Wood and metal finishing

Question 1

Give 4 reasons for finishing wood

Answer 1

preventing moisture absorption
protect against decay
protect against insect attack
enhance appearance

Question 2

Describe the three ways in which natural wood degrades

Answer 2

Wet rot - alternating wetness and dryness means that moisture is absorbed into the wood and remains below the surface when I dries = breaking down wood fibres and spreading of fungus (wood becomes soft wet and crumbly)

Dry rot - different, fast spreading fungus (wood becomes dry, soft and powdery)

Insect attack - woodworm and beetles attack wood and lay eggs in cracks = hatched larvae eat into wood creating tunnels
when beetle forms it eats its way out

Question 3

What must be done before applying a wood finish?

Answer 3

make surfaces smooth either by planing and or sanding with glass paper
remove all dust with cloth

Question 4

What do most metals (apart from steels) have that provides some barrier against environmental effects?

Answer 4

oxide layer

Question 5

Describe coppers oxide layer

Answer 5

copper originally has a bright reddish colour but over time a greenish protective oxide layer, or patina, forms

Question 6

Describe steels (but not stainless steels) oxide layer

Answer 6

porous allowing moisture to penetrate the metal leading to rust

Question 7

What happens when two different metals are joined together and why?

Answer 7

electrical current as all metals have a natural voltage

Question 8

What happens if water is present at the join of two different metals?

What is this reaction?

Answer 8

electrochemical cell and one of the metals will corrode

current flow through the water from the anode (more actively corroding) to the cathode (less actively corroding)

Question 9

What is cathodic protection used for?

Answer 9

controlling rate of corrosion by making the metal the cathode and the anode a sacrificial metal to protect the base

Question 10

Give a real life example of when cathodic protection is used

Answer 10

protecting steel structures buried in soil or water such as pipelines

Question 11

What are the 2 methods of cathodic protection?

Answer 11

impressed current

sacrificial anodes

Question 12

Describe the impressed current method of cathodic protection

Answer 12

component to be protected is connected to electrical power supply
impressed direct current flows from an inert electrode through any liquid to the component to be protected i.e. electrode is cathode and metal anode is protected

Question 13

Describe the sacrificial anode method of cathodic protection

Answer 13

more electrochemically active metal is wrapped around or joined to less active metal to provide resistance to corrosion