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mod 6 > 6.4 corrosion > Flashcards

6.4 corrosion Flashcards

1
Q

What reactions are corrosion?

A

electrochemical or chemical reactions

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2
Q

What does corrosion create?

A

Salts and oxides which replace the metal

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3
Q

When does corrosion form on a metal?

A

When there is a difference in electrode potentials in contact with an electrolyte

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4
Q

What is an electrolyte?

A

A solution that produces an electrically conducting fluid

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5
Q

Acid and alkalis react with metals to form what?

A

Metallic salts (corrosion)

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6
Q

Where is sulphuric acid found?

A

Batteries

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7
Q

What element is extremely active in corrosion of aluminium?

A

Sulphuric

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8
Q

What is used to prepare a surface for painting?

A

Chromium or phosphorus

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9
Q

What metals are more subject to damage from acid and alkalis?

A

Ferrous metals

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10
Q

Is aluminium more prone to alkalines or acids?

A

Alkalines

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11
Q

Why does aluminium corrode when left on concrete?

A

Because water washes out lime from cement which forms alkaline

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12
Q

What is erosion?

A

Destruction through mechanical action

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13
Q

Why does corrosion occur?

A

Because of the metas tendancy to return to its natural state

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14
Q

Why do gold and platinum not corrode easily?

A

They are chemically pure in their natural state

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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

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16
Q

What do different parts of the surface act as?

A

An anode and a cathode

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17
Q

What joins the anode and cathode of a metal together?

A

The internal metal surface as it forms an electrical surface

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18
Q

What does the corrosion process involve in terms of chemical changes?

A

Anodic and Cathodic reactions

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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

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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

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21
Q

What is the cathode protected from?

A

Corrosion

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22
Q

What do some metals including stainless steel and titanium produce?

A

A passive film which prevents corrosion

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23
Q

Why is the passive film invisible?

A

Because it is so tightly bound to the surface

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24
Q

What happens when a corrosion film is loose and porous?

A

The electrolyte can easily penetrate and continue the corrosion process

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25
Q

What are most alloys made up of?

A

Small crystaline regions (grains)

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26
Q

What does corrosion form on?

A

Less resistant surface regions

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27
Q

Where else can corrosion form?
What can this cause?

A

At boundary’s between regions
Pits and Intergranular corrosion

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28
Q

What makes a metal corrode faster?

A

If electrons are lost easier

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29
Q

What metals corrode quickly due to easy release of electrons?

A

Magnesium and aluminium

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30
Q

What metals don’t lose electrons easy?

A

Noble metals such as silver and gold

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31
Q

What two metals produce the passive film?
Becsuse of this what are they known as?

A

Aluminium and Titanium
Corrosion Resistance

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32
Q

For corrosion to form, what three things must be present?

A

An electrical potential difference
A conductive path between the two areas of potential difference
Electrolyte or fluid covering both areas

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33
Q

How can you control corrosion?

A

By cleaning the surface because moisture accumulates dirt and grease

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34
Q

When does galvanic corrosion occur?

A

When two dissimilar metals come into electrical contact with an electrolyte

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35
Q

When do metals from the less noble group corrode easier?

A

When they are in contact with more noble metals

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36
Q

What makes the corrosion more active?

A

The more further apart they are on the galvanic scales

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37
Q

If something is more noble, is it less or more susceptible to corrosive attack?
Why?

A

Less
Higher number on galvanic scale

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38
Q

If something is less noble, does this mean it is less or more susceptible to corrosive attack?
Why?

A

More
Lower number on galvanic scale

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39
Q

What are the top three on the more noble side?

A

Platinum
Gold
Silver

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39
Q

What are the top three on the less noble side?

A

Magnesium
Zinc
Aluminium

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40
Q

What is electrolytic corrosion caused by?

A

An electrolyte and the flow of an electric current between two metals caused by Ann externally induced EMF

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41
Q

In the case of large motor and generator bearings, what can induced EMF’s result in?

A

Current flow in the bearing resulting in bearing corrosion.

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42
Q

What does bearing corrosion look like?

A

Pinhole type pits

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43
Q
A
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44
Q
A
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45
Q
A
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46
Q

What is electrolytic corrosion caused by?

A

An external source of current

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47
Q

When can galvanic corrosion not occur

A

Where there is no potential difference between two metals
Without electrically conductive joints
Without connection of electrolytes

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48
Q

What does a microbial attack consist of?

A

Bacteria fungi or moulds

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49
Q

Why is a microbial attack common in fuel tanks?

A

Because the water and fuel interface with each other

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50
Q

Aerobic corrosion needs what to live?

A

Oxygen

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51
Q

Why does aerobic corrosion accelerate corrosion?

A

Oxidising sulphur to produce sulphuric acid

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52
Q

What does bacteria living adjacent to the metal release that promotes corrosion?

A

Metabolic products

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53
Q

What is different about anaerobic bacteria?

