In simple terms, erosion corrosion is caused due to the relative movement between fluid particles and solid surfaces in a corrosive environment. As this form of corrosion affects critical equipment in aviation and other industries, preventive measures are needed at design stage itself to minimize such deterioration. Coatings play an important role in minimizing surface deterioration due to erosion corrosion.
Erosion corrosion is primarily an aggravation of the rate of corrosion damage on a metallic substrate due to a relative movement between corrosive fluid particles and the solid surface. In the fluid power system, the localized turbulence at higher velocities—initiated by early erosive pitting occurring on the pipe’s internal surfaces—can cause a rapid increase in the erosion-corrosion rate and finally lead to dangerous leaks. This can be further aggravated due to faulty installation and imperfections in the workmanship. As the initial erosion leads to a loss of protective films present on the metal surface, a very high rate of corrosion will be observed in due course. Erosion can also occur even when the fluid is primarily static and the moving equipment parts are partially or fully immersed in the fluid.
Oil and gas wells that pump sand bearing crude oil frequently face the problem of erosion damage to pipelines and equipment components. Erosion corrosion is frequently observed on aluminum alloys and copper alloys. At some threshold value of fluid velocities, fluid particles begin to break the protective film and corrosion starts at a rapid rate. These lighter alloys form a protective film in a mildly wet and corrosion-prone environment. There is a limiting value of fluid velocity above which erosion damage and related corrosion accelerates very fast. This type of corrosion induced by particle velocity is commonly termed as "erosion corrosion, which includes electrochemical metal dissolution and mechanical attack due to particle impingement. The fluid in this case includes liquids as well as gases.
The Mechanism of Erosion Corrosion
The erosion-corrosion mechanism could be explained as:
- A mass transport type of metal loss, or
- A phase transport type of metal loss
In a mass transport type of metal loss, the convective transfer rate of mass at the metal-fluid interface determines the rate of erosion corrosion. For example, when steel gets deteriorated due to the flow of water containing the dissolved oxygen, the convective flow of oxygen determines the initial metal loss, and oxygen diffusion rate into the layer of rust determines the metal loss subsequently. Metal loss is generally uniform across the surface in such cases.
In the case of a phase transport type of metal loss, the rate of wetting of the substrate by the liquid phase and vapor phase of the corrosive fluid would determine the metal loss. Wetting is determined by the flow rate of the vapor phase and liquid phase of the fluid. In the case of boiler tubes in a power generation plant, for example, the water phase separates from the steam phase, and the turbulence in flow increases erosion corrosion in certain zones of the boiler.
Initial attack on the protective films or the layers of coatings is entirely induced by velocity and turbulence of the flow and contaminant particles. This can occur only when the fluid velocity has exceeded a critical value. The flow generated-damage further creates additional local pressure surges and high shear stresses on the surface. The deterioration is aggravated by bubble bursting and the movement of gritty particles and hot gas vapors.
Erosion corrosion affects critical plant equipment, such as:
- Internal combustion engine components
- Gas compressors
- Industrial blowers and high-speed fans
- Aviation equipment
- Marine equipment
- Offshore oil and gas production systems
- Heat exchangers
- Hydraulic components
- Automotive and mining equipment
- Chemical process plants
- Power generation equipment
Inside the gas turbine compressors of aircraft, erosion corrosion can cause a reduction of efficiency due to the reduction of compression ratio. Power generation from steam turbines may be affected by erosion corrosion due to water droplets impinging on the rotor blades of turbines. High temperature turbo expanders—designed for power recovery—suffer due to particle impingement, causing a drop in turbine efficiency.
Erosion Due to Cavitation
Deterioration due to cavitation can be treated as a special category of erosion corrosion. It is a result of the collapsing and bombarding of vapor bubbles on the metallic surfaces of components and inner periphery of pipes and tubes. Low-pressure zones created at the pump suction, hydraulic actuator inlets and other points in the circuit cause the formation of vapor bubbles, which later collapse at a point where fluid pressure increases due to the reduced internal diameter of the pipe. The pressure at which liquid vaporizes depends on temperature and properties of the liquid.
Cavitation causes the abrupt destruction of the protective films because of the imploding vapor bubbles in a liquid. These implosions can cause severe vibration and severe shock waves. These surface deteriorations can look like sharp-edged pits at close proximity.
