spore: profile
id: chromium-oxide-coating
spore_version: 0.2
family: oxide-ceramics
name: Chromium oxide thermal spray coating (Cr2O3, chromia)
abbreviation: Cr2O3
role: Hard ceramic wear and seal-face coating, sprayed onto steel parts
category: coatings
type: anodize
summary: |
  Chromium(III) oxide (chromia, Cr2O3) is sprayed onto steel by atmospheric plasma spraying to form a hard ceramic layer. Vickers microhardness of the plasma-sprayed Metco 6156 coating was 1496 HV300 g in one test and 1762 HV300 g in another, per Odhiambo et al. The coating is brittle, so it chips under point impact and its thickness is limited before it spalls, according to the HTS Coatings technical data table.
best_for: Wear-resistant, sliding and abrasive contact faces on steel, such as printing anilox rolls and mechanical seal faces, where the part does not take impact
weakest_at: 'Brittleness: chromia chips under point-loaded impact or edge loading, and thick build-up risks spalling (HTS Coatings)'
identity:
  formula: Cr2O3
  cas: 1308-38-9
  crystal_system: trigonal (corundum structure)
  space_group: R-3c (No. 167)
  source: wikipedia-cr2o3
  where: 'Chembox: CAS number, crystal structure (corundum) and space group sections'
  designations:
    - system: Trade name
      code: Metco 6156
      source: odhiambo2021
      where: 'Experimental details, first paragraph: commercially available powder (Metco 6156)'
tags:
  environments: []
  stresses: []
  applications: []
properties:
  - benchmark: density
    label: Density of crystalline chromium(III) oxide
    value: 5.22
    unit: g/cm3
    scope: material
    conditions:
      basis: bulk crystalline Cr2O3 at 25 degC as printed in the Merck Index entry
      not_a: sprayed coating density
    method: Value printed in the experimental-properties section of the PubChem record, citing the Merck Index
    status: ground
    source: pubchem-cr2o3
    where: Chemical and Physical Properties > Experimental Properties > Density, first entry (5.22 g/cm3 at 25 degC)
  - benchmark: melting_point
    label: Melting point of chromium(III) oxide
    value: 2435
    unit: °C
    scope: material
    conditions:
      basis: bulk crystalline Cr2O3 as printed in the Merck Index entry
    method: Value printed in the Chembox properties of the Wikipedia article (melting point 2435 °C, printed as 2,435 °C); the PubChem record prints the same figure from the Merck Index
    status: ground
    source: wikipedia-cr2o3
    where: 'Chembox Properties, melting point: 2,435 °C (2435 °C in the infobox source)'
  - benchmark: vickers_hardness
    label: Typical hardness of atmospheric plasma sprayed chromia
    value: 900
    value_high: 1200
    bound: range
    unit: HV300
    scope: material
    conditions:
      form: atmospheric plasma sprayed coating (typical APS)
      basis: range printed for typical APS coatings in the technical data table
    method: Range printed in the HTS Coatings technical data table, Hardness (Vickers) row
    status: ground
    source: hts-chrome-oxide
    where: 'Technical data table, Hardness (Vickers) row: 900-1200 HV300 typical APS'
  - benchmark: rockwell_c_hardness
    label: Hardness of chromia coating in Rockwell C
    value: 62
    value_high: 72
    bound: range
    unit: HRC
    scope: material
    conditions:
      form: chromia coating, range depends on formulation as printed
    method: Range printed in the HTS Coatings technical data table, Hardness (HRC) row
    status: ground
    source: hts-chrome-oxide
    where: 'Technical data table, Hardness (HRC) row: 62-72 HRC depending on formulation'
  - benchmark: vickers_hardness
    label: Microhardness of plasma sprayed Metco 6156, first coating
    value: 1496
    unit: HV300 g
    scope: material
    conditions:
      feedstock: Metco 6156 chromium oxide powder
      process: atmospheric plasma spray, no bond coat, 550 A arc current
      load: 300 g
      dwell: 15 s
    method: Mean Vickers microhardness on polished cross-sections, as stated in the microhardness section
    status: ground
    note: The conclusion section of the same paper prints 1497 HV for this coating, one unit higher.
