Influence of Incorporated Cl− in NiO on the Pitting Susceptibility of Nickel

Abstract

The pitting susceptibility of nickel in 0.08M C1-solutions (pH 4.0 Na2SOD has been determined by both potentiodynamic and potentiostatic techniques, the former giving a pitting potential and the latter an induction time, rind, for pit initiation. The potentiostatic approach is preferred because of the good reproducibility of results. Samples were prepassivated in non-C1-or C1-containing solutions and then potentiostatically polarized in 0.08M CI-at different potentials and rind measured. For either pretreatment, r~nd decreased exponentially as the potential of pitting increased, i.e., log 1/r ~V. Oxides on samples prepassivated in C1 solution contain incorporated C1-and are significantly more resistant to pitting than those formed in C1--free solutions. This increase in pitting resistance for the C1 -treated samples is in contrast to their decreased resistance to open-circuit breakdown. C1-incorporation results in -2% expansion of the NiO lattice leading to a more defective and thus less stable film towards open-circuit breakdown. The ri,d results strongly suggest that C1-incorporated in the oxide lattice (as distinct from C1-in the electrolyte) is not a precursor to pit initiation since it actually increases the resistance to pitting.In a previous paper (1), it was shown that formation of the passive oxide film on nickel in a C1-containing electrolyte resulted in the incorporation of C1-into the oxide lattice. While Auger sputter profiles indicated that the C1-was mainly distributed in the outer layers of the oxide film, the results were not compatible with a simple adsorption of C1-on the oxide surface (cf. 2-7). Indeed, the C1-signal increased substantially as the first oxide layer was sputtered away, and there appeared to be approximately twice as much C1-in the second layer as in the first. The amount of chloride in the film was highly dependent on the [C1-] in solution and the potential of anodization. The maximum homogeneous atomic concentration of C1-which could be achieved was ~-,5%. (This concentration is determined assuming a uniform layer-by-layer distribution of C1-.) The concentration of incorporated C1-quickly reached a steady-state value (~---15s) if the films were formed in a C1-containing solution. On the other hand, incorporation into an initially C1--free NiO film, achieved by subsequent anodization in a C1-containing solution, was dependent on the state of perfection of the passive film. The results suggested that C1-incorporation into an existing passive film occurred via local breakdown-repair events. These breakdown events were not induced by the presence of CI-, but were simply due to chemical dissolution of the oxide at local defects, i.e., the process that gives rise to the corrosion current even in the absence of C1-. Local reformation of the film in the C1-containing solution therefore resulted in a local C1-incorporation. A more perfect oxide film has fewer defects and therefore a slower rate of C1-uptake.While the previous paper (1) indicated that C1-can incorporate into the NiO lat...

Changes in Oxide Films on Nickel during Long‐Term Passivation

“…When this is taken into account, the kinetics of conversion of the prior film are similar in the presence or absence of C1-ion, i.e., the C1-does not appear to accelerate the process of film breakdown. This is consistent with the model, which suggests that C1-has no direct influence on breakdown of the passive oxide film but simply interferes with local repassivation events which are continuously occurring at numerous sites on the oxide surface (12,18). It is also in agreement with the observation that addition of C1-to a solution in which a passivated sample is held potentiostatically does not result in any measurable increase in i, until t > t,,d, the induction time at which pitting begins (20).…”

“…3. The eventual current is somewhat higher than that observed for breakdown in pH 1.0 Na2SO4, but this is simply due to C1-incorporation into the oxide film making the film more defective, i.e., producing more sites at which breakdown will occur at any instant in time (17)(18)(19). When this is taken into account, the kinetics of conversion of the prior film are similar in the presence or absence of C1-ion, i.e., the C1-does not appear to accelerate the process of film breakdown.…”

Influence of Chloride on Nanoscale Electrochemical Passivation Processes

“…Cl − 2p XPS data suggest that NiCl 2 is not present, and Cl − incorporation in the inner layer remains controversial. 28,66,67 We present herein an investigation where the timedependent electrochemical passive film growth on binary Ni-22 wt % Cr is studied using single frequency EIS (SF-EIS) to examine passivation in situ, coupled with ex situ X-ray photoelectron spectroscopy, AFM imaging and analysis of the evolution of the surface topography and roughness. The passivation was achieved in dilute acidified NaCl solution, which was used in tandem with chloride-free acidified Na 2 SO 4 at the same pH to understand the impact of chloride anions on the nucleation and growth and of passive films and their resultant topography.…”

“…Prior literature has helped to identify the chemical, electronic, and physical structure of passive films on Ni–Cr, but very little work has been reported aimed at understanding the surface of Ni–Cr alloys during the early to intermediate stages of passivation and film growth in acidic and chloride-containing solutions. , Limited prior studies found Cl – may interact with Ni­(OH) 2 via a ligand exchange process, where the hydroxide may form Ni­(OH)­Cl. Cl – 2p XPS data suggest that NiCl 2 is not present, and Cl – incorporation in the inner layer remains controversial. ,, …”

Spatial Interactions among Localized Corrosion Sites

“…Both oxide film properties and MnS inclusion dissolution can be affected by chloride ions and acidity. 11,[15][16][17][18] In contrast, the ohmic potential drop will decrease the interfacial potential over some distance from the first pit and thereby decrease stable pit generation rates which are often strongly potential dependent. 19,20 We wish to determine how long each effect persists, the relative strength of each, and the distances from the sustained initial pit site that are affected.…”