EFFECTIVE CORROSION PREVENTION STRATEGY FOR ABOVEGROUND STORAGE TANK SOIL-SIDE BOTTOM USING VAPOR CORROSION INHIBITORS AND CATHODIC PROTECTION

Sujay Math and Chuck Barris, Zerust Oil and Gas

Keywords: Cathodic Protection, Current Distribution, Vapor Corrosion Inhibitors, Aboveground Storage Tank.

ABSTRACT

A mixed metal oxide (MMO) anode grid cathodic protection (CP) system is a state-of-the-art technique for corro -sion prevention of soil-side bottom (SSB) of an aboveground storage tank (AST). In the anode grid system, anode spacing is determined based on the depth of the anode from the tank bottom. Specifically, the anode spacing, number of power feeds, and sand bed resistivity are all considered and adjusted as needed to achieve uniform CP current distribution throughout the tank bottom. Changes in the sand resistivities directly affect the CP current distribution and inadequate CP distribution leads to unprotected areas of the tank bottom. The use of Vapor Cor-rosion Inhibitor (VCI) in combination with CP is shown to have a synergistic effect in achieving the NACE CP crite -ria. VCI liquids are highly conducive, and when used for the AST SSB application, VCIs lower the sand pad resis-tivity. The volatile component of VCI is advantageous in protecting areas of the tank bottom with inadequate CP distribution or when the bottom loses contact with the sand pad due to its flexing.

VCI and CP are an effective corrosion control technique as standalone; however, their benefits as a combined strategy are not fully investigated. In this paper, investigations are carried out to understand the role of VCI in changing the sand bed resistivity and its effect on the CP current distribution for AST SSB corrosion protection. A test setup large enough to mimic tank bottom anode grid system was constructed and CP current was applied to steel plate. The cathodic current densities occurring at different distances from the anode was measured and effective polarization was verified in simulated clean sand and VCI electrolytes. The addition of VCI lowered the electrolyte resistivity and consequently reduced the CP system voltage requirements. The current required to achieve effective polarization and uniform CP current distribution was lower for VCI electrolyte.

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2019 Department of De-fense – Allied Nations

Technical Corrosion Con-Paper No. 2019-0000

INTRODUCTION

Above ground storage tank (AST) soil-side bottom (SSB) corrosion continues to affect tank integrity and service life. The state-of-the-art corrosion prevention technique is to use a MMO anode grid cathodic protection (CP) sys -tem for SSB protection. For older tanks that have the original bottom which are unserviceable, a new bottom is in-stalled 100 mm to 300 mm (4 to 12 inches) above the old bottom and a HDPE liner is placed in between with sand backfill. Tank owners are continuously challenged to meet regulatory requirements, even with an active CP sys -tem. In a grid CP system, the anodes are spaced based on the depth of the anode from the tank bottom (1). To achieve uniform CP current distribution throughout the tank bottom, the anode spacing, and the number of power feeds are designed based on the resistivity of the sand bed. API 651 (2) document on CP design guidelines rec-ommends using a washed clean sand, free from anions which could potentially cause corrosion. The recom-mended clean sand could result in high resistive sand pad for the CP system. Over the life cycle of the tank bot-tom CP system, the sand resistivity changes due to the ingress of rain and flood water or due to the loss of mois -ture and excessive drying. CP design guidelines suggest accommodating for the changes in soil resistivity over their design life, however this is not always considered. The change in the sand resistivities directly affect the CP current distribution, inadequate CP current distribution will lead to unprotected areas of the tank bottom (3). The CP criteria are set by NACE SP0193 (4) standard which is adopted by APIa and U.S. Department of Transporta-tion agencies. The API 653 (5) document sets the out-of-service tank floor inspection intervals based on the floor scan data of the previous inspection. The tank owners benefit from longer inspection intervals because the cost of down time for the stored product is expensive. During periodic CP surveys if the CP potential results show areas of the tank bot -tom that have inadequate protection, the tank owner is mandated by regulators to seek replacement of the CP system or the tank bottom floor itself, if extensive corrosion is observed. Any replacement required before the next scheduled out-of-service inspection interval will be costly for the tank owners.

Vapor Corrosion Inhibitors (VCIs) are used either by themselves or in combination with CP systems to protect the bottom plates of ASTs (6) (7). The use of VCI in combination with CP is shown to have a synergistic effect in achieving the NACE CP criteria (8) (9). VCIs are highly conductive due to their ionic strength. When used in the tank bottom soil side application, VCIs lower the resistivity of the sand bed and can improve uniform current distri -bution. The volatile component of VCI is advantageous in protecting areas of the tank bottom plate that have inad-equate CP or when the bottom loses contact with the tank pad (10). In this paper, investigations are carried out to understand the role of VCI in changing the sand bed resistivity and its effect on the CP current distribution for AST soil-side tank bottom corrosion protection.

