Protection of wood

Protection  of wood

The two main types of protection for woods are barrier and chemical.

Barrier coatings can be applied to the wood to both preserve and seal the wood these coatings are usually made from epoxy resins and Oils. For example Tung oil penetrates the wood then hardens to form a hydrophobic layer up to 5mm thick.

Chemical protection can be classified into 3 different types of categories. These are;

• Oil Bourne preservatives – The most common oil-type preservatives are creosote, pentachlorophenol, and copper naphthenate. Occasionally, oxine copper and IPBC (3-iodo-2-propynyl butyl carbamate) also are used for aboveground applications. The conventional oil-type preservatives, such as creosote and pentachlorophenol solutions, have been confined largely to uses that do not involve frequent human contact. The exception is copper naphthenate, a preservative that was developed more recently and has been used less widely. Oil-type preservatives may be visually oily, or oily to the touch, and sometimes have a noticeable odour. However, the oil or solvent that is used as a carrier makes the wood less susceptible to cracks and checking. This type of preservative is suitable for treatment of glue-laminated stringers for bridges where cracks in the stringers could alter the bridges’ structural integrity.
• Water Bourne preservatives – Waterborne preservatives react with or precipitate in treated wood, becoming “fixed.” They resist leaching. Because waterborne preservatives leave a dry, paintable surface, they are commonly used to treat wood for residential applications, such as decks and fences. Waterborne preservatives are used primarily to treat softwoods, because they may not fully protect hardwoods from soft-rot attack. Most hardwood species are difficult to treat with waterborne preservatives. These preservatives can increase the risk of corrosion when metals contact treated wood used in wet locations. Metal fasteners, connectors, and flashing should be made from hot-dipped galvanized steel, copper, silicon bronze, or stainless steel if they are used with wood treated with waterborne preservatives containing copper. Aluminum should not be used in direct contact with wood treated with waterborne preservatives containing copper. Borates are another type of waterborne preservative. However, they do not fix in the wood and leach readily if they are exposed to rain or wet soil. Borate treatment does not increase the risk of corrosion when metals contact preservative-treated wood. Examples of water bourne preservatives are ; Chromated Copper Arsenate (CCA),Ammoniacal Copper Zinc Arsenate (ACZA), Alkaline Copper Quaternary (ACQ) Compounds, Copper Azoles (CBA-A and CA-B), Borates, Other Waterborne Preservatives
• Light organic solvent preservatives – This class of timber treatments use white spirit, or light oils such as kerosene, as the solvent carrier to deliver preservative compounds into timber. Synthetic pyrethroids are typically used as an insecticide, such as permethrin, bifenthrin or deltamethrin. While still using a chemical preservative, this formulation contains no heavy-metal compounds. With the introduction of strict volatile organic compound (VOC) laws in the European Union, LOSPs have disadvantages due to the high cost and long process times associated with vapour-recovery systems. LOSPs have been emulsified into water-based solvents. While this does significantly reduce VOC emissions, the timber swells during treatment, removing many of the advantages of LOSP formulations.

 

Most commonly used chemicals for Oil bourne preservatives is Creosote. Creosote is Coal-tar and is one of the oldest known wood preservatives. It is made by carbonising coal at high temperatures. Unlike most of the other oil preservatives Creosote is not dissolved in oil, but is naturally a mixture of organic molecules giving it its place in the oil bourne preservatives. The creosote gives wood a dark brown to black colour. Cresote However, creosote-treated wood has advantages to offset concerns with its appearance and odor. It has a lengthy record of satisfactory use in a wide range of applications and is relatively inexpensive. Creosote is effective in protecting both hardwoods and softwoods and improving the dimensional stability of the treated wood.

 

Most commonly used chemicals for Water bourne preservatives is CCA. CCA is Chromated Copper Arsenate can be used on wood above ground below ground and in contact with sea or freshwater.It usually has a greenish colour due to the chromium. It has a basic chemical composition of 47.5% Chromium trioxide 18.5 Copper oxide and 34%Arsenic pentoxide dissolved in water. Copper acts a primary fungicide, arsenic is a secondary fungicide and also an insecticide and Chromium acts as U.V resistance (as wood is a natural polymer). The risk of chemical exposure from wood treated with CCA is minimized after chemical fixation reactions lock the chemical in the wood. The treating solution contains hexavalent chromium, but the chromium reduces to the less toxic trivalent state within the wood (Hexavalent Chromium is carcinogenic, new regulations are enforcing that other alternatives need to be looked at). This process of chromium reduction also is critical in fixing the arsenic and copper in the wood.

 

Ecosystem Sensitivities

Although largely undocumented, some preservatives may be more appropriate than others in sensitive ecosystems). For example, CCA has a much lower copper content than other waterborne preservatives (except the borates). Although there is no evidence at this time to suggest that any of the wood preservatives leach enough copper to harm terrestrial or freshwater ecosystems, CCA may pose less of a threat to aquatic ecosystems than preservatives with more copper. Similarly, preservatives without arsenic may pose less of a threat to mammals than those that do contain arsenic. Once again, there is no evidence that wood preservatives containing arsenic harm people or other mammals if they are used as intended. Wood treated with oil borne preservatives often produces an oily surface sheen when installed in stagnant freshwater environments. This may be unacceptable in some situations. Waterborne preservatives may be more appropriate when the treated wood will have extensive contact with freshwater

 

Selecting a Wood Preservative

The type of preservative that is most appropriate depends on the species of wood being treated, the type of structure, the cost, the availability of treated wood, and the specific area where the wood will be used. Generally, hem-fir (hemlock and fir) and southern pine can be treated adequately with any of the commercial wood preservatives, although copper napthenate has not been standardized for use with hem-fir. CCA is not recommended for treatment of Douglas-fir, which is more readily treated with oil-type or ammoniacal preservatives. CCA is not recommended for treating hardwoods that will contact the ground.

