6.4 corrosion Flashcards
(166 cards)
1
Q
What reactions are corrosion?
A
electrochemical or chemical reactions
2
Q
What does corrosion create?
A
Salts and oxides which replace the metal
3
Q
When does corrosion form on a metal?
A
When there is a difference in electrode potentials in contact with an electrolyte
4
Q
What is an electrolyte?
A
A solution that produces an electrically conducting fluid
5
Q
Acid and alkalis react with metals to form what?
A
Metallic salts (corrosion)
6
Q
Where is sulphuric acid found?
A
Batteries
7
Q
What element is extremely active in corrosion of aluminium?
A
Sulphuric
8
Q
What is used to prepare a surface for painting?
A
Chromium or phosphorus
9
Q
What metals are more subject to damage from acid and alkalis?
A
Ferrous metals
10
Q
Is aluminium more prone to alkalines or acids?
A
Alkalines
11
Q
Why does aluminium corrode when left on concrete?
A
Because water washes out lime from cement which forms alkaline
12
Q
What is erosion?
A
Destruction through mechanical action
13
Q
Why does corrosion occur?
A
Because of the metas tendancy to return to its natural state
14
Q
Why do gold and platinum not corrode easily?
A
They are chemically pure in their natural state
15
Q
Where do all corrosive attacks occur and why?
A
On the surface of the metal because it is considered an electrode in contacts eith an electrolyte
16
Q
What do different parts of the surface act as?
A
An anode and a cathode
17
Q
What joins the anode and cathode of a metal together?
A
The internal metal surface as it forms an electrical surface
18
Q
What does the corrosion process involve in terms of chemical changes?
A
Anodic and Cathodic reactions
19
Q
What happens in an anodic reaction?
A
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
20
Q
What happens in the corresponding cathodic reaction?
A
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
21
Q
What is the cathode protected from?
A
Corrosion
22
Q
What do some metals including stainless steel and titanium produce?
A
A passive film which prevents corrosion
23
Q
Why is the passive film invisible?
A
Because it is so tightly bound to the surface
24
Q
What happens when a corrosion film is loose and porous?
A
The electrolyte can easily penetrate and continue the corrosion process
25
What are most alloys made up of?
Small crystaline regions (grains)
26
What does corrosion form on?
Less resistant surface regions
27
Where else can corrosion form?
What can this cause?
At boundary's between regions
Pits and Intergranular corrosion
28
What makes a metal corrode faster?
If electrons are lost easier
29
What metals corrode quickly due to easy release of electrons?
Magnesium and aluminium
30
What metals don't lose electrons easy?
Noble metals such as silver and gold
31
What two metals produce the passive film?
Becsuse of this what are they known as?
Aluminium and Titanium
Corrosion Resistance
32
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
33
How can you control corrosion?
By cleaning the surface because moisture accumulates dirt and grease
34
When does galvanic corrosion occur?
When two dissimilar metals come into electrical contact with an electrolyte
35
When do metals from the less noble group corrode easier?
When they are in contact with more noble metals
36
What makes the corrosion more active?
The more further apart they are on the galvanic scales
37
If something is more noble, is it less or more susceptible to corrosive attack?
Why?
Less
Higher number on galvanic scale
38
If something is less noble, does this mean it is less or more susceptible to corrosive attack?
Why?
More
Lower number on galvanic scale
39
What are the top three on the more noble side?
Platinum
Gold
Silver
39
What are the top three on the less noble side?
Magnesium
Zinc
Aluminium
40
What is electrolytic corrosion caused by?
An electrolyte and the flow of an electric current between two metals caused by Ann externally induced EMF
41
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.
42
What does bearing corrosion look like?
Pinhole type pits
43
44
45
46
What is electrolytic corrosion caused by?
An external source of current
47
When can galvanic corrosion not occur
Where there is no potential difference between two metals
Without electrically conductive joints
Without connection of electrolytes
48
What does a microbial attack consist of?
Bacteria fungi or moulds
49
Why is a microbial attack common in fuel tanks?
Because the water and fuel interface with each other
50
Aerobic corrosion needs what to live?
Oxygen
51
Why does aerobic corrosion accelerate corrosion?
Oxidising sulphur to produce sulphuric acid
52
What does bacteria living adjacent to the metal release that promotes corrosion?
