Degradation Flashcards
Define degradation.
Loss of structural, mechanical or physical properties over long periods.
How can degradation be caused?
Chemical or mechanical processes.
Give an example of chemical degradation.
Corrosion of steel to form rust is a process of chemical degradation.
Give an example of mechanical degradation.
Propagation of micro-cracks during cyclic loads (i.e. fatigue).
Are mechanical and chemical degradation often coupled?
Yes for example steel corrosion crack of metals, where fatigue of fractures in metal is favoured by a corrosive process taking place near the tip of the crack.
Name 3 types of mechanical degradation?
1) Fracture
2) Fatigue
3) Creep
What type of deformation is creep?
Permanent deformation is similar to plastic deformation.
When does creep happen?
In metals at high temperatures (>0.4Tm).
What type of systems experience creep?
Turbine rotors in jet engines and steam generators.
When testing for creep what is kept constant?
Stress and temperature.
When testing for creep what is measured?
The strain is measured and plotted as a function of time.
What are the 3 different areas of a creep curve?
1) Primary
2) Secondary
3) Tertiary
What is the primary area on a creep curve?
Strain hardening takes place and deformation becomes difficult.
What is the secondary area on a creep curve?
Competition between strain hardening and recovery. Longest duration.
What is the tertiary area on a creep curve?
Strain rate increases as failure or rupture occurs.
What is corrosion?
A reaction between a metal and its environment.
What are the losses to the UK economy due to corrosion?
5% of GDP.
What is dry corrosion?
Fast reaction between the surface metal and atmospheric oxygen.
Is aluminium reactive?
Yes very and forms an impermeable oxide layer which protects it from further oxidation.
Is copper reactive?
Yes as it reacts with oxygen in the atmosphere to form a protective layer. The green patina is formed of varying mixtures of copper chlorides, sulphides, sulphates and carbonates, depending upon the environment.
Which electrode undergoes corrosion?
Electrons leave the anode, therefore, corrosion occurs.
What happens at the cathode?
Electrons arrive and the positive electrolyte ions precipitate.
What is standard electrode potential?
Two materials are coupled, the one corroding (the anode) is that with the lowest electrode potential.
What is rust also known as?
Iron hydroxide.
How does mill scale form?
Hot steel reacts with oxygen in the air to produce an oxide film called mill scale.
What does corrosion require?
Oxygen and Water
Why doesn’t steel reinforcement bars in concrete corrode?
The cathodic reaction formed OH but typically cement solution having a high pH meaning they are already rich in OH ions.
