Plasma electrolytic oxidation (PEO) is the same with anodizing in principle, in which a valve metal component is immersed in an electrolyte, and electrical potential is applied to convert the surface metal into an oxide layer. However, PEO employs different compositions of electrolyte and electric regime from conventional anodizing, so the ceramic coating generated on metal presents superior mechanical, chemical, thermal and electrical properties to other commercial coatings.

Generally, PEO is applied to valve metals including aluminum, magnesium, titanium, zirconium and its alloys etc. During the PEO process, surface micro-discharges occur continuously, and it results in growing various crystalline oxides converted from substrate metal and/or incorporated with chemical elements stemmed from electrolyte, which is dense or functional ceramic layer.

Since PEO were invented in Russia about four decades ago, several organizations in the world such as institutes, universities and businesses have carried out its research and development in their way. Although the PEO recipes they have are different a little from each other, the resulting oxides have the similar characteristics in high hardness, excellent wear/abrasion resistance and corrosion resistance, and the process is environmentally friendly without hazardous and toxic chemical elements in electrolyte and coating layer.

Naain also has conducted intensive R&D independently, and secured our own creative and proven processes of PEO. We have been trying to apply PEO to the military and commercial applications, and can get vast industrial experience in this field. We have given the most economical and feasible solution to customers, and accomplished lots of successful applications.

The PEO coating layer consists of amorphous alumina, boehmite (-Al2O3), and corundum (α-Al2O3). Highly durable coating is due to higher portion of corundum which is much harder, while conventional anodizing layer is mostly boehmite crystalline as shown in figures. The ratio of corundum and amorphous alumina (or boehmite) can be controlled, and also the amorphous layer can be optimized in compositions and morphology to give a specific function depending on a coating purpose. Comparison of a conventional anodizing and PEO is as follows.

Items Anodizing Plasma Electrolytic Oxidation
Cross Section of Coating Layer & Characteristics of surface crack
Electrolyte Acid Solution
(Sulfuric acid, Phosphoric acid etc.)
Alkaline Solution
(KOH etc.)
Electrical Regime DC (or AC etc.)
Current Density 0.5 – 10A/dm2
Potential: < 200VDC
AC (or DC etc.)
Current Density 5 – 20A/dm2
Potential (max) 200 – 700VAC
Necessary Facility Waste Treatment facility Chiller for electrolyte
  • Hard to change composition and nano-structure of anodizing layer
  • Limited application to cast aluminum alloy with high silicon
  • Various Colors: 2nd or 3rd step processes
  • Coating thickness(max.) ∼50µm
  • Easy to change composition & nano-structure of PEO layer (Naain’s know-how)
  • Limited application to aluminum alloys with Pb(lead) and Sn(tin)
  • Intrinsic Colors (Black, Gray, Ivory and White) : Single Step
  • Coating Thickness(max) ∼300µm
As already well known, PEO coatings are recognized for high hardness, wear resistance, and corrosion resistance, although the properties are strongly dependent on alloy compositions of substrate metal as well as electrolyte chemicals and electrical conditions, as shown in tables.

Alloy Substrates Microhardness, Hv 100g
Aluminum Alloys Al-Cu-Mg: 2024, 2014, 2618(Ni), 2017 1400 ~ 2100
Al-Mg-Si: 6061, 6063 1300 ~ 1800
Al-Mg, Al-Mg-Zn: 5052, 5083, 3003 1200 ~ 1500
Al-Zn-Mg: 7075, 7175, 7050 1200 ~ 1600
AlSiMg, AlSiCuFe, AlSiCuMgNi: AC4A, AC4B, AC4C(A356.0), AC8B, ADC12(A384) etc 600 ~ 1100
Mg Alloy AZ91D, AZ31B 350 ~ 800
Ti and its Alloy Ti-6Al-4V, Grade 4 & 5 400 ~ 600
The properties in this content are just general values for the coatings conducted under typical conditions of PEO process. The characteristics of PEO coating depend strongly on various factors such as chemical composition of substrate alloy, electrolyte composition, electrical regime, design of counter-electrode, shape of metal component to be coated etc. Therefore, according to customers’ needs, the optimized PEO coating layers in durability and functionality can be generated.