Metabolic products
53
What is different about anaerobic bacteria?
It doesn't need oxygen to survive
54
The metabolism of Anearobic bacteria requires them to obtain support how?
by oxidising inorganic compounds
55
What are fungi?
Inorganic materials that feed on organic materials
56
What slows the growth of microbes but does not kill them?
Low humidity
57
What is the ideal temperature for growth of microbes?
20 degrees to 40 degrees
58
What is the ideal humidity for growth of microbes?
85% to 100%
59
Where does microbial growth occur?
At the interface of water and fuel
60
What colour is the fungus on the bottom of the tank when dry?
Brown
61
How can microbial corrosion be minimised?
Drain fuel water
Inspections for milky white substance
Apply biocide
62
What two other areas are susceptible to microbial corrosion?
Toilets and Gallies
63
Even if the contamination is not corrosive, why can it still be bad?
It can attract and hold moisture promoting microbial growth
64
What is Stress Corrosion Cracking?
The growth of crack formation in a corrosive environment
65
When can Stress corrosion cracking fail when subjected to?
Cyclic loading and tensile stress
66
Where can stresses come from?
Something that has been pressed into a structural part with tight tolerance
67
Where does the corrosion attack in a SCC focus on?
The end of a crack
68
Where is a common place for stress corrosion?
Between rivets in a stress skin
Tapered pipe fittings
Pressed in bushings
69
What types of inspections may reveal SCC?
Visual and dye
70
Where does filiform corrosion occur?
On metals surfaces with an organic coating
71
How is filiform corrosion identified?
By worm like traces
72
When does filiform corrosion occur?
When the relative humidity 78% to 90% and the air is slightly acidic
73
What is an advanced state of filiform corrosion?
Intergranular corrosion
74
What metals is pitting corrosion common on?
Magnesium and aluminium
75
What is pitting corrosion?
A localised form of corrosion that produces holes
76
Why can the corrosion holes be harmful?
They act as stress risers and aid in stress corrosion
77
Which way does pitting corrosion extend?
Vertically
78
Is there an indication of pitting corrosion?
No
79
What does intergranular corrosion look like?
Cracking
80
How is moisture allowed to enter through intergranular corrosion?
Chemical and electrolytic attack along grain boundaries
81
How can you prevent intergranular corrosion?
Paint or primer or Al-clad
82
What is an advanced form of intergranular corrosion?
Exfoliation corrosion
83
When is exfoliation corrosion most prone to occur?
In wrought products
84
What does machined edges, damaged edges or unprotected edges
Exposed grain ends
85
How does the corrosion in exfoliation corrosion proceed?
Parallel to the grain surfaces
86
What is fretting corrosion?
Erosion corrosion that vibrates and slips
87
What does fretting corrosion appear as?
Pits or grooves surrounded by dust or powder
88
What does the rubbing action do?
Remove the protective film
89
How is fretting identified?
Black or Grey brown powder
90
How can you reduce fretting corrosion?
Lubricant
91
What is crevice corrosion?
Attacks metal through stagnant solution in crevices
92
Why are dust and sand detrimental in regards to crevice corrosion?
Because they accumulate moisture
93
What can assist in the penetration process of the joint area?
Joint flexing
94
What chemical reaction does mercury attack in?
Amalgamation
95
What should you do if there is magnesium on an aircraft?
Not touch it
96
What are more prone to corrosive attacks?
Thicker structural components
Because of the variations in their compositions
97
What else does corrosive attacks depend on?
Heat treatment during fabrication
98
What types of stresses will effect the types and rates of corrosion?
Operational
99
Aircraft under cyclic loadings are vulnerable to what?
Stress corrosion cracking
100
What is a major factor in corrosion?
The composition of the material
101
Why are costal environments very corrosive?
Becsuse they have lots of salts in the air which firm electrolytes
102
What maximises resistance to corrosion?
Proper heat treatment
103
What are noble metals good for?
Thermal and electrical conductivity
Resistance to surface attack
104
What colour tarnish can occur if the metal is not cleaned?
Brown or black
105
What metal is corrosion resistant?
Copper and copper alloys
106
What are attacks on copper limited to?
Stains and tarnish
107
Are stains and tarnish on copper dangerous?
No
108
What does severe copper corrosion look like?
What us it treated with once removed?
