6.4 corrosion Flashcards
what is corrosion
chemical or electrochemical reactions cause metals to be oxidised, forming salts and oxides
where does corrosion form
on metals where there is a a difference in electrode potentials between 2 parts of a metal structure which are in contact with an electrolyte
what acid is aluminum susceptible to
sulphuric acid
what is aluminium more vulnerable to: alkaline or acid
alkaline
do noble metals corrode easily?
no
is metal an electrode or electrolyte
electrode
water is the electrolyte
how is aluminium and titanium naturally protectedf
they produce a passive oxide film which is impermeable and prevent further surface oxidisation
what are the 3 requirements for corrosion to occur
electrical potential difference
conductive path
electrolyte
what is one of the best ways to prevent corrosion
cleanliness of the surface
what is galvanic corrosion
two dissimilar metals (more noble and less noble) make electrical contact in the presence of an electrolyte
what is electrolytic corosion
corrosion due to eleectrolyte and emf
where is microbiological corrosion commonly found
fuel tanks
what are the ideal growth conditions for micro-organisms
humidity of 85%-100%
temps of 20-40c
how can microbial corrosion be prevented
scheduled draining of water in fuel tanks
inspections for milky white substance
biocide
what other areas are microbial corrosion common
galleys
toilets
how is stress corrosion caused
cyclic loading
tensile stress
where is a common place for stress corrosion
between rivets in a stressed skin
around pressed in bushings
tapered pipe fittings
what is filiform corrosion
worm like trace of corrosion
crevice corrosion
beneath the paint film
when does filiform corrosion occur
when relative humidity is 78-90%
if filiform corrosion is not removed what happens
it can lead to interangular corrosion
what is the most common type of corrosion on aluminum and magnesium
pitting corrosion
what is pitting corrosion
holes on surfaCE
why is pitting corosion dangerous
decreases material strength
what is intergranular corrosion
cracking which tends to accelerate over time
a combination of chemical and electrolytic actions attacj material along grain boundary
what is exfoliation
advanced form of intergranular
in which products is exfoliation more common
wrought products
what is fretting corosion
occurs due to vibration and slipping
how does fretting work
rubbing removes protective layer on material
how is fretting identifyied
black or grey/brown powder/paste
what is crevice corrosion
attacks metal through stagnant water in crevice
where can crevice corrosion be seen
around the edges of nuts and rivet heads
what is mercury corrosion
mercury can corrode various metals and alloys
especially aluminium
what is the chemical reaction in mercury corrosion
amalgamation
are thicker structural components more or less susceptible to corrosion
more susceptible due to variations in their composition
what are corrosion prone areas example
exhaust
battery compartment
lav and gallies
bilge areas
wheel well
landing gears
external skin
is aluminium generally corrosion resistant
yes
what does anodising do
increases the thickness of oxide film on aluminium
naturally protective film
is titanium susceptible to corrosion
no it is highly corrosion resistant
what is a special form of oxygen concentration cell corrosion or crevice corrosion that occurs on metal surfaces
filiform corrosion
what is exfoliation corrosion
advanced form of intergrangular
if intergranular is left untreated what happens
becomes exfoliation