 

References

http://www.fs.fed.us/t-d/pubs/htmlpubs/htm06772809/page02.htm#cca

http://en.wikipedia.org/wiki/Wood_preservation

Steel rebars in concrete

Task: Literature review steel rebar in cement

Question – For your individual blog portfolio, write a brief literature on the corrosion of steel rebar’s in concrete. You should consider the environment that exists within the concrete and the corrosion mechanisms that may occur. You do not need to review corrosion prevention methods, as these will be covered in a later lecture

Books and journals read

• Corrosion: Volume 1, L.L Shreir, R.A. Jarman, G.T Burstein, Butterworth – Heinemann, 1994 (3^rd Ed)
• Corrosion of steel in concrete: Understanding investigation and repair, John.P.Broomfield, Spon Press, 1996
• The influence on water on the performance of a concrete, K.C Hover,Elseveir, 2011
• Durability of steel re-enforced concrete in chloride environments: An Overview, Xinming Shi, Ning Xie, K.Fortune, Jing Gong, Elsevier, 2011

Environments and Corrosion mechanisms that occur in OPC

The environments in which steel rebar exist in ordinary Portland cement concrete was identified and discussed in two of the literatures I read these literatures discussed the pH of the environment and more specifically what this did to the steel rebar held within the concrete ‘When we say that concrete is alkaline we mean that it contains microscopic pores with high concentrations of soluble calcium, sodium and potassium oxides. These oxides form hydroxides which are very alkaline when water is added. This creates a very alkaline condition (pH12-13)’ (Broomfield, 1996). This paragraph from J.Broomfield is back up and expanded on ‘Concrete made with ordinary Portland cement is alkaline material having a pH of 12.6-13.5. Steel embedded in such material will be passive however, like most alkaline materials concrete will react with the acid gases in the atmosphere e.g Sulphur dioxide and carbon dioxide will react reducing the alkalinity’ (L.L Shreir, 1994). Both L.L Shreir & J.Broomfield both agree that alkaline environment creates a passive layer but the passive layer can be degraded by Carbon and Sulphur dioxide. ‘However the principle is fairly simple, especially where chlorides present. At some suitable site steel surface (often thought to be a void in the cement paste or a sulphide inclusion in the steel), the passive layer is more vulnerable to attack and an electrochemical and potential difference attacks chloride ions. Corrosion is initiated and the acids are formed: Hydrogen sulphide from the Sulphide MnS inclusion and the HCl from the chlorine irons if present. Iron dissolves and the iron in solution reacts with water’ (Broomfield, 1996).

Fe²⁺ + H₂O to FeOH⁺ + H⁺

MnS +2H⁺ to H₂S + Mn²⁺                       

Other impurities or contaminants which can cause failure are Chlorides. Which is given an overview in (Xianming Shi, 2011)  where it is stated ‘Chloride induced rebar corrosion is one of the major forms of environmentally attack to reinforce concrete, which may lead to the reduction in strength, severability and aesthetics (Xianming Shi, 2011)’. Chlorine can either be introduced at the casting stage usually as setting accelerators or they can be diffused in from the outside environment ‘Chlorides can diffuse into concrete as a result of:

• Sea salt spray and direct sea water wetting
• Deicing salts
• Use of chemicals’ (Broomfield, 1996)

These chlorides form little galvanic cells which cause the corrosion of the steel rebar; once the corrosion has started the reaction is exacerbated causing further increased corrosion.

The influence of water on the performance of concrete is key and can be examined using different software ‘The power of water in influencing permeability, and thus the rate that salt water intrusion to the reinforcing steel can be examined via the ‘life-365 software’ (Hover, 2011) this is backed up with the statement ‘ In very dry concrete corrosion may not occur even at very high Cl- Concentrations as the waters is missing from the corrosion reaction’ (Broomfield, 1996) this shows that reducing the things needed for corrosion to take place reduces/suppresses the rate of corrosion.

Bacterial corrosion occurs when cement is in contact with soil. Bacteria converts sulphur and sulphides to sulphuric acid. Different bacterial species attack in different ways ‘ Ferrobacilli that attacks the sulphides in steel’ (Broomfield, 1996)

Stray induced corrosion historically was blamed for all concrete corrosion the main causes were ‘Stray current induced corrosion was the direct current flowing through reinforcing steel due to the DC traction systems on trams (street cars) and electric trains’ (Broomfield, 1996) these are limited with cathodic protection systems.

Macro or micro cells

Bbliography

Broomfield, J., 1996. Corrosion   of steel in concrete : Understanding, investigation and repair. s.l.:Spon   Press.

Hover, K., 2011. Construction and Building Materials.   The influence of water on the performance of concrete, pp. 3003-3012.

L.L Shreir, R. J. G. B., 1994. Corrosion : Volume   1. 3rd ed. s.l.:Butterworth – Heineman.

Xianming Shi, N. X. K. F. J. G., 2011. Construction   and Buildng materials. Durability of steel reinforced concrete in chloride   environments: An overview, pp. 126-138.

Corrosion prevention through alloying

Corrosion prevention by alloying

 

1. State which alloying elements of stainless steel are added to improve corrosion resistance and also what elements are added to prevent particular forms of corrosion.

Stainless Steels comprise a number of alloying elements according to the specific grade and composition. The main elements added to different grades of stainless steel are C, Mn, Cr, Ni, Mo, N, Ni Cu, Ti ,P ,S ,Se ,Nb ,Si, Co, Ca. These are all added for different reasons some are to do with strengthening and forming precipitates whereas other are for corrosion resistance. The main alloying elements added for corrosion resistance are listed below these are linked with effects on mechanical properties.

Chromium (Cr): Chromium is added to steel to increase resistance to oxidation. This resistance increases as more chromium is added. ‘Stainless Steels have a minimum of 10.5% Chromium (traditionally 11 or 12%). This gives a very marked degree of general corrosion resistance when compared to steels with a lower percentage of Chromium. The corrosion resistance is due to the formation of a self-repairing passive layer of Chromium Oxide on the surface of the stainless steel.

Nickel (Ni): Nickel is added in large amounts, over about 8%, to high Chromium stainless steels to form the most important class of corrosion and heat resisting steels. These are the Austenitic stainless steels, typified by 18-8 (304/1.4301), where the tendency of Nickel to form Austenite is responsible for a great toughness (impact strength) and high strength at both high and low temperatures. Nickel also greatly improves resistance to oxidation and corrosion.

Molybdenum (Mo): Molybdenum, when added to chromium-nickel austenitic steels, improves resistance to pitting and crevice corrosion especially in chlorides and sulphur containing environments.

Nitrogen (N): Nitrogen has the effect of increasing the Austenite stability of stainless steels and is, as in the case of Nickel, an Austenite forming element. Yield strength is greatly improved when nitrogen is added to stainless steels as is resistance to pitting corrrosion.

Copper (Cu): Copper is normally present in stainless steel as a residual element. However, it is added to a few alloys to produce precipitation hardening properties or to enhance corrosion resistance particularly in sea water environments and sulphuric acid.

Titanium (Ti): Ttitanium is added for carbide stabilization especially when the material is to be welded. It combines with carbon to form titanium carbides, which are quite stable and hard lo dissolve in steel, which tends to minimise the occurrence of inter-granular corrosion. Adding approximately 0.25 / 0.60% titanium causes the carbon to combine with titanium in preference to chromium, preventing a tie-up of corrosion-resisting chromium as inter-granular carbides and the accompanying loss of corrosion resistance at the grain boundaries. However, the use of titanium has gradually decreased over recent years due to the ability of steelmakers to deliver stainless steels with very low carbon contents that are readily weldable without stabilisation.

