anodes
For effective implementation of a CP system, the current generated by anodes is a determining factor to achieve effective cathodic polarization on the tank bottom. The industry standard practice, API (1) 651 document on CP for AST bottoms recommends using a clean sand for the tank pad material. The recommendation of washed clean sand pad material is to reduce the contaminants that could cause corrosion. As per API 6511, a sand resistivity value of 10,000 ohm-cm or greater is recommended. This resistivity test is conducted in laboratory at saturated condition (100% saturation). Most tank owner specifications restrict the sand moisture content to 10% or lower when installed in the field under tank bottoms. Hence, the resultant field installed sand resistivity could be anywhere from 30,000 ohm-cm or higher. The clean sand even though helps in reducing the contaminants from sand pad, it certainly challenges the principles of Ohms law for generating sufficient CP current due to the resultant high resistivity. With a high resistive sand pad, the anode groundbed resistance to tank (structure) is high, which in turn increases the total circuit resistance of the CP system. As per Ohm’s law, the higher the circuit resistance the lesser will be the current generated by that system. Uniform CP current distribution throughout the entire surface of the tank bottom is also difficult to achieve with high resistive sand pad material.
(1) American Petroleum Institute (API) 200 Massachusetts Avenue NW Suite 1100 Washington, DC 20001-5571 USA
The use of high resistive sand has also led CP designers to use a constant voltage rectifier so that adequate CP current is generated. For example, in a newly installed CP system for an AST bottom, the constant voltage is set at higher values to overcome the circuit resistance and generate adequate CP current to achieve protective cathodic polarization. During commissioning of the CP system, the dry sand is clean of contaminants and high in resistance (30,000 ohm-cm or greater), however, over time due to intrusion of moisture from climatic events the resistance of sand deceases. With the rectifier set at a constant voltage and with the decrease in sand pad resistivity due to moisture intrusion, the CP anodes will generate more current and operate at higher anodic voltages.
In a Pourbaix2 diagram of any metal in water, electrolysis of water molecule takes place and hydrogen (H+) and oxygen (O2-) ions are generated at an electrode potential of 1.229 VSHE. At the cathode, reduction takes place and hydrogen gas is evolved. At the anode, oxidation takes place and oxygen gas is evolved. Oxygen is a cathode depolarizer3, and in the case of close coupled CP systems, the oxygen evolution from MMO anodes depolarizes the cathode (tank bottom) polarized potentials. When the tank bottom plate flexes, CP cannot reach areas of the bottom plate that loses contact with the electrolyte.
Past experiences 4,5 with CP systems have indicated problems such as uneven CP current distribution to the tank bottom surface, oxygen gas evolution and acid generation at anode. Oxygen gas is an universally-known cathodic depolarizer, however it is not clear how the oxygen evolved at the CP system anodes accumulates in the void spaces under tank bottoms and cause excessive corrosion. Oxygen in the presence of moisture is easily dissolved and dissolved oxygen is known to cause sever pitting attack on carbon steel at concentrations lower than 50 parts per billion.6 For AST bottom plates, pitting corrosion is the main hazard as pitting leads to leaks and operational downtime. The atmospheric oxygen can also enter underneath AST bottoms either through the CP monitoring ports, leak detection ports or through the chime area. The presence of oxygen in void spaces under tank bottoms could be detrimental to AST bottom plate integrity, and hence, measures should be adopted to prevent void space corrosion where CP is ineffective.
