Metal Finishing Flashcards
Why do metals need finishing?
Most metals have an oxide layer which provides a barrier against environmental effects
But steels have a porous oxide layer which allows moisture to penetrate the metal leading to rust
Finishing is required to protect metals from environmental attack
Cellulose and acrylic paints
Used on low cost metals
Surface of the metal is cleaned and degreased which ensures the primer can ‘grip’ onto the surface
Red oxide primer is often used followed by and undercoat and then a final coat
Can be applied by brush or spray
Provide colour and a textured finish
Provide a barrier against corrosion
Specialist paints can provide special graphic effects e.g. Pearlescence
Electroplating
The product and donor are placed in a container with an electrolyte solution
Direct current is applied and the product attract the donor metal
Donor metals include - zinc, silver, gold, copper and tin
Protective layer, more hard wearing and enhanced aesthetics
Polymer dip coating
Metal is heated to 230 degrees C
Hot product is dipped into a tank of fine polymer powder with air blowing through it (fluidisation bath)
This provides an even coating
Heat melts the polymer powder over the product which is then air cooled
Used for coat hooks, outdoor play equipment frames and tool grips
Metal dip coating - tin plating
Metals are dipped into a tank of molten plating metal (donor metal)
Provides a barrier against corrosion
Metals must be clean and degreased mild acid is used and flux applied to prevent oxidation before coating
Tin plating - pass sheets of steel through a tank of molten tin at 320 degrees C. Often used to provide a non-corrosive coating to food cans
Powder coating
Product is coated is negatively charged and positive thermoset polymer resin is the sprayed via an airgun
The product is then baked in an oven - the heat melts the powder to give an even coating
Aesthetic appeal - wide range of colours
Protection from corrosion
More hard wearing than paint - hard and durable
Used for - washing machine bodies, dishwater bodies, refrigerator bodies, metal gates, fencing and railways
Metal varnishing
The metal is polished to a shine and grease is removed
Varnish is applied via spray or a fine brush
Protects the metal from corrosion and allows the metals colour to show through (aesthetically pleasing)
Used on aluminium, brass and copper
Sealants
Sealant is applied with a cloth or machine pad to produce a film that is then allowed to cure for up to 15 mins
Then buffed with a cloth to a shine
Tough polymer-based sealant coatings protect polished surfaces from decay and tarnishing
Often used in the automotive industry against the effects of weathering, road salts and insect attack
Preservatives
Applied via a cloth, spray or immersion of the product
Silicon sealants are classified as preservatives - used on machine beds to provide a smooth, snag-free surface to allow materials to be pushed across the bed more easily and safely
Provide medium to long term protection for metal surfaces
Used on moulds and dies for preventing fingerprints showing on the surfaces, minor atmospheric corrosion or condensation build up
Anodising
Current flows through electrolyte solution from the anode to the cathode
The part to be treated is the anode - oxide layer builds up on this part to produce the anodised finish
The product can then be finished with a clear lacquer or varnish to seal the finish and provide further protection against scratches
Commonly used for aluminium products to enhance the natural oxide layer, resulting in increased hardness and toughness
Can also provide colour
Used in the outer casings of torches, external hard drive covers and casings for phones
Cathodic protection
All metals have a natural voltage and when two metals are joined together there is a current
If water is present the result is an electrochemical cell and one of the metals will corrode
The cathode is less easily corroded whilst the anode is more easily corroded
A sacrificial metal is made to be the anode to protect the cathode
Impressed current - the component to be protected is connected to an electric supply. The current flows through an inert electrode which protects the component from corrosion
Sacrificial anodes - a sacrificial metal is used to protect the valued metal.
On large structures the sacrificial metal is monitored for signs of corrosion and replaced when the base metal is no longer protected
Used for submerged steel structures, pipelines, ship hulls, jetties and offshore platforms
Metal finishes
Cellulose and acrylic paints
Electroplating
Polymer dip coating
Metal dip coating
Powder coating
Metal varnishing
Sealants
Preservatives
Anodising
Galvanising
Dipping steel into molten zinc at 460 degrees C
Metal is cleaned and degreased - mild acid may be used
Flux is applied to prevent oxidation before galvanising
Used for beams, gates, animal pens and low carbon steel car bodies
Benefits of galvanising
Significantly prolongs the life of the steel
It chemically bonds to the steel to form an abrasion resistant layer
Even if scratched it won’t corrode as steel can’t corrode adjacent to the zinc
Zinc is recyclable, reusable and a low maintenance finish
