Passivators critical concentration

The passivating action of an aqueous solution within porous concrete can be changed by various factors (see Section 5.3.2). The passive film can be destroyed by penetration of chloride ions to the reinforcing steel if a critical concentration of ions is reached. In damp concrete, local corrosion can occur even in the presence of the alkaline water absorbed in the porous concrete (see Section 2.3.2). The Cl content is limited to 0.4% of the cement mass in steel-concrete structures [6] and to 0.2% in prestressed concrete structures [7]. 

Amorphous Fe-3Cr-13P-7C alloys containing 2 at% molybdenum, tungsten or other metallic elements are passivated by anodic polarisation in 1 N HCl at ambient temperature". Chromium addition is also effective in improving the corrosion resistance of amorphous cobalt-metalloid and nickel-metalloid alloys (Fig. 3.67). The combined addition of chromium and molybdenum is further effective. Some amorphous Fe-Cr-Mo-metalloid alloys passivate spontaneously even in 12 N HCl at 60° C. Critical concentrations of chromium and molybdenum necessary for spontaneous passivation of amorphous Fe-Cr-Mo-13P-7C and Fe-Cr-Mo-18C alloys in hydrochloric acids of various concentrations and different temperatures are shown in Fig. 3.68 ... 

Fig. 3.68 Critical concentrations of chromium and molybdenum necessary for spontaneous passivation of amorphous Fe-Cr-Mo-13P-7C and Fe-Cr-Mo-18C alloys in hydrochloric acid of various concentrations and temperatures ...

Figure 43. Average critical concentration fluctuation Pa on a completely active surface against Ni2+ ionic concentration.91 T = 300 K. (Reprinted from M. Asanuma and R. Aogaki, Nonequilibrium fluctuation theory on pitting dissolution. II. Determination of surface coverage of nickel passive film, J. Chem. Phys. 106,9938,1997, Fig. 13. Copyright 1997, American Institute of Physics.)...

The specific objectives of the experiment were to determine critical concentrations of Pt necessary for stable passivity of the surface alloy in IN H2SO4 and to aid understanding of the mechanisms of noble metal redistribution. 

Add efficient inhibitors for certain systems since it is believed to assist the formation of a stronger, more stable or more readily repaired passive film. (Miller)24 Phosphates and other inorganic or organic corrosion inhibitors, used in fairly corrosive surroundings, decrease the effects of the SCC. A minimal critical concentration of some oxidizing inhibitors such as nitrites is absolutely necessary to avoid pitting. 

Passivator ions acting as oxidizers are adsorbed on the substrate and reduced easily, thus enlarging the cathode surface area. An optimum passivator solution concentration should exceed some critical value and, the higher the passivator concentration, the easier it is adsorbed, and the smaller the anodic areas on the substrate will be. This promotes an increase in anodic polarization and total passivation of the substrate. If the passivator concentration is lower than some critical value, it initiates local corrosion of the substrate. 

As shown in Fig. 3(a), the efficiency of passivating inhibitors depends strongly on their presence at a sufficient (critical) concentration. Below this critical concentration (curve I) a bistable situation may arise, in which the corrosion potential can either sit in the passive (Ej.) or active ( ) region, resulting generally in pitting. Because of this problem, which can even cause an increase in the corrosion... 

The development of pits starts with a crack or a hole of atomic dimension in the passive film caused, e.g., by tensions or by local chemical dissolution of the fihn. Permanent pitting corrosion can start above a critical potential and a critical concentration of the chloride ions. Above these critical values repassivation is prevented by the adsorption of the aggressive anions in the crack or the hole. The small dimensions of the crack or hole stabilize the large potential drop between active and passive surface. 

K iron is first immersed in dilute chromate solution for several minutes, it remains passive in concentrated nitric acid without the initial reaction to form HNO2. The passive film is preformed by chromate, and nitrous acid is no longer necessary as depolarizer to reach /critical, but is needed only in concentration sufficient to maintain the film already present. 

Thomas looked for lead (as a constituent of lead azelate) at the steel-rust interface in an attempt to confirm this theory. Samples coated with lead-based paint were exposed for three years and then cross-sections were examined in a LAMMS however, lead was not detected at the interface. As Thomas points out, this finding does not eliminate the mechanism as a possibility lead could still be present but in levels below the 100 ppm detection limit of the LAMMS [30,31]. Appleby and Mayne have shown that 5 to 20 ppm of lead azelate is enough to protect pure iron [25]. The levels needed to protect rusted steel would not be expected to be so low, because the critical concentration required for anodic inhibitors is higher when chloride or sulphate ions are present than when used on new or clean steel [35]. Possibly, a level between 20 and 100 ppm of lead azelate is sufficient to protect the steel. Another point worth considering is that the amounts of lead that would exist in the passive film formed by complex azelates, suggested by Appleby and Mayne, has not been determined. The lead soaps/lead azelate theory appears to be the most likely mechanism to explain how red-lead paints protect rusted steel. 

Current-voltage trends in the dissolution-passivation range are restituted by assuming Tafel kinetics for iron dissolution and a chemical rate-determining step for chromium. Extension to 3D lattices [192] leads to similar conclusions but highlights a critical concentration pf, 2 (the percolation threshold for interactions up to the second nearest neighbor) on a 3D lattice in better agreement with the transition of Fe-Cr to stainless steel behavior. 

On pure aluminum, Hebert and Alkire [68] proposed that passivity breakdown occurred when a critical concentration of aluminum chloride is attained in the crevice environment. Indeed, they showed convincingly (Fig. 25) that the presence of aluminum cations in the solution caused passivity breakdown for a pH and chloride concentration at which solutions containing only sodium cations did not. 

---