Colin Bateman discusses solving complex challenges with high velocity thermal spray (HVTS).
When faced with metal wastage, asset owners and operators can address the corrosion mechanisms with a corrosion-resistant alloy (CRA) barrier. The technique used to apply this CRA is determined by the shutdown time available to carry out the application.
Thermal spray technology has been used for the application of CRA since the 1980s. However, it was quickly recognised that the thermal spray process itself can negatively affect the condition of the material being sprayed. The resulting cladding, when using traditional metal alloys and commercially available thermal spray equipment, was not able to create a sufficient barrier to corrosive media.
Permeability coupled with internal stress and lower bond strength with the base metal creates a path for corrosion and premature failure. These early failures resulted in an understandable and rather universal distrust of early iterations of commercially available thermal spray technology.
Engineers and material scientists have successfully developed a solution to this problem by redesigning the equipment used to apply the metal cladding, the process technology, and the alloy of the feedstock material. The result is a high velocity thermal spray (HVTS) alloy cladding.
Case study
During a routine inspection, a US refinery identified a problem with its second stage de-salter and overhead accumulator vessels.
A nickel-copper thermal spray coating was applied in both vessels approximately 20 years ago. Initially, the coating performed as expected. However, after some time, localised damage was evident on the bottom third of both vessels, leading to deep pitting and metal wastage beyond the existing corrosion allowance.
Thermal Spray Coating and Weld Failures
This highlighted the fact that typical thermal spray coatings are not suitable for internal protection of mission critical process equipment due to their permeability, weaker bond strength and propensity for cracking. These ‘low velocity’ thermal spray systems cannot produce flat and tightly packed particle sizes or nano-scale grain structures, and this leads to the coating’s failure due to corrosion and/ or permeation.
In addition, following weld repairs adjacent to the failing thermal spray, a crack had formed on its heat affected zone (HAZ). In 2017, refinery engineers decided they needed a more permanent solution.
An HVTS alloy cladding solution was chosen to stop corrosion for the expected life of the asset without any further maintenance anticipated for at least the next 15 years.
HVTS technology uses alloy materials, which offer erosion-corrosion protection, even in high-temperature and high-pressure service up to 1371°C/2500°F.
The cladding option also offered significant time savings compared with weld overlay. As a result, the refinery project manager welcomed the solution and HVTS was applied in 2019.
The bottom third of the overhead accumulator, including the stem pipe with a vortex breaker and a flange, were protected with HVTS. Regular inspections have shown no deterioration of the cladding since application.
HVTS technology has been proven to succesfully address complex challenges associated with metal wastage and corrosion in critical process equipment like that described above. The recent ingenuity of engineers and material scientists in the field has succesfully mitigated the historical distrust of commercially available thermal spray technology and the associated permeability issues, internal stress, and inadequate bond strength with its propensity for cracking.
Colin Bateman is director of business development EMEA at IGS.
