Corrosion with sulfuric acid occurs because of three main factors: temperature, concentration and composition of material. These factors affect two main properties of sulfuric acid, its activity rate and oxidation rate. Activity rate means how well sulfuric acid dissolves, or breaks things down, and oxidation rate means how easily it can cause change in the electric charges, which allows new reactions and more corrosion. Metal rusting is an example of oxidation causing the reaction of iron with water to form iron oxide, or rust. Both properties increase corrosion with sulfuric acid and both become more powerful with increasing temperature and concentration of the sulfuric acid solution.
The type of material plays an important factor when considering sulfuric acid and corrosion. Even diluted sulfuric acid at low temperatures will cause organic materials to corrode, but not metals. Carbon based materials, such as skin, are severely corrosive when exposed to sulfuric acid, because of their organic composition. Acid burns are actually like melting in a hot fire; the carbon is changing to carbon dioxide, and the heat develops from the sulfuric acid mixing with the water trapped in organic substances. This removal of water, or dehydration, causes corrosion because the cells’ water is being ripped out, destroying them in the process.
The activity rate and oxidation rate of sulfuric acid are affected by the temperature. With more heat comes more power to dissolve and cause reactions; thus, more corrosion. With metals, oxidation does not occur with diluted sulfuric acid because not enough of the acid is allowed to break up. This is because sulfuric acid has two hydrogen atoms that need to be separated for most oxidation reactions to occur with metals. Under the same conditions, low heat and low concentration, most metals will not corrode, but sulfuric acid can become very corrosive at high temperatures.
Above 212° Fahrenheit (100° Celsius), concentrated sulfuric acid starts automatically releasing another hydrogen atom, freeing both of the hydrogen atoms. This allows oxidation to occur, which corrodes most metals by forming a metal sulfate and hydrogen gas. At more than 302° Fahrenheit (150° Celsius), the activity rate becomes extreme, and corrosion with sulfuric acid is unstoppable. Even tantaline, an alloy developed to not corrode in a high-temperature concentrated sulfuric acid solution, will rapidly corrode under those conditions.
A bizarre event does occur in “water-free” concentrated sulfuric acid. In this state, found only in a foam form, most metals experience less corrosion with sulfuric acid because the hydrogen uses water to separate, or disassociate, from the sulfuric acid. Without water, the sulfuric acid losses it oxidation capabilities, and corrosion can be caused only by acid activity, which is still extremely high but does not affect materials in which no water is available. One reason why sulfuric acid is used every day in various industries is to remove water from products and materials. Nearly any water-containing material, even sugar crystals, become more dehydrated when exposed to concentrated sulfuric acid.
