Modern Technology Of Disposal Range Of Materials Engineering Essay

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Modern technology has at its disposal a wide range of materials commonly used for structures or mechanical components. Among these materials are metals and alloys, plastics, rubber, ceramics, composites, wood, e.t.c. and the selection of appropriate material for a given application is the important responsibility of the design engineer.

The prevailing environmental condition during service is given a great deal of attention where metals are involved. Most metals are chemically unstable in corrosive environments, hence the tendency to revert to their more stable unrefined mineral forms. This phenomenon is the process by which corrosion occurs. The integrity of engineering components made of metals and alloys can be seriously compromised by the action of corrosion. Corrosion of metals can be rarely completely avoided, but can be curtailed to ensure the material attain their expected lifetime without risk. More often than not sacrificial and barrier coatings are used to protect metals against corrosion.

Cadmium has been the corrosion resistant material of choice coatings during a long time because of its unique properties. Amongst its many excellent properties are: lubricity, self-healing, anti-seizure, sacrificial and barrier protection. But cadmium is toxic and carcinogenic, therefore its use will be limited in the future due to environmental regulations and alternative coatings that are environmentally friendly are being developed.

Aluminium-based coatings have proven to be capable replacement for cadmium [1-3]. Because they have shown to yield better cathodic corrosion protection of carbon steel in urban, industrial, and particularly in marine climates than other common coating materials such as zinc [ 4-10]. Methods of application of aluminium coatings on steel are not a threat to human and the environment as in the case of cadmium. In addition, aluminium's ranking on the scale of corrosion potentials of metals makes it a good choice for sacrificial protection of steel. These qualities and many others have put aluminium in good stead to protect steel even in a galvanic assembly.

Therefore, the focus of this work is to:

Investigate the barrier and sacrificial protection properties of some commercial Al-based coatings with a view to suggesting an alternative to cadmium as a barrier and sacrificial coating material for steel in simulated seawater environment (0.6 M NaCl solution).

Predict based on the findings of the experiments, the most suitable of the commercially Al-based coating that can provide sufficient and reliable galvanic protection for steel in 0.6 M Nacl solution.

2 Introduction to the Fundamentals of Corrosion

The significant technical challenges, risks, and high cost directly related to corrosion provide strong motivation for engineers, designers, and other technical personnel to develop a firm understanding on the elementary bases of corrosion. Understanding the essentials of corrosion is necessary not only for identifying corrosion mechanisms, but also for preventing corrosion by right corrosion protection means and for predicting the corrosion behaviour of metallic materials in service conditions. Understanding the process of corrosion is vital to the development of a knowledge-based design of corrosion resistant alloys and to the prediction of the enduring behaviour of metallic materials in corrosive atmospheres.

There are two major areas usually distinguished in the corrosion of metals and alloys. The first area is where the metal and alloy is exposed to a liquid electrolyte, usually water, and thus naturally called aqueous corrosion. The second is where corrosion takes place in a gaseous atmosphere, often called oxidation, high-temperature corrosion, or high temperature corrosion. These two areas are referred to as wet and dry corrosion respectively. This dissimilarities finds its origin (and its validation) in some fundamental differences in the mechanisms, in particular the electrochemical nature of reactions taking place in aqueous solution (or in non-aqueous electrolyte), as compared to the formation of thick oxide layers in air or other oxidizing atmospheres, at high temperature with swift transport processes by solid diffusion through a growing oxide. The separation between the two areas, however, should not be overemphasized, because there are also similarities and correlations, for example:

The initial stages of reaction involve the adsorption of chemical species on the metal surface that can be described by the Gibbs equation for both liquid and gaseous environments.

The nucleation and growth phenomena of oxide layers and other compounds

The use of surface analytical techniques

2.1 Thermodynamics

The object of thermodynamics is to examine the driving force for corrosion. Thermodynamics sets the background of what is possible and what is not. It foretells the direction in which changes of the system can occur. The only reactions that can take place unexpectedly are those that will lower the energy of the system. If thermodynamics prediction show that a reaction cannot occur, the reaction will definitely not occur. In spite of this, thermodynamics does not provide information on the rate at which a reaction will occur; that is the area of kinetics.

