Failures of metals and alloys

What can change mechanical properties 

Temperature 

When does brittle fracture occur in what material

Lower temperature crystalline planar surface

What is fatigue failure

Failure by cyclic loading at stresses below ultimate tensile stress and usually below yield strength

What is fatigue stress caused by

Nucleation and growth of a crack

When does fatigue failure occur

When crack reaches critical size for the load

What are 4 types of fatigue testing

Bend test, beam loading, Wohler rotational bending, cyclic uni-axial loading

What is cyclic loading usually

Compressive and tensile or only tensile, sometimes only compressive

What is Wohler rotational bending

When a material is help in a chuck with a weight on the end whilst rotating

What is the mean stress with no tensile load in a Wohler bending test

0

What do macro fatigue failures show as

Beach marks

What do micro fatigue failures show as

Striations

Where do cracks run from in fatigue failures

Free surfaces when stress concentrations exist- change in geometry and weld edges

What is Miners law

Damage fraction is fraction of life used up by an event or series of events

What 3 things is Miners law based on

Linear accumulation of damage, loading is fully reversed with no mean stress or strain, change in loading level does not alter proportion of life remaining

What happens to stress and strain if its at a third of its life used

At any other component it is equivalent to a third of the component at any other stress level 

Where can corrosion fatigue occur

In aqueous, gaseous and liquid metal systems

What does corrosion fatigue do and when does it happen

Happens spontaneously and turns metal alloys back to natural states

When do metals/ alloys display corrosion resistance

When they have a protective surface film or coating

Why does corrosion fatigue happen

Second law of thermodynamics, spontaneous process occurs if there is an increase in entropy

What is entropy

Degree of disorder

What is corrosion driven by

Thermodynamically but also need to know kinetics of reactions to assess corrosion risk to structures 

What is life cycle costing

Investing in quality materials to provide long term savings on operations and maintenance

Why is cost from corrosion increasing

Average cost of structures rising, failure to educate about materials, materials are wanted to work in more hostile environments, false economies in material selection

4 corrosion classification environments 

Dry oxidation, emulsions, aerosols and mist, aqueous corrosion 

Where does dry oxidation corrosion happen

Hot flue pipes and scaling

Where does emulsion corrosion happen

Oil and water mixes

Where does aerosols and mist corrosion happen 

Coastal salt air and chemical vapours 

Where does aqueous corrosion happen 

Most common natural and synthetic environments 

What is corrosion

An electrochemical process

What 4 things does corrosion involve

Removal of electrons, consumption of electrons, oxidation at the anode, reduction at the cathode 

What 4 things are needed for aqueous corrosion 

Anode, cathode, metallic path electrically connecting anode and cathode, conductive electrolyte that the anode and cathode must be immersed in 

What shows the conservation of energy in corrosion

Anode and cathode processes occur at the same rate, anode becomes acidic, cathode becomes alkaline unless the material is washed away 

General corrosion equation for oxidation of metal at the anode

Metal= metal cation + electrons

What do cations do in solution

React with solution to form hydroxides and may then form oxides which precipitate

Equation for cations which precipitate

Cation + water = metal oxide + hydronium

After electrons flow to the cathode, what happens at the cathode

Reduction of oxygen or hydrogen

Equation for reduction of oxygen at cathode 

Water + oxygen + electrons = hydroxyl anions or water + electrons = hydroxyl anions + hydrogen 

3 types of corrosion 

Uniform attack, localised corrosion, galvanic attack 

What is uniform attack

Oxidation and reduction reactions occur randomly across the surface

Rate of uniform attack for steel in sea water

0.1mm/yr

What is localised corrosion

Corrosion when there are a distinct anodic and cathodic regions

What does localised corrosion cause

Pitting corrosion

Which corrosion can be spotted easily and which is hard to spot

Uniform attack easily spotted, localised corrosion difficult to spot

What is galvanic attack 

Occurs when two metals having different compositions are electrically coupled whilst exposed to an electrolyte 

How to limit galvanic attack

2 metals chosen close together in the galvanic series and electrically insulate dissimilar metals from each other

What is galvanised steel

Steel with a zinc coating, hot dipped or electroplated

How does galvanised steel work

Zinc acts as a barrier

What does zinc do in galvanised steel if there is a scratch

oxide choke- steel below corrodes then zinc fills in the gap

Why is galvanised steel suitable for ships

White finish so does not then need to be finished

What is a galvanic series

Ranking of reactivity of metals and alloys in environments, if they are far apart, larger voltage difference so more current flow and corrosion 

What does pitting corrosion lead to

Perforation failures

What is pitting failures related to

Passive surfaces with defects or microstructural variations

What is oxygen differential

Most localised attack driven by oxygen availability for cathode reaction

What happens to anode and cathode in oxygen differential

Cathode in high oxygen area, anode in low oxygen area

Why are coating effects used

Breaks the electrical circuit and insulates and dries the surface

What is erosion corrosion

Combined action of chemical attack and mechanical abrasion or wear as a consequence of fluid motion 

What is erosion corrosion harmful to 

Metals that passivate 

How to increase erosion corrosion rate

Increasing fluid velocity

When are solutions more erosive

When bubbles and suspended particulate solids are present

What is cavitaion

Mechanically dominated corrosion from bubbles collapsing on a solid surface

How do decrease cavitation

Chose a more cavitation resistant alloy, use a more hydrodynamic propeller design

What law does corrosion rate obey 

Faradays law where charge= number of electrons x faradays constant x number of moles reacted

What types of corrosion obey faradays law 

All aqueous corrosion like pitting, crevice, stress corrosion cracking, corrosion fatigue