Figure 1VCI delivery mechanism underneath the tank bottom plate resting on sand pad

a American Petroleum Institute (API)2

Figure 1 shows the VCI liquid delivery mechanism underneath the tank bottom plate resting on a sand pad. The VCI liquid is injected typically through the existing leak detection ports on the concrete ring wall. VCI molecules vaporize from the sand pad base and adsorb to the soil-side tank plate to provide effective corrosion protection.

EXPERIMENTAL PROCEDURE

The experimental investigations were divided into two stages. In the first stage, corrosion of coupons was evalu -ated in potable water, 3.5% NaCl solution and 5% VCI solution electrolytes. A commercially available amine car-boxylate based VCI was used for testing purposes. The VCI material had an added soluble corrosion inhibitor (SCI) in its formulation; the pH of the VCI solution was in the range of 9 to 10. The concentrations of NaCl and VCI solutions were prepared by adding known amounts of salts to potable water. The resistivities of the elec-trolytes were determined using the 4−pin Miller soil box and the Nilsson 400 soil resistivity meter. The standard weight loss coupon of dimensions 50 mm x 89 mm (2-inch x 3.5-inch) were used for corrosion assessments.

In the second stage, the CP current distribution for tank bottom plates in simulated clean sand electrolyte and VCI electrolyte was evaluated. A test cell, large enough to represent field installed anode grid system pattern and to obtain convenient current-density gradient on steel plate, was used. Figure 2 shows the test setup schematics for CP current distribution. A wooden box of dimension 3 m x 0.9 m x 0.39 m (10 ft x 3 ft x 1.3 ft) was used. The in -side of the box was lined with dielectric liner and was completely isolated from external circuits. MMO anode rib -bon was used for applying cathodic protection current, the MMO anode was installed in a grid system with spac-ing of 1.5 m (5 ft) between the anodes, a parallel anode was also installed along the length of the plate. The an -ode ribbon intersection points were isolated with a dielectric material and connection to anode was made exter -nally for respective measurements. A36 grade steel plate of dimensions 3000 mm long x 150 mm wide x 6.35 mm thick (120-inch x 6-inch x 0.25-inch) was used to represent the tank bottom plate. A36 grade steel coupons of di-mensions 50 mm x 50 mm (2-inch x 2-inch) were spaced at 0.3 m (1 ft) intervals and connected to the steel plate with an electric wire. The current flowing between the plate and coupon was measured using a microammeter. A portable DC power supply (rectifier) was used to power the CP system. Copper-copper sulfate reference elec-trode (CSE) was used for potential measurements at various locations on the steel plate.

(a) top view

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(a) side view

Figure 2Water bath test setup to measure potential and current distribution along the steel plate

EXPERIMENTAL RESULTS

The corrosion of coupons was evaluated by immersion test for a period of 365 days. Figure 3 shows visual corro-sion of coupons at Day-0 and Day-365 for 3 different electrolytes. The Day-0 image was taken immediately after the coupons were immersed in 3 different electrolytes and after 365 days of continuous immersion corrosion was observed on potable water and 3.5% NaCl solution coupons, whereas the 5% VCI solution coupon had no visible corrosion. This test gives qualitative analysis information of electrolytes and their apparent corrosivity on steel coupons.

(a) Day-0 (b) Day-365

Figure 3Corrosion of coupons in immersion test with different electrolytes

Figure 4 shows the electrical resistivity of the same 3 different electrolytes performed using miller soil box test. The potable water had relatively higher resistivity and when NaCl and VCI salts were added to make solution the electrical resistivity decreased significantly.

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Potable Water 3.5% NaCl 5% VCI Potable Water 3.5% NaCl 5% VCI

Figure 4Resistivity measurements of different electrolytes using Miller soil-box test

Cathodic Protection Current Distribution

A large-scale test was conducted to evaluate cathodic protection current distribution on steel plates in simulated electrolytes representing clean sand and clean sand plus VCI. Figure 5 shows the electrical resistivity of simulated electrolytes representing clean sand and sand plus VCI electrolytes.

Clean Sand 2.5% VCI0

5000

10000

15000

20000

25000

30000

Res

istiv

ity, o

hm-c

m

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Figure 5Resistivity of simulated clean sand and VCI electrolytes using Nilsson 400 soil resistivity meter in water bath setup

The electrical resistivity of electrolytes as shown in Figure 5 was measured using a soil resistivity meter in the large-scale water bath test setup. The A36 grade steel coupon at 0.3 m (12 inch) spacing were used for 4-pin electrolyte resistivity test method. The simulated clean sand electrolyte was a mixture of deionized water and potable water which had low ionic conductivity, resulting in a high resistivity electrolyte. VCI powder was added to the simulated clean sand electrolyte at a later stage and completely dissolved. The resulting net concen-tration of VCI was 2.5 % in the water bath. The 4-pin electrolyte resistivity test method was repeated to calculate the resistivity of 2.5% VCI electrolyte. VCI molecules increased the ionic strength of the resulting electrolyte and aided in electrical conductivity, thus yielding a low resistivity electrolyte.

