Historical iron corrosion samples. Series of glass tubes with 70-year-old samples of iron under different conditions, demonstrating the effect on corrosion. The years vary between 1907 and 1910. At top, the iron was sealed in air dried with phosphorus pentoxide (P2O5), showing no corrosion. Water and air (in second tube) are needed for corrosion (formation of rust, or hydrated iron oxide). Sterilisation did not prevent corrosion (third tube). Dilute alkali (ammonium hydroxide, NH4OH) inhibits corrosion (fourth tube), as does potassium dichromate (K2Cr2O7, final tube). Photographed in 1979, at the National Physical Laboratory, Teddington, UK.