Corrosion in aqueous solution is an electrochemical activity where the corroding metal is an electrode in contact with an electrolyte. The processes taking place at the metal surface are electrode processes and with the appreciation of this, it is possible to understand the principle of potential-pH (E-pH) diagrams. These diagrams allow us to envisage in a practical and easy way the domains of stability of chemical species (solid phases and dissolved species) and to know at a glimpse what corrosion reaction can occur in a given metal-solution system.

It is necessary to stress the fact that thermodynamics can only be a first step in understanding the corrosion behaviour of a given metal/aqueous solution system. Thermodynamics provides the guide lines. Using it, the possible destinations are clearly known [11].

Figure 1: Pourbaix diagram-E-pH of Aluminium [12]

2.2 Kinetics

Once thermodynamics are understood, it becomes necessary to advance to examine which reactions, among those reactions sanctioned by thermodynamics, will occur and at what rate.

The kinetics of aqueous corrosion examines the relations between the current and the potential connected with each of the two or more electrochemical reactions establishing the mixed system characteristics of corrosion .The current-potential curves (I-E), the corrosion potential, and the corrosion current form the core of the kinetic approach. Here activation energy of the reactions comes into play. The theory of mixed potential is essential. Corrosion occurs if both an anodic and a cathodic reaction take place, each reaction comprising chemical species that relate to a different oxidation-reduction system. Because of this, the corrosion potential is not an equilibrium potential but rather is thought to be a mixed potential.

2.3 Electrochemical Basis of Metal Corrosion

Corrosion of metals is caused by the electrochemical reaction between a metal (or an alloy) and an aqueous phase. It takes place according to a complex electrochemical process that is linked to the atomic structure of matter. Matter is built up from elementary particles that carry electrical charges, namely ions and electrons, and form particles that are electrically neutral, namely atoms and molecules. In metals, the electrical environment of atoms is made up of free electrons capable of moving throughout the metal [13].

In the aqueous phase, which is a solution, the following species can be found:

Positive ions (cations) and negative ions (anions),

Neutral molecules such as water and various unbroken compounds.

At the interface between metal and water, the transfer of electrical charges leads to electrochemical reactions (figure 2):

The metal atom is oxidised and forms Mn+ ions that are released in the aqueous phase. This creates a flux of electrons within the metal in the direction solution → metal. The resulting anodic oxidation current ia flows from the metal to the solution.

The ions or molecules of the aqueous phase are reduced, which means that they take up electrons from the metal and get transformed into another chemical species. This creates a flux of electrons within the direction metal → solution. The resulting cathodic current ik flows from the solution to metal.

2.4 Elementary Electrochemical reactions of Corrosion

The corrosion of a metal is the result of two simultaneous reactions that are in electrical equilibrium:

the oxidation of the metal, resulting in a loss of electrons, according to the fundamental reaction

M → Mn+ + ne-

it results in an anodic current ia that flows in the direction

metal → solution

The reduction of an ion present in the aqueous solution according to the fundamental reaction

Xn- → X + ne-

it results in a cathodic current ic that flows in the direction

solution → metal

The reactions of oxidation and reduction proceed at distinct sites of the metal surface. The surface at which oxidation takes place is called anode. It carries negative charges and is designated by the sign (-); the resulting current is called anodic current. Reduction reaction takes place on a surface called the cathode, designated by the sign (+); the reducing current is the cathodic current.

Except where connected to the electrodes of a generator, the metal is electrically neutral, which means that electron and current fluxes are in equilibrium:

∑ ia = ∑ ic [13]

Figure 2: Electrochemical reactions at the metal-solution interface [14].