Vapor corrosion inhibitors (VCIs) are injected in the sand pad to provide protection for the soil side tank bottoms7-9 and have been extensively studied in a recently completed Pipeline Research Council International (PRCI)(2) study.8 The study reported that use of amine carboxylate based VCI in combination with CP does not adversely affect the CP system in meeting the NACE CP criteria.8,9 The study reported VCIs lower the resistivity of sand pad due to their ionic strength and decrease the circuit resistance of CP system.9 The low resistivity helps the anodes to generate current easily and meet the NACE CP criteria. The limited literature information on VCI plus CP combination studies mainly involved the use of MMO anodes and were found to have a synergistic effect8-10. One study shows that certain VCIs improve the cathodic polarization process and reduce the CP current required to achieve effective cathodic protection10.The laboratory studies on galvanic anode compatibility with VCIs show that VCIs can de-passivate the oxide layer on zinc and keep it active to function as an anode for galvanic CP systems used in interstitial space of AST bottoms.11
In this paper, experimental investigations are carried out to understand the evolution of oxygen at MMO anode surface and its concentration in the void spaces. The effect of oxygen concentration in void space versus corrosion growth rate with differing oxygen concentration is investigated. This paper also explores VCIs as a measure to prevent oxygen concentration corrosion in the void space of the tank bottom plate.
(2) Pipeline Research Council International (PRCI), 15059 Conference Center Drive, Suite 130, Chantilly, VA. USA 20151.
EXPERIMENTAL PROCEDURE An oxygen sensor from Apogee Instruments† was used to monitor differing oxygen levels in the void space. The oxygen sensor works on the concept of galvanic cell and a millivolt reading is obtained from the sensor for differing oxygen levels. The oxygen sensor was calibrated at atmospheric oxygen concentration (which is 21%), the sensor was then placed in the test setup to monitor varying oxygen levels. An atmospheric corrosion electrical resistance (ER) probe from Metal Samples† was used to monitor the corrosion rate in the void space, a weight loss corrosion coupon was also placed in the void space for corrosion evaluation purposes. The ER Probe and corrosion coupon were carbon steel alloy C1010 grade. Figure 2 shows the oxygen sensor, atmospheric ER probe and weight loss coupon.
Figure 2: Atmospheric ER Probe, Oxygen Sensor and Weight loss Coupon
Figure 3 shows the test setup used to evaluate oxygen concentration levels in the void space generated by the MMO anode in an ICCP system. A one-gallon jar was used to configure an ICCP system with potable water as an electrolyte. A36 grade carbon steel coupon acting as cathode was suspended at the water line and an MMO ribbon anode with net effective length of 1-feet was placed on the bottom of the jar. Constant CP current of 5 mA was applied and the resultant net current density on the MMO ribbon anode was 5 mA/ft. Copper-copper sulfate reference electrode (CSE) was used for potential measurements. The oxygen sensor was placed in the void space and the test setup was sealed to prevent escape of oxygen from the jar. The test was run for 40 days and oxygen concentrations in the void space were recorded. The On-potential values with a fixed reference electrode was recorded every 6 hours for 40 days.
Figure 4 shows the test setup used to evaluate different oxygen concentrations vs. corrosion rates in the void space. Three separate one-gallon jar setups were used; (i) Atmospheric Oxygen, (ii) High Oxygen and (iii) An amine carboxylate based VCI as a mitigation measure to counter the corrosion in the void space due to high oxygen concentration. Each test setup consisted of an oxygen sensor, atmospheric corrosion probe and weight loss coupon. Deionized (DI) water was used as electrolyte in test setups (i) and (ii); in test setup (iii), 5% VCI was mixed in DI water. To avoid complexity of generating oxygen gas in the void space from MMO anode and maintain safety, oxygen gas was directly injected into the void space and the system was sealed in the test setups (ii) and (iii). The intent was to test the corrosion behavior in the void space in the presence of high oxygen and not to generate oxygen. It was found that the system could not be sealed hermitically due to the oxygen sensor and ER probe attachments and periodic loss of oxygen occurred, at these periodic loss occurrences oxygen gas was reinjected to maintain high oxygen concentrations in the void space. In test setup (i), a small vent was provided to allow atmospheric oxygen to enter into the gallon jar and oxygen concentration was monitored.
