Long steel pipes used in energy, oil and gas, chemical processing, and underground storage systems may operate under high pressure while continuously contacting corrosive media. The steel pipe can provide the required structural strength, but its internal surface may not provide sufficient long-term corrosion resistance.

Internal pipe laser cladding offers a practical solution by depositing a corrosion-resistant nickel-based alloy onto the pipe ID. The process creates a metallurgically bonded coating while maintaining relatively low dilution, shallow substrate penetration and limited thermal deformation.

A recent industrial application involving approximately 12-meter-long N80 and P110 steel pipes demonstrated that an Inconel 625-type nickel alloy could be deposited over the entire internal surface. The finished coating thickness exceeded 0.6 mm, with an average uniform corrosion rate of only 0.0018 mm/year and no detected pitting under the reported test conditions.

Why Do Long Steel Pipes Need Internal Corrosion-Resistant Cladding?

Structural steels such as N80 and P110 provide the mechanical strength required for demanding pipe applications, but severe corrosive environments can limit their service life.

A typical example is salt cavern compressed air energy storage (CAES). Saturated brine remaining inside underground salt caverns can strongly corrode steel injection and production pipes. In the studied project, the target service life exceeded 30 years. The required uniform corrosion rate in saturated brine at 60°C was below 0.01 mm/year, while the maximum pitting rate had to remain below 0.015 mm/year.

Instead of manufacturing the complete pipe from an expensive corrosion-resistant alloy, a functional structure can be created:

Steel Pipe for Mechanical Strength + Internal Ni-Based Cladding for Corrosion Resistance

This combines the structural properties of the base pipe with a corrosion-resistant internal surface.

What Is Internal Pipe Laser Cladding?

Internal pipe laser cladding uses a specialized ID laser head to deliver laser energy and metal powder to the inner surface of a tube or pipe.

The laser melts the supplied alloy powder together with a shallow layer of the steel substrate. Rapid solidification then creates a dense coating metallurgically bonded to the base material.

Compared with processes involving larger thermal input, laser cladding provides advantages such as a narrow heat-affected zone, shallow penetration, low dilution and limited deformation.

These characteristics are particularly important when applying expensive nickel-based corrosion-resistant alloys to long steel pipes.

Why Is Long-Pipe Internal Laser Cladding Difficult?

Cladding the outside of a shaft is relatively straightforward compared with processing several meters inside a pipe. Long-pipe ID cladding introduces additional optical, mechanical and process-control challenges.

Limited Internal Space

The laser beam, powder delivery system and processing head must operate inside a confined cylindrical space.

The internal cladding head must therefore provide accurate laser delivery and powder convergence while fitting within the available pipe diameter.

Long Working Depth

Working depth becomes a major engineering constraint for long pipes.

In the reported application, pipe length was approximately 12 m. Instead of processing the complete length from one side, the system was designed for approximately 6.5 m single-side working depth, allowing the pipe to be processed from both ends.

This approach makes full-length ID cladding possible without requiring a 12-meter-long single-direction cladding head.

Maintaining Constant Stand-Off Distance

The distance between the cladding nozzle and the instantaneous pipe surface is a critical parameter.

Changing this distance changes the laser defocus, powder convergence position, and laser spot geometry. These changes directly influence powder melting, substrate fusion and dilution.

For long-pipe laser cladding, the mechanical system must therefore maintain a stable nozzle-to-surface distance throughout the processing length.

Stable Rotational and Linear Motion

Full-surface internal cladding requires coordinated pipe rotation and longitudinal movement of the cladding head.

In the reported system, pipe rotation was controlled within 25–75°/s, with a rotational speed error below 0.5°/s. Stable motion was necessary to maintain a consistent laser scanning speed and cladding track.

Material Selection for Corrosion-Resistant Pipe ID Cladding

The coating alloy should be selected according to the actual corrosive medium, temperature, pressure, required service life and compatibility with the steel substrate.

