Anodizing uses an electric current to develop a thick, hard, corrosive-free layer on metals such as aluminum by oxidizing. It gives a structure firmly fixed to the metal, which does not chip or peel off in any conditions. As compared to normal paint, for example, or even plating, anodized aluminum works well due to its highly ordered porous nature. Moreover, it’s more flexible than conventional concrete and enables the application of secondary processes such as coloring and sealing.
Following anodizing, Micro Arc Oxidation (MAO) is an advanced layer-forming technique. Originally, the USSR applied plasma material to protect the Mir space station; PEO is considerably better at that. It forms relatively hard-wearing layers, excellent electrical insulation, and improved resistance to corrosion. In addition, MAO is more long-lasting than standard anodizing procedures and increases material endurance in severe conditions.
Get more about micro arc oxidation vs anodizing in this guide.
What Is Micro Arc Oxidation(MAO) or Plasma Electrolytic Oxidation (PEO)?
Micro Arc Oxidation(MAO)
Plasma Electrolytic Oxidation or micro arc oxidation is used to develop ceramic coatings. Design manufacturers also introduce micro-arc oxidation (MAO) and micro-arc oxidation (MAO) coating.
High valve, light metals, and their alloys are the key beneficiaries of the concept. It acts to form a barrier layer, typically a porous layer on the substrate surface.
The intended layer can be designed to accommodate several different reagents for further modification. The end product is a coating that has improved performance. PEO coatings are more hard as compared to anodizing or steel. They give 2 to 4 times better hardness, and protection.
The PEO process employs an electrolyte dilute-type solution specifically designed for the process. Some additional reagents are added when more advanced features are required. A voltage of 200V or more is applied. It causes surface plasma discharges of high voltage.
These discharges create the appropriate conditions for crystallization. Plasma raises the temperature and pressure; required for the oxidation process. This leads to the formation of an oxide crystal layer which is a metallographic feature. The plasma-based process improves the coating performance to a very large extent.
Key Benefits of Micro Arc Oxidation Coatings
MAO Parts
When comparing micro arc oxidation vs anodizing, the PEO process has several benefits over traditional anodizing techniques. Plasma is used to grow more rigid ceramic phases and crystallization. Moreover, it gives chemical passivity, and reactivity to the coating.
Most PEO ceramics are chemical passive materials. The structure permits the penetration of different elements. It makes the coating to be tailor-made to suit a given requirement. Low stiffness provides good bonding when subjected to force.
PEO coatings avoid crack formation at the edge. Furthermore, these coatings give a better and more long-lasting substratum.
PEO is applied to metals such as aluminum, titanium, and magnesium. It is also efficiently utilized on valve metals including zirconium. Optimally, it gives wear resistance, corrosion protection, and high thermal stability.
Three Process Stages of Micro Arc Oxidation Coating
Micro Arc Oxidation Coating
PEO involves three process steps: The primary surface treatments are as follows: Substrate Oxidation, Plasma Modification, and incorporation of Electrolytes.
Stage 1: Substrate Oxidation
In this stage, the component is immersed in a solution known as an electrolyte. The solution is often a weak, chrome-free aqueous liquor. The applied voltage is high, in the range of 200 volts. This produces discharges that alter the substrate polarity. The discharges form growth formations according to the requirements of the coatings.
Stage 2: Plasma Modification
Plasma Electrolytic Oxidation of Magnesium
The electrical discharges then vaporize and heat the surface of the metallic alloy. It results in the generation of plasma on the surface of the component. The plasma offers high temperature and pressure. These conditions develop a crystalline oxide layer. The oxide may contain corundum, periclase rutile, and many other elements. In this stage, no crack formation happens making it ideal than the conventional anodizing process.
Stage 3: Incorporation of Electrolyte Element
In this case, the plasma creates pores in the oxide layer. These pores let elements from the electrolyte seep in. This in turn assists in the control of the desired characteristics of the coating. Special reagents are included to improve functionality and improve properties such as corrosion and heat transfer coefficients.
