Power Generation Industry

Metal Sprayed coatings are used extensively in the Power Generation industry. 

Power stations, turbines and electrical towers are all subject to hostile environments which are often prone to wear, corrosion, as well as other diminishing factors. The use of Metal/Thermal spray coatings can improve the efficiency of the Power Generation industry greatly. The burning of fuels at high temperatures and the use of some combustible items in Power Stations creates wear in the boiler tubes, this can lead to operational break downs. Depending on the operating conditions (temperature, size and type), materials such as Inconel 625, high chrome amorphous/ nanocrystal line material, Chrome Carbide Nickel Chrome HVOF sprayed powder and Aluminium etc. can be used on components in the power generation industry to combat these conditions. Steam and land-based turbines also owe their operating efficiency to Metal Spray. Almost every component of a gas turbine engine for example, has some type of coating on it to prevent the damage caused by wear and temperature.

Steam and Land-Based Turbines

Steam and land-based turbines owe their operating efficiency to Metal/Thermal Spray. The video below shows a HVAF system spraying a WC-Co-Cr coating being applied on-site onto steam turbine blades in order to protect them from droplet erosion.

Almost every component of the gas turbine engine has some type of coating on it to prevent the damage caused by wear and temperature. HVAF and HVOF coatings are used in the hot section of the engine to coat the shroud with high temperature metals that will not damage the edge of the engine’s turbine blades, yet provide the tight clearance necessary for the engine to function properly. HVAF and HVOF coatings are also applied to the rotating sleeves or bearing found in the hot section of the engine. Common coatings used in this application include T-800 and Chromium Carbide Nickel Chrome coatings.

The above video shows a Kermetico HVAF AK7 spraying Tungsten Carbide on a turbine floor.

Wind Turbines

From the early era of inland wind towers, thermal spray coatings have been widely used. The tower sections are generally coated with zinc or aluminium in areas where there is a higher probability of coating damage during erection or use of the tower. This includes, but is not limited to - at the flanged ends, around access hatches and ladder supports.

As the towers went off-shore, this meant that they were situated in a harsher corrosion environment which called for a tougher corrosion-resistant coating. Many of the major manufacturers are specifying Metal Sprayed zinc or zinc/aluminium coatings on their towers, nacelles, hubs and slewing rings. The industry is increasingly using Thermal Spray Aluminium (TSA) coatings on monopiles, jacket structures and boat landings (taking learnings from the decades of use in the oil and gas industry).

The above video shows the zinc Arc spray coating process of wind towers (turbines) from blast, thermal spray zinc, prime and top coat.

Pelton Buckets

Pelton Buckets suffer high wear on the internal surface due to silt erosion and cracking on the thinner bucket surface areas cause an imbalance in operation. These are protected by HVOF Spraying or Arc Spraying with 420 Stainless Steel.

For more information on the Turbine Industry, Click Here.

Electrical Towers

Electrical Towers straddle the landscape and in a lot of cases are very remote; this requires the protection from the varying climates that occur during the 4 seasons. Metal sprayed Zinc has been used not only on new towers but also for the repair of corrosion damaged leg repairs in situ.

Anti-Corrosion on Transmission Towers

Unimec Chooses ARC140 System

Reason for use: Corrosion protection of high voltage power transmission towers.

Unimec, based in Romania, manufactures metal hardware for low and medium voltage power lines, has been using the Metallisation Arc140 system (superseded model) for many years to spray small parts that historically have been hot-dip galvanised or zinc plated.

Metallisation’s Romanian distributor, Straaltechniek Minex, who delivered the first Arc spray 140 system to Unimec, have since been cooperating with the company with technical support and materials supply. Unimec uses the Arc spray 140 system to metal spray small pylon components to protect them from corrosion. The components include brackets, hinges, horizontal supporting consoles, stretching consoles, aluminium clamps and supporting rods. Metal spraying the components with zinc will ensure the longevity of the individual parts.

At the inception of Unimec, these small components were hot-dip galvanised or mainly electro zinc plated, which meant Unimec had to send the parts out to a third party. Both of these processes are commonly used for coating smaller parts for corrosion protection.

