Process Vessel Corrosion Frequently Asked Questions (FAQs)
Answers provided by Colin – IGS Corrosion Mitigation SME
Q: How do you carry out surface preparation and cleaning prior to HVTS application?
A. We have a durable two-stage process that is very robust in the sense that it provides the cleanliness that we need. In the first stage, we blast the vessel with SA two-and-a-half almost white metal, and then we inspect to ensure it is suitable for the thermal spray application.
Once we’re satisfied, in the second stage, each of our technicians performs surface preparation, blast setup, and thermal spray setup for a specific area. Then, they do a second blast, which we call a profile blast, and we get a profile that is normally an average anchorage profile of 125 microns.
After that, we cover that specific area with high-velocity thermal spray within a set time period, which depends on the internal conditions in the asset. Both of these procedures effectively remove any contamination on the surface. However, if we are aware that there is a lot of salt contamination, for example, we might wash the surface prior to the first blast, but that is very rare.
High Velocity Thermal Sprays are not affected in the same way as organic coatings at high levels of salt content. The same applies if there is oil on the substrate. It would have to be removed as you would for any internal coating or corrosion barrier. Fundamentally, the shot blast processes are very robust and deliver the cleanliness and profile needed for the application.
Q: How can you prevent porosity/permeability or ensure the coating is not porous?
A. The application process itself eliminates the risk of permeation, and thus thermal spraying is similar to other processes such as shotcrete that gradually builds up in thickness. The thermal spray technician sprays the surface using a gun that goes through the surface with 50% overlaps and deposits 30 to 60 microns of high-gloss films that break up each pass over the surface. The gun goes over the surface specifically 10 to 12 times to build up to our normal thickness.
Since the same point on the surface is covered 10, 12, or 15 times to build up the thickness, it is impossible to have a pinhole or any flaw in that location. The process generates a dense microstructure due to its high speed, effectively eliminating any interconnected permeability that may be present.
The porosity in the system is very low, at less than 1%, due to the high-speed process. The oxide level is low due to the alloy modification, so we have eliminated any permeability. The process follows the procedures that we have developed to eliminate any patency in the system. If we had a risk of permeability or problems with that, we would not have had the success we have had with application case histories. It is not necessary to do a post-application inspection to look for small holes because they do not exist where the coating has been applied unless all the coating has actually been physically lost. You will not have any problem with permeability in the coated section.
Q: Is it possible to apply HVTS inside a pipe?
A. It’s possible, and currently, we do the applications by hand. We have some automation and we are working on more so we are looking at systems where we can lower a 24-inch ID with a fully automated process that has been developed by IGS. We’re pretty far down the road of that development and we’re close to having a system that’s ready for field use.
For process vessels, if the spout/nozzle is larger than eight inches, we can completely protect it with our standard procedures. If it’s less than 8 inches depending on the length and size, we can use alternative methods to protect them with the nozzle inserts or sleeves using a corrosion-resistant alloy sleeve or a non-metallic sleeve. Or some customers will cut or replace a nozzle with a corrosion-resistant alloy.
Q: Is HVTS effective in the environment with hydrochloric acid (HCL)?
A. Yes, we have conducted some applications with HCL. However, to determine if the HVTS system is suitable for a particular environment, we need to understand the details of that environment.
The HVTS process would deposit the alloy in a manner consistent with its behaviour if it were forged. Therefore, we can determine which alloy would be appropriate for a specific environment, such as whether a 65% or a custom 185 or 276 would work in that environment. We can use HVTS to install the appropriate alloy whenever we have access, providing long-term protection. We have a sophisticated laboratory at our facility in Richmond, Virginia, where we can conduct metallographic analysis using a scanning electron microscope to experiment.
It’s necessary to examine the specifics of your application to determine if we should conduct tests to prequalify and verify suitability.
Q: Is HVTS recognised as a permanent repair or a temporary repair under the API code?
A. In general, when we talk about API codes and barriers against corrosion, the focus is on mechanical integrity or pressure retention. I am not sure if HVTS is specifically included in the API code or if the high-pressure metal coating is recognized as a corrosion barrier. However, the coating is so effective that it eliminates the need for welding overlays or physical coatings like plate coatings. If you have questions about how the API codes apply to our coating, I recommend speaking with my colleagues in the USA, who can provide more nuanced information. As long as the coating remains intact and provides a barrier with no probability of failure, it will last for the useful life of the asset.