6.4 Corrosion Flashcards
Why does corrosion occur in metals?
Corrosion occurs because of the tendency for metals to return to their natural state.
How does corrosion occur in metals?
Corrosion attacks metal by chemical or electrochemical action and converts it into a metallic compound, such as an oxide, hydroxide or sulphate.
Substances that cause corrosion are called corrosive agents.
Why do gold and platinum not corrode?
Gold and Platinum do not corrode easily since they are noble metals (do not lose elections easily)
They are also chemically un-combined in their natural state.
What is meant by the term active metals?
Those (metals) which tend to lose electrons easily.
(Magnesium and Aluminium)
What is meant by the term ‘noble metals’?
Those (metals) which do not lose electrons easily (Gold and Platinum)
Name the two general types of corrosion?
Pitting Corrosion
Intergranular Corrosion
Filiform Corrosion
Exfoliation Corrosion
Fretting Corrosion
Fatigue Corrosion
Microbial Corrosion
Stress Corrosion
Galvanic Corrosion
Crevice Corrosion
Explain how / why microbial corrosion occurs?
Microbial attack includes actions of bacteria, fungi or moulds.
They deplete oxygen and/or oxidise inorganic compounds and sulphur
How does stress corrosion occur?
Stress corrosion is an intergranular cracking of the metal caused by a combination of stress and corrosion.
Stress nay be caused by internal or external loading.
What kind of corrosion is caused after a battery acid spill?
Chemical corrosion
Name the corrosive agents that give the best conditions to start corrosion?
Corrosive agents cause corrosion.
Acids
Alkalis
Salts
The atmosphere
Water
Micro-organisms
How would you recognise corrosion on the surface of steel?
A red rust colour
How would you recognise corrosion on the surface of aluminium?
A white-to-grey powdery colour
What would the word “smoking” imply when referring to corrosion around rivets?
Fretting corrosion ( Damage can occur at the interface of two highly loaded surfaces, which are not supposed to move against each other)
Engine Pylon example
What form of corrosion is likely to occur after improper heat treatment?
Stress corrosion which can be transgranular or intergranular
How would you identify filiform corrosion on an aircraft?
Filiform corrosion is identified by it’s characteristic worm-like trace or corrosion products beneath the paint film.
Where is exfoliation corrosion most likely to occur?
Exfoliation is most prone to occur in wrought products such as extrusions, thick sheet sand, and thin plates which have elongated flat type grain structure.
What causes stress corrosion cracking?
Caused by a combination of stress and corrosion, stress may be cuased by internal or external laoding.
Produced by non-uniform deformation during cold working
+ Unequal cooling from high temperatures
+ By internal structural rearrangement involving volume changes
What type of corrosion occurs when two dissimilar metals meets?
Galvanic corrosion
Which aluminium alloys are most likely to develop intergranular corrosion?
High strength aluminium alloys like 2024 and 7075 can develop intergranular corrosion if they have been improperly heat treated and are exposed to corrosive environments.
(Not quenched quickly enough) Dendritics grow ..
What type of corrosion is exfoliation corrosion?
Exfoliation corrosion is an advanced form of intergranular corrosion where the surface grains of a metal are lifted up by the force of expanding corrosion products occuring att he grain boundaries just below the surface
What form of corrosion occurs when two highly loaded surfaces which are not supposed to move against each other, move due to vibration?
Fretting Corrosion
What causes fatigue corrosion?
Fatigue corrosion is caused by the combined effects of cyclic stress and corrosion.
If the cadmium plating on a steel bolt fails before the bolt what kind of corrosion is this?
Galvanic Corrosion - Two dissimilar metals make electrical contact in the presence of an electrolyte
Microbial attack includes action of bacteria of which there are two types, name both?
Bacteria may be either aerobic or anaerobic
(Aerobic require oxygen to live)
What is the definition of an electrolyte?
A chemical, either a liquid or a gas, which conducts eletrical current by releasing ions that unite with ions on the electrodes.
An examply of an electrolyte is water
What relative humidity at does Filiform corrosion occur and how can it be removed?
Filiform corrosion occurs when the relative humidity of the air is between 78% and 90% and the surface is slightly acidic.
Filiform corrosion can be removed by using glass bead blasting material with portable abrasive equipment and/or sanding.
What is corrosion?
Corrosion is an electrochemical attack on a metal that changes some of the metal into its salts.
Corrosion destroys the strength of the metal
What is oxidation?
The chemical action in which a metallic element is united with oxygen.
Electrons are removed from the metal in this process.
Be aware of where elements occur
The metal that is attacked is or _____ suffers an ______ change and the corrosive agent is reduced and suffers a _____ change
oxidised
anodic
cathodic
In terms of flow of electrons:
Conventional flow
Electron flow
Conventional flow is from positive to negative
Electron flow is from negative to positive
The cathode is the ______ charged electrode
negatively
The anode is the _____ charged electrode
positively
What provides the path for corrosion in electrochemical action
The path is provided by the electrolyte, which may be a surface film of moisture, which contains such pollutants as acids, salts , or other industrial contaminations
Pitting Corrosion
White, powdery deposit that can extend vertically
This corrosion can be the starting point of intergranular corrosion
Caused by contaminations during the alloying process
intergranular Corrosion
Intergranular corrosion is an attack along the grain boundaries of the metal with little to no indication on the surface.
Caused by improper heat treatment then exposed to corrosive environments like 2024 or 7075
Filiform Corrosion
Special form of oxygen concentration cell corrosion or crevice corrosion - organic coating ssystem
Worm-like trace beneath paint film
Occurs when the relative humidity of the air is between 78% and 90% and the surface is slightly acidic
Can be removed by using a glass bead blasting material with portable abrasive equipment and/or sanding
Exfoliation Corrosion
Exfoliation corrosion is an advanced form of intergranular corrosion where the surface grains of a metal are lifted up by the force of expanding corrosion.
Occurs in wrought products such as extrusions, thick sheet sand, thin plates which have elongated flat type grain stucture.
Only way to remove is to cut out panel or area.
Fretting corrosion
Damage can occur at the interface of two highly loaded surfaces, which are not supposed to move against each other,.
Fretting can cause severe pitting
Dampening of vibration, tightening of joints, application of a lubricant or installation of a fretting resistant material between the two surfaces can reduces fretting corrosion
Engine pylons most likely place to occur ( smoking rivet)
Fatigue Corrosion
Fatigue corrosion is caused by the combined effects of cyclic stress and corrosion.
Microbial Corrosion
Microbial attack includes actions of bacteria, fungi, or moulds.
They accelerate corrosion by oxidising sulphur to produce sulphuric acid. Also deplete oxygen supply or release metabolic products.
Microbial corrosion can be minimised with a maintenance program which includes frequent sump draining, tank inspection, total removal of microbial growth, and application of bio-cide with ‘soak periods’
Stress Corrosion
Stress corrosion cracking is an intergranular cracking of the metal which is caused by a combination of stress and corrosion, stress may be caused by internal or external loading.
Applying protective coatins, stress relief heat treatment, using corrosion inhibitors, or controlling the environment may reduce stress corrosion.
Galvanic Corrosion
Galvanic corrosion occurs when two dissimilar metals make electrical contact in the presence of an electrolyte.
