Passivation, and Inhibition

These formulations frequently are used after acid cleaning treatments to leave the surfaces in a condition (called passive ) that resists corrosion from the ambient atmosphere. Note, however, that the metal is passive in an electrochemical sense only while it is in contact with the solvent. Upon exposure to the air in the plant, passivating conditions rapidly cease to exist. 

Phosphate System. This system consisted of 0.25% NaH2P04,0.25% NS2P04, and 0.5% NaN02. The system also contained dissolved air. Without air, the polarization curves did not show spontaneous passivation however, this behavior is present with air and phosphate. 

Sodium Carbonate. This solution consists of 1% NaCOs with 0.5% NaN02. The polarization curves also show that this system achieves spontaneous passivation oniy when air is present. 

Ammonia/Hydrazine. Polarization curves did not indicate spontaneous passivation however, one specimen did develop a dark gray coating in the presence of air. The results of the exposure tests are shown in Table 1. 

From these tests, only the phosphate/nitrite solutions achieved any passivation without air. All of the systems did benefit from introduction of the second oxidizer (air). Additional studies evaluated citrate/iron/nitrite (as after a Citro-Soiv treatment), phosphate/hydrazine, and carbonate/gluconate/phosphate. The citrat ron/nitrite was a very effective passivator, but the other two solutions were not as effective. The conclusion is that the more powerful oxidizers are the more effective passivating solutions. 

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Mills and Mabbutt have pointed out that accurate determination of Rn requires that the statistical distribution of the intensity of the fluctuations should have a Gaussian distribution about the mean intensity [130]. These authors observed that in situations of competition between passivity and inhibition, a non-Gaussian, bimodal distribution could occur, and suggested that a second parameter in addition to Rn might be appropriate. 

C. Wagner, Passivity and inhibition during the oxidation of metals at elevated temperatures, Corros. Sci. 5 (1965) 751-764. 

C. Wagner. Passivity and Inhibition during the Oxidation of Metals at Elevated Temperatures. Corrosion Science, 5 751-764, 1965. 

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