Electrolytic applied coatings (electro-plating)

Electroplating is a process that uses electric current to reduce dissolved metal cations so that they form a thin coherent metal coating on an electrode. Electroplating is primarily used to change the surface properties of an object (e.g. abrasion and wear resistance, corrosion protection, lubricity, aesthetic qualities, etc.), but may also be used to build up thickness on undersized parts.

The process used in electroplating is called electrodeposition. It is similar to a galvanic cell acting in reverse. The part to be plated is the cathode of the circuit. In one technique, the anode is made of the metal to be plated on the part. Both components are immersed in a solution called an electrolyte containing one or more dissolved metal salts as well as other ions that permit the flow of electricity. A power supply supplies a direct current to the anode, allowing them to dissolve in the solution. At the cathode, the dissolved metal ions in the electrolyte solution are reduced at the interface between the solution and the cathode, such that they "plate out" onto the cathode. The rate at which the anode is dissolved is equal to the rate at which the cathode is plated, vis-a-vis the current through the circuit. In this manner, the ions in the electrolyte bath are continuously replenished by the anode.

Electroplating increases life of metal and prevents corrosion. Cleanliness is essential to successful electroplating, since molecular layers of oil can prevent adhesion of the coating.

Electroplating is widely used in various industries for coating metal objects with a thin layer of a different metal. The layer of metal deposited has some desired property, which the metal of the object lacks. For example, chromium plating is done on many objects such as car parts, bath taps, kitchen gas burners, wheel rims and many others for the fact that chromium is very corrosion resistant, and thus prolongs the life of the parts. Electroplating has wide usage in industries. It is also used in making inexpensive jewellery, such as silver-plated and gold-plated necklaces, bracelets and rings.

Thermally sprayed metal coatings are deposits of metal which has been melted immediately prior to projection onto the substrate. Zinc and aluminium are by far the most widely used for protecting steel against corrosion. Zinc may be used in thickness as low as 25 µm, while aluminium should be 75 µm or above. Pre-treatment should be blast cleaning to Sa 3.

Thermal metal spray requires high attention to personal safety:

• Protect the eyes from the intense light created while melting the metal
• Protect from inhaling metal gases (example zinc-fever)
• Protect against high level of noise
• Protect the entire body against hot melted metal

Thermal metal spray operator must be specially trained, both for quality and for safety reasons.

Three main stages of the thermal metal spraying process:

• The metal (zinc or aluminium) is melted at high temperature at the tip of the spray gun
• A jet of air or gas will break up the molten metal to small droplets and propel them forwards towards the prepared substrate
• The molten metal particles will hit the prepared substrate and form a metallic film

The metal droplets will cool quickly in the air, so the distance between the spray nozzle and the substrate will usually be kept smaller than what is the case for airless spraying. Even so, the molten droplets will not flow entirely together to form a continuous film over the substrate. A thermal metal spray film will end up having a lot of voids and pores, and will be more porous than for example a zinc-rich silicate primer. If overcoated, a mist coat is a necessity.

The metal which shall be sprayed is supplied as either a fine powder or as a continuous wire. If powder is used, this will be kept in a powder cup mounted on top of the spray gun and gravity-fed into the gun just behind the nozzle (a bit like a hopper-gun), where it is melted. If metal wire is used, this will be supplied in large coils which is fed continuously from the back of the gun and into the nozzle where it is melted.

The all-important heat to melt the metal which is sprayed is created in a few different ways, and the most common and important ones are:

• Flame spraying (also called gas spray)
• Arc spraying
• Plasma spraying

General

The heat is created by burning a mixture of oxygen and a flammable fuel gas at the tip of the spray gun. The flame spray gun thus needs a constant supply of oxygen, fuel gas and metal to be sprayed:

• A single continuously moving wire is passed through the spray gun and melted by a conical jet of burning oxy-fuel gas
• The wire tip enters the cone, melts, atomizes and is propelled onto the substrate
• Alternatively, metal powder is fed into the flame from a powder cup or hopper
• Flame temperature usually 2,700 – 3,100 ˚C.

General purpose oxy-fuel wire spray guns are capable of spraying all available metallic wires like zinc, aluminium, steel, stainless steel, copper, molybdenum and various alloys.

Application

Distance to object, correct pressure and mix of fuel/compressed air is important for obtaining a uniform coat with a surface that is not too rough, and with minimum pores and oxides. Correct distance between gun and object is normally 25-30 cm. If the distance is too short the metal may get burnt, while to long distance can create molten particles and a rough surface. The required adjustments for oxy/fuel gas and compressed air can be found in the operating manual, and may vary depending on the different coating materials.