However, the protection granted by these processes was not proving adequate in some of the more highly corrosive environments that the pylons were being installed. Now with the Arc spray 140 metal spray system (superseded model), Unimec has total control over the production and quality of all components and can deliver them to meet customers’ timescales. Also, due to the harsh environment the components face, Unimec has found that metal spraying offers much greater anti corrosion protection than galvanising or electro zinc plating.

Due to the volume of parts Unimec sprays the company has to run two daily shifts to meet the demand for metal sprayed components. Each component is grit blasted to SA 2.5 and then coated with 50 to 100 microns of zinc.

Major advantages of metal spraying are that coatings are available for almost instant use, with no drying or curing times, and there is no risk of damaging the component through heat distortion. The thickness of the coatings can be locally controlled by the operator, allowing variations in the level of corrosion protection depending on the environment. With increasing transportation costs becoming an issue, the process can easily be installed in-house. This not only reduces costs but also increases internal control over quality, production planning and shorter response times.

Arc Spray Process

The Metallisation Arc spray process is normally used to protect large steel structures such as, vessels, tanks, buildings and bridges, but is also proving itself to be a viable option for smaller components as described here.

In the Arc spray process the raw material, in the form of a pair of metallic wires, is melted by an electric arc. This molten material is atomised by a cone of compressed air and propelled towards the work piece. The molten spray solidifies on the component surface to form a dense, strongly adherent coating suitable for corrosion protection or component reclamation. Sprayed coatings may also be used to provide wear resistance, electrical and thermal conductivity.

Straaltechniek Minex has been a Metallisation distributor for over ten years and is proud of its knowledgeable pre-sales and after-sales support. Adrian Hentulescu, Technical Director for Straaltechniek Minex, says: “Unimec needed a supplier that understood the problems corrosion can cause and could supply appropriate advice as well as the most flexible and reliable equipment.

The Arc spray equipment has enabled Unimec to increase its output and respond much quicker to its clients demands, which is great news for all concerned.”

Boiler Tubes

The application of a Metal Sprayed coating can considerably enhance the life and efficiency of boiler tubes that are subjected to high heat levels, erosion, corrosion and heat cycling. Boiler Water Wall Tubes, Super Heater Tubes, Boiler Components in the burner area as well as high temperature furnace components are all subject to thermal sprayed coatings.

The Arc Spray and HVOF applied coatings has provided this protection.

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Some boilers have high sulphur content that warrants the use of Ni Cr based material, if the ash content is high - a high hardness Arc Spray coating is needed. Another popular material is a Chrome Carbide Nickel Chrome applied using the HVOF technique which can operate up to 870°C with good corrosion, abrasion, particle erosion and hot gas resistance and also has a high hardness characteristic.

High Temperature Corrosion and Wear of Boiler and Waste Incinerator Tubes

HVAF Thermal Spray Chromium Carbide

Boiler coating is extremely important to mitigate corrosion and wear in boilers and waste incinerators. HVAF chromium carbide has proven to be the excellent coating for the application.

Circulating Fluidized Bed Erosive Wear

Circulating fluidized bed (CFB) boilers are used widely in coal-fired power plants because of their advantages of highly efficient combustion of a wide variety of solid fuels, in-bed sulfur capture, and relatively low NOx emission. Hard coal gangue is the primary cause of erosive failures in economizer boiler tubes at high temperature. Erosive wear resistance of the boiler tubes’ surface is necessary to lengthen the continuous operation lifetime. The size of the coal gangue particles in CFB is on the millimeter scale, and the particle velocity is from several to tens of meters per second.

Boiler Cold End Sulfuric Acid Dew Point Corrosion

Using fuels containing sulfur yields a potential hazard of sulfur corrosion at the cold end of the boiler. The severity depends on many factors like percentage of sulfur in the fuel, excess air, moisture in the flue gas, etc. The problem is most severe for waste incinerators.

