Methodologies to Evaluate Compatibility between Cathodic Protection ...

Paper No.

11567

Methodologies to Evaluate Compatibility between Cathodic Protection and Vapor Corrosion Inhibitors for Tank Bottom Applications

Pavan K. Shukla(1) Southwest Research Institute® San Antonio, Texas, 78238 USA

Sujay Math Zerust Oil and Gas Beachwood, Ohio, 44122 USA

ABSTRACT Prevention of soil-side corrosion of the bottom plates of Aboveground Storage Tanks (ASTs) is a major challenge for the oil and gas storage industry. AST bottom plates are generally 0.25 inch (6.35 mm) thick A36 carbon steel. Literature suggests that the corrosion rate of the soil–side bottoms can be up to 200 mpy (5 mm/year). The soil-side surfaces of the bottom plates are usually protected by an impressed current cathodic protection (CP) system. However, when the bottom plate flexes the tank fill levels are varied, creating air gaps between the bottom plate and the tank bed leading to reduced CP effectiveness and potential corrosive conditions for the tank bottoms. Furthermore, in many situations, CP could become ineffective due to the poor ionic conductivity within tank beds and failure of anodes. Limited research and fieldwork have shown that vapor corrosion inhibitors (VCIs) by themselves or in combination with CP can be used to protect the bottoms of ASTs from external corrosion. VCIs could reduce corrosion incidents by 70–90 percent and decrease the incidence of pitting corrosion. This paper discusses methodologies to evaluate the compatibility between VCI and CP for the tank bottom application. Experimental work is reported to evaluate the CP criteria applicability in presence of VCIs, the work also provides a guideline for selecting an effective corrosion mitigation strategy for combined VCI and CP systems. In summary, VCI and CP, used in combination, are found to have a synergistic interaction to mitigate tank–bottom corrosion to acceptable levels. Key words: Vapor Corrosion Inhibitors, Cathodic Protection, Above Ground Storage Tanks, CP Criteria

(1)

Current affiliation: Savannah River National Laboratory, Aiken, SC, 29808, [email protected]

©2018 by NACE International. Requests for permission to publish this manuscript in any form, in part or in whole, must be in writing to NACE International, Publications Division, 15835 Park Ten Place, Houston, Texas 77084. The material presented and the views expressed in this paper are solely those of the author(s) and are not necessarily endorsed by the Association.

1

INTRODUCTION Aboveground Storage Tanks (ASTs) are one of the key assets for owners and operators in the oil and gas transmission industry. Protecting these assets is an economic burden and hence the operators are in continuous struggle to protect and extend the service life of these storage tanks. The soil-side corrosion of the bottom plates of ASTs is a major challenge and directly impacts the service life of tanks. API 6531 makes it mandatory to determine the tank bottom integrity to prevent leakage of storage fluids that could cause environmental damage. Inspection frequency to verify tank integrity is mandated once every 10–20 years, provided the in–service periodic inspections assure continued tank integrity. The tank bottom plate corrosion rates normally determine the frequency of inspection, with higher corrosion rates of the bottom plates the frequency of inspection may be as little as 10 years. The tank owners take precautionary measures to keep the tank in–service for the mandated 20–year inspection cycle, as frequent inspection cycles cause downtime and loss of revenue which is significant in vindicating cost to benefit economics. Cathodic Protection (CP) is a well–known technique commonly used for AST bottom plate soil–side corrosion protection. However, with past experiences due to poorly designed CP systems or failure of CP systems before their intended design life, continued tank integrity is not ensured. Literature information shows that several other factors such as microbiologically influenced corrosion (MIC), improper current distribution from the anodes and increased resistance of the tank pad material leads to ineffective CP systems. When the liquid stored in the tank is transported, the storage tank fill levels change causing the bottom plates to flex and lose contact with the sand electrolyte. This flexing of bottom plates is a known phenomenon and widely observed during routine inspection cycles. CP is ineffective in areas where the tank bottom loses contact with the sand electrolyte. Vapor Corrosion Inhibitors (VCIs) can be used either by themselves or in combination with CP systems to protect the bottom plates of ASTs, and address deficiencies of CP systems. VCIs are especially useful in protecting bottom plate surface areas that are difficult to reach using CP. Literature information27 suggests that VCIs can protect the corrosion of metals by adsorbing at the metal surface; the corrosion protection mechanism is through formation of a chemical barrier that reduces oxidation of the metals. VCIs form an invisible layer on the metal surface and act as a transparent coating. The key advantage of using VCIs is that inhibitors can reach metal surfaces in small confined spaces that could be difficult to protect using CP. The corrosion protection mechanism by CP, on the other hand, is electrochemical in nature, and requires contact of metal surfaces with the electrolyte to function. VCI application for the tank bottom is just over a decade old and has started to gain traction as an alternative corrosion control measure by itself or in combination with a CP system. Literature suggests that VCIs are effective in reducing corrosion incidents by 70–90 percent and decrease the incidence of pitting corrosion in most aggressive environmental conditions. Limited experience indicates that the VCI usage increases tank service life approximately 3–5 times and reduce maintenance and replacement costs. While the VCI technology has a potential to reduce tank bottom corrosion, reduce maintenance costs, and provide extended service life, the key question remains regarding compatibility between VCI and CP. The underlying corrosion prevention mechanism of VCI is chemical action whereas CP is electrochemical in nature; when used in combination, a compatible synergistic interaction between the two-corrosion control measure could be extremely advantageous. This paper explores several methodologies to evaluate the compatibility between VCI and CP for the tank bottom application, and evaluate CP criteria that are most appropriate for a given VCI with specific electrochemical characteristics. CORROSION CONTROL CRITERIA FOR AST BOTTOM PROTECTION Cathodic Protection The objective of CP is to reduce AST bottom corrosion rates by less than 1 mil/yr (25 µm/yr). Two standards are generally practiced by the AST operators: (i) NACE SP0193,8 and (ii) API 651.9 NACE ©2018 by NACE International. Requests for permission to publish this manuscript in any form, in part or in whole, must be in writing to NACE International, Publications Division, 15835 Park Ten Place, Houston, Texas 77084. The material presented and the views expressed in this paper are solely those of the author(s) and are not necessarily endorsed by the Association.

