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Vapor Corrosion Inhibitors for Tank Bottom Corrosion Control

Soil-side corrosion of aboveground storage tank (AST) bottoms is a major challenge. ASTs typically include active cathodic protection (CP) systems to mitigate this corrosion.

A study was recently completed by the Pipeline Research Council International (PRCI) to determine corrosion mitigation performance of vapor corrosion inhibitors (VCIs) for the soil-side of aboveground storage tank (AST) bottom plates. PRCI is a nonprofit research organization that is comprised of energy pipeline operating and service companies located worldwide. The study details are published in a report.1 This article provides a summary of the study.

Soil-side corrosion of the bottom plates of ASTs is a major concern and maintenance issue. ASTs are typically supported by concrete ring walls or sometimes by compacted dense gravel aggregate. The tank bottoms are typically bare steel and supported by a sand pad of varying thickness. The sand is expected to be in accordance with API 6512 or operator-developed standards. In North America, most ASTs include active cathodic protection (CP) systems to protect the tank bottoms. Review of industry experience and literature indicated that soil-side corrosion could occur at elevated rates and CP alone may be insufficient. This is because CP could partially or completely fail, or CP may not reach the entire surface of the plate—as in the case of local soil settlement and associated depressions which could form due to repeated flexing of the bottom plates. In addition, a tank without a CP system could experience elevated corrosion depending on the pad conditions.

To address these issues, VCIs have been applied under ASTs for many years and are being promoted as alternative corrosion control measures. Tank operators needed an independent study to establish the efficacy and applicability of VCIs. Specifically, the operators needed a basis to determine life cycle applicability of VCIs for their assets, both from an integrity management perspective and for regulatory compliance. 

There were three key questions that were asked: (i) are VCIs effective in mitigating corrosion comparable to a working CP system, (ii) what is the best way to apply and monitor efficacy of VCIs, and (iii) are VCIs compatible with CP? The study was conducted to address these questions, with the objective of rigorously evaluating and documenting the effectiveness of VCIs. The scope included literature review, laboratory experiments, and field testing.

VCIs have been used for corrosion mitigation in numerous applications for decades. However, their application for tank bottom corrosion control began in the early 2000s; literature exists on application of VCIs such as recent work by Pynn and Abed.3 VCI technical specialists have devised methods to apply VCIs into a tank pad. Application methods are dependent on tank operating conditions that could include a tank being in-service, out-of-service, and during construction. The application relies upon the process of chemical volatilization and diffusion for distribution of VCIs throughout a tank pad. VCIs’ volatilization obviates a need for direct access to the bottom plate surface. VCIs could be injected and distributed in the sand pad for an in-service tank, and at the pad surface for an out-of-service tank. The most common ways to introduce VCIs in the pad include liquid slurry (prepared by mixing potable water with VCI) injection and dry powder application.

A VCI chemistry must be contained adjacent to the tank bottom to be available for corrosion control. A typical AST includes a concrete support ring wall surrounding the pad that is underlain with a containment liner. The bottoms are typically A36 steel and are constructed on the pad surface, then the plates extend onto the concrete ring wall. The chime of the tank in contact with the ring wall can be sealed with a sealant. This creates sufficient containment under ASTs for VCIs to be available long-term. When VCIs are delivered and released either within interstitial space or at the pad surface, volatilization coupled with diffusion occurs until equilibrium, determined by partial vapor pressure and concentration gradients, is reached. The mechanism for corrosion control is the formation of molecular-level inhibitor layer over the entire plate surface. Inhibitors adsorb on the steel surface and then suppress both metal dissolution and reduction reaction. The mechanism is like the action of inhibitors that are used widely, and recognized by regulators, for protection of pipelines from internal corrosion.

VCI Effectiveness Study

Laboratory testing was conducted to determine efficacy of two commercially available VCIs. Field sand samples from an existing tank pad were procured and used. The bottom plate at the sand sampling site experienced severe soil-side pitting; therefore, the samples were considered corrosive. Two sets of VCI experiments were set up. For the first set, control and VCI effect experiments were set up in plastic tubs that were filled with the sand, dosed with VCIs, and sealed to avoid escaping VCIs. A36 steel coupons were placed in contact with sand, and in the vapor space of the tubs. Electrical resistance (ER) probes were also placed in some of the tubs. The coupons were extracted after several months and analyzed for corrosion. For the second set, VCI and CP compatibility experiments were set up in glass beakers using field sand dosed with VCIs. Mixed metal oxide and Mg-based anodes were used in impressed current CP (ICCP) and galvanic CP systems, respectively. A36 coupons were used in the CP compatibility experiments. Various electrochemical techniques such as potentiostatic polarization and galvanic coupling were employed to evaluate VCI and CP compatibility.

