Page 190 - CW E-Magazine (14-5-2024)

Special Report Special Report

While powder coatings may be cost- the cathode as the gas ions collide with Fusion bonding Table 1: Typical list of metal coatings on steel(10). Present trends are related to
effi cient in the long-term, the initial the cathode. These atoms are attracted Coatings of low melting metals such Process Coating DFT [Mils] Use/Limitation advanced phosphorus-free surface pre-
start-up costs can be signifi cant. The to the anode and coat the substrate. This as tin, lead, zinc, and aluminium may Aluminizing Aluminium 1-6 Factory Process treatment, the development of silanes,
coating process requires special spray method can be used for non-conducting be applied by cementing the metal as Cathode sputtering Metals Ceramics 4 Special Applications and nanoparticles in coating systems.
booths, ovens and spraying equipment. as well as conducting materials. The a powder on the substrate then heating Diffusion coatings Metals Silicates 0.01-30 Special Applications Smart coatings, which can sense the pH
This can also limit the size of objects major disadvantages are the heating of the substrate to a temperature above the Electroplating Aluminium 0.25 Wire, Sheet. Small of corrosive species and release anti-cor-
that can be coated. It is also diffi cult, or the substrate and low deposition rates. melting point of the coating metal. Cadmium 0.15-0.5 Parts rosive chemicals just in time to counter
even impossible, to achieve thin coating Some of the most commonly used metals Chromium 0.005-20 the attack, are also being developed.
layers. Furthermore, the fi nished surface deposited by sputtering are aluminium, Explosion bonding Copper 0.01-30
is not the smoothest when compared copper, chromium, gold, molybdenum, This method produces a bond bet- Brass 0.07-0.1 It is essential to understand the
to other coating methods. Projects that nickel, platinum, silver, tantalum, titanium, ween two metals by the exertion of a Gold 0.03-0.8 advantages and disadvantages of each
require a coating thickness of less than tungsten, vanadium, and zirconium. strong force that compresses the two Silver 0.1-1.0 coating type to select the one most suited
six mils should rely on another coating metals suffi ciently to develop a strong Iron >125 for a particular application.
process(8). Diffusion coating interfacial interaction(1, 9). Lead 0.25-50
This method requires a preliminary Nickel 0.1-2.2 REFERENCES
Nitriding step followed by thermal treatment and Metal cladding Platinum 0.1-1.0 1. Schweitzer, P.A., ‘Corrosion of
Nitriding can improve the surface diffusion of the coating metal into the The most common method is roll Palladium 0.1-0.2 Linings & Coatings’, Corrosion
corrosion resistance of steel parts along substrate. A commercial material known bonding that produces full-sized sheets Rhodium 0.01-0.02 Engineering Handbook, II Edition,
with increasing wear and fatigue resis- as galvannealed steel is made by coating of clad (coated) material. The bond Tin 0.2-2.0 CRC Press, (2007).
tance. In this process, which was inven- steel with zinc followed by heat treat- formed is partly mechanical and partly Zinc 0.1-1.0 2. Fontana M.G, Green N.D, Corro-
ted in the early 1900s, atomic nitrogen ment and the formation of an iron-zinc metallurgical; consequently, metallurgi- Explosion bonding Metals 60-750 Plates, Tube sheets, sion Engineering (1967) McGraw-
penetrates the steel surface and reacts inter-metallic coating by diffusion. cally incompatible materials cannot be Strip Hill Book Co., New York, USA.
with substrate atoms (predominantly produced. Flame spraying Aluminium 4-8 Porous x 3. Coatings for Material Protec-
iron) to produce nitrides. The metallic Flame spraying Zinc 2-5 Porous x tion. Edited by Makhlouf, A.S.H.,
nitrides are hard and enhance the wear The coating metal is melted and kept CONCLUSION Tin 3-15 - Woodhead Publishing., UK
resistance of the surface. The hardness in the molten condition until it strikes For industrial applications, paint Metals 5-60 Low melting Alloys (2014).