Phosphorus (P): Phosphorus is usually added with sulphur, to improve machinability. The Phosphorus present in Austenitic stainless steels increases strength. However, it has a detrimental effect on corrosion resistance and increases the tendency of the material to crack during welding.

Sulphur (S): When added in small amounts Sulphur improves machinability. However, like Phosphorous it has a detrimental effect on corrosion resistance and weldability.

2. Make a bullet point list of Alloying strategies that can be used to prevent dezincification

• Alloying copper and Zinc in the correct weight percentage’s i.e. 85% Cu 15%Zn is regarded resistant to de-zincification.
• Alloying the brass with 0.020 – 0.6% As or P are considered resistant to de-zincification
• Other posssiblities can include Cathodic protection, liners or coatings

 

References

C,Campbell (2008). Elements of Metallurgy and Engineering Alloys. USA: ASM International.

High Temperature Surface Modification

High Temperature Surface modification

Can a high temperature reaction with the environment be beneficial?

Yes –  a high temperature reaction with the enivronment can be beneficial and many surface technology processes use this for example Carburising and Nitriding. Both these processes hardnen the materials ssurface by increasing the temperature around them and introducing another element.

 

Are there instances where surfaces are deliberetly modified in this  way?

Yes – these are named above but there are also many more for example, , Carbonitriding, Cyaning & Ferrocarburizing. Flame hardneningis also a high temperature surface modification where the surface hardening process in which heat is applied by a high temperature flame followed by quenching jets of water. It is usually applied to medium to large size components such as large gears, sprockets, slide ways of machine tools, bearing surfaces of shafts and axles, etc. Steels most suited have a carbon content within the range 0.40-0.55% this changes the presence of phases rather than adding an addition to the surface).

 

How is this achieved and what are the benefits?

The general process of high temperature hardneing are the same and carburizing is the most commonly used. Carburizing is achieved by heating steel in a furnace. By means of a carbonaceous medium (gas or salts) the outside layer of a carbon poor component is enriched with carbon by means of carbon diffusion. The increase of carbon content causes the material to harden. The result is a hard and wear resistant surface with a tough core. The temperature is relatively high since in austenite, the high temperature modification of iron, carbon is more soluble than in ferrite, the low temperature modification of iron. In carbonitriding besides carbon, nitrogen is also dissolved in the material surface. The carbonitriding layer is thinner than a carbonised layer.

The Advantages are ;

• Hard layer
• tough core
• Good adherence by diffusion
• Cheap process

Disadvantages

• Dimensional deviations by the high temperature process and the hardening treatment
• Selective increase of the angle points;
•  Expensive machining.
•  The dimension of the work piece is limited to the size which fits the hardening furnace.

Portfolio Task 2 – Component failures

Portfolio Task 2

FiliForm corrosion of lacquer coated steel

Fig.1 Showing Filiform corrosion on a Steel component coated in lacquer

Filiform corrosion is a type of localized corrosion that is often associated with aluminum and magnesium alloys that have an organic coating. This type of corrosion has in the above Fig.1 instance has occurred on zinc coated steel. Filiform corrosion can be exacerbated when the component is subject to high humidity, e.g. greater than about 75% and temperatures at or slightly above room temperature. The corrosion appears as thread-like filaments under the coating. The corrosion products cause the coating to bulge giving the surface the appearance shown above. The filaments proceed from points where the coating is no longer continuous. Numerous coating systems are susceptible. Condensates containing halides, sulfates, carbonates, or nitrates have been associated with filiform corrosion. Damage to the metal tends to be limited but the effect on appearance tends to be detrimental.

 

Fig.2 Showing the mechanism of  Filiform corrosion

The mechanism has a number of characteristics that are similar to Crevice Corrosion, e.g. differential aeration and hydrolysis of metal ions resulting in increasing acidity in the region of dissolution. This type of corrosion has the following characteristics.

• The coating allows oxygen and water to migrate through it.
• The concentration of dissolved oxygen becomes highest at the back of the head near the region of the tail. This region becomes the cathode.
• Oxygen becomes depleted at the head. This region becomes the anode.
• Corrosion is driven by the potential difference between these regions, a potential difference which can rise to several tenths of a volt.
• Metal ion formation and dissolution proceeds at the head while oxygen is reduced closer to the tail.

Thus, the worm-like or thread-like structure that is formed has two parts that participate in the corrosion process, the front of the head in which metal dissolves and the region behind the head in which oxygen is reduced. Farther back is an inactive region in which metal oxide and metal hydroxide have formed a precipitate. Hydrogen bubbles can be formed if the head becomes very acidic. The propagating head region continues to move under the coating into new areas leaving behind a thin trail of corrosion under the coating. The threads can measure less than 1 millimeter across. Multiple threads or worms can appear under the coating. When two propagating heads meet, the propagation tends to stop. When a propagating head approaches the inactive tail it tends to be deflected.

The corroding alloy can affect the appearance of the filament. See above Fig.1 fill form corrosion of Filiform corrosion of iron can results  in a head containing a greenish fluid (Fe(II)) and a tail containing a reddish precipitate (Fe(III)).

The occurrence of fill form corrosion has been reported to be decreased when the following actions are took.

• Application of more than one layer of a coating
• Use of a chromate containing conversion coating or primer on aluminium
• Use of a zinc containing primer on steel
• Reducing the relative humidity or maintaining a low relative humidity when storing items made of susceptible alloys

Stress corrosion cracking of a 70/30 brass fitting

 

Fig.3 Showing stress corrosion cracking/ ‘Season Cracking of a 70/30 Brass fitting

Stress corrosion cracking or “season cracking” occurs only in the presence of a sufficiently high tensile stress and a specific corrosive environment. For brasses the environment involved is usually contains ammonia or closely related substances, but atmospheres containing between 0.05% and 0.5% of sulphur dioxide or nitrites by volume can also cause stress corrosion cracking.

Stress corrosion cracking in brass is usually localized and, if ammonia has been involved, may be accompanied by black staining of the surrounding surface. The fracture surface of the crack may be stained or bright, according to whether the crack propagated slowly or rapidly. The cracks run roughly perpendicular to the direction of the tensile stress involved. For example, drawn brass tube that has not been stress relief annealed has a built-in circumferential hoop-stress; consequently exposure to an ammoniacal environment is liable to cause longitudinal cracking this is show above in Fig.3.

The mechanism of stress corrosion craking is that they display the phenomenon of subcritical crack growth, i.e. small surface flaws propagate (usually smoothly) under conditions where fracture mechanics predicts that failure should not occur. Cracks develop and propagate well below K_Ic. In fact, the subcritical value of the stress intensity, designated as K_Iscc, may be less than 1% of K_Ic.

