Coatings MCQ Quiz - Objective Question with Answer for Coatings - Download Free PDF

Explanation:

Surface Coating Process

• Surface coating is a process in which a protective or functional layer is applied to the surface of a material (substrate) to improve its properties, such as corrosion resistance, wear resistance, appearance, or other functional characteristics. This layer can be applied using various techniques, one of which is hot dipping.

Hot Dipping:

• Hot dipping is a widely used surface coating process in which a metal substrate is immersed into a bath of molten material (usually zinc, aluminum, or tin) to create a protective coating. This process is commonly used for corrosion protection and is widely applied in industries for galvanizing steel and other metals.
• The substrate to be coated is cleaned thoroughly to remove any contaminants, such as grease, oil, or oxides. It is then dipped into a molten bath of the coating material. The molten material adheres to the surface of the substrate and forms a uniform layer. After withdrawal from the molten bath, the coated material is cooled to solidify the coating.

Steps Involved in Hot Dipping:

1. Surface Preparation: This involves cleaning the substrate to remove dirt, grease, and oxides. Common cleaning methods include pickling (acid cleaning), alkaline cleaning, and abrasive cleaning. Proper surface preparation ensures good adhesion of the coating.
2. Hot Dipping: The cleaned substrate is immersed in a molten bath of the coating material. The coating material typically includes metals like zinc (for galvanization), aluminum, or tin.
3. Cooling and Solidification: After dipping, the substrate is withdrawn from the molten bath and allowed to cool. The coating solidifies, forming a durable and protective layer on the surface.

Advantages of Hot Dipping:

• Provides excellent corrosion resistance, especially in steel and iron components.
• Forms a metallurgical bond between the substrate and coating, resulting in high durability.
• Relatively low-cost method for large-scale applications.
• Can be applied to a wide range of materials and shapes.

Applications:

• Galvanization of steel and iron components to protect them from rust and corrosion.
• Coating of electrical components to improve conductivity and resistance to oxidation.
• Used in the construction industry for coated roofing sheets, pipes, and wires.

Additional InformationOption 1: Tumbling

It is a finishing process used to smooth, polish, or deburr the surface of a material. In tumbling, the workpieces are placed in a rotating barrel or vibratory tumbler along with abrasive media. This process is used to improve the surface finish of components but does not apply a coating.

Option 4: Pickling

Pickling is a surface treatment process used to remove impurities, such as oxides, rust, and scale, from the surface of metals. It involves immersing the metal in an acid solution (e.g., hydrochloric acid or sulfuric acid).

Explanation:

Electroplating and Its Opposite Process

Definition: Electroplating is a process that uses electrical current to reduce dissolved metal cations so that they form a coherent metal coating on an electrode. This process is commonly used to deposit a layer of material, such as a metal, onto a surface for various purposes including corrosion resistance, aesthetic improvements, and reducing friction.

Working Principle: In electroplating, the object to be plated is made the cathode (negative electrode) in an electrolysis cell. The anode (positive electrode) is typically made of the metal to be plated. A solution containing a salt of the metal to be plated is used as the electrolyte. When an electric current is passed through the cell, metal cations from the electrolyte are reduced at the cathode surface, forming a thin layer of metal. Simultaneously, metal atoms from the anode dissolve into the electrolyte to maintain the concentration of metal cations.

Advantages:

• Provides corrosion resistance to the plated object.
• Enhances the appearance of the object.
• Improves wear resistance and reduces friction.

Disadvantages:

• Requires careful control of the plating process to achieve uniform coating.
• Involves hazardous chemicals that require proper handling and disposal.

Applications: Electroplating is widely used in various industries, including automotive, electronics, jewelry, and manufacturing, to enhance the properties and appearance of products.

