Chapter 9 - Corrosion | Corrosion Prevention Flashcards
What are the methods of controlling corrosion?
- Judicious design
- Protective coatings
- Corrosion inhibitors
- Monitoring
What is judicious design?
Meaning you are well informed about the material and its geometry and configuration
What are protective coatings?
Passive metals (oxide films) Paint Polymer coatings
What are corrosion inhibitors?
These are chemicals added to the corrosive medium that retard the rate of the corrosion reaction
What is monitoring?
Means with protective maintenance or regular replacement
how can you avoid or control corrosion?
- Make design changes to prevent trapped fluids
- Change design to avoid crevice corrosion
What do coating systems consist of?
- A primer
- An intermediate coat
- One or more topcoats
Characteristics of the primer?
Adheres to the substrate and resists corrosion
Characteristics of the intermediate coat?
Low moisture and vapour transfer rate
Characteristics of the Top coat?
Resistant to ion penitration
Alkali resistant
insulator
What are the 3 types of surface coatings?
Metallic, inorganic and organic
What are the 3 ways in which they can protect?
- Exclusion
- Sacrificial protection
- Inhibition
What are inhibitor coatings?
This is the use of chemical inhibitors to decrease the rate of corrosion processes
What are some examples of inhibited coatings
Priming paints, oils & greases
In order to be fully exclusive what does a coating have to do?
It has to fully cover the surface completely and resist mechanical damage
What are the two ways sacrificial coatings act?
- where coating is continuous they act as excluders
- when coating is discontinuous they provide cathodic protection
Examples of sacrificial coatings?
Steels protected by Zn, Cd or Al
Copper protected by Zn
Why must sacrifiacial coatings be thick enough to last the life of the component they are protecting?
Because as they protect they corrode away.
What are the types of metallic coatings?
- Electroplated coatings
- Hot-dipped coatings
- Cementation & oxide coatings
- Sprayed coatings
how does electroplating work?
An electrochemical process where metal ions are transferred from a solution and are deposited as a thin layer onto surface of a cathode.
which materials can be electroplated?
Virtually all metals
E.G. Cr, Zn, Sn, Fe, Pb, Cu
how do electroplated coatings protect?
By exclusion or or by sacrificial
How do hot dipping coatings work?
By immerising the metal to be protected in a bath of molten metal e.g. Zn, Sn, Pb
Characteristics of hot dipped coatings?
generally thicker than electroplated coatings and also more uneven and less uniform
Characteristics of an electroplated coating?
Uniform coating along faces due to accurate control but varied thickness at corners
How does the cementation process work?
The metal to be protected is heated in contact with an element that will slowly diffuse into its surface
What is ‘Sherardising’?
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
How doe the process of sprayed metals work?
Modified gas torches are used to spray metals (powder or wire) to coat a component
Which metals tend to be used in spraying?
Al & Zn