Green to blue copper salts
Chromic acid solution
109
What do most metals form when reacting with oxygen?
An oxide layer
110
What does zinc and zinc coatings carry?
Zinc Patina which increases in thickness
111
When does dry corrosion occur?
After a direct chemical attack
112
What is dry corrosion explained by?
The absorption mechanism
113
Is corrosion uniform or non uniform in dry corrosion?
Uniform
114
Wet corrosion takes place where the relative humidity is above what?
60%
115
What corrosion can wet corrosion result in?
Pitting and stress corrosion
116
Does the more noble metal remain protected or unprotected?
Protected
117
Does saltwater have a higher or lower conductivity?
Higher
118
What substance increases the rate of corrosion?
Chloride
119
What is wet corrosion?
It occurs due to an electrochemical attack in the presence of a moisture or conducting medium
120
In wet corrosion, where do the products ammuculate?
On the cathode
121
Where is the corrosion produced on wet corrosion?
The anode
122
Is corrosion uniform in wet corrosion?
no
123
On what type of surfaces does wet corrosion occur?
Heterogeneous
124
What is direct chemical attack?
If an alkaline or acidic liquid comes into contact with a metal
125
What is the nature of a direct chemical attack?
Electrochemical
126
Is there a current flow in direct chemical attack?
No
127
Are there anodic and cathodic in direct chemical attack?
No
128
What can skydrol cause?
Corrosion and embrittlement of titanium alloys above 120 degrees
129
What are aircraft external surfaces covered in?
Protective finish
130
how can you identify corrosion caused by moisture and corrosive agents?
by a bulging in the aircraft skin
131
What do you need to keep bilge areas free from?
Dirt
132
How do battery compartments cause corrosion?
Because of the fumes from overheated batteries are hard to contain and spread to internal structures
133
Pitting of al clad occurs when?
When accumulation of salts and pollution and electrolytes
134
Is aluminium corrosion resistant?
Yes
135
When airborne pollutants or industrial pollutants what do all aluminium alloys require?
Protection
136
What is the corrosion product if aluminium?
White to grey powdery material
137
How can you remove the white powedery deposit of aluminium?
Brushing it with something softer than the material
138
Some aluminium parts are costed with what type of coating?
Anodised
139
What does anodising do to aluminium?
It thickens the oxide film
140
How can the anodised aluminium be restored when undergone damage?
Chemical surface treatment
141
Is copper and its alloys corrosion resistant?
Yes
142
What colour is the product of corrosion on copper?
Bluish green
143
What corrosion is copper susceptible to?
Surface and intergranular
144
What is copper's chemical activity like?
Low when coupled with most metals and greatly accelerates corrosion
145
Is magnesium susceptible to corrosion?
Which one in particular?
Yes
Pitting
146
What metal corrosion is the easiest to detect?
Magnesium
147
How does magnesium corrosion show?
The paint will peel up, and then white spots show which develop into moulds or whiskers
148
How do you fix corrosion on magnesium?
Remove corrosion and apply chemical coatings
149
Is Titanium corrosion resistant?
Highly
150
What can result in the degradation of titanium at high temperatures?
Repeated contact with chlorinated solvents
151
What corrosion is titanium most susceptible to?
Galvanic
152
What can contact with zinc and cadmium plated parts promote?
Cracking
153
When are nickel alloys used? (Good resistance against what?)
In challenging environments so need good resistance against temperature changes
154
What corrosion is linked to nickel alloys?
Pitting, surface and crevis
155
What are nickel corrosion products?
Green powdery deposits
156
What enhances a corrosion in a marine environment?
A nickel undercoat
157
Why does corrosion originate in Chromium coatings?
Why does corrosion originate in Chromium coatings?
158
What does chromium posting do?
What does chromium posting do?
159
What does attack on Cadmium look like?
What does attack on Cadmium look like?
160
How do you know when the cadmium plate is not protecting its core anymore after cracking?
Iron rust appears.
161
Is cadmium and nickel plating still protecting when cracks are present?
Yes
162
What percentage of chromium does CRES contain?
12% or more
163
What will happen to CRES when undergoing harsh environments?
They will pit
164
What corrosion is CRES susceptible to?
Crevice, SCC and galvanic
165
How can corrosion in stainless steel be prevented?
By keeping it clean