    source: odhiambo2021
    where: Microhardness section (Vickers microhardness values for 550A)
  - benchmark: vickers_hardness
    label: Microhardness of plasma sprayed Metco 6156, second coating
    value: 1762
    unit: HV300 g
    scope: material
    conditions:
      feedstock: Metco 6156 chromium oxide powder
      process: atmospheric plasma spray, no bond coat, 650 A arc current
      load: 300 g
      dwell: 15 s
    method: Mean Vickers microhardness on polished cross-sections, as stated in the microhardness section
    status: ground
    note: The conclusion section of the same paper prints 1763 HV for this coating, one unit higher.
    source: odhiambo2021
    where: Microhardness section (Vickers microhardness values for 650A)
  - benchmark: bond_strength
    label: Bond strength of atmospheric plasma sprayed chromia on steel
    value: 4000
    value_high: 6000
    bound: range
    unit: psi
    scope: material
    conditions:
      form: atmospheric plasma spray on steel
      basis: range printed in the technical data table
    method: Range printed in the HTS Coatings technical data table, Bond strength row
    status: ground
    source: hts-chrome-oxide
    where: 'Technical data table, Bond strength row: 4,000-6,000 psi on steel (APS)'
  - benchmark: bond_strength
    label: Tensile bond strength, first coating (550A)
    value: 12.24
    unit: MPa
    scope: material
    conditions:
      feedstock: Metco 6156
      process: atmospheric plasma spray, no bond coat, 550 A
      thickness: 348 um as printed
      failure: adhesive
    method: Tensile adhesion test in the paper, which says it followed the ASTM C633 procedure
    status: ground
    source: odhiambo2021
    where: Table 4, averaged tensile bond strength results (550A, 12.24 MPa, adhesive failure)
  - benchmark: bond_strength
    label: Tensile bond strength, second coating (650A)
    value: 23.6
    unit: MPa
    scope: material
    conditions:
      feedstock: Metco 6156
      process: atmospheric plasma spray, no bond coat, 650 A
      thickness: 391 um as printed
      failure: adhesive
    method: Tensile adhesion test in the paper, which says it followed the ASTM C633 procedure
    status: ground
    source: odhiambo2021
    where: Table 4, averaged tensile bond strength results (650A, 23.60 MPa, adhesive failure)
  - benchmark: bond_strength
    label: Bond strength of Rokide C combustion-sprayed rod coating
    value: 4545
    unit: psi
    scope: material
    conditions:
      product: Rokide C chrome oxide ceramic rod (mostly Cr2O3 with silica and alumina), combustion rod spray
      form: combustion-sprayed rod coating
    method: Value printed in the Rokide C technical bulletin, Mechanical section
    status: ground
    source: rokide-c-tds
    where: 'Coating features, Mechanical: Bond Strength 4545 psi'
  - benchmark: porosity
    label: Porosity of plasma sprayed Metco 6156, first coating (550A)
    value: 8
    unit: '%'
    scope: material
    conditions:
      feedstock: Metco 6156
      process: atmospheric plasma spray, 550 A
    method: Porosity by image analysis of SEM micrographs, as stated in the paper
    status: ground
    note: The results section text is garbled for this value; the correlation section prints 8% in full.
    source: odhiambo2021
    where: 'Correlation of the results and the discussion: porosities 8% and 4% for 550A and 650A'
  - benchmark: porosity
    label: Porosity of plasma sprayed Metco 6156, second coating (650A)
    value: 4
    unit: '%'
    scope: material
    conditions:
      feedstock: Metco 6156
      process: atmospheric plasma spray, 650 A
    method: Porosity by image analysis of SEM micrographs, as stated in the paper
    status: ground
    note: The results section text is garbled for this value; the correlation section prints 4% in full.