Deterioration is observed in parts and components of high-velocity fluid power system components such as pumping elements, impellers, pipes, control valves, pipe bends, and tee sections and actuator components.
Engineering Approach for the Prevention of Erosion Corrosion
A corrective and preventive approach may involve the following.
- Design modifications to ensure minimum turbulence in the fluid flow such as:
- Increasing pump suction pipe diameters wherever feasible (to reduce bubble formation), as well as internal diameters of other pipes and tubes.
- Reducing the suction pipe lengths to reduce pressure drops.
- Redesign the system to reduce the flow rate as well as turbulence; ensure net positive suction head.
- Reduce the number of pipe joints on the suction side.
- Maintenance effort to control fluid leakages from joints, suction-side joint leaks in particular; use appropriate joint seals and packing.
- Control the entry of particulate contaminants, moisture and air bubbles.
- Use strainers and filters or conditioners (centrifuges) to minimize contaminants in the fluids.
- Install fluid coolers as needed.
- Allow fluid to settle down in settling tank, so that water and air can be removed and sludge and emulsion, if any, can be separated.
- Select erosion corrosion-resistant materials compatible with environment.
- Use coatings and other surface treatments.
- Consider catholic protection against corrosion.
While we adopt any combination of solutions, we need to look at the overall impact of the solution and the long-term performance implications with cost effectiveness.
- It is generally advisable to ensure laminar flow in the systems. Rough internal surfaces and restrictions promote turbulence. Sudden changes in the direction of flow may be avoided.
- Inlet pipes and return pipes to tanks should be placed in such a way that the flow doesn’t hit the tank walls.
- Baffle plates may be used if required.
- Alignment of pipes and fittings must be accurate.
- Baffle plates that bear the impingement of particles should be easy to replace.
- Magnetic plugs can be used to separate out the iron and steel particles in the fluids.
- Water traps may be used in a compressed air utility system to allow moisture to be separated from the compressed air to control the erosion corrosion of system components.
Considerations for Coating Application
While adopting coating as a preventive solution, the compatibility of the coating with the fluid used and the surfaces needs to be studied. Factory-applied layers of coatings need to be supplemented by in situ application of coatings. Factors to be kept in mind for the selection of coating include:
- Severity of impingement of particles
- Angle of impingement
- Corrosiveness of the environment
- Providing erosion protection
- Size of particles causing erosion
- Temperature capability to decide about the coating process suitability
- Other substrate properties
The application technique is chosen on the basis of:
- Substrate properties
- Required thickness of coating
- Masking of substrate before coating
- Economics and one-time cost of process
Testing of Coatings
An erosion corrosion-resistant coating is usually inspected and tested for:
- Erosion resistance as per ASTM G76
- Abrasion resistance by Taber abrasion simulated test
- Sand jet test for erosion resistance for sand particles
- Water jet erosion measurement
- Elasticity test such as micro penetration and impact tests
- Uniformity of thickness of the coating
- Cavitation erosion resistance as per ASTM G32,
- Coating holidays, if any
- Bond strength of coating
- Coating material density
- Hardness of coating film
Carbide and Nitride coatings
Coatings applied on precision component parts include:
- Tungsten carbide coatings
- Chromium carbide coatings
- Titanium nitride coatings
Carbides are generally used for oxidation resistance as well as erosion-corrosion resistance. Flame spray techniques are used for depositing the tungsten carbide on metals as a porous coating. Porosity is filled by using ceramics or polymers. High velocity oxy fuel technique is also used for coating the tungsten carbide onto component surfaces. Supersonic gas stream at elevated temperatures is used for creating thick deposit on the component substrate. This coating can be honed and finished to a superfine polished layer.
A plasma coating technique is also used for hard tungsten carbide and ceramic-type coatings. In this technique, argon gas is partly ionized through an electric arc, which produces a stream of hot gas flame. Tungsten carbide powder or ceramic powder is injected into the hot gas flame. The plasma coating gun can be controlled through electronic controllers or robots as per the needs of the configuration of the component to be coated.
- Titanium nitride is used for aircraft and spacecraft for low-angle erosion protection.
- Chromium carbide coatings are used for erosion protection at very high temperatures.
- Tungsten carbide coatings provide erosion protection and corrosion protection at higher temperatures under extreme erosion conditions.
