This is my first time post. My name is Tom and I'm a highschool chemistry teacher and also a science writer. I mention this because I wasn't completely sure where to publish this post. Anyway, I have a batchelors in pure chemistry so that's the level that I'm supposed to be able to comprehend. I'm looking for help to understand something that I need to explain in an article. It needs to be explained to a highschool chemistry level audience but I want to understand it at a higher level so that my explanation is good.
Basically, the article is about aluminium and I'm interested in the contrast between aluminium and iron in the matter of corrosion. The simple explanation is that aluminium doesn't corrode because it quickly forms an oxide layer which adheres to the surface of the metal and thereby prevents further corrosion. The next simple explanation is that iron goes on rusting right through because its oxide layer does not adhere to the surface of the metal but rather flakes away, thereby exposing fresh metal to continued corrosion.
These explanations feel like an over-simplification. The process of rusting is obviously very complex and I've read in certain places that iron oxide adheres to iron with comparative strength to the bond between aluminium and its oxide. What is further suggested (by Wikipedia, which I don't hold up as a bastion of accuracy), is that in fact it is often the acidity of the water that bridges the two halves of the electrochemical cell that promotes rusting. The process is said to produce iron hydroxide ions. These are said to be soluble in water, unlike plain iron oxide, which is not. What is being suggested here is that the difference between the aluminium and iron oxides is not that they adhere with markedly different strengths to their respective metals but that iron oxide is more prone to dissolving in acid and forming the species iron hydroxide, which acts as an intermediary in the overall corrosion process.
Another thing on which I'm seeking clarity is the fact that aluminium does corrode by the galvanic or dissimilar metal process. In these cases, aluminium sets up a galvanic cell via an adjacent and less reactive metal and the standard salt water bridge. What intrigues me about this is the implication that another metal is required for the oxidation process to proceed whereas iron can set up a galvanic cell without the presence of a second, dissimilar metal. In the latter case, I have read that iron plays the anode while oxygen gas serves as the cathode.
So, my questions are:
1) Is it that iron oxide flakes away exposing fresh metal to further electrolytic corrosion or is it that ferrous oxide is more prone to dissolving in acids and producing species which facilitate further oxidation?
2) If aluminium oxide does adhere more tenaciously to aluminium than iron to its own oxide, why is that?
3) Basically, why is it that iron will go on and on rusting when aluminium stops after the formation of a thin layer?
Many thanks in advance for any help rendered.