For the salt-cavern pipe application, a Ni-Cr-Mo-Nb nickel-based alloy was selected to provide corrosion protection while allowing laser deposition onto N80 and P110 steel.

Why Inconel 625 Is Used for Internal Pipe Cladding

The composition of the deposited alloy corresponded to the composition range of UNS N06625, commonly associated with Inconel 625-type Ni-Cr-Mo-Nb alloys.

Measured principal elements included:

Maintaining these alloying elements throughout the functional coating is one reason dilution control becomes particularly important during Inconel 625 laser cladding.

Powder Requirements for Internal Laser Cladding

Stable powder delivery is essential for maintaining consistent coating geometry during several meters of continuous ID cladding.

The nickel-based powder used in the reported process was approximately spherical, with:

Particle size: 53–150 μm
Bulk density: 4.55–4.75 g/cm³
Hall flow rate: 14.5–15.5 s/50 g

Suitable powder morphology and flowability support stable feeding and help maintain consistent deposition during continuous processing.

Internal Laser Cladding Head Design

Long-pipe cladding requires an ID head specifically designed for confined and deep working environments.

Beam Delivery Inside the Pipe

The study used a specialized internal focusing and reflective-output laser cladding head. This architecture allows the laser energy to be delivered to the internal pipe wall rather than continuing along the pipe axis.

Coaxial Powder Feeding

The head incorporated coaxial powder feeding, with the powder convergence point located within the depth-of-focus region of the output laser beam.

Correct alignment between the laser and powder stream is necessary for stable melting and deposition.

Nozzle-to-Surface Distance Control

Maintaining the designed stand-off distance helps preserve:

Laser Spot Geometry + Powder Focus + Melt-Pool Geometry + Dilution

For deep ID processing, this requires sufficient rigidity and accurate positioning of the long-arm cladding system.

Long-Duration Processing Stability

Full-surface cladding of a 12 m pipe involves a large processing area. The system must therefore maintain stable motion, laser delivery, powder feeding and nozzle positioning over long continuous processing cycles.

Key Process Parameters for Long-Pipe Internal Laser Cladding

There is no universal parameter set for every internal pipe. Pipe diameter, substrate, coating alloy, powder characteristics, desired thickness and productivity all influence process development.

However, several parameters are especially important.

Laser Power

The engineering study used a 6 kW continuous-wave fiber laser at 1080 nm. After process trials, laser power of at least 4.5 kW was selected for this specific application.

Power must be sufficient to melt the powder and create metallurgical bonding, but excessive energy increases substrate penetration and dilution.

Scanning Speed

Conventional laser cladding speeds cited in the study were approximately 6–20 mm/s. This was too slow for the large internal surface area of long pipes.

The developed high-speed process operated at approximately:

120–150 mm/s (7.2–9.0 m/min)

while producing a single-pass coating thicker than 0.8 mm.

This combination of deposition thickness and scanning speed was important for industrial productivity.

Powder Feed Rate

Powder feed must be matched with laser power and scanning speed.

Too much powder for the available energy can cause incomplete melting, while insufficient powder can reduce coating thickness and production efficiency.

The source study identifies powder feed rate as part of the coordinated parameter set but does not report one universal feed-rate value for all operating conditions.

Stand-Off Distance and Defocus

Stand-off distance determines the relationship between the workpiece surface, laser focus and powder convergence point.

Even when laser power and scanning speed remain unchanged, an unstable stand-off distance can alter the actual energy distribution and deposition conditions.

Why Low Dilution Is Critical for Inconel 625 Pipe Cladding

In corrosion-resistant laser cladding, the objective is not deep penetration.

The laser must melt enough steel to establish metallurgical bonding, while minimizing the amount of Fe-rich substrate entering the nickel-based coating.

The reported process achieved a substrate melting depth of less than 0.15 mm. Layer-by-layer chemical analysis also showed that the principal Ni, Cr, Mo, Nb and Fe contents remained nearly unchanged through most of the coating thickness.

