1. CORROSION and CHEMICAL PROCESSES
7/2/2018 7/2/2018
CORROSION and CHEMICAL PROCESSES
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2. Durability is a key material attribute, but is one of the more difficult attributes to characterize, quantify, and use for selection
It is a function not just of the material but of the environment in which it operates
There are many mechanisms, some general, some peculiar to particular materials and environments
Material combinations and configurations play a role

4. The most stable form of most elements is an oxide
If the oxide adheres to the material, it gains weight in either a linear or parabolic way
If the oxide is volatile, the sample loses weight in a linear way

5. The oxygen concentration in % required to maintain steady burning
Limiting Oxygen Index (LOI)
Underwriters Laboratory (UL) rating
The oxygen concentration in % required to maintain steady burning
Response to a 1.6 mm polymer strip that is ignited while being held horizontally or vertically
Codes such as HB (horizontal burn) describe the response

6. Polymers and elastomers age when exposed to light and oxygen, causing loss of strength, stiffness, and toughness as well as discoloration
Additives such as anti-oxidants, light stabilizers, and fluorescent whitening agents are used to counteract this

7. Oxidation rates rise steeply with rising temperatures
To enable a material to operate at high temperatures, precautions must be taken to limit the affects of oxidation
Oxide coatings can be applied to metals like cast irons, steels, or nickel alloys to prevent material degradation at high temperatures
Alloying elements such as chromium and silicon form oxides that become passive and limit continued oxidation

8. Corrosion by acids and alkalis is an electro-chemical reaction
Figure 17.6
A chemical compound that dissociates in water increases either the hydrogen ion concentration or hydroxyl ion concentration
The pH level of an environment can initiate corrosion by stimulating a reaction in which a metal dissociates into a metal ion and releases electrons

9. The value is relative to a standard, the hydrogen standard potential
Figure 17.8
Each metal has its own characteristic potential called the standard reduction potential
The value is relative to a standard, the hydrogen standard potential

10. If two metals are connected together in a cell, a
potential difference appears between them
If a current is allowed, electrons
flow from the anode to the cathode –
causing corrosion of the anode
The reduction potential of each
metal dictates which is the anode and
which is the cathode – the metal with the
lower potential acts as the anode
In the figure, the iron corrodes while
copper ions plate out onto the copper
following the cathodic reaction
Figure 17.9

11. Judicious design, meaning informed material choice and choice of geometry and configuration
Protective coatings, passive and active
Corrosion inhibitors, chemicals added to the corrosive medium that retard the rate of corrosion reaction
Monitoring, with protective maintenance or regular replacement

15. Allow for uniform attack
Avoid fluid trapping
Suppress galvanic attack
Avoid crevices
Consider cathodic protection
Beware of stress corrosion and corrosion-fatigue
Design for inspection and maintenance

16. Self-generated coatings
Passive coatings
Separate the material from the corrosive environment and are inherently corrosion resistant
Active coatings
Sacrificially corrode to protect the metal from corrosion
Self-generated coatings
Rely on alloying in sufficient concentrations so that a protective film forms spontaneously

17. Polymer powder spraying Enameling
Figure 17.17
Paint spraying
Hot dip galvanizing
Electroplating
Metal flame spraying
Polymer powder spraying
Enameling

18. Corrosion inhibitors reduce the rate of attack when dissolved or dispersed in a corrosive medium
Some work by lowering the pH or by coating the part and suppress either the anodic or the cathodic step of the reaction
The choice of inhibitor depends on both the material and the environment

19. Regular inspection allows early indications of corrosion to be detected
Maintenance – painting, recoating, or repair – can then be carried out to minimize its down-time and risk of failure
The design of the system should allow for inspection of all vulnerable surfaces and permit access for maintenance
Replacement as prescribed, regular intervals often used for safety-critical components

20. SUMMARY Corrosion will slowly, inevitably lead to material failure
Corrosion involve chemical reactions with:
Oxygen
Water
Solvents
Acids, alkalis etc.
Light (UV or otherwise)
All involve the creation of FREE RADICALS
For metals – we use a few alloys (Cr, Al, Si, Ti) which form a protective and strong outer oxide layer
For polymers – UV absorbing chemicals and free radical absorbing chemicals are mixed into the polymer