Why corrosion of aluminum is an advantage for the metal and why the corrosion of iron a serious problem

To answer your first question, when the outer layer of aluminum interacts with oxygen, it forms an aluminum oxide layer that is passive. While the outer layer corrodes, it provides a protective coating that protects the inner layer from further deterioration. As a result, aluminum corrosion is beneficial.

To answer your second question, corrosion of iron is a serious problem because almost all things in our civilization is made of iron or one of its alloys, from automobiles to homes, tools to prosthetics.

When you look back at the past of corrosion research, you'll notice that it all began with the question "why does iron rust?" The most powerful organization of corrosion engineers, NACE, crunched the numbers, and the total amount of iron (only in the United States) is in the billions!

When you consider how inexpensive iron is in comparison to other metals, you can see why it is so widely used; and when you consider all of the developments in metallurgy over the last century, you can see how scientists have worked to improve its sturdiness: new alloys with improved corrosion resistance and strength; treatments to postpone corrosion and even attempt to replace it.

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I deeply apologize if this is a late or overdue answer. I also hope this answers your original question and if it doesn't, I deeply apologize.

Other than that, I hope this was helpful and I hope you have a pleasant evening :)