Sulfuric acid can have contradictory effects on steel, forming a protective layer at certain concentrations, but causing corrosion and hydrogen gas at others. The best reaction is the formation of iron sulfate, but corrosion is common. A steel case lined with iron sulfate is recommended to prevent corrosion and hydrogen gas. Low levels of hydrogen gas can cause changes in the shape of the steel.
Sulfuric acid is a strong acid that can respond 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 the steel and the steel will be fine. Other concentrations will produce corrosion and the steel will slowly deteriorate. When steel is corroded, sulfuric acid often forms hydrogen gas, which promotes corrosion. Low levels of hydrogen gas can cause gouging or warping of the steel.
The best reaction of sulfuric acid on steel is that the acid begins to create a layer of iron sulfate which protects the steel from the acid and causes no corrosion and little metal loss. This will only occur if the sulfuric acid concentration is between 70% and 99.5%. At a concentration in this range, it can be stored in a non-protective, unlined steel case.
As with most substances, the most common reaction of sulfuric acid on steel is corrosion. Whether the concentration is below or above the safe concentration range, the sulfuric acid will begin to corrode the steel. Corrosion is typically slow, but very high concentrations can rapidly corrode steel. To keep the sulfuric acid at the safest concentrations, a steel case lined with iron sulphate should be used, and the lining should be refilled as needed if possible.
When sulfuric acid reacts on steel, it’s not just a loss of metal. A byproduct of this corrosion is hydrogen gas, which works by stripping away the protective iron sulfate layer. This happens because the hydrogen in the sulfuric acid breaks free to make room for the iron in the steel, which causes the hydrogen to become a gas. This causes the corrosion of the steel to accelerate and, if not repaired, will make further corrosion inevitable. For this reason it is necessary to check the steel case and to fill the protective layer often.
A low level of hydrogen gas may not doom the metal to complete deterioration, but could instead cause changes in the shape of the steel. If the hydrogen gas is light, when it removes the iron sulphate, the reaction between the sulfuric acid and the steel will automatically produce more iron sulphate. This constant up and down of corrosion, shielding, and hydrogen gas means that the steel will typically gouge or warp in random directions. At this point, the steel is often unusable, especially if it served as a pump to move the sulfuric acid.
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