Steam generating boilers use different types of fuels containing sulphur. The higher the percentage of sulfur, the higher will be the risk of cold-end corrosion in the boiler. The sulphur converts to sulphur dioxide during combustion. Depending upon the other impurities present in the fuel and excess air levels, some portion of the sulphur dioxide gets converted to sulphur trioxide. Sulphur dioxide and trioxide combine with moisture and forms sulphurous and sulphuric acids.

The basic reactions taking place are:

S + O2 → SO2

SO2 + O2 ↔ SO3

H2O + SO2 ↔ H2SO3

H2O + SO3 → H2SO4

Condensation of these acids results in metal wastage and boiler tube failure, air preheater corrosion and flue gas duct corrosion.

The amount of SO₃ produced in boiler flue gas increases with an increase of excess air, gas temperature, residence time available, a number of catalysts like vanadium pentoxide, nickel, ferric oxide, etc. and the sulfur level in fuel. The flue gas dew point temperature increases steeply from 90°C to 135°C (194 to 275°F)  with the sulfur percentage increasing up to 1%. A further increase in the sulfur percentage in the fuel gradually increases the dew point temperature of 135°C to 165°C (275 to 329°F) at 3.5% sulfur in the fuel.

The industry uses wear and corrosion resistant coatings widely to mitigate these risks.

High Velocity Thermal Spray Boiler Tube Coatings

Kermetico HVAF Chromium Carbide Coating Properties

Thermally sprayed Cr₃C₂-NiCr coatings combat high-temperature wear due to the high wear resistance imparted by the hard carbide particles and the high temperature oxidation resistant nature of the Cr₂O₃ oxide formed over both phases.

While WCCoCr 86/10/4 provides better wear resistance when temperatures are under 510°C (950°F), high velocity sprayed chromium carbide ceramic coatings mitigate abrasive and erosive wear at temperatures up to 750°C (1,380°F). The corrosion resistance is provided by the NiCr matrix while the wear resistance is provided mainly by the carbide ceramic phase.

A Typical Microstructure of a Kermetico HVAF Sprayed Chromium Carbide Coating.

HVAF and HVOF Sprayed Inconel-Type Coatings

Despite less wear resistant than chromium carbide, an Inconel-type coating is a popular choice for dew-point corrosion protection of boilers.

HVAF Inconel-type coatings provide excellent hot corrosion resistance due to Cr₂O₃ formation, high density and high bond strength.

For further information, Click Here.

Laser Cladding for Materials Processing Hammers - Tungsten Carbide reinforced Nickel Chromium Silicon Boron

Materials processing hammers are used in industry to pulverise hard and abrasive materials into a fine powder. The hammers are made from manganese steel, which although hard wearing, is prone to impact wear and tear.

Laser cladding the impact areas of the hammers is a cost effective method of applying a nickel based material reinforced with a high volume fraction of tungsten carbide, which provides a wear and impact resistant coating.

Laser cladding is a process that falls into the range of hard-facing solutions, which can be used to increase corrosion resistance, wear resistance or impact performance of metallic components, using a method of applying a fully dense, metallurgically bonded and virtually pure coating.

One of the major benefits of laser cladding is the ability to finely control the heat input to the substrate and the coating material, which allows a deposit of a two phase Metal Matrix Composite Structure. This means the coating can have a softer, lower melting point material (the matrix) where a harder wearing, higher melting point material (the hard phase) is suspended. The matrix material is typically a nickel based alloy, which provides a tough, ductile and impact resistant surface, while being wear resistant at elevated temperatures.

Laser clad hammers have been proven to last over three times longer than standard hammers, which reduced downtime and improves productivity.

Materials - Main Deposit: Tungsten Carbide reinforced Nickel Chromium Silicon Boron

Method

Preparation: The surface of the hammer components need to be cast and shot blasted to remove any contamination prior to being laser clad.

Equipment: Metallisation MET-CLAD system

Application of Laser Cladding

The images below show cage crushing machine material processing hammers in a granulated coal injection (GCI) plant. In this instance, coal is pulverised down by the hammers to produce a very fine powder, which is then blown into the blast furnace as an alternative to more expensive coke.

