The old adage “an ounce of prevention is worth a pound of cure” applies particularly well to practice of corrosion control. In most, but certainly not all, cases, corrosion is preventable. However, many industries still focus their efforts on outdated methods for surface preparation that amount to chasing corrosion rather than eradicating the causes of underfilm corrosion.
Research and development in the field of corrosion engineering has led to the creation and implementation of effective new technologies focused on strategic prevention of corrosion, and form a key component of cutting-edge corrosion protective coating operations. A new category of corrosion control products: decontamination surfactants, functions as the core of high-performing corrosion prevention systems that outperform in terms of cost- and labor-savings, and significantly simplify surface preparation and coating processes.
“Cure” is a passive approach that entails waiting for corrosion to appear, using frequent coatings inspections and maintenance in an attempt to mitigate corrosion damage (not corrosion as a phenomenon), followed by removal of the visible evidence of corrosion by blasting to a merely visual standard of hygiene, then coating metal surfaces with paint or coatings. This scenario is analogous to wiping the blood and pus out of a wound, then covering it with a bandage. Without disinfection, the wound is likely to fester painfully. Likewise, failure to remove all existing soluble salts and non-soluble microcontaminants results in inevitable underfilm corrosion that eventually compromises protective coatings.
“Prevention” is a dynamic approach that entails adopting measures that remove the highest number of factors that cause corrosion from the equation, not simply palliate the problem once it begins. The systematic elimination of known root causes (known corrosion triggers that initiate corrosion reactions) both inside and outside metal surfaces is key. In the case of corrosion, especially in aggressive industrial environments, this means taking steps to a) eradicate corrosion triggers existing in metal surfaces in and b) protect metal surfaces from contact with corrosion triggers existing in the environment.
SUBSTRATE PREPARATION VITAL TO HIGH PERFORMANCE PROTECTION
“The most important single factor influencing the life of a paint is the proper preparation of the metal surface.” (Uhlig and Revie. Corrosion & Corrosion Control.)
While coatings capabilities continue to advance, coating performance remains inextricably dependent on adhesion. Although preventable, corrosion continues to occur because conventionally prepared metal surfaces do not provide an optimally receptive surface for coating adhesion. The bottom line: coatings cannot adhere to contaminated surfaces. Surface contamination and site circumstances directly correlate to adhesion failures.
Year after year, NACE studies state that surface preparation failure is a major cause of corrosion problems. Even layers of theoretically impermeable coatings are vulnerable because optimal adhesion to the substrate and the absence interference materials is crucial to coating adhesion and performance. Ineffective surface preparation cuts coating performance and longevity in half, and leaves metal assets vulnerable from both sides of the coating. Economic consequences of failure are a strong motivation for metal asset owners, coatings contractors, and corrosion engineers to address this issue in an advanced, and comprehensive, manner. The latest trend in corrosion control in aggressive environments, metal decontamination surfactants are designed to fill an existing gap in feasibly achieving optimal surface preparation for corrosion control and prevention.
OBSTACLES TO ACHIEVING IDEAL SURFACE PREPARATION
To achieve optimal performance, coatings must perfectly and permanently match the surface and pores of the surface. The distance between the surface of the substrate and the coating should be as small as possible, with no microcontamination between substrate and coating to prevent perfect adhesion. Reliable, fail-safe surface decontamination in the field is therefore critical to creating an optimally receptive surface for coating.
Coating over contaminated surfaces may seem unavoidable because the limitations of conventional metal surface preparation processes in dealing with microcontamination. Coating life can be reduced by 30 percent, 50 percent, and 75 percent in such cases. Coatings cannot fill the gap in effective surface preparation; “surface-tolerance” does not extend to application over steel with chloride or sulfate contamination.
Situational variations (meteorological, geographical, seasonal, etc.) that confound attempts perfect or near-perfect surface preparation. Coatings applied over-the-ditch are highly vulnerable to adhesion failures due to the presence of strongly bonded microcontaminants and salts embedded in blasted surfaces. Because conventional surface preparation systems cannot adequately relieve microcontamination, organizations have settled for undesirable compromise between economical and physical feasibility that exclude the possibility of achieving the ideal objectives of surface preparation.
(Read on for more on the what and why of decontamination surfactant technology, or contact me to test / challenge CleanWirx technology onsite).