Micro Arc Oxidation Vs Anodizing: Why MAO is More Beneficial than Traditional Anodizing?
Micro arc oxidation vs. anodizing
MAO does not necessitate as elaborate a piece of equipment as in other methodologies. Moreover, it does not require a high-temperature furnace, plasma guns, and vacuuming. The preparation or pretreatment step is easy, sometimes it only involves the removal of greases from the surface. Conventional anodizing involves a large number of steps needed in the surface preparation process.
Factors Affecting the Quality of MAO coating
Unlike other surface treatment techniques, the micro arc oxidation aluminum or another metal process can be easily regulated, and the parameters to control coating characteristics can be changed. So, it warrants the applied coatings have set functional desired features.
1. Voltage & Current Density
The coating thickness formed can be varied by controlling the received voltage and current density. Higher values increase the rate of growth and excessive sparking. Arc initiation is affected by either low voltage or low current density.
2. Frequency
Higher frequencies also give better, more uniform layer thickness. At lower frequencies, the coatings produced are rough and comprise a coarser structure and poor uniformity.
3. Duty Cycle
“Duty cycle” is a time measure during which the current is on. A higher duty cycle is associated with a higher level of disturbance produced during the process which influences the coating characteristics.
4. Electrolyte Composition
The type of electrolyte used affects the coating formation. Through silicates, thickness, and coating hardness can be increased. Besides, through phosphates, corrosion can be reduced. Some other additives, minerals, and salts may alter the coating performance further.
5. Additives
Pigment, particles, and other incorporated substances improve some coating aspects in question. For instance, pigments act as coloring agents and particles improve the abrasion property of the coating.
6. pH
The deposition dynamics of the coating also depend on the bath pH, whether acidic or alkaline; pH alters the microstructure of the resulting bath.
What Is Anodizing?
Anodizing
Anodizing is an electrochemical procedure that forms an anodic oxide coating on a metallic component. It contributes to the growth of the natural oxide thickness. It improves corrosion and wear properties and the appearance of parts.
The process is carried out by submerging the metal piece(Job Material) in an electrolyte bath. A direct electric current is then run through the bath to deposit a thin oxide layer on the metal’s surface.
Anodizing is most frequently used on aluminum alloy. (Read more about anodized aluminum). However, you can also use it for titanium and magnesium metals. Particularly, it is useful in production services that require a product to be durable, and pleasing appearance.
Types of Anodizing
Types of Anodizing
- Type I – Chromic Acid Anodizing
- Type II – Sulfuric Acid Anodizing
- Type III – Hardcoat Anodizing
Fundamental Steps in the Anodizing Procedure
Knowledge of each phase allows the anodizing process to meet certain project requirements in terms of both function and aesthetics. Here are the most common steps involved in the anodizing process;
1. Cleaning and Pre-Treatment
Pre-Treatment Anodic Cleaning
The preliminary cleaning process prepares the metal surface. It helps wash out impurities, oils as well as contaminants. You can use detergents, and alkaline solutions, for cleaning the metal.
2. Anodizing
Upon cleaning, the metal part is immersed in an electrolyte solution. The electrolytic bath contains sulfuric or chromic acid. An electric current is passed through it to oxidize it. The metal’s surface reacts to form a passive aluminum oxide layer that remains adherent to it. The oxide thickness and properties vary with parameters such as time, temperature, and current.
3. Coloring (Optional)
Different Anodizing Colors
For aesthetic purposes, anodized parts can be treated through a process of dip coloring. These parts are put in dye solutions. It enters the porous oxide layer’s surface and provides a variety of colors.
4. Sealing
The sealing process measures the corrosion integrity of metal parts. It also stops the pores of the oxide layer. There are different sealing methods:
- Hot DI Sealing: Components are soaked in deionized water to a temperature of 96-100°C. This helps to hydrate and swell the oxide layer thus sealing it.
