Lecture 8

Question 1

Anodic inhibitors

Answer 1

a) Anodic precipitation inhibitors
- form insoluble precipitate with metal ions
- Precipitate seal the anodic site
- PO4^3-
- shifts anodic line left and up
b) Oxidising Inhibitors
- Anodic inhibitor are oxidising agents
- Increase cathodic reaction
- pushes cathodic rate higher and Ecorr value into passive region

Question 2

Adv of anodic inhibitors

Answer 2

Small amounts of anodic inhibitor can reduce corrosion rate by many orders of magnitude

Question 3

Disadv of anodic inhibitors

Answer 3

• If passive layer incomplete small area exposed to electrolyte
• Metal is polarised to a higher potential due to inhibitor and will undergo rapid dissolution
• Inhibitors can be toxic

Question 4

Cathodic inhibitors

Answer 4

a) Precipitation of insoluble materials at the cathode
- React with hydroxide ions to form insoluble barrier
- e.g. Ca2+
b) Cathodic poisoning
- In cases where cathodic reaction is the reduction of hydrogen ions or direct reduction of water to produce hydrogen gas, certain metals can be added which prevent H gas formation and thus slow down rate
- e.g. Arsenic
- Can lead to hydrogen embrittlement though

Question 5

Advantages of cathodic inhibitors

Answer 5

-As Ecorr is reduced we do not see accelerated anodic attack if too little inhibitor used

Question 6

Disadvantages of cathodic inhibitors

Answer 6

• Less efficient than anodic inhibitors since latter method intercepts the most damaging corrosion reactions
• Also could introduce risk of H embrittlement

Question 7

Mixed inhibitors

Answer 7

Zinc phosphate
Zn for cathodic
phosphate for anodic

Question 8

Adsorption inhibitors

Answer 8

• reactive end that binds to metal
• unreactive tail that sticks out from metal
• Bulky polymer chain lowers rate of oxygen diffusion thus inhibiting cathodic reaction