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Questions and Answers
What is the purpose of metallic coatings?
To protect the base metal from corrosion
Which type of metallic coating involves the base metal acting as the anode?
Galvanization is a process of coating ____ on Fe/steel objects.
Zn
Inorganic coatings are also known as chemical conversion coatings.
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Match the following coatings with their descriptions:
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What triggers corrosion in metals?
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Corrosion is the destruction of a metal or an alloy due to electrochemical attack.
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Explain the difference between dry corrosion and wet corrosion.
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Corrosion involves __________ of the metal.
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Match the following reactions with their corresponding environments during corrosion:
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What is differential aeration corrosion?
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What is the name of the corrosion that occurs when two dissimilar metals are in contact with each other and a conducting solution?
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Stress corrosion is caused by tensile stress and a specific environment.
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___________ corrosion is the localized and intense corrosion of a metal, often resulting in the formation of pits or holes.
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What is passivation of a metal?
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What type of chemicals in the air usually cause the corrosion product to be formed as an oxide or carbonate?
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What are the properties of a corrosion product that forms a perfect barrier layer?
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What does the Pilling-Bedworth ratio (PBR) assess in relation to metal passivation?
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Passivation involves the volume occupied by the metal oxide compared to the volume of the metal, expressed as the specific volume ____.
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True or False: Pure metals generally exhibit greater resistance to corrosion compared to impure metals.
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What is the measure of acidity of a medium known as?
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What are the principal components of an electroplating process?
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Chromium anodes are used in chromium plating.
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What is the driving force in electroless plating?
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Electroless plating can be used to plate non-conductors, for example, __________.
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Match the following properties with their respective plating type:
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Study Notes
Corrosion Science and Management
Introduction to Corrosion
- Corrosion is the destruction and consequent loss of a metal or an alloy due to chemical or electrochemical attack on its surface by its environment.
- Metal corrosion occurs when a metal is oxidized or loses electrons.
- Corrosion is like cancer, and the reasons for it are many.
Types of Corrosion
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Dry Corrosion or Direct Chemical Attack:
- Occurs in the absence of a liquid medium.
- Relatively rapid process.
- Corrosion product is formed at the place of oxidation.
-
Wet Corrosion or Electrochemical Corrosion:
- Occurs in the presence of a liquid medium (usually water).
- Relatively slow process.
- Corrosion product is usually formed at a place different from the point of oxidation.
Electrochemical Theory of Corrosion
- Corrosion involves the formation of tiny electrochemical or galvanic cells on the metal surface.
- The metal acts as an electronic conductor, and the liquid medium (water) helps in the conduction of ions.
- There will be spontaneous oxidation and reduction reactions leading to the formation of corrosion products.
- At the Anodic Area:
- Metal gets oxidized (M → Mn+ + ne-).
- Metal ions enter the liquid medium.
- Electrons produced migrate to the cathodic area through the metal.
- At the Cathodic Area:
- Environment (liquid medium) on the metal surface can be acidic, alkaline, or neutral.
- Electrons are absorbed by different mechanisms depending on the nature of the environment.
Formation of Anodic and Cathodic Areas
- Conditions for formation of anodic and cathodic areas:
- When two or more metals are in contact with each other.
- When a metal is exposed to different oxygen or air concentrations.
- When some area of a metal is under stress.
- When the metal is impure.
- When different phases of a metal are at the surface.
Galvanic Corrosion or Differential Metal Corrosion
- Occurs when two dissimilar metals are in contact and also in contact with a conducting solution (mostly water).
- The metal with higher tendency to get oxidized (with lower reduction potential) becomes anode and corrodes.
- Factors that decide the rate of galvanic corrosion:
- Difference in electrode potentials of the two metals.
- Relative areas of anode and cathode.
- Environmental factors like pH, temperature, concentration of oxygen, and oxidizing agents.
- Passivation of the metal.
Minimizing Galvanic Corrosion
- Proper selection of metals with least difference in their electrode potentials.
