List the mechanical properties considered when selecting materials
- strength - tensile strength
- stiffness - elastic modulus
- toughness - fracture resistance
- hardness - wear resistance
- fatigue resistance
- creep resistance
Explain the effect that temperature has on mechanical properties
The tensile strength and elastic modulus of metals decrease with increasing temperature. Also, at low temperatures less than 10C metals that are normally ductile can fail in a brittle manner
List the categories that corrosion can be classified into
- uniform
- galvanic
- pitting
- intergranular
- stress
- erosion
- fatigue
- high temperature oxidation
- hydrogen embrittlement
List the four components necessary for an electrochemical cell
- anode (corroding electrode)
- cathode (passive, non corroding electrode)
- conducting medium (electrolyte)
- completion of the electric circuit (through the material)
Cathodic areas can arise in what ways
- dissimilar metals
- corrosion products
- inclusions in the metal (such as slag)
- less well-aerated areas
- areas of different concentration
- differentials strained areas
Describe uniform corrosion
Uniform wastage of material by corrosion
Describe galvanic corrosion
If dissimilar metals are placed in contact, in an electrolyte, the corrosion rate if the anodic metal will be increased as the metal lower in the electrochemical series will readily act as a cathode
Describe pitting
Very localised form of corrosion that forms pits in the metal surface
Describe intergranular corrosion
Preferential corrosion of material at the grain(crystal) boundaries
Describe erosion corrosion
Increased rate of attack caused by a combination of erosion and corrosion - high velocity or turbulence
Describe high temperature oxidation
Corrosion is normally associated with aqueous solutions, but oxidation can occur in dry conditions. Carbon and low alloy steels oxidize at high temperatures.
Describe hydrogen embrittlement
Loss of ductility caused by the absorption (and reaction) of hydrogen in a metal
What factors must be considered when selecting materials for corrosion resistance
- temperature
- pressure
- pH
- presence of trace impurities
- amount of aeration
- steam velocity and agitation
- heat transfer rates
What methods can be applied to protect materials from corrosion
- isolation of the anode from the electrolyte
- cathodic protection
- use of inhibitors
- minimization of corrosion
What are the most important characteristics to be considered when selecting materials of construction
- Mechanical properties
- The effect of high and low temperatures on mechanical properties
- Any special properties required (thermal conductivity, electrical resistance, magnetic properties)
- Ease of fabrication
- Availability in standard sizes
- Cost