    source: odhiambo2021
    where: 'Correlation of the results and the discussion: porosities 8% and 4% for 550A and 650A'
  - benchmark: porosity
    label: Porosity of standard atmospheric plasma sprayed chromia
    value: 5
    bound: upper
    unit: '%'
    scope: material
    conditions:
      form: standard APS coating, not the dense sintered-powder anilox grade
    method: Upper limit printed in the HTS Coatings technical data table
    status: ground
    source: hts-chrome-oxide
    where: 'Technical data table, As-sprayed porosity row: up to 5% on standard APS'
  - benchmark: porosity
    label: Total porosity of Rokide C combustion-sprayed rod coating
    value: 8.4
    unit: '%'
    scope: material
    conditions:
      product: Rokide C chrome oxide ceramic rod, combustion rod spray
    method: Value printed in the Rokide C technical bulletin, Physical section
    status: ground
    source: rokide-c-tds
    where: 'Coating features, Physical: Total porosity 8.4%'
  - benchmark: max_service_temp
    label: Maximum service temperature for wear service
    value: 540
    unit: °C
    scope: material
    conditions:
      service: dry abrasive wear service
    method: Value printed in the HTS Coatings technical data table, metric row
    status: ground
    source: hts-chrome-oxide
    where: 'Technical data table, Max service temp (metric row): 540 °C wear service'
  - benchmark: max_service_temp
    label: Maximum service temperature for sealed corrosive service
    value: 177
    value_high: 205
    bound: range
    unit: °C
    scope: material
    conditions:
      service: sealed coating in corrosive chemical environments
    method: Range printed in the HTS Coatings technical data table, metric row
    status: ground
    source: hts-chrome-oxide
    where: 'Technical data table, Max service temp (metric row): 177-205 °C sealed corrosive service'
  - benchmark: coating_thickness
    label: Typical thickness of thermal sprayed chromia
    value: 75
    value_high: 640
    bound: range
    unit: µm
    scope: material
    conditions:
      process: typical thermal spray chromia coating
    method: Range printed in the HTS Coatings technical data table, Typical thickness row
    status: ground
    source: hts-chrome-oxide
    where: 'Technical data table, Typical thickness row: 3-25 (75-640 µm)'
  - benchmark: density
    label: Bulk density of Rokide C ceramic rod
    value: 4.5
    unit: g/cc
    scope: material
    conditions:
      product: Rokide C chrome oxide ceramic rod, mostly Cr2O3
      note: a rod product figure, not the density of pure chromia
    method: Value printed in the Rokide C technical bulletin, Physical section
    status: ground
    source: rokide-c-tds
    where: 'Coating features, Physical: Bulk Density 4.5 g/cc'
  - benchmark: particle_size
    label: Feedstock particle size, Metco 6156 powder
    value: -35
    value_high: 15
    bound: range
    unit: µm
    scope: material
    conditions:
      product: Metco 6156 powder as received, sieve range as printed
    method: Size range printed in the Experimental details section of the paper
    status: ground
    source: odhiambo2021
    where: 'Experimental details, first paragraph and Table 1: particle size -35 to +15 um'
corrosion:
  - environment: Sealed chromia in acidic and alkaline solutions
    headline: The HTS page says sealed coatings hold up through 30-day immersion across pH 0-10 with no loss of wear resistance.
    effect: The HTS page attributes this to a Journal of Thermal Spray Technology study of plasma sprayed chromia and alumina sealed with aluminum phosphates. The primary paper could not be opened for this profile, so the result rests on the HTS summary.
    rating: low
    conditions:
      sealant: aluminum phosphate (per the HTS page)
      test: immersion in acidic and alkaline solutions, as summarized by HTS
      basis: secondary summary of a primary study
    source: hts-chrome-oxide
    where: Sources section (Journal of Thermal Spray Technology entry) and Where it doesn't section
  - environment: Unsealed plasma sprayed coating with open porosity
    headline: 'Open porosity is the weak point: standard APS coatings run up to 5% porosity, so sealing is specified for aggressive chemistry.'
