Metal finishes.
The use of finishes · Technical principles
Revision notes
Metal Finishing
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.
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