Corrosion

Question 1

What does corrosion consist of?

Answer 1

A chemical reaction with a transfer of electrons from one chemical species to another.
-Metal atoms characteristically give up electrons. OXIDATION

Question 2

Where does oxidation occur?

Answer 2

Oxidation occurs at the anode, Oxidation is an anodic reaction

Question 3

What is the other chemical reaction involved in corrosion?

Answer 3

Electrons generates from each metal atom must be transferred and become part of a second chemical species. This is termed a REDUCTION reaction.

Question 4

Where does reduction occur?

Answer 4

The location at which reduction occurs is called the cathode.

Question 5

What is the REDOX reaction for the corrosion of zinc in an acidic solution?

Answer 5

The oxidation reaction is Zn → Zn 2 + +2 e-
The reduction reaction i 2H + +2 e- → H2

Question 6

What is electrode potential?

Answer 6

Not all metals oxidise with the same ease. A current will pass. This current can be termed the electrode potential of an electrochemical cell

Question 7

What is a galvanic couple?

Answer 7

Arrangement is called a galvanic couple. Two metals electrically connected in a liquid electrolyte when one corrodes (anode) while the other acts as a cathode

Question 8

What is the emf series

Answer 8

The electromotive force (emf) series is generated by coupling standard hydrogen electrode and ranking according to voltage

Question 9

Explain the emf series

Answer 9

Metals higher on the emf series are more cathodic meaning they are least likely to react (corrode) and metals lower down on the series are more anodic which means that they react more easily. When standard half cells are coupled, the metal lowest in the emf series will corrode.

Question 10

What are the terms for the following; CPR=KW/⍴At?

Answer 10

CPR is the corrosion penetration rate
W is the weight loss after time t (mg)
⍴ is the density of the material (g/cm^3)
A is the exposed area (cm^2)
K is a constant, for mm/year this value is 87.6

Question 11

What is the galvanic series?

Answer 11

A series which represents the relative reactivities of metals/alloys in seawater. No voltages are provided with this series.

Question 12

What are the means of corrosion prevention?

Answer 12

-Materials selection
-Lower temperature
-Apply physical barriers
-Add inhibitors
-Cathodic (sacrificial) protection

Question 13

Explain material selection for corrosion prevention

Answer 13

-Use metals that are relatively unreactive in the corrosion environment
-Use metals that passivate (form a thin adhering oxide layer) which slows corrosion.

Question 14

Explain lowering temperature for corrosion prevention

Answer 14

Lowering the temperature reduces the rate of oxidation and reaction. Practically very hard to implement

Question 15

Explain applying physical barriers for corrosion prevention

Answer 15

We can add films/coatings which will protect the material from corrosion as the material will not be in contact with the corrosive enrivonment

Question 16

Explain adding inhibitors for corrosion prevention

Answer 16

These are substances that decreases the reactivity of the electrolyte solution. Usually used in a closed system such as radiators and boilers etc.

Question 17

Explain cathodic (sacrificial) protection for corrosion prevention

Answer 17

Involves supplying electrons from an external source to the metal to be protected making it a cathode. Corrosion reaction is forced to occur in the reverse (reduction) direction.
There are two methods for this:
- Attach a more anodic material to the one to be protected
-Use a sacrificial anode

Question 18

What are the forms of corrosion?

Answer 18

-Uniform attack
-Stress corrosion
-Erosion-corrosion
-Pitting
-Crevice
-Galvanic
-Intergranular
-Selective leaching

Question 19

Explain a uniform attack

Answer 19

Oxidation & reduction reactions occur uniformly over surfaces

Question 20

Explain a galvanic attack

Answer 20

Dissimilar metals are physically joined in the presence of an electrolyte – the more anodic metal corrodes.

Question 21

Explain a crevice attack

Answer 21

Corrosion occurring in narrow and confined spaces

Question 22

Explain pitting

Answer 22

The downward propagation of small pits and holes

Question 23

Explain intergranular corrosion

Answer 23

This is corrosion that occurs along grain boundaries

Question 24

Explain Erosion-Corrosion

Answer 24

Arises from the combined action of chemical attack and mechanical abrasion or wear as a consequence of fluid motion. Can be identified by surface grooves and waves having contours that are characteristic of the flow of the fluid

Question 25

Explain stress corrosion

Answer 25

Results from the combined action of an applied tensile stress and a corrosive environment. Some materials that are inert in a particular corrosive medium become susceptible to stress corrosion when a stress is applied. Small cracks form and then propagate in a direction perpendicular to the stress applied.

Question 26

Explain selective leaching corrosion

Answer 26

This attack can be found in solid solution alloys. It occurs when one constituent is removed as a consequence of the corrosion process.