BRIEF OVERVIEW OF MICROCONTAMINATION ISSUES
Osmosis:Contaminants remaining on a surface prevent bonding of the paint and set up osmotic driving forces. Moisture collects in areas of poor adhesion or crosslinking, water causing a swelling in coating ﬁlm and subsequent additional water penetration. Water-soluble salts and non-soluble sulfates are notorious for causing osmotic blistering of coatings in immersion service and accelerate corrosion in atmospheric service if allowed to remain on a substrate before coating. Water molecules separate polar bonds holding resin particles together, leading to bond expansion of 20 - 50% or more, thus negating the bond necessary to ﬁlm adhesion.
Alkalis: Surfaces exposed to caustics or other alkalis are difficult to prepare for optimal coating adhesion. Often coatings react with trace alkalis remaining on the surface. Even vinyl resins, considered among the most alkali resistant of synthetic resins, are affected by alkali dust or solution that has contaminated a steel surface, even though cleaned by normal procedures such as abrasive blasting.
Sulfides: Sulfides react directly with iron without the need for water to form heavy iron sulfide scale, which oxidizes rapidly to form iron oxide. Iron sulfide is cathodic to steel, and can also accelerate existing corrosion reactions where sulfide contamination exists. Sulfides are non-water soluble contaminants with strong ionic bonds to surfaces that are extremely difficult to break.
Soluble Salts: Chlorides, sulfates and salt particles present in most site situations are difficult to remove. Rinsing a clean blasted area with clean water will not remove the salt particles and contamination in the voids of the blasted pipe. Remaining salts attract water and pose a continued risk because in actual practice, no coatings are 100% water vapor and water impermeable. Salts drive osmosis. Coatings applied over chloride-contamination evidence poor adhesion because of the hygroscopic nature of sodium chloride. Soluble salts, particularly chlorides and sulfates, not only initiate and accelerate corrosion of steel, but also become deeply embedded within corrosion products.
Current industrial standards make allowances for a limited presence of remaining soluble salts, suggesting the inability of conventional methods to eradicate these contaminants.
Effect of Salts on Corrosion and Coatings
Studies demonstrate that the presence of both chlorides and sulfates cause loss of steel mass proportional to the amount of chloride or sulfate present. Rapid “rust-back” of freshly blasted surfaces in moderate to high humidity is universally observed, with up to 50 percent decrease in effective life of lining systems, “attributed primarily to surface contamination by soluble salts not removed by blast cleaning” (Appleman).
Sodium bicarbonate-based decontaminant surfactant technology solubilizes, reacts and disperses strongly bonded soluble salts, non-soluble sulfides and microcontaminants, simultaneously producing surfaces reflecting extremely high levels of metal hygiene, with zero ionic contaminants detected (as determined by sensitive potassium ferricyanide testing), both improving and stabilizing metal substrate conditions to create an optimally receptive surface proven to promote maximum coating and lining adhesion and performance, including under insulation. Hygiene results exceed SSPC/NACE and industry standards, not just minimizing but permanently eradicating microcontaminants, as evidenced by sensitive ferricyanide testing and SEM analysis.
Moreover, case studies and pilot tests of first- and second-generation decontamination surfactants conducted between 1992 and 2011 on crude oil tanks, brine pit facility piping and fuel storage tanks in Wyoming, Texas and California suggest significantly improved surface hygiene results and a direct correlations between metal decontamination surfactant use and greatly improved coating / lining performance.
The decontamination surfactant system is integrated into surface preparation procedures following blast cleaning or concurrently in WAVB operations. Acid gel is applied to surfaces and allowed to dwell approximately thirty minutes to solubilize and disperse contaminants, followed by short alkaline rinse to stabilize surface. Resulting air-dried surface is ready for coating reception.
The decontamination system is biodegradable and contains less than 1% volatile organic compound. Dry decontamination surfactant formulations may be mixed onsite, decreasing overall shipping costs and storage space for offshore, field repair and maintenance. In addition, labor, consumables, and energy costs are reduced through process simplification inherent to surfactant decontamination, allowing the entire surface preparation process to accomplish in 10 hours an optimized version of the results one would expect following 100 hours or more of more conventional methods. Decontamination allows surface preparation and coating processes to proceed in a single, uninterrupted phase rather than the current model of time-consuming daily repetition. (Illustration 1)
SEM EVIDENCE OF EFFICACY
Image 1 (left)
Contaminated coupon, post treatment of upper left half.
Image 2 (left)
SEM imagery of contaminated control region A.
Figure 1: (left)
Region A (contaminated control) EDS results.
Image 3 (left)
SEM imagery of decontaminated control region A.
Figure 2 (left)
Region A (decontaminated) EDS results.
The decontaminant surfactant has been successfully used on several major projects. Metal hygiene (cleanliness) was confirmed using potassium ferricyanide test procedures with results as noted.
