Sulfuric acid (H₂SO₄) is a strong mineral acid widely used in fertilizer manufacturing, metal processing, and chemical synthesis. Its highly corrosive nature allows it to aggressively attack most organic and inorganic materials. Understanding which materials resist this compound is paramount for safe handling, storage, and industrial equipment design. Resistance is always dependent on the acid’s specific state, as no single material is impervious across all temperatures and concentrations.
How Acid Concentration Changes Reactivity
The chemical behavior of sulfuric acid changes dramatically depending on its concentration. When diluted below approximately 70%, the acid acts as a typical strong acid. It dissolves reactive metals through a single displacement reaction, where the metal reacts with hydrogen ions to produce a metal sulfate salt and release hydrogen gas.
Conversely, highly concentrated sulfuric acid (typically above 90%) behaves more like a strong oxidizing and dehydrating agent. This allows it to react with non-metals and less-reactive metals, such as copper, by oxidizing them rather than relying on hydrogen ion activity. Containment materials must be evaluated against both the corrosive nature of the dilute acid and the oxidizing power of the concentrated form.
Metals That Resist Dissolution Through Passivation
Certain metals resist sulfuric acid through passivation, a unique self-protecting mechanism. This process involves the metal reacting immediately upon contact with the acid to form a thin, non-porous, impervious layer on its surface. This protective film, often a sulfate or oxide, separates the bulk metal from the corrosive liquid, halting further dissolution.
Lead resists acid attack by forming a layer of lead sulfate (PbSO₄) when exposed to moderate concentrations of sulfuric acid. This highly insoluble film provides excellent corrosion protection, which is why lead was widely used in early industrial acid production chambers and piping.
Resistance in stainless steels, such as high-grade alloys like 316L, relies on the formation of a protective chromium oxide film. This oxide layer allows these alloys to be used up to moderate acid concentrations, though they may fail in hot, highly concentrated acid environments due to the solution’s aggressive oxidizing nature.
Specialized high-nickel alloys, like Alloy 20, handle aggressive conditions where standard stainless steel fails. These alloys contain high percentages of nickel, chromium, and molybdenum, enhancing the stability of the passive film and resisting localized corrosion. The addition of copper also contributes to their superior performance in certain sulfuric acid concentrations and temperatures.
Highly Resistant Polymers and Carbon Materials
Synthetic materials resist acid by relying on intrinsic chemical stability rather than a reactive surface layer. Fluoropolymers, such as Polytetrafluoroethylene (PTFE) and Perfluoroalkoxy alkanes (PFA), are highly resistant, exhibiting near-universal chemical inertness. Their molecular structure, characterized by strong carbon-fluorine bonds, makes them highly stable and resistant to attack across a broad spectrum of acid concentrations and temperatures.
These fluorinated plastics are often used as liners for steel pipes and vessels requiring maximum protection, particularly in high-temperature or high-concentration applications. Other polyolefins, like High-Density Polyethylene (HDPE) and Low-Density Polyethylene (LDPE), offer good resistance to dilute sulfuric acid at ambient temperatures. However, they suffer from softening or chemical degradation when exposed to highly concentrated acid or elevated temperatures.
Carbon and graphite materials, especially in impervious forms, are valued for their chemical resistance and thermal conductivity in specialized equipment like heat exchangers. Their stable carbon structure resists attack from most concentrations of sulfuric acid unless aggressive oxidizing agents, such as oleum (fuming sulfuric acid), are present. These materials are suitable for handling hot, concentrated acid where many metals and polymers would fail.
The Role of Ceramics and Glass
Traditional containment materials like ceramics and glass offer substantial resistance due to their stable, inorganic, and non-metallic composition. Borosilicate glass, commonly used in laboratory glassware, is highly resistant to chemical attack by sulfuric acid across various concentrations. The low alkali content and boron oxide in borosilicate glass contribute to its exceptional stability and resistance to leaching.
Advanced ceramics, such as high-alumina and silica materials, exhibit excellent acid-proof properties, making them suitable for industrial applications like chemical stoneware and vessel linings. They resist dissolution because the acid struggles to break down their robust, crystalline silicate or oxide structures.
While glass and ceramics are reliable for handling most sulfuric acid solutions, their inherent brittleness and susceptibility to thermal shock present design challenges in large-scale industrial settings. Although their resistance to dissolution is high, their mechanical properties often limit their use compared to the flexibility offered by advanced alloys and polymers.