    effect: The HTS page lists sealing with epoxy, aluminum phosphate or polymer impregnation for aggressive chemical environments. The Odhiambo paper finds that the coating with higher porosity (8%) had the lower tensile bond strength (12.24 MPa against 23.60 MPa at 4%), so porosity also weakens the bond to the steel. This is an adhesion result, not a corrosion test.
    rating: moderate
    conditions:
      basis: qualitative corrosion ranking from the HTS page; porosity and bond comparison from the Odhiambo paper
    source: hts-chrome-oxide
    where: Porosity bullet in Where it doesn't; Porosity row of the technical data table
  - environment: High temperature in wear or sealed corrosive service
    headline: The table gives a wear ceiling of 540 °C and a sealed corrosive ceiling of 177 to 205 °C.
    effect: The HTS page says the sealed corrosive ceiling is the limit in chemical service and advises stepping over to Cr3C2-NiCr for higher-temperature wear. The page does not say what fails above these ceilings.
    rating: moderate
    conditions:
      basis: service ceilings as printed by HTS Coatings
    source: hts-chrome-oxide
    where: Technical data table, Max service temp row; Where it doesn't section
risks:
  hazards:
    - text: PubChem's GHS summary for CID 517277 says the substance 'does not meet GHS hazard criteria for 57.8% (1112 of 1924) of all reports'. Its hazard statements are H302, H317, H319 and H360, with the signal word Danger and pictograms GHS07 and GHS08.
      ghs:
        - H302
        - H317
        - H319
        - H360
      source: pubchem-cr2o3
      where: Safety and Hazards, GHS Classification (CID 517277)
    - text: A Johnson Matthey safety data sheet for Katalco 71-5, a Grade 2 chromium(III) oxide classified by an EU registrant, is stricter. It lists H314 (causes severe skin burns and eye damage), H340, H350 (may cause cancer), H361 and H371, with the signal word Danger. It is a different product, but it shows that classifications for this oxide disagree between sources.
      ghs:
        - H302
        - H312
        - H332
        - H314
        - H334
        - H317
        - H340
        - H350
        - H361
        - H335
        - H371
        - H373
        - H411
      source: jm-katalco-sds
      where: 'Section 1, classification and labelling: hazard statements and classification table'
  failure_risks:
    - mode: Chipping under impact
      trigger: Point-loaded impact or edge loading
      consequence: Chromia is a ceramic, so impact chips the coating. The HTS page says to specify it for sliding, erosive and corrosive duty, not impact.
      severity: moderate
      source: hts-chrome-oxide
      where: Limitations list, Brittle bullet
    - mode: Spalling of thick build-up
      trigger: Build-up beyond the practical limit on cylindrical work
      consequence: The HTS page says 'Thickness limited to roughly 25-30 mils on cylindrical work before residual stress risks spalling'. Typical thickness is 75 to 640 µm.
      severity: moderate
      source: hts-chrome-oxide
      where: Limitations list, thickness bullet; Technical data table, Typical thickness row
    - mode: Service above the ceiling for the duty
      trigger: Service above 540 °C for wear, or above 177 to 205 °C in sealed corrosive service
      consequence: The table gives these as ceilings. The HTS page does not describe the failure mode above them.
      severity: moderate
      source: hts-chrome-oxide
      where: Technical data table, Max service temp row
  handling:
    - text: For the Cr2O3-TiO2 blend Metco 6485 from the same maker (not pure chromia), keep the powder in its original container in a dry location, tumble it gently before use, and store open containers in a drying oven to prevent moisture pickup.
      source: metco-6485-ds
      where: Section 4.2, Handling Recommendations
    - text: The HTS page says finishing requires diamond wheels, so plan the cost and lead time.
      source: hts-chrome-oxide
      where: Introduction, closing caveats paragraph
  mitigations:
    - text: Seal the coating with epoxy, aluminum phosphate or polymer impregnation for aggressive chemical service.