2.5 Atmospheric Corrosion of Aluminium

Due to its exceptional resistance to atmospheric corrosion, the use of aluminium in construction, civil engineering, electrical power transmission lines and aviation has been increasing considerably since 1930. Nowadays, aluminium is the second most common metal, after steel, to be exposed to weathering, in all climate and geographic zones.

The resistance of aluminium to atmospheric corrosion has been a very important issue, and, since the early 1930s, has attracted a great deal of attention from corrosion experts working with the major aluminium producers in Europe and North America. The first tests of aluminium in outdoors corrosion testing stations were performed in the United States in 1931 by America Society of Testing Materials (ASTM) [15].

Atmospheric corrosion has been the subject of many publications [16]. The first was by E.Wilson, who reported results of observations made on electrical cables exposed in London over 24 years, beginning in 1902 [17]. Since 1945, many national and international conferences in Europe and in the United States have been devoted to the atmospheric corrosion of aluminium (as well as that of other common metals and alloys such as steel, copper, e.t.c).

From the very beginning of the industrial history of aluminium, several outdoor applications were executed, some of which are still in use: the Eros statue, erected in 1893 on the top of the monument dedicated to the memory of Lord Shaftesbury at Piccadilly Circus in London [18], and the roof sheet of the San Gioacchino Church in Rome, built in 1898. Other applications, mainly in the building sector, were to follow during the 1920s and 1930s both in Europe and in the United States.

The wide spread use of aluminium began in the 1950s in the building sector: claddings, curtain walls, roofing sheet, metallic fittings, as well as in civil engineering: street signs, road signs, street furniture, e.t.c. These applications use both cast and wrought (flat rolled and extruded) products. These products can be used without any protection or can be protected by anodising, painting, lacquering, e.t.c.

The present knowledge of the resistance of aluminium to atmospheric corrosion has solid foundations, based on two complementary approaches:

Testing in outdoor corrosion stations in Europe and the United States (where most, and the oldest, of these stations can be found), often for a very long time: 10, 20 years, and sometimes even longer, and under three classic atmospheric conditions- marine, industrial and rural;

Experience with applications over several decades: some of the oldest and most prestigious have been reported by aluminium producers and transformers.

The understanding of the atmospheric corrosion of metals is based on theoretical foundations that are special case of corrosion of metals and alloys.

2.6 Nature of Atmospheric Corrosion

Atmospheric corrosion is the deterioration of a metal (or an alloy) by the atmospheric environment to which it is exposed. This corrosion is initiated by the simultaneous attack by rainwater or condensing water, oxygen contained in the air, and atmospheric pollutants. Atmospheric corrosion is a special type of corrosion because the electrolyte is represented by thin film moisture, whose thickness does not exceed a few hundred micrometers. It can be assumed that such a film is always saturated with oxygen, and the diffusion is not hindered. This type of corrosion may be intermittent, because it stops when the surface of the metal is no longer humid. When immersed in water or in a salt solution, the metal is in permanent contact with the electrolyte, but the corrosion may be slowed down by the weak diffusion of oxygen to cathodic sites.

The first theoretical explanation of atmospheric corrosion of metals was given by Vernon [14] and Hudson [19] starting in 1923 and was completed later by Rozenfeld in the 1960s [20] and by Greadel for aluminium in the 1980s [21]. Vernon introduced the concept of the critical degree of moisture, the threshold below which practically no corrosion will occur. The value of this threshold depends on several factors such as the nature and concentration of atmospheric pollutants and the metal's surface condition.

2.6 The electrochemical reactions in the corrosion of Aluminium

The fundamental reactions of the corrosion of aluminium in aqueous medium have been the subject of many studies [11, 12]. Basically, the oxidation of aluminium in water proceeds according to the reaction:

Al → Al3+ + 3e-

Metallic aluminium, in oxidation state 0, goes in solution as trivalent cation Al3+ when losing three electrons.

