Sulfuric acid corrosion is influenced by temperature, concentration, and material composition. The activity and oxidation rates of sulfuric acid increase with temperature and concentration, causing corrosion. Organic materials, such as leather, are severely corrosive due to dehydration. Concentrated sulfuric acid above 302°F is unstoppable, and even tantalum corrodes rapidly. Waterless concentrated sulfuric acid causes less corrosion due to the lack of water, but still has high activity. Sulfuric acid is used to remove water from materials.
Corrosion with sulfuric acid occurs due to three main factors: temperature, concentration and composition of the material. These factors influence two main properties of sulfuric acid, its activity rate and oxidation rate. The activity rate tells you how well sulfuric acid dissolves or breaks things down, and the oxidation rate tells you how easily it can cause changes in electrical charges, which allows for new reactions and more corrosion. Metal rust is an example of oxidation that causes iron to react with water to form iron oxide, or rust. Both properties increase sulfuric acid corrosion, and both become more potent with increasing temperature and concentration of the sulfuric acid solution.
The type of material plays a factor when considering sulfuric acid and corrosion. Even dilute sulfuric acid at low temperatures causes corrosion of organic materials, but not metals. Carbon-based materials, such as leather, are severely corrosive when exposed to sulfuric acid, due to their organic composition. Acid burns are actually like melting in a hot fire; the carbon is transformed into carbon dioxide, and the heat develops from the sulfuric acid mixing with the water trapped in the organic substances. This removal of water, or dehydration, causes corrosion because the cells’ water is driven out, destroying them in the process.
The activity rate and oxidation rate of sulfuric acid are affected by temperature. With more heat comes more power to dissolve and cause reactions; therefore, more corrosion. With metals, oxidation does not occur with dilute sulfuric acid because not enough acid is allowed to split. This is because sulfuric acid has two hydrogen atoms that must be separated for most oxidation reactions to occur with metals. Under the same conditions, at low temperatures and low concentrations, most metals do not corrode, but sulfuric acid can become very corrosive at high temperatures.
Above 212° Fahrenheit (100° Celsius), concentrated sulfuric acid begins to automatically release another hydrogen atom, freeing both hydrogen atoms. This allows oxidation to occur, which corrodes most metals to form a metal sulfate and hydrogen gas. At more than 302° Fahrenheit (150° Celsius), the rate of activity becomes extreme and sulfuric acid corrosion is unstoppable. Even tantalum, an alloy developed not to corrode in a high-temperature concentrated sulfuric acid solution, will also corrode rapidly under those conditions.
A bizarre event occurs in “waterless” concentrated sulfuric acid. In this state, found only as a foam, most metals suffer less corrosion with sulfuric acid because the hydrogen uses water to separate or dissociate from the sulfuric acid. Without water, sulfuric acid loses its oxidizing capabilities and corrosion can only be caused by acid activity, which is still extremely high but does not attack materials where water is not available. One of the reasons sulfuric acid is used every day in various industries is to remove water from products and materials. Nearly all materials containing water, even sugar crystals, become more dehydrated when exposed to concentrated sulfuric acid.
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