Corrosive substance
A corrosive substance is one that will destroy or irreversibly damage another substance with which it comes in contact. The main hazards to people include damage to eyes, skin and tissue under the skin, but inhalation or ingestion of a corrosive substance can damage the respiratory and gastrointestinal tracts. Exposure results in chemical burn.
Terms
The word 'corrosion' is derived from the Latin verb corrode which means 'to gnaw' indicating how these substances seem to 'gnaw' their way through the flesh. Sometimes the word 'caustic' is used as a synonym, but by convention 'caustic' generally refers only to strong bases, particularly alkalis, and not acids, oxidizers, or other non-alkaline corrosives. The term 'acid' is often used imprecisely for all corrosives.
A low concentration of a corrosive substance is usually an irritant. Corrosion of non-living surfaces such as metals is a distinct process. For example, a water/air electrochemical cell corrodes iron to rust. In the Globally Harmonized System, both rapid corrosion of metals and chemical corrosion of skin qualify for the "corrosive" symbol.
Corrosives are different from poisons in that corrosives are immediately dangerous to the tissues they contact, while poisons may have systemic toxic effects that require time to become evident. Colloquially, corrosives may be called "poisons"; but the concepts are technically distinct. There is nothing, however, which precludes a corrosive from being a poison, that is, there are substances that are both corrosives and poisons.
Corrosivity
Common corrosives are strong acids and strong bases, or concentrated solutions of certain weak acids and weak bases. Their action on living tissue is based on acid-base catalysis of ester and amide hydrolysis. Both corrosive acids and corrosive bases are able to defat skin by catalyzing the hydrolysis of fats, which are chemically esters. Proteins are chemically amides, which can also be hydrolyzed by acid-base catalysis. Strong acids and bases denature proteins and also hydrate easily. Hydration removes water from the tissue and is significantly exothermic. For example, concentrated sulfuric acid causes thermal burns in addition to chemical burns. Strong oxidizing agents, such as concentrated hydrogen peroxide, can also be corrosive to tissues and other materials, even when the pH is close to neutral. Nitric acid is an example of a strong acid that is also a strong oxidizer, making it significantly more corrosive than one would expect from its pKa alone.
There are also more specific corrosives. Hydrofluoric acid, for example, is initially painless in lower concentrations but easily permeates tissue to selectively attack bone, making it extremely hazardous to work with. It is technically a weak acid, but it produces fluoride ions (the actual corrosive species) after the acid is painlessly absorbed. Although zinc chloride solutions are also regularly acidic (by the Brønsted definition), the zinc cation also specifically attacks hydroxyl groups as a Lewis acid. This explains the ability of zinc chloride solutions to react with cellulose and corrode through paper and silk.
Common types of corrosive substances
Common corrosive chemicals are classified into:
- Strong acids — the most common are sulfuric acid, nitric acid and hydrochloric acid (H2SO4, HNO3 and HCl, respectively).
- Some concentrated weak acids, for example acetic acid
- Solutions of Lewis acids with specific reactivity, e.g. solutions of zinc chloride
- Caustics or alkalis, such as sodium hydroxide (NaOH) and potassium hydroxide (KOH)
- Alkali metals in the metallic form (e.g. elemental sodium) and hydrides of alkali and alkaline earth metals hydrate to give caustics
- Some concentrated weak bases, such as ammonia when anhydrous or in a concentrated solution
- Dehydrating agents such as phosphorus pentoxide, calcium oxide, anhydrous zinc chloride, also elemental alkali metals
- Strong oxidizers such as concentrated hydrogen peroxide
- Electrophilic halogens: elemental fluorine, chlorine, bromine and iodine, and electrophilic salts such as sodium hypochlorite or N-chloro compounds such as chloramine-T[1]; halide ions are not corrosive
- Organic halides and organic acid halides such as acetyl chloride and benzyl chloroformate
- Acid anhydrides
- Alkylating agents such as dimethyl sulfate
- Some organic materials such as phenol ("carbolic acid")