two different ways the turbine blade is protected from oxyge

two different ways the turbine blade is protected from oxygen during manufacturing.

Solution

[1] Direct Chemical Attack or Chemical or Dry Corrosion
Whenever corrosion takes place by direct chemical attack by gases like\' oxygen, nitrogen and
halogens, a solid film of the corrosion product is formed on the surface of the metal which protects
the metal from further corrosion. If a soluble or volatile corrosion product is formed, then the metal
is exposed to further attack. For example, chlorine and iodine attack silver generating a protective
film of silver halide on the surface. On the other hand, stannic chloride formed on tin is volatile and
so corrosion is not prevented.
Oxidation corrosion is brought about by direct action of oxygen at low or high temperatures
on metals in the absence of moisture. Alkali metals (Li, Na, K, etc.) and alkaline earth metals
(Mg, Ca, Sn, etc.) are readily oxidized at low temperatures. At high temperatures, almost all metals
except Ag, Au and Pt are oxidized. Alkali and alkaline earth metals on oxidation produce oxide
deposits of smaller volume. This results in the formation of a porous layer through which oxygen can
diffuse to bring about further attack of the metal. On the other hand, aluminium, tungsten and
molybdenum form oxide layers of greater volume than the metal from which they were produced.
These non-porous, continuous and coherent oxide films prevent the diffusion of oxygen and
hence the rate of further attack decreases with increase in the thickness of the oxide film.
The protective or non-protective nature of the oxide film is determined by a rule known as the
Pilling-Bedworth rule. The ratio of the volume of the oxide formed to the volume of the metal
consumed is called the Pilling-Bedworth rule. According to it, if the volume of the oxide layer is
greater than the volume of the metal, the oxide layer is protective and non-porous. On the other hand,
if the volume of the oxide layer formed is less than the volume of the metal, the oxide layer is non
protective and porous.
[2] Electrochemical Theory or Wet Corrosion
According to the electrochemical theory, the corrosion of a metal in aqueous solution may be
a two-step process, one involving oxidation and another reduction. It is known that two metals
having different electrode potentials form a galvanic cell when they are immersed in a conducting
solution. The emf of the cell is given by the difference between the electrode potentials.
When the electrodes are joined by a wire, electrons flow from the anode to the cathode.   
The oxidation reaction occurs at the anode, i.e. at the anode the metal atoms lose their electrons to
the environment and pass into the solution in the form of positive ions.
Fe Fe2+ + 2e-
3
Thus, there is a tendency at the anode to destroy the metal by dissolving it as ions. Hence
corrosion always occurs at anodic areas.
The electrons released at the anode are conducted to the cathode and are responsible for
various cathodic reactions such as electroplating (deposition of metals), hydrogen evolution and
oxygen absorption:
(i) Electroplating: The metal ions at the cathode collect the electrons and deposit on the
cathode surface. Cu2+ + 2e- Cu
(ii) Liberation of hydrogen: In an acid solution, (in the absence of oxygen) hydrogen ions
accept electrons and hydrogen gas is formed. 2H+ + 2e- H2
In a neutral or alkaline medium, (in the absence of oxygen) hydrogen gas is liberated with the
formation of OH- ions. 2H2O + 2e- H2 + 2OH-
(iii) Oxygen absorption: In the presence of dissolved oxygen and in an acid medium, oxygen
absorption reaction takes place. 4H+ + O2 + 4e- 2H2O
In the presence of dissolved oxygen and in a neutral or weakly alkaline medium, OH- ions are
formed. 2H2O + O2 + 4e- 4OH- Thus it is clear that the essential requirements of electrochemical
corrosion are as follows: (a) Formation of anodic and cathodic areas. (b) Electrical contact between
the cathodic and anodic parts to enable the conduction of electrons. (c) An electrolyte through which
the ions can diffuse or migrate. This is usually provided by moisture.