HS2440 Week 3 Asyn_Electrochemical corrosion basics (copy) Flashcards
The three things necessary for corrosion to occur
Soluble salts, oxygen and water
examples of corrosion of iron and bronze
- laminating and pitting
- bronze is more stable and has a range of differing corrosion products
Order of stability of metals
- The reactivity series of metal
- Each metal has a unique energy level, the electrochemical series measures this
- measures in terms of reactivity, the more negative the number, the more reactive the metal is
What happens when a pure metal is plunged into water
- it dissolves, leaving positive ions in the water, and it gains an electronegative surface charge, as electrons are produced but do not mix in the water
- with very reactive metals, a lot of ions and electrons are produced
- a point is reached when some of the ions are redeposited onto the metal, picking up electrons
- an equilibrium of desolution and redeposition is reached, this is the reversible equilibrium point, represents the energy level in a metal
- theoretical, because it requires totally pure metal and water
Different metals dissolve more or less in water, they have different equilibrium point (reactivity)
- describe what happens if the reaction is polarised
by removing electrons, more ions are produced, in order to build up the correct amount of electrons
- the reaction becomes polarised, and moves away from its equilibrium, reversible state
- when a metal is in its equilibrium state it is not corroding or picking up electrons, when it is polarised, the metal is corroding
What is electrolytic corrosion
- an electrochemical reaction, involving electrons and ions, for example in the equilibrium state, where ions are produced, and electrons are accumulated on the surface of a metal
- any salt soluble in water is an electrolyte, and the ions in water is required for electrolytic corrosion
- oxygen is usually dissolved in water
- impurities in a metal, or an alloy, can produce the energy difference which is required for corrosion
Diagram showing the reaction between two metals of different energy levels in an electrolyte
- the more reactive metal produces more electrons comparatively
- when connected by a wire, electrons flow from the more negative area, to where there are fewer electrons
- ## a voltmeter can measure the flow of electrons, and therefore the energy difference between the two metals
How to standardise the reactiveness of one metal compared to another
A half cell (diagram showing setup)
- electrons produced by zinc move over to the copper, where they build up
- electrons cannot build up on the surface, so the cooper ions in solution pick up the electrons and become metallic copper!
- we lose zinc and copper deposits build up
- the salt bridge balances the ionic charge so + = - as copper ions are being lost and zinc ions produced
- from this we understand that zinc is less stable than copper, and from the voltmeter we know the energy difference between the two
Measuring potential difference using half cells: standard conditions
How to create a scale for measuring energy levels?
What Volts and Amp means for corrosion of a metal
Using SHE to zero the scale
- electrons collect on the platinum, there are hydrogen ions in the solution, which use the hydrogen ions to produce gas
- magnesium has a negative value, copper would have a positive value, it draws ions
- Hydrogen creates the artificial zero, metals are either negative or positive in comparison to it
Summery of using hydrogen to measure energy of metals
How the energy scale can be used to estimate how metals will corrode
Galvanic Series
For marine environemnts
