Hazardous materials
Hazardous materials: UN Class 8 Corrosive Substances By Colin Deiner, chief director, disaster management and fire brigade services, Western Cape Government
It’s important to understand that acids and alkalis are normally transported at very high concentrations
T
he series on hazardous materials continues this month where we will be covering corrosive substances ie UN Hazard Class 8. This covers substances that attack, through a chemical action, epithelial tissue of skin or mucous membranes, which they touch or are capable of destroying or damaging any other materials they come into contact with when released from their containment. This hazard class also covers substances that form a corrosive liquid in the presence of water or produce a corrosive vapour in the presence of moisture in the air. Corrosives are generally either acids, bases, alkalis or salts but can also include other substances that are not necessarily classified in this class, such as chlorine or any other oxidising, toxic and corrosive gasses. Acids Acids will generally react with a wide range of metals and cause corrosion of these metals, which will produce Hydrogen gas. Hydrogen is lighter
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than air and can, in confined spaces, create a flammable or explosive atmosphere. The possibility of this happening will depend solely on the particular type of acid and the metal concerned. It will also depend on the concentration of the acid and the ambient temperature. The fact that acids are often corrosive to metal or flesh also indicates that they can present a level of toxicity. The corrosiveness of the substance and the quantities involved should be the primary considerations of any incident commander dealing with an incident involving the release of acid. Some of the more important inorganic or mineral acids that may be encountered are: Nitric acid (HNO3) Nitric acid is considered a concentrated acid as it is available in a 70 percent solution Nitric acid is available in 100 percent concentrations and this is known as ‘fuming Nitric acid’). The manner in which Nitric acid will behave is dependent on the concentration of
the substance. A diluted solution will display typical properties of an acid; however, if it is more concentrated, it will exhibit more corrosive properties. When Nitric acid reacts to Copper it will produce Nitrogen dioxide, which is a toxic gas. Not all metals will, however, react this way. Iron, Aluminium and Chromium react rapidly with Nitric acid to form a layer of oxide. The oxide formed will be insoluble even in highly concentrated acid and will therefore prevent further corrosion from taking place. When exposed to a fire, Nitric acid readily decomposes producing Nitrogen dioxide and Oxygen. Hydrochloric acid (HCl) Hydrochloric acid is manufactured by dissolving Hydrogen chloride gas in water. When Hydrochloric acid is heated it releases highly pungent and irritating Hydrogen chloride fumes. Hydrogen chloride dissolves very readily in water. Inhaling these fumes causes the formation of Hydrochloric acid in the lungs causing irritation and possible oedema. Hydrochloric acid is not oxidising but does exhibit all the characteristics of a typical acid. Sulphuric acid (H2SO4) Sulphuric acid can generally be found in two forms: • 98 percent Sulphuric acid and • Fuming Sulphuric acid, Oleum or Pyrosulphuric acid; names given to Sulphuric acid containing dissolved Sulphur trioxide (SO3)] Concentrated Sulphuric acid it has the ability to remove and/or absorb large quantities of water and, in so doing, causes a massive exothermic reaction sufficient to boil the added water. Diluting Sulphuric acid with water is therefore an extremely risky Volume 5 | No 3