      source: hts-chrome-oxide
      where: Porosity bullet in Where it doesn't
    - text: Apply over a corrosion-resistant bond coat for chemical service, as the HTS page describes for its own process.
      source: hts-chrome-oxide
      where: Introduction paragraph
uses:
  - application: mechanisms
    device: Mechanical seal faces, pump sleeves and impellers
    example: The HTS page lists mechanical seal faces, pump sleeves and impellers in abrasive slurry among the parts it coats with chromia.
    source: hts-chrome-oxide
    where: Introduction paragraph (We use it for ...) and Applications list
  - application: mechanisms
    device: Flexographic printing anilox rolls, laser engraved after coating
    example: The HTS page says chromia is the industry-standard surface for flexographic and gravure printing rolls because it takes laser engraving cleanly.
    source: hts-chrome-oxide
    where: Applications list and Where it earns its keep
  - application: mechanisms
    device: Wire drawing capstans, draw blocks and sheaves
    example: The Saint-Gobain Rokide C bulletin lists wire drawing capstans, wire draw blocks and sheaves among the parts for which the rod is used.
    source: rokide-c-tds
    where: Applications list (right column)
supply:
  - subject: Chromium (chromite ore, the feedstock for chromia)
    producers:
      - country: South Africa
        share_percent: 45.1
        year: 2025
      - country: Turkey
        share_percent: 17.6
        year: 2025
      - country: Kazakhstan
        share_percent: 13.7
        year: 2025
      - country: India
        share_percent: 5.9
        year: 2025
      - country: Brazil
        share_percent: 3.9
        year: 2025
      - country: Zimbabwe
        share_percent: 3.9
        year: 2025
      - country: Finland
        share_percent: 3.7
        year: 2025
    recycling: 'USGS: ''recycled chromium (contained in reported stainless-steel scrap receipts) accounted for 21% of apparent consumption'' in the United States in 2025.'
    substitutes: []
    source: usgs-mcs2026-chromium
    where: World Mine Production and Reserves table, 2025e column (world total printed as 51,000 thousand t; the table header gives chromium content); shares calculated by Space Mushroom from the seven countries listed; Recycling paragraph. Other countries (3,000 thousand t) are excluded from the shares.
documents:
  - title: 'HTS Coatings: Chrome oxide technical data table (web product page)'
    type: datasheet
    url: https://htscoatings.com/coatings/chrome-oxide
    year: 2026
  - title: Saint-Gobain Rokide C (Chrome Oxide) ceramic rod technical bulletin, copy hosted by Alloy Coating Supply
    type: datasheet
    url: https://store.alloycoatingsupply.com/wp-content/uploads/2023/11/Rokide-C-Rods-Technical-Data-Sheet.pdf
    year: 2023
  - title: Influence of plasma spraying current on the microstructural characteristics and tribological behaviour of plasma sprayed Cr2O3 coating (open access)
    type: paper
    url: https://www.elsevier.es/en-revista-boletin-sociedad-espanola-ceramica-vidrio-26-articulo-influence-plasma-spraying-current-on-S0366317520300340
    year: 2021
  - title: 'USGS Mineral Commodity Summaries 2026: Chromium'
    type: report
    url: https://pubs.usgs.gov/periodicals/mcs2026/mcs2026-chromium.pdf
    year: 2026
  - title: 'PubChem: Chromium(III) oxide (CID 517277), GHS classification and experimental properties'
    type: database
    url: https://pubchem.ncbi.nlm.nih.gov/rest/pug_view/data/compound/517277/JSON
    year: 2026
  - title: 'Oerlikon Metco data sheet DSM-0387.1: Chromium Oxide - Titanium Oxide Powders (Metco 6485)'
    type: datasheet
    url: https://www.oerlikon.com/ecoma/files/DSM-0387_Cr2O3_40TiO2.pdf?download=true
    year: 2024
  - title: Johnson Matthey (C&L) safety data sheet, Cr - Chromium (III) Oxide, Katalco 71-5 (IUCLID 6 export)
    type: sds
    url: https://matthey.com/documents/161599/166998/Cr-Chromium-III-Oxide-Katalco-715.pdf/dcfbe5f7-6cc9-a7c9-0d92-7d2339ca4759
    year: 2020
gaps:
  - No vacuum, radiation, thermal-vacuum or flight data for chromia coatings were found in any source we hold; this profile has no space application.
