Conversion coating explained

A conversion coating is a chemical or electro-chemical treatment applied to manufactured parts that superficially converts the material into a thin adhering coating of an insoluble compound. These coatings are commonly applied to protect the part against corrosion, to improve the adherence of other coatings, for lubrication, or for aesthetic purposes.

Types

The most common conversion coating processes for metal parts with industrial use include

• Chromate (aluminum, steel)
• Phosphate (steel)
• Bluing (steel)
• Black oxide (steel)
• Anodizing (aluminum)
• Stannate (magnesium)
• Molybdate (zinc, zinc-nickel)
• Zirconate (steel, aluminum, magnesium, galvanized steel).
• Titanate (steel, aluminum, magnesium).
• Plasma electrolysis (aluminum, magnesium, titanium)

Non-metallic substrates

Conversion coatings have been studied for non-metallic substrates, such as for protection of fluorozirconate glasses used in optoelectronics.

Regulations

US military specifications for conversion coatings include MIL-DTL-5541, MIL-DTL-81706, and MIL-DTL-5574, all dealing with aluminum.

See also

• Rust converter  - tannate conversion coating

References

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Notes and References

1. A. A. O. Magalhães, I. C. P. Margarita, O. R. Mattos (2004): "Molybdate conversion coatings on zinc surfaces". Journal of Electroanalytical Chemistry, volume 572, issue 2, pages 433-440.
2. M. A. Gonzalez-Nunez, C. A. Nunez-Lopez, P. Skeldon, G. E. Thompson, H. Karimzadeh, P. Lyon, and T.E.Wilks (1995): "A non-chromate conversion coating for magnesium alloys and magnesium-based metal matrix composites". Corrosion Science, volume 37, issue 11, pages 1763-1772.
3. "http://www.rit.edu/affiliate/nysp2i/sites/rit.edu.affiliate.nysp2i/files/pdfs/plating_or_conversion_coatings_final2e.pdf"
4. "http://core.materials.ac.uk/repository/eaa/talat/5202.pdf"
5. "Fastener Surface Conversion", In: FastenerData.co.uk
6. A. P. Rizzato, C. V. Santilli, S. H. Pulcinelli, Y. Messaddeq, and P. Hammer (2004): "XPS Study of the Corrosion Protection of Fluorozirconate Glasses Dip-Coated with SnO2 Transparent Thin Films". Journal of Sol-Gel Science and Technology, volume 32, pages 155–160.
7. I. Milošev and G. S. Frankel (2018): "Review—Conversion Coatings Based on Zirconium and/or Titanium". Journal of the Electrochemical Society, volume 165, issue 3,pages C127-C144.
8. Lishi Wang, Tao Feng, Shanwen Yu, Yihang Cheng, Zhixiang Bu and Xinbin Hu (2020): "Fluoride effect on plasma electrolytic oxidation coating formed on Mg-Al alloy in alkaline electrolytes". Materials Research Express, volume 7, issue 1, article 016412.
9. Raoling Wang (2018): "Chromium-free aluminum conversion coating agent, aluminum material, and surface conversion coating treatment method". Patent WO2018006270, filed on 2016-07-05, granted on 2018-01-11.
10. Charles E. Tomlinson (1998): "Chromate-free conversion coatings for metals". US Patent 5759244, filed on 1996-10-09, granted on 1998-06-02, expired on 2016-10-09.
11. Liu Feng, Shan Dayong, Song Yingwei, Liu Xianbin, and Han Enhou (2012): "Highly corrosion-resistant ceramic coating solution prepared through magnesium alloy surface micro-arc oxidation and application thereof". Chinese patent 101994145, filed on 2009-08-19, granted on 2012-05-23.
12. J. A. Wharton, G. D. Wilcox, and K. R Baldwin (1996): "Non-Chromate Conversion Coating Treatments For Electrodeposited Zinc-Nickel Alloys". Transactions of the IMF - The International Journal of Surface Engineering and Coatings, volume 74, issue 6, pages 210-213.