Galvanic cell:

• A galvanic cell, also known as a voltaic cell, is an electrochemical cell that derives electrical energy from spontaneous redox reactions taking place within the cell. It essentially converts chemical energy into electrical energy. The key difference between a galvanic cell and electroplating lies in the direction of electron flow and the nature of the reactions involved.
• In a galvanic cell, two different metals are connected by a salt bridge or a porous disk between the individual half-cells. Each metal is immersed in an electrolyte solution. The metal that is more reactive (anode) undergoes oxidation, losing electrons, while the less reactive metal (cathode) undergoes reduction, gaining electrons. This flow of electrons through an external circuit generates electrical energy.
• In contrast, electroplating is an electrolytic process that requires an external power source to drive the non-spontaneous chemical reactions. In electroplating, the object to be plated is the cathode, and the metal to be deposited is the anode. When current is applied, metal cations from the electrolyte are reduced at the cathode, forming a coating.
• Thus, while electroplating involves the deposition of metal ions onto a surface using an external power source, a galvanic cell generates electrical energy through spontaneous redox reactions. This fundamental difference makes galvanic cells the opposite of the electroplating process.

Explanation:

Organic Coating:

• In organic coatings (such as paints and varnishes), pigments are used to provide color, opacity, and protection. However, pigment particles tend to agglomerate (clump together), leading to poor dispersion and settling issues. This negatively affects:
  • Coating uniformity
  • Storage stability
  • Aesthetic appearance
  • Durability

Role of Colloidal Stabilisers:

• Colloidal stabilisers are added to enhance pigment dispersion by preventing the particles from aggregating.

They work by providing:

• Electrostatic stabilization (charge repulsion)
• Steric stabilization (physical hindrance due to polymer chains)

This results in:

• Improved stability of pigment suspensions
• Longer shelf life of coating materials
• Even and smooth finish during application

Explanation:

Hot dipping:

• Hot dipping involves immersing a base metal into a molten coating metal. For proper adhesion and durability of the coating, the molten metal must:
  • Form an alloy (metallurgical bond) with the base metal at the interface.
  • Have good wettability (not lower wettability).
  • Not necessarily have a higher melting point than the base metal (e.g., zinc melts at a lower temp than steel in galvanization).
  • Not evaporate completely (must remain to form the coating).

Forming an alloy at the interface with the base metal is critical for several reasons:

1. Strong Bonding: When the coating material forms an alloy with the base metal, it creates a metallurgical bond that is much stronger and more durable compared to a simple mechanical bond. This strong bond ensures that the coating adheres well to the base metal, providing long-lasting protection.

2. Enhanced Properties: Alloying can significantly enhance the properties of the base metal. For example, in the case of galvanizing (coating steel with zinc), the zinc forms a series of alloy layers with the steel, which not only protects the steel from corrosion but also improves its overall mechanical properties.

3. Improved Corrosion Resistance: The formation of an alloy at the interface can create a more uniform and dense coating, which is more effective at preventing the penetration of corrosive elements. This is particularly important in environments where the coated metal is exposed to harsh conditions.

4. Thermal Stability: Alloys generally have better thermal stability compared to pure metals. This means that the coated metal can withstand higher temperatures without degrading, which is essential for applications where the metal will be exposed to heat.

5. Reduced Defects: Alloying at the interface can help to reduce defects such as cracks, voids, and porosity in the coating. This leads to a more uniform and defect-free coating, which is crucial for ensuring the longevity and performance of the coated metal.

Explanation:

Cure additives:

• In organic coatings, cure additives act as catalysts that accelerate the curing (hardening or setting) process of the coating, which may be triggered by heat, UV light, or chemical reactions. These additives are critical in reducing drying time and improving the efficiency of the coating process.

Other options serve different purposes:

• Colloidal stabilisers: Prevent settling or coagulation in suspensions.
• UV stabilisers: Protect the coating from degradation due to UV radiation.
• Plasticisers: Increase flexibility and workability of the coating.

Explanation:

A protective coating on the surface of metals and alloys isolates them from corrosive environment, thereby preventing corrosion. There are different types of coating:

• Metallic coating
• Organic coating
• Inorganic coating

Inorganic coatings: Glass, cement, ceramic and chemical conversion coatings.

Chemical conversion: Anodizing, oxide, chromate, phosphatizing.

Explanation:

Galvanizing:

• Galvanizing protects the underlying iron or steel in the following main ways:
• The zinc coating, when intact, prevents corrosive substances from reaching the underlying steel or iron.
• The zinc protects iron by corroding first. For better results, the application of chromates over zinc is also seen as an industrial trend.
• In the event the underlying metal becomes exposed, protection can continue as long as there is zinc close enough to be electrically coupled. After all of the zinc in the immediate area is consumed, localized corrosion of the base metal can occur.
• GI pipes have a coating of zinc metal on bare steel pipe, through a molten zinc bath, ensuring natural corrosion resistance, even in outdoor conditions.
• Galvanizing is done to all metal but most generally it is done on low carbon steel.