Futher examination of metallographic sections through cracked areas will usually show a markedly intergranular crack pattern in simple alpha brasses. Stress corrosion cracks in alpha-beta brasses run transgranularly through the beta phase or, occasionally, along the alpha-beta interface. The cracks look discontinuous in metallographic sections, as they divert above or below the plane of the section to pass round the alpha phase.

The occurrence of Stress corrosion cracking (Season cracking) decreased when the following actions are took.

Material change – Provided that service and manufacturing process requirements permit, improved resistance to stress corrosion cracking can be achieved by selecting the less susceptible brasses – low zinc rather than high zinc alloys; nickel silver rather than simple brass; aluminium brass rather than admiralty.

Reducing internal stresses (stress relieving) – Cold working operations such as pressing, spinning, drawing and bending leave internal stresses which, unless removed or substantially reduced by stress relief heat treatment, can lead to stress corrosion cracking. The optimum time and temperature for stress relief depends upon the alloy but will lie within the range ½ to 1 hour at 250-300°C.

Installation of finished components – careless fitting in assembly and installation can induce stress corrosion cracking. Poor alignment, gaps at joints and over tightening of bolts are obvious examples of bad practice in this respect. One that is not so often recognised is the practice of screwing taper-threaded connectors into parallel-threaded brass valves. Especially when ptfe tape is used to seal the thread, it is all too easy to over tighten such joints to a point where a very high circumferential “hoop” stress is generated in the female member. There have been many examples of subsequent longitudinal stress corrosion cracking of the valve ends as a result of contact with quite low concentrations of ammonia in service.

The Environment – The control of the environment in which brass is used may seem an impractical way of ensuring freedom from stress corrosion cracking in service, in view of the wide range of service conditions under which brass articles and components are in daily use, but it is possible to avoid unnecessary exposure to ammoniacal contamination. One source of such contamination that has caused brass fittings, overstressed in assembly, to crack in service is some varieties of foamed plastic insulating material in which amines or other ammonia-related chemicals are used as foaming or curing agents. Chilled water valves in air conditioning units are most likely to be affected since these are subjected to condensed moisture as well as the ammoniacal chemicals.

References

Chamberlain,J Tretheway,KR (1988). Corrosion for the students of science and engineering. New York: J.Wiley and Son,Inc.

http://www.corrosionclinic.com/types_of_corrosion/filiform_corrosion_underfilm_corrosion.htm

Portfolio Task 1 – Anodes and Cathodes

Portfolio Task 1

 

Introduction

It is usually possible to identify different regions of a corroding metal surface at which the anodic and cathodic reactions have occurred. For corrosion to occur 4 essential components can be identified;

The Anode – The anode corrodes by loss of electrons from electrically neutral metal atoms to form ions. The ions may remain in solution or react to form an insoluble corrosion product. (The latter is most common) this can usually be identified as:

M   =   M^z+   +   ze^–

In which the number of electrons taken from each atom is governed by the valency of the metal, commonly z= 1, 2 or 3. It is said that this is oxidation occurs at the anode and results in an a loss of material

The cathode – The cathode does not normally corrode, although it may suffer damage under certain conditions. A common reaction that takes place at the cathode is

O₂ + 2H₂O + 4e^– à 4OH^–

It is said that this is reduction occurs at the cathode and results in a gain of material (this in in the form of oxidation reduction or hydrogen evolution.

An electrolyte – This is the name given to the solution, which must conduct electricity, and be in contact with both the anode and the cathode

Electrical connection – The anode and cathode must be in electrical contact for a current to flow in the corrosion cell, a physical connection is not necessary if the anode and cathode are part of the same metal.

Fig.1 showing a galvanic cell (with 4 essential components)

Fig.2 below shows the electrochemical series for different metals. The higher up on the tables the metals occur the more stable/noble the element is. This table can be used to work out which metal would preferentially corrode in a galvanic cell. If two metals were submerged in electrolyte and electrically connected the less noble of the two metals (the one lower down on the electrochemical series table) would become the anode and corrode and the other metal would experience oxidation reduction or hydrogen evolution and become the cathode. The larger the potential difference in an electrochemical reaction the bigger the quicker the corrosion rate.

Fig.2 The electrochemical series

In this corrosion laboratory a sample of agar gel containing sodium chloride was doped with 10ml of potassium ferricyanide solution and 2 ml of phenol phthalein solution. These chemicals react with the corrosion byproducts at both the anode and cathode, the potassium ferrocyanide turns blue in the presence of iron ions and phenol phthalein turns pink in the presence of hydroxide ions. Differential aeration corrosion cells were set up in the agar trays and the images are explained below.

Evans Drop Experiment

Fig.3 showing a mild steel sheet with a water droplet

Electrochemical reduction reactions that produce OH^– occur at the edges due to readily available oxygen from the air, making this the cathode. Electrochemical oxidation reactions occur at the middle of the drop due to the lack of oxygen leading to corrosion this is the anode. Below Fig 3 shows the reaction.

Fig.4 showing the differential aeration cell

Iron nail with zinc coated tip

Fig.5 showing a Iron nail with zinc coated tip

In this instance the iron nail is more noble/stable than zinc on the electrochemical series table(see fig.2). As shown above the iron then becomes the cathode and is surrounded by the pink OH- activated phenol phthalein. The Zinc anode is surronded in a white substance, this is Zn ions. The potassium ferrocyanide does not react with zinc as it does with iron, this is why the anode is not blue in this instance.

Steel nail with copper coated tip

Fig.6 showing an iron nail with copper coated tip

Fig.6 shows that the copper coated part of the nail is more noble than zinc on the electrochemical series(see fig.2) and is therefore the cathode and is surrounded by the pink OH ions activated phenol phthalein. The middle section of the nail is the anode and is blue as the Fe ions have activated the potassium ferrocyanide.

Iron nail connected to a piece of zinc

Fig.7 showing an iron nail connected to a piece of zinc

Fig.7 shows galvanic corrosion very similar to fig.5 the main difference is that in fig.5 the zinc was coated onto the nail. In this case the Zinc is connected via a wire. The image shows that again due to the electrochemical series the zinc sacrifices itself and corrodes preferentially to the iron nail.

A Iron nail partially copper coated and connected to a piece of zinc

Fig.8 showing an iron nail partially copper coated and connected to a piece of zinc

Fig.8 is very similar to fig.7 the only difference is that there is 3 different metals which are in the galvanic cell. Fig.8 shows that corrosion still happens at the zinc anode as this is lowest on the electrochemical series. After the  zinc is fully corroded the iron would then become the anode and the copper the cathode.

