Corrosion Mitigation Strategies & Corrosion Barrier Selection: IGS HVTS Cladding as an Alternative to Organic Coatings
Production plants have a wide range of corrosion barrier options available to address internal metal wastage from corrosion, including metallic claddings applied with high velocity thermal spray (HVTS) and organic coating systems. The selection process of a corrosion barrier for internal corrosion mitigation of process equipment should take several factors into account. These factors include the design and operation of the internal process environment as well as the ownership costs, or lifecycle costs, related to the installation of a particular corrosion barrier.
When making this type of large-scale decision, the key factors to consider extend far beyond the initial investment in the corrosion barrier material and application. Key commercial decisions for selected barrier systems should be made with the lifecycle cost, or cost of ownership, in mind. This lifecycle cost/cost of ownership represents the overall cost of maintaining the applied system as an effective corrosion barrier over the remaining operational or design life of the asset. Different corrosion barrier technologies will present different lifecycle costs (e.g., organic coatings have maintenance costs that are not a consideration with an HVTS cladding system). These costs are not only measured in dollars, but also in additional downtime and aggravation. This article highlights and discusses the many factors that may greatly alter the lifecycle cost and performance of metallic claddings applied with HVTS versus organic coating systems.
BREAKOUT: Key commercial decisions for selected barrier systems should be based on the lifecycle cost, or cost of ownership, rather than the cost of purchase.
Costs Related to Installation
Installation cost is more than the material and labor cost of the application of a corrosion barrier in a process asset, particularly when an asset is in-situ and part of the process train. Other considerations have a far greater bearing on the true cost of installation. The economic argument for a more robust and permanent solution becomes an obvious consideration when these other factors are taken in to account as part of a cost-benefit analysis.
FACTORS THAT CAN INCREASE
INSTALLATION TIME AND COSTS
FOR ORGANIC COATING SYSTEMS
at a glance
- Steel Surface Temperature
- Cool/Damp Environments
- Humid Environments
- Surface Preparation
- Application Time
- Cure Time
- Scaffold Removal/Furniture Installation
- Restarting the Process
STEEL SURFACE TEMPERATURE
- If the steel surface temperature is too cold, the organic material will be too viscous to wet out the prepared substrate. The applicator will be unable to control the material thickness and initial gel and cure times will extend. Where a system contains VOCs (volatile organic compounds), they will take longer to flash off.
- If the steel surface temperature is too hot, the material will be too fluid. Film buildup will be limited, runs and drips will be likely, and multiple coats will be necessary. Accelerated cure times may lead to evaporation of solvents and other chemical constituents, changing the applied material’s chemical composition and structure.
BREAKOUT: Metallic claddings applied with HVTS are not affected by steel surface temperature in terms of application process or rate. No flow or chemical cure is required for metallic claddings applied with HVTS.
- If the environment is cool and/or damp, moisture may be trapped beneath or between layers of the applied coating, leaving voids and weaknesses that increase the risk of premature failure.
- If the environment is cool and/or damp and high solids content organic systems are being used, there will be a high risk of the applied coating developing a surface amine blush/bloom. The coating surface would need to be washed and prepared in between additional coat applications.
BREAKOUT: There is no risk of amine blush or bloom with metallic claddings applied with HVTS as no chemical reaction takes place. Due to the high air velocities and the single, continuous application process used for metallic claddings applied with HVTS, there is no risk of moisture entrapment or contamination between layers.
- If the environment is high in humidity, it precludes the application of organic coating systems due to moisture contamination (condensation) of the applied coating and flash rusting and/or gingering on steel surfaces.
BREAKOUT: To prevent environments high in humidity from stopping work, the application process for metallic claddings applied with HVTS may be adapted in typical cases.
- If the steel surface cleanliness is not tested, organic coating systems are likely to have a reduced working life. High surface salt concentrations lead to accelerated osmotic pressures in immersion service, pulling moisture through the coating to the substrate surface. This phenomenon increases with higher operating temperatures. If the vessel has been exposed to a saline environment, the surface may need multiple washes to meet the specification requirements.
- If in certain cases the steel surface is not washed with deionized water to remove substrate or surface salts, the organic coatings are likely to have a reduced working life as described in the previous bullet point.
BREAKOUT: Metallic claddings applied with HVTS are not impacted by surface salt contamination in the same way organic systems are.
- If an organic coating that is not 100% solids is not given enough time for entrapped solvents to evaporate or disperse, localized blistering/voids will occur from the release of the solvents after partial curing. If a solvented epoxy is used, these areas will be a future failure points.
BREAKOUT: Metallic claddings applied in a single, continuous coat with HVTS are dry, require no cure time, and contain no VOCs.
- If the ambient temperature is low during the final curing of the applied organic coating corrosion barrier, it takes longer for the chemical reactions to complete and the coatings to cure. The lower the temperature, the more time is needed. Where heating is required, there are additional costs and logistics to consider.
- If the ventilation is not adequate during the final curing of an applied solvented coating, the VOC elements will not be effectively liberated.
BREAKOUT: No cure time is required for metallic claddings applied with HVTS.
SCAFFOLD REMOVAL/FURNITURE INSTALLATION
- If organic systems are damaged by impact during scaffold removal and/or vessel internal furniture installation, the damage must be repaired. Otherwise, the damaged location would become a future failure point and an area of accelerated pitting corrosion. The repair process requires surface preparation, coating reapplication and additional cure time. During a scaffolding removal and furniture installation process, inspection and application teams should be kept on standby due to the significant risk of damage to the barrier.