  - 'The Saint-Gobain chromia data sheet, the Saint-Gobain #300 series chromium oxide data sheet, the Cincinnati Thermal Spray CT1302-1 data sheet, the American Elements SDS and the Journal of Thermal Spray Technology paper by Leivo et al. on sealed chromia could not be opened (403, access denied or not found), so their figures are not used.'
  - The two MDPI papers on plasma sprayed Cr2O3 (Crystals 2023 and Coatings 2023) returned access denied, so their porosity, hardness and bond figures are not used.
  - The Odhiambo paper states the feedstock as Blended in Table 1 but as sintered and crushed in its text, and gives 99.7 in Table 1 but 99% purity in its text; neither composition figure is recorded as a property.
  - Odhiambo paper results section porosity values are garbled (8.% and 4.%); the correlation section values of 8% and 4% are used instead.
  - Density of a sprayed pure chromia coating was not found; the 5.22 g/cm3 value is for bulk crystalline Cr2O3, and the 4.5 g/cc value is for the Rokide C rod, which is mostly Cr2O3.
  - Coefficient of thermal expansion, thermal conductivity and total emittance for Rokide C are printed in exponent or estimate form and are left out (Rokide C bulletin, Thermal section).
  - Atomic weights of chromium and oxygen and the crustal abundance of chromium were not read in any source we hold.
  - Critical-mineral list status for chromium was not confirmed in the fetched USGS text, so no critical_lists entry is given.
  - No UNS, EN, DIN, JIS or AISI designation was found for chromia powder or coating; only the Metco 6156 trade name is printed in a source.
  - The sealed-chromia corrosion result rests on the HTS summary of the primary paper; no coating corrosion rate in mm per year or mass loss was found.
  - Pure-chromia supplier SDS for hazards was not reachable; hazard data rest on PubChem and on one registrant classification for a different product (Katalco 71-5).
  - Non-canonical benchmark names are used for hardness (vickers_hardness, rockwell_c_hardness), bond strength (bond_strength), porosity, coating thickness (coating_thickness) and particle size (particle_size).
sources:
  hts-chrome-oxide:
    title: 'Chrome oxide thermal spray coating: technical data table, applications and sources (HTS Coatings)'
    id: HTS Coatings product page, /coatings/chrome-oxide
    url: https://htscoatings.com/coatings/chrome-oxide
    year: 2026
    kind: web
    license: supplier web page, linked not reproduced
  wikipedia-cr2o3:
    title: Chromium(III) oxide, Wikipedia
    id: Wikipedia article Chromium(III) oxide
    url: https://en.wikipedia.org/wiki/Chromium(III)_oxide
    year: 2026
    kind: web
    license: CC BY-SA 4.0 (Wikipedia)
  rokide-c-tds:
    title: Rokide C (Chrome Oxide) ceramic rod for thermal spraying, Saint-Gobain Coating Solutions technical bulletin
    id: Saint-Gobain Coating Solutions Rokide C bulletin, copy on Alloy Coating Supply
    url: https://store.alloycoatingsupply.com/wp-content/uploads/2023/11/Rokide-C-Rods-Technical-Data-Sheet.pdf
    year: 2023
    kind: datasheet
    license: manufacturer document, linked not reproduced
  odhiambo2021:
    title: Influence of plasma spraying current on the microstructural characteristics and tribological behaviour of plasma sprayed Cr2O3 coating
    id: doi:10.1016/j.bsecv.2020.03.007 (Bol. Soc. Esp. Ceram. Vidr. 60, 338-346, 2021)
    url: https://www.elsevier.es/en-revista-boletin-sociedad-espanola-ceramica-vidrio-26-articulo-influence-plasma-spraying-current-on-S0366317520300340
    year: 2021
    kind: paper
    license: open access, as listed on the journal page
  usgs-mcs2026-chromium:
    title: Mineral Commodity Summaries 2026, Chromium