Inspection

The following shall be inspected:

• Environmental condition, normally maximum RH of 85% and a steel temperature at least 3 ˚C above dew point
• Surface preparation to minimum Sa 2 ½ according to ISO 8501-1
• Surface roughness 50-75 microns
• Functionable equipment
  • No worn out nozzles
  • Flow meters installed and correctly adjusted
  • Pressure valve installed and correctly adjusted
• Correct spray distance under application
• Uniform surface without "lumps" or burned areas
• Assessment of correct thickness of coating as specified
• Adhesion test with results according to requirements. Normal values obtained are 10-15 N/mm² by pull-off adhesion. Note! When carrying out adhesion test, assure that the glue does not penetrate the thermally sprayed coating (sealing of pores may be necessary). If adhesive penetrates it may give incorrect results (higher values). 

General

The heat is created by an electric arc between two electrodes at the tip of the spray gun. This is the same principle used during electric welding, the arc melts the welding electrode and bonds the two surfaces together. The arc spray gun thus needs a supply of high voltage DC electrical current, the metal to be sprayed and the highly pressurized air which propels the molten metal forward.

What is an arc?

• A pair of metal wires are electrically energized so an arc is struck across the tips when brought close together in the spray gun
• The arc melts the wire ends
• Compressed air is blown across the arc to atomize and propel the auto-fed metal wire particles onto the prepared work piece
• Temperature in the electric arc is approx. 5,500 ˚C.

Application

Distance to object and adjustments are similar to flame spray.

Special care when spraying aluminium and aluminium alloys (e.g. AlMg5) since they are exothermic. When exposed to the high temperature in the arc at 5500 °C, an exothermic reaction starts between the Al/AlMg5 and oxygen and the melted particles increases in temperature from 1100 °C to approximately 2100 °C. This is the reason for why the spray jet is glowing. This high temperature is reached when the distance from the nozzle is 22-23 cm with the result that the sprayed material gets a micro welding point on the tip of the roughness on the blast cleaned steel. This is also the reason for higher adhesion value requirements for these coatings than for non-exothermic metals like Zn and Zn alloys.

Inspection

 The following shall be inspected:

• Environmental condition, normally maximum RH of 70% and a steel temperature at least 3 ˚C above dew point. RH may be specified lower than for flame spray due to risk of oxidation on the peaks of the blast cleaned steel
• Surface preparation to minimum Sa 2 ½ according to ISO 8501-1
• Surface roughness 50-75 microns
• Functionable equipment
  • No worn out nozzles
  • Nozzles correctly adjusted
  • Voltage, ampere and compressed air are correctly adjusted according to operating manual
• Correct spray distance under application
• Uniform surface without "lumps" or burned areas
• Assessment of correct thickness of coating as specified
• Adhesion test with results according to requirements. Normal values obtained are 10-15 N/mm² by pull-off adhesion. Note! When carrying out adhesion test, assure that the glue does not penetrate the thermally sprayed coating (sealing of pores may be necessary). If adhesive penetrates it may give incorrect results (higher values). 

• Abrasion resistant coatings are used to improve or modify the surface hardness of a component and hence improve its performance and lifespan. The most commonly applied coatings are Tungsten Carbide and ceramic coatings. Depending on the application and the part, machining is often carried out after the coating has been applied. The application of a fluoropolymer with the abrasion resistant plasma coating can be used to give a hard wearing coating with release properties.
• Thermal-spray technology is commonly used for structural components by building up a protective coating layer on their surfaces. Choosing a suitable sprayed metal can improve corrosion resistance, oxidation resistance, wear resistance and/or heat insulation, and thus extend the life of protected components
• These coatings are typically applied using plasma spraying

• An electric arc is formed between a water-cooled anode and cathode
• A gas is fed into the electric arc (e.g. helium gas, argon, hydrogen, nitrogen, or mixtures of these)
• The gas is heated in the arc and forms a mixture of ions and electrons, called plasma
• Metal powder is transported into the plasma stream by a carrier gas
• The powder melts and is propelled at great velocity towards the substrate
• There is an intense generation of heat in the plasma, typically 10,000 – 25,000 ˚C

• “Normal” flame and arc-sprayed metal coatings are usually very porous, perhaps even more porous than zinc-silicate primers, and must be treated in the same way when over-coated
• Popping is very much a real problem, so a tie-coat or a mist-coat technique must be involved
• Sometimes the metallic coating is left with only a tie-coat or mist-coat (“sealer”). This will penetrate the pores, reduce the total area of exposed metal and smoothen the surface texture
• In other situations, a full paint system is used on top of metallic coatings

Sealing only:

• Polyamide cured epoxy

Full paint system, use a system suitable for zinc-silicate primers, such as:

1. Polyamide cured epoxy as mist coat
2. Polyamide cured epoxy
3. Suitable topcoat (Polyurethane)