Nanostructure of Surface Films on Ni18P Alloy in Sulfate Solutions by the Maximum Entropy Method

Abstract

NiP alloys are very often used in industry, due to their outstanding performance in corrosion and wear. Alloys with high phosphorus content (≥16 atom % P) are amorphous and show high corrosion resistance in both neutral and acidic solutions irrespective of the presence of chloride ions. The reason for this behavior is attributed to the formation of a “P-enriched surface layer” whose exact nature is still under debate. In this work, an iterative algorithm based on the application of maximum entropy method on nondestructive angle-resolved X-ray photoelectron spectroscopy data has been applied to the investigation of the surface layer grown on Ni18P alloys following mechanical polishing and anodic polarization in sulfate solutions. The results show that the outermost region of the examined alloy has a complex layered structure: (1) an uppermost hydrocarbon contamination layer about 1 nm thick, which includes also adsorbed water; (2) a nickel (poly)phosphate layer of about 1 nm; (3) a highly phosphorus-enriched interface being about 2 nm thick with a marked phosphorus concentration gradient, from 70 to 20 atom %; and (4) bulk alloy with the stoichiometric composition. These findings, together with the chemical state of the different phosphorus compounds, allow us to conclude that the high corrosion and wear resistance of NiP alloys might be ascribed to the presence of a thin, self-repairing nickel (poly)phosphate layer grown on a strongly P-enriched interface. Because the Auger parameter of P at the interface is similar to that of elemental P, it might be also concluded that the interface is enriched in elemental phosphorus

Molecular dynamics simulations of amorphous Ni–P alloy formation by rapid quenching and atomic deposition

“…A P-enriched 10 ± 3 Å thick surface layer, with about 40% P, has been shown to exist on mechanically polished electroless Ni 82 P 18 [49], in agreement with the simulation result in figure 7. Because nickel depletes preferentially from the surface, upon corrosion testing the layer grows to 20 ± 6 Å, and it becomes further enriched in phosphorus (46% P), [49].…”

“…A P-enriched 10 ± 3 Å thick surface layer, with about 40% P, has been shown to exist on mechanically polished electroless Ni 82 P 18 [49], in agreement with the simulation result in figure 7. Because nickel depletes preferentially from the surface, upon corrosion testing the layer grows to 20 ± 6 Å, and it becomes further enriched in phosphorus (46% P), [49]. Even on the mechanically polished sample there is a layer of nickel phosphate, so that corrosion cannot be excluded from playing a role in forming this layer during the polishing.…”

Stainless steels: Passive film composition, pitting potentials, and critical chloride content in concrete

“…[ 22,23,26 ] Thickness and composition of the passive films formed on the three alloys and the composition of the metal interface beneath the film have been calculated using a three‐layer model. [ 27 ] The calculations were based on the integrated XPS intensities of the different components that were corrected for Scofield's photo‐ionization cross‐sections, [ 28 ] the instrument transmission function, [ 29 ] and the inelastic mean free path.…”