Anodizing

• Anodizing is an electrolytic passivation process used to increase the thickness of the natural oxide layer on the surface of metal parts
• The process derives its name from the fact that the aluminium part to be coated becomes the anode in an electrolytic cell.
• Anodizing refers to conversion coating of the surface of aluminium and its alloys to porous aluminium oxide.
• Anodic films are most commonly applied to protect aluminium alloys, although processes also exist for titanium, zinc, magnesium, zirconium etc
• This differentiates it from electroplating, in which the part is made the cathode.
• Aluminium is ideally suited to anodizing, although other non-ferrous metals such as magnesium and titanium also can be anodized.
• The corrosion results suggest that the sealing treatment can significantly improve the corrosion resistance of anodized Mg alloys.
• In fact, the sealing technique can also be applied to a phosphate coating to obtain improves corrosion performance.

Explanation:

Parkerizing: 

• It is a process of applying an anti-corrosion and lubricating phosphatized surface treatment.
• Parkerizing is an electrochemical process that creates a protective iron-phosphate layer on the outer surface of steel.
• Parkerizing, bonderizing, phosphating, or phosphatizing is a method of protecting a steel surface from corrosion and increasing its resistance to wear through the application of a chemical phosphate conversion coating.
• Parkerizing is usually considered to be an improved zinc or manganese phosphating process.
• Parkerizing is commonly used on firearms as a more effective alternative to protect against rust.
• It is also used extensively on automobiles to protect unfinished metal parts from corrosion.
• The Parkerizing process cannot be used on non-ferrous metals such as aluminium, brass, or copper.
• It similarly cannot be applied to steels containing a large amount of nickel, or on stainless steel.

Explanation:

Commonly used metallic corrosion-resisting coatings:

• Hot dipping (galvanizing)
• Electroplating
• Cladding
• Metal spraying
• Cementation

Galvanizing:

• In this process, mild steel or iron is coated with zinc.
• For hot dip galvanizing, the workpieces are initially pickled in hot sulphuric or cold hydrochloric acid to clean the surface, and then fluxed with zinc chloride and ammonium chloride.
• After this they are dipped in molten zinc.
• Sometimes a small quantity of aluminum is added which gives a bright appearance and uniform thickness.
• The temperature of the zinc bath is usually maintained between 450° and 465°C.
• The hot-dipped workpieces are then quenched in a water bath.
• Galvanizing is done for structural work, bolts, nuts, pipes, and wires, which are exposed to different atmospheric conditions.
• This method is highly reliable.
• It can withstand severe working conditions and the cost is low.

Explanation:

Cementation:

• It is a commonly used metallic corrosion-resisting coating technique.
• There are three types of cementation processes for protecting metal surfaces:
  • Sherardising (Zinc coating)
  • Calorising (Aluminium coating)
  • Chromising (Chromium coating)

Sherardising:

• In this process, the workpieces are initially prepared by acid pickling or grit-blasting.
• They are then placed in a rotating steel barrel containing zinc powder and heated to a temperature of around 370°C.
• The time taken for the coating depends on the thickness of the coat.
• The heated powder bonds to the ferrous workpiece by diffusion and forms a hard even layer of iron/zinc inter-metallic compound.
• The surface of the sherardizes components will be slightly rough which provides a good grip for subsequent painting.

Calorising:

• This process is very similar to sherardising but the powder used is aluminium, and the heating temperature is between 850°C and 1000°C.
• This is used to protect steel components from corrosion.
• This process requires a higher temperature and higher humidity than sherardising.

Chromising:

• This provides a chromium-rich surface.
• The work to be chromised is baked with aluminium oxide and chromium powder at a temperature of 1300°C to 1400°C in an atmosphere of hydrogen to prevent oxidation of chromium.
• The process is expensive, and due to this reason, it is used only in places where extreme protection is required.
• This coating caused by the action of the acids in the atmosphere protects the surface of the copper.

