Sulfuric acid is a strong acid capable of responding in many different ways, and the reaction of sulfuric acid on steel can sometimes be contradictory. At certain concentrations, sulfuric acid will form a protective layer on steel and the steel will be fine. Other concentrations will yield corrosion and the steel will slowly deteriorate. With corroding steel, sulfuric acid often forms hydrogen gas, which furthers the corrosion. Low levels of hydrogen gas can cause the steel to groove or warp.
The best reaction of sulfuric acid on steel is that the acid begins to create an iron sulfate layer that protects the steel from the acid and causes no corrosion and little metal loss. This will only occur if the sulfuric acid’s concentration is between 70 percent and 99.5 percent. At a concentration in this range, it can be stored in a non-protective and unlined steel case.
As with most substances, the more common reaction of sulfuric acid on steel is corrosion. Regardless of whether the concentration is lower or higher than the safe concentration range, sulfuric acid will begin corroding the steel. The corrosion typically is slow, but very high concentrations may quickly eat through the steel. To hold sulfuric acid at the safer concentrations, a steel case lined with iron sulfate should be used, and the lining should be replenished as needed, if possible.
When sulfuric acid on steel reacts, there is more than metal loss occurring. A byproduct of this corrosion is hydrogen gas, which works at removing the iron sulfate protective layer. This happens because the hydrogen in the sulfuric acid releases to make room for the iron in steel, which causes the hydrogen to become gas. This causes the steel’s corrosion to speed up and, if not fixed, will make further corrosion unavoidable. For this reason, the steel case should be checked and the protective layer replenished often.
A low level of hydrogen gas may not doom the metal to full deterioration, but it may instead cause changes in the steel’s shape. If the hydrogen gas is slight then, by the time it removes any of the iron sulfate, the reaction between the sulfuric acid and steel will yield further iron sulfate automatically. This constant up and down of corrosion, protection and hydrogen gas means the steel typically will groove or warp in random directions. By this point, the steel is often unusable, especially if it acted as a pump to move the sulfuric acid around.