- Taking care to ensure that small areas like screws, bolt-nut, etc. become cathodes but not anodes.
- Preventing contact between the two metals using proper insulating materials.
Differential Aeration Corrosion
- Corrosion of a metal due to difference in air or oxygen concentration on its surface.
- The area with lower oxygen concentration becomes anode and corrodes.
- Examples:
- Waterline Corrosion: Corrosion of a metal partially dipped in water.
- Pitting Corrosion: Localized and intense corrosion of a metal leading to the formation of a pit or pin hole.### Stress Corrosion
- Metallic objects fabricated using various mechanical operations (bending, pressing, drawing, hammering, riveting) experience stress.
- If the metal is not properly annealed, stress remains, and the atoms and grain boundaries at the stressed part exhibit greater reactivity, becoming anodes and corroding.
- Stress corrosion occurs due to the combined effect of tensile stress and a specific environment.
- Cracks appear at stressed parts, especially in hard metals and alloys, and can deepen into the metal, leading to intergranular stress corrosion.
Caustic Embrittlement
- A type of stress corrosion found in mild steel boilers.
- Necessary conditions for caustic embrittlement are:
- Hairline cracks due to stress
- Presence of caustic alkali (NaOH)
- Stressed part of a boiler will have fine cracks, which can react with water containing Na2CO3 to produce NaOH, leading to a concentration cell and the dissolution of the metal.
Factors Influencing the Rate of Corrosion
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- Nature of the metal (electrode potential):
- Some metals have a higher tendency to corrode than others
- Electrode potential can be used to understand corrosion tendency
- Lower the reduction potential, more is the tendency of the metal to get oxidized (corroded)
-
- Nature of Corrosion Product:
- Corrosion product is the result of the chemical reaction between the metal and the environment
- A suitable environment is necessary for corrosion to occur
- The corrosion product formed on the metal surface can be an oxide or carbonate
-
- Purity and physical state of the metal:
- Pure metals show greater resistance to corrosion compared to impure metals
- Impurities can facilitate the formation of tiny galvanic cells, promoting corrosion
- Physical state refers to the size of the crystals in the metal, with smaller grain size leading to more grain boundaries and increased corrosion rate
-
- pH of the medium:
- pH is a measure of acidity of the medium
- Lower pH increases the corrosion rate, as H+ ions help in cathodic reaction and dissolve corrosion products
-
- Temperature of the medium:
- Increase in temperature increases the corrosion rate
- Reasons for this include increased conductance of the medium, increased solubility of corrosion products, and breakdown of protective films
-
- Relative areas of anode and cathode (Area effect):
- According to electrochemical theory, the metallic part that acts as an anode corrodes
- Smaller anodic area and larger cathodic area lead to severe corrosion of anodic area (pitting)
Corrosion Control
-
Techniques to safeguard metals from corrosion include:
- Coating technique: creating a barrier between the metal and the environment
- Metallic coatings: coating the base metal with another metal (anodic or cathodic)
- Inorganic coatings: converting the metal surface into an inorganic compound like oxide or phosphate
-
Important techniques for controlling corrosion include:
- Galvanization: coating Zn on Fe/steel objects
- Tinning: coating tin on iron or steel
- Phosphating: creating a conversion coating on the metal surface
- Anodizing: creating a thick oxide layer on the metal surface### Coatings for Corrosion Protection
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Coatings provide corrosion resistance and have additional advantages:
- Provide electrical insulating property to the surface
- Enhance the adherence of paint and enamel to the surface
Phosphate Coating or Phosphating
- Phosphating involves developing a phosphate layer on the surface by reacting metal with dilute phosphoric acid solution
- Phosphating can be done by:
- Dipping a cleaned object in phosphating solution
- Spraying phosphating solution on the object
- Applying the solution using a brush