Aluminum oxide-ceramic coatings as well as chromium oxide-ceramic coatings are used as an economical alternative for erosion protection for light metal alloys. Zirconium oxide ceramics are used for high-temperature applications like the tips of hot gas turbine blades for erosion corrosion protection.
Cermet coatings are used for controlling steam turbine erosion and the problem of fouling of compressors, handling ethylene gases. Hardened aluminum-ceramic coatings are used to minimize the hard particle erosion in aqueous environments. Specially formulated ceramic coatings have corrosion resistance against chemicals handled at higher temperatures in chemical and other process industries.
Polymer-aluminum-ceramic coatings are extensively used in aviation. They provide rain erosion protection for aluminum alloy surfaces. They also act as a replacement of hard anodized layers. They are effective against rain droplet corrosion and erosion, and they are applied as sealed coatings. Smooth, aerodynamic-sealed ceramic-aluminum coatings are applied in place of diffused coatings of cadmium-nickel for coating the gas path components of aircraft.
Metallic-aluminum-ceramic coating systems, applied in place of cadmium plating on surfaces of steel, provide erosion protection in a highly acidic chemical environment, such as in case of SO2 gas. They are used as coatings on high-strength steels as well as aluminum alloys.
Molybdenum disulfide (MoS2) is used for erosion protection and anti-seize applications. Metal-filled polymers are used for erosion and corrosion protection of aluminum and magnesium alloy surfaces.
Ceramic-Filled Epoxy System
Epoxy resins are produced by mixing different types of raw materials to build an epoxy system of the required properties, such as:
- Film strength
- Curing time
- Water resistance
- Abrasion resistance
- Chemical resistance
The epoxy group and amine group’s molecules must react together, to be able to form a thermoset cross-linked resin system.
Epoxy resins with ceramic filling are used as coatings in chemical process industries, to protect surfaces that are prone to severe erosion corrosion. Such coatings contain hard particles of ceramics in the epoxy binders, creating ceramic composites that have excellent chemical resistance and mechanical properties. These hard ceramic particles in coatings rub with the entrained contaminants in fluids, without getting eroded, thus ensuring a long life of the coating as well as the substrate.
The limitation of the ceramic-filled epoxy coating is its problematic application method. It is generally not amenable to the spray system of application. Epoxy resins and the hardeners used could be corrosive or sticky on the skin. Hence, the nitrile rubber gloves should be used while handling epoxy resins and the coatings (along with barrier cream and cotton under gloves).
Epoxies are also used for high temperature erosion-resistant application in aviation, oil and gas, chemical, automotive and other critical process industries. Ceramic microspheres, filled in epoxies, provide strength and exceptional adhesion to metallic substrates, with good corrosion resistance as well as abrasion-erosion protection.
These are also used in marine applications for reducing the erosion of ballast and water tanks. Proprietary epoxy coating formulations applied on pump components, inside surfaces of piping and valves, are designed to improve pumping efficiency by ensuring a smooth flow and by reducing the losses due to erosion and local turbulence. Specially formulated epoxy systems are used for coating the parts that remain continuously immersed in chemicals at elevated temperatures.
Ceramic metal composites are also used for the restoration of high-value corroded and eroded metal components. Repair can restore eroded engines, gear boxes, bearings, cylinder blocks, liners, casings and flanges.
Polyurethanes and Other Coatings
Polyurethane coatings are used to coat the strike areas of airplanes, along with leading edges. They are effective against rain erosion as well as gritty particle erosion. They are also resistant to phosphate ester hydraulic fluids as well as certain de-icing chemicals. Specially formulated polyurethane coatings are also used for the walkways.
For extreme chemical resistance required in chemical and petrochemical industries, Fluoro polymer coatings, such as ethylenechloro-trifluoroethylene (ECTFE), are applied.
Coatings for Cavitation
In many fluid handling systems, cavitation also contributes to metal loss along with normal particulate erosion. Flexible coatings of polyurethane or flexible epoxies, applied on metallic components, reduce cavitation-induced corrosion by absorbing the energy released by collapsing bubbles and pressure surges.
A systematic engineering approach is essential for solving the problems related to erosion corrosion of critical equipment assets. Design modification can minimize the turbulence in the flow to minimize erosion and cavitation in most cases. The selection of suitable coatings can minimize the surface damage due to erosion corrosion and ensure the productive longevity of equipment.