Most metals naturally form an oxide layer when exposed to oxygen. However, the effectiveness of this layer as a barrier varies significantly between metals.
Copper: Forms a green patina. This layer is stable and protective, preventing further corrosion of the metal underneath.
Mild Steel: Forms a porous oxide layer (rust). Because it is porous, moisture and oxygen can penetrate the layer, causing the metal to continue corroding until it fails.
Stainless Steel: Forms a stable, non-porous chromium oxide layer. This creates an airtight seal that prevents any further corrosion.
Exam Tip: Never refer to a paint finish simply as a “spray.” Spraying is the method of application; the finish is the paint itself (e.g., cellulose or acrylic).
Metal Finish
Method of Application
Purpose / Typical Use
Cellulose & Acrylic Paints
Metal is cleaned and degreased. A primer (often red oxide) is applied to create a "keyed" surface. Undercoat and top coats are applied via brush or spray.
Adds color and texture. Provides a barrier against corrosion. Low-cost finish for steel.
Electroplating
Component and donor metal are placed in an electrolyte solution. A direct current is applied, depositing metal ions from the donor onto the component.
Improves corrosion resistance. Hardwearing and decorative (e.g., chrome, zinc, nickel).
Polymer Dip Coating
Metal is heated and dipped into a fluidized bed of polymer powder. The powder melts on contact, forming an even plastic coating.
Tough, impact-resistant. Used on wire products (dish racks) and outdoor equipment.
Metal Dip Coating
Cleaned metal is dipped into molten metal. Includes tin plating and galvanizing (zinc).
Excellent corrosion protection. Tin for food cans; galvanizing for structural steel.
Powder Coating
Component is electrically charged. Charged thermoset powder is sprayed on, then baked in an oven to melt and cure the finish.
More durable than paint. Used on white goods (fridges), gates, and railings.
Metal Varnishing
Metal is polished and degreased. A clear coat is applied by brush or spray.
Protects the surface while keeping the natural metal color visible. Used on brass and copper.
Anodizing (A-level)
Aluminium acts as the anode in an acid electrolyte. A controlled oxide layer forms, which can then be dyed.
Increases hardness and corrosion resistance. Used on high-end electronics and flashlights.
Cathodic Protection
Cathodic protection is an electrochemical technique used to prevent corrosion. When two different metals are in contact in the presence of moisture, they form an electrochemical cell. The more reactive metal becomes the anode (and corrodes), while the less reactive metal becomes the cathode (and is protected).
Methods of Protection
Impressed Current
An external power supply provides a continuous flow of electricity to the structure.
Usage: Large-scale infrastructure like underground pipelines and storage tanks.
Sacrificial Anodes
A block of a more reactive metal (like zinc or magnesium) is attached to the main structure.
The sacrificial metal corrodes away entirely to save the main structure.
Usage: Ship hulls, offshore oil rigs, and underground pylons.
Finishes can improve appearance, durability, corrosion resistance and surface performance.Vacuum forming: a heated thermoplastic sheet is drawn over a former and held while it cools.Injection moulding: granules are melted and driven by a screw through a nozzle into a closed mould.Extrusion forces softened material through a shaped die to produce a continuous cross-section.Blow moulding: air expands a hot polymer parison against the inside of a mould.Rotational moulding coats the inside of a heated rotating mould before the hollow product cools.Compression moulding uses heat and pressure to form a measured charge inside a mould.A pattern forms a cavity in sand before molten metal is poured, cooled and removed.Die casting injects molten metal into a reusable metal die under pressure.Investment casting forms a ceramic mould around a sacrificial pattern for detailed components.MIG welding feeds a consumable wire electrode into a gas-shielded weld pool.TIG welding uses a non-consumable tungsten electrode and separate filler where required.Spot welding clamps sheet between electrodes and creates localised heat through electrical resistance.Steam softens lignin so timber can be bent around a former and held until dry.Thin veneers are glued and clamped around a former to create strong curved components.Flexography uses a relief plate and rollers to transfer ink to the substrate.Offset lithography transfers the image from plate to blanket cylinder and then to the substrate.Screen printing pushes ink through open areas of a mesh stencil onto the substrate.A focused laser follows a programmed path while assist gas helps clear the cut.Milling removes material with a rotating multi-point cutter while the work is accurately positioned.In turning, the workpiece rotates while a cutting tool removes material to create cylindrical forms.FEA divides a model into elements so stresses and deformation can be calculated.CFD predicts how fluids move around or through a product.A go/no-go gauge quickly checks whether a feature lies within its permitted tolerance.Vernier callipers measure external, internal and depth dimensions accurately.A micrometer uses a precision screw to measure small dimensions accurately.A tensile test records how a standard specimen responds as an increasing axial load is applied.Hardness tests compare resistance to permanent indentation under controlled conditions.Anthropometric data provides measured human dimensions for designing appropriate product sizes and clearances.Ergonomics applies knowledge of people to improve comfort, safety, usability and performance.Bauhaus design favoured functional forms, modern materials and design suited to industrial production.Art Deco is recognisable through geometric decoration, symmetry, luxury materials and machine-age influence.Memphis challenged restrained modernism with bold colour, pattern, geometry and playful forms.Arts and Crafts valued skilled making, natural inspiration, material honesty and resistance to poor-quality mass production.The life-cycle model helps explain how sales, competition and redesign priorities change over time.Sustainable design decisions should reduce resource use and keep products and materials useful for longer.
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