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The environmental impact of hard anodising

Anodising coating methods have been in use for a century to enable the use of light metals in extreme environments. But as designers are looking to become green and manufacture sustainably, the environmental impact of anodising has come into concern.

What is the hard anodising process?

Anodising is a surface coating technology that electrochemically produces a protective outer oxide layer for light metals like aluminium, magnesium and titanium. This oxide layer provides the metal with corrosion-resistance, durability and a porous surface that allows the use of various sealants for further protective features. The anodising process involves submerging the metal in an electrically conducting solution, typically sulphuric acid and chromic acid, and introducing an electric current. This causes a chemical reaction with oxygen and the metal’s surface to form the outer oxide layer.

Hard anodising is an alternative method of anodising that creates a thicker oxide layer due to changes in the coating process. By lowering the temperature of the submersion fluid and increasing the voltage applied, the anodised coating becomes thicker and more abrasion resistant. As a result, hard anodising provides greater features of abrasion and thermal resistance, but loses the porosity of standard anodising.

The two forms of anodising suit different applications depending on their need for a decorative finish against extreme resistance;

Hard anodising applications

  • Semiconductor
  • Aerospace
  • Automotive
  • Industrial
  • Oil & gas
  • Medical
  • Defence
  • Sports equipment

What is the environmental impact of hard anodising?

Anodising and hard anodising pose an environmental risk due to the resultant byproducts; degraded sulphuric acid and aluminium hydroxide. These each pose different risks and challenges and are extremely hazardous if not disposed of properly. Regulatory bodies such as the EPA have introduced regulations on the disposal of this waste to prevent environmental and health risks. An example is the fact that chromic acid anodising has been a historic choice for aerospace applications, but is now outlawed under European REACH regulations.

Degraded sulphuric acid poses significant environmental damage if it is dumped into a wastewater system. It dissolves when mixed with water, making it difficult to treat and separate. It can then cause short and long-term damage to plants, aquatic life and animals as it is highly corrosive. Because of this, dedicated wastewater treatment processes need to be incorporated, such as electrodialysis, that can be both expensive and resource consuming.

PEO: a cleaner and greener coating method

The widespread use of hard anodising makes the potential environmental harm of its waste products an inevitable problem for the future. As industries move towards greener and cleaner processes, alternative coating methods are required that avoid sacrificing the crucial protective properties of hard anodising.

Plasma electrolytic oxidation (PEO) is an evolution of anodising that provides better results with reduced risk to the environment. It involves the formation of a strong outer layer for light metals through plasma discharges in an electrolyte solution. The resultant surface is 2-4x the strength of anodised surfaces, creates a consistent coating for complex geometries, and provides all the material properties of anodising and more.

PEO has no organic compounds, VOCs, strong acids, or heavy metals used during the process. It uses 99% distilled water and creates no toxic waste products. In fact, the leftover byproducts are no more hazardous than run-off water from a domestic washing machine. This places PEO as a surface coating technology for the future that improves upon the features of anodising at zero cost to the environment.

Get in touch with the Keronite engineers today, to see if Keronite can deliver a custom PEO-induced ceramic coating that helps your components excel.

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About the author

Robin Francis

About the author

Robin Francis

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Bio
Dr. Robin Francis is a material science specialist with a M.A. and PH.D in inorganic chemistry from Oxford University. He is the Chief Technology Officer at Keronite, world leading developer of advanced surface treatments for engineering metals. Robin is a recognised expert in Plasma Electrolytic Oxidation technology, offering his insight and contribution in various engineering journals. Examples of these journals include, “thermal swing coatings for future sustainable heavy-duty IC engines” and, “the incorporation of particles suspended in the electrolyte into plasma electrolytic oxidation coatings on Ti and Al substrates.”