3: Corrosion
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corrosion of metals
the destructive and unintentional attack on a metal based on oxidative electrochemical processes involving liquid electrolytes
inverse of corrosion
electroplating
electroplating vs corrosion
electroplating: metal ions to metal corrosion: oxidation
electrochemical reaction
a chemical rxn involving electron charges. it involves pairs of rxn which are called RedOx rxn. in our case, it involves solids, electrons, and electrolytes (soln containing mobile ions). can happen in two separate location, connected by a conductor.o
oxidation happens where?
anode
reduction happens where?
cathode
describe oxidation
• at anode • an element, often metal, loses electrons (creating free electrons) in an oxidation rxn. thus, they become more positive. • an oxidation rxn can also be the Gian of oxygen or loss of H. • material is removed from the anode (corrosion)
give example of oxidation rxn (Al, w 3e-)
Al -> Al^(3+) + 3e-
describe reduction
• at cathode • an element or molecule gains electrons (capturing free electron) in reduction rxn thus, they become more negative • red rxn can also be loss of oxygen or gain of H • material is added to cathode (electroplating)
give example of reduction rxn (Al, w 3e)
Al^(3+) + 3e- -> Al
def electrochemical rxn
electrochemical rxns involve the flow of electrons that “mediates” two sides of a reactions allowing it to occur. Electrical contact is essential. it is driven by thermodynamics
name two *well-controlled different ways you can have electrochemical rxn (the set-up)
• same metal with a battery driving force • different metals which produce a potential
name a less controlled way an electrochemical rxn can occur
corrosion in pipes, corrosion of zinc in acid soln
what is the reference cell, what metal do we use ? why do we need it ?
• the standard hydrogen half cell • need: in order to predict the potential of any galvanic couple, the potential of each standard half-cell is measured against a reference half cell: the standard hydrogen half cell
def standard half cell
pure metal electrode immersed in a 1M soln of its ions at 25C
EMF series: all rxn are represented as what type of rxn?
reduction
the potentials for emf are valid in what conditions ? What can we use if these conditions aren’t met ?
emf: 1M soln at 25C, zero current the Nernst equation provides the cell potential and different temperatures and ion concentrations. it still requires zero current.
galvanic series is for what conditions
in sea water
same electrode material, two concentrations, which corrodes (high or low) ?
lower concentration will corrode to produce more electrons to even out the concentration of electrons
what parts of metals corrode fastest? if a metal has a tear how does this change corrosion ?
corners have lower electron concentration, thus, they corrode faster at bottom of tear, less electrons, corrode faster
if a metal plate is in a soln of its ions, not connected to anything, what happens ?
technically nothing on macro scale on micro scale, constant equilibrium between ox and red rxns. r,red = r,oxid = io/nF io = exchange current density which is an internal current corresponding to the equilibrium rate of the red and ox rxn
def polarization
change in electrode potential caused by some effect
when an electrochemical cell is doing work and current is flowing through it, what happens to the potential of the electrodes ? why ?
the potential of each electrode change bc of polarization effects
def overvoltage
the difference between closed circuit voltage (Vw) and the open circuit voltage (Vemf) n = Vw-Wemf