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Unmasking chloride attack on the passive film of metals.

ABSTRACT: Nanometer-thick passive films on metals usually impart remarkable resistance to general corrosion but are susceptible to localized attack in certain aggressive media, leading to material failure with pronounced adverse economic and safety consequences. Over the past decades, several classic theories have been proposed and accepted, based on hypotheses and theoretical models, and oftentimes, not sufficiently nor directly corroborated by experimental evidence. Here we show experimental results on the structure of the passive film formed on a FeCr₁₅Ni₁₅ single crystal in chloride-free and chloride-containing media. We use aberration-corrected transmission electron microscopy to directly capture the chloride ion accumulation at the metal/film interface, lattice expansion on the metal side, undulations at the interface, and structural inhomogeneity on the film side, most of which had previously been rejected by existing models. This work unmasks, at the atomic scale, the mechanism of chloride-induced passivity breakdown that is known to occur in various metallic materials.

SUBMITTER: Zhang B

PROVIDER: S-EPMC6028649 | biostudies-literature | 2018 Jul

REPOSITORIES: biostudies-literature

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Unmasking chloride attack on the passive film of metals.

Zhang B B Wang J J Wu B B Guo X W XW Wang Y J YJ Chen D D Zhang Y C YC Du K K Oguzie E E EE Ma X L XL

Nature communications 20180702 1

Nanometer-thick passive films on metals usually impart remarkable resistance to general corrosion but are susceptible to localized attack in certain aggressive media, leading to material failure with pronounced adverse economic and safety consequences. Over the past decades, several classic theories have been proposed and accepted, based on hypotheses and theoretical models, and oftentimes, not sufficiently nor directly corroborated by experimental evidence. Here we show experimental results on ...[more]

PMID: 29967353

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Project description:The bimodal-grain-size 7075 aluminium alloys containing varied ratios of large and small 7075 aluminium powders were prepared by spark plasma sintering (SPS). The large powder was 100 ± 15 ?m in diameter and the small one was 10 ± 5 ?m in diameter. The 7075 aluminium alloys was completely densified under the 500 °C sintering temperature and 60 MPa pressure. The large powders constituted coarse grain zone, and the small powders constituted fine grain zone in sintered 7075 aluminium alloys. The microstructural and microchemical difference between the large and small powders was remained in coarse and fine grain zones in bulk alloys after SPS sintering, which allowed for us to investigate the effects of microstructure and microchemistry on passive properties of oxide film formed on sintered alloys. The average diameter of intermetallic phases was 201.3 nm in coarse grain zone, while its vale was 79.8 nm in fine grain zone. The alloying element content in intermetallic phases in coarse grain zone was 33% to 48% higher than that on fine grain zone. The alloying element depletion zone surrounding intermetallic phases in coarse grain zone showed a bigger width and a more severe element depletion. The coarse grain zone in alloys showed a bigger electrochemical heterogeneity as compared to fine grain zone. The passive film formed on coarse grain zone had a thicker thickness and a point defect density of 2.4 × 1024 m-3, and the film on fine grain zone had a thinner thickness and a point defect density of 4.0 × 1023 m-3. The film resistance was 3.25 × 105 ?cm2 on coarse grain zone, while it was 6.46 × 105 ?cm2 on fine grain zone. The passive potential range of sintered alloys increased from 457 mV to 678 mV, while the corrosion current density decreased from 8.59 × 10-7 A/cm2 to 6.78 × 10-7 A/cm2 as fine grain zone increasing from 0% to 100%, which implied that the corrosion resistance of alloys increased with the increasing content of fine grains. The passive film on coarse grain zone exhibited bigger corrosion cavities after pitting initiation compared to that on fine grain zone. The passive film formed on fine grain zone showed a better corrosion resistance. The protectiveness of passive film was mainly determined by defect density rather than the thickness in this work.

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Unmasking chloride attack on the passive film of metals.

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Unmasking chloride attack on the passive film of metals.

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