A worked iron nail

Fig.9 showing a worked uncoated iron nail

Fig.9 above shows that the worked areas of the nail have acted as a anode and started to corrode. The areas which have been worked less show no corrosion and act as the cathode. Thermodynamic laws tell us of a  strong tendancy for high energy states to transform into low energy states. It is the tendancy of all metals to recombine with elements present in the environment that lead to corrosion. In the case of fig.9 above the corrosion of the worked areas is due to the higher energy states trying to achieve a lower energy state by corroding.

5 different types of corrosion

5 types of corrosion

Intergranular corrosion (weld decay and knife line attack)

Intergranular corrosion occurs when the grain boundary area is attacked because of the presence of precipitates in these regions. Grain boundaries are the preferred site for precipitation and segregation observed in many alloys. Segregates and precipitates from are physically distinct from the remainder of the material with their own thermodynamic energies. Intermetallic (formed from metal atoms) and Compounds (formed between a metal and a non-metal i.e Fe₃C) In principle any metal with intermetallic or compounds are susceptible to intergranular corrosion

Intergranular ccorrosion has been most commonly reported for austenitic stainless steels, but it can also occur in ferritic and two phase stainless steels, as well as in nickel based corrosion resistant alloys. Aluminium alloys may suffer severe intergranular corrosion. Aluminium alloys rely on control of precipitates at the grain boundaries and also with the grain to strengthen the alloy. These are most commonly found as precipitates CuAl₂ and Fe Al₃ these are cathodic , Mg₅Al₈ and MgZn₃ are anodic to the surrounding material. With the presence of these constituents (anodic of cathodic) creates localised galvanic series when there is an electrolyte presence. If this occurs corrosion happens, if the precipitate is anodic they will corrode and leave pours material. If the precipitates are cathodic the surrounding material will start to deteriorate, both these outcomes will weaken the materials. Intergranular corrosion has also been observed in zinc die-cast and also lead, the most significant problem has occurred in austenitic stainless steels where it has been commonly referred to as weld decay due to the frequency of failures related to the welding of the material. In type 304 S/S (18Cr 8Ni Fe bal) when carbon content is as low as 0.03% the γ phase is stable. For 304 with a higher concentration of carbon there is present the stable γ phase and a mixed carbide which has the formula (FeCr)₂₃C₆ and is know as chromium carbide. The proportion of the carbide present depends on various things including chemical composition, heat treatment and quenching medium. If heat treated at >1000°C and water quenched carbide formation is suppressed. If the material would then be either further heat treated or welded exposing the material to temperatures of 650°C – 800°C, there would be a considerable likelihood the carbide precipitation would occur. This is called sensitizing and is very susceptible to corrosion. Below 600°C the rate of diffusion of chromium carbide is too low. >12% Cr vastly improves steels corrosion resistance however the precipitation of chromium out of the grain can lower the chemistry to <12. This chromium depleted region is very anodic and severe attack occurs adjacent from the grain boundary if the metal becomes in contact with electrolyte.  A practical example of where these kinds of failures have occurred is on heated pressure vessel i.e steam generators.

To mitigate/reduce the susceptibility of a metal such as 304SS/S to corrode

• A low carbon steel could be use i.e 304L
• A post weld heat treatment can be applied to dissolve the precipitates back into solution this heat treatment would need to be carried out at >1000°C
• A small addition of Ti or Nb can be added to form carbides which preferentially occur before chromium carbide (This doesn’t stop the formation of knife line attack on re heat-treating or re welding on the fusion line).

Stress corrosion cracking

Stress corrosion cracking is a term given to a intergranular or transgranular cracking of the metal under static tensile stress and a specific environment. It can lead to unexpected sudden failure of normally ductile metals subjected to a tensile stress, especially at elevated temperature in the case of metals. SCC is highly chemically specific in that certain alloys are likely to undergo SCC only when exposed to a small number of different chemical environments. The chemical environment that causes SCC for a given alloy is often one which is only mildly corrosive to the metal otherwise. Hence, metal parts with severe SCC can appear bright and shiny, while being filled with microscopic cracks. This factor makes it common for SCC to go undetected prior to failure. SCC often progresses rapidly, and is more common among alloys than pure metals. The specific environment is of crucial importance, and only very small concentrations of certain highly active chemicals are needed to produce catastrophic cracking, often leading to devastating and unexpected failure

Certain austenitic stainless steels and aluminium alloys crack in the presence of chlorides, mild steel cracks in the presence of alkali (boiler cracking) and nitrates, copper alloys crack in ammoniacal solutions (season cracking). This limits the usefulness of austenitic stainless steel for containing water with higher than few ppm content of chlorides at temperatures above 50 °C. Worse still, high-tensile structural steels crack in an unexpectedly brittle manner in a whole variety of aqueous environments, especially containing chlorides. With the possible exception of the latter, which is a special example of hydrogen cracking, all the others display the phenomenon of subcritical crack growth, i.e. small surface flaws propagate (usually smoothly) under conditions where fracture mechanics predicts that failure should not occur. Cracks develop and propagate well below K_Ic. In fact, the subcritical value of the stress intensity, designated as K_Iscc, may be less than 1% of K_Ic.

SCC is the result of a combination of three factors – a susceptible material, exposure to a corrosive environment, and tensile stresses above a threshold. If you eliminate any one of these factors SCC initiation becomes impossible. There are, consequently, a number of approaches that we can use to prevent SCC, or at least to give an acceptable lifetime. In an ideal world a stress corrosion cracking control strategy will start operating at the design stage, and will focus on the selection of material, the limitation of stress and the control of the environment. The skill of the engineer then lies in selecting the strategy that delivers the required performance at minimum cost. In this context we should appreciate that a part of the performance requirement relates to the acceptability of failure. For the primary containment pressure vessel in a nuclear reactor we obviously require a very low risk of failure. For the pressed brass decorative trim on a light switch, the occasional stress corrosion crack is not going to be a serious problem, although frequent failures would have an undesirable impact on product returns and the corporate image. The conventional approach to controlling the problem has been to develop new alloys that are more resistant to SCC. This is a costly proposition and can require a massive time investment to achieve only marginal success.

Corrosion fatigue

There are many similarities between corrrocsion fatigue and stress corrosion cracking, but the significant difference is that corrosion cracking Is extreamly non specific. Mechanical fatigue affects all metals, causing them to fail at stress levels well below those at which static stress leads to failure. In aqueous environments it is frequently found that a metals fatigue resistance is reduced. This makes corrosion fatigue common and a dangerous form of corrosion.

The stages of development

• The formation of slip bands leads to intrusions and extrusions of material
• The nucleation of an embryo crack approx. 10 microns long
• Extensions of the crack in a favourable direction
• Macroscopic crack propagation in a direction perpendicular to maximum tensile stress leeding to failure.

