Major corrosion of lead and lead-based welds can occur in soft water that contains dissolved oxygen or dissolved oxygen and carbon dioxide together. When both gases are present, the severity of the corrosion will mainly depend on the carbon dioxide concentration.
Lead will be corrosion-resistant at hardness levels in excess of approximately 12 °F, low levels of dissolved carbon dioxide and a slightly base pH through passivation via a lead carbonate film. On the other hand, in extremely hard water, the high balancing CO2 will make this water corrosive once more even when it is in carbonate stability conditions.
Lead hydroxycarbonate Pb3(CO3)2(OH)2 is a normal corrosion product. This product does not adhere well and, therefore, affords little protection.
When water contains phosphates (a few mg · L–1), a layer of hydroxyphosphate Pb5(PO4)3OH forms. This layer is more adhesive and, therefore, provides greater protection. The use of lead pipes to carry drinking water is now prohibited because the acceptable limit of lead dissolved in water is very low (< 10 ppb). With regard to existing pipes, please refer to the strategy for fighting lead in heavy metals removal.