    id: USGS MCS 2026, February 2026
    url: https://pubs.usgs.gov/periodicals/mcs2026/mcs2026-chromium.pdf
    year: 2026
    kind: report
    license: public domain (USGS)
  pubchem-cr2o3:
    title: 'PubChem compound record, Chromium(III) oxide (CID 517277): GHS classification and experimental properties'
    id: CID 517277; CAS 1308-38-9
    url: https://pubchem.ncbi.nlm.nih.gov/rest/pug_view/data/compound/517277/JSON
    year: 2026
    kind: database
    license: public domain (NCBI)
  metco-6485-ds:
    title: Chromium Oxide - Titanium Oxide Powders (Metco 6485), Oerlikon Metco data sheet DSM-0387.1
    id: DSM-0387.1 (c) 2024 Oerlikon Metco
    url: https://www.oerlikon.com/ecoma/files/DSM-0387_Cr2O3_40TiO2.pdf?download=true
    year: 2024
    kind: datasheet
    license: manufacturer document, linked not reproduced
  jm-katalco-sds:
    title: Cr - Chromium (III) Oxide C&L Katalco 71-5, classification and labelling (IUCLID 6 export)
    id: Johnson Matthey C&L record, Grade 2 Chromium (III) Oxide, generated 11/12/2020
    url: https://matthey.com/documents/161599/166998/Cr-Chromium-III-Oxide-Katalco-715.pdf/dcfbe5f7-6cc9-a7c9-0d92-7d2339ca4759
    year: 2020
    kind: datasheet
    license: registrant document, linked not reproduced
license: CC-BY-4.0
updated: '2026-11-02'
review:
  status: draft
  method: assistant
  checked_values: 19
  unreachable: 7
  caveats: 3
  note: Written by an AI assistant on 2026-11-02 from fetched pages only; 19 benchmark values carry a source read in this run. Seven further sources were blocked or not found and are listed in gaps.
story:
  recipe:
    line: Chromia is chromium(III) oxide, Cr2O3. It is made from chromite ore by converting it through sodium dichromate, which is reduced with sulfur at high temperature. The Metco 6156 feedstock used in the Odhiambo paper is a sintered and crushed powder.
    parts:
      - name: Chromium
        symbol: Cr
        role: The oxide-forming metal. Chromium sits at the centre of the corundum framework that the oxide forms.
      - name: Oxygen
        symbol: O
        role: Fills the close-packed oxide framework with chromium in its octahedral holes, which gives a hard, brittle ceramic.
  risk_groups:
    - title: For people
      level: 0
      verdict: Classification differs between sources
      lead: Chromium(III) oxide is classed as hazardous by some sources and not by others. The sources disagree, so handle the powder as a possible sensitizer and reproductive hazard until your own safety data sheet says otherwise.
      points:
        - label: The powder
          line: PubChem's summary says the substance does not meet GHS hazard criteria for 57.8% (1112 of 1924) of all reports. The rest carry hazard statements H302, H317, H319 and H360.
        - label: Stricter registrant entry
          line: A Johnson Matthey classification for Katalco 71-5, a Grade 2 chromium(III) oxide, lists H314, H340, H350, H361 and H371 with the signal word Danger. That is a different product, but it shows how far classifications can differ.
        - label: Powder dust
          line: The Oerlikon blend data sheet advises a dry place and an original container. It does not give a dust exposure limit for chromia.
    - title: If a coating fails
      lead: These points describe the coating's own limits, not people's exposure.
      points:
        - label: Impact
          line: Chromia is brittle. Point impact or edge loading chips the coating, so the HTS page says to specify it for sliding, erosive and corrosive duty, not impact.
        - label: Thickness
          line: The HTS page says 'Thickness limited to roughly 25-30 mils on cylindrical work before residual stress risks spalling'.