of the nitride layer reaches up to 65 RC the substrate to be coated. Aluminium coatings are slowly being replaced by Ceramics - 4. Suzuki, I., 1989. Corrosion Resis-
while the substrate hardness is usually and zinc are applied in this manner. other coating methods. Some paints may Fusion bonding Metals 60-750 Plates/Tubes tant Coatings Technology, Marcel
around 45 RC. The corrosion resistance Flame-sprayed aluminium has a lower contain toxic elements and VOCs, mak- Galvanizing Zinc 0.5-5.0 Max length 80’ Dekker, New York, USA.
of the nitride layer is excellent in many density than pure aluminium because of ing them harmful to the environment. 5. Porter, F.C., 1994. Corrosion
environments. It is possible to induce voids in the coating. Their durability is also lower than other Gas plating Metals 0.01-70 Special Application Resistance of Zinc and Zinc Alloys,
both nitrogen and carbon atoms into coating methods, as they are likely to Metal cladding Aluminium 10-300 Sheets, Plates, Strips, Marcel Dekker, New York, USA.
the metallic surface simultaneously. Plasma spraying fade, peel or fl ake off due to prolonged Brass 60-750 Tubes, Transition 6. Satas, D., 1991. Coatings Techno-
This process is called nitrocarburizing This method is similar to fl ame environmental exposures. Copper 60-750 Joints logy Handbook, Marcel Dekker,
or carbonitriding, depending on which spraying except that forms of heating Lead - New York, USA.
element is predominant in penetrating. other than a fl ame are used. The combination of barrier and gal- Magnesium - 7. Schweitzer, P.A., 1988. Corrosion
vanic protection that metallic coatings and Corrosion Protection Hand-
Corrosion engineers should consider Vacuum and vapour deposition provide offer a form of corrosion pro- Plasma spraying Metals Ceramics 0.01-100.0 Better Quality, High book, 2nd ed., Marcel Dekker,
nitriding as a method to mitigate corro- This method is used primarily for the tection that is unparalleled in terms of 0.25-25.0 temp. melting metals New York, USA.
sion, particularly when the corrosion is formation of metallic coatings on non- application, cost and versatility. They are Vacuum deposition Metals Ceramics 0.01-3.0 Special applications 8. Leidheiser, R., Jr. 1987. “Coat-
likely to be accompanied by mechanical conductive substrates. Common deposi- an effective way to protect metals such - ings, in Corrosion Mechanisms”,
damage. ted coatings using this method include as steel (which is technically a metal Vapour deposition Aluminium 0.5-1.0 Special applications F. Mansfi eld, Ed., Marcel Dekker,
aluminium coatings on plastics and alloy) from degradation due to corrosion. Chromium 0.1-1.0 New York, USA, pp. 165-209.
Cathodic sputtering rhodium coatings on mirrors. This protection allows vulnerable met- Iron - 9. Tator, K.B., 1988. “Coatings”,
This method is carried out under als to be used in a number of demand- Nickel 1.0-100.0 Corrosion and Corrosion Protec-
a partial vacuum. The substrate to be Gas plating ing applications that would otherwise be Graphite - tion Handbook”, 2nd ed., P.A.
coated is attached to the anode. Argon, Some metal compounds can be deemed unsuitable. ‘X’ – Needs Sealing Schweitzer, Ed., Marcel Dekker,
or a similar inert gas, is admitted at low decomposed by heat to form the metal. important to remember that all coatings ticles, formation of smart coatings, New York, USA, pp. 453-489.
pressure. A discharge is initiated, and the Typical examples are metal carbonyls, Metal coatings, if used correctly, can are susceptible to failure. and fi eld trials of cold sprays are some 10. Pludek, V.R. Design and corro-
positively charged gas ions are attracted metal halides, and metal methyl com- be effective in prolonging the service signifi cant developments taking place in sion control, (1977), Macmillan
to the cathode. Atoms are dislodged from pounds. life of many metal assets. However, it’s Multilayer deposition of nanopar- the fi eld of advanced corrosion protection. Press Ltd.

190 Chemical Weekly May 14, 2024 Chemical Weekly May 14, 2024 191

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