Explanation:

Cure additives:

• In organic coatings, cure additives act as catalysts that accelerate the curing (hardening or setting) process of the coating, which may be triggered by heat, UV light, or chemical reactions. These additives are critical in reducing drying time and improving the efficiency of the coating process.

Other options serve different purposes:

• Colloidal stabilisers: Prevent settling or coagulation in suspensions.
• UV stabilisers: Protect the coating from degradation due to UV radiation.
• Plasticisers: Increase flexibility and workability of the coating.

Explanation:

• The reduction of oxide ore with carbon at high temperatures is known as Smelting.
• It is carried out in the presence of a reducing agent in a blast furnace.
• The reducing agent mostly used is carbon.
• The carbon reacts with oxygen from ores to produce CO₂.
• Along with reducing agents like Coke, heat is also used to treat the metals.
• Some metals are extracted from their ores via melting but some other processes are employed for ores in the form of oxides, etc.
• Smelting is used to extract metals such as iron, copper, etc.
• Smelting of pig iron takes place in a blast furnace which converts it to steel.
• The steps of  the process are:

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• Iron oxide must be reduced in order to obtain iron and this can be done only in a blast furnace.
• It is the molten iron from the blast furnace, which is a large and cylinder-shaped furnace charged with iron ore, coke, and limestone.
• Pig iron has a very high carbon content, typically 3.5 - 4.5%.
• A vertical shaft furnace that produces liquid metals by the reaction of a flow of air introduced under pressure into the bottom of the furnace with a mixture of metallic ore, coke (blast furnace fuel), and flux fed into the top.
• In a blast furnace, coke (C) reacts with oxygen to form carbon monoxide, which then reacts with iron oxide to form carbon dioxide and pig iron.
• Pig iron is obtained by the chemical reduction of iron ore. This process of reduction of the iron ore to pig iron is known as smelting
• Produced pig iron is used for subsequent processing into steel, and they are also employed in processing lead, copper, and other metals.

Hence, smelting involves reduction, melting of metal, and formation of slag.

 Hence, the incorrect statement regarding Smelting is: Oxidation of ore takes place. 

Additional Information

Galvanizing:

• In this process, mild steel is coated with zinc
• For hot dip galvanizing, the workpieces are initially pickled in hot sulphuric or cold hydrochloric acid to clean the surface, and then fluxed with zinc chloride and ammonium chloride
• After this, they are dipped in molten zinc. Sometimes a small quantity of aluminium is added which gives a bright appearance and uniform thickness
• The temperature of the zinc bath is usually maintained between 450^oC and 465^oC

Explanation:- 

• Corrosion is one of the greatest adversaries that metals face.
• It is a natural occurrence caused by the reaction of metal and environmental factors.
• One of the most effective ways of corrosion prevention is using metals that are not prone to corrosion.
• Some of the methods used to prevent corrosion are as follows: 

1. Galvanization: Galvanized metal is coated with a thin layer of zinc to protect it against corrosion. The zinc oxidizes when it is exposed to air creating a protective coating on the metal surface.
2. Alloying: It is the method of improving the properties of a metal by mixing the metal with another metal or nonmetal. When iron is alloyed with chromium and nickel in stainless steel is obtained. Stainless steel does not rust at all.
3. Painting (Protective Coating): Rusting of iron can be easily prevented by coating the surface with paint which protects iron from air and moisture
4. Greasing/Oiling: When some grease oil is applied to the surface of an iron object, then air and moisture cannot come in contact with it, and hence rusting is prevented.
5. Metal Plating:  plating is almost similar to painting. Instead of paint, a thin layer of metal is applied to the metal you want to protect. The metal layer prevents corrosion and adds an aesthetic finish.
6. Corrosion inhibitors: Corrosion inhibitors are chemicals applied to the surface of the metal that react with the metal or the surrounding gases to inhibit or suppress the electrochemical processes that lead to corrosion.
7. Sacrificial coatings:  a coat of a metal that is likely to oxidize is added on the surface of the metal you want to protect. You can either use cathodic protection in a process known as galvanizing or use anodic protection.

• There’s a rule of thumb that the corrosion rate of a metal doubles for every 10°C increase in temperature. Thus, if the corrosion rate is 10 mpy (mils per year) at 30°C, expect it to be 20 mpy at 40°C, 40 mpy at 50°C, etc