6.4 corrosion m Flashcards
when does corrosion from on a metal?
whenever there is a difference in electrode potentials between parts of metal structure when in contact with an electrolyte
what do alkalis react with metals to create?
metallic salts (corrosion)
wat is used as a surface treatment for painting?
phosphoric acid solution
wat are ferrous metals subjected to but what is more vulnerable?
subject to damage but aluminium is more vulnerable to strong alkaline solutions than acids
corrosion should be distinguished from erosion but how is this done?
mechanical action
why does corrosion occur?
because of metals tendency to return to its original state
d noble metals corrode easily?
no because they are chemically pure in there natural state
where do all corrosive attacks start?
on the metal surface
what is the metal surface known as?
as electrolyte
what does the corrosion process involve?
two chemical changes
anodic and cathodic
what does the anode look like?
like it is being eten away
what happens in an anodic reaction?
electrolyte donates electrons into the internal circuit so the anode is oxidized
wat happens in a cathodic reaction?
receive electrons from anode and the electrons reduce the oxygen and water monocles into hydroxide ions and mixes with metal ions
what are examples of metals that produce corrosion products?
stainless steal
titanium
wen can the electrolyte easily penetrate and continue the corrosion process?
when the corrosion film is lose and prosperous
where does corrosion tend to form?
on less resistant surfaces but can also happen in boundary’s in between layers
what can corrosion in boundary layers result in?
pits and intergranular corrosion
what metals tend to lose electrons easily?
magnesium and aluminum
do metals corrode faster when thy lose more or fewer electrons?
when they lose more more easily
why may metals be corrosive resistant?
when they produce a passive oxide film on there surface
for corrosion to form what requirements must be met?
an electrical potential difference with the metal
a conductive path between the two areas of potential difference
some form of electrolyte or fluid covering the two areas
can corrosion be prevented?
no but can be controlled
what is one of the best way to control corrosion?
cleanliness
when does galvanic corrosion?
when two dissimilar metals make electrical contact in the presence of electrolyte
metals from what group corrode easier?
less noble groups
what metals will more actively corrode on the galvanic scale?
further apart on the galvanic scale
what is electrolytic corrosion?
when electrolyte and the flow of an electrical current between two metals
what does galvanic corrosion describe?
the interaction between two different metals
wat is an electrolytic corrosion caused by?
external source of current
wat does microbiological corrosion include?
actions of bacteria fungi or mold found in the fuel tanks
bacteria can be classified in to two category’s what are they?
aerobic
anaerobic
what does aerobic bacteria require?
require oxygen to live
how dos aerobic bacteria accelerate corrosion?
by oxidizing Sulphur to produce sulfuric acid
when does anerobic oxygen live?
can only survive when free oxygen is no present
what is fungi?
micro organisms that feed on organic materials
what slows the growth of microbes?
low humidity
what are ideal growth conditions for most micro organisms?
temperatures of 20 °C - 40 °C
relative humidity of 85% - 100%.
where does microbiological growth occur?
a the interface of water an fuel
what does the growth of fungus produce?
organic acids
alcohols
esters
Microbial corrosion can be minimized with a maintenance programmed that includes what?
A scheduled draining of water from the fuel tanks.
Inspections that look for a milky-white substance, indicating that microbial growth is present, followed by removing the microbial growth.
An application of biocide to prevent further growth.
wa are the two main areas susceptible to corrosion?
toilets and galleys
what is stress corosion?
growth of crack formation in corrosive environment
what can stress corrosion lead to?
unexpected and sudden failure of normally ductile metal alloys when subjected to tensile strength or cyclic loading
wat is a common place for stress corrosion?
between rivets in a stressed skin
what can sow the extent of stress corrosion?
dye penetrant
what is filiform corrosion?
form of oxygen-concentration cell corrosion or crevice corrosion that occurs on metal surfaces with an organic coating
how is filiform corrosion identified?
a worm like trace of corrosion produced beneath the paint film
when does filiform corrosion occur?
when the relative humidity of the air is 78% - 90% and the surface is slightly acidic
where does filiform corrosion start?
at breaches in the coating system as well as in fastener heads and extends along the surface of the painted sheet underneath the coating
what can happen if filiform corrosion is not removed?
can lead to intergranular corrosion
what is the most common type of corrosion on aluminum and magnesium?
pitting corrosion
wat is pitting corrosion?
a localized form of corrosion that produces holes in the material
what can holes in materials from pitting corrosion also cause?
act as stress risers and acid in forming stress croon
how is intergranular corrosion presented?
as cracking and tends to accelerate over time
what are the protective coatings for intergranular corrosion?
paint
primer
cladded alliminum
exfoliation corrosion is an advanced form of what?
intergranular corrosion
where is exfoliation most prone?
extrusions
thick sheets
thin plates with elongated flat type grain structure
what is fretting corrosion?
special type of erosion corrosion which occurs vibration and slipping
how is fretting identified?
black or gray brown powder paste
what helps reduce rubbing friction?
lubricate or sealant
what is crevice corrosion?
attacks the metal surface through as stagnant solution
describe crevice corrosion?
sever localized corrosion at narrow gaps between assemblies
is mercury corrosion common on aircraft?
not commonly found
what is the attack on aluminum through mercery called?
amalgamation
how does a mercury attack happen?
mercury rapidly attacks the grain boundary’s of the aluminum
what is corrosion?
deterioration of a metal due to its reaction between different metals and their operational environment
are thicker or thinner materials more prone to corrosion?