This demonstrates the desired relationship:

Shallow Substrate Melting → Low Dilution → Stable Alloy Composition → Corrosion-Resistant Coating

Coating Thickness and Post-Machining

As-clad thickness and final usable coating thickness should not be treated as the same specification.

In this application, the high-speed process produced a single-pass layer thicker than 0.8 mm. The surface was then mechanically ground and polished.

The required final coating thickness was above 0.5 mm, while actual measurements after finishing showed a thickness of at least 0.6 mm. The finished internal surface was also processed to meet a roughness requirement below Ra 6.3.

This machining allowance should be considered when defining the original deposition thickness.

Common Defects in Internal Pipe Laser Cladding

Porosity

Gas entrapment, unstable powder delivery or melt-pool instability can produce pores inside the coating.

Cracking

Thermal stresses generated during rapid heating and cooling can cause cracking and compromise corrosion protection.

Lack of Fusion

Insufficient local energy can prevent complete bonding between deposited alloy and substrate.

Delamination

Poor interfacial bonding can allow the coating to separate from the steel pipe under mechanical loading.

Oxide Inclusions

Oxidation or contamination during deposition can introduce inclusions into the coating.

Missing Cladding Areas

Unstable motion, powder interruption or incorrect track overlap may leave portions of the internal surface insufficiently covered.

Excessive Melting and Collapse

Excessive heat input can destabilize the melt pool and cause local coating geometry to collapse.

Substrate Deformation

Although laser cladding provides localized heat input, inappropriate parameters can still cause unwanted thermal deformation.

Surface Oxidation

Insufficient shielding can oxidize the deposited surface and affect coating quality.

These were among the defect types specifically considered during inspection of the industrial pipes.

How to Inspect Long-Pipe Internal Laser Cladding Quality

Industrial ID cladding requires more than checking whether the coating looks continuous at the pipe entrance.

Visual and Borescope Inspection

Because much of the coating cannot be directly observed, a borescope can inspect the complete internal surface for cracks, pores, missing coating, oxidation and other visible defects.

The reported pipes showed none of the specified defect categories during full-surface inspection.

Coating Thickness Measurement

Thickness measurements confirmed a finished coating thickness of ≥0.6 mm.

Chemical Composition Analysis

Composition testing verifies that dilution has not reduced the key corrosion-resistant alloying elements outside the required range.

Metallographic Inspection

The project specified that pores larger than Φ0.3 mm were unacceptable.

Cross-sectional metallographic examination found a maximum pore size of only Φ0.09 mm, while the largest inclusion measured approximately 0.08 × 0.03 mm.

Mechanical Testing

Mechanical testing showed that the laser-clad N80 and P110 pipes continued to meet the required pipe mechanical properties.

Flattening Test

Flattening provides a more demanding evaluation of coating-substrate bonding.

Even when specimens were flattened until the inner pipe walls contacted each other, the laser-clad alloy did not delaminate from the steel substrate. This supported the conclusion that good metallurgical bonding had been achieved.

Corrosion Performance of Inconel 625 Laser-Clad Pipe

Corrosion testing was conducted under conditions representing the salt-cavern application:

Temperature: 60°C
Solution: Saturated NaCl
Total pressure: 9 MPa
Flow velocity: 1.5 m/s
Test duration: 168 h

The three samples produced an average uniform corrosion rate of:

0.0018 mm/year

and no pitting was detected.

This was well below the project’s specified uniform corrosion limit of 0.01 mm/year under the stated conditions.

Engineering a 12-Meter Internal Pipe Laser Cladding System

Long-pipe ID cladding requires an integrated system rather than an isolated laser head.

The engineering configuration described in the study combined:

Fiber Laser + Internal Cladding Head + Powder Feeder + Cooling + Long-Axis Head Travel + Pipe Rotation + Motion/PLC Control + Shielding Gas

The long-arm support structure maintained the cladding head position, while controlled pipe rotation provided stable circumferential scanning.

