Chapter 5: Corrosion Flashcards
why do metals corrode?
metals want to have a more stable state
types of attack mechanism
chemical attack
electro-chemical attack
what is direct chemical attack?
aka oxidation
current or electrons travel through the metal for only a short distance
what is electro-chemical attack?
there is anodic and cathodic area in presence of electrolyte
current or electrons flow through more distances than oxidation
what is an electrolyte?
liquid containing ions or charged particles
difference between anodic and cathodic area
anodic area is more reactive (area where it corrodes) than cathodic area
why do oxidation occur in metals?
some metal form oxides to protect the metal (Cr, Al)
requirements for electrolytic corrosion to occur
1) presence of electrolyte in contact with metal
2) potential difference between anode and cathode
3) conducting path for electrons
which side is corroded and protected? and which is -ve and +ve?
anode will be corroded (-ve)
cathode will be protected (+ve)
definition of anode and cathode
anode: oxidation occurs (give out electrons)
cathode: reduction occurs (uses up electrons)
anodic reaction formula
M –> M++ + 2e-
cathodic reaction formula
H2O + 2e + 1/2 O2 —> 2(OH)–
resulting reaction formula
M2+ + 2(OH)– —> M(OH)2
how is rusting of steel controlled by?
- moisture
- dust (hygroscopic)
- pollutants (SO2, turns in to H2SO4 to attack the steel)
difference between oxidation and electrolytic corrosion
oxidation: dry corrosion
- occur at higher temperatures
- short distances
electrolytic corrosion: wet corrosion
- occur in the presence of water/moisture
- longer distances
what is uniform corrosion? and what causes it?
most common form, least damaging
caused by direct chemical attack (acids, oils)
occurs uniformly over the entire surface
what is localised corrosion?
all other forms of corrosion (except direct chemical attack)
occurs at specific locations
spreads quickly, makes the steel brittle
when does galvanic corrosion occur, and what are the requirements?
occurs when 2 dissimilar metals are in contact
requires the presence of:
electrolyte
conductive path
what is the emf series?
ability of metal to resist corrosion (higher better, more corrosion resistance)
gold (more cathodic)
platinum
silver
copper
HYDROGEN
Tin
Nickel
Cadmium
Iron
Zinc
Aluminium
Magnesium (more anodic)
what is galvanic series?
ranking of metals relative to their electrochemical reactivity in sea water
rate of galvanic corrosion depends on?
difference in pd in the emf series, further away the 2 metals are, greather the pd, faster the corrosion will occur
area ratio:
large anode area (uniform corrosion, slower rate)
small anode area (localised corrosion, faster rate)
where is crevice corrosion found?
with stagnant solution, occurs on gaps
how is crevice corrosion formed?
crevices form in inaccessible corners by bad design which reduces oxygen within the area (becomes anodic)
how does crevice corrosion work?
concentration cell form due to difference in oxygen concentration in electrolyte
more oxygen - cathodic
less oxygen - anodic
how can crevice corrosion be prevented?
- welding instead of rivet or bolts
- using non-absorbing gaskets
- remove accumulated deposit
- design to avoid stagnant areas
what is pitting corrosion?
localised attack produces small holes or pits in metal
difficult to detect (pits covered by corrosion products)
very destructive
causes of pitting corrosion
started by a break in oxide film
water factors:
acidity
low oxygen concentration
high concentration of Cl
how does stress corrosion occur?
effect is accelerated when operating with high stress
causes of stress corrosion
pd occurs when stress levels are different
area with higher stress become anodic
often in cold worked materials
how to prevent stress corrosion?
- removing stress through design (round corners) or through annealing
- remove/neutralise the environment
what is intergranular corrosion?
localised attack on or beside grain boundaries
causes of intergranular corrosion
caused by precipitates (Chromium Carbide) that are concentrated at grain boundaries, making it anodic
how to prevent intergranular corrosion?
- high temp. HT to redissolve chromium carbide particles
- rapid cooling in CCR
- lower carbon content to < 0.03%
- add stabilisers (Nb, Ti)
what is microbiological corrosion?
corrosion caused by biological organisms (fungi, bacteria)
how does microbiological corrosion work?
organisms grow on metallic surface
area beneath organism (anodic)
causes of microbiological corrosion
- chemical attack on metals by the by-products of microbes
- microbial attack on organic materials
corrosion control methods
1) correct choice of material
2) design to minimise effect of corroding agents
3) cathodic protection
4) use of protective coatings
5) environmental control
types fo alloy for corrosion control and its effect
Cu: resistance to acid and salt solutions
Cr, Al, Si: forming passive oxide film
Ti & Nb: resistance to intergranular corrosion
Mo: resistance to crevice and pitting
what does a good design mean to prevent corrosion?
to prevent any liquids from staying and making contact with the part (allow it to spill off)
what is cathodic protection?
supplying the material with electrons and forcing it to be a cathode
what is sacrificial anode and how does it work?
sacrificial anode corrodes instead of the material attached to it
- sacrificial anode supplies electrons to the material
- prevent anode reaction at material
- Zn/Mg
- will be consumed and must be replaced
3 main type of coating for corrosion?
metallic
organic
inorganic
types of metallic coating
1) electroplating
2) coating in liquid state
3) cladding
types of inorganic coating
1) ceramic
2) glass
types of organic coating
1) paint
2) varnish
3) lacquer
environmental control methods to prevent corrosion
1) lowering temperature (corrosion rate increases with temperature)
2) decrease liquid velocity (fast flowing fluid can cause erosion corrosion)
3) remove oxygen from fluids (corrosion rate increases with oxygen concentration)
4) reduce ion concentration
5) adding inhibitors to electrolytes (slow down anodic or cathodic reaction)
how does adding inhibitors to electrolytes slow down corrosion?
- forming thin film / bulky precipitate to coat and protect it
- combine with corrosion product to form passive layer
