How do oxide formations affect voltaic cells?
There are several questions in your text.
First, you imagine that &quot;a metal's ability to form an oxide layer is related to its reactivity&quot;. No. It is not. Aluminum is rather reactive with water. But sodium is more. And metallic sodium is not protected by any oxide layer. On the contrary. Sodium reacts dangerously with water. Aluminum is a strange metal, as it forms an oxide that has the same density as the metal. So this oxide can perfectly cover the surface of the metal.
Then you ask about the difference of reactivity between zinc and nickel. I cannot answer this question, as I don't know enough of the nickel chemistry.
Then you say that aluminum oxide reacts with acids. It is right, but this reaction is extremely slow. If some aluminum piece or foil is put into a solution of $\ce{HCl}$ made by mixing the concentrated solution ($36$%) by an equal amount of water, you will have to wait a couple of minutes before the oxide layer is dissolved according to : $$\ce{Al2O3 + 6 HCl -&gt; 2 AlCl3 + 3 H2}$$ And this last reaction is so slow that the expected reaction producing hydrogen gas happens only two or three minutes later according to $$\ce{2 Al  +  6 HCl  -&gt;  AlCl3  +  3 H2}$$ In this case, the production of gas is violent and highly exothermic, but it only happens with delay.
At the end, you ask why &quot;I even got a voltage in the aluminum cell if the oxide layer does not allow any aluminum ions to be transferred&quot;. This is a good question, for which I don't have a real answer. It may be due to some default of the alumina layer.