What Is Anodizing: The Complete Guide

Probably, you want to learn more about metal anodizing. Well, in this guide, you will find all information about anodizing process. If you want to be an expert in anodizing process read this guide.

What Is Anodizing?

Anodizing is an electrochemical procedure in which a metal surface is transformed into a durable, corrosion-resistant, anodic oxide and decorative finish through electrolytic passivation. It increases the natural oxide layer’s thickness on the metal parts’ surface. Metals that can be anodized are magnesium and titanium, aluminum, zinc and tantalum.

History Of Anodizing

In 1993, the industry first used anodizing to safeguard duralumin seaplane parts from rusting. It was catalogued in British defence specifications and was known as the Bengough-Stuart process. The process advanced; Gower and O’Brien branded the first sulphuric acid anodizing process in 1927. This, therefore, became the primary common anodizing electrolyte. In 1993 in Japan, Oxalic acid anodizing was first branded, then used in Germany later for architectural uses. Again, in the 1970s and 1960s, anodizing of aluminium became known but has been replaced with powder coating and plastics which are way cheaper.

History Of Anodizing
Anodizing Type 1 (Using Chromic Acid)

Anodizing Type 1 (Using Chromic Acid)

It is an electrochemical process that promptly regulates the oxidation of an aluminium surface and forms a thin aluminium oxide film. The end aluminium oxide film is an electrically resistant material. Compared to Type II and Type III anodizing, Type I generates a thinner film and manufactures increased drawing and forming characteristics. For aluminium with more minor alloying elements, this is a suitable process. It is preferably used for flight-critical aluminium elements such as landing gear subject to high stresses.

Anodizing Type II (Using Sulphuric Acid)

Anodizing Type II is an electrolytic process in which electricity and a sulphuric acid electrolytic solution impact oxidation of the aluminium surface to create a thin film of aluminium oxide. The process is metallurgically bonded by converting the aluminium surface to aluminium oxide. Additionally, the layer produced by the process is porous and thus is sealed to enclose pores to minimize additional attacks on the aluminium. Next, the element is dipped in near-boiling de-mineralized water to seal the component. The size of the aluminium component grows as the anodized layer forms both on the top surface and penetration.

Anodizing Type II (Using Sulphuric Acid)
Anodizing Type III (Hard Coat)

Anodizing Type III (Hard Coat)

Anodizing Type III is an electrochemical process that converts the surface of a plain metal into appealing outer that is durable and corrosion-resistant. The oxide layer is thick and stiff, thus suitable for engineering applications. Majorly applied in military devices, cookware and electrical equipment. The thickness of the hard coat differs depending on the type of alloy to be anodized. Ranges from 0.0005 to 0.0030″.

Anodizing In Organic Acid

The electrolytic passivation process transforms the surface of the metal by maximizing the thickness of the natural oxide layer using organic acid. The process can result in a yellowish integral colour finish with no die if it is done in weak acids with high current densities, high voltages and solid refrigeration.  Anodizing in some organic acids can result in a loose situation, where the current leads the acid to damage the metal more than normal impacting scarring and large holes.  If the voltage or current is passed too high, flaming can start.

Anodizing In Organic Acid
Anodizing Phosphoric Acid

Anodizing Phosphoric Acid

It is the pre-treatment for surface adhesive bond priming and is primarily applied in the aerospace sector. The thickness of the anode ranges from 0.1µm to 5.0µm and looks like a blurred sparkling film on the metal surface. Remember, the anodized finish is unsuitable for decoration and its durability is nominal.

Common Thickness In Anodizing

Depending on the anodized layer’s thickness, the metal will be more or less resistant. Instead of the standard thickness of 5-10 µm for decorative pieces, it can be maximized to 50 µm for the parts to be resistant. Uses of the anodized metal depend on the oxide layer’s thickness which include:

25-29  microns; Class 25 – Used in marine and industrial conditions.

25-30  15-19 microns; Class 15 – This is a standard class. Used for outdoor only.

5-9  micron and 10-14 micron – Belongs to Class 5-10 and applied for indoor use only.