The reaction is balanced by a concurrent reduction in ions present in the solution, which capture the discharged electrons. In common aqueous media with a pH close to neutral such as fresh water, seawater, and moisture it can be shown by thermodynamic theory that only two reduction reactions can occur:

Reduction of H+ protons:

3H+ + 3e- → 3/2H2

H+ protons result from the dissociation of water molecules:

H2O ↔ H+ + OH-

Reduction of oxygen dissolved in water:

In alkaline media: O2 + 2H2O + 4e- → 4HO-

In acidic media: O2 + 4H+ + 4e- → 2H2O

Generally, the corrosion of aluminium in aqueous media is the sum of two electrochemical reactions, oxidation and reduction:

Al → Al3+ + 3e-

3H+ + 3e- → 3/2H2

Al + 3H+ → Al3+ + 3/2H2

Or

Al + 3H2O → Al (OH)3 + 3/2H2

This reaction is followed by a change in the oxidation state of aluminium which, from the oxidation state 0, in the metal is changed into the oxidation state of alumina (+3), and by an exchange of electrons, since aluminium loses three electrons that are picked up by 3H+.

3 Galvanic Corrosion

Galvanic corrosion (also called "dissimilar metal corrosion" or "bimetallic corrosion") is the corrosion damage induced when two dissimilar metal are coupled in a corrosive electrolyte. In a bimetallic couple, the less noble material becomes the anode and tends to corrode at an accelerated rate, in comparison to the uncoupled condition and the more noble material will act as the cathode in the corrosion cell. A necessary condition for galvanic corrosion to occur is that there is an electrolytic connection between the metals, so that a closed circuit is established (figure 3). The sacrificial corrosion of metals such as zinc, magnesium and aluminium has thus become a widespread method of cathodically protecting steel structures and components.

Fig.3 Galvanic corrosion showing electrolytic connection between two metals [einar badal] p 95. Igalv (galvanic current-anode and galvanic current-cathode respectively)

3.1 Galvanic series

The potential of a metal in a solution is associated to the energy that is released when the metal corrodes. Variation in corrosion potentials of dissimilar metals can be measured in specific environments by gauging the direction of the current that is generated by the galvanic action of these metals when exposed in a given environment. The order of metals in a galvanic series based on observations in seawater, as shown in figure 4, is frequently used as a first estimate of the probable direction of the galvanic effects in other environments.

In a galvanic couple between any two metals in a galvanic series, corrosion of the metal higher in the list is likely to be enhanced, while corrosion of the metal lower in the list is likely to be reduced. Metals with more positive corrosion potentials are called noble or cathodic, and those with more negative corrosion potentials are referred to as active or anodic.

Values of potential can change from one solution to another or in any solution when swayed by such factors as temperature, aeration, and velocity of movement. Accordingly, there is no way, other than by direct measurements in the exact environment of interest, to predict the potentials of the metals and consequent direction of galvanic effect in that environment. As an example, zinc is normally very negative or anodic to steel at ambient temperatures, as indicated in the galvanic series shown in figure 4. But, the potential difference decreases with an increase in temperature until the potential difference may be zero or actually reversed at 60oC [23, 24].

Figure 4: Galvanic series for seawater. Dark boxes indicate active behaviour of active-passive alloys [11].

3.2 Predicting Galvanic corrosion

The most common method of predicting galvanic corrosion is by immersion testing of the galvanic couple in the environment of interest. Although, very time consuming, this is the most appropriate method of investigating galvanic corrosion. Initially, screening tests are carried out to eliminate as many as possible candidate material. These screening tests comprise of one or more of the following three electrochemical techniques: potential measurements, current and polarization measurements.

3.2.1 Potential Measurement

Potential measurements are made to construct galvanic series of metals and alloys. As a first estimate, the galvanic series is a helpful tool. However, it has serious limitations. Metals and alloys that form passive films will give varying potentials with time and are therefore difficult to position in the series with certainty. Also, the galvanic series does not give information on the polarization characteristics of the materials and so is not helpful in predicting the probable degree of galvanic effect.

