Homework 4

Question 1

Briefly explain the difference between oxidation and reduction electrochemical
reactions

Answer 1

Oxidation is the process by which an atom gives up an electron (or electrons) to become
a cation. Reduction is the process by which an atom acquires an extra electron (or electrons) and
becomes an anion

Question 2

Which reaction occurs at the anode and which at the cathode?

Answer 2

Oxidation occurs at the anode; reduction occurs at the cathode

Question 3

Briefly explain why, for a small anode-to-cathode area ratio, the corrosion rate will be higher than for a large ratio?

Answer 3

for some given current flow associated with the corrosion reaction,
for a small area ratio the current density at the anode will be greater than for a large ratio

Question 4

Briefly describe the two techniques that are used for galvanic protection.

Answer 4

sacrificial anode- is electrically coupled to the metal piece to be protected, which
anode is also situated in the corrosion environment. It is a metal or alloy that
is chemically more reactive in the particular environment. the anode preferentially oxidizes,
and, upon giving up electrons to the other metal, protects it from electrochemical corrosion

An impressed current from an external dc power source provides excess electrons to
the metallic structure to be protected.

Question 5

Difference between equation 17.32 & 17.33?

Answer 5

When there is a reduction of O you use 17.32 and vice versa for 17.33.

Question 6

Using Eq. 17.32 & 17.33, how do you determine if oxide scales will be protective?

Answer 6

If it is >1 Nonprotective.
if it is 12 protective.
If it is <2 nonprotective

Question 7

What is V_2 and V_1 on equation 17.18, 17.19, 17.20?

Answer 7

V_2 is the higher potential on table 17.1.

Question 8

How do you predict the possibility of corrosion?

Answer 8

Use table 17.2, if within the same brackets it will not corrode. If not within the same brackets the most active material will react.

Question 9

Briefly describe the phenomenon of passivity. Name two common types of alloy that
passivate

Answer 9

Passivity is the loss of chemical reactivity, under particular environmental conditions, of
normally active metals and alloys. Stainless steels and aluminum alloys often passive.

Question 10

For each form of corrosion other than uniform, do the following: Describe why, where, and the conditions under which the corrosion occurs

Answer 10

Galvanic Corrosion- When 2 metals or alloys having difference compositions are electrically coupled while exposed to an electrolyte. Usually on surface.
Crevice Corrosion- Consequence of concentration differences of ions or dissolved gasses in the electrolyte solution and between 2 regions of the same metal piece

Question 11

r each form of corrosion other than uniform: Cite three measures that may be taken to prevent or control it.

Answer 11

cathodic protection, Anodization, Protection of Iron with Zinc