2

SP0193 recommendation for corrosion control of carbon steel tank bottoms is based on achieving either of these two criteria: (i) a polarized potential of at least –850 mV relative to a copper sulfate reference electrode (CSE); (ii) a minimum of 100 mV of cathodic polarization from the native (opencircuit) potential of carbon steel with a stable reference electrode. Both criteria are based on the polarized potential of carbon steel. In NACE SP0193, special considerations are required when stray currents or electrical gradients exists at the measurement location, a criteria other than the polarized potential criteria may be used. The use of coupons or electrical resistance (ER) probes is suggested to check for effectiveness of CP systems. API 651 recommendations for CP-based corrosion control of carbon steel tank bottom are similar to NACE SP0193 criteria. As in NACE SP0193, API 651 guidelines also require considerations for measurement techniques when polarized potential criteria are used. Special considerations are required for voltage (IR) drop errors and electrical gradients caused from the anodes, when performing potential measurements. API 651 highlights as tank content fill vary, the tank bottom surface area contacting the electrolyte could change. As a result, the tank-to-soil potential measurements could be affected by changes in the tank fill levels. In API 651, the corrosion rate monitoring of bottom plates is recommended and the end of service life is considered when the bottom plate thickness reaches 0.1 inch (2.54 mm). Vapor Corrosion Inhibitors without and with CP VCIs, when used by themselves, a direct measurement of AST bottom corrosion rate is most appropriate. The corrosion rate criterion of 1 mil/yr (25 µm/yr) can be used as it will be consistent with the CP criteria of achieving polarized potential that implicates achieving polarized potential at the steel surface in turn reduces the corrosion rates to less than 1.0 mpy (25 µm/yr). However, for practical purposes, a tolerable corrosion rate of 2.0 mpy (50 µm/yr) is considered an acceptable criterion when VCIs are used without any CP. This will provide a service life of 75 years for 0.25-inch (6.35 mm) thick carbon steel plate with residual thickness of 0.1 inch (2.5 mm), considering uniform corrosion. When VCI and CP are used in combination as a corrosion mitigation technique, the question arises which CP criteria should be used? This question is complicated by the fact that VCIs mitigate the corrosion by chemical action and CP by cathodically polarizing the steel surface. In addition, application of VCIs may change the metal surface characteristics, which in turn can change the open circuit potential of the bottom plate in contact with sand electrolyte. In either case, one of the CP criteria must be satisfied if the storage tank is regulated by PHMSA of the U.S. Department of Transportation in the continental U.S. It is recommended that the basis for the criterion should be the corrosion rate of 1.0 to 2.0 mpy (25 to 50 µm/yr). Similar to VCI only application, 2.0 mpy corrosion rate will yield a service life of 75 years for 0.25-inch (6.35 mm) thick carbon steel plate considering uniform corrosion and the service life ends at 0.1-inch (2.5 mm) thickness of the bottom plate. VCI COMPATABILITY WITH CP SYSTEMS Most of the storage tank bottoms have an active CP system installed during the construction phase for soil–side corrosion protection of the tank bottom plates. The CP system components such as anodes, reference electrodes and electrical wiring cables are buried in the sand base. When a VCI, either in powder or slurry form is installed into the sand base, the potential effect of VCI on the performance of the CP system and/or the CP components is not known. The underlying mechanisms of VCI interaction with the CP system is a complex dynamic process and is time−dependent. The performance of individual CP components can however be tested to understand their functionality in presence of VCIs. Native Potential Shift of Bottom Plate Due to VCI Application VCI molecules contact the metal surface and form a chemical barrier layer. This effect is common in both the liquid and vapor phase application of VCIs. The inhibitors form a protective hydrophobic film of ©2018 by NACE International. Requests for permission to publish this manuscript in any form, in part or in whole, must be in writing to NACE International, Publications Division, 15835 Park Ten Place, Houston, Texas 77084. The material presented and the views expressed in this paper are solely those of the author(s) and are not necessarily endorsed by the Association.