        - label: Temperature
          line: The table sets the wear ceiling at 540 °C and the sealed corrosive ceiling at 177 to 205 °C.
    - title: Handling and storage
      points:
        - label: Storage
          line: Keep Metco 6485 powder in its original container in a dry location, and dry any open container in an oven before use. That data sheet is for a chromia-titania blend, not pure chromia.
        - label: Finishing
          line: Finishing needs diamond wheels, so the HTS page says to plan the cost and lead time.
  why: 'Chromium(III) oxide, Cr2O3, is a corundum-structure oxide that is sprayed onto steel to make a hard, wear-resistant layer. The HTS Coatings technical table gives a typical Vickers hardness of 900 to 1200 HV300 for plasma sprayed coatings, and the Odhiambo paper measured 1496 HV300 g and 1762 HV300 g for two coatings sprayed at different currents. The trade-off is brittleness: the same sources say chromia chips under impact and must be sealed for chemical service. The HTS page points to sliding, abrasive and corrosive duty as where it is used.'
  uses:
    - where: Printing anilox rolls
      line: The HTS page says chromia is the industry standard surface for flexographic and gravure printing rolls because it takes laser-engraved cells cleanly. It also lists mechanical seal faces and pump sleeves.
    - where: Pump and seal parts
      line: The Saint-Gobain Rokide C bulletin lists pump sleeves, pump shafts, impellers, mechanical seal faces and ball valves among the parts it coats. The rod is mostly Cr2O3 with silica and alumina, so it is not pure chromia.
    - where: Wire drawing
      line: Wire drawing capstans, draw blocks, sheaves and pulleys are also listed for the Rokide C rod. No space or flight use was found in any source we hold.
  can_do:
    - label: Wear
      line: Resist sliding and abrasive wear on steel parts. The HTS page calls chromia the hardest common oxide ceramic in thermal spray.
    - label: Finish
      line: Take a laser-engraved cell pattern for printing rolls, which is why it is used for anilox rolls.
    - label: Chemistry
      line: Hold up in alkaline or mildly acidic media when the coating is sealed, according to the HTS page.
    - label: Seal faces
      line: Work as a seal face where the part does not take impact.
  compare: Chromia is hard and brittle. Its melting point is 2435 °C as bulk crystal, and a typical plasma sprayed coating has a Vickers hardness of 900 to 1200 HV300. The HTS technical table caps wear service at 540 °C. Adding titania to chromia makes the coating less brittle, according to the Oerlikon data sheet for a chromia-titania blend. The Oerlikon sheet does not give hardness values for the blend that can be compared here, so this profile compares only pure chromia figures.
  compare_rows:
    - label: Melting point
      benchmark: melting_point
    - label: Vickers hardness (typical APS)
      benchmark: vickers_hardness
    - label: Maximum service temperature (wear)
      benchmark: max_service_temp
  handling:
    - The coating powder is not classed as hazardous by every source. The sources disagree, so read the supplier's safety data sheet for the exact product.
    - Keep the powder dry and in its original container, and dry open containers in an oven before use (Oerlikon blend data sheet).
    - Finish with diamond wheels, and plan the cost and lead time.
  good_for:
    - Wear on sliding and abrasive surfaces, such as seal faces and printing rolls. The HTS page says the coating outlasts steel in these duties.
    - Corrosion in mild acid and alkali once the coating is sealed. The HTS page quotes 'hold up through 30-day immersion across pH 0-10'.
  watch_out:
    - Brittleness. Point impact chips the coating, and the HTS page says not to specify it for impact.
    - Porosity. Standard plasma sprayed coatings run up to 5% porosity, and the Odhiambo paper ties higher porosity to lower bond strength.
    - Temperature. The wear ceiling in the table is 540 °C and the sealed corrosive ceiling is 177 to 205 °C.
  verdict: Chromia is the hard, brittle choice for sliding wear and seal faces, and it works only if impact is kept out and the coating is sealed for chemistry.