Alkaline Inhibitive Solutions

The most common alkaline inhibitors used in conservation are sodium hydroxide, sodium carbonate, and sodium sesquicarbonate. Solutions containing these alkalis will prevent the corrosion of iron in oxygenated water as long as they are in concentrations sufficient to maintain a pH which passivates the iron (i.e., makes it chemically inactive) through the formation of an oxide film on the metal. In general, iron can be passivated in a chloride-free solution with a pH above 8.

In inhibitive solutions with a pH of less than 8 the presence of oxygen will increase the rate of deterioration, the corrosion will be localized and the attack will be even more intense than if no inhibitor had been used. Passivation of iron is difficult or impossible at a pH below 8, relatively easy at a pH above 8, and very easy between pH 10 and 12. Iron will corrode by hypoferrate formation in solutions above pH 13 that are free from oxidizing agents. Thus, if iron is stored in an alkaline solution with the pH maintained between 10 and 13, the iron will remain passivated and will not corrode.

A 5% sodium carbonate (pH 11.5) or 5% sodium sesquicarbonate (pH 9.7) storage solution is sufficient for most iron objects if chlorides are not abundant. At high chloride concentrations, prolonged storage in either of these two solutions is not advisable, unless additional alkali is added or the solution is changed often. Because the pH of these solutions is borderline to the corrosion domain, they are not recommended for long-term storage of iron objects from a marine environment. They can be used only for short-term field use or other temporary laboratory storage. reference

Numerous pages of the Corrosion Doctors Web site discuss passivation related topics:

Beer, Biomaterials, Blocking, Calcareous deposits, Electrochemical noise, Electrode passivation, Galvanized, Inhibitors, Iron, Nickel aluminum bronze, Oxidizers, Passivation layer, Passive curve, Passivity, pH, Pickling, Pitting, Potentiodynamic polarization, Rouging, Stainless steels, Steel, Stress corrosion cracking, Surface contaminants