6N.23

[905290504]

Question: Suggest two metals that could be used for the cathodic protection of a titanium pipeline. (b) What factors other than relative positions in the electrochemical series need to be considered in practice? (c) Often copper piping is connected to iron pipes in household plumbing systems. What is a possible effect of the copper on the iron pipes?

I'm just kind of confused on this question and don't really understand whatever concepts it's trying to get at. If anyone could help me out, I'd really appreciate it!
thanks!

[BrittneyMyint1D]

Hi! I believe this question is asking us to compare the standard reduction potentials and analyzing its effects. For the first question, we want to prevent titanium from corroding/oxidizing, so we want two metals that have standard reduction potentials below the standard reduction potential of titanium. This is because a more positive reduction potential leads to reduction, while a more negative value would lead to oxidation. So, for the first part, just find a metal that has a lower standard reduction potential than titanium. This concept is also similar for part c, where you would compare the standard reduction potentials of copper and iron. If you look at the values, iron has a more negative value, which means it would be oxidized. Hope this helps!

[Zaid Bustami 1B]

If left exposed to the air, many metals corrode over time - you probably know that rusting is an example of corrosion, and it involves the oxidation of iron in the presence of water and oxygen. Corrosion involves the oxidation of the metal, so in order to prevent corrosion you need to coat the metal in a material that will prevent it from oxidizing. This means that the material you use to coat the metal needs to have a low enough reduction potential such that the oxidation of the metal is no longer spontaneous. From what I understand, coating the metal is called cathodic protection because in the process you turn the metal from an anode (prone to oxidation) into a cathode, and thus make it resistant to oxidation. So in the case of titanium, in order to give it cathodic protection you need to coat it with a metal with a reduction potential lower than that of titanium, which is -1.63V.

Some reasonable choices would be aluminum or magnesium since they aren't highly unstable or radioactive. Cesium probably wouldn't be as good a choice because it's liquid near room temperature, but anyways you really just want to pick materials that are reasonably stable and that you could coat the metal with easily. These are some of the practical considerations you need to make for part b), because even if something like uranium has a lower reduction potential than titanium it isn't necessarily the material you want to use because, in the case of uranium, it's radioactive and not really safe to have near a pipeline. Also, some elements could be more expensive than others, so it's also something to consider.
For part c) what they want you to get at is seeing if some form of a reaction will occur between copper and iron. To see if any corrosion will occur, you just need to compare their reduction potentials (also I wrote all the possible reduction reactions for Fe, since any of its possible oxidation states may form):
Cu²⁺ + 2e^- → Cu, E^o=+0.34
Fe²⁺ + 2e^- → Fe, E^o=-0.44V
Fe³⁺ + 3e^- → Fe, E^o=-0.04V
Since the reduction potential for copper is greater than both those of iron, we know that the contact between the copper and iron pipes will eventually result in the copper oxidizing the iron in the presence of enough water to form an electrolyte (a solution that can conduct electrons/electricity). The copper will corrode the iron as a result.