Grade 9 Technology: Preserving Metals

Metal Classification and Fundamentals of Oxidation and Rust

• Ferrous Metals:

  • Defined by the presence of the element iron ($\text{Fe}$) in the metal composition (e.g., steel).
  • The presence of iron makes ferrous metals significantly more prone to undergo oxidisation when exposed to the environment.
  • Ferrous metals are generally less expensive than non-ferrous metals but require active preservation against corrosion.

• Non-Ferrous Metals:

  • Metals that do not contain iron molecules in their chemical structure.
  • Examples include aluminium ($\text{Al}$), copper ($\text{Cu}$), zinc ($\text{Zn}$), and gold ($\text{Au}$).
  • Non-ferrous metals are more expensive than ferrous metals.
  • Non-ferrous metals do not rust, making them naturally resistant to iron oxidation processes.

• Oxidation and Rust Formation:

  • Oxidation: The chemical reaction of a metal with oxygen.
  • Rust: The specific corrosion product formed on iron and iron alloys through the process of oxidation.
  • Rust occurs when iron reacts with both oxygen and water in the surrounding environment.

• Environmental Factors Accelerating Rust on Steel Objects:

  • Contact with acid or an acidic environment.
  • Sea spray in the air (salt-laden marine atmospheres).
  • Climates that are misty, have a high rainfall, or are very humid (environments with a high level of moisture in the atmosphere).

Environmental and Economic Context of Metal Preservation

• Mining and Resource Consumption:

  • Mining raw metallic ores carries severe negative impacts on the natural environment, including habitat destruction and resource depletion.
  • Recycling metallic materials and equipment significantly reduces the global demand for virgin mining.

• Role of Preservation:

  • Extending the working lifespan of ferrous and non-ferrous components through preservation directly prevents premature disposal.
  • Protection techniques preserve structural integrity, minimizing material degradation and the environmental burden of replacement.
  • The primary preservation techniques applied to metals include painting, galvanising, and electroplating.

Metal Preservation by Painting

• Overview and Characteristics:

  • Painting the exposed surface is the most common method employed to prevent rust from forming on metal.
  • Serves as an inexpensive and accessible approach to preserving ferrous metals.

• Protective Mechanism:

  • Paint forms an impermeable physical barrier between ambient air and the underlying ferrous metal, cutting off contact with oxygen and moisture.

• Application Requirements and System Structure:

  • Effective protection requires thorough and correct surface preparation of the metal prior to painting.
  • The specific paint formulation must be chosen correctly to suit the operating environment and purpose of the metal.
  • Complete protection requires a multi-coat application consisting of three distinct layers:
  • Primer: The base layer applied directly to the prepared bare metal to promote adhesion and inhibit corrosion.
  • Undercoat: The intermediate layer applied over the primer to build thickness and provide uniform color and body.
  • Topcoat: The final exterior layer that seals the system against weather, wear, and environmental moisture.

• Vulnerabilities of Painted Coatings:
  • The fundamental weakness of paint is that any small area where the paint chips, scratches, or flakes off leaves the bare metal exposed.
  • The exposed spot comes into direct contact with ambient air containing oxygen and water, allowing rust to form and undermine the surrounding coating.

Metal Preservation by Galvanising

• Definition of Galvanising:

  • A method used to protect iron or steel by covering it with a protective exterior layer of zinc ($\text{Zn}$).
  • Zinc is selected because it is highly resistant to corrosion and acts as both a barrier and a sacrificial shield.
  • Once iron and steel have been galvanised, they resist corrosion even under direct exposure to ambient air and moisture.

• Primary Methods of Galvanising:

  • Hot-dip galvanising.
  • Electroplating.

• Hot-Dip Galvanising Process:

  • Involves immersing cleaned iron or steel into a bath of molten zinc.
  • Produces a corrosion-resistant, multi-layered coating consisting of metallurgical zinc-iron alloy layers topped with pure zinc metal.
  • While steel is immersed in the molten zinc bath, a metallurgical reaction occurs between the iron in the steel and the molten zinc.
  • This reaction is a diffusion process, causing the coating to form perpendicular to all surfaces, which guarantees a completely uniform thickness throughout the entire component.