3

adsorbed molecules on the metal surface, which provides a barrier to the dissolution of the metal in the electrolyte. VCIs that are used for storage tank bottom soil-side application are chemical−based, for example, amine carboxylate−based inhibitors. The liquid−phase formulations of the VCIs, when in contact with the steel, could shift the open circuit potential (OCP) of steel in either an electropositive or an electronegative direction.10 Krissa et. al.10 used two different VCIs and monitored the change in open circuit potential for an extended period. The authors reported that both the VCIs shifted the OCP in an electropositive direction without application of CP. Krissa et. al.10 also reported that the shift in OCP change with application of CP. For one VCI, the changes in OCP remained in a positive direction with the application of CP, whereas for other VCI, the OCP change was in a negative direction when compared to no-CP OCP. This complicates the selection of a CP criterion when both CP and VCI are used in combination. For example, if –850 mV polarized potential criterion is to be used, the VCI with electropositive OCP shift may result in a larger CP current when compared with the original OCP without VCI. Therefore, a careful consideration of the CP criteria is needed when VCI and CP are to be used in combination. Reference Electrodes The common reference electrode recommended by API 651 and NACE SP0169 for tank bottom application is the copper/copper sulfate reference electrode (CSE). The CSE electrode is not very stable in chloride rich sand environments and alternative reference electrodes are recommended such as silver/silver chloride (SSC), and zinc/ zinc sulfate (ZRE). As a common practice for extended service life monitoring, CSE and ZRE are installed together where zinc acts as a stable reference point. The stability of various reference electrodes was evaluated by Krissa et. al. 11 in another study in which they used various commercially available reference electrodes, including Cu/CuSO4, bentonite–clay clad Cu/CuSO4, and bentonite-clay clad Zn/ZnSO4 reference electrodes. The stability of these reference electrodes was evaluated with a silver/silver chloride electrode and compared to the theoretical potential difference between the tested electrodes. Krissa et. al.11 found no significant differences among reference electrodes in VCI dosed sand systems for the test duration. The authors recommended longer test duration to check for long–term stability of the reference electrodes. Anodes The mixed metal oxide (MMO) coated titanium anodes are the state-of-the art impressed current CP system anodes that are widely used for tank bottom soil–side corrosion protection. The MMO anodes are installed either in direct contact with the sand or in coke backfilled sleeves. The anode placement positions are designed for uniform current distributions at the tank bottom. The anode is buried in a clean sand which has relatively high resistivity, the cathodic protection is provided by proximity of the anodes to the tank bottom surface. The effect of VCIs on the anodes has not been tested, however the MMO anodes are tested in various aggressive environments and the mixed metal oxide layer is known to withstand wide ranges of pH and can be operated at higher voltages without breakdown.12 Funahasi et.al.12 discussed that when current from anode discharges to a high resistivity electrolyte, such as sand, the consumption rate of MMO coating is much higher than in a wet electrolyte at the same anode current density. This phenomenon is important as the tank sand base could become dry overtime and therefore become highly resistive. When VCIs are injected in the sand base, the resistivity would decrease significantly and could help reduce the consumption rate of the anodes at a given anode current density, thus extending the service life of the MMO anodes.