A

It doesn’t need oxygen to survive

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54
Q

The metabolism of Anearobic bacteria requires them to obtain support how?

A

by oxidising inorganic compounds

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55
Q

What are fungi?

A

Inorganic materials that feed on organic materials

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56
Q

What slows the growth of microbes but does not kill them?

A

Low humidity

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57
Q

What is the ideal temperature for growth of microbes?

A

20 degrees to 40 degrees

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58
Q

What is the ideal humidity for growth of microbes?

A

85% to 100%

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59
Q

Where does microbial growth occur?

A

At the interface of water and fuel

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60
Q

What colour is the fungus on the bottom of the tank when dry?

A

Brown

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61
Q

How can microbial corrosion be minimised?

A

Drain fuel water
Inspections for milky white substance
Apply biocide

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62
Q

What two other areas are susceptible to microbial corrosion?

A

Toilets and Gallies

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63
Q

Even if the contamination is not corrosive, why can it still be bad?

A

It can attract and hold moisture promoting microbial growth

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64
Q

What is Stress Corrosion Cracking?

A

The growth of crack formation in a corrosive environment

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65
Q

When can Stress corrosion cracking fail when subjected to?

A

Cyclic loading and tensile stress

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66
Q

Where can stresses come from?

A

Something that has been pressed into a structural part with tight tolerance

67
Q

Where does the corrosion attack in a SCC focus on?

A

The end of a crack

68
Q

Where is a common place for stress corrosion?

A

Between rivets in a stress skin
Tapered pipe fittings
Pressed in bushings

69
Q

What types of inspections may reveal SCC?

A

Visual and dye

70
Q

Where does filiform corrosion occur?

A

On metals surfaces with an organic coating

71
Q

How is filiform corrosion identified?

A

By worm like traces

72
Q

When does filiform corrosion occur?

A

When the relative humidity 78% to 90% and the air is slightly acidic

73
Q

What is an advanced state of filiform corrosion?

A

Intergranular corrosion

74
Q

What metals is pitting corrosion common on?

A

Magnesium and aluminium

75
Q

What is pitting corrosion?

A

A localised form of corrosion that produces holes

76
Q

Why can the corrosion holes be harmful?

A

They act as stress risers and aid in stress corrosion

77
Q

Which way does pitting corrosion extend?

A

Vertically

78
Q

Is there an indication of pitting corrosion?

A

No

79
Q

What does intergranular corrosion look like?

A

Cracking

80
Q

How is moisture allowed to enter through intergranular corrosion?

A

Chemical and electrolytic attack along grain boundaries

81
Q

How can you prevent intergranular corrosion?

A

Paint or primer or Al-clad

82
Q

What is an advanced form of intergranular corrosion?

A

Exfoliation corrosion

83
Q

When is exfoliation corrosion most prone to occur?

A

In wrought products

84
Q

What does machined edges, damaged edges or unprotected edges

A

Exposed grain ends

85
Q

How does the corrosion in exfoliation corrosion proceed?

A

Parallel to the grain surfaces

86
Q

What is fretting corrosion?

A

Erosion corrosion that vibrates and slips

87
Q

What does fretting corrosion appear as?

A

Pits or grooves surrounded by dust or powder

88
Q

What does the rubbing action do?

A

Remove the protective film

89
Q

How is fretting identified?

A

Black or Grey brown powder

90
Q

How can you reduce fretting corrosion?

A

Lubricant

91
Q

What is crevice corrosion?

A

Attacks metal through stagnant solution in crevices

92
Q

Why are dust and sand detrimental in regards to crevice corrosion?

A

Because they accumulate moisture

93
Q

What can assist in the penetration process of the joint area?

A

Joint flexing

94
Q

What chemical reaction does mercury attack in?

A

Amalgamation

95
Q

What should you do if there is magnesium on an aircraft?

A

Not touch it

96
Q

What are more prone to corrosive attacks?

A

Thicker structural components
Because of the variations in their compositions

97
Q

What else does corrosive attacks depend on?

A

Heat treatment during fabrication

98
Q

What types of stresses will effect the types and rates of corrosion?

A

Operational

99
Q

Aircraft under cyclic loadings are vulnerable to what?

A

Stress corrosion cracking

100
Q

What is a major factor in corrosion?

A

The composition of the material

101
Q

Why are costal environments very corrosive?

A

Becsuse they have lots of salts in the air which firm electrolytes

102
Q

What maximises resistance to corrosion?

A

Proper heat treatment

103
Q

What are noble metals good for?

A

Thermal and electrical conductivity
Resistance to surface attack

104
Q

What colour tarnish can occur if the metal is not cleaned?

A

Brown or black

105
Q

What metal is corrosion resistant?

A

Copper and copper alloys

106
Q

What are attacks on copper limited to?

A

Stains and tarnish

107
Q

Are stains and tarnish on copper dangerous?

A

No

108
Q

What does severe copper corrosion look like?
What us it treated with once removed?

A

Green to blue copper salts
Chromic acid solution

109
Q

What do most metals form when reacting with oxygen?

