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1
Question
Answer
2
Question
What are the effects of oxygen percentage and humidity on the rate of corrosion?
Answer
In the air, if the availability of oxygen is high, the rate of corrosion is higher due to the formation of an oxygen concentration cell. Part of the metal getting less O2 becomes the anode and undergoes corrosion while the other parts are unaffected. Corrosion increases in humid air compared to dry air because humid air tends to dissolve acidic gases like O2, CO2, and SO2, which increases the pH.
3
Question
How does temperature affect the rate of corrosion?
Answer
Corrosion is more at high temperatures because the diffusion of gases increases with the rise in temperature, which accelerates the corrosion process. For example, this effect is observed in caustic embrittlement.
4
Question
What is the effect of pH on the rate of corrosion and how does it differ in acidic and alkaline mediums?
Answer
In acidic pH, the rate of corrosion is higher because the electrochemical corrosion mechanism proceeds by the evolution of hydrogen at the cathode. In alkaline or neutral medium, corrosion takes place by absorption of oxygen, forming an oxide film that acts as a cathodic part. Such a film adheres to the metal surface and the rate of corrosion is governed by the corrosive product.
5
Question
What is the main principle behind the impressed current method for corrosion protection?
Answer
The impressed current method applies current in the opposite direction to the corrosion current, thereby nullifying the effect of the corrosion current on the base metal, converting it from anode to cathode.
6
Question
How is the impressed current obtained in this corrosion protection method?
Answer
The impressed current is obtained by using a DC source along with an insulating anode such as platinum or graphite that is embedded underground.
7
Question
To what extent is the impressed current applied in protecting metallic structures?
Answer
The impressed current is applied to the whole system of the metallic structure via wires.
8
Question
What role does the backfill made of gypsum play in the impressed current method?
Answer
The backfill made of gypsum helps in increasing electrical contact with the soil.
9
Question
How can large metallic structures be protected using the impressed current method?
Answer
For large metallic structures, multiple anodes can be connected in series using the same impressed current method.
10
Question
What is the purpose of alkaline neutralisation in relation to acidic vapours?
Answer
Alkaline neutralisation is used to neutralise acidic vapours of acids (like HCl, H2SO4) and gases such as SO2 and CO2, which make the environment acidic. Adding a base to the environment neutralises it, reducing corrosion.
11
Question
What does the 'Sacrificial Anode Method' aim to protect, and how?
Answer
The Sacrificial Anode Method aims to protect metal from corrosion by connecting it via a wire to a more reactive metal. The more reactive metal corrodes (sacrifices itself) instead of the base metal, hence protecting it.
12
Question
Why is the metal connected to the base metal in the Sacrificial Anode Method described as 'more reactive'?
Answer
Because this more reactive metal undergoes corrosion preferentially, sacrificing itself, which prevents the base metal from corroding.
13
Question
What happens when the sacrificial metal in the Sacrificial Anode Method is completely corroded?
Answer
It is replaced by a new piece to continue protecting the base metal from corrosion.
14
Question
What are typical applications of the Sacrificial Anode Method mentioned in the notes?
Answer
It is used for protecting storage tanks using zinc (Zn) blocks and buried iron pipes with magnesium (Mg) blocks.
15
Question
What is the purpose of deaeration in modifying an environment?
Answer
Deaeration is the process of controlling the oxygen content in a corrosive environment to reduce the percentage of oxygen present, which helps control corrosion.
16
Question
How can the reduction in oxygen content be achieved during deaeration?
Answer
The reduction in oxygen content can be achieved by regulating parameters such as temperature and pressure or by mechanical stirring.
17
Question
What chemicals are commonly used for deactivation in a corrosive environment?
Answer
Sodium sulfite (Na2SO3) and hydrazine (N2H4) are commonly used chemicals added for deactivation.
18
Question
Write the chemical reactions for the addition of sodium sulfite and hydrazine in deactivation.
Answer
Na2SO3 + 1/2 O2 → Na2SO4 and N2H4 + O2 → N2 + 2H2O.
19
Question
What is dehumidification and why is it important in a corrosive environment?
Answer
Dehumidification is the process of controlling the percentage of humidity in a corrosive environment. Lowering humidity reduces the amount of condensed water and thus helps in controlling corrosion.
20
Question
What substances are used to achieve dehumidification by absorbing moisture?
Answer
Substances such as silica gel and alumina gel are used to absorb moisture in the vicinity to achieve dehumidification.