Chapter 9 - Corrosion | Corrosion Prevention

Question 1

What are the methods of controlling corrosion?

Answer 1

• Judicious design
• Protective coatings
• Corrosion inhibitors
• Monitoring

Question 2

What is judicious design?

Answer 2

Meaning you are well informed about the material and its geometry and configuration

Question 3

What are protective coatings?

Answer 3

Passive metals (oxide films)
Paint
Polymer coatings

Question 4

What are corrosion inhibitors?

Answer 4

These are chemicals added to the corrosive medium that retard the rate of the corrosion reaction

Question 5

What is monitoring?

Answer 5

Means with protective maintenance or regular replacement

Question 6

how can you avoid or control corrosion?

Answer 6

• Make design changes to prevent trapped fluids

- Change design to avoid crevice corrosion

Question 7

What do coating systems consist of?

Answer 7

• A primer
• An intermediate coat
• One or more topcoats

Question 8

Characteristics of the primer?

Answer 8

Adheres to the substrate and resists corrosion

Question 9

Characteristics of the intermediate coat?

Answer 9

Low moisture and vapour transfer rate

Question 10

Characteristics of the Top coat?

Answer 10

Resistant to ion penitration
Alkali resistant
insulator

Question 11

What are the 3 types of surface coatings?

Answer 11

Metallic, inorganic and organic

Question 12

What are the 3 ways in which they can protect?

Answer 12

• Exclusion
• Sacrificial protection
• Inhibition

Question 13

What are inhibitor coatings?

Answer 13

This is the use of chemical inhibitors to decrease the rate of corrosion processes

Question 14

What are some examples of inhibited coatings

Answer 14

Priming paints, oils & greases

Question 15

In order to be fully exclusive what does a coating have to do?

Answer 15

It has to fully cover the surface completely and resist mechanical damage

Question 16

What are the two ways sacrificial coatings act?

Answer 16

• where coating is continuous they act as excluders

- when coating is discontinuous they provide cathodic protection

Question 17

Examples of sacrificial coatings?

Answer 17

Steels protected by Zn, Cd or Al

Copper protected by Zn

Question 18

Why must sacrifiacial coatings be thick enough to last the life of the component they are protecting?

Answer 18

Because as they protect they corrode away.

Question 19

What are the types of metallic coatings?

Answer 19

• Electroplated coatings
• Hot-dipped coatings
• Cementation & oxide coatings
• Sprayed coatings

Question 20

how does electroplating work?

Answer 20

An electrochemical process where metal ions are transferred from a solution and are deposited as a thin layer onto surface of a cathode.

Question 21

which materials can be electroplated?

Answer 21

Virtually all metals

E.G. Cr, Zn, Sn, Fe, Pb, Cu

Question 22

how do electroplated coatings protect?

Answer 22

By exclusion or or by sacrificial

Question 23

How do hot dipping coatings work?

Answer 23

By immerising the metal to be protected in a bath of molten metal e.g. Zn, Sn, Pb

Question 24

Characteristics of hot dipped coatings?

Answer 24

generally thicker than electroplated coatings and also more uneven and less uniform

Question 25

Characteristics of an electroplated coating?

Answer 25

Uniform coating along faces due to accurate control but varied thickness at corners

Question 26

How does the cementation process work?

Answer 26

The metal to be protected is heated in contact with an element that will slowly diffuse into its surface

Question 27

What is ‘Sherardising’?

Answer 27

A combination of a fine Zn powder and sand are tumbled in a heated container. Zn diffuses into the surface to produce a fine layer of zinc rich compounds

Question 28

How doe the process of sprayed metals work?

Answer 28

Modified gas torches are used to spray metals (powder or wire) to coat a component

Question 29

Which metals tend to be used in spraying?

Answer 29

Al & Zn