- Constituents of phosphating solution and their functions:
- Free phosphoric acid: reacts with metal and produces metal sulphate
- Metal phosphates of zinc and manganese: help increase adherence of phosphate layer to metal surface and decrease porosity of the coating
- Nitrates, chromates, peroxide: oxidizing agents that help in oxidation of metal and formation of phosphate layer
- pH of the bath is maintained between 1.8-3.2
- Temperature of the bath is maintained around 35°C
- Phosphating involves three steps:
- Dissolution of metal (undergoing oxidation)
- Metal ions combining with phosphate ions to form metal phosphate
- Deposition of an insoluble phosphate layer
- Properties of phosphate coatings:
- Coatings are porous and cannot guarantee 100% protection against corrosion by themselves
- Coatings contain zinc and manganese phosphates
- Coatings are generally grey in colour but can be given desired colour using dyes
- Applications:
- Phosphate coating is given as an undercoat (under layer) before painting car bodies, refrigerators, and washing machines
- Phosphate coatings are generally rough and porous, making them a good undercoat to paint
- Phosphating is done on nails, screws, bolt-nuts, drums, etc. made of steel
Anodizing of Aluminium or Anodized Coatings
- Anodizing is a process of developing an oxide layer on a metallic object by induced oxidation, making the object anode and passing current
- Electrode reactions occurring during anodizing of Al process:
- At anode: 2Al + 3H2O → Al2O3 + 6H+ + 6e-
- At cathode: 6H+ + 6e- → 3H2↑
- Overall reaction: 2Al + 3H2O → Al2O3 + 3H2↑
- On anodizing, Al2O3 is formed on the surface as a porous layer
- The layer may be made compact by sealing: blocking pores by dipping the anodized object in hot water or exposing it to steam
- Applications:
- Metals such as Al, Mg, Ti, etc. are anodized to control corrosion
Cathodic Protection
- Cathodic protection involves providing electrons from an external source to make the metal or alloy remain as cathode
- Sacrificial Anode Protection:
- The metal to be protected from corrosion is converted into cathode by connecting it to a metal which is anodic to it
- Metals like Mg, Al, and Zn are used as anodes, which undergo corrosion and supply electrons to the protected metal
Corrosion Testing by Weight-loss Method
- Weight-loss tests are used to measure corrosion
- Clean metal coupon or a standard test piece is measured, weighed, and exposed to the corrodent for a known time
- The piece is then taken out, cleaned to remove corrosion products, and reweighed
- The rate of corrosion of the metal (R) is calculated using the equation: CPR = KW / ATD
- Units of corrosion penetration rate (CPR) can be converted to depth of corrosion or penetration
Metal Finishing
- Metal finishing is the process of modifying the surface properties of a metal
- Importance of metal finishing:
- Corrosion resistance
- Particular physical properties (electrical conductivity, thermal conductivity, and magnetic)
- Particular mechanical properties (hardness, heat and wear resistance, and solderability)
- To rebuild damaged parts or worn-out surfaces of machinery
- In modern engineering, metal finishing is used to manufacture electronic components, semiconductors, and metal articles with specific properties
- Electroplating process:
- Deposition of a metal or alloy on a conducting surface through electrolysis
- Principal components:
- Electroplating bath containing a conducting salt that furnishes the ions of the metal to be deposited
- Electronically conducting anode (usually made of the metal to be deposited)
- Electronically conducting cathode (the object to be plated)
- Inert vessel containing the electrodes and aqueous electrolyte
- D.C. electric power source
- Example: Electroplating of copper
- Types of plating:
- Chromium plating (hard, wear-resistant, and corrosion-resistant)
- Electroless plating (controlled autocatalytic deposition of a metal from its salt solution by a reducing agent without electrical energy)
- Applications:
- Electroless copper plating is used in the manufacture of printed circuit boards (PCBs)
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Description
This quiz covers the importance of metals and alloys in various engineering fields, their properties, and corrosion science concepts. Learn about the characteristics of common metals and their applications.