5-10  20-24 microns; Class 20 – Used in aggressive, industrial and marine conditions

Common Thickness In Anodizing
Metal Anodizing Quality Standards

Metal Anodizing Quality Standards

The following are the metal anodizing quality standards:

MIL-A-8625; it characterizes the three types of anodizing; Type I, Type II and Type III

MIL-A-63576; it defines the need for an electrochemical process for forming an anodic coating on metals.

Applications Of Anodizing

The following are the applications of anodizing:

  • Jewellery and artwork; anodized products such as titanium create wedding rings and other jewellery.
  • Residential and commercial building applications; anodized products such as aluminium are used in architecture to provide durable, decorative, less-maintenance ceilings, floors, escalators, lobbies, staircases and roofs.
  • Motor vehicle components; anodizing components are used to manufacture motor vehicle components because they harm the air and the environment less.
  • Home appliances
  • Architectural and structural applications
  • Food preparation equipment
  • Sporting boats and goods
Applications Of Anodizing
Advantages Of Anodizing

Advantages Of Anodizing

Some of the main advantages include:

  • Durability; the anodized finish does not flake, chip or peel for a long time.
  • Ease of maintenance; cleaning is easy as it can be done using water and mild detergent to restore its original appearance.
  • Colour stability; the sealing process in anodizing helps maintain the finish’s new look.
  • Health and safety; the process is not harmful to human health as the finish is stable chemically, on-toxic, does not decompose and is heat-resistant.
  • Aesthetics; it provides the maximum number of gloss and colour choices and reduces colour variations.
  • Cost; maintenance and initial cost are lower and hence valuable in the long run.

Disadvantages Of Anodizing

Some of the main disadvantages of anodizing process are:

  • As compared to higher-level painted products, anodizing does not give warranties.
  • Colour selection is limited.
  • A retouch of an anodized finish that is highly mutilated either in the field or shop is more observable than air dry touch-up paint for a painted finish.
Disadvantages Of Anodizing

How Anodizing Process Works

How Anodizing Process Works
How Anodizing Process Works

The following is a step-by-step anodizing process;

Cleaning

Acid or alkaline is used to remove dirt, grease and oil on the surface of the metal to be anodized.

Pre-treatment

The metal is pre-treated through marinating and cleansing to give it a visible finish. Pre-treatment is done in two ways; brightening and etching.

Etching: hot sodium hydroxide solutions are used to remove a thin layer of the metal to form an attractive matte finish.

Brightening: the surface of the metal is smoothened to form the desired finish by a mixture of nitric and phosphoric acids

Anodic Oxide

The metal is submerged in an acid electrolyte bath, and an electric current is passed through. An aluminium serving as an electrode is set up inside the anodizing tank; hence oxygen ions are freed from the electrolyte to mix with the aluminium particles on the anodized surface. The result is now ready for the next step of colouration.

Colour Anodizing

Colouring can be done using four methods: integral, interference, organic dyeing and electrolytic colouring. Electrolytic is the popular method used for colouring the anodized metal.

Electrolytic Colouring

The anodized metal is dipped in an inorganic metal salt bath. As the metal salts oxides in the pores of the metal, an electric current is passed through. The colour of the anodized metal will vary according to the time taken to dip and the chemical state of the bath. Majorly anodized colour finishes are stainless, brown, bronze, gold, and black.

Sealing

Sealing is the final step of anodization. It involves closing the pores of the metal anodized to prevent water leakage and corrosion. It also prevents staining and scratching of the metal surface. Sealing can be done in three ways; a hot method, a cold method and a combination of hot and cold forms.

Anodizing Vs Electroplating

Anodizing
Anodizing

Anodizing is an electrochemical process in which a metal undergoes electrolytic passivation to increase the thickness of the natural oxide layer. Normally, the metal being anodized in the process acts as the anode. Remember, the metal surface is transformed into a durable, corrosion-resistant, anodic oxide and decorative finish. The materials that can be anodized are; aluminium and titanium. Magnesium, niobium, tantalum and zinc.