First Generation Decontamination Surfactant
1. The first project was the interior of an 8-foot diameter steel circulating water pipe at Public Service Company of New Mexico’s San Juan Generating Station, Waterflow, NM. The total surface area cleaned totaled approximately 49,000 square feet. Decontaminant surfactant was applied post blast to the interior pipe surface using “wet jet” abrasive blast equipment and deionized water. A production rate as high as 500 square feet per nozzle hour was achieved. The surfactant decontaminant blast was followed by an 8,000-psig wash with deionized water. The wash rate was approximately 750 square feet per nozzle hour. The pipe interior was then tested for soluble salts using a potassium ferricyanide test procedure and a “zero-detectable” level was confirmed.
2. A later project was the interior of 135-foot diameter steel floating top crude tank at Sinclair Oil Corporation’s Refinery at Sinclair, Wyoming. Decontaminant surfactant was applied post blast to the interior tank surface using the “wet jet” blast equipment and deionized water. A production rate of over 330 square feet per nozzle hour was achieved. Decontaminant surfactant blast was followed by a 5,000-psi wash with deionized water. A potassium ferricyanide test procedure confirmed that a “zero-detectable” level of ionic contamination was achieved.
3. Sinclair Pipeline Company, Sinclair WY (conducted in 1992, using first generation decontamination surfactant): Repeated abrasive blasting cycles (NACE #2) on a 150-foot diameter crude oil tank could not maintain visual standard required for the installation of lining/coating per the coating manufacturer's specifications. Decontamination surfactant was applied, with the subsequent result of zero detectable levels of ionic contaminants as determined by sensitive potassium ferricyanide testing. Lining/coating was applied without incident on decontaminated surface. 15 years later tank was opened for inspection, but required no recoating or repair maintenance of lining/coating.
4. Pilot Test, Equistar Brine Pit Facility Piping, Markham TX (conducted in 1995, using first generation decontamination surfactant): Refurbishing and re-coating of three similar brine pit project sites was chosen for a pilot test of the effect of surfactant decontamination on protective coating performance. Traditional blast cleaning and coating was completed on two sites. Third site was subjected to traditional blast clean, surfactant decontamination, and identical coating. Across eleven years of monitoring, the two control sites had required three maintenance recoats. The decontaminated site required no coating maintenance.
Second Generation Decontamination Surfactant
5. Contractor for USN Fuel Storage, San Diego CA (conducted in 2011, using second generation decontamination surfactant): Eight 125,000 barrel tanks had been extremely contaminated resulting in extreme corrosion. Months of multiple abrasive applications and alternative solutions were attempted with no progress in meeting inspection standards for coating. Standards were finally met after first application of decontamination surfactant.
Uhlig, Herbert Henry, and R. Winston Revie. Corrosion and corrosion control: an introduction to corrosion science and engineering. Hoboken, NJ: J. Wiley, 2008.
Materials Performance. "Assessing the Risk of Coating Failure from Residual Soluble Salts." Materials Performance 1 Mar. 2017: 1. <http://www.materialsperformance.com/articles/coating-linings/2017/03/assessing-the-risk-of-coating-failure-from-residual-soluble-salts>
Frezel, Lydia M., PhD. “Study to Determine the Level of Salt Mitigation as related to the Accuracy of the Measurement and Cost Benefits”. NSRP/ASE Surface Preparation and Coatings Panel (SP-3), 18 Feb. 2011. <http://www.arpinstruments.biz/2010_Salt_Mitigation_Final_Report.pdf>.
Appleman, B.R., "Painting Over Soluble Salts: A Perspective" Journal of Protective Coatings & Linings, Oct 1987, pp. 68-82.
Johnson, W.C., "Corrosion Failure from Water-Soluble Contaminants on Abrasives," Journal of Protective Coatings & Linings, Sept. 1990, pp. 54-59.
Tator, Kenneth B., P.E. "Coating Deterioration." ASM Handbook, Volume 5B, Protective Organic Coatings, Sept. 2015, pp. 462-472.
Rodriguez, V., Castaneda, L., & Luciani, B. (1998, January 1). “Effect of Contaminants on the Performance of Fusion Bonded Epoxy”. NACE International.
Schilling, M. S. (2009, January 1). “Osmotic Blistering-Coatings Under Pressure”. NACE International.
United States Army Corps of Engineers. “Engineering and Design: Painting, New Construction, and Maintenance”. Washington, D.C.: The Corps, 1995.
Hatle, Loren L., and Jack R. Cook, P.E. “Achieving “Zero-Detectable” Soluble Salt Contamination on Steel Substrates”. Oct. 1992.