Wednesday, 1 January 2014

Lead Corrosion in Acids

Lead forms a protective film, which if undisturbed preserves the metal below this layer.

The corrosion resistance of lead is based on its ability to readily form a tenacious coating of a reaction product. This then becomes a protective coating. Protective coatings on lead may form as the result of exposure to sulphates, oxides, carbonates, chromates, or chemical complexes.
Handbook of Corrosion Data, by Bruce D Craig, p26

Lead is resistant to corrosion especially “with solutions containing sulphate ions, such as sulphuric acid.”

However, the new or bright metal reacts quickly with a variety of alkalis and many organic (although not most inorganic) acids.  ...Lead is not stable in nitric and acetic acids, nor in alkalis. The metal does not resist nitric acid. Lead corrodes rapidly in acetic and formic acids.” (Handbook of Corrosion Data, by Bruce D Craig, p.29)

Lead has very limited resistance to acetic acid.... Dilute [acetic acid], even at room temperature attacks lead at rates exceeding 1.3mm/year. These rates increase rapidly with increasing aeration and velocity However … acetic acid … has little effect at strengths of 52% to 70%.

The corrosion rate in acid increases rapidly in the presence of oxygen and also in oxygen in combination with soft waters such as rain and distilled water. Corrosion increases at the rate approximately proportional to the oxygen content of the water.”
Handbook of Corrosion Data, by Bruce D Craig, p.26, 29

This another good reason to avoid vinegar as a cleaning agent for leaded windows.

Lead dissolves in organic acids (in the presence of oxygen). Lead also dissolves in quite concentrated alkalis (≥10%) because of the characteristic of the lead salts that can act as either an acid or an alkali. These salts are soluble in the presence of water and oxygen.

Alkali salts are soluble hydroxides of alkali metals and alkali earth metals, of which common examples are:
  • Sodium hydroxide (often called "caustic soda")
  • Potassium hydroxide (commonly called "caustic potash")
  • lye (generic term, for either of the previous two, or even for a mixture)
  • Calcium hydroxide (saturated solution known as "limewater")
  • Magnesium hydroxide is an example of an atypical alkali since it has low solubility in water (although the dissolved portion is considered a strong base due to complete dissociation of its ions).

Although this has been a rather technical posting, these data show that lead is subject to rapid attack by both organic (and some inorganic) acids and alkalis in relatively low concentrations when in the presence of aerated water. However in normal environmental conditions the protective reaction layer avoids much of this vulnerability.