Electroplating
Electroplating

Electroplating is an analytical and industrial process that uses electrical energy to coat one metal on another. An electrochemical cell holding two anodes is dipped in the same electrolyte. The metal to be electroplated is the one used as the cathode. Remember, the anode can either be an inert electrode or metal to be applied to the cathode.

Electroplating is used chiefly for the prevention of corrosion and decoration applications.

Materials You Can Anodize

Anodizing Aluminium
Anodizing Aluminium

Aluminium is dipped into the acid electrolyte bath, and an electric current is passed through the form.  An aluminium serving as an electrode is set up inside the anodizing tank; hence oxygen ions are freed from the electrolyte to mix with the aluminium particles on the surface is anodized.

Anodizing Titanium
Anodizing Titanium

The titanium part acting as a positive conductor is dipped in a molten electrolyte solution; trisodium phosphate. Water molecules experience hydrolysis when an electrical current is spread and is separated into oxygen and hydrogen. A thin layer of titanium oxide is formed by forcing the oxygen to the titanium surface using electrical power.

Anodizing Magnesium
Anodizing Magnesium

The electrolytic process manipulates the magnesium surface to a hydroxide film with decorative, protective and functional characteristics. The process uses an alkali-rich molten bath and a direct current. The direct current is either turned off or partially altered to create the compound on the magnesium surface.

Anodizing Niobium
Anodizing Niobium

The process involves dipping the niobium metal into an electrolytic bath and passing an electric current forming an oxide layer on the surface of the metal. The surface of the metal and the oxide layer is light strike, thus leading to two light rays refracted, which strengthen each other and, depending on the oxide layer’s thickness, produce different colours.

Anodizing Tantalum
Anodizing Tantalum

Tantalum is anodized the same way as niobium and titanium. The tantalum part acting as a positive conductor is dipped in a molten electrolyte solution; sodium citrate. Water molecules experience hydrolysis when an electrical current is spread and is separated into oxygen and hydrogen.

What Is Anodizing

People Also Ask:

What Is The Purpose Of Anodizing?

Anodizing metals’ purpose is to make them durable and corrosion-resistance, increase aesthetic features and counter-scratching the surface.

Will Anodized Aluminium Rust?

The anodized aluminium will not rust because a protective film is created on the surface, increasing corrosion resistance.

Can You Remove Anodizing?

Anodizing can be removed from the surface of the metal with safety provisions and genuine materials, chemicals and elbow grease.

Which Is Better; Between Anodizing And Powder Coating?

Anodizing is better than powder coating as it gives the metal an appealing look, tight dimensional margins and corrosion-resistant characteristics.

What Is The Difference Between Anodizing And Galvanizing?

Galvanizing prevents metal from rusting by coating the surface with zinc, while anodizing transforms the metal surface into a durable, corrosion-resistant, attractive and anodic oxide finish.

Is There Recommended Thickness For Anodizing?

The recommended thickness range for anodizing is 40 to 60µm; beyond approximately 50µm, the wear resistance of anodized metal reduces relatively.

How Do You Check Quality Of Anodized Parts?

Quality is evaluated by the testing level of resistance to corrosion and abrasion.

The quality is unsatisfactory if tested with alcohol, and the metal surface bleeds.

What Is Anodizing Barrier Layer?

It refers to pores formed on the aluminium surface due to the escape of extra positive ions, which geometrically form patterns and erode the layer.

How Do You Add Color During Metal Anodizing?

Colouring during metal anodizing is done in three methods which are:

Electrolytic Colouring

This process involves fixing metal oxides in the aluminium layer using alternating current. At the bottom of the pores of the coating, metal ions are deposited. Depending on the dispensation of the metal, the end tone and brightness will be achieved according to the metallic salt used.

Anodic Colouring

Uses direct current for colouration. The colour will be solid and result in bronze and dark grey from bronze and light grey

Combined Colouring

The electrolytically coloured oxide layers can be re-coloured, preserving the metallic impact with inorganic or organic dyes. The colour range that this process can attain is considerably increased.

More Resources:

What Is Anodizing

Anodizing Process

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