The system diagram on page 3 of the paper illustrates how these subsystems work together to maintain the laser, powder and workpiece relationship during deep internal cladding.

Full-Length Cladding Strategy for 12 m Pipes

A long pipe does not necessarily require an ID cladding head capable of reaching the entire pipe length from one end.

For the approximately 12 m pipes in this project, the system was designed for a 6.5 m single-side processing depth.

The pipe could therefore be processed from one side and then reversed to complete the remaining internal surface.

This approach can significantly reduce the mechanical difficulty associated with extremely long cantilevered ID cladding structures.

Industrial Application in Compressed Air Energy Storage

The process progressed beyond laboratory validation into industrial production.

According to the paper, more than 500 approximately 12-meter-long injection and production pipes, together with corresponding pipe ends, sealing areas and couplings, received the nickel-based laser cladding treatment.

These components were applied in a 300 MW-class compressed air energy storage power station, which reached full-power grid connection on January 9, 2025.

The application demonstrates that long-pipe internal laser cladding can be scaled from process development to repetitive industrial manufacturing.

Other Applications of Long-Pipe Internal Laser Cladding

The same basic engineering concept can be considered for other long tubular components where the substrate provides structural strength and the internal surface requires additional corrosion or wear protection. The following are broader potential applications rather than applications validated by this particular paper.

Oil and Gas Tubing and Casing

Internal laser cladding can provide localized or full-length corrosion-resistant surfaces for tubing and casing exposed to demanding production environments.

Chemical Process Pipes

Nickel-based ID coatings may be considered where steel process piping must resist aggressive chemical media.

Mining and Slurry Transport Pipes

Where internal surfaces experience combined corrosion and wear, suitable cladding alloy systems may extend pipe service life.

Hydraulic Cylinders and Large Bores

ID laser cladding can also be applied to cylindrical components requiring internal surface repair or functional coating.

Energy Storage and Underground Gas Systems

The salt-cavern project demonstrates the direct relevance of internal corrosion-resistant cladding to underground energy infrastructure.

How to Develop a Stable Industrial Pipe ID Cladding Process

Industrial process development should begin with the pipe ID, total length, wall thickness and base material, followed by the actual corrosion environment and required service life.

The coating alloy and final thickness can then be defined. Because subsequent grinding or polishing removes material, the required as-clad thickness should include an appropriate machining allowance.

Next, the ID cladding head and working depth must be matched to the pipe geometry. Laser power, scanning speed, powder feed rate, stand-off distance and track strategy should then be optimized together.

Particular attention should be given to low dilution and shallow substrate penetration. After deposition, coating thickness, chemical composition, visible defects, metallurgical quality and bonding should be inspected. For critical applications, the coating should ultimately be validated under corrosion conditions representative of its intended service environment.

In other words:

Pipe Geometry → Material Selection → ID Head Design → Process Parameters → Dilution Control → Post-Machining → Inspection → Corrosion Validation

Conclusion

Internal pipe laser cladding provides an effective route for combining the structural strength of long steel pipes with the surface performance of corrosion-resistant nickel alloys such as Inconel 625.

However, successful long-pipe ID cladding involves much more than placing a laser head inside a tube. Working depth, stand-off distance, pipe rotation, linear travel, powder feeding, scanning speed, dilution and post-machining must operate as one integrated process.

The 12 m pipe application examined here demonstrates the industrial potential of this approach: high-speed internal laser cladding produced a finished coating of at least 0.6 mm, shallow substrate melting below 0.15 mm, strong metallurgical bonding and an average uniform corrosion rate of 0.0018 mm/year with no detected pitting under the reported test conditions.

For industrial long-pipe applications, the key objective is therefore not simply higher laser power or faster deposition. It is achieving a repeatable balance of production efficiency, low dilution, coating integrity and long-term corrosion protection.