Protecting Reformer Components Against Carburisation

Yet even specialist high-temperature alloys can be vulnerable to carbon ingress under carburising conditions.

Carburisation occurs when carbon from the process environment enters a metallic material at elevated temperature. Over time, this can alter the microstructure of the alloy and contribute to changes in its properties and performance.

For operators of hydrogen, ammonia, methanol and syngas/HyCO plants, the concern is therefore not simply the presence of carbon.

It is the effect prolonged carbon ingress can have on a high-value component expected to remain in service for many years.

If carburisation is contributing to degradation, protecting the surface can help protect the investment already made in the underlying alloy.

Diffusion aluminising provides a surface-engineering approach designed to create a barrier between the process environment and the substrate, increasing resistance to carburisation in appropriate high-temperature applications.

What Is Carburisation?

Carburisation is a high-temperature degradation mechanism involving the absorption and diffusion of carbon into a metallic material.

When an alloy is exposed to a sufficiently carbon-rich environment at elevated temperature, carbon can enter through the surface and diffuse into the material.

Once inside the alloy, carbon can react with carbide-forming elements.

This can result in the formation and growth of internal carbides and changes to the material’s microstructure.

The extent and rate of carburisation depend on the interaction between factors including:

• Temperature
• Carbon activity
• Process gas composition
• Alloy composition
• Surface condition
• Exposure time
• Existing protective scales

Carburisation is therefore not simply a surface deposit that can be removed.

It involves changes occurring within the material itself.

Why Does Carburisation Matter in Reformer Systems?

Reformer equipment is expected to operate reliably for extended periods.

The alloys used in these systems are selected to provide a carefully balanced combination of high-temperature properties.

If carbon progressively penetrates the material and changes its microstructure, that balance can be affected.

Depending on the alloy, operating environment and extent of attack, carburisation can contribute to changes in properties such as:

• Ductility
• Toughness
• Mechanical behaviour
• Dimensional stability
• Resistance to further environmental degradation

The precise consequences vary considerably between materials and applications.

However, the underlying lifecycle issue remains the same:

Environmental attack at the surface can ultimately affect the performance and useful life of the complete component.

Why Are Reformer Tubes Susceptible?

Reformer tubes operate under particularly demanding conditions.

They must withstand high temperatures while simultaneously carrying mechanical loads and interacting with process environments.

High-temperature alloys are selected specifically to meet these requirements.

But alloy selection is always a balance between different properties.

The material must provide sufficient:

• High-temperature strength
• Creep resistance
• Metallurgical stability
• Fabricability
• Environmental resistance

No single alloy offers unlimited resistance to every degradation mechanism.

Where process conditions promote carbon ingress, the surface can therefore benefit from additional protection.

Carburisation Begins at the Surface

Before carbon can affect the interior of a component, it must first cross the interface between the process environment and the metallic surface.

That makes the surface an important line of defence.

If a stable barrier can be maintained between the process gas and the substrate, the transfer of carbon into the alloy can be reduced.

This is where diffusion aluminising can provide additional protection.

Rather than changing the entire component material, surface engineering modifies the region directly exposed to the environment.

The substrate retains the properties required for structural performance, while the engineered surface provides additional resistance to carbon ingress.

How Diffusion Aluminising Protects Against Carburisation

Diffusion aluminising enriches the surface region of a metallic component with aluminium.

During treatment, aluminium diffuses into and reacts with the substrate at elevated temperature, creating a metallurgically bonded aluminide diffusion layer.

Under appropriate operating conditions, this aluminium-rich surface can develop a stable alumina (Al₂O₃) scale.

The alumina layer acts as a barrier between the process environment and the underlying alloy.

By reducing direct interaction between carbon-containing gases and the substrate, the diffusion-treated surface can restrict carbon ingress and increase resistance to carburisation.

The basic protection mechanism can be viewed as:

Carbon-containing process environment

↓

Protective alumina scale

↓

Aluminium-rich diffusion layer

↓

High-temperature substrate alloy

The objective is to address the degradation mechanism where it begins — at the component surface.

Why a Metallurgically Bonded Layer Matters

High-temperature reformer components experience prolonged thermal exposure and potentially significant changes in operating conditions throughout their service life.

Any surface protection system must therefore be suitable for demanding high-temperature operation.

Diffusion aluminising differs from a simple deposited coating.

During processing, aluminium diffuses into the substrate and forms intermetallic phases within the surface region.

The resulting diffusion layer is metallurgically bonded to the component.

This makes diffusion coatings particularly suitable for high-temperature applications where long-term environmental protection is required.

The precise structure and chemistry of the diffusion layer depend on the substrate alloy and treatment specification.

Carburisation and Metal Dusting: Related but Different

Carburisation and metal dusting both involve interaction between carbon and metallic materials at elevated temperature.

However, they should not be treated as the same degradation mechanism.

Carburisation

Carbon diffuses into the alloy and promotes internal carbide formation and microstructural change.

Metal Dusting

Under susceptible conditions, carbon-related reactions can result in much more severe breakdown of the alloy, producing carbon, carbides and fine metallic particles.

Metal dusting can therefore lead to substantial localised material loss.

Understanding which mechanism is occurring is important because the operating conditions, material response and resulting damage can differ significantly.

Related: Metal Dusting in Reformer Systems: Causes, Consequences & Prevention

Carburisation and Oxidation

Oxidation and carburisation may appear to be very different problems, but protective oxide formation plays an important role in controlling both.