3.2.2 Measurement of Galvanic Current

Measurement of current between coupled metals is based on the use of a zero-resistant milli- ammeter. Zero-resistance electrical continuity between the members of the galvanic couple is kept electronically, while the resulting current is measured with the ammeter. Use of this technique should consider certain limitations. First, when localized corrosion such as pitting or crevice corrosion is feasible in the galvanic couple, long induction periods may be required before these effects are observed. Test periods must be of sufficient duration to take this effect into consideration. Also the measured current is not always a true measure of the real corrosion current, because it is the algebraic addition of the currents due to the anodic and cathodic reactions. When cathodic current are appreciable at the mixed potential of the galvanic couple, the measured galvanic current will be substantially lower than the true current. Therefore, large differences between the true corrosion rate calculated by weight loss and that obtained by galvanic current measurements have been observed [11]

3.3.3 Polarization Measurement

Polarization measurement on the members of a galvanic couple can provide accurate information concerning their behaviour. The polarization curves and the mixed potential for the galvanically coupled metals in a given environment can be used to determine the degree of the galvanic-corrosion effects as well as the type of corrosion.

A crucial application in the use of polarization measurements in galvanic corrosion is the prediction of localised corrosion. Polarization techniques and critical potentials are used to measure the vulnerability to pitting and crevice corrosion of metals and alloys coupled in chloride solutions. In addition, this method is valuable in predicting galvanic corrosion among three or more coupled metals and alloys.

4 Corrosion Protection by Coatings

Through the application of coatings, corrosion is prevented by one of the following three main mechanisms or by combination of two of them:

Barrier effect, where any physical contact between the corrosive medium and metallic material is prevented

Cathodic protection, where the coating material acts as a sacrificial anode

Inhibition/passivation, including cases of anodic protection

4.1 Metallic Coatings

Metallic coatings can be divided in two groups, the cathodic coatings, which are nobler than the substrate, and the anodic ones, which are active than the substrate, i.e. the coatings that have, respectively, a higher and a lower corrosion potential than the substrate in the environment in question. The cathodic coatings regularly act by the barrier effect only, but for some combinations of substrate and environment the substrate can also be anodically protected (on uncovered spots). The anodic coatings will in addition to the barrier effect offer cathodic protection of possible coating defect or parts of the surface where the coating is imperfect and the substrate is exposed to the corrosive environment. The main difference between a cathodic and an anodic coating is the behaviour at such a defect as shown in figure 5. In the situation of a cathodic coating (a), the substrate is subject to galvanic corrosion in the coating defect. The corrosion may be rather intense because the area ratio between the cathodic coating and the anodic spot of bare substrate is usually very high. In the other case (b), only a cathodic reaction occurs on the bare substrate (i.e. it is protected cathodically), while the coating is liable to a corresponding galvanic corrosion distributed over a larger surface area. So as to protect the substrate, low porosity, high mechanical strength and continuous adhesion are even more necessary for a cathodic than for an anodic coating.

Figure 5: Localization of corrosion at a defect in a metal coating on steel. a. cathodic coating. b. Anodic coating [22 p283].

With respect to steel, Ag, Cr, Ni and Pb are cathodic, while Cd and Zn are anodic in most environments. The polarity of Al and Sn in relation to steel varies from one environment to another.

4.1.1 Aluminium in Contact with Steel.

Even though steel corrodes faster than aluminium in environments such as seawater, marine and industrial atmospheres, and water containing SO2, and soft water, aluminium is the material that primarily corrodes when these two materials are galvanically coupled in the said environments. The reason for this contradiction is that the corrosion potential of steel is higher than aluminium when the materials are separated. On coupling the two, a mixed potential is established which is lower than the separate steel potential and above the corrosion potential of aluminium. Particularly in seawater, the corrosion of aluminium is accelerated significantly by galvanic contact. But the situation may be more complex in some other environments; aluminium may in one case be anodic and in another be cathodic to steel. In pores in thermally sprayed aluminium on steel in moist environments, the following trend may occur in sequence:

The coating acts as a cathode due to the oxide, which prevents conductance of ions but allows electronic conductance to some degree

Due to increased pH at the cathode, solubility of the aluminium oxide increases. Hence, metallic aluminium is exposed to the solution and the coating becomes nobler than steel and begins to act as an anode.

