Sol-gel coatings, a safe alternative to chrome plating

Since the beginning of the 20th century, hexavalent chromium derivates have been used to prevent corrosion and to improve the adherence of further coatings on zinc, magnesium, steel and aluminum alloys. Chromium (VI), or hexavalent chromium, is one of the most stable oxidation states of chromium. It very seldom occurs in nature and most hexavalent chromium compounds are manufactured products. Chromates are salts that contain hexavalent chromium and have the ability to provide excellent corrosion resistance even when the surface is scratched or damaged. They dilute into the corrosive media and migrate to the bare metal surface, forming compounds that avoid corrosion. The chrome plating process takes place usually in acid media; solutions contain chromic acid, chromates or dichromates, with a pH value around 2. On the metal surface some anodic and cathodic areas initially appear, where dissolution of the metal and hydrogen formation respectively occurs. Then, the hexavalent chromium Cr6+, a strong oxidizing agent, it is reduced to trivalent chromium Cr3+. In the cathodic areas where H2 is formed, the pH increases and favors the chromate precipitation on the surface of the metal.

However, in spite of their excellent anticorrosion properties, the production, transportation, stocking, and application of hexavalent chromium are considered very high risk activities. It has been proved with «no need for further information and/or testing» [1] that these compounds are extremely dangerous for the environment and the ecosystems, and mortal diseases such as lung cancer or liver and kidney failure are likely to be caused by its exposure [2]. Consequently, the use of hexavalent chromium, (alongside with the use of lead, mercury and cadmium) was banned in 2006 by the European Union in the automotive and electronic industries and the development of safer alternatives is one of the main objectives of the industry.

Smart coatings are new and safe alternative to chrome plating. Smart coatings are those able to respond to the environmental stimuli [3], and they are designed to fulfill specific requirements of both conventional and hi-tech demands. In the recent times, surface functionalization is one of the most developing fields, and includes a wide variety of techniques such as Layer-by-layer assembly or ceramic coatings obtained by sol-gel process. A glass can be defined as an amorphous compound obtained by the melted combination of siliceous and alkali carbonates. However, since the last century, new synthetic routes have been developed to allow the preparation of glassy materials without melting through the sol-gel method.

Sol-gel coatings have important advantages over conventional processes [4]:

  • The temperature at which the sol–gel occurs is generally low, close to room temperature.Thus thermal volatilization and degradation of entrapped species, such as organic inhibitors, is minimized.
  • Sol-gel allows to cast coatings in complex shapes, since liquid precursors are used. It also allows to produce thin films without the need for machining or melting.
  • The sol–gel is considered a “green” coating technology: It uses compounds that do not introduce impurities into the end product as initial substances; this method is waste-free and excludes the stage of washing.

 

chrome plating

Corrosion ends to appear in chrome plated steel.

Image: Coyau / Wikimedia Commons, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=21520228

[1] European Chemicals Bureau, European Union Risk Assessment Report CAS-No.: 1333-82-0, 7775-11-3, 10588-01-9, 7789-09-5 and 7778-50-9 EINECS-No.: 215-607-8, 231-889-5, 234-190-3,232-143-1 and 231-906-6. 2005, European Communities: Luxembourg.

[2] European Chemicals Agency, Services to support the assessment of remaining cancer risks related to the use of chromium- and arsenic-containing substances in Applications for Authorization, Final report for Hexavalent Chromium. 2013, Consortium ETeSS: Helsinki.

[3] Makhlouf, A.S.H., Handbook of smart coatings for materials protection. 2014: Elsevier.

[4] Wright, J.D. and N.A. Sommerdijk, Sol-gel materials: chemistry and applications. Vol. 4. 2000: CRC press.

© Nadetech Innovations 2017

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