M6.4 Corrosion Flashcards
What’s formed when alkalis or acids react with metals
Metallic salts
Why does corrosion occur
occurs because of the metals’ tendency to return to their natural state
Why aren’t noble metals as susceptible to corrosion
because they are chemically pure in their natural state.
What happens in relation to the anode and cathode in corrosion
Anode - donates electrons
Cathode- receives electrons from anode
What metals can produce a passive film
Stainless steel
Pure aluminium
Titanium
What three requirements must be met In order for electrolysis corrosion to form
Potential difference in the metal
Electrolyte
Conductive path between the two areas of potential difference
What’s galvanic corrosion
when two dissimilar metals make electrical contact in the presence of an electrolyte which causes erosion to occur
How does the galvanic scale affect the rates of corrosion
The further they are away from each other on the galvanic scale the faster the reaction will occur
What’s the difference between galvanic corrosion and electrolytic corrosion.
Electrolytic corrosion is caused by an external current whereas galvanic describes the difference between two metals
What’s microbiological corrosion
Corrosion caused by bacteria or fungi growing due to water and fuel meeting
What two sections can bacteria be classified into
Aerobic and anaerobic
How does low humidity affect the growth of fungi
It slows down the growth
What are the ideal conditions for microorganisms
20-40 degrees and 85-100% humidity
How can Microbiological corrosion be minimised
with a maintenance programme that includes:
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
What main areas of the aircraft are affected by microbiological corrosion
The galley,toilets and fuel tank
What’s stress corrosion caused by
Due to the surface being put under tension due to a load being applied to it
What’s filliform corrosion
oxygen-concentration cell corrosion or crevice corrosion that occurs on metal surfaces with an organic coating system
How can filiform corrosion be identified
Worm like trace under paint film
Under what environmental conditions does filiform corrosion usually occur under
78-90% humidity and slightly acidic surface
What can happen if filiform corrosion isn’t removed
Can lead to inter-granular corrosion
What’s the most common type of corrosion on magnesium and aluminium
Pitting
What’s pitting corrosion
localised form of corrosion that produces holes in the material
How is intergranual corrosion created
Due to not being quenched within the given 10 second time period after being heated
What type of metal is exfoliation corrosion occur in
Wrought metals
What’s fretting corrosion
Corrosion caused by two surfaces under a load vibrating or slightly rubbing together
What can be done to prevent fretting from occurring
Applying a lubricant or a sealant
What causes crevice corrosion
Stagnant water in a scratch or divot of a surface which causes and imbalance of o2 which causes a potential difference
What’s the chemical reaction called when mercury attacks aluminium
Amalgamation
In what conditions does wet erosion occur in
Humidity exceeding 60%
What’s skydrol’s importance in chemical attacks
Found within hydraulic fluid and cam cause sever corrosion and embrittlement of titanium alloys at temps of above 120degress
What are some factors that affect metal corrosion and the rate in which it occurs
Presence of a dissimilar, less corrodible metal (galvanic corrosion)
Anode and cathode surface areas (galvanic corrosion)
Temperature
Heat treatment and grain direction
Presence of electrolytes (hard water, saltwater, battery fluids etc.)
Availability of oxygen
Presence of different concentrations of the same electrolyte
Presence of biological organisms
Mechanical stress on the corroding metal
Time of exposure to a corrosive environmen
What are the corrosion prone areas on an aircraft
Under galley
Toilets
Exhaust areas
Undercarriage
Battery compartment
Bilge areas
Wheels and landing gear
External skin
What types of corrosion is copper particularly susceptible to
Intergranualr and surface
Whats produced when copper begins to corrode
A blueish greenish costing on surface
Is magnesium susceptible to corrosion if so what type
Pitting and yes extremely
Is titanium susceptible to corrosion and if so what
No
Is nickel susceptible to corrosion if so what type
No unless under extreme temp
Is cadmium and zinc susceptible to corrosion
Yes
Evident due to white to brown to black mottling of the surface
