$($a$)$ Why do stainless steels have superior corrosion resistance? Please indicate the type of corrosion that the stainless steel in Figure Q$5$ is experiencing, describe the mechanism behind it$,$ and suggest a solution.

$(5$ marks$)$

Figure Q$5$

$($b$)$ What testing method can be used to study the ductile$-$to$-$brittle transition of materials? Please draw a diagram to illustrate the ductile$-$to$-$brittle transition of an austenitic steel and a martensitic steel.

$(5$ marks$)$

$($c$)$ With the aid of sketches, $($i$)$ describe the definition of "tacticity" and $($ii$)$ distinguish between syndiotactic and atactic configurations.

$(3$ marks$)$

$($d$)$ With the aid of a sketch, explain "Boltzmann Superposition Principle" in viscoelastic solid.

$(4$ marks$)$

$($e$)$ Compute the number$-$average molecular weight for a polypropylene, $-(C_3{H}_6{)}_{n^{-}},$ for which the degree of polymerization is $25,000.(C$:$12\frac{g}{m}ol,H$:$1\frac{g}{m}ol).(3$ marks$)$