3: Corrosion Flashcards
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
activation polarization
electrochemical reactions are surface reactions which include a series of steps, one of which is rate limiting.
activation is related to the concept of passing the energy barrier for the slowest step.
no overvoltage = no net rxn
overvoltage = driving force for rxn
def concentration polarization. when is it an issue ? what does it affect ?
exists when the reaction rate is limited by diffusion in solution.
if the reaction rate is low or the concentration of species is high, diffusion is not rate limiting.
However, when the rate is high or concentration is low, three may be a depletion zone next to electrodes.
affects only the cathode.
study corrosion rates from polarized data
def passivity
some metals form a thin oxide film which serves as a barrier to corrosion (mainly chromium, iron, nickel, titanium, alloys)
this occurs at anode only
def passive region
oxide layer forms on surface, preventing passage of current
def transpassive region
the potential is high enough to break the oxide layer
name forms of corrosion
- uniform
- galvanic
- crevice
- pitting
- intergranular
- selective leaching
- erosion-corrosion
- stress corrosion
def uniform attack
- form of corrosion
relatively homogeneous corrosion over the whole surface
def galvanic corrosion. avoided yb what ?
occurs when two metals or alloys having different composition are electrically coupled while being exposed to an electrolyte
how can u avoid galvanic corrosion
- choosing metals that are close in galvanic series (low emf potential)
- avoid unfavourable anode-cathode area ratios (have a large anode)
- electrical insulation
- connect to a their, more anodic, more metal
def crevice corrosion
corrosion that occurs because of the concentration differences in ions: occurs in regions of low concentration
def pitting corrosion
concentration difference driven like crevice corrosion, however the corrosion pit forms as a deep well, driven by gravity
def intergranular corrosion
occurs at grain boundaries often when metals are heated between 500C and 1000C
often associated with welding
def selective leaching
happens when one element of an alloy is selectively removed by a corrosion process.
ex: dezincification of brass leaving porous copper with poor mechanical properties
def erosion-corrosion corrosion
occurs from the combined action of chemical attach and mechanical abrasion from fluid motion.
especially harmful to alloys that normally passivate (removes protective layer)
define stress corrosion
some metals which normally resist a certain corrosive environment may corrode when stress is applied in addition to the corrosive environment.
small cracks form and propagate in a direction perpendicular to the stress. the stress does not need to be externally applied.
def hydrogen embrittlement
not exactly corrosion but a form of degradation. some metal alloys, specifically steel, experience significant reduction in ductility and tensile strength when atomic hydrogen penetrate the structure of the material.
it is typically described as a failure mode. may be induced by cathodic protection.
how to prevent corrosion
- select material that doesnt corrode in application environment
- make environment less corrosivve
- use chemical corrosion inhibitors in the environment which chemically bing the surface and prevent corrosion
- use of protective coating
- make parts of interest a cathode
cathodic prevention
create electrical connection to a more anodic metal which will corrode preferentially (Zn, Mg). the metal to be oxidized is called a sacrificial anode, can also be force by applying an external potential (rectifier)
surface oxidation
oxide-reduction reactions are also possible in solid-gas environment (without liquid electrolyte). the oxidation of a metal by atmospheric oxygen is called dry corrosion
to obtain good conformal oxide coating, volume of oxide should be the same as volume of metal it is replacing
dry corrosion
oxidation of a metal by atmospheric oxygen
pilling-bedworth ratio evaluates what
to obtain conformal oxide coating, the volume of the oxide should be the same as the volume of the metal it is replacing
surface oxidation:
PB ratio is:
A. <1
B. =1
C. >1
A = oxide takes less vol than metal
B = oxide takes same volume
C = oxide take greater volume than metal
kinetic of surface oxidation:
well adhering films, growth is limited by….
growth kinetics is what shape/relationship
for well adhering films: oxide growth limited by ionic diffusion, which is described by Fick’s law. The growth kinetics for these oxides is parabolic
for surface oxidation, for films that are porous or flake off, oxidation rate = ?, growth is ? PB is ?
oxidation rate constant
growth is linear
PB is <1 or >2
for surface oxidation, for very thin oxides (<100nm), growing close to room temp, what growth kinetics are observed
logarithmic
