Metal finishes Flashcards
What is the reason for adding a finish to metals?
- protection or aesthetic reasons for applying a finish
- If the metal forms a protective layer by itself, it may not be finished
What are 5 types of finishes?
Galvanise
Paint/varnish
Cathodic protection
‘black it’ - blacking
Powder coat
What happens to aluminium without finishes?
Forms an oxide layer
Tough
Dull grey colour
What happens to copper without finishes?
Forms an oxide layer
Tough
Turns green
Very expensive material
May finish for aesthetic reasons - customer doesn’t like the green colour
What happens to steel without finishes?
Finish on steels with carbon content <0.4% (e.g. Mild or low carbon steel, high carbon steel won’t oxidise for a long time)
Oxide layer will form but will be washed off easily by rain or by wiping it
Describe the process of painting?
- Clean it (in abrasive solutions)
- Prime it (seals surface and makes paint go on smoother) (x1)
- Under coat (x1)
Top coat (sometimes coloured) (x2) - Can paint using electrostatic charging where the metal in charged (usually negative) and charge a spray nozzle -ve so the paint is attracted to the metal
Describe the process of powder coating?
- Grind up plastic pellets to a powder (usually polyester)
- Pump air into it to create a fluidising bath
- Clean the metal
- Heat the metal gently
- Dip the metal into the plastic powder fluidising bath
- The longer it stays in the thicker the coat
When removed it forms a plastic coating around the whole metal surface
Describe the process of galvanising (only with steel)?
- Clean the metal thoroughly
○ Give it an acid bath
○ Flux the surface to keep it clean whilst heating - Heat zinc to molten liquid
- Dip the metal into the molten fluid at an angle so air can escape from tubular pieces
- Iron in the steel reacts with the zinc and forms a very strong and hard wearing layer due to strong bonds
Maintenance free and very long lasting finish
What does anodising aluminium do?
- Can’t paint aluminium easily (will flake off as it doesn’t bond very well)
- Instead, it will need to be anodised
- Makes a natural layer of aluminium oxide what can protect the layer
- Anodising induces a much thicker oxide layer
- After anodising, it can be dyed or coloured (oxide layer is very porous) which is much tougher than painting
- Submerge aluminium in sulfuric acid whilst running a charge through it
It is a highly controlled oxidation process
Describe the process of anodising aluminium?
- Clean surface by submerging it in a detergent bath
- Etching - a thin layer of Al is removed to form a smooth flat surface
- Rinse the Al
- Submerged into the sulfuric acid bath whilst a charge is run through it and it oxidises
- Causes the growth of the oxide layer dependant to the time it is left in
- Rinsed in de-ionised water
- Usually dyed to make it more aesthetic as the oxide layer is very porous
Seal the porous surface to make it very durable and make the colour long lasting
Describe the process of blacking?
- Extremely thin layer
- Clean the metal
- Place in a black oxide layer
- Rinse with distilled water (if not then will cause rusting)
- Immediately, dip into penetrating sealant and leave for a suitable time
Shake off excess oil and allow to dry for a day before handling
Describe the process of enamelling?
- Clean metal
- Coat with glass powder
- Heat metal to around 850°C
- Glass powder will fuse together
- Cool to provide a hard wearing surface finish
Brittle surface which can chip or crack if bent
Describe the process of electroplating?
- Pass a current through an electrolyte solution
- Electrodes are dipped in to solution
- One is connected to metal and the other is connected to the solution
- When a current flows, the electrolyte splits and metal ions are deposited on the metal and electrode
- This creates a thin layer
What are the advantages and disadvantages of electroplating?
- Protects object from tarnishing and corrosion
- Can change objects properties (e.g. Layer can be conductive or magnetic)
x Long process
x Shows surface imperfections e.g. Pitting markets or scratches