Using the MET-CLAD system, Metallisation can develop coatings that are tailored to suit a range of specific crushing applications.

To create an extremely strong wear and impact resistant coating for the CGI plant, the components of the hammers are laser clad with Tungsten Carbide reinforced Nickel Chromium Silicon Boron using a fine, accurately controlled laser beam.

The laser cladding process utilises a precisely focused high power laser beam to create a tightly controlled weld pool into which a metallic powder is applied. The powder is carried by a stream of inert shielding gas, which is blown coaxially through the laser beam. The highly accurate nature of the laser beam allows fully dense cladding with minimal dilution and a perfect metallurgical bond.

The very low heat input, associated with a laser, means the tungsten carbide particles remain un-melted and so retain their harness, which adds to the wear resistance of the coating.

Due to the high level of accuracy and control, laser cladding enables the cost effective application of high performance alloys to tackle a wide range of engineering issues. Typical deposition rates are between 60 and 100 g/min around 3-6 kilograms per hour, depending on the material being deposited and the geometry of the work piece.

To apply a laser clad coating the cladding head has to be fed the appropriate with four key things; a laser beam, process gasses, powder and cooling water. The Metallisation MET-CLAD laser cladding control console provides integration and control of the complex component parts. The MET-CLAD system is a simple to use control system with touch screen HMI and is based on the Metallisation HVOF and Plasma control concept.

The control console offers mass-flow control of the laser shielding and powder feed gas for repeatable cladding. The laser can also be housed and controlled within the cladding console.

The control interface for production operations is simple, but it can be drilled down to a great level of complexity for coating development. Repeatable operations are easily programmed or they can be linked to a barcode system for even simpler programming. The process gases are mass flow controlled for repeatability of the coating process.

Comparison of Coatings

The image above shows a Laser Clad hammer on the left and a conventionally coated hammer on the right.

Comparison of Coating Processes

The following table gives a broad comparison of coating processes. The data shown is based on typical applications and parameters. There can be exceptions to this data, dependent on the specific applications and parameters, MSSA will be happy to offer advice for specific applications.

Conclusion

Laser cladding is capable of producing coatings with a combination of excellent toughness and abrasive wear resistance. Due to the low heat input the tungsten carbide particles remain un-melted, which means they retain their hardness and wear resistance.

Laser clad hammers have been proven to last over three times longer than standard hammers, resulting in reduced downtime and increased productivity.

Silencer Protection

This massive silencer for a 5 megawatt Power Generation Plant in Finland has been protected against corrosive attack by metal spraying. The Plant is installed at a new limestone processing complex and the sulphurous environment would be particularly damaging to the steel fabrication. After grit blasting the silencer was aluminium sprayed to a depth of 150 microns (0.006”).

Thermal Spray Zinc on Mabey Bridge Wind Towers

Thermal spray zinc coating manually applied to a traditional cylindrical wind tower.

Metallisation Arc Spray systems are used to apply zinc coatings to the insides and outsides of wind towers. The long supplies packages make spraying the internal areas of long towers a more simple and efficient process. The hand-held pistol is flexible to move around the spray areas and has a manageable throughput.

Metallised zinc coatings are applied predominantly to areas that may be damaged during assembly and installation. The robust zinc coating will give sacrificial corrosion protection to areas where the paint coatings may be damaged.

Thermal Spray Zinc on Andresen Towers Wind Towers

Metallisation ARC528E systems with 1500A energisers are robot mounted to produce high volume, high quality repeatable coatings to give corrosion protection and increase bolt joint friction. The 1500A systems enable high production rates required to keep up with the demand for this unique design of wind tower.

HVAF Spraying onto Steam Turbine Blades

The video above shows a HVAF system spraying a WC-Co-Cr coating being applied on-site onto steam turbine blades in order to protect them from droplet erosion.

HVAF Spraying on a Turbine Floor

The above video shows a Kermetico HVAF AK7 spraying Tungsten Carbide on a turbine floor.

For more information on Metal Spray equipment or consumables, call us on 07 3823 1004, or email us using our contact form.

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