These can take place on an Active: freely corrodes, Immune: When a material is protected cathodically and Passive: when a metal is protected by an oxide generated surface coating.

The environment plays a significant role in the fatigue of high-strength structural materials like steel, aluminum alloys and titanium alloy what can be done to mitigate it.

Hydrogen damage corrosion

The mechanism starts with lone hydrogen atoms diffusing through the metal. At hightemperatures, the elevated solubility of hydrogen allows hydrogen to diffuse into the metal (or the hydrogen can diffuse in at a low temperature, assisted by a concentration gradient). When these hydrogen atoms re-combine in minuscule voids of the metal matrix to form hydrogen molecules, they create pressure from inside the cavity they are in. This pressure can increase to levels where the metal has reduced ductility and tensile strength up to the point where it cracks open (hydrogen induced cracking, or HIC). High-strength and low-alloy steels, nickel and titanium alloys are most susceptible. Austenised iron is also susceptible. Tests have shown that austenitic stainless steels, aluminum (including alloys), copper (including alloys, e.g. beryllium copper) are not susceptible to hydrogen embrittlement along with few other metals.

Hydrogen embrittlement can occur during various manufacturing operations or operational use – anywhere that the metal comes into contact with atomic or molecular hydrogen. Processes that can lead to this include cathodic protection, phosphating, pickling, and electroplating. A special case is arc welding, in which the hydrogen is released from moisture (for example in the coating of the welding electrodes; to minimize this, special low-hydrogen electrodes are used for welding high-strength steels). Other mechanisms of introduction of hydrogen into metal are galvanic corrosion, chemical reactions of metal with acids, or with other chemicals (notably hydrogen sulfide in sulfide stress cracking, or SSC, a process of importance for the oil and gas industries).

Microbial corrosion

Microbial corrosion (also called microbiologically-influenced corrosion or MIC) is corrosion that is caused by the presence and activities of microbes. This corrosion can take many forms and can be controlled by biocides or by conventional corrosion control methods. There are a number of mechanisms associated with this form of corrosion. Most MIC takes the form of pits that form underneath colonies of living organic matter and mineral and bio deposits. This biofilm creates a protective environment where conditions can become quite corrosive and corrosion is accelerated. These biofilms can allow corrosive chemicals to collect within and under the films. Thus the corrosive conditions under a biofilm can be very aggressive, even in locations where the bulk environment is noncorrosive. Corrosion (oxidation of metal) can only occur if some other chemical is present to be reduced. In most environments, the chemical that is reduced is either dissolved oxygen or hydrogen ions in acids. In anaerobic conditions (no oxygen or air present), some bacteria (anaerobic bacteria) can thrive. These bacteria can provide the reducible chemicals that allow corrosion to occur. In addition to the use of corrosion resistant alloys, control of MIC involves the use of biocides and cleaning methods that remove deposits from metal surfaces. Bacteria are very small, and it is often very difficult to get a metal system smooth enough and clean enough to prevent MIC.

References

Chamberlain,J Tretheway,KR (1988). Corrosion for the students of science and engineering. New York: J.Wiley and Son,Inc.

Degraded Object & Personal Reflection

Degradation homework for personal Blog

Lecture 1

1)Identify a item that appears to have degraded and take a photograph of it

2)Write a brief paragraph on what has caused the object to degrade

Fig.1 A draining colander

Fig.1 above shows corrosion on a stainless steel kitchenware colander.

The two main reasons that this colander has corroded are; Material selection and colander design. This colander is made from a low grade of stainless steel. If it was made from a higher grade e.g. an austenitic 18/8 stainless steel this may give a higher corrosion resistance. The Colander design has a circular seam on the underside, this seam will hold water and further fuel the aqueous corrosion. To prevent the colander from corroding material can be changed. For the application of a kitchen colander the steel could be swapped to use a high melting point plastic The design could be changed to that the colander has no seams this would reduce the corrosion but it would not eliminate it.

 

Reflection

In reflection on the paragraph I wrote at the start of the semester above I still agree that the corrosion mechanism is aqueous corrosion. I still agree that a change in material type may have given better corrosion resistance and also that the design of the colander could be improved. The main key point which I have missed and the reflection has given me chance to talk about is the environment to which the colander has been exposed. As talked about this semester 4 things are needed for corrosion to take place these are ; Anode, Cathode, metallic path for the electrons to travel and an electrolyte. I have not discussed the electrolyte that the colander has been exposed to. This colander was washed and dried in a dishwasher. The dishwasher is a very harsh environment for a metal and has a variety of ways of accelerating the corrosion process.

• Increasing the temperature to boost corrosion reaction kinetics therefor corrosion
• Applying cyclic temperature ranges.
• Acidifying the corrosive medium.
• Increasing the corrosivity of the corrosive medium by other chemical changes (e.g. salt concentration, degree of aeration, addition of oxidizing species, etc.).
• Alternate wetting and drying (can lead to a surface concentration effect of corrosive species).
• Increasing the relative humidity.
• Cycling through different humidity ranges/different degrees of moisture exposure.

All the accelerants for corrosion listed above were present whilst the colander was in the dishwasher. I feel that it would be impossible to fully eliminate corrosion on this low grade stainless steel colander but by controlling the accelerating factors above the rates of corrosion can be dramatically reduced.

Adhesion wear and Abrasive wear

Adhesive wear, Abrasive wear, metal fatigue and corrosion wear

There are many terms used to describe wear (fretting, pitting, spalling, scuffing, scoring, abrasion, and many others). This suggests that several physical phenomena are involved. There are essentially four main types of wear: adhesive, abrasive, surface fatigue and corrosive-wear.

The complexity of the wear process is compounded by a number of influential factors, including a) Metallurgy Variables (i.e. hardness, toughness, microstructure, chemical composition), and b) Service Variables (i.e. contacting material, pressure between contacting parts, relative speed of the parts, temperature of the parts, surface finish of the parts, atmosphere) and others such as c) Lubrication and d) Corrosion.

Adhesive Wear (sometimes called scuffing or galling) involves several steps. When two surfaces press together, the microscopic bumps on the surfaces squeeze together and form a solid junction when the atoms of the two surfaces bond. When one surface moves relative to the other the weaker material breaks at some distance from the junction and forms a lump of wear material.

Introduction

Enhanced surface durability is desired for new bearing materials, including corrosion resistant materials. Development of advanced bearing materials is generally focused on material microstructure for rolling contact fatigue resistance. The challenge is to develop surface durability attributes that complement fatigue resistance attributes.