4

Corrosion Rate Monitoring The use of mass–loss coupons and ER probes are commonly used methods to monitor the corrosion rates of tank bottom plates. When VCI is injected in the sand base, there is time dependency for uniform distribution of the inhibitor through sand medium; the coupons and ER probes react to the inhibitor injection and inhibitor presence is observed by the change in corrosion rate. Adelakin et. al.3 in their field research study used 10 ER probes distributed at different locations under a tank bottom and periodically monitored the change in corrosion rates for an extended period. In their study, two different corrosion rates were used as a basis to evaluate the effectiveness of VCIs. The NACE SP0193 recommended CP corrosion mitigation rate of 1.0 mpy and a tolerable corrosion rate of 2.0 mpy. The data from the 10 ER probes showed that there is an initial transient time for VCI to induce its effectiveness in reducing corrosion rates. With extended monitoring, 8 probes were below the 1.0 mpy threshold and the remaining 2 probes were below 2.0 mpy even after 3 years of VCI injection.3 The test tank used by Adelakin et. al.3 did not have an active CP system and the corrosion mitigation was achieved with VCI only. Selecting CP Criteria for Corrosion Mitigation of the combined system As an industry practice, the following three CP criteria are predominantly used: 1. –850 mVCSE current–applied potential, 2. –850 mVCSE polarized potential, and 3. 100 mV polarization (formation or decay) For practical purposes, –850 mVCSE current–applied and polarized potentials are commonly referred as on– and instant–off potentials, respectively. The PRCI–sponsored work showed that achieving a certain CP criterion does not guarantee complete corrosion mitigation for a buried pipeline, and the effectiveness of each criterion depends on environmental conditions, such as soil resistivity.13–15 Song et. al.13–15 provided a detailed analysis of the three CP criteria in different conditions. The authors identified that corrosion rates could be higher than 1.0 mpy at approximately 5 percent of the locations, even if the −850 mV on– and off–potential criteria or the 100–mV polarization potential criteria were satisfied. Barlo et al.16 determined that the cathodic protection criteria can be overly conservative in some cases, inadequate in others, and can be inconsistent among themselves. Barlo et al.16 also determined that the 100–mV polarization criterion is the most generally valid and applicable criterion to prevent corrosion in various soils and can be explained in terms of anodic polarization behavior. In addition, the anodic polarization behavior may be useful to approximate the potential and polarization requirements for cathodic protection. The use of VCI has shown promising results as a corrosion control technique. When CP and VCI are used in combination, depending just on CP criteria to verify the corrosion mitigation effectiveness may not be the best cost–effective strategy. The addition of VCI to the CP system makes it complicated to rely on a certain CP criterion, as the chemistries of various VCIs differ and their effect could complicate justification of CP criteria. The use of coupons and ER probes are an acceptable technique for measuring corrosion rates of tank bottoms and pipelines and using this technique for combined VCI+CP systems is justifiable. However, for regulatory requirements and the immediate needs of the corrosion industry in the US, additional work is needed to establish effectiveness of the CP criteria when used with VCI and develop guidelines on applicability of these criteria to various VCI chemistries. EXPERIMENTAL We evaluated two different VCIs, identified as VCI–A and VCI–B, for their compatibility with the CP system. Electrochemical tests were conducted using a potentiostat on A36 hot–rolled carbon steel ©2018 by NACE International. Requests for permission to publish this manuscript in any form, in part or in whole, must be in writing to NACE International, Publications Division, 15835 Park Ten Place, Houston, Texas 77084. The material presented and the views expressed in this paper are solely those of the author(s) and are not necessarily endorsed by the Association.

5

samples by applying CP current. Potable water from a local supply was used for all the tests as a control electrolyte, and a 5% concentration of VCIs were used for testing purposes. Mixed metal oxide (MMO) strip anode was used for applying impressed CP current. An A36 steel bar of 0.5-inch (12.7 mm) diameter was used; only the cross section of the bar was exposed to the electrolyte for a known surface area of 0.196 inch2 (1.26 cm2) and the rest was coated with an epoxy. The sample surface was prepared to have a clean shinny surface using #1500-micron sand paper, before starting each of the test runs. A portable DC power supply (rectifier) was used to power the CP system. Copper-copper sulfate reference electrodes (CSE) were used for potential measurements. The corrosion potential of the cathode and anode was continuously monitored with a fixed common CSE and the multiplexed feature of potentiostat during the testing.

Reference electrode DC Power Supply

Anode Cathode

(a) Corrosion cell setup to test VCI plus CP compatibility

(b) A36 Steel (Cathode)

(c) Exposed surface area of cathode

Figure 1. Experimental setup for VCI plus CP system compatibility measurements. ©2018 by NACE International. Requests for permission to publish this manuscript in any form, in part or in whole, must be in writing to NACE International, Publications Division, 15835 Park Ten Place, Houston, Texas 77084. The material presented and the views expressed in this paper are solely those of the author(s) and are not necessarily endorsed by the Association.

