Oxidizing Solutions

Various chromate compounds, such as potassium chromate, potassium dichromate, and sodium chromate make effective storage mixtures. They are more reliable than alkaline inhibitors as long as the concentration and pH are maintained at safe levels. Chromate solutions prevent corrosion by forming a very thin passivating film of ferric oxide and chromic oxide on the surface of the metal. This oxidizing solution creates an environment where a much wide range of pH's and electrode potentials of the surface of the metal is in the passivation range of the metal. It must be emphasized, however, that the chromate solution must be alkaline. The natural alkalinity of chromate, pH 9.1 to 9.3, is an important factor in passivating iron. Dichromates, with a pH of less than 7, are much more acidic than chromates and will not passivate iron unless an alkali is added. The addition of alkali, e.g. NaOH, converts dichromates to chromates and establishes the natural pH of chromate.

Like the alkaline inhibitors, the protection offered by a chromate solution will be suppressed if the pH is too low. Should this occur, the chromate can stimulate an intense localized attack on the surface of the metal and create pits covered with membranous blisters of iron hydroxide. The iron hydroxide inhibits contact between the chromate and the iron surface and allows the anodic reaction to take place. It is necessary that the pH of the chromate solution be maintained in a range of 9.0 to 9.5; if the pH of the solution falls below 9, corrosion of the object will be worse than if no inhibitors were used. The concentration of chloride in chromate solutions is not as critical a factor as it is in alkaline inhibitors, as long as there is free hexavalent chrome (Cr⁺⁶) present in the solution. For this reason chromate solutions are particularly suited for the storage of iron from chloride-contaminated environments.

The pH of the chromate solution must be checked regularly, as some of the chromate in the solution is reduced and the solution may become acidic. If this occurs, additional alkali must be added to convert the dichromates to chromate and re-establish the natural pH range of 9.0 to 9.5. 5. Chromate solutions have the serious disadvantage of being highly toxic if ingested and inhaled. Chromates are strong irritants and some are highly flammable if they come in contact with organic material. Chromate solutions should not be discharged into city sewage lines or natural drainages since they kill beneficial bacteria. Most cities have regulations concerning the disposal of solutions containing hexavalent chrome and a conservation laboratory must, of course, comply with them. If proper disposal cannot be arranged, chromate solutions should not be used. For these reasons, although they are considered excellent for the safe, long-term storage of iron, chromate solutions are not generally used.

If chromate solutions are used, they must be disposed of safely. There are several methods of treating a chromate solution for disposal. One method is described in Pearson:

• The chromate solution is acidified with concentrated sulfuric acid to a pH of 4.
• Sodium metabisulfite is added until the solution turns bright green.
• This reduces the hexavalent chrome to trivalent chrome.
• The solution is then neutralized with a 40% sodium hydroxide solution to precipitate out chromium hydroxide.
• The chromium hydroxide is allowed to settle as a sludge in the bottom of the vat.
• The solution is drained into the sewer lines and the chromium hydroxide, which is insoluble, is disposed in a chemical dump.

Excellent long-term storage results have been achieved with a 0.1N solution of potassium dichromate (K₂Cr₂O7) with sodium hydroxide Many wrought iron artifacts recovered from 16th century Spanish ships have been stored in this solution for more than three years with no apparent corrosion. The pH may have to be adjusted to and maintained at 9.0 to 9.5 by the addition of sodium hydroxide as the chrome is reduced. Sodium chromate also provides good results and has the added benefit of being cheaper than potassium dichromate. (reference)

Numerous pages of this 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