One can categorize surface deterioration into three basic modes: wear, scuffing and fatigue. The term “adhesive wear” is commonly used when failed surfaces appear to have undergone plastic flow due to local “adhesion” at the interface. When attempting surface fatigue simulation tests with advanced bearing materials having corrosion resistant properties, local adhesive events have been found to prevent operation when the EHD film thickness (h) is small relative to surface roughness height (?). If the chemistry of the material does not allow the formation of surface films from reactions with the oil, adhesive wear can supersede surface deterioration due to surface initiated fatigue. In addition, material properties that affect plastic flow, like hardness, seem to influence the onset of adhesive wear. The mechanisms that control the ability of a surface to handle high normal and tangential stress and to recover subsequent to local damaging events are a mystery. Testing for these mechanisms and associated surface durability attributes is essential for material development and assurance of performance in service.

It was found that surface failure by adhesive wear is initiated at microscopic sites of insufficient surface film lubrication or at sites of debris encounters. With limited chemical reactivity between lubricating oil and some corrosion resistant materials, local adhesion events, which are not able to recover, propagate into broad patches of adhesive wear damage. With sufficient sliding velocity and contact stress, adhesive wear can transition into a major scuffing event. A scuffing event is characterized by a rapid rise in friction and temperature. These tribological features, as measured with an adhesive wear test method, correlate with experience in full-scale bearing tests.

At the heart of surface durability is material compatibility with lubricating oil chemistry to form surface films, which prevent local adhesion. The adhesive wear test method described below progressively increases the degree of asperity encounter at the interface under rolling/sliding conditions. The test method invokes tribological interactions, which are measured in terms of friction (traction), gentle polishing wear of surface features, adhesive wear events and scuffing. The test protocol described below is an attempt to simulate the adhesive wear mechanisms that believed to occur in rolling element bearings.

Abrasive Wear occurs when a hard particle digs into a softer surface and plows out material. This is called two-body abrasion. Three body abrasion may happen when free particles are trapped between the two surfaces. As the surfaces move relative to each other, the hard foreign particles plow out material from the softer surface areas.

Abrasive wear occurs when a hard rough surface slides across a softer surface.^[  ASTM International (formerly American Society for Testing and Materials) defines it as the loss of material due to hard particles or hard protuberances that are forced against and move along a solid surface.

Abrasive wear is commonly classified according to the type of contact and the contact environment.^[10] The type of contact determines the mode of abrasive wear. The two modes of abrasive wear are known as two-body and three-body abrasive wear. Two-body wear occurs when the grits or hard particles remove material from the opposite surface. The common analogy is that of material being removed or displaced by a cutting or plowing operation. Three-body wear occurs when the particles are not constrained, and are free to roll and slide down a surface. The contact environment determines whether the wear is classified as open or closed. An open contact environment occurs when the surfaces are sufficiently displaced to be independent of one anotherDeep      ‘groove’ like surface indicates abrasive wear over cast iron (yellow arrow indicate sliding direction)

There are a number of factors which influence abrasive wear and hence the manner of material removal. Several different mechanisms have been proposed to describe the manner in which the material is removed. Three commonly identified mechanisms of abrasive wear are:

1. Plowing
2. Cutting
3. Fragmentation

Plowing occurs when material is displaced to the side, away from the wear particles, resulting in the formation of grooves that do not involve direct material removal. The displaced material forms ridges adjacent to grooves, which may be removed by subsequent passage of abrasive particles. Cutting occurs when material is separated from the surface in the form of primary debris, or microchips, with little or no material displaced to the sides of the grooves. This mechanism closely resembles conventional machining. Fragmentation occurs when material is separated from a surface by a cutting process and the indenting abrasive causes localized fracture of the wear material. These cracks then freely propagate locally around the wear groove, resulting in additional material removal by spalling

Abrasive wear can be measured as loss of mass by the Taber Abrasion Test according to ISO 9352 or ASTM D 1044.

Metal Fatigue is the third mechanism of wear. It occurs when continuous sliding, rolling, or impacting motions subject a surface to repeated stress cycling. The stress cycle starts with very small cracks on or near the surface. These cracks spread and eventually link up to form a free wear particle. Surface fatigue is also involved in spalling and pitting wear. Surface fatigue wear depends strongly on the stress at the surface and the roughness of the surface.

Corrosive Wear involves both chemical activity and mechanical action. It is a modifier of adhesion, abrasion or fatigue wear. The presence or the rate of formation of an oxide film can greatly alter the wear characteristics of a material. The oxide usually has properties quite different from the base metal, therefore, the wear rate is affected by the oxide layer. Failure may occur at the oxide-metal interface.

Referencing

http://www.cryotron.com/varicold-wear.htm

http://en.wikipedia.org/wiki/Wear

PVD, CVD & Thermal Spraying

Thermal Spraying

History

It has long been recognized that fluids may be broken up into very fine particles by a stream of high velocity gas emanating from a nozzle. Early experiments using this atomizing approach appear to have been directed at producing metallic powders rather than coatings. It was left to Schoop to appreciate the possibility that a stream of metallic particles, formed from a molten source, could produce a coating. Myth has it that Schoop developed the concept when playing “soldiers” with his son and observing the deformation of lead pellets being fired from a toy cannon against a brick wall. Whatever the rationale, it can be stated that the pioneer work of Schoop resulted in the discovery and development of metal spraying and subsequently the “Thermal Spray Process”. The first spray technique developed by Schoop was the outcome of experiments in which molten metal was poured into a stream of high velocity gases. Schoop’s apparatus consisted of a compressor supplying air to a heated helical tube. The heated air was used to pressurize a crucible filled with molten metal and eject it out as a fine spray that would adhere to a suitable surface. This system was bulky, primitive and inefficient; however, the concept did lead to the development of portable and user friendly equipment. There are no further accounts of molten metal spraying by , it appears that his efforts were directed at developing and improving powder and wire flame spraying. However, work by others continued as a 1924 Dutch patent, describing equipment for spraying low point metals, was granted to Jung and Versteeg. Mellowes Ltd commercialized the process in the UK. Their system consisted of a gun, a furnace, an air compressor and a fuel supply. The gun had many air and gas valves, a heating chamber

Types of flame spraying

There are many types of thermal coating as shown below in Fig.1

       

Fig.1 showing different thermal spraying methods

Fig.2 Showing a schematic of the basic principle of powder flame deposition

powder flame spraying is a thermal spray process in which the material to be sprayed is in powder form. Powder flame spraying is probably the simplest of all the spray processes to describe – feed a powder through the center bore of a nozzle where it melts and is carried by the escaping oxy-fuel gases to the work piece. Unfortunately, this approach yields coatings high in oxides and with void contents approaching 20 volume percent (v/o). However, coating quality can be improved by feeding air to the nozzle through a small jet, which reduces the pressure in a chamber behind the nozzle. This chamber is connected to the powder feed hopper. In this way a gentle stream of gas is sucked into the gun and carries powder with it. A typical gun is

illustrated in Fig 3.

