HVAS: the solution for protecting Oil & Gas components

In every refinery, every petrochemical plant, and every offshore platform, there comes a moment when asset management, maintenance, and operations must confront the same critical question: what do we do with components that show signs of degradation but cannot be moved or replaced before the next scheduled shutdown?

A 40-meter atmospheric distillation column cannot be dismantled. A hydrocracking reactor cannot be transported. A tube-and-shell exchanger installed on a rack 15 meters high cannot be removed without weeks of site work. Yet all these components, operating under severe service, develop problems over time such as internal corrosion from naphthenic acids, sulfidation, H₂S and CO₂ attack, and catalyst erosion in FCC units.

Classic options such as full replacement, in-situ weld overlay, or welded metal lining  are technically valid but economically prohibitive: additional weeks of shutdown, post-weld heat treatment (PWHT), metallurgical risks, and the need for combustible gases in confined spaces.

In this scenario, HVAS technology — High Velocity Arc Spray — is today one of the most mature and strategic solutions for the corrosion protection of critical components in oil & gas. Developed and offered by Flame Spray as a leading technology for the petrochemical sector, HVAS combines application speed, portability for on-site interventions, excellent metallurgical coating properties, and operational safety in a single process.

In this article we look at the characteristics of HVAS, where it is applied, and why it is becoming an increasingly common choice for the protection of critical infrastructure in the oil industry.

The characteristics of HVAS coating

HVAS coatings stand out for four metallurgical characteristics that make them particularly suited to corrosion protection in oil & gas:

Density and impermeability

The typical porosity of an HVAS coating is below 2%, the result of the dynamic compaction generated by the high impact velocity. Each particle "spreads" the molten material into every microscopic asperity of the surface, creating a gas-tight barrier essential for the protection of pressure vessels and heat exchangers.

Superior adhesion

Bond strength values measured according to the ASTM C633 standard regularly exceed 35 MPa (over 5,000 psi), often with excellent performance on properly prepared steels. Adhesion is predominantly mechanical in nature, resulting from the anchoring of high-energy particles onto the micro-asperities of the blasted surface.

Low thermal alteration of the substrate

The heat generated by the arc remains concentrated in the wires and fine particles. The substrate stays below 100 °C, eliminating the risk of:

• Dimensional distortion
• Formation of a Heat Affected Zone (HAZ)
• Metallurgical alteration of sensitive materials (hardened steels, heat-treated alloys)
• The need for post-deposition heat treatments (PWHT)

This is one of the most underrated yet decisive advantages in oil & gas applications: coating an existing component without altering its original metallurgy is what makes it possible to work on heavy structures already in service.

Continuity and uniformity over large surfaces

HVAS is characterized by high deposition rates (up to 10-15 kg/h of deposited material depending on the material and process parameters). This makes it possible to cover large surfaces in compressed timeframes, with thickness uniformity ensured by robotic control systems or specialized operators.

 

The HVAS material family: the flexibility of wire

One of the most strategic advantages of HVAS is the flexibility in choosing the filler material, made possible by the availability of solid wires and flux-cored wires that allow even complex compositions to be deposited.

Ni-Cr-Mo superalloys for sour service

• Inconel® 625 – The reference NiCrMo alloy for protection against H₂S, CO₂, chlorides, and acidic high-temperature environments. Compatible with sour service and offshore applications.
• Hastelloy® C-276 – A superalloy with very high corrosion resistance, suited to the most severe services (mineral acids, hydrochloric acid, reducing-oxidizing conditions).
• Alloy 825 and other Ni-Fe-Cr formulations for specific applications.

 

Special stainless steels

• 316L – Austenitic stainless steel for general protection and dimensional restoration.
• 430 – Ferritic stainless steel for specific applications with good high-temperature resistance.
• Duplex and super-duplex – For services involving chlorides and requiring mechanical strength.

Cermet and high-hardness materials (via flux-cored wires)

• Tungsten carbide (WC) cermet coating – Using flux-cored wires with a carbide-rich core, HVAS can achieve hardness values above 1,000 HV0.3, suited to withstanding severe erosion (e.g., catalyst in FCC units).
• Amorphous and nanocrystalline alloys – Some flux-cored wire formulations, thanks to rapid post-impact cooling, generate glassy or nanocrystalline structures with exceptional resistance to corrosion and wear.
  

