Which corrodes faster at 5% NaCl solution at room temperature: Steel or Aluminum? Is it true that Al has high corrosion resistance? Or is it just a myth as stated in this article.

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    $\begingroup$ Come on. At this point it would be better to do the experiments and report. It seems that whatever you are doing you run its virtual version here. I more or less explained to you that Al is a sacrificial anode to iron. Al cannot have a stronger resistance than iron, nor obviously to stainless steel. The point is that aluminium oxide has pretty good mechanical features and does not go away as iron rust. As long the iron (steel) is covered no oxidation can proceed (aluminium is already oxidised and iron is cover protected). Actually it is what you should have found as basically you did the exp. $\endgroup$
    – Alchimista
    Jul 26 '17 at 13:54
  • $\begingroup$ Actually, as I ask several questions, I am conducting a thorough experimentation. I am asking to verify some of the results, just to know if I am doing the right procedures, since I am very new in this field and my brain cannot comprehend all reasoning behind my project. $\endgroup$
    – Acid
    Jul 26 '17 at 14:00
  • $\begingroup$ Besides, I didn't specified that the Al is coupled with steel in my question. $\endgroup$
    – Acid
    Jul 26 '17 at 14:02
  • $\begingroup$ It is not really the number of question... I am nobody here to tell they were too much. But looks like you ignored the previous answer and explanations, as there should be no reason to reask basically the same, and pointing to a link the quality of which is indeed low. $\endgroup$
    – Alchimista
    Jul 26 '17 at 14:04
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    $\begingroup$ Corrosion usually refers to objects. A bar of Al corrodes slower than a bar of iron. But because it gets self passivated. Al it easy to oxidise, tough. It is always the same argument.................. $\endgroup$
    – Alchimista
    Jul 26 '17 at 14:10

I've decided to answer only because the confusion about "what is readily/faster oxidised/corroded between..." is quite common.

Given a couple of elements/substances in a given state, than the one with lower oxidation potential is the one which is readily oxidised (this is a rather obvious statement).

If I take corrosion = oxidation, then Al gets easily corroded than Fe or steel. This will be evident if we were to make a nice dispersion of very very fine particles of the metals and compare their behaviour, for instance weight gain by oxygen intake at given times.

However, although the chemistry underlying is obviously the same, corrosion is not a synonymous of oxidation, in the sense that it refers to the loosing of the integrity (shape, mechanical properties, even electrical properties) of an object.

If the Al or steel of the question are, instead of very very fine particles, in form of bars, rods, etc., the oxidation process is still easy in case of aluminium. HOWEVER, while iron oxide may take longer to form (at the VERY beginning of the experiment), its mechanical and coating properties are poor. It can continuously detach from the surface it originates from, and so doing lets the underlying iron exposed to the corrosive environment. Things go like this, until the complete object is destroyed - or at least lost is original function.

Conversely, Aluminum oxide has a compact structure and relatively good coating properties. It does not go away. Therefore, as the outer layer of the Al gets oxidised but not detached, an inert passivating layer is formed, and oxidation/corrosion of the object stops.

This may lead someone to say that Al is corroded less than Fe, but now it should be clear that no one means that is indeed harder to oxidise Al as compared to iron.

As the intrinsic properties of Al and Al oxide stay the same, the same happens if we have aluminium coating an inner metal, for instance steel or iron as in the discussion above or in the previous question by the same user. As such, Al is an effective protection layer for steel.

Finally and related: if Al doesn't cover the iron object, but it is merely in contact with it, still it maintains a protective function. A galvanic cell is form, of which Al is the anode.( Look for sacrificial anode). As still Al can get passivated, a more effective sacrificial anode will be Mg, both in terms of even lower oxidation potential and especially reactivity of the oxide. It gets literally consumed and must be replaced, but meantime you won't have to buy a new boat or engine :)

I hope this makes some clarity.

(Note that the very very fine particles are intended to reduce the mass/surface ratio, little particles can self-passivate, too)

Edited: this is a general picture. Technologically/Material science: I do expect lot of details, much of which shall be on the practical attainment of the passivating layer, its morphology, its resistance to diffusion. It seems well known that aluminium oxide undergoes pitting corrosion upon prolonged permanence in solutions containing chloride ions, such as that of the question.


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