6.4 corrosion Flashcards
What reactions are corrosion?
electrochemical or chemical reactions
What does corrosion create?
Salts and oxides which replace the metal
When does corrosion form on a metal?
When there is a difference in electrode potentials in contact with an electrolyte
What is an electrolyte?
A solution that produces an electrically conducting fluid
Acid and alkalis react with metals to form what?
Metallic salts (corrosion)
Where is sulphuric acid found?
Batteries
What element is extremely active in corrosion of aluminium?
Sulphuric
What is used to prepare a surface for painting?
Chromium or phosphorus
What metals are more subject to damage from acid and alkalis?
Ferrous metals
Is aluminium more prone to alkalines or acids?
Alkalines
Why does aluminium corrode when left on concrete?
Because water washes out lime from cement which forms alkaline
What is erosion?
Destruction through mechanical action
Why does corrosion occur?
Because of the metas tendancy to return to its natural state
Why do gold and platinum not corrode easily?
They are chemically pure in their natural state
Where do all corrosive attacks occur and why?
On the surface of the metal because it is considered an electrode in contacts eith an electrolyte
What do different parts of the surface act as?
An anode and a cathode
What joins the anode and cathode of a metal together?
The internal metal surface as it forms an electrical surface
What does the corrosion process involve in terms of chemical changes?
Anodic and Cathodic reactions
What happens in an anodic reaction?
The electrode donates electrons to the internal circuit so the anode is oxidised. So the metals atoms of which it is composed become positive ions and dissolve in the electrolyte
What happens in the corresponding cathodic reaction?
The cathode receives the electrons (via the internal circuit) and reduce the dissolved oxygen and water molecule to hydroxide ions. The hydroxide reacts with the metal ions to form rust
What is the cathode protected from?
Corrosion
What do some metals including stainless steel and titanium produce?
A passive film which prevents corrosion
Why is the passive film invisible?
Because it is so tightly bound to the surface
What happens when a corrosion film is loose and porous?
The electrolyte can easily penetrate and continue the corrosion process
What are most alloys made up of?
Small crystaline regions (grains)
What does corrosion form on?
Less resistant surface regions
Where else can corrosion form?
What can this cause?
At boundary’s between regions
Pits and Intergranular corrosion
What makes a metal corrode faster?
If electrons are lost easier
What metals corrode quickly due to easy release of electrons?
Magnesium and aluminium
What metals don’t lose electrons easy?
Noble metals such as silver and gold
What two metals produce the passive film?
Becsuse of this what are they known as?
Aluminium and Titanium
Corrosion Resistance
For corrosion to form, what three things must be present?
An electrical potential difference
A conductive path between the two areas of potential difference
Electrolyte or fluid covering both areas
How can you control corrosion?
By cleaning the surface because moisture accumulates dirt and grease
When does galvanic corrosion occur?
When two dissimilar metals come into electrical contact with an electrolyte
When do metals from the less noble group corrode easier?
When they are in contact with more noble metals
What makes the corrosion more active?
The more further apart they are on the galvanic scales
If something is more noble, is it less or more susceptible to corrosive attack?
Why?
Less
Higher number on galvanic scale
If something is less noble, does this mean it is less or more susceptible to corrosive attack?
Why?
More
Lower number on galvanic scale
What are the top three on the more noble side?
Platinum
Gold
Silver
What are the top three on the less noble side?
Magnesium
Zinc
Aluminium
What is electrolytic corrosion caused by?
An electrolyte and the flow of an electric current between two metals caused by Ann externally induced EMF
In the case of large motor and generator bearings, what can induced EMF’s result in?
Current flow in the bearing resulting in bearing corrosion.
What does bearing corrosion look like?
Pinhole type pits
What is electrolytic corrosion caused by?
An external source of current
When can galvanic corrosion not occur
Where there is no potential difference between two metals
Without electrically conductive joints
Without connection of electrolytes
What does a microbial attack consist of?
Bacteria fungi or moulds
Why is a microbial attack common in fuel tanks?
Because the water and fuel interface with each other
Aerobic corrosion needs what to live?