6

Potentiostatic17 polarization tests were conducted on A36 steel and MMO anode at the cathodic and anodic potentials observed during the CP system testing. Individual tests were conducted on A36 steel electrode and MMO anode electrodes separately acting as working electrodes, graphite electrode was used as the counter electrode during the polarization test. The A36 steel was cathodically polarized where as MMO anode was anodically polarized at OCP and on-potential values observed from the CP system testing. The cathodic and anodic currents generated at different potentials were recorded. A 1 L size 4-neck flask was used for the test setup shown in Figure 1. For VCI resistivity measurements, the 4−pin miller soil box was used and resistivity was measured using the Nilsson 400 soil resistivity meter at different concentrations. The resistivity measurements were also conducted using a liquid conductivity meter. RESULTS AND DISCUSSION The resistivity and pH data of the VCI solutions measured are shown in Figure 2. The results indicate that the resistivity of both VCIs are nearly identical. However, the pH of VCI–A was found to be 2 units higher than VCI–B. When the VCIs are added to potable water, based on the inherent chemistries of individual VCIs, the ionic strength of the resultant solution changes. The change in pH is due to remaining free H+ and OH– ions after the VCI reaction products are formed and the solution has reached an equilibrium. If the resultant equilibrium solution has more free hydrogen ions, i.e., [H+] then the pH of the solution will decrease, similarly if the resultant equilibrium solution has more free hydroxyl ions, i.e., [OH–], the pH of the solution will increase. It is to be noted that the low resistivity of VCIs is due to the high ionic strength of VCI molecules and not due to contaminates such as chlorides or sulfates.

Figure 2. Resistivity and pH with different concentration of VCIs ©2018 by NACE International. Requests for permission to publish this manuscript in any form, in part or in whole, must be in writing to NACE International, Publications Division, 15835 Park Ten Place, Houston, Texas 77084. The material presented and the views expressed in this paper are solely those of the author(s) and are not necessarily endorsed by the Association.

7

Figure 3 shows the effect of impressed current cathodic protection on– and Instant–off potentials on the A36 steel electrode and MMO anode before and after addition of VCIs. Two separate experimental setups were used and VCIs were added to each respective setup. The OCP of the steel electrode and MMO anode was continuously monitored at 4 secs sampling rate for 165 hours. The data in Figure 3 can be divided into six sections as discussed below: Section 1: In Figure 3, OCP of A36 carbon steel electrode and MMO anode were measured. Once the potable water was added to the corrosion cell, measurements were conducted after steady–state OCP was reached. Section 2: Cathodic protection current was applied using a constant potential DC power supply (rectifier) and the current–applied potentials, i.e., on–potentials, of the steel electrode and MMO anode were recorded. The DC power supply was set to a constant 3.0 V output, and depending on the total circuit resistance encountered by the CP system, the current output obtained was 5 mA. The cell EMF observed was also 3.0 V (current applied). The on–potentials were allowed to reach steady−state; CP system was turned off after steady–state was reached. Section 3: The instant–off potential of steel electrode and MMO anode were recorded immediately after the CP system was turned off, the instant–off potentials were observed to be more electronegative than the –850 mV CSE and satisfied the NACE CP criteria. As per NACE CP criteria, the instant-off potentials should be recorded within 0.2–1 second after the DC power supply is turned off. In this study, the multiplexed Potentiostat with 4 channel monitoring could record potentials with 4 sec intervals. If the instant−off potentials measured after the 4 second delay are able to meet NACE CP criteria, then instant−off potentials measured after 0.2–1 secs would also meet the criteria. The OCPs of the electrodes were monitored until stable readings were obtained. Section 4: VCIs were added to the potable water in their respective setups and change in OCP was recorded, the net concentration of the electrolyte in the 1 L corrosion cell was 5 % VCI by volume. The effect of pH on the shift in OCP was observed after the addition of VCIs. Sufficient time was allowed for the OCP of both VCI systems to stabilize and reach equilibrium. Exposure to VCI-A shifted the equilibrium OCP of steel electrode and MMO anode in electronegative direction. Exposure to VCI-B shifted the equilibrium OCP of MMO anode in electronegative direction, whereas the equilibrium OCP of steel electrode was nearly same to as observed before adding VCI B. This change in OCP was due to the change in electrode-electrolyte interface resistance, which changes the magnitude of exchange current densities occurring on the cathode and anode surfaces, respectively. Section 5: The CP system was turned on and the same constant potential of 3.0 V was applied to both VCI systems. The cell EMF (difference in potential between cathode and anode) remained same as 3.0 V (as applied) whereas the total circuit resistance offered by the CP system changed depending on the VCI type. The current output generated for VCI-A CP system was 9 mA and for VCI-B CP system was 3 mA, respectively. The current outputs were recorded after 24 hours, it was observed that the immediate CP applied current outputs were higher for both VCIs and with cathodic and anodic polarization the steady state current reached was lower. It was also observed that the stable on–potentials on the cathodes were lower (less electronegative) for VCI+CP system as compared to potable water CP system without VCIs (Section 2). If only the shifts on cathodes are observed without considering the anode potentials, cell EMF and circuit resistance offered by VCI+CP systems, the results will be deceiving and may lead to drawing erroneous conclusions on satisfying the NACE CP criteria. However, as evident from the graph the instant–off potentials observed for potable water CP system and the VCI+CP systems are similar irrespective of the on–potentials on the cathodes. The 4-sec instant–off potentials were more electronegative than the –850 mVCSE and satisfied the NACE CP criteria. The 100–mV polarization decay NACE CP criteria was also satisfied for the VCI + CP systems. ©2018 by NACE International. Requests for permission to publish this manuscript in any form, in part or in whole, must be in writing to NACE International, Publications Division, 15835 Park Ten Place, Houston, Texas 77084. The material presented and the views expressed in this paper are solely those of the author(s) and are not necessarily endorsed by the Association.