The CP current distribution test was measured in the large-scale water bath setup. The total surface area of the steel plate with coupons was 0.47 m2 (5.15 ft2), if a design current density of 1-2 mA/ft2 was used, the total current required would be 5.15 to 10.3 mA. Three different current output levels were chosen 40 mA, 10 mA and 5 mA for testing purpose, and applied using a constant current rectifier. Figure 6 shows the CP current density distribution and polarization behavior from a single MMO anode acting at center on a 10 feet wide steel plate. Figure 6a rep -resents the simulated clean sand electrolyte and Figure 6b represents the simulated clean sand plus VCI elec -trolyte.

In Figure 6a, the CP current distribution for simulated clean sand electrolyte shows higher current densities occur-ring at the center of the plate, near the anode. The polarization levels were also high at the center, indicating the current from the anode is concentrated at the center. For effective cathodic protection the current from the anode reaching the plate should meet the NACE CP polarization criteria. The polarization curves in Figure 6a that are above the 100-mV polarization line indicate the effective CP current distribution area. For 5 mA of applied current, the spread of effective CP is approximately 1 foot in both directions similarly for 10 mA of applied current, the spread of effective CP is approximately 2 feet in both directions. When the current applied was increased to 40 mA, the current distribution spread widened, and the polarization curve remained above the 100-mV polarization line. In Figure 6b for VCI electrolyte, the CP current densities were lower at the center compared to the simulated clean sand electrolyte and the polarization levels increased for distances away from the center anode, indicating effective CP current distribution.

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a) Simulated clean sand electrolyte

b) Simulated clean sand plus VCI electrolyteFigure 6

Current density distribution and polarization from a single MMO anode at center on a 10 feet wide steel plate. The current density curves labeled as “C.D.” and polarization curves labeled as “Polz.”

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a) Simulated clean sand electrolyte

b) Simulated clean sand plus VCI electrolyteFigure 7

Current density distribution and polarization from an MMO anode grid at 5 feet anode spacing on a 10 feet wide steel plate. The current density curves labeled as “C.D.” and polarization curves labeled as “Polz.”

Figure 7 shows the CP current density distribution and polarization behavior from an MMO anode grid system at 5 feet on center spacing, acting on a 10 feet wide steel plate. Figure 7a represents the simulated clean sand elec-trolyte and Figure 7b represents the simulated clean sand plus VCI electrolyte. In Figure 7a, the CP current distri -bution for simulated clean sand electrolyte shows uniform current densities occurring along the length of the plate. The polarization levels were also uniform along the length of the plate, indicating the current from the anode grid is spreading uniformly. The magnitude of polarization levels achieved was proportional to the magnitude of current applied. For effective cathodic protection the current from the anode reaching the plate should meet the NACE CP polarization criteria. The polarization curves in Figure 7a that are above the 100-mV polarization line indicate ef-fective CP current distribution. For 5 mA of applied current effective CP polarization is not achieved and for 10 mA of applied current the CP polarization levels are just above the 100-mV polarization line. When the current output is increased to 40 mA effective CP polarization is achieved. Similarly, in Figure 7b for the VCI electrolyte, effective CP polarization is achieved for 5 mA, 10 mA and 40 mA current output levels. The addition of VCI to the simulated clean sand electrolyte lowered the resistivity of resultant electrolyte, as a result the anode ground bed resistance was also lowered. This effect enabled the uniform current-spread and achieve effective CP polarization levels for lower current output of 5 mA, which was not the case for high resistive simulated clean sand electrolyte.

DISCUSSIONS

Sand pad quality directly effects the corrosivity of the AST bottom plate and hence clean sand is recommended for tank bottoms during installation. The API-651 standard document sets threshold limits on the sand pad quality and uses electrically resistivity of the sand pad material to determine its corrosivity. The electrical resistivity value of 10,000 ohm-cm or greater is recommended with threshold limits set on chlorides and sulfates. The resultant clean sand resistivity can be as high as 50,000 ohm-cm. The clean sand is good from a reduced corrosivity stand-point; however, the clean sand also increases the electrical resistivity which could potentially cause problems from a CP current distribution standpoint. In other words, with increased sand resistivity the CP current will not distrib-ute uniformly to the entire surface of the tank soil-side bottom, leaving unprotected areas of the tank bottom. From

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a CP design perspective, lower anode ground bed resistance is desired as this helps in increased anode current output.

