What Are the Effects of Sulfuric Acid on Metal?

The effects of sulfuric acid on metal depend on a number of factors, including the type of metal, the concentration of the acid, and the temperature. Dilute sulfuric acid will, in theory, react with any metal that lies above hydrogen in the reactivity series by displacing hydrogen from the acid, releasing it as a gas and forming the sulfate salt of the metal. The metals that come into this category include the alkali metals, such as sodium and potassium, and the alkaline earth metals, like magnesium and calcium, as well as many other common metals, such as iron, nickel, and zinc. Since hydrogen has very low solubility in water and acids, it will produce bubbles; the resulting effervescence is greater with the more reactive metals. Diluted sulfuric acid (H₂SO₄) and magnesium, for example, will react vigorously: Mg + H₂SO₄ -> MgSO₄ + H₂.

In practice, not all of these metals will react with sulfuric acid under normal circumstances. Although the pure metals will react, some elements, when exposed to air, quickly acquire a layer of oxide. The effects of this acid on metal oxides vary, but in some cases, the oxide layer is chemically very inert and will prevent any reaction from taking place. For example, although titanium is above hydrogen in the reactivity series, it normally has a thin coating of titanium dioxide that renders it unreactive toward sulfuric and most other acids. Aluminum also forms a protective oxide layer; however, sulfuric acid and aluminum will react after some delay to produce hydrogen gas and aluminum sulfate.

Another factor that can affect the combination is the solubility of the salt, or metal sulfate, formed by the reaction. Some metal sulfates — for example, those of iron, zinc and aluminum — are very soluble in water or acids while others — like the sulfates of calcium and barium — are not. When the sulfate has low solubility, the reaction will quickly slow down or stop as a protective layer of sulfate builds up around the metal.

Pure sulfuric acid does not react with metals to produce hydrogen, since the presence of water is required to allow this reaction to take place. The concentrated sulfuric acid used in laboratories is normally 98% acid and 2% water; the small quantity of water present allows these reactions to proceed in some cases, albeit slowly. If a more dilute solution is used, the reaction is much more rapid. Stainless steel, at low temperatures, is not corroded significantly by the acid at concentrations above about 98%. At industrial plants, it is sometimes stored in steel tanks; however, corrosion is rapid if the water content is higher.

The effect of sulfuric acid on metal elements that are below hydrogen in the reactivity series is different, as they cannot displace hydrogen from the acid. These metals include copper, mercury, silver, gold and platinum. They will not react with dilute sulfuric acid, or with the concentrated acid at room temperature.

Concentrated sulfuric acid, however, acts as an oxidizing agent when hot and this it to react with copper, mercury, and silver. In the case of copper, for example, the following reaction takes place: Cu + 2H₂SO₄ -> CuSO₄ + SO₂ + 2H₂O. Gold and platinum will not react with sulfuric acid at all.