The corrosion products of aluminium gradually block the pores in the coating, the galvanic element is made ineffective and the coating becomes stable [ 22]

5 Cadmium Sacrificial Coatings of Steel and its Alternatives

Some publications about alternative to cadmium have shown that aluminium and modified aluminium can provide improved sacrificial protection to steel. The limitation of these discoveries lies in the cost of deposition of the coating on steel rather than the anti-corrosion properties. To further assist with the identification of cadmium replacements, key technical performance, environmental, and economic data have been collected and summarised in table 1. These data are presented in a qualitative matrix in which the performance of alternatives is rated with respect to that of cadmium coatings [25]

5.1 Previous Studies on Cadmium replacement

5.1.1 Ion Vapour Deposition (IVD) Aluminium

Ion vapour deposition is a physical vapour deposition technique also referred to as "ion plating", first reported as such by Mattox in early 1960s [26-28]. This technique is used widely in the aerospace industry for applying pure aluminium coatings to various parts, mainly for corrosion protection. The use of IVD aluminium coatings does in fact date back to the early 1970s [29] and the deposition process has changed very little since then. McDonnell Douglas Aerospace (MDA)-St. Louis [30-35] started to eliminate many cadmium and zinc plating processes and replaced them with the IVD process. MDA developed production equipment for low cost aluminium plating [36-39], this equipment utilized the ion plating process and the technique became known as Ivadizing. .

The IVD process is similar to modern PVD systems, but with one main difference: during plating, the substrate is held at a high negative potential (1 kV) with respect to the vacuum chamber and evaporation source. This potential produces a DC glow discharge of inert argon gas in the deposition chamber for substrate cleaning by argon ion bombardment (called sputter cleaning). Following glow discharge cleaning, aluminium wire is evaporated by being

Cadmium

replacement

Specifications

Corrosion control performance

Environment-assisted cracking

Coating lubricity

Environmental/

worker health regulations

Cost/

Performance factors.

Zn, Zn-Ni,

Zn-Co, or ion vapour deposited aluminium coating with lubricious topcoat

Commercial;

MIL-C-83488 with MIL-C-81751

Better/comparable

Worse/comparable

Comparable or more lubricious

Better, although some lubricious topcoats may be subject to volatile organic compound regulations

Significantly higher cost

Solid alloy without coating (e.g., Nickel alloy, stainless steel)

QQ-N-21,QQ-N-286 (nickel alloy); AISI 304, AISI 316 (stainless steel)

Better (stainless steel may pit)

Better

Not applicable; stainless steel and nickel alloys have been used as fasteners

Better, no coating process

Initial cost significantly higher; life cycle cost may be comparable

Zinc plating

ASTM B 633

Comparable/worse

Worse

Less/comparable

Better, although the type II or III chromate post-treatment is hazardous

Initial costs lower; zinc coatings generate voluminous white corrosion products

Zinc-nickel plating

ASTM B 841; commercial

Comparable

Worse/comparable

Less/comparable

Better, although nickel is included in environmental protection agency "Toxic 17" list; chromate post-treatment hazardous

Initial costs may be higher; life cycle costs comparable

Tin-zinc plating

Commercial

Comparable/worse

Worse/comparable

Less/comparable

Better, chromate post-treatment hazardous

Initial costs may be higher; life cycle costs comparable

Ion vapour deposited aluminium

Commercial MIL-C-83488

Comparable

Worse/comparable

Less; ion vapour deposited aluminium coatings on threaded fasteners can gall

Better, chromate post-treatment hazardous

Significantly higher costs.

