Acids that “eat” metal initiate a powerful chemical transformation, converting the solid metal into a dissolved compound called a salt. Unlike general corrosion, such as the slow rusting of iron, acid dissolution is a rapid, active process where the metal atoms are chemically stripped away. The speed and extent of this reaction depend entirely on the specific properties of both the acid and the metal involved.
How Acids Dissolve Metal
The core process by which an acid dissolves a metal is a reduction-oxidation (redox) reaction, involving the transfer of electrons between the metal atoms and the acid solution. For the reaction to begin, the metal must be more chemically reactive than the hydrogen ions supplied by the acid.
When a metal is submerged, its atoms readily lose electrons (oxidation), turning the solid metal atoms into positively charged metal ions that disperse into the liquid acid solution. Simultaneously, the positively charged hydrogen ions (\(\text{H}^+\)) from the acid gain the electrons lost by the metal, which is the reduction half of the reaction.
In many common acid reactions, the reduced hydrogen ions then pair up to form neutral hydrogen gas (\(\text{H}_2\)), often visible as bubbles rising from the metal surface. The metal ions combine with the acid’s negative ions, forming a dissolved metal salt. The reaction continues until either the metal is completely consumed or the acid is spent.
The Most Powerful Metal-Dissolving Acids
Different acids attack metals through slightly different chemical pathways, leading to varying levels of corrosiveness. Common mineral acids like hydrochloric acid (\(\text{HCl}\)) and dilute sulfuric acid (\(\text{H}_2\text{SO}_4\)) are corrosive because they are excellent sources of hydrogen ions. They follow the standard redox mechanism, where the metal is oxidized and hydrogen gas is produced. These acids are highly effective against reactive metals such as zinc, iron, and magnesium.
A far more aggressive class of acids includes those that are also strong oxidizing agents, such as nitric acid (\(\text{HNO}_3\)). Nitric acid’s dissolving power comes not just from its hydrogen ions, but from the nitrate ion (\(\text{NO}_3^-\)) itself, which is a powerful electron acceptor. Instead of reducing the hydrogen ions to form hydrogen gas, the nitric acid reduces to various nitrogen oxide gases, such as nitrogen dioxide (\(\text{NO}_2\)) or nitric oxide (\(\text{NO}\)). This oxidizing ability allows nitric acid to dissolve metals like copper and silver, which resist the hydrogen-producing reaction of non-oxidizing acids.
The most potent acid mixture is aqua regia, Latin for “royal water,” a combination of concentrated nitric acid and concentrated hydrochloric acid, typically mixed in a 1:3 ratio. While nitric acid serves as a powerful oxidizer to convert the metal into ions, the chloride ions from the hydrochloric acid stabilize the metal ions by forming highly soluble complex ions. This synergistic action allows aqua regia to dissolve “noble metals” like gold and platinum, which are otherwise inert to nearly all single acids.
Why Certain Metals Do Not React
Not all metals succumb to the power of acids, and their resistance is determined by two main chemical factors: inherent nobility and surface passivation.
Metals like gold, platinum, and palladium are known as noble metals because of their extremely low chemical reactivity. They are positioned low on the reactivity series, meaning their atoms hold onto their electrons very tightly and are highly unwilling to be oxidized into ions. This lack of chemical drive provides them with natural corrosion resistance, as the energy required to strip the electrons is greater than the energy released by the acid’s reaction.
Other metals, notably aluminum and chromium, achieve resistance through a different mechanism called passivation. When these metals are exposed to oxygen in the air or to certain oxidizing acids, they instantly form a microscopically thin, dense layer of metal oxide on their surface. This oxide layer, which is typically a few nanometers thick, is chemically inert and completely non-porous. It acts as an impenetrable shield, protecting the bulk of the underlying metal from coming into contact with the corrosive acid.