The question of what materials an acid cannot “burn through” refers to chemical stability against extreme corrosion, not combustion. Acids chemically degrade or dissolve materials through corrosion or dissolution. This degradation occurs when the acid reacts with the material, breaking down its chemical bonds and turning the solid into soluble ions or salts. We are investigating the materials that resist this chemical breakdown even under the most aggressive conditions.
The Chemistry of Acid Corrosion
Acid corrosion is fundamentally an electrochemical process driven by the high concentration of hydrogen ions (H+) in the acid solution. These ions seek to react with the atoms of the exposed material by accepting electrons from the surface. This process, known as oxidation, converts the solid material’s atoms into positively charged ions that then dissolve into the acid solution.
The strength of an acid is often measured by its pH level, but corrosiveness depends on the acid’s ability to participate in these electrochemical reactions. For metals, acids facilitate a redox reaction where the metal is oxidized and H+ ions are reduced to hydrogen gas. Non-metallic materials, like polymers, degrade if the acid can break their molecular bonds. A material’s resistance comes from either a lack of reactive sites or the formation of a protective barrier.
Highly Acid-Resistant Polymers and Ceramics
Non-metallic materials offer robust acid resistance due to their inherently stable chemical structures. Ceramics like borosilicate glass, commonly used in laboratory glassware, owe their resistance to their high silica content. Silica is already a highly oxidized compound, making it chemically reluctant to undergo further oxidation by most acids. This glass is highly resistant to practically all acids, with notable exceptions being hydrofluoric acid and hot, concentrated phosphoric acid.
Polytetrafluoroethylene (PTFE), widely known by the brand name Teflon, is among the most chemically inert polymers available. Its exceptional resistance stems from its molecular structure, which consists of a long chain of carbon atoms saturated with extremely strong carbon-fluorine bonds. These bonds are so stable that they are largely non-reactive to almost all corrosive chemicals. This makes PTFE a preferred material for seals, liners, and containers in highly corrosive chemical processing environments.
The Role of Passivation in Resistant Metals
Metallic resistance to acid often relies on a mechanism known as passivation, which differs from the inherent stability of polymers and ceramics. Passivation occurs when a metal spontaneously forms a thin, dense, and non-reactive layer of metal oxide on its surface when exposed to air or an oxidizing acid. This protective oxide layer acts as a barrier, preventing the underlying bulk metal from further electrochemical attack.
Metals like stainless steel, which contains chromium, form a chromium oxide layer that is highly resistant to many acids. Titanium and aluminum also exhibit this self-protecting quality, making them useful in corrosive environments. Noble metals like gold and platinum possess an inherent stability, meaning they are chemically unreactive without relying on an oxide layer. However, even these metals can be challenged by specific chemical mixtures.
The Ultimate Test: Superacids and Aqua Regia
The limits of acid resistance are tested by extreme chemical agents like superacids and Aqua Regia. Superacids are defined as acids stronger than 100% pure sulfuric acid, and the strongest known example is fluoroantimonic acid. This substance is billions of times stronger than sulfuric acid, possessing the power to protonate nearly any organic compound. Fluoroantimonic acid will rapidly dissolve glass and most metals, but must be stored in containers lined with fluoropolymers like PTFE or PFA.
Aqua Regia, Latin for “royal water,” is a fuming mixture of concentrated nitric acid and hydrochloric acid, typically in a 1:3 ratio. This mixture is potent because the nitric acid acts as a powerful oxidizer, while the chloride ions form stable, soluble complexes with the metal ions. Aqua Regia is famous for its ability to dissolve the noble metals gold and platinum, which resist either acid alone. However, certain metals, such as tantalum and iridium, still resist dissolution by Aqua Regia due to the immediate formation of a highly insoluble and protective surface film.