Module 8 Corrosion and Failure Analysis Objective Two

Question 1

Uniform Corrosion (general corrosion)

Answer 1

occurs where the material is all the same

Question 2

Galvanic corrosion, needs three simultaneous conditions

Answer 2

1. there must be dissimilar materials having different corrosion voltages (reduction potentials)
2. the materials must be immersed in a common electrolyte
3. there must be an electrical conducting path to connect the materials

Question 3

Crevice and Pitting corrosion

Answer 3

may occur instead of uniform corrosion when the surface has shielded areas such as severe gouges, large inclusions/defects, crevices, etc.

Question 4

Pitting

Answer 4

an extremely localized corrosion attack that results in holes in the metal that may lead to perforation and leakage
the pit is a whole with a depth to width ratio >1

Question 5

Electrolyte Concentration Gradient

Answer 5

behaves similarly to the oxygen concentration cell.
In shielded areas, the electrolyte ions may react with the corroded metal, which reduces the concentration of electrolyte.

Question 6

Temperature Gradient:

Answer 6

occurs when there’s a differential temperature over a short distance. The hotspot will have higher activity (anode) and will corrode more compared to the cooler regions (cathode)

Question 7

Velocity Gradient (velocity cell)

Answer 7

can be created from a high velocity fluid that can erode the passive oxide layer (erosion corrosion)
The second way is when the fluid is carrying suspended particles and the velocity is slow. Particles can settle to the bottom of the pipe, can set up a concentration cell

Question 8

Cavitation Corrosion

Answer 8

Result of collapsing bubbles. As fluid goes through a constriction, the velocity increases, which causes the pressure to drop. if there is a volatile phase in the fluid, bubbles will form in the low pressure zone. When the fluid re-enters the low velocity region, the pressure increases and the bubbles implode violently causing a shockwave. This can generate stress greater than the yield strength

Question 9

Fretting Corrosion

Answer 9

results from a slight motion or vibration between two parts that wears away the surface oxide.

Question 10

Atmospheric Corrosion

Answer 10

The corrosion/degradation of a material exposed to the air and its pollutants.
Responsible for more failures, and the costs resulting from atmospheric corrosion are greater than from any other type of corrosion.

Question 11

Dry atmospheric corrosion

Answer 11

Occurs in the absence of moisture. Occurs when metals form an oxide film in the presence of oxygen.

Question 12

High temperature corrosion

Answer 12

Reactivity of the oxygen in the air increases. Initially, an oxide layer forms, but it eventually gets so thick that the compressive forces cause it to crack. Further oxidation can occur between the cracks

Question 13

Damp atmospheric corrosion

Answer 13

occurs when the relative humidity of the atmosphere is above 70% and a very thin film of moisture forms on metal surfaces. The film is a stronger electrolyte then air, increases the transfer of corrosion current

Question 14

Wet atmospheric corrosion

Answer 14

occurs when visible pockets or layers of water are formed on the metal surface. Crevices or other traps cause water to form into pools, leading to corrosion even when most surfaces are dry.

Question 15

Stray current corrosion

Answer 15

cause by an external source of current. Stray current corrosion is not influenced to any great extent by environmental conditions such as pH and entrained oxygen.

Question 16

Biological Corrosion

Answer 16

corrosion caused by bacteria

Question 17

Aerobic bacteria

Answer 17

require oxygen to live and grow

Question 18

Anaerobic bacteria

Answer 18

thrive best in the absence of oxygen

Question 19

facultative bacteria

Answer 19

grow in the presence or absence of oxygen

Question 20

Sulphate Reducing Bacteria

Answer 20

cause high corrosion rates in an anaerobic environment, production of sulphide or hydrogen sulphide reacts with iron and cause pitting directly under the bacteria colony

Question 21

Iron bacteria

Answer 21

aerobic and grow well in environments with trace amounts of oxygen. Doesn’t directly take part in corrosion

Question 22

Slime forming Bacteria

Answer 22

produce a dense mass of slime on a solid surface. Function similarly to iron bacteria.

Question 23

Intergranular corrosion

Answer 23

the grain boundaries become anodic and the grains become the cathode. Caused by the grain boundary precipitates. These cause change in chemical composition creates very small galvanic cells

Question 24

Exfoliation

Answer 24

a form of intergranular corrosion most commonly seen in high strength (PH) aluminium alloy extrusions. Occurs when the grains are elongated in the forming direction and there is some increase in alloy content at the grain boundaries usually due to improper heating

Question 25

Stress Corrosion Cracking

Answer 25

The intergranular or transgranular cracking of a material due to the combination of two factors

• stress can be well below the yield point
• specific environmental/material combinations