Oxygen
Why does aerobic corrosion accelerate corrosion?
Oxidising sulphur to produce sulphuric acid
What does bacteria living adjacent to the metal release that promotes corrosion?
Metabolic products
What is different about anaerobic bacteria?
It doesn’t need oxygen to survive
The metabolism of Anearobic bacteria requires them to obtain support how?
by oxidising inorganic compounds
What are fungi?
Inorganic materials that feed on organic materials
What slows the growth of microbes but does not kill them?
Low humidity
What is the ideal temperature for growth of microbes?
20 degrees to 40 degrees
What is the ideal humidity for growth of microbes?
85% to 100%
Where does microbial growth occur?
At the interface of water and fuel
What colour is the fungus on the bottom of the tank when dry?
Brown
How can microbial corrosion be minimised?
Drain fuel water
Inspections for milky white substance
Apply biocide
What two other areas are susceptible to microbial corrosion?
Toilets and Gallies
Even if the contamination is not corrosive, why can it still be bad?
It can attract and hold moisture promoting microbial growth
What is Stress Corrosion Cracking?
The growth of crack formation in a corrosive environment
When can Stress corrosion cracking fail when subjected to?
Cyclic loading and tensile stress
Where can stresses come from?
Something that has been pressed into a structural part with tight tolerance
Where does the corrosion attack in a SCC focus on?
The end of a crack
Where is a common place for stress corrosion?
Between rivets in a stress skin
Tapered pipe fittings
Pressed in bushings
What types of inspections may reveal SCC?
Visual and dye
Where does filiform corrosion occur?
On metals surfaces with an organic coating
How is filiform corrosion identified?
By worm like traces
When does filiform corrosion occur?
When the relative humidity 78% to 90% and the air is slightly acidic
What is an advanced state of filiform corrosion?
Intergranular corrosion
What metals is pitting corrosion common on?
Magnesium and aluminium
What is pitting corrosion?
A localised form of corrosion that produces holes
Why can the corrosion holes be harmful?
They act as stress risers and aid in stress corrosion
Which way does pitting corrosion extend?
Vertically
Is there an indication of pitting corrosion?
No
What does intergranular corrosion look like?
Cracking
How is moisture allowed to enter through intergranular corrosion?
Chemical and electrolytic attack along grain boundaries
How can you prevent intergranular corrosion?
Paint or primer or Al-clad
What is an advanced form of intergranular corrosion?
Exfoliation corrosion
When is exfoliation corrosion most prone to occur?
In wrought products
What does machined edges, damaged edges or unprotected edges
Exposed grain ends
How does the corrosion in exfoliation corrosion proceed?
Parallel to the grain surfaces
What is fretting corrosion?
Erosion corrosion that vibrates and slips
What does fretting corrosion appear as?
Pits or grooves surrounded by dust or powder
What does the rubbing action do?
Remove the protective film
How is fretting identified?
Black or Grey brown powder
How can you reduce fretting corrosion?
Lubricant
What is crevice corrosion?
Attacks metal through stagnant solution in crevices
Why are dust and sand detrimental in regards to crevice corrosion?
Because they accumulate moisture
What can assist in the penetration process of the joint area?
Joint flexing
What chemical reaction does mercury attack in?
Amalgamation
What should you do if there is magnesium on an aircraft?
Not touch it
What are more prone to corrosive attacks?
Thicker structural components
Because of the variations in their compositions
What else does corrosive attacks depend on?
Heat treatment during fabrication
What types of stresses will effect the types and rates of corrosion?
Operational
Aircraft under cyclic loadings are vulnerable to what?
Stress corrosion cracking
What is a major factor in corrosion?
The composition of the material
Why are costal environments very corrosive?
Becsuse they have lots of salts in the air which firm electrolytes
What maximises resistance to corrosion?
Proper heat treatment
What are noble metals good for?
Thermal and electrical conductivity
Resistance to surface attack
What colour tarnish can occur if the metal is not cleaned?
Brown or black
What metal is corrosion resistant?
Copper and copper alloys
What are attacks on copper limited to?
Stains and tarnish
Are stains and tarnish on copper dangerous?
