Corrosion Monitoring Techniques for Measuring VCI Efficiency for Tank Bottom Protection
Measuring VCI Efficiency Overview
In this article, the following topics will be reviewed:
- VCIs for Tank Bottom Corrosion
- Corrosion Monitoring System
- Weight Loss Coupons
- Ultrasonic Thickness (UT) Scanning System
- Electrical Resistance (ER) Probes
- ER Probe Potential Environmental and Electrical Interferences
- ER Probe Data for VCI Replenishment Timeline
- Establishing Baseline Corrosion to Determine VCI Effectiveness
- VCI Testing for Replenishment Forecast
VCIs for Tank Bottom Corrosion
Corrosion of the underside surfaces of aboveground storage tanks is an ever-present drawback that can lead to costly environmental concerns as a result of product leakage.
Zerust Vapor Corrosion Inhibitors (VCIs), as stated in API 655, can be used for the protection of aboveground storage tank bottoms from soil-side corrosion. provide sufficient corrosion prevention to the soil-side bottom surfaces of steel storage tanks when applied correctly and analyzed regularly.
Corrosion Monitoring System
It is recommended that an appropriate corrosion monitoring system be implemented during VCI installation to monitor the rate of corrosion and ensure that the VCI is replenished as needed to continue to maintain optimal corrosion protection.
Currently, the methods used to determine the corrosion rate on the soil-side bottom plates of an AST are Saturated Low Frequency Eddy Current (SLOFEC), Magnetic Flux and Ultrasonic Thickness Scanning System per API 653 inspection intervals. These methods typically requires the tank to be out of service and therefore, data can only be obtained at certain intervals that are usually years apart.
Due to the excessive timeframe for an internal tank scan, other online corrosion rate monitoring devices should be considered to evaluate the corrosiveness of the tank soil-side bottom environment. Several corrosion rate monitoring options have been listed in the sections below.
Weight Loss Coupons
Zerust weight loss coupons are made of SAE 1008 carbon steel. It is recommended to install coupons at the time of VCI installation and experience six months of exposure prior to weight loss analysis. Following a visual inspection of the installed coupons, each unit needs to be cleaned, weighed and analyzed per standards published under ASTM G1. Coupons need to be replaced and analyzed every six months for continued monitoring.
Ultrasonic Thickness (UT) Scanning System
UT testing is often times performed by third-party contractors. It is recommended that corrosion rates be established prior to VCI installation by the contractor for a baseline.
Electrical Resistance (ER) Probes
To establish a baseline rate of corrosion, it is recommended to install a minimum of 4 ER probes, consisting of a carbon 1010 steel element, under the floor of each tank at least 6 months prior to VCI injection.
Data points collected on a regular basis (once per month, etc.) will give the most insight into the corrosion rates occurring at the probes measuring element. Graphing metal loss as a function of time can yield a corrosion rate value equal to the slope of line between readings.
One method to determine this value is applying linear regression to a reasonable amount of data points taken over a reasonable amount of time. The limitations for this technique vary depending on the quality of data provided by the instrument. In some cases, the data may not follow a linear trend, and in extreme cases, no trend can be found in the data.
Listed below are some of the possible environmental and electrical interferences that can impact data from the ER probe readings.
ER Probe Potential Interferences
- Corrosion byproducts
- Temperature Changes
- Insufficient Connection
- Cathodic Protection Current
- Stray Current
- EMF Signals
ER Probe Data for VCI Replenishment Timeline
In Figure 1 below is an example of how VCI data can be plotted to monitor and determine the replenishment timelines.
Establishing Baseline Corrosion to Determine VCI Effectiveness
In Figure 2 below is ER Probe data showing how the baseline corrosion prior to implementation of VCIs can be established and monitored to determine the effectiveness of the Zerust VCI solution in corrosion reduction.
VCI Testing for Replenishment Forecast
Another way to determine the replenishment time and efficiency of VCI application for in- service tanks is testing the air, soil, and water and assessing what amount of anti-corrosive chemicals remain. The air and water samples taken can be compared to ER probe readings in an effort to “fingerprint” corrosion patterns on a specific site.
This data can be used to better forecast solution reinjection well in advance of an acceleration in tank bottom corrosion.
Electrical resistance probes, weight loss coupons and VCI samplings are necessary parameters in determining both VCI presence and effectiveness, and more importantly, the replenishment timelines or any substantial changes in the corrosiveness of the environment beneath storage tank floors as well as in the pipelines where the VCI technology could be implemented.
Our Team is Here to Help
The staff at Zerust Oil & Gas are here to custom design a corrosion mitigation solution that is specific to your given asset. Contact us today to find out how we can help extend the life of your equipment.