8

Figure 3. Effect of VCIs on CP System “OCP” “On” and “Instant-off” Potentials


9

(a) Potential versus time

(b) Current versus time Figure 4. Potentiostatic polarization data for VCIs at OCP and on-potential


10

Table 1 Test data for various VCI and CP system parameters Test Solution Potable Water VCI–A VCI–B

Electrode

OCP (V)

On– Potential (V)

Anode

0.15

1.25

Cathode

-0.71

-1.80

Anode

0.10

1.60

Cathode

-0.80

-1.50

Anode

0.15

1.75

Cathode

-0.75

-1.25

DC Power Supply Potn. Curr. (V) (A)

Instant– off Potential (V)

Polariza tion (V)

-1.032

0.322

3.0

-1.068

0.268

-1.023

0.273

Cell EMF (V)

Circuit Resist. (Ω)

0.005

3.05

610

3.0

0.009

3.1

344

3.0

0.003

3.0

1000

Section 6: The CP system was turned off and the OCP was monitored until stable potentials were reached. The OCP of cathodes and anodes returned to as observed OCP in Section 4 i.e., before applying CP current. The MMO anode depolarization was observed to be faster for VCI–A after the CP system was turned off, this probably is due to the high pH of VCI–A and abundant availability of [OH–] ions. For Section 7, the CP system on / off was repeated and similar trends were observed as seen in Section 5 and Section 6. Table 1 shows the test data for different parameters for VCI and CP systems. The data in Table 1 show the potentials measured for different test solutions and the polarization achieved after applying CP. The polarization achieved for all systems were more than 100 mV and the instant-off potentials were more electronegative than the –850 mVCSE. The cell EMF was calculated by the stable on–potential difference between cathode and anode for the respective test solution. The constant DC potential of 3 V was applied and the corresponding current generated was recorded in units of Amps. The circuit resistance of the CP systems was calculated by dividing the cell EMF by the current generated by the CP system circuit. The VCI-B CP system encountered more circuit resistance than VCI-A CP system. Figure 4 shows the potentiostatic polarization plot for VCI and CP systems. The stable OCP and on–potentials recorded in Table 1 are applied to each electrode for 60 minutes and the corresponding current generated is recorded for the different test solutions. At the stable OCP, the corrosion potential is at equilibrium with the solution and no net current flows to or from the surface of the electrode. In a hindsight, a dynamic equilibrium condition exists at the surface of the electrode where the rate in the forward direction is equal to the rate in the reverse direction for the same redox reaction; indicating the exchange current density. When the on–potentials are impressed, the anode becomes more electropositive and generates anodic current, whereas, the cathode becomes more electronegative and receives cathodic current. Figure 4(b) shows the current vs. time plot for each electrode, when the OCP is applied the net current is zero. When the on–potentials are impressed, VCI–A generates more anodic and cathodic currents compared to the potable water electrodes. VCI–B generates marginally more anodic current and nearly same cathodic current compared to the potable water electrodes. This suggest that the resistance offered by the VCI–B is higher at the electrode−electrolyte interface compared to potable water and VCI–A. The potentiostatic polarization test verifies the results tabulated in Table 1 and as observed in Figure 3. Effective corrosion mitigation for the combined VCI and CP system It is evident that the CP criteria can be satisfied in the presence of VCIs by the test results seen in Table 1. However, it is also evident that VCIs tend to shift the open circuit (native) potentials of cathode and anode depending on their inherent chemistries. The current applied on–potentials are also shifted due to the change in circuit resistance of the CP system. Thus, care should be taken in selecting a ©2018 by NACE International. Requests for permission to publish this manuscript in any form, in part or in whole, must be in writing to NACE International, Publications Division, 15835 Park Ten Place, Houston, Texas 77084. The material presented and the views expressed in this paper are solely those of the author(s) and are not necessarily endorsed by the Association.