- Mid-Temperature Sealing: The activity of nickel acetate solutions is at the level of 60-80°C. It responds to the need for balance between performance and energy, providing reasonable anticorrosive protection.
- Cold-Temperature Sealing: In low-temperature solutions, at 20-30°C, parts are sealed with cobalt salts. The method is economical in terms of energy and suitable for temperature-sensitive components.
5. Rinsing and Drying
The parts are then rinsed with water after the sealant has been set to eliminate any chemical deposits on them. The last step is critical since it makes sure there’s no residue left behind. Finally, the parts are then dried to complete the anodizing process Part.
Features and Anodizing Variables
Anodizing relies on four primary parameters:
- Electrolytic Concentration
- Cell Temperature in the cell
- Current density, and
- Time of Electrolysis.
1. Electrolyte Composition
The acids present in the electrolyte influence the oxide layer. Sulfuric acid is used for general anodizing while chromic acid is suited for thin coatings providing unique performances.
2. Cell Temperature
Anodizing speed is affected by the temperature of the electrolyte bath. Commonly, it can be observed the hardness of any coating subject to be applied reduces with an increase in temperature. Explorations also have postulated that high temperatures increase layer porosity.
3. Current Density
The present density provides information on the rate of formation of the coating and evenness. The rates are increased at higher densities but surface smoothness is compromised.
4. Time
There is a straight correlation between anodizing time and coating thickness. Whereas the processes involve treating the workpiece with the anodizing bath for a short period, high anodizing periods generate thicker oxide layers, and intensify the wear barrier’s necessities.
Plasma Electrolytic Oxidation (PEO) vs. Hard Anodizing
Hard Coat Aluminum Anodizing
Anodizing of metals is an improvement on regular metal wear, but it has its drawbacks. Anodic oxide layers can spill when exposed to high temperatures. This sort of vulnerability typically has an impact on performance in challenging circumstances. This is well provided for by PEO as it has the right microstructure for the situation. The non-uniform layering on the coating serves to minimize the formation of cracks and increase thermal stability. PEO has high hardness, and strong adhesion, and can be made to conform to a particular surface.
Comparing micro arc oxidation vs anodizing, no doubt that anodizing is inferior in terms of corrosion resistance. Its ceramic matrix characterizes the material as chemically inactive. PEO coatings may sustain anything that is beyond 2,000 hrs in the salt spray test. On the other hand, anodizing lasts for only 200 hours. Of the several properties that may be attributed to PEO, corrosion resistance is most effective for magnesium alloys.
As acknowledged by the regulators, anodizing is safe; however, it produces toxic waste. It forms aluminum hydroxide and uses up degenerated sulfuric acid. These byproducts generate dangerous emissions and are typically challenging to treat. On the other hand, PEO is eco-compatible. It incorporates 99% distilled water and produces no hazardous waste.
Moreover, PEO does not have an affinity for organic compounds, VOCs, strong acids, and heavy metals. The remaining byproducts are as harmless as the water that is washed away by the washing machine. Thus, PEO is safer to meet all the environmental regulations than the anodizing treatments.
Conclusion – Micro Arc Oxidation Vs Anodizing: Which One To Choose?
To sum up, the above analysis has shown that micro-arc oxidation vs anodizing; both processes provide significant benefits. Anodizing is a well-tried method of corrosion protection and provides a variety of surface treatments, especially for Al. But Plasma Arc Oxidation, with the complexity of the process, has much better wear and heat resistance, as well as durability in more extreme conditions for high-performance uses.
In general, anodizing is used more frequently and is cheaper for the typical requirements, however, in extreme conditions, PAO surpasses all to achieve exceptional corrosion protection for such materials as magnesium or titanium. So, deciding, which one to use depends on the certain project needs, the material type to be used, and what performance levels you are expecting.
