Question

Material Science- Corrosion is one of the processes that affects the reliability of all engineering components. It is very complicated process and many factors are affecting it. However, when it comes to the fundamentals, it is not as complicated as one think. Basically, corrosion occurs in metals and their alloys because they are with electrons that are not saturated in bond states. Because of this, the work function plays a critical role in determining the corrosion susceptibility of metallic materials.

1) Alternate way of presenting corrosion susceptibility of metals is to compare standard chemical potential of electrons in metals or standard half cell voltage of metals. What is the standard chemical potential of electronics and the standard half-cell voltage? Can you prepare table showing the a few elemental metals and their standard chemical potential as well as half-cell potential?

2) The standard half-cell voltage does indicate whether material in your hand is corrosion prone or not. However, there are a few exceptions on this such as Cr and Al. In case of Cr, its potential is very much in the negative range but is very good at withstanding corrosion (that is a basis of stainless steel). Briefly explain why this is the case.

3) The worst form of corrosion failure is the Galvanic corrosion. Explain what it is and why it is so problematic for the component reliability.

4) Crevice corrosion can be analyzed in the frame work of Galvanic corrosion even if there is no obvious component that can create Galvanic couple with your piece. Briefly explain what type of condition can make Crevice corrosion to be similar to Galvanic corrosion.

Answer 1

Chemical electronic potential:

The ability of a species to gain electron/s is called chemical electronic potential.

Standard cell potential:

Th ability of a species to undergo reduction (at cathode) i.e. gain of electrons which cause potential difference at 25 degree C and 1M concentration of electrolyte is called standard half cell potential.

Standard half cell potential of some metal ions:

Al³⁺ + 3e --------------> Al (s)              E0 = - 1.66 V

Cr³⁺ (aq.) + 3 e -------------> Cr (s)     E0 = - 0.744 V

Zn²⁺ (aq.) + 2 e --------------> Zn (s) E0 = - 0.76 V

Cu²⁺ (aq.) + 2e -------> Cu (s)          E0 = + 0.34 V

Ni²⁺(aq.) + 2e ------------> Ni (s)       E0 = - 0.257

Fe²⁺ (aq.) + 2 e ----------> Fe (s)      E0 = - 0.447

(2) Yes.

Smaller the redction potential larger is its ability to undergo oxidation i.e corrosion in the presence of atmospheric conditions.

Though Cr and Al are more reactive i.e ready to lose electrons, they are resistance to corrosion becuase of their ability to form protective oxide film on their surface.Once this oxide film is formed on the surface, it prvents the exposer of remaining part of the metal to the atmosphere sothat it stops the corrosion.