• Advantages and Applications of Hot-Dip Galvanising:

  • Relatively Inexpensive: Highly cost-effective for large components and structural elements.
  • Exceptional Toughness: The resulting zinc layer is thick and metallurgically bonded to the substrate.
  • Extended Outdoor Durability: Highly suitable for heavy-duty outdoor use over prolonged lifespans ranging from $20\text{ to }50\text{ years}$.

• The Three Main Steps of Hot-Dip Galvanising:

  • Step I: Surface Preparation: Cleaning and degreasing to remove all surface contaminants, oxides, and scale.
  • Step II: Galvanizing: Submerging the component into the molten zinc bath.
  • Step III: Post-Treatment and Inspection: Cooling, passivating, and examining coating thickness and uniformity.

• Detailed Step-by-Step Industrial Sequence for Hot-Dip Galvanising:
  • Hot Caustic Degreasing: The steel is immersed in a heated alkaline bath where paint, oil, and grease residues are removed.
  • Hydrochloric Acid Pickling: The component is bathed in hydrochloric acid ($\text{HCl}$) where rust and mill scale are chemically stripped away.
  • Water Rinse: Cleanses the acid and dislodged particulate matter from the steel surfaces.
  • Hot ZAC Preflux: The surface is treated in a zinc ammonium chloride (ZAC) flux solution, fluxing the surface to prevent pre-dip oxidation and promote metallurgical bonding.
  • Zinc Bath: The fluxed steel is submerged directly into molten zinc, where the diffusion reaction takes place and the steel is fully galvanised.
  • Chromate Quench: The hot galvanised component is quenched in a chromate solution to cool it rapidly and provide surface passivation.

Metal Preservation by Electroplating

• Fundamental Electroplating Concepts:

  • Electroplating: The process of depositing one metal onto another metal through electrolysis.
  • Electrolysis: A process where an electric current is passed through an electrolyte to drive a chemical reaction.
  • Electrolyte: A solution containing dissolved positive and negative ions that conducts an electric current.

• Characteristics of Electroplated Finishes:

  • Electroplated coatings are usually thin.
  • Produce a highly reflective, mirror-like aesthetic finish.
  • Because electroplated coatings are thin, they are primarily suitable for indoor use rather than severe outdoor weathering.

• Circuit Architecture and Mechanism of Electroplating:

  • Direct Current Power Source: A battery supplies the electrical potential required to drive ion migration.
  • Cathode (Negative Terminal): The object being plated (e.g., a penny) is connected to the negative end of the battery, making it the negatively charged cathode.
  • Anode (Positive Terminal): A strip of the coating metal (e.g., zinc) is connected to the positive end of the battery, acting as the positively charged sacrificial anode.
  • Electrolyte Bath: A metal salt solution containing dissolved ions of the plating metal (e.g., zinc ions).
  • Electron Flow: Electrons travel from the negative terminal of the battery through the cathode, traverse through the solution/circuit, move up through the anode, and return into the positive terminal of the battery.
  • Ion Migration and Deposition:
  • Positively charged zinc ions in the solution are electrostatically attracted to the negatively charged cathode.
  • Upon contacting the cathode, these positively charged ions gain electrons, reduce to neutral metal atoms, and firmly attach themselves as a solid metal layer on the surface.
  • Anode Replenishment:
  • As zinc ions deposit onto the cathode, more positively charged zinc ions are pulled off the solid zinc anode and dissolve into the electrolyte bath, continuously replenishing the solution.

• Common Plating Metal Examples:
  • Unplated Base: A regular copper penny substrate before plating.
  • Nickel Plating: A penny plated in nickel, producing a bright, silvery, corrosion-resistant surface finish.
  • Gold Plating: A penny plated in gold, creating a lustrous, non-reactive decorative layer.