Figure 5: Water bath setup for cyclic temperature variation from 72 F to 110 F
The three test setups were placed in a large water bath as shown in Figure 5 and the temperature was
cycled between 72 F to 110 F (22 to 43 C) to simulate daily variation in temperature. The electric bucket heater and circulation pump were connected to temperature controlled switch with temperature range
set. The switch would automatically turn on at 72 F (22 C) and turn off at 110 F (43 C). The temperature variation allowed the electrolytes inside the jar to condensate on the ER probe and corrosion coupon in the void space. This process allowed for any corrosion to occur. The water bath test was conducted for 110 days and the test was stopped for further evaluation of the corrosion coupons. Pitting corrosion analysis was performed on the weight loss coupons by scanning coupons’ surfaces. The scanned surfaces were used to estimate material wastage and pit depths.
RESULTS AND DISCUSSION In an AST bottom scenario, the oxygen generated at the MMO anode can be trapped in void spaces where the bottom plate is elevated from the sand pad due to flexing. The concentration of oxygen in a void space can be at the atmospheric oxygen level or it can be higher if the oxygen cannot diffuse through the sand pad to the ambient. Figure 6 shows the oxygen concentration levels (green symbols) generated by the MMO anode in an ICCP system, shown in Figure 3. The data was collected every 6 hour for 40 days. As seen in Figure 6, the oxygen concentration increased over time in a sealed system and stabilized in the range of 34% to 36%. It was evident that an MMO anode when operated at a current output of 5 mA/linear-feet generates oxygen, mainly due to the electrolysis of water molecule.
Figure 6: Oxygen concentration in void space vs. time
Figure 6 also shows the change in On-potential (y2-axis) values with time. As seen in the figure, the On-potential values decreased over time. For the CP system test experiment a constant current of 5 mA was applied and the resistivity of the electrolyte (potable water) was not changed. With constant current entering the cathode, constant voltage gradients are created around the cathode with electrolyte resistance being the same, from Ohm’s law principles. Since the reference electrode location is fixed, the On-potential measured at the cathode is not expected to change significantly without the change in electrolyte resistivity. However, as the oxygen concentration levels increased in the void space and stabilized over time, a certain amount of oxygen dissolved back in to the electrolyte and a new equilibrium was reached with higher amounts of dissolved oxygen in the electrolyte. The increase in dissolved oxygen levels in the electrolyte depolarize the cathodic protection polarized potentials as evident in Figure 6. The corrosion phenomenon with different oxygen concentrations in the void space was further evaluated.
Corrosion at Atmospheric Oxygen Concentration (21%) Figure 7 shows ER probe metal loss (mils) due to corrosion in a void space at atmospheric oxygen concentration (21%). The oxygen concentration values were calculated from the millivolt readings
measured from the oxygen sensor. Due to the cyclic change in temperature from 72 to 110 F, and change in relative humidity in the glass jar test setup, the oxygen sensor had ±0.5% measurement error. As seen in Figure 7, the ER probe metal loss (mils) in void space increased over time at atmospheric oxygen. The cyclic variation of the temperature and relative humidity in the glass jar test setup caused the condensate to form on the metal surface and cause corrosion.
Figure 7: ER probe corrosion in void space at atmospheric oxygen concentration
Corrosion at High Oxygen Concentration (27% to 29%) Figure 8 shows ER probe metal loss (mils) due to corrosion in a void space at high oxygen levels in the range of 27% to 29%. The high oxygen concentrations were achieved by injecting oxygen in the test setup, and it simulates the oxygen generated by electrolysis of water due to CP system anodes. The oxygen concentration values were calculated from the millivolt readings measured from the oxygen
sensor. Due to the cyclic change in temperature from 72 to 110 F, and change in relative humidity in the glass jar test setup, the oxygen sensor had ±0.5% measurement error in measured oxygen concentration levels. As the oxygen levels decreased due to losses from the test setup, small amount of oxygen gas was reintroduced to maintain the desired levels. As seen in Figure 8, the ER probe metal loss (mils) in a void space increased over time at high oxygen concentrations. The cyclic variation of the temperature and relative humidity in the glass jar test setup caused the condensate to form on the metal surface and cause corrosion.