Fig.3 Showing s powder flame deposition gun

This concept was developed by Fritz Schori in the early 1930s. However, the amount of powder that can be supported by a gas stream depends on many factors including powder characteristics. If air is not used then the density of the supporting gas influences the feed rate and, for any particular powder there is an optimum amount that can be carried in a gaseous stream. It depends upon the velocity and volume of the gases used. The usefulness and criticality of flow meters and pressure gauges are governing factors. This has not radically changed since the days of Schoop. While there have been changes in nozzle and air cap design, replacement of the air turbine with an electrical motor and even the use of barrel valves the basic principal, however, remains the same “push or pull a wire into a flame, melt and atomize it and deposit the molten droplets to form an adherent coating”.

CVD Coating

Chemical vapour deposition or CVD is a generic name for a group of processes that involve depositing a solid material from a gaseous phase and is similar in some respects to physical vapour deposition (PVD).PVD differs in that the precursors are solid, with the material to be deposited being vaporised from a solid target and deposited onto the substrate.

Types of CVD Processes

CVD covers processes such as Atmospheric Pressure Chemical Vapour Deposition (APCVD), Low Pressure Chemical Vapour Deposition (LPCVD), Laser Chemical Vapour Deposition (LCVD), Photochemical Vapour Deposition (PCVD), Chemical Vapour Infiltration (CVI) and Chemical Beam Epitaxy (CBE)

How Does CVD Work?

Precursor gases (often diluted in carrier gases) are delivered into the reaction chamber at approximately ambient temperatures. As they pass over or come into contact with a heated substrate, they react or decompose forming a solid phase which and are deposited onto the substrate. The substrate temperature is critical and can influence what reactions will take place.

Coating Characteristics

CVD coatings are typically:

•         Fine grained

•         Impervious

•         High purity

•         Harder than similar materials produced using conventional ceramic fabrication processes

CVD coatings are usually only a few microns thick and are generally deposited at fairly slow rates, usually of the order of a few hundred microns per hour.

CVD Apparatus

A CVD apparatus will consist of several basic components (See fig.1 below).

1.            Gas delivery system – For the supply of precursors to the reactor chamber

2.            Reactor chamber – Chamber within which deposition takes place

3.            Substrate loading mechanism – A system for introducing and removing substrates

4.            Energy source – Provide the energy/heat that is required to get the precursors to react/decompose.

5.            Vacuum system – A system for removal of all other gaseous species other than those require the reaction.

6.            Exhaust system – System for removal of volatile by-products from the reaction chamber.

7.            Exhaust treatment systems – In some instances, exhaust gases may not be suitable for release into the atmosphere

8.            Process control equipment – Gauges to monitor process parameters such as pressure, temperature and time.

Fig.1 Showing CVD Equipment

Precursors

Materials are deposited from the gaseous state during CVD. Thus precursors for CVD processes must be volatile, but at the same time stable enough to be able to be delivered to the reactor. Generally precursor compounds will only provide a single element to the deposited material, with others being volatilised during the CVD process. However sometimes precursors may provide more than one. Such materials simplify the delivery system, as they reduce the number of reactants required to produce a given compound.

Materials That Can be produced by CVD Processes

CVD is an extremely versatile process that can be used to process almost any metallic or ceramic compound. These are Elements, Metals and alloys, Carbides,  Nitrides, Borides Oxides & Intermetallic compounds

Applications

CVD has applications across a wide range of industries such as, Coatings – Coatings for a variety of applications such as wear resistance, corrosion resistance, high temperature protection, erosion protection and combinations thereof. Semiconductors and related devices – Integrated circuits, sensors and optoelectronic devices. Dense structural parts – CVD can be used to produce components that are difficult or uneconomical to produce using conventional fabrication techniques. Dense parts produced via CVD are generally thin walled and maybe deposited onto a mandrel or former. Optical Fibres – For telecommunications.  Composites – Preforms can be infiltrated using CVD techniques to produce ceramic matrix composites such as carbon-carbon, carbon-silicon carbide and silicon carbide-silicon carbide composites. This process is sometimes called chemical vapour infiltration or CVI. Powder production – Production of novel powders and fibres

PVD coatings

PVD is the abbreviation of Physical Vapor Deposition. It is a process carried out in high vacuum at temperatures between 150 and 500 °C.

The process

The PVD processes include arc evaporation, sputtering, ion plating, and enhanced sputtering.

The high-purity, solid coating material (metals such as titanium, chromium and aluminium) is either evaporated by heat or by bombardment with ions (sputtering). At the same time, a reactive gas (e.g. nitrogen or a gas containing carbon) is introduced; it forms a compound with the metal vapour and is deposited on the tools or components as a thin, highly adherent coating. In order to obtain a uniform coating thickness, the parts are rotated at uniform speed about several axes. Fig.1 below shows some examples of some PVD coated tool tips

The properties of the coating (such as hardness, structure, chemical and temperature resistance, adhesion) can be accurately controlled.

Fig.1 Showing a PVD process TiN tool tip

Advantages:

• PVD coatings are sometimes harder and more corrosion resistant than coatings applied by the electroplating process. Most coatings have high temperature and good impact strength, excellent abrasion resistance and are so durable that protective topcoats are almost never necessary.

• Ability to utilize virtually any type of inorganic and some organic coating materials on an equally diverse group of substrates and surfaces using a wide variety of finishes.

• More environmentally friendly than traditional coating processes such as electroplating and painting.

• More than one technique can be used to deposit a given film.

Disadvantages:

• Specific technologies can impose constraints; for example, line-of-sight transfer is typical of most PVD coating techniques, however there are methods that allow full coverage of complex geometries.

• Some PVD technologies typically operate at very high temperatures and vacuums, requiring special attention by operating personnel.

• Requires a cooling water system to dissipate large heat loads.

REFERENCES

(1) Frank J. Hermanek, Thermal Spray Terminology and Company Origins, First Printing, 2001, ASM International, Materials Park, OH

(2) W. E. Ballard, Metal Spraying and the Flame Deposition of Ceramics and Plastics, Fourth Revised, Edition, 1963, Charles Griffin and Company Limited, London, England

(3) The Metals Handbook, Eighth Edition, Volume1, 1961, American Society for Metals, Metals Park, OH

(4) A. P. Alkimov, V. F Kosarev, A. N. Papyrin, “A Method of Cold Gas-Dynamic Deposition”, 1990, Sov. Phys. Dokl., Vol 35

(5)  http://www.azom.com/article.aspx?ArticleID=1558