一周年庆典－－THERMAL SPRAYING OF ALUMINIUM AND ZINC

BMA004

THERMAL SPRAYING OF ALUMINIUM AND ZINCThe term thermal spraying cover several processes used for applying melted droplets of metals, alloys, plastics, carbides and various oxides onto a substrate. The word metallizing is also used to describe these processes. Metals comprise a minor part of the approx. 250 substances, which may be applied through thermal spraying, however, metals are very important from a corrosion protection point of view. Among the metals, aluminum and zinc are dominating.
The break-through in the use of thermal spraying came in the 1960s when the aircraft and space industries demanded protective coatings with extreme properties.
Before thermal spraying, the substrate must be completely clean. Steel surfaces must be abrasive blasted to Sa 3 (or Sa 2.5-3) using a sharp abrasive. No grease or oil must be present and this degree of cleanliness must be maintained throughout the application process. The surface profile may also be specified and commonly a factor defined as Ry = 50-85 (or sometimes 80-120)µm is used.
The following processes and media are used:

SPRAYING PROCESS:	MEDIA USED:
Flame spraying	Metal as wire or powder
High velocity flame spraying	Metal as powder
Electric arc spraying	Metal as wire
Plasma spraying	Metal as powder

Flame spraying (using wire)
The heat source for melting the metallic coating material is burning gas. Most commonly used is acetylene and propane, however, hydrogen, stabilized methyl-acetylene, LNG and other industrial gasses may also be used. All these gasses are mixed with oxygen for maintenance of the combustion.
The flame temperature varies from 1900 to 3100℃
dependent on the gas used. Acetylene will give a flame temperature sufficiently high to melt steel and the other metals/alloys, which are supplied in wire form. Propane can only be used for thermal spraying of media with relatively low melting points, e.g. zinc, aluminum and lead.
To have equal temperature around the wire, this is usually fed through the flame nozzle in the center of the gas flame.
The burning gasses by themselves are not able to vaporize and transport the melted metal to the substrate and therefore compressed air is supplied. This compressed air, fed through an opening between the flame nozzle and the outer mantle, surrounds, limits and directs the flame so only the end of the wire is melted. The melted metal is then vaporized by the compressed air and projected onto the substrate.
A feeding mechanism feeds the wire through the spray gun consisting of a compressed air turbine and feeding rollers. The feed speed can be regulated. The spray gun is usually held at a distance of 12 – 20 cm from the substrate.
Flame spraying (using powder)
The process, also called high velocity flame spraying, is a relatively new process. The working principle involves a propane flame (mixed with oxygen) with very high energy level melting and projecting metal particles onto a substrate. The particle speed is approx. 800 – 1000 m/s and the flame temperature approx. 2700℃. The metal is supplied to the spray gun in powder form.
The process covers DFTs ranging from 50µm up to 500µm.
The capacity varies from 4 to 11 kg/h.
High velocity flame spraying is mainly used for thin, pore free (<1% porosity) coatings with excellent adhesion. Also for abrasion resistant coatings with extreme hardness. Replacement of hard chromium.
Electric arc spraying
Two metal wires (both homogeneous and hollow wires are used) are fed through a spray gun so they will cross each other when exiting in front of the spray gun. Both wires must have identical speeds. Through the gun the wires are in contact with contractors (pipes or blocks) coupled to an electrical rectifier. Here electricity is transferred to the wires and they are surrounded by an ionized field. In the crossing point between the wires and electric arc will form melting the metal in the wires. The arc is maintained as long as the metal is electrified. The particle speed is approx. 150 – 250 m/s and the temperature in the electric arc is approx. 5500℃.
From a nozzle at the rear of the arc (crossing point) compressed air is emitted, vaporizing the molten metal and projecting the metal vapor onto the substrate.
The process covers DFTs ranging from 300&micro;m up to 3000&micro;m.
The capacity when used for anti-corrosion spraying is 50-55 kg/h for zinc and 12-16 kg/h for aluminum.
Electric arc is used for:
---Repair spraying of machine parts within the construction, pulp and paper, airplane engine etc. industries.
---As a means of creating either a friction increasing or a friction decreasing coating.
---Spraying of corrosion protection coatings consisting of either zinc, zinc/aluminum alloys or aluminum.
Plasma spraying
Plasma is an assemblage of positive ions and unbound electrons in which the total number of positive and negative charges is almost exactly equal.
In general, the plasma will also contain some proportion of neutral atoms or molecules. The properties of plasma are sufficiently different from those of solids, liquids, and gases for it to be considered to be a fourth state of matter.
Plasmas can also exist in solids. For example, the fixed ions and free electrons in an electrically conducting metal constitute a plasma, as do the free electrons and mobile positive “holes” in a semiconductor.
The electric field of an isolated charged particle diminishes as the square of the distance from the particle. In plasma, however, this field is modified because the electrons are free to move into the vicinity of positive ions and away from other electrons. The field of each isolated particle is thus partially shielded by its immediate neighbors. Over a sufficiently large distance, wherein the fields of many individual charges are able to cancel each other, this shielding becomes complete. This distance, called a Debye length, is a measure of the distance over which an individual charged particle can exert an effect. Volumes greater in radius than a Debye length must be approximately neutral. The Debye length is equal to (6.9 times the square root of (T/n)) centimeters, where T is the temperature of the electrons in Kelvin and n (e) is the number of electrons per cubic centimeter. For a body of particles to behave as plasma, its dimensions must be large compared to the Debye length.
Any displacement of the electrons relative to the ions over a distance of the order of a Debye length gives rise to strong electrostatic fields that accelerate the electrons back towards the ions. Upon reaching their original positions, the electrons will have kinetic energy equal to the potential energy acquired in their displacement and will consequently overshoot, continuing to oscillate about an equilibrium position until this energy is dissipated. This simple harmonic oscillation is common to all plasmas. A particular frequency called the plasma frequency is determined by the time it takes for a particle with thermal speed to travel a Debye length. For electrons, the plasma frequency (in cycles per second) is approximately 9000 times the square root of the number of electrons per cm3.
Where plasma comes into contact with a solid object, a region of charge separation develops in which charge neutrality is not preserved. The thickness of this region, called the sheath, is about equal to the Debye length. The sheath arises because, at the same temperature, the electrons in plasma have a much higher average velocity than do the ions (most of which have positive net charge). If an object in contact with plasma has no net current flowing through it, then electrons and positive ions must be reaching it at the same rate. This balance can occur only if the average velocity of the approaching electrons is reduced and that of the approaching positive ions increased by the formation of a layer of net negative charge near the object and a layer of net positive charge one Debye length into the plasma. If an electric field is applied across plasma formed by an electron-emitting cathode, then, in the absence of a magnetic field, essentially the entire potential drop occurs across the sheath at the cathode. The bulk of the plasma remains at a uniform potential.
Another important property of plasma is its electrical conductivity. Because of its large number of free electrons, gaseous plasma generally offers little resistance to the passage of an electric current. The resistance that does occur arises from the collisions of electrons with each other and with ions. Because the probability of such collisions occurring decreases with increasing electron velocity, the electrical resistance of plasma decreases at higher temperatures – exactly opposite to the behavior of a metal. This characteristic has profound effects when attempts are made to heat plasma by passing a current through it. As the plasma gets hotter, there is a proportional decrease in the power input. Thus it is very difficult to raise the temperature of hydrogen plasma much above 10000000 K by this method (called ohmic heating).
Plasma is often surrounded by or embedded in a magnetic field. A charged particle moving in a magnetic field experiences a force perpendicular to the directions of both the motion and the field. Thus a charged particle in a uniform magnetic field moving with velocity components perpendicular and parallel to the field traces out a helix centered on a magnetic line of force. The frequency of oscillation is proportional to the magnetic field strength times the particle’s charge-to-mass ratio. This is called the cyclotron frequency. For electrons it is equal to 1.76 * 10 million * B radians per second, where B is the magnetic field strength in gauss. For ions, the equivalent quantity is called the ion cyclotron frequency, but it is different for each species. The radius of the cyclotron helix is called the Larmor radius and is equal to the velocity perpendicular to the magnetic field divided by the cyclotron frequency. The cyclotron motion of a charged particle produces a magnetic field parallel to but opposing the initial field within its orbit. This effect, called diamagnetism, is an important property of plasma. Plasma tends to exclude a magnetic field suddenly applied at its periphery, so a magnetic field pressure can be used to balance the thermal pressure and to contain plasma.
If both electric and magnetic fields are present in the plasma, a particle drift velocity perpendicular to both fields is superimposed on the cyclotron rotation. Other types of drift motion occur if the magnetic field is inhomogeneous, or if a gravitational field is acting on the particles.
The plasma spray gun consists of a circular copper anode and a water-cooled wolfram cathode. A gas (e.g. helium, argon, hydrogen, nitrogen or mixes of these) is emitted into the electric arc formed between the anode and the cathode. The gas becomes superheated and split into ions and electrons. Inside the plasma spray, which has formed, we then introduce metal powder in a carrier gas. The ions and the electrons will merge and the extreme heat (10000 – 25000℃) will melt, vaporize and project the powder onto the substrate.
Thermal spraying of metals – especially aluminum – is increasingly being specified within the offshore industry. Due to the cost involved, these processes are mainly specified for areas with difficult access after the platform has been taken into service or for areas subjected to high temperatures like, e.g. flare booms. Thermal spraying of the underwater area on fishing vessels has been carried out for decades in Norway.
The energy costs per kg steel for electric arc spraying is much lower than for any of the other methods and if the cost is set at a factor of 1, the corresponding energy cost for f.i. high velocity flame spraying would be in the range of factors 80-100.
At the moment, for all purposes in all industries, electric arc and high velocity flame spraying are the processes increasing the most.