Figure 8: ER probe corrosion in void space at high oxygen concentration
Corrosion Mitigation with VCI at High Oxygen Concentration (27% to 29%) Figure 9 shows ER probe metal loss (mils) corrosion mitigation in a void space at high oxygen levels in the range of 27% to 29% plus VCI. The high oxygen concentrations were achieved by injecting oxygen in the test setup, and it simulates the oxygen generated by electrolysis of water due to CP system anodes. The oxygen concentration values were calculated from the millivolt readings measured from the oxygen
sensor. Due to the cyclic change in temperature from 72 to 110 F, and change in relative humidity in the glass jar test setup, the oxygen sensor had ±0.5% measurement error in measured oxygen concentration levels. As the oxygen levels decreased due to losses from the test setup, small amount of oxygen gas was reintroduced to maintain the desired levels. As seen in Figure 9, the metal loss in mils on the ER probe did not increase over time in the void space of the test setup. The cyclic variation of the temperature and relative humidity in the glass jar test setup caused the condensate to form on the metal surface. The presence of VCI in the void space mitigated the corrosion in the presence of high oxygen and moisture. The mechanism by which VCI provides corrosion protection is by dissolving the VCI molecule in the condensate water and adsorbing to the metal surface, known as chemisorption. The VCI molecule takes precedence over the oxygen molecule in the condensate water during chemisorption process, thus preventing oxidation of steel.
Figure 9: ER probe corrosion mitigation with VCI in presence of high oxygen concentration
Corrosion Rate of Weight Loss Coupons The corrosion rates of weight loss coupons were calculated after test completion. The weight loss measurements provided the average corrosion rate for the test period of 110 days. Figure 10 shows the respective corrosion rates of weight loss coupons in the three test setups. The weight-loss coupons corroded at atmospheric and high oxygen conditions, but no corrosion occurred on coupons exposed to high oxygen plus VCI condition. The coupons’ corrosion behavior agreed with the ER probe data: the VCI prevented corrosion occurrence on both samples. A statistical comparison could not be made between the atmospheric oxygen and the high oxygen coupons as the sample size for each condition was one, however a qualitative conclusion was drawn that coupons corroded in the void space in the presence of oxygen without VCIs. Pitting corrosion was observed on the coupon surfaces and further investigation was done to evaluate pit depths on both ER probes and weight loss coupons.
Figure 10: Corrosion rate of weight loss coupons in void space with differing oxygen
concentrations and VCI Pitting Corrosion The pitting corrosion data is presented in Figure 11. The data are presented in 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. As seen in the figure, the front face of the high oxygen condition coupon exhibits most loss of material, followed by atmospheric oxygen condition, and no corrosion of the high oxygen plus VCI coupon. The same observation is made on the back faces of the mass-loss coupons: the back face of the high oxygen condition exhibits most loss of material, followed by the atmospheric oxygen coupon, and no loss of material of the high oxygen plus VCI. The ER probe material elements were also scanned for material wastage quantification; the scanned data are presented in form of color maps in Figure 11. As seen in the figure, the blue area cover most of the ER probe surface for the high oxygen condition, followed by the atmospheric oxygen ER probe element. The ER probe element of the high oxygen plus VCI showed no signs of corrosion, therefore, it was not scanned. The ER probe elements’ material wastage correspond with the coupons, indicating that ER probes provide in-situ continuous information about corrosivity of the test conditions. The deepest pit information from the scanned data in Figure 11 was obtained. The deepest pit depths are listed in Table 1. The pit depth on the front face of the high oxygen condition was the deepest, whereas the pits on the back faces were of same depths for both atmospheric and high oxygen conditions. The pit depth data should be used with cautions as it does not contain extent of corrosion.