VCIs lower the resistivity of the sand pad when injected as a slurry in to the pad base. The resultant low resistivity is due to the ionic strength of the VCI molecules which aid in conducting electricity and decrease the net electri -cally resistivity. With VCI slurry injection the sand pad resistivity could fall below the API recommended value of 10,000 ohm-cm. This can be observed as failing the API recommended guidelines for sand quality, but it needs to be understood that VCIs are not corrosive; rather they inhibit the corrosion of steel. Adding VCIs to sand is advan-tageous from a CP design standpoint, as the lower resistivity will lower the CP anode ground bed resistance and increases the anode current output. With increased current output, the polarization levels and CP current distribu-tion achieved is uniform, as seen by the test results in Figure 7. The lower resistivity will aid in decreased circuit resistance on the CP system and hence lower rectifier voltage output is required for a constant current rectifier output. The current output required to achieve 100-mV polarization CP criteria was also reduced.

In summary, addition of the VCI slurry to sand pad is advantageous from a corrosion inhibition standpoint. VCI molecules vaporize from the sand base and adsorb to surfaces of steel plate that loses contact due to flexing. CP current cannot reach areas of the tank plate that has no contact with the sand base. From a CP system perfor -mance standpoint, VCI lowers the resistivity of sand and aids in uniform CP current distribution; thus, the current required to achieve a minimum polarization level is also reduced. Hence using VCI and CP in combination proves to be an effective corrosion protection strategy for aboveground storage tanks resting on sand pads.

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CONCLUSIONS

1. VCIs lower the electrical resistivity of electrolytes due to their ionic strength. Generally, the lower electrical resistivity is indicative of corrosivity; however, this concept does not apply to sand plus VCI electrolytes. This is evident by the coupon immersion test results showing corrosion on potable water and 3.5% NaCl electrolytes, whereas no corrosion is observed on 5% VCI electrolyte coupon, for 365 days of continuous immersion. It is noted that the electrical resistivity of 3.5% NaCl and 5% VCI electrolyte were similar.

2. Addition of VCIs to the simulated clean sand electrolyte lowered the resultant electrical resistivity of the electrolyte. This effect aided in lower CP anode ground bed resistance and resulted in uniform CP current distribution and polarization levels on the tank bottom plate. The current required to achieve NACE recommended 100-mV criteria for effective CP was lower for the VCI electrolyte.

3. Using VCI and CP in combination proves to be an effective corrosion protection strategy for aboveground storage tank soil-side bottoms resting on sand pads, based on the test results.

REFERENCES

1. NACE CP- 4 Manual. Cathodic Protection Specialist Course Manual. Houston,TX. : NACE International, 2000. NACE CP-4 Manual.

2. API 651. Cathodic Protection of Aboveground Petroleum Storage Tanks, Fourth Edition. Washington D.C. : American Petroleum Institute, 2015. API 651: 49 CFR 195.565.

3. The Role of Current Distribution in Cathodic Protection. Holler, H.D. Washington D.C. : Journal of Research of the National Bureau of Standards, 1951, Vol. 47, No. 1. Paper no. 2220.

4. NACE SP0193. External Cathodic Protection of On-Grade Carbon Steel Storage Tank Bottoms. Houston, TX : NACE International, 2016. NACE SP0193.

5. API 653. Tank Inspection, Repair, Alteration, and Reconstruction of Above Ground Storage Tanks, Fifth Edition. Washington D.C. : American Petroleum Institute, 2014. API 653: 49 CFR 195.432(b).

6. Shukla, P.K., et al. Vapor Corrosion Inhibitors Effectiveness for Tank Bottom Plate Corrosion Control. Chantilly, VA : PRCI Inc., 2018. Report Catalog Number PR–015–153602-R01.

7. External Corrosion Protection of Underside Bottom of Above Ground Storage Tank Using Vaporized Corrosion Inhibitors. Adelakin, T.K., Math, S and Lindemuth, D. Houston, TX : NACE CORROSION, 2017. Paper No. 9544.

8. Metholodies to Evaluate Compatabiltiy between Cathodic Protection and Vapor Corrosion Inhibitors for Tank Bottom Applications. Math, S and Shukla, P.K. Houston, TX : NACE CORROSION, 2018. Paper No. 11567.

9. Galvanic Anode Cathodic Protection System Performance in the Presence of Vapor Corrosion Inhibitors for Aboveground Storage Tank Application. Math, S and Shukla, P.K. Houston, TX. : NACE CORROSION, 2019. Paper No. 12787.

10. Corrosion Protection of Soil Side Bottoms of Aboveground Storage Tanks. Lyublinski, E, et al. Houston, TX. : NACE CORROSION, 2014. Paper No. 4337.

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