Glass reacts slowly with aqueous sodium hydroxide solutions at ambient temperatures to form soluble silicates. Because of this, glass joints and stopcocks exposed to sodium hydroxide have a tendency to “freeze”. Flasks and glass-lined chemical reactors are damaged by long exposure to hot sodium hydroxide, which also frosts the glass.
Sodium hydroxide does not attack iron at room temperatures, since iron does not have amphoteric properties (i.e., it only dissolves in acid, not base). Nevertheless, at high temperatures (e.g. above 500 °C), iron can react endothermically with sodium hydroxide to form iron(III) oxide, sodium metal, and hydrogen gas.[22] This is due to the lower enthalpy of formation of iron(III) oxide (−824.2 kJ/mol compared to sodium hydroxide (-500 kJ/mol), thus the reaction is thermodynamically favorable, although its endothermic nature indicates non-spontaneity.
A few transition metals, however, may react vigorously with sodium hydroxide.In 1986, an aluminium road tanker in the UK was mistakenly used to transport 25% sodium hydroxide solution,[23] causing pressurization of the contents and damage to the tanker. The pressurization was due to the hydrogen gas which is produced in the reaction between sodium hydroxide and aluminium.