Table1:Cadmiumreplacementidentificationmatrix[25]

continuously fed into resistant-heated crucible. As the aluminium vapour passes through the glow discharge, a portion of it becomes ionized. This, in addition to collision with the ionized argon gas, accelerates the aluminium vapour towards the substrate surface, resulting in improved coating density, adhesion and uniformity. Al-magnesium recently deposited with this technique, have been reported to have displayed improved corrosion resistance at approximately 20-wt% Mg [40].

5.1.2 Sermetels Coatings

Figueroa [ ] described sermetel type of aluminium-based coatings as ''aluminium particles in a chromate/phosphate inorganic binder''. Others [41], believed aluminium is used by its incorporation in a powder or flake form in an inorganic slurry based on phosphates or chromates to produce sermetel coatings which has found use on landing gear components as a substitute for cadmium.

The classes of sermetel coatings are determined by the degree of post curing or surface alteration to the cured coating [42]. Post curing indicates a further heating requirement after curing, while burnishing (surface alteration) indicates a mechanical abrading or buffing of the finished surface after curing. Details of the techniques for applying sermetel coatings are still sketchy. K.R.Baldwin [41], gave an indication that sermetel CR962 and sermetel CR984-LT were sprayed as slurry. It was also reported [26], that sermetel 1140/962 were applied as two spray coats, each being cured at 3150C with a thickness of 70µm in a non-electrolytic process.

Figueroa [43] evaluated the suitability of Sermetel 1140/962 as a sacrificial coating for steel in comparison with Zn-14%Ni and cadmium; the coatings were subjected to open-circuit potential measurements in quiescent 3.5% NaCl solution for 1200h exposure period under slow strain rate test. It was observed that the final potential recorded for sermetel 1140/962 (-650mV SCE) is too close to that of freely corroding steel. Hence the coating was not considered active enough to protect the steel substrate, while Zn-14% Ni give a final potential of -960 mV (SCE) with a network of fine intergranualr cracks due to selective leaching along the columnar grains. In the same report, sermetel 1140/962 was found to be more reliable than Zn- 14% Ni in terms of re-embitterment.

5.1.3 Thermal Spray Coatings - HVOF and Arc Spray

Thermal spraying is a generic term which explains a group of processes in which metallic, ceramic, cermets (and some polymeric) materials in the form of powder, wire, or rod are fed to a torch or gun with which they are heated to near (or a bit above) their melting point.

The resulting molten or semi-molten droplets of the material are accelerated in a gas stream and projected against the surface to be coated (i.e. substrate). On impact, the droplets flow into thin lamellar particles, which adhere to the surface, overlapping and interlocking as they solidify. The total coating thickness is usually generated in multiple passes of the spraying device [44].

Figure 5: Schematic of the general thermal spray process [l1]

A number of thermal spraying technologies are used commercially (each with their own distinct features and advantages/disadvantages).

The main technology groups are:

Flame spraying

Electric arc (wire-arc) spraying

Plasma spraying

High velocity oxy-fuel (HVOF)

Detonation gun

Plasma transferred arc

Thermal spray coatings, primarily zinc and aluminium, have been successfully used to prevent corrosion in a wide range of applications. Steel structures and components that have been sprayed by zinc or aluminium includes TV towers, antennae, radars, bridges, light poles, girders, ski lifts, and numerous other similar structures. In addition, thermal spray coatings, primarily aluminium, offer years of protection in marine applications, such as buoys and pylons. Aluminium spraying has been used in offshore oil rigs for well head assemblies, flare stacks, walkways, and other structural steel components. Shipboard components, both above and below deck, also use aluminium spraying for protection against corrosion. The United States in particular, uses aluminium spraying extensively to combat corrosion [45]. There are numerous approved applications for sprayed metal coatings aboard Navy ships [46].

