# Passivation of metals in acids

It is commonly taught in chemistry textbooks that metals such as $$\ce{Fe, Al, Cr}$$ is "passivated" in cold concentrated $$\ce{H2SO4}$$ and $$\ce{HNO3}$$ due to them forming oxide films on the surface that prevents the bulk metal from reacting further. However, all those metals can dissolve in dilute solutions of the aformentioned acids, and dilute $$\ce{HCl}$$ as well.

However in metals such as $$\ce{Ti}$$ and $$\ce{Sn}$$ show somewhat reversed characteristics compared to iron or chromium. For example, according to Wikipedia:

Titanium is capable of withstanding attack by dilute sulfuric and hydrochloric acids, chloride solutions, and most organic acids. However, titanium is corroded by concentrated acids.

For tin, Greenwood and Earnshaw's Chemistry of the Elements state that:

Dilute $$\ce{H2SO4}$$ and $$\ce{HCl}$$ show little, if any, reaction but dilute $$\ce{HNO3}$$ produces $$\ce{Sn(NO3)2}$$ and $$\ce{NH4NO3}$$. Hot concentrated $$\ce{HCl}$$ yields $$\ce{SnCl2}$$ and $$\ce{H2}$$ whereas hot concentrated $$\ce{H2SO4}$$ forms $$\ce{SnSO4}$$ and $$\ce{SO2}$$.

My question is that why some metals are corroded by dilute acids but not by concentrated acids; while other metals do the exact opposite but with the same mechanism?

My attempt to answer is that for the cases of $$\ce{Fe, Al, Cr}$$, concentrated acids oxidize them so quickly so that a thick layer of oxide is rapidly formed. The oxide is structured in a way so that the acid ions/molecules cannot get past it to attack the unreacted metal. In a diluted solution the oxidation does not occur as fast and the rate of formation of oxide layer is slower than rate of metal dissolving.

In $$\ce{Ti, Sn}$$'s case, the oxides $$\ce{TiO2, SnO2}$$ are more acidic but for some reason not as thick. Because they are more acidic, they require higher acid concentration to be dissolved, thus those metals only dissolve in concentrated acids rather than dilute.

Any help would be greatly appreciated, especially if you can give reliable journal papers or books.

• – Mithoron Jun 14 at 19:30
• It is necessary to separate the case of $\ce{HCl}$ from the other acids like $\ce{H2SO4}$ and $\ce{HNO3}$ which may be oxidizing, because $\ce{HCl}$ is only an acid and does not or cannot act as an oxidizing agent. $\ce{HCl}$ cannot make an oxide layer. it cannot produce passivation. Nevertheless, the fact is that $\ce{HCl}$ is unable to react with some oxides like $\ce{Al2O3}$ and $\ce{Cr2O3)$ I am not sure there is a reason for this absence of reaction. – Maurice Jun 14 at 19:36
• Hot concentrated H2SO4 may be an oxidizer, but whether hot or cold, Fe, Al and Cr will dissolve in it, not be passivated by it. HCl is also an active corrosive. HNO3 is the unique oxidizing passivator or corroder. – James Gaidis Jun 14 at 20:10