Materials for cathodic protection

• Zinc-aluminum alloy (Zn-Al 85/15) – Sacrificial coating for cathodic protection of structures exposed to marine environments, aggressive atmospheric conditions, or splash zones.

Cobalt-based alloys

• Stellite – Suited to components subject to contact wear and high-temperature erosion.

Material selection is always the result of an integrated technical analysis: fluid composition, operating temperature, partial pressures of H₂S/CO₂, pH, presence of particulates, substrate material. Flame Spray's in-house metallurgical laboratory supports customers in this choice, validating the solution through adhesion testing, micrography, porosity analysis, and service simulation.

 

The real differentiator: on-site intervention

While the metallurgical properties of the HVAS coating are already outstanding, the technology's real strategic advantage for the oil & gas sector lies in its on-site applicability.

System portability

HVAS equipment is compact and portable, and can be moved and set up even in confined spaces, on scaffolding inside distillation columns, inside reactor shells, and in all the complex geometries typical of process plants. To operate, it essentially requires:

• Electrical power
• Compressed air
• Metal wires (consumable)

It does not require oxygen, combustible gases (propane, kerosene), or pressurized cylinders. This is significant in two respects:

• ATEX/HAZOP safety: the absence of combustible gases drastically simplifies site management in classified zones.
• Logistics: fewer consumables, less transport, fewer special work permits.

 

No need for disassembly

The internal and external surfaces of columns, exchangers, vessels, and piping can be coated directly in place, without the need to disassemble or move the component. This:

• Eliminates disassembly and reinstallation costs
• Reduces the risks associated with lifting and transporting heavy components
• Drastically shortens component downtime
• Makes it possible to work within already-planned turnaround windows, without extending the plant shutdown

 

Intervention in complex geometries and at height

Flame Spray's specialized operators can work:

• Inside columns and reactors, on temporary work platforms
• At height on skids and pipe racks
• In confined spaces with dedicated ventilation systems
• On cylindrical, spherical, and converging conical geometries

Operational safety

The HVAS process is considered one of the thermal spray processes best suited for safe on-site application thanks to:

• Absence of combustible gases
• Moderate substrate temperature (no incandescence, no thermal risk to personnel and nearby materials)
• Integrable extraction and filtration systems
• Compatibility with standard work permits

 

HVAS vs. alternative solutions: an honest comparison

Characteristic	HVAS	Weld Overlay	HVOF
Bond strength	>35 MPa	Metallurgical welding	>70 MPa
Heat Affected Zone	None	Significant	None
PWHT required	No	Yes (in many services)	No
Substrate dilution	No	YesNo	No
On-site applicability	Excellent	Difficult in complex geometries	Limited (requires combustible gases)
Deposition rate	High (10–15 kg/h)	Variable	Medium
Combustible gases on site	No (air + electricity only)	Yes (sometimes)	Yes (oxygen + fuel)
Applicable thicknesses	From 0.3 mm to several mm	Variable	0.1–0.5 mm
Available materials	Wide range of metals and cermets (with flux-cored wires)	Limited to weldable alloys	Wide (powders)
Environmental compliance	High	Medium	High

 

HVAS vs. Weld Overlay

Weld overlay is a technically valid solution but has significant limitations for on-site work:

• Dilution: the filler material mixes with the substrate; the coating's pure chemical properties are only achieved after a certain thickness. HVAS, by contrast, maintains chemical purity from the first layer.
• Residual stress and cracking risk: weld overlay introduces significant tension; HVAS operates in a compressive state.
• PWHT: often mandatory in sour service, greatly complicating field work.
• Execution time: significantly longer on large surfaces.

HVAS vs. HVOF

HVOF and HVAS share high-velocity and high-density characteristics, but serve different scenarios:

• HVOF is the reference choice for shop-coated components with the highest hardness and finish requirements (shafts, valves, control valve trim).
• HVAS is the reference choice for corrosion-resistant coatings on large surfaces and for on-site applications where execution speed, logistical simplicity, and on-site safety are priorities.

The two technologies are complementary, not competing: the choice depends on the component, the performance objective, and the application context.