Countless immersion applications have also employed zinc or aluminium spraying, for example, dams and sluice gates. Aluminium has also been used to control chemical corrosion in such applications as storage tanks for fuels or other corrosive liquids. The interiors of railroad hopper cars are often sprayed to protect them against sulphuric acid (H2SO4) corrosion attack when hauling coal [11]

The electric-arc (wire-arc) spray process uses metal in wire form. This differs from the other thermal spray processes in that there is no external heat source such as gas flame or electrically induced plasma .Heating and melting occur when two electrically opposed charged wires, comprising the spray material, are fed together in such a manner that a controlled arc occurs at the intersection. The molten metal on the wire tips is atomised and forced onto a prepared substrate by a stream of compressed air or other gas. This set up is shown in figure 6

Figure 6: Typical electric arc spray device [11]

Electric-arc coatings are widely used in high-volume, low-cost applications such as use of zinc corrosion-resistant coatings. In a more common application, metal-face moulds can be made using a fine spray attachment available from some manufacturers [11]

5.1.4 Electroplated Aluminium

The electrodeposition of aluminium coating from aqueous solutions is impossible due to the hydrogen discharge which occurs before the deposition of the metal. High vacuum techniques such as chemical vapour deposition (CVD) and physical vapour deposition (PVD) are slow and expensive. Electrodeposition from molten salts is widely used for aluminium production (process Heroult-Hall) [47], but high temperatures required, higher than aluminium melting point, makes this process useless for plating. Therefore, there are limited numbers of publications on the use of electroplated aluminium as sacrificial coatings.

Ionic liquids (ILs) are a relatively new class of compounds characterised by high conductivity, extremely low vapour pressures, low viscosity, low toxicity, non flammability, high thermal stability, wide electrochemical advantage and being liquid in a wide range of temperatures [48, 49]. These classes of liquids (ILs) are currently being used to deposit aluminium coatings [47, 50, 51], some of which have proven to have promising anti-corrosion properties. A.P Abbott [52] claimed that these properties make ILs the ideal media for electro reduction, at a room or close to the room temperature, of highly electropositive metals which cannot be electrodeposited from aqueous media. ILs has been used for electro reduction of pure aluminium and aluminium alloys [53-55]. Stefano et al.[47] obtained aluminium coatings at room temperature from 1-butyl-3-methyl-imidazolium heptachloroaluminate ([BMIM]Al2Cl7) and this proved an effective protection of carbon steel against wet corrosion after subjecting the coatings to salt spray test and potentiodynamic polarization test in 3.5% NaCl. Liu et al. [50] has also deposited aluminium on mild steel from 1-ethyl-3-methyli-midazolium chloride [EMIm] Cl/AlCl3. The outcome was a well adherent aluminium coating on mild steel which can resist even mechanical scratching.

Barchi et al,[51] studied electrodeposited layers of aluminium as anti-corrosion protective coatings on metallic substrates. Aluminium was deposited on carbon steel substrates from chloroaluminate based ionic liquids (ILs). Open-circuit potential and potentiodynamic polarization techniques as well as free corrosion tests, carried out in salt-spray cabinet were used to evaluate the coating. This investigation showed that the aluminium coatings obtained from the ILs provides nearly the same corrosion resistance as Ni-Cr, with advantage to being totally free of harmful metals.

Kautek [1], investigated the galvanic interaction between carbon steel and six galvanic coating materials including electroplated aluminium, cadmium, zinc and duplex combinations with copper, nickel and tin. Galvanic couples obtained from these materials were immersed in argon and air saturated sulphate and chloride solutions of various acidity. Potentiodynamic techniques and continuous monitoring of the galvanic current were employed to determine the compatibility of the different galvanic couples as well as other anti-corrosion properties of the coatings. It was revealed that steel can cathodically be protected, not only by cadmium and zinc, but also by aluminium in moist, urban, industrial and marine atmospheres.

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