The field sand was corrosive and caused pitting on A36 coupons in the control experiments. The control experiment coupons were compared with the VCI experiment coupons; the comparison showed that VCIs mitigated pitting when the vendor-recommended dosages were used. Specifically, pitting of A36 decreased substantially in the presence of vendor-recommended dosages of the two VCIs; however, the pitting rates were not mitigated to the extent specified in NACE SP01934 and NACE SP01695 for demonstrating adequate CP. Experiments were also conducted at dosages lower than what is recommended, and included 10% and 1% of the recommended values. These dosages were selected to determine if there is a threshold level that will trigger reinjection of VCIs after initial dosing. With the 10% and 1% VCI dosages, pitting was equally severe in the control and VCI-exposed coupons, indicating that any large deviation from vendor-recommended dosages will render the VCIs ineffective.

ER probes are designed to measure the surface average corrosion rate and are sometimes used to measure tank pad corrosivity. Suitability of the ER probes for monitoring the effectiveness of VCIs was evaluated by comparing ER-probe derived corrosion rates with the surface average corrosion rates of the coupons. ER-probe corrosion rates were within the range of the coupons’ surface average corrosion rates. Further, in both VCI exposed coupons and ER probe data, the effect of VCIs was evident by a reduction in surface average corrosion rates compared to the control. ER probes are suitable enough to monitor VCIs’ effectiveness. It is recognized that pitting is the dominant contributor to overall corrosion and failure of the plates. While there was agreement between ER-probe and coupon corrosion rates, ER probes cannot measure pitting rates. ER probe and coupon agreement indicates that there exists a correlation that can be derived between the surface average and pitting corrosion rates. The correlation could be used to infer changes in the pitting rates using the ER-probe rates.

There could be situations where VCIs are used in combination with CP. VCIs and CP provide protections by different mechanisms: VCIs by chemical action and CP by cathodic polarization. An advantage of using the combination is that inhibitors can reach metal surface areas that could be difficult to protect using CP. Additional advantages include VCIs providing protection to a plate portion that has lost contact with the tank pad and where the CP system is unavailable due to power loss, damage to, or downtime associated with CP.

The study found that VCIs are compatible with an ICCP system, and do not adversely affect delivery of CP to the plate. However, VCI and CP compatibility is complicated by the fact that the native potential of A36 steel shifts with exposure to VCIs. Additional changes in the native potential could occur after polarization of a VCI-exposed plate. These changes in native potential should be considered when operators use a specific VCI and select one of the SP0193 CP criteria to meet regulatory requirements. For galvanic anode CP systems, the data was limited and varied widely; therefore, no conclusion could be drawn on the compatibility.

Summary

VCIs were found to be effective in mitigating pitting of A36 steel exposed to corrosive sand when recommended dosages were used. VCIs significantly reduced the tendency of pitting, but pitting rates were not mitigated to the extent specified in NACE SP0193 and NACE SP0169 for demonstrating adequate CP. Nonetheless, use of VCIs could provide protection, and thus service life extension, for the tanks without CP or where CP systems have either failed or degenerated. ER probes, designed to measure the surface average corrosion rate, can be used to monitor both the plate corrosion rate and efficacy of VCIs. VCIs are compatible with ICCP systems, but changes in native potentials of A36 steel must be considered when using VCIs in combination with CP. This work demonstrates proof-of-concept of the VCI technology, and additional work would help optimize operating and monitoring parameters associated with the technology.

Acknowledgments

The authors acknowledge PRCI and its members for funding and in-kind support, and VCI manufacturers for providing products for the study.

References

1 P. Shukla, et al., “Vapor Corrosion Inhibitors Effectiveness for Tank Bottom Plate Corrosion Control,” PRCI, Inc., Report Catalog Number PR–015–153602-R01, 2018.

2 API 651, “API Recommended Practice 651: Cathodic Protection of Aboveground Petroleum Storage Tanks” (Washington, DC: American Petroleum Institute, 2014).

3 C.R. Pynn, K. Abed, “Compatibility and Interactions Between Cathodic Protection and a Vapor Phase Corrosion Inhibitor,” MP 57, 4 (2018): pp. 38-42.

4 NACE SP0169, “Control of External Corrosion on Underground or Submerged Metallic Piping Systems” (Houston, TX: NACE International, 2013).

5 NACE SP0193, “External Cathodic Protection of On-Grade Carbon Steel Storage Tank Bottoms” (Houston, TX: NACE, 2016).

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