6.4 Flashcards
Does not include corrosion susceptibility
what is corrosion
a natural phenomenon where chemical or electro chemical reactions cause metal to oxidise
what needs to be present for corrosion to happen
difference in electrode potentials and an electrolyte
what metals are less likely to corrode
noble metals such as gold and platinum
what metal can corrode easily when in contact with concrete covered in water and why
aluminium because the water washes lime out of the concrete and lime is alkali which corrodes the aluminium
what are the 2 chemical changes in corrosion
anodic and cathodic
out of anodic and cathodic what loses material in corrosion
the anodic metal loses the material
what metals have a passive film to prevent corrosion and what is it
titanium and stainless steel because they have a thin oxide layer
what metals have a loose and porous film which can be easily penetrated by electrolyte
magnesium and aluminium
what is the best way to control corrosion
to have good cleanliness
what is galvanic corrosion
2 dissimilar metals make electrical contact in the presence of an electrolyte
what kind of metals corrode easier
less noble
what is the difference between galvanic and electrolytic corrosions
electrolytic requires an electrical current to be passed through both metals for any corrosion to take place
where is microbiological corrosion usually found
in fuel tanks
what does the micro organisms do to causes corrosion
produce corrosive chemicals
how can microbiological corrosion be minimised
maintaining a maintenance programme which includes water checks looking for a milky appearance
what is stress corrosion
intergranular cracking caused by the combination of stress and corrosion
how can stress corrosion be revealed
through ndt methods other than visual inspections
what does BVID mean
barely visible impact damage
what is filiform corrosion
worm like corrosion under paint surface which leads to intergranular
how can filiform corrosion be removed
bead blasting or sanding at the early stages
what is pitting corrosion and what materials does it occur in
white powdery deposit that extends vertically happens in magnesium and aluminium
what is intergranular corrosion
corrosion that spreads along the metal grain boundaries and seriously affects structural integrity
what is exfoliation corrosion
advanced form of intergranular the surface grains gets lifted by the expanding corrosion products
what is fretting corrosion
where 2 surfaces rub the protecting oxide layer off each other allowing fresh active metal on the surface
how is fretting corrosion identified
a black or grey deposit
what is crevice corrosion
where stagnant solutions get in the crevices causing corrosion
how is crevice corrosion reduced
by sealing joint
what is mercury corrosion
mercury is very reactive on aluminium and eats through it the process is amalgamation
what affects corrosion
what different metals are in contact, environmental and operational conditions, size of material, operational stress, composition of metal alloys
what is a sign of corrosion in copper and copper alloys
green to blue copper salts
what is a direct chemical attack
when alkaline or acidic liquid contacts metal
what affects the rate of corrosion for a direct chemical attack
the formation of the protective film, if the film has been mechanically removed, flexing of the metallic surface and the changes in temperature
what are some key areas prone to corrosion
exhaust, battery compartment, toilet and galley areas, bilge areas
why does corrosion happen in dry corrosion
there is direct chemical attack in the absence of moisture
If the corrosion takes place due to electrochemical attack in the presence of moisture or conducting medium what type of corrosion is this
wet corrosion
