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ENGINEERING, AE 2017 HSC 17 MC

In which of the following does an impervious oxide surface layer provide corrosion resistance for the base metal?

  1. Zinc coating of steel in underground applications
  2. Carbon fibre panels in automotive body applications
  3. Powder coating of steel structures in marine applications
  4. Nickel-based alloys in high temperature aeronautical applications
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`D`

Show Worked Solution

→ B is incorrect as carbon fibre is not a base metal.

→ A and C may form protective layers, however they are the result of a base metal being coated with a different anodic metal.

→ D is the correct answer as Nickel-based alloys form an impervious oxide layer to prevent corrosion without the need to coat them with a different metal.

`=>D`


♦♦♦ Mean mark 14%.

ENGINEERING, AE 2018 HSC 25d

Identify factors that contribute to stress corrosion cracking in aluminium alloy aircraft components.   (2 marks)

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→ Stress corrosion cracking develops in a corrosive environment (moisture).

→ Sustained tensile stress is also a contributing factor and can be applied both externally or internally through residual tensile stress.

Show Worked Solution

→ Stress corrosion cracking develops in a corrosive environment (moisture).

→ Sustained tensile stress is also a contributing factor and can be applied both externally or internally through residual tensile stress.

ENGINEERING, AE 2022 HSC 24b

The image shows corroded screws on the body of an aircraft.

Identify this type of corrosion and explain how it can occur.   (3 marks)

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→ The corrosion shown in the image is galvanic corrosion

→ It has occurred since the metal used for the body of the plane is not the same as the metal used for the bolts. 

→ This causes galvanic corrosion as the bolts become an anode to protect the cathodic fuselage from rusting. This can also be referred to as dissimilar metal corrosion.
 

Pit and crevice corrosion

→ Although less likely, the image could also show pit and crevice corrosion (concentration cell corrosion), as it is occurring around the cracks between sheets of the fuselage.

→ This occurs when a metal is placed in an electrolyte of varying concentration. In this case there is a lower concentration of oxygen in the crack, causing the tip of the crack to become the anode and corrode.

Show Worked Solution

→ The corrosion shown in the image is galvanic corrosion

→ It has occurred since the metal used for the body of the plane is not the same as the metal used for the bolts. 

→ This causes galvanic corrosion as the bolts become an anode to protect the cathodic fuselage from rusting. This can also be referred to as dissimilar metal corrosion.
 

Pit and crevice corrosion

→ Although less likely, the image could also show pit and crevice corrosion (concentration cell corrosion), as it is occurring around the cracks between sheets of the fuselage.

→ This occurs when a metal is placed in an electrolyte of varying concentration. In this case there is a lower concentration of oxygen in the crack, causing the tip of the crack to become the anode and corrode.


Mean mark 54%.

ENGINEERING, AE 2020 HSC 16 MC

Which of the following contributes to pitting or crevice corrosion in aircraft component joints?

  1. Water levels
  2. Alloy concentrations
  3. Oxygen concentrations
  4. Levels of static electricity
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`C`

Show Worked Solution

Pit and crevice corrosion occurs when:

→ The anode and cathode are both in contact with an electrolyte, and

→ The concentration is lower at the anode.

→ Therefore, oxygen concentrations contribute as oxygen is an electrolyte which is often in contact with aircraft component joints.

`=>C`


Mean mark 54%.

ENGINEERING, AE 2021 HSC 23a

Due to a reduction in air travel, an airline company has had to store some of its unused aircraft.

Explain why the desert is a suitable place to store the unused aircraft.   (3 marks)

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→ Aircraft are susceptible to corrosion as they are constructed from aluminium. Specifically, crevice corrosion in the joints between the aluminium sheets.

→ While the aircraft are in storage they should be kept in a dry area to prevent any moisture from entering the crevices.

→ A desert provides this environment at a low cost.
 

Successful answers could also include:

→ Deserts also have fewer insects/wildlife which could nest in the aircraft.

→ The surface of the desert is also hard and dry so paving is not required.

Show Worked Solution

→ Aircraft are susceptible to corrosion as they are constructed from aluminium. Specifically, crevice corrosion in the joints between the aluminium sheets.

→ While the aircraft are in storage they should be kept in a dry area to prevent any moisture from entering the crevices.

→ A desert provides this environment at a low cost.
 

Successful answers could also include:

→ Deserts also have fewer insects/wildlife which could nest in the aircraft.

→ The surface of the desert is also hard and dry so paving is not required.