11

certain CP criteria when VCI and CP are used as combined corrosion mitigation strategy. Most of the above ground storage tank bottoms are bare steel and hence the 100−mV polarization criterion is preferred over the –850 mVCSE instant-off potential criterion for effective cathodic protection. When VCI and CP systems are used in combination, the shift in the native potential of cathode should be accounted for, when calculating the polarization formation. It is beneficial to perform the 100−mV polarization decay if the native potentials are not known for the tank bottom. When the –850 mVCSE instant-off potential criterion is selected for cathodic protection of the combined system, the shift in the native potential of cathode becomes important if the shift is more electronegative and close to –850 mVCSE. As, by merely achieving the –850 mVCSE instant-off potential criterion does not guarantee effective corrosion control. Additional measures should be taken to cathodically polarize the tank bottom by 100−mV minimum, from its open circuit potential. As per NACE SP0169,18 a commonly used benchmark for effective external corrosion control is a reduction in the corrosion rate to 1 mpy (0.25 mm per year) or less. The basis of satisfying NACE CP criteria is an indication of reducing the corrosion rate to 1 mpy or less. VCIs, as discussed earlier, could not be subjected to a polarization criterion and should depend on reducing corrosion rates to less than 1 mpy for effective corrosion mitigation. The common industry practice for monitoring corrosion rates is by using weight−loss coupons, ER Probes or UT probes. Thus, for a combined VCI and CP system, either the NACE CP criteria or the corrosion rate monitoring can be used, or both provided potential and corrosion rate measurements complement each other. The underlying intent is to provide effective corrosion protection for soil-side tank bottoms. CP by itself may not always be sufficient, and may not provide effective corrosion protection due to the limitations discussed in the paper. VCIs could complement the CP system by providing protection in vapor space and rest of the bottom plate in the event of CP failure or when CP is either ineffective or partially effective. SUMMARY 1. OCPs of the steel electrode and MMO anode changed with the addition of VCI to the electrolyte. This was due to the change in electrode–electrolyte interface resistance, which changes the magnitude of exchange current densities occurring on the cathode and anode surfaces, respectively. 2. On–potentials of the steel electrode and MMO anode changed with the addition of VCI to the electrolyte. This was due to the change in circuit resistance of the VCI + CP system, the cell EMF remained constant before and after adding VCIs to the CP systems. For a constant 3 V applied potential, VCI-A generated 9 mA output, whereas VCI-B generated 3 mA output. The potable water CP system without VCIs generated 5 mA output for the same 3 V applied potential. 3. Potentiostatic polarization tests conducted on cathode and anode electrodes shows the magnitude of current generated for the applied potentials in potable water and VCI systems. On the contrary, the magnitudes of anodic current leaving the anode surface and cathodic current entering the cathode surface will shift the potentials of anode and cathode accordingly. 4. Potentiostatic polarization tests confirm that the new OCP in the presence of VCIs is true and should be used for CP testing. The new OCP is the equilibrium potential in the presence of VCIs where the net current generated is zero. 5. Potentiostatic polarization tests confirm the On-potential shifts observed during CP testing in the presence of VCIs, which was due to the change in magnitudes of currents occurring on cathode and anode in the presence of VCIs. 6. The amine carboxylate based–VCIs tested with the CP system showed synergetic behavior in achieving the –850 mVCSE instant-off potential and 100 mV polarization decay criteria when ©2018 by NACE International. Requests for permission to publish this manuscript in any form, in part or in whole, must be in writing to NACE International, Publications Division, 15835 Park Ten Place, Houston, Texas 77084. The material presented and the views expressed in this paper are solely those of the author(s) and are not necessarily endorsed by the Association.