A

An oxide layer

110
Q

What does zinc and zinc coatings carry?

A

Zinc Patina which increases in thickness

111
Q

When does dry corrosion occur?

A

After a direct chemical attack

112
Q

What is dry corrosion explained by?

A

The absorption mechanism

113
Q

Is corrosion uniform or non uniform in dry corrosion?

A

Uniform

114
Q

Wet corrosion takes place where the relative humidity is above what?

A

60%

115
Q

What corrosion can wet corrosion result in?

A

Pitting and stress corrosion

116
Q

Does the more noble metal remain protected or unprotected?

A

Protected

117
Q

Does saltwater have a higher or lower conductivity?

A

Higher

118
Q

What substance increases the rate of corrosion?

A

Chloride

119
Q

What is wet corrosion?

A

It occurs due to an electrochemical attack in the presence of a moisture or conducting medium

120
Q

In wet corrosion, where do the products ammuculate?

A

On the cathode

121
Q

Where is the corrosion produced on wet corrosion?

A

The anode

122
Q

Is corrosion uniform in wet corrosion?

A

no

123
Q

On what type of surfaces does wet corrosion occur?

A

Heterogeneous

124
Q

What is direct chemical attack?

A

If an alkaline or acidic liquid comes into contact with a metal

125
Q

What is the nature of a direct chemical attack?

A

Electrochemical

126
Q

Is there a current flow in direct chemical attack?

A

No

127
Q

Are there anodic and cathodic in direct chemical attack?

A

No

128
Q

What can skydrol cause?

A

Corrosion and embrittlement of titanium alloys above 120 degrees

129
Q

What are aircraft external surfaces covered in?

A

Protective finish

130
Q

how can you identify corrosion caused by moisture and corrosive agents?

A

by a bulging in the aircraft skin

131
Q

What do you need to keep bilge areas free from?

A

Dirt

132
Q

How do battery compartments cause corrosion?

A

Because of the fumes from overheated batteries are hard to contain and spread to internal structures

133
Q

Pitting of al clad occurs when?

A

When accumulation of salts and pollution and electrolytes

134
Q

Is aluminium corrosion resistant?

A

Yes

135
Q

When airborne pollutants or industrial pollutants what do all aluminium alloys require?

A

Protection

136
Q

What is the corrosion product if aluminium?

A

White to grey powdery material

137
Q

How can you remove the white powedery deposit of aluminium?

A

Brushing it with something softer than the material

138
Q

Some aluminium parts are costed with what type of coating?

A

Anodised

139
Q

What does anodising do to aluminium?

A

It thickens the oxide film

140
Q

How can the anodised aluminium be restored when undergone damage?

A

Chemical surface treatment

141
Q

Is copper and its alloys corrosion resistant?

A

Yes

142
Q

What colour is the product of corrosion on copper?

A

Bluish green

143
Q

What corrosion is copper susceptible to?

A

Surface and intergranular

144
Q

What is copper’s chemical activity like?

A

Low when coupled with most metals and greatly accelerates corrosion

145
Q

Is magnesium susceptible to corrosion?
Which one in particular?

A

Yes
Pitting

146
Q

What metal corrosion is the easiest to detect?

A

Magnesium

147
Q

How does magnesium corrosion show?

A

The paint will peel up, and then white spots show which develop into moulds or whiskers

148
Q

How do you fix corrosion on magnesium?

A

Remove corrosion and apply chemical coatings

149
Q

Is Titanium corrosion resistant?

A

Highly

150
Q

What can result in the degradation of titanium at high temperatures?

A

Repeated contact with chlorinated solvents

151
Q

What corrosion is titanium most susceptible to?

A

Galvanic

152
Q

What can contact with zinc and cadmium plated parts promote?

A

Cracking

153
Q

When are nickel alloys used? (Good resistance against what?)

A

In challenging environments so need good resistance against temperature changes

154
Q

What corrosion is linked to nickel alloys?

A

Pitting, surface and crevis

155
Q

What are nickel corrosion products?

A

Green powdery deposits

156
Q

What enhances a corrosion in a marine environment?

A

A nickel undercoat

157
Q

Why does corrosion originate in Chromium coatings?

A

Why does corrosion originate in Chromium coatings?

158
Q

What does chromium posting do?

A

What does chromium posting do?

159
Q

What does attack on Cadmium look like?

A

What does attack on Cadmium look like?

160
Q

How do you know when the cadmium plate is not protecting its core anymore after cracking?

A

Iron rust appears.

161
Q

Is cadmium and nickel plating still protecting when cracks are present?

A

Yes

162
Q

What percentage of chromium does CRES contain?

A

12% or more

163
Q

What will happen to CRES when undergoing harsh environments?

A

They will pit

164
Q

What corrosion is CRES susceptible to?

A

Crevice, SCC and galvanic

165
Q

How can corrosion in stainless steel be prevented?

A

By keeping it clean

mod 6 (12 decks)

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