Revamps 2026 Issue

Figure 3 HVTS application in 2021

Figure 2 Original coating delamination

• Equipment geometry and accessibility. Weld overlay and alloy cladding should be viewed as complementary technologies rather than direct substitutes. WOL is generally preferred in cases where structural rein- forcement is required or where equipment has exceeded minimum thickness requirements. In contrast, HVTS is well-suited for corrosion protection when the substrate remains structurally sound, particularly when application time is limited or thermal input must be minimised. An engineering-based evaluation of these factors is essential to determine the most appropriate solution for a given application. Case Study 1: Ammonia scrubber vessel alloy cladding An ammonia scrubber vessel at a Japanese refinery expe - rienced recurring coating failures over multiple mainte- nance cycles. The unit was subject to periodic inspection at approximately four-year intervals, during which degrada- tion of previously applied coating systems was consistently observed. The operating conditions of the vessel were as follows: • Operating pressure: 0.49 MPa. • Operating temperature: 75°C. • Process environment: hydrogen sulphide (H₂S), ammonia (NH₃), and water. Such environments are known to promote localised corro- sion mechanisms, including pitting and under-deposit cor- rosion, particularly where coating integrity is compromised. Inspection findings indicated that previously applied thermal spray coatings exhibited partial peeling and delam- ination across localised areas of the vessel surface (see Figure 2 ). These failures resulted in repeated maintenance requirements and increased operational risk. The degrada- tion mechanism was associated with: • Ingress of corrosive species through coating defects. • Progressive loss of adhesion between coating and substrate. • Localised corrosion at the coating-substrate interface. The internal geometry of the vessel further complicated application, limiting accessibility and contributing to varia- bility in coating performance. Conventional thermal spray coatings applied to the vessel demonstrated limitations in

this service environment. Typical characteristics include: • Adhesion strength in the range of 15-35 MPa. • Coating porosity typically between 2-8%. • Lower particle velocities (200-250 m/s), resulting in less dense coating structures. These characteristics can lead to the formation of perme- able pathways within the coating, enabling corrosive media to reach the substrate and initiate degradation processes. Additionally, the combined effects of chemical exposure and thermal cycling contributed to repeated coating delam- ination over successive inspection intervals. An engineering evaluation was conducted to determine an appropriate mitigation strategy. As the vessel retained suf- ficient structural integrity and had not exceeded minimum wall thickness requirements, a corrosion-resistant barrier approach was considered appropriate. HVTS alloy cladding was selected based on its ability to provide (see Figure 3 ): • High adhesion strength (>50 MPa). • Low porosity (<0.5%). • Dense cladding microstructure resulting from high- velocity particle impact (up to approximately 1,200 m/s). These characteristics reduce permeability and improve resistance to disbondment under corrosive service conditions. Prior to application, all existing degraded coating mate- rial was removed, and surface preparation was performed to ensure appropriate substrate condition. Given the con- fined geometry of the vessel, application procedures were adapted to maintain consistent coating quality throughout the internal surface. The alloy cladding was then applied to a total surface area of approximately 8.1 m². Results and performance Following alloy cladding application in 2021, the vessel was returned to service. A subsequent inspection was conducted during the next scheduled turnaround in 2025. Inspection included visual examination and localised thick- ness measurements in accordance with standard industry practices. The inspection findings confirmed: • No observable cladding delamination or peeling. • No visible evidence of corrosion activity beneath the cladding. • Stable cladding condition across the treated surface.

Revamps 2026