thicker
what will effect the rate and type of corrosion?
operational stresses
what is a major factor in corrosion?
composition of the material
what are aircraft materials selected on?
the material strength and the cost
corrosion resistance is a secondary consideration
what contaminates are in air and gas?
chlorides
surfaces
nitrates
what climates increase corrosion?
warm moisture found climates
why are noble metals used in aircraft assemblies?
because of there resistance to surface attack and there improved electrical and thermal conductivity
how can copper corrosion be removed?
mechanically removed then the area is treated with a chromic acid solution
can corrosion occur in dry conditions?
yes
what do most metals form in dry room temp conditions?
a thin oxide layer
what does aluminum carry?
a thin but highly protective oxide layer
how is dry corrosion explained?
by absorption mechanisms
what type of surfaces does dry corrosion occur on?
heterogenous and homogenous
is dry corrosion uniform?
yes
is dry corrosion a quick or slow process?
slow process
where does wet corrosion occur?
when the relative humidity exeads 60%
what can wet corrosion result in?
pitting and stress cracking corrosion
does corrosion happen quicker in freshwater or saltwater?
saltwater
when is corrosion known as wet corrosion?
takes place from electrochemical attack with presence of moisture of conducting medium
what type of surfaces does wet corrosion occur on?
heterogenous surfaces
what is wet corrosion explained by?
electrochemical mechanism
is we corrosion uniform?
no
where does wet corrosion happen but where does it accumulate?
takes place at anode but accumulates at cathode
what is a direct chemical attack?
when an alkaline or acid liquid contacts metal
when there is no current flow or definitive anodic or cathodic area ho can a direct chemical attack still happen?
formation of protective film on the surface
mechanical removal of protective films
flexing on the metal surface
changes in temperature
what can cause sever corrosion and embrittlement of titanium alloys but above what temp?
phosphate ester hydraulic fluid
when above 120 degrease Celsius`
what can happen when materials are placed under to much stress?
corrosion can occur and the material can crack
what must happen to corrosion prone areas?
must be cleaned, inspected and treated more frequently than less corrosive areas
what re corrosion prone areas?
exhaust areas
battery components
lavs and gallys
bilge areas
wheel wells and landing gear
external skin areas
why is a battery compartment a corrosion prone area?
fumes from overheated battery’s
what must we do with led battery’s?
clean and neutralize frequently
why are lavs and galleys corrosive?
spilled food and waste products spilled on the floor
what must we do to bilge areas to prevent corrosion?
keep clean and free of contaminates
what is the wheel well and landing gear exposed to?
mud
salt
gavel
flying debris
when inspecting landing gears and wheel wells where should attention be focused?
high strength steel compartments
exposed surfaces
exposed seiches
areas where water can be trapped
wheels
rigid tubing
what are aircraft surfaces usually covered with?
a protective finish but it is still susceptible to corrosion
are areas subject to high operational areas more or less likely to corrode?
more likely
what can cause damage to the original surface protection?
trimming
drilling
reverting
id aluminum corrosive resistant?
generally corrosive resistant
what corrosion types can attack aluminum ?
surface
galvanic
pitting
intergranular
what are some aluminum parts connected with?
anodized coating
how can the white to gray powdery material be removed form aluminum?
mechanical polishing with a softer material
what is the corrosive product of aluminum?
white to gray powder
how can the white to gray powder be removed?
mechanical polishing with a matirial softer than the metal
what are some aluminum parts protected by?
a anodized coating
what do anodizing do?
increases the thickness of the oxide film
what will happen if the anodized coating is damaged in service?
the costing will be partially restored by chemical surface treatment
what can degrade the performance of copper and copper alloy components?
minor amounts of corrosion on copper
what is the product of corrosion on copper?
bluish green coating on the surface
what type of corrosion is copper susceptible to?
surface and intergranular corrosion
what is magnesium’s susceptibility to corrosion?
highly susceptible
what should happen when the protective coting fails on magnisium?
should be repaired promptly and completed repaired to avoid serious structural damage
is magnesium corrosion easy to detect?
yes possibly the easiest
what does corrosion on magnesium look like?
lifting of the paint film as white spots on the surface which rapidly develop in to snow like mounds or whiskers
what does correction of corrosion on magnesium look like?
remove corrosion by applying chemical conversion coat and protective finish
does titanium corrode easily?
no highly corrosive resistant
when may titanium see degradation of its structural prophies?
when in repeated contact with chlorinated solvents at high temps
what does titanium do to adjacent parts?
cause sever corrosion through galvanic corrosion
how can the prevention o the titanium part crossing adjacent surfaces be done?
galvanically isolated through barrier coatings or films
what can promote cracking in titanium?
contact with zinc or cadmium plated parts
when are alloys with a nickel basis often used?
challenging environments that require good resistance against high and low temperatures
how can corrosion on nickel alloys be identified?
green powdery deposit on the surface
what does cadmium coating protect?
steel parts and provides compatible surface when a part contacts another material
what does a corrosive attack on cadmium look like?
white to brown to black motting on the surface
when does the cadmium plate protect the metal up until?
until actual iron rust appears
when will steel begin to pit?
when exposed to harsh corrosive environments
what type of corrosion is stainless steel highly suseptiable to?
crevice and stress corrosion cracking in moist salt laden environments and can cause galvanic corrosion for any other metal they are in contact with
how can corrosion of stainless steel be prevented?
by keeping it clean