Typical applications in the Oil & Gas sector

Distillation columns (atmospheric and vacuum)

Distillation columns operate under conditions that combine naphthenic acids, high-temperature sulfidation, and aggressive multiphase flows. The protection of internal walls, trays, distributors, and pump-around zones is critical to ensuring the column's integrity over time. An HVAS coating in Ni-Cr-Mo alloy, applied in the field, acts as an impermeable metallurgical barrier, preventing wall thinning and avoiding the need to replace heads or internals.

Hydrocracking and hydroprocessing reactors

The internal surfaces of reactors operate in acidic environments (low pH) with sulfur, chlorides, and other corrosive species at high pressure and temperature. HVAS provides on-site corrosion protection without the need for costly heavy structural material solutions.

Heat exchangers and condensers

Tube sheets, tube bundles, and the external shells of exchangers are subject to galvanic corrosion, pitting, and attack from fluids containing H₂S or sour water. HVAS makes it possible to coat the tube sheet in place, sealing corrosion initiation points and restoring chemical resistance without pulling the bundle.

FCC Units (Fluid Catalytic Cracking)

With catalyst flowing at high velocity, unit components undergo severe erosion. Applying HVAS coatings with tungsten-carbide-based flux-cored wires provides the hardness needed to withstand catalyst impact.

Storage tanks and refinery piping

Tanks, separators, and piping subject to atmospheric or chemical corrosion can be protected with layers of corrosion-resistant alloy applied internally or externally, extending the vessel's integrity. Critical piping sections or valves can be coated in place to increase their resistance to aggressive media.

Representative use case: protecting a distillation column during a refinery turnaround

The following use case describes a representative application scenario of the type of intervention that Flame Spray regularly carries out with HVAS technology, reconstructed from typical operational cases in the industry.

The scenario
A refinery with an atmospheric topping unit is approaching its scheduled multi-year shutdown. Routine inspections carried out in the period before the turnaround revealed, on the upper section of an atmospheric distillation column, wall thinning caused by naphthenic acid corrosion and high-temperature sulfidation. The metallurgical diagnosis confirmed the nature of the degradation mechanism and ruled out structural defects, but presented the asset manager with a strategic choice.

The options under consideration
Option A — Replacement of the upper section of the column

• Significant capital costs (fabrication, transport, lifting, installation)
• Fabrication lead time potentially incompatible with the turnaround window
• Need for hot work on site and PWHT
• Risks associated with disassembly/reassembly

Option B — Internal weld overlay in Inconel 625 alloy

• Long execution times for the surface to be treated
• Need for combustible and flammable gases in a confined space
• Risk of distortion and thermal residual stress
• Problematic on-site PWHT

Option C — On-site HVAS coating with NiCrMo alloy

• Work can be carried out within the already planned turnaround window
• No post-deposition heat treatment
• No combustible gas in confined spaces (simplifying HSE procedures)
• Ability to work on scaffolding inside the column with a team of specialized operators

The HVAS intervention
The typical operational sequence for an on-site HVAS intervention on a distillation column includes:

1. Preliminary planning: technical site survey, component analysis, definition of the areas to be coated, material selection (e.g., NiCrMo flux-cored wire), drafting of the qualified procedure.
2. Site setup: installation of internal scaffolding, dedicated ventilation and extraction systems, qualification of operators specialized in confined spaces.
3. Surface preparation: blasting to grade Sa 3 per ISO 8501-1 to ensure the optimal mechanical anchoring profile.
4. HVAS application: coating deposition according to the qualified procedure, with real-time control of thickness and uniformity.
5. Non-destructive testing: thickness verification (ultrasonic testing), coating continuity (holiday detection), visual inspection of the microstructure.
6. Documentation and traceability: intervention certificate, coating technical data sheets, map of treated areas, photographic report.

The result
The strategic advantage for the operator is not only metallurgical, but above all operational and economic. Without seeking to quantify specific results — which vary significantly depending on the component, the service environment, and the materials used — on-site HVAS intervention typically makes it possible to:

• Significantly extend the service life of the protected component, acting as a metallurgical barrier against the degradation mechanism
• Contain overall costs compared to replacement or weld overlay solutions
• Keep to the planned turnaround window, avoiding an extension of the plant shutdown
• Defer or eliminate the need to replace the asset at the next turnaround
• Reduce HSE risks related to hot work and combustible gases in confined spaces

The real extent of the benefit is always the result of a technical analysis conducted case by case by the Flame Spray engineering team.

Discover the HVAS service