12

constant voltage was applied. This suggests that VCIs do not negatively affect the CP systems in meeting the required CP criteria. 7. The combined effect of VCI and CP system is beneficial and their synergistic interaction could be harnessed to mitigate soil-side tank bottom corrosion for achieving effective corrosion protection. ACKNOWLEDGEMENTS The authors would like to thank various storage tank operators, cathodic protection companies and VCI manufacturers for their inputs on this extensive research. REFERENCES 1. API 653 (latest revision): Tank Inspection, Repair, Alteration, and Reconstruction of Above Ground Storage Tanks, Fifth Edition. CFR Section(s): 49 CFR 1 95 .432(b). 2. K. Abed, P. Panchal, and K. Gandhi. “Evaluation of Impressed Current Cathodically Protected API1 650 Tank Bottoms in the Presence of Vapor Phase Corrosion Inhibitor.” Conference Proceedings of CORROSION 2016 Conference. Paper No. 7600. (Houston, TX: NACE, 2016). 3. T. K. Adelakin, S. Math and D. Lindemuth, “External Corrosion Protection of Underside Bottom of Above Ground Storage Tank Using Vaporized Corrosion Inhibitors”. Corrosion 2017 Conference, Paper No. 9544. (Houston, TX: NACE, 2017). 4. E. Lyublinski, G. Ramdas, Y. Vaks, T. Natale, M. Posner, K. Baker, R. Singh, and M. Schultz. “Corrosion Protection of Soil Side Bottoms of Aboveground Storage Tanks.” Conference Proceedings of CORROSION 2014 Conference. Paper No. 4337. (Houston, TX: NACE, 2014). 5. E. Lyublinski, K. Baker, T. Natale, M. Posner, G. Ramdas, A. Roytman, and Y. Vaks. “Corrosion Protection of Storage Tank Soil Side Bottoms Application Experience.” Conference Proceedings of CORROSION 2015 Conference. Paper No. 6016. (Houston, TX: NACE, 2015). 6. C.R. Pynn and K. Abed. “Compatibility & Interactions between Cathodic Protection and a Vapor Phase Corrosion Inhibitor.” Conference Proceedings of CORROSION 2017 Conference. Paper No. 9232. (Houston, TX: NACE, 2017). 7. T. Whited, X. Yu, and R. Tems. “Mitigating Soil-Side Corrosion on Crude Oil Tank Bottoms Using Volatile Corrosion Inhibitors.” Conference Proceedings of CORROSION 2013 Conference. Paper No. 2242. (Houston, TX: NACE, 2013). 8. SP0193 (formerly RP0193) (latest revision): External Cathodic Protection of On-Grade Carbon Steel Storage Tank Bottoms. (Houston, TX: NACE, 2016). 9. API 651 (latest revision): Cathodic Protection of Aboveground Petroleum Storage Tanks, Fourth Edition. CFR Section(s): 49 CFR 195.565 10. L. Krissa, J. Dewitt, P. Shukla and A Nordquist. “Experimental Studies to Determine Effects of Vapor Corrosion Inhibitors for Mitigating Corrosion in Casing”. Corrosion 2016 Conference, Paper No. 7801. (Houston, TX: NACE, 2016).


13

11. L. Krissa, J. Dewitt, P. Shukla and Xihua He. “Chemical and Electrical Stability of Reference Electrodes in Sand Bed Dosed with Volatile Corrosion Inhibitors”. Corrosion 2017 Conference, Paper No. 9635. (Houston, TX: NACE, 2017). 12. M. Funahashi and H. Wu. “What You Need to Know About Coated Metal Anodes”. Corrosion 2013 Conference, Paper No. 2107. (Houston, TX: NACE, 2013). 13. F. Song and H. Yu. “Evaluation of Global Cathodic Protection Criteria–Part 1: Criteria and Relevance with Cathodic Protection Theory.” Proceedings of the CORROSION 2012 Conference. Paper No. C2012–0001163. (Houston, TX: NACE, 2012). 14. F. Song and H. Yu. “Evaluation of Global Cathodic Protection Criteria–Part 2: Effectiveness of the −850 mV On- and Off-Potential Criteria.” Proceedings of the CORROSION 2012 Conference. Paper No. C2012–0001164. (Houston, TX: NACE, 2012). 15. F. Song and H. Yu. “Evaluation of Global Cathodic Protection Criteria–Part 3: Effectiveness of the −100 mV Polarization Criterion and Various Off-Potentials with Higher Resistivity Soils, Elevated Temperatures, and Soils with Bacteria.” Proceedings of the CORROSION 2012 Conference. Paper No. C2012–0001165. (Houston, TX: NACE, 2012). 16. T.J. Barlo, N. G. Thompson, A. J. Markworth, J. H. Holbrook, W. E. Berry, “An Assessment of The Criteria For Cathodic Protection Of Buried Pipelines,” Report Catalog Number L51439e. (Falls Church, VA: PRCI, Inc. 1983). 17. ASTM International. ASTM G5 (latest revision), “Standard Test Method for Conducting Potentiodynamic Polarization Resistance Measurements.” (West Conshohocken, PA: ASTM International 2014). 18. SP0169-2013 (formerly RP0169) (latest revision): "Control of External Corrosion on Underground or Submerged Metallic Piping Systems" (Houston, TX: NACE, 2013).


14