No
What does severe copper corrosion look like?
What us it treated with once removed?
Green to blue copper salts
Chromic acid solution
What do most metals form when reacting with oxygen?
An oxide layer
What does zinc and zinc coatings carry?
Zinc Patina which increases in thickness
When does dry corrosion occur?
After a direct chemical attack
What is dry corrosion explained by?
The absorption mechanism
Is corrosion uniform or non uniform in dry corrosion?
Uniform
Wet corrosion takes place where the relative humidity is above what?
60%
What corrosion can wet corrosion result in?
Pitting and stress corrosion
Does the more noble metal remain protected or unprotected?
Protected
Does saltwater have a higher or lower conductivity?
Higher
What substance increases the rate of corrosion?
Chloride
What is wet corrosion?
It occurs due to an electrochemical attack in the presence of a moisture or conducting medium
In wet corrosion, where do the products ammuculate?
On the cathode
Where is the corrosion produced on wet corrosion?
The anode
Is corrosion uniform in wet corrosion?
no
On what type of surfaces does wet corrosion occur?
Heterogeneous
What is direct chemical attack?
If an alkaline or acidic liquid comes into contact with a metal
What is the nature of a direct chemical attack?
Electrochemical
Is there a current flow in direct chemical attack?
No
Are there anodic and cathodic in direct chemical attack?
No
What can skydrol cause?
Corrosion and embrittlement of titanium alloys above 120 degrees
What are aircraft external surfaces covered in?
Protective finish
how can you identify corrosion caused by moisture and corrosive agents?
by a bulging in the aircraft skin
What do you need to keep bilge areas free from?
Dirt
How do battery compartments cause corrosion?
Because of the fumes from overheated batteries are hard to contain and spread to internal structures
Pitting of al clad occurs when?
When accumulation of salts and pollution and electrolytes
Is aluminium corrosion resistant?
Yes
When airborne pollutants or industrial pollutants what do all aluminium alloys require?
Protection
What is the corrosion product if aluminium?
White to grey powdery material
How can you remove the white powedery deposit of aluminium?
Brushing it with something softer than the material
Some aluminium parts are costed with what type of coating?
Anodised
What does anodising do to aluminium?
It thickens the oxide film
How can the anodised aluminium be restored when undergone damage?
Chemical surface treatment
Is copper and its alloys corrosion resistant?
Yes
What colour is the product of corrosion on copper?
Bluish green
What corrosion is copper susceptible to?
Surface and intergranular
What is copper’s chemical activity like?
Low when coupled with most metals and greatly accelerates corrosion
Is magnesium susceptible to corrosion?
Which one in particular?
Yes
Pitting
What metal corrosion is the easiest to detect?
Magnesium
How does magnesium corrosion show?
The paint will peel up, and then white spots show which develop into moulds or whiskers
How do you fix corrosion on magnesium?
Remove corrosion and apply chemical coatings
Is Titanium corrosion resistant?
Highly
What can result in the degradation of titanium at high temperatures?
Repeated contact with chlorinated solvents
What corrosion is titanium most susceptible to?
Galvanic
What can contact with zinc and cadmium plated parts promote?
Cracking
When are nickel alloys used? (Good resistance against what?)
In challenging environments so need good resistance against temperature changes
What corrosion is linked to nickel alloys?
Pitting, surface and crevis
What are nickel corrosion products?
Green powdery deposits
What enhances a corrosion in a marine environment?
A nickel undercoat
Why does corrosion originate in Chromium coatings?
Why does corrosion originate in Chromium coatings?
What does chromium posting do?
What does chromium posting do?
What does attack on Cadmium look like?
What does attack on Cadmium look like?
How do you know when the cadmium plate is not protecting its core anymore after cracking?
Iron rust appears.
Is cadmium and nickel plating still protecting when cracks are present?
Yes
What percentage of chromium does CRES contain?
12% or more
What will happen to CRES when undergoing harsh environments?
They will pit
What corrosion is CRES susceptible to?
Crevice, SCC and galvanic
How can corrosion in stainless steel be prevented?
By keeping it clean