In an oxidising environment, a stable protective oxide can slow continued reaction between the atmosphere and substrate.

In a carburising environment, a sufficiently protective barrier can reduce carbon transfer into the alloy.

Aluminium-rich diffusion surfaces are valuable because they can promote formation of a stable alumina scale under appropriate conditions.

This provides an environmental barrier protecting the material beneath.

Related: High-Temperature Oxidation in Reformer Systems

Which Reformer Components Can Be Affected?

The risk of carburisation depends on the operating environment rather than simply the name of the component.

Potentially susceptible equipment can include selected:

Reformer Tubes

Reformer tubes operate for extended periods under combinations of temperature, stress and process chemistry.

Where carburisation contributes to material degradation, diffusion aluminising can provide additional environmental resistance.

Catalyst Tubes

High-temperature catalyst-containing tubes can also experience demanding process environments in which carbon-related degradation must be considered.

Headers and Collectors

Depending on their position, temperature and exposure to process gases, selected headers and collector components may encounter carburising conditions.

Heat Exchanger Components

High-temperature heat exchanger equipment handling carbon-containing process streams may also experience conditions where carburisation presents a material challenge.

Associated Process Components

Additional piping, fittings and other metallic components should be assessed according to their individual material and operating environment.

The presence of high temperature alone does not mean aluminising is required.

The actual degradation mechanism must first be understood.

Protecting High-Value Reformer Tubes

Reformer tubes represent a significant investment in specialist materials and manufacturing.

Where carburisation becomes a life-limiting mechanism, replacing the entire tube means replacing material that may otherwise retain significant mechanical value.

Surface engineering provides another approach.

By increasing resistance to carbon ingress, diffusion aluminising can help protect the underlying alloy from a degradation mechanism that begins at its surface.

Where technically appropriate, this can form part of a strategy to maximise the useful service obtained from the tube.

The Lifecycle Value of Carburisation Protection

The value of protecting a reformer tube should not be assessed solely by comparing the price of aluminising with the initial cost of an untreated component.

The wider lifecycle can include:

• Component manufacture
• Surface treatment
• Inspection
• Maintenance
• Replacement frequency
• Removal and installation
• Associated plant intervention

Where carburisation contributes to premature component degradation, increasing resistance to carbon ingress may help reduce the lifecycle consequences associated with that degradation.

The purpose of aluminising is therefore not simply to add another process to component manufacture.

It is to help protect the value already contained within the component.

Supporting Longer Useful Service

Component life is determined by multiple interacting factors.

These can include:

• Creep
• Mechanical loading
• Thermal cycling
• Metallurgical ageing
• Process conditions
• Oxidation
• Carburisation
• Other corrosion mechanisms

Diffusion aluminising does not eliminate these other factors.

Instead, it addresses the environmental surface degradation element where appropriate.

If carburisation is one of the mechanisms limiting useful life, increasing resistance to carbon ingress can support the wider objective of keeping the component in service for longer.

Material Efficiency and Component Life

Reformer tubes and other high-temperature process components often contain substantial quantities of valuable alloying materials.

Manufacturing replacement components requires additional:

• Raw materials
• Alloy production
• Energy
• Manufacturing
• Transportation
• Installation

Extending useful service life can therefore reduce the frequency with which those resources are required.

This provides a secondary material-efficiency benefit alongside the primary objectives of component integrity, reliability and lifecycle economics.

Start With the Cause of Degradation

Surface engineering should be selected to solve an identified problem.

Before considering aluminising, the key questions include:

Is carburisation actually occurring?

Which area of the component is affected?

What is the substrate material?

What temperatures are involved?

What is the process environment?

How quickly is degradation progressing?

Is carburisation limiting component performance or useful life?

Only once the degradation mechanism is understood should an appropriate surface treatment be specified.

This is why Diffusion Alloys approaches high-temperature protection as an application-engineering challenge rather than simply a coating sale.

Developing the Appropriate Protection System

Successful diffusion coating requires an understanding of the relationship between:

Substrate + Component Geometry + Operating Environment + Degradation Mechanism + Coating Specification

Diffusion Alloys has decades of experience developing and applying diffusion coatings to components used in demanding industrial environments.

When reviewing a carburisation application, our technical team can consider:

• Substrate alloy
• Component dimensions and geometry
• Surfaces requiring treatment
• Operating temperature
• Process gas composition
• Existing degradation
• Service history
• Required coating characteristics

This allows diffusion aluminising to be assessed against the actual component and its operating conditions.

Protect the Surface to Protect the Alloy

Carburisation begins at the interface between a valuable high-temperature alloy and its process environment.

If carbon is allowed to penetrate the surface, degradation can progress into the material itself.

Engineering that surface provides an opportunity to interrupt the process before significant carbon ingress occurs.

Diffusion aluminising creates a metallurgically bonded aluminium-rich surface capable, under appropriate conditions, of developing a protective alumina barrier.

For reformer operators, the objective is bigger than preventing carbon diffusion:

Protect the alloy. Preserve component performance. Extend useful service life.

Discuss Your Carburisation Application

If reformer tubes or other high-temperature components are experiencing carburisation, metal dusting or related carbon-driven degradation, Diffusion Alloys can review the material, component and operating environment to determine whether diffusion aluminising may provide an appropriate protection solution.