1.5-Metal finishing Flashcards
Metal finishing- examples of metals that have a protective layer
- most metals have an oxide layer which provides some barrier e.g. copper has a bright reddish colour but overtime a greenish protective oxide layer forms on the metal
- steels have a porous oxide layer allowing moisture to penetrate the metal. The porous layers lead to layers of crumbling rust
Metal finishes- cellulose and acrylic paints- method of application&purpose
- paints used on low cost metals e.g. steel. Surface of metal cleaned and degreased which ensures primer coat has a sound surface to key to. Red oxide primer often used, followed by an undercoat
- provides colour/textured finish, barrier against corrosion, specialist paints provide texture or special graphic effects
Metal finishes- electroplating- method of application&purpose
- metals including silver, gold, zinc, copper and tin. Product and donor material placed in container with an electrolyte solution. Direct current appliedand product attracts donor metal
- protective layer, hardwearing finish, enhanced aesthetics
Metal finishes- polymer dip coating- method of application&purpose
- metal product heated to 230 degrees then dipped into tank of fine polymer poweder which has air blowing through it to provide an even coating. The retained heat from the product melts the polymer powder over the product which is then air cooled to set evenly
- wire coat hangers, coat hooks, dishwasher racks, outdoor play equipment frames
Metal finishes- metal dip coating- method of application&purpose
- metals coated by dipping product into a tank of molten plating metal. Metal must be clean and degreased. Can be tin plating (pass sheets through tank of molten tin) or zinc plating (dipping steel into moulten zinc)
- barrier against corrosion, tin plating- non corrosive coatig to food cans, zinc plating- industrial and agricultural applications e.g. beams, gates. Used in car bodies
Metal finishes- powder coating- method of application&purpose
- product is statically charged (negative). Thermoset polymer resin (positive) sprayed thwough an airgun. Charge results in a strong attraction between powder and product. Product baked in oven- heat melts the poweder to give an even coating
- protection from corrosion, aesthetical appeal. used on washing machine bodies, dishwater bodies, metal gates and fencing
Metal finishes- metal varnishing- method of application&purpose
- metal polished to a shine any surface grease removed. Varnish applied by spray or brush
- protects the metal and allows the colour of the base metal to show through. Used on aluminium, brass and copper
Metal finishes- sealants- method of application&purpose
- sealant applied with a cloth or machine pad to produce a film that is allowed to cure for 15 mins. Then buffed with a cloth to shine
- tough polymer based sealant coatings protect polished surfaces from decay and tranishing. Used in automative industry against the effects of weathering, road salts and insect attack
Metal finishes- preservatives- method of application&purpose
- applied by wiping on with cloth, spray or immersion of product
- silicon sealants can be classed as preservatives and can be used on machine beds for smooth, snag free surface
- temporary in process or final post processing, medium to long term, protection for metal surfaces
- used on moulds and dies to prevent fingertips showing on surfaces, condensation build up
Metal finishes- anodising- method of application&purpose
- electric current passes through sulphuric acid electrolyte solution to be treated to a negative cathode. As current flows from the part to be reated (positive anode) to the negative cathode, aluminium oxide layer builds up on the treated part, producing the anodised finish. Anodised products can be finished with clear laquer or varnish to seal the finish and provide protection agaisnt surface scratches
- aluminium products to enhance natural oxide layer- increased hardness/toughness. Colour and used in outer castings of tourches, external hard drive covers for phones
How do metals corrode
- all metals have a natural voltage and when two different metals are joined together, there is an electric current. If water present this results in an electrochemical cell and one of the metals will corrode- current flows through the water from the anode to the cathode
- r.g. two aluminium sheets held together with brass screws will form an electrochemical cell in the presence of rainwater. Electric current will flow between the anode and the brass due to voltage between the metals
What can be used to prevent corrosion of metals and how is this used
Cathodic protection- this controls the rate of corrision by
-making the metal the cathode of an electrochemical cell- the anode is a sacrificial metal which ismore easily corrorded so the anode corrodes while base metal protected
What products is cathodic protection used on
protect steel structures buried in soil or immersed in water e.g. pipelines, ship hulls, jetties and offshore platforms
Methods of cathodic protection- impressed current
- component to be protected is connected to an electrical power supply
- impressed direct current flows from an inert electrode through any liquid to the component to be protected e.g. a buried pipeline recieves direct curret from an electrode buried in the ground. Pipe becomes anode and electrode is cathode. Cathode corrodes overtime and corrosion is monitored
Methods of cathodic protection- sacrificial anodes
- uses a sacrifical metal to protect the metal product of value
- a more electrochemically active metal wrapped around or joined to less active metal to provide resistance to corrosion
- on large structures, the sacrificial metal is monitored for signs of corrosion and replaced when the base metal is no longer protected