BREAKOUT: Scaffold removal and furniture installation pose a negligible risk for metallic claddings applied with HVTS.
RESTARTING THE PROCESS
- Organic coating systems typically require a gradual and controlled process start-up, and this results in additional system downtime. A slow restart is often necessary to ensure that the coating is cured and solvents are gradually released, and to prevent thermal and/or pressure shock from damaging the coating. This damage during start-up would shorten the expected lifetime of a coating at the very beginning of its service.
BREAKOUT: Metallic claddings applied with HVTS do not have any special start-up parameters.
Costs Related to Inspection and Repair
Once the corrosion barrier system application process is complete, system performance is monitored on a Risk Based Inspection (RBI) basis. Scheduled inspections, and scheduled/unscheduled repairs of the installed corrosion barrier system, add even more to the lifecycle cost/cost of ownership.
FACTORS THAT CAN INCREASE
INSPECTION AND REPAIR TIME AND COSTS
FOR ORGANIC COATING SYSTEMS
at a glance
- Internal Visual Inspections
INTERNAL VISUAL INSPECTIONS
- If inspections must be performed visually, which is the case with an organic coating protective barrier, they could be problematic because:
- Online inspections of the substrate metal wall thickness only indicate when the internal coating has failed, when metal wastage/wall loss has already started to occur;
- Typically point-location, non-destructive testing (NDT) techniques using ultrasonic transducer (UT) technology are utilized instead of high-tech inspection techniques;
- It is not possible to assess the internal coating condition over a wider area; and
- There are typically blind spots where external inspection is not possible due to access or internal/external furniture (e.g., saddles).
BREAKOUT: Metallic claddings applied with HVTS can be inspected/verified online using UT inspection technologies. Vessel internal inspection intervals have typically been extended to eight to 10 years.
Costs Related to Shutdown and Cleaning
The inspection process itself can be highly problematic for an organic coating system. Depressurization, increased temperatures for cleaning processes such as steam out, cyclic service (temperature or pressure), and other aspects of shutdown and cleaning of an asset can greatly increase inspection cycle costs. While any single cleaning phase could be damaging, issues incurred over repeated cleaning cycles may significantly increase the lifecycle costs of an organic system.
FACTORS THAT CAN INCREASE
SHUTDOWN AND CLEANING TIME AND COSTS
FOR ORGANIC COATING SYSTEMS
at a glance
- Controlled Depressurization
- Controlled Cleaning Processes
- Vessel Entry
- Pressure Boundary Repairs/Lining Reapplications
- If the depressurization during shutdown is not properly controlled, the absorbed expanding gases and liquids exert significant pressures within the coating, as well as at the coating/substrate interface. Blisters are generated and the coating fails. With increased service temperatures/pressures and cyclic service, organic coating systems become even more susceptible to permeation and the requirement for a controlled shutdown becomes even more important.
BREAKOUT: Issues related to controlled depressurization do not impact metallic claddings applied with HVTS.
- If the vessel cleaning processes are not carefully controlled, they can severely damage internal organic coatings. For example, steam cleaning often destroys organic coatings by exposing them to high temperatures. Some epoxies with 100% solids can survive short-term exposures to high temperatures, but each new exposure stresses and weakens the coating and creates points of future failure.
BREAKOUT: There are no issues related to unit shutdown that affect metallic claddings applied with HVTS.
- If humans enter a vessel to carry out an inspection, they pose a high risk to the organic coating, no matter how careful or well trained they are. Damage from footwear, inspection equipment, scaffolding (if required), tools, and furniture removal/replacement are a common cause of localized organic coating damage.
BREAKOUT: Organic coatings susceptible to localized impact damage present a high risk during routine inspections.
UNPLANNED PRESSURE BOUNDARY REPAIRS AND LINING REAPPLICATIONS
- If the internal corrosion barrier cannot be monitored for integrity without an internal, visual inspection, localized failure points from the initial application or in-service damage might not be identified. Internal inspections may reveal unforeseen damage to the vessel shell, leading to unplanned maintenance.
- If vessel shells are pitted or corroded or they have metal wastage, they must be mechanically repaired to restore the pressure boundary (e.g., using weld metal build-up). Shutdown schedules may be significantly impacted. Additionally, this creates heat-affected zones (HAZ) that may require post-weld heat treatment (PWHT).
BREAKOUT: Unplanned repairs dramatically increase the cost of failure of an internal corrosion barrier by extending the critical path of the shutdown schedule and delaying production resumption.
Taking in to consideration the true lifecycle costs of an applied corrosion barrier, the actual cost of ownership of an organic coating—when considered for the design life of the vessel or beyond—is significantly higher than typically anticipated. Clients should strongly consider:
- Extensive repairs and recoating requirements at every inspection;
- T&I with the added risk of significant cost in the case of a damaged or failed organic coating leading to wastage of the vessel shell;
- The requirement for costly and lengthy mechanical repairs (e.g., weld build-up); and
- Potential loss of containment if the loss of wall thickness is not identified during inspections and vessel replacement.
Upgrading the internal surfaces of a process vessel with the application of a nobler alloy, such as a NiCrMo-based material, prevents further metal wastage. If applied to the correct standards (i.e., a true HVTS process) a permanent barrier may be established that would maintain the asset integrity until the end of the vessels operating life.