CODES AND STANDARDS RELEVANT TO THERMAL SPRAYING OF METALLIC COATINGS. Relevant codes and standards in Europe include the following.
BS EN 22063	Metallic and Other Inorganic Coatings – Zinc, Aluminum and Their Alloys.
BS 5493	Protective Coating of Iron and Steel Structures Against Corrosion, Handling, Transport, Storage and Erection.
DIN 8566	Teil 1 und 2. Zusatse fur das thermische Spritzen.
ISO 1463	Metal and Oxide Coatings – Measurement of Coating Thickness – Microscopical Method.
ISO 2063	Metallic Coatings – Protection of Iron and Steel Against Corrosion – Metal Spraying of Zinc, Aluminum, and Alloys of These Metals.
ISO 2064	Metallic and Other Non-Organic Coatings – Definitions and Conventions.
ISO 2176	Non-Magnetic Coatings on Magnetic Substrates – Measurement of Coating Thickness – Magnetic Method.
ISO 4624	Paints and Varnishes – Pull-off Test for Adhesion.
ISO 8503-1/4	Preparation of Steel Substrates Before Application of Paints and Related Products – Surface Roughness Characteristics of Blast-Cleaned Steel Substrates.
NPD	Guidelines for Corrosion Protection of Installations.
NS 476	Rules for the Approval of Surface Treatment Inspectors.
NS 1975	Rules for the Approval of Surface Treatment.
SS 2626	Thermal Spraying Equipment – Requirements and Testing.