I already posted this question 2rd time now someone didn\'t answer the whole thi

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I already posted this question 2rd time now someone didn't answer the whole thing this is a 2rd refund please just answer all parts:

You are required to answer all parts of the question. Our professor gave it as you see it as a single question, so to save to getting a refund and posting again just please.

I know the rule is 1 question at a time in material science at our university this is a single question!!

And correctly clear writting please.

it would be a pain for me to separate them, since I have a fair bit of them.

And finally please do not write on a piece of paper then take a single photo with your mobile phone its going to be blured we can't see it use a photo copier. otherwise take multiple close up photos if mobile is all you have.

8. (a) Describe how pitting corrosion can occur, and specify the factors that can affect the rate of galvanic corrosion. (4 marks) (b) Explain how a T6 heat treatment can be effectively used to achieve the maximum strength offered by an Al-4wt%Cu alloy produced by casting. (Note: the as-cast Al-4wt% ingot contains 4vol% of fairly large non-equilibrium Al2Cu particles, and T6 involves solution treatment, quenching and artificial t treatment. ageing.) (4 marks (c) Explain the roles of Si in aluminium wrought alloy Al-1wt%Si-1wt%Mg and aluminium casting alloy Al-7wt%Si-0.4wt%Mg, respectively. (2 marks) (d) A piece of Zn metal is submerged into an aqueous solution containing 0.4 mole/litrc of Zn and having a pH of 11, but without any dissolved oxygen. Determine whether this piece of zinc is corroded by the solution, and describe how the potential of the Zn /Zn would change with increasing the current density 0.059 flowing through it n this situation. (e 0.059 pH, and some information can be found from Table i on page 70. (7 marks

Explanation / Answer

( a). Mechanism of Pitting Corrosion

The more conventional explanation for pitting corrosion is that it is an autocatalytic process. Metal oxidation results in localised acidity that is maintained by the spatial separation of the cathodic and anodic half-reactions, which creates a potential gradient and electromigration of aggressive anions into the pit. For example, when a metal is present in an oxygenated NaCl electrolyte, the pit acts as anode and the metal surface acts as cathode. The localised production of positive metal ions in the pit gives a local excess of positive charge which attracts the negative chlorine ions from the electrolyte to produce charge neutrality. The pit contains a high concentration of MCl molecules which react with water to produce HCl, the metal hydroxide, and H+ ions, accelerating the corrosion process.In the pit, the oxygen concentration is essentially zero and all of the cathodic oxygen reactions take place on the metal surface outside the pit. The pit is anodic and the locus of rapid dissolution of the metal. The metal corrosion initiation is autocatalytic in nature however its propagation is not.

This kind of corrosion is extremely insidious, as it causes little loss of material with the small effect on its surface, while it damages the deep structures of the metal. The pits on the surface are often obscured by corrosion products.

Pitting can be initiated by a small surface defect, being a scratch or a local change in composition, or a damage to the protective coating. Polished surfaces display higher resistance to pitting.

(b).

Heat-treatment is of major importance since it is commonly used to alter the mechanical

properties of cast aluminum alloys. Heat-treatment improves the strength of aluminum

alloys through a process known as precipitation-hardening which occurs during the heating

and cooling of an aluminum alloy and in which precipitates are formed in the aluminum

matrix. The improvement in the mechanical properties of Al alloys as a result of heat

treatment depends upon the change in solubility of the alloying constituents with

temperature.

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