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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.

SUBMITTER: Scorciapino MA

PROVIDER: S-EPMC6645160 | biostudies-literature | 2017 Nov

REPOSITORIES: biostudies-literature

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

Scorciapino Mariano Andrea MA Fantauzzi Marzia M Crobu Maura M Navarra Gabriele G Elsener Bernhard B Rossi Antonella A

ACS omega 20171110 11

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 entro ...[more]

PMID: 31457337

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Project description:When constructing discrete (binned) distributions from samples of a data set, applications exist where it is desirable to assure that all bins of the sample distribution have nonzero probability. For example, if the sample distribution is part of a predictive model for which we require returning a response for the entire codomain, or if we use Kullback-Leibler divergence to measure the (dis-)agreement of the sample distribution and the original distribution of the variable, which, in the described case, is inconveniently infinite. Several sample-based distribution estimators exist which assure nonzero bin probability, such as adding one counter to each zero-probability bin of the sample histogram, adding a small probability to the sample pdf, smoothing methods such as Kernel-density smoothing, or Bayesian approaches based on the Dirichlet and Multinomial distribution. Here, we suggest and test an approach based on the Clopper-Pearson method, which makes use of the binominal distribution. Based on the sample distribution, confidence intervals for bin-occupation probability are calculated. The mean of each confidence interval is a strictly positive estimator of the true bin-occupation probability and is convergent with increasing sample size. For small samples, it converges towards a uniform distribution, i.e., the method effectively applies a maximum entropy approach. We apply this nonzero method and four alternative sample-based distribution estimators to a range of typical distributions (uniform, Dirac, normal, multimodal, and irregular) and measure the effect with Kullback-Leibler divergence. While the performance of each method strongly depends on the distribution type it is applied to, on average, and especially for small sample sizes, the nonzero, the simple "add one counter", and the Bayesian Dirichlet-multinomial model show very similar behavior and perform best. We conclude that, when estimating distributions without an a priori idea of their shape, applying one of these methods is favorable.

Dataset Information

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

Publications

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

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