No Pitting Corrosion observed on ER probe exposed surface
Figure 11: Pitting Corrosion of ER probe and weight loss coupon
Table 1
Pit Depths for Various Coupons
Parameter Conditions
Atmospheric Oxygen High Oxygen High Oxygen Plus VCI
Front Face Maximum Pit Depth
85 µm 132 µm Nil
Back Face Maximum Pit Depth
100 µm 100 µm Nil
ER Probe Pit Depth 263 µm 174 µm Nil
DISCUSSION
Corrosion of AST soil side bottom plates is a major issue and historically CP systems have been used. Conceptually the CP systems should work for AST soil side bottom protection, but operational issues associated with flexing of the tank bottom plate due to change in fill volume levels results in loss of contact between the bottom plate and the sand pad electrolyte, making CP ineffective in those areas. The CP anode spacing is a critical factor in CP current distribution for complete coverage of the bottom plate in contact with the electrolyte. The change in sand pad resistivity also effects the CP current distribution. The regulatory requirements and NACE CP criteria requirements for effective CP has necessitated tank operators to achieve protective polarized levels on the tank bottoms. This practice has led to polarizing CP anodes to higher polarized potentials. However, the polarized potentials at the anodes when exceed the 1.229 VSHE, electrolysis of water occurs and generates oxygen gas. The oxygen gas rises to the surface of tank bottom and dissolves in the electrolyte at the cathode surface depolarizing the polarized
potentials to non-protective levels. Seeing the non-protective levels, the logical conclusion from a CP standpoint would be to increase the CP rectifier outputs to achieve protective polarized potentials. However, the high rectifier outputs also increase the polarized potentials on anodes and leads to more oxygen generation at the anode surface, which further depolarize the cathodic protection potentials. The process becomes a vicious cycle until rectifier reaches maximum limits. The generated oxygen can also accumulate in the void spaces underneath the tank bottoms when contact is lost with the electrolyte. The concentration of oxygen in the void space can be at atmospheric oxygen levels or can be higher if the oxygen cannot escape through the sand pad to the ambient. The cyclic variation of temperature due to climatic changes cause the moisture to condensate in the void spaces underneath the tank bottom. The presence of oxygen and moisture accelerates the pitting corrosion of tank bottom steel in the void spaces as seen by the test results reported in this study. Pitting leads to the tank bottom plate leaks and negates the purpose of having effective cathodic protection system. Cathodic protection is most effective when operated at low anode polarized potentials that prevent water electrolysis. A low resistivity of sand pad is most conducive for having a low anode polarized potential, as this will allow CP anodes to generate sufficient current at lower voltages to achieve effective cathodic protection without much oxygen generation. There is a wrong industry practice of using water-dripping tubes installed under the ASTs to lower the resistivity of the sand pad, water can bring contaminants and cause corrosion of bottom plates along the water dripping tubes. The investigations carried out in this paper show VCIs having beneficial effects in preventing pitting corrosion in presence of oxygen and moisture. VCIs, when used in combination with CP systems, lower the resistivity of the sand pad due to their ionic strength. The low resistive environment decrease the CP anode ground bed resistance to the tank bottom and the CP system can be operated at lower anode polarized potentials for achieving uniform current distribution and effective polarization throughout the tank bottom. The lower anode polarized potentials also reduce the oxygen generation process. If in certain cases the anodes are operated at higher polarized potentials and oxygen is generated, the presence of VCI molecule takes precedence over the oxygen molecule in the condensate water. This provides effective corrosion mitigation in the void space and to the rest of the bottom plate in the event of CP being ineffective or partially effective.
CONCLUSION
1. MMO anodes for CP systems when operated at high polarized potentials that exceed the 1.229 VSHE, the electrolysis of water occurs and generates oxygen.
2. The oxygen gas generated rises to the tank bottom, dissolves in the electrolyte at the cathode
surface, and could result in depolarizing the polarized potentials to non-protective levels.
3. The presence of oxygen and moisture in the void spaces accelerates the pitting corrosion of tank bottom plate as seen by the test results in this study. The pitting leads to tank bottoms plate leaks and negates positive effects of CP.
4. VCIs, when used in combination with CP systems, lower the resistivity of the sand pad. The low resistive environment decrease the anode ground bed resistance to the tank bottom and CP system can be operated at lower anode polarized potentials for achieving effective polarization.
5. The investigations carried out in this paper show VCIs having beneficial effects in preventing
pitting corrosion due to oxygen and moisture in the void space. The use of VCI and CP in combination proves advantageous as both technologies complement each other to increase the overall life of an AST bottom plate from corrosion.
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