{"id":82171,"date":"2021-06-03T00:00:00","date_gmt":"2021-06-03T00:00:00","guid":{"rendered":"https:\/\/www.corrosionpedia.com\/2021\/06\/03\/anodize-this-the-brilliance-of-anodizing"},"modified":"2021-06-01T22:41:12","modified_gmt":"2023-12-09T18:57:59","slug":"anodize-this-the-brilliance-of-anodizing","status":"publish","type":"post","link":"https:\/\/www.corrosionpedia.com\/anodize-this-the-brilliance-of-anodizing\/2\/6819","title":{"rendered":"Anodize This: The Brilliance of Anodizing"},"content":{"rendered":"

The anodizing<\/a> process enables the formation of a brilliant, stable and durable oxide layer<\/a> upon certain metals, which minimizes wear and corrosion damage to the underlying metallic substrate. The thick anodic<\/a> oxide layer also serves as an effective base for applying an additional colored coating<\/a> layer to further enhance the surface protection, luster and aesthetics of a substrate.<\/p>\n

Here we'll take a look at anodizing, how it works and why it's the preferred metal finishing process for aluminum, titanium and similar metals and alloys<\/p>\n

The Anodizing Process<\/h2>\n

Anodizing involves an electrochemical process<\/a> that enhances the ability of metallic surfaces to absorb oxygen by having the surface immersed in an acid solution and connecting a voltage source<\/a> across the metallic object to be anodized.<\/p>\n

\"AA demonstration of anodization
Source: Jasper Nance<\/a>
<\/span><\/span> <\/span><\/p>\n

Thus, the anodic oxidation of metals such as aluminum, zinc, cadmium, magnesium and titanium and their alloys enables the creation of a hard layer of their respective metallic oxide<\/a> (e.g., aluminum oxide<\/a>, magnesium oxide, titanium oxide<\/a>, etc.). These stable oxides strongly adhere to the metal substrate<\/a> without the tendency to flake or fall off that is observed in the case of ferrous surfaces that become rusted<\/a> during oxidation. (For an introduction to anodizing and other methods, read The 5 Most Common Types of Metal Coatings that Everyone Should Know About.<\/a>)<\/p>\n

Classifications of Anodizing<\/h2>\n

There are several anodizing classifications:<\/p>\n

Hard anodizing<\/h3>\n

The hard anodization process enables the formation of an oxide film of greater thickness, which is generally on the order of twenty micrometers to a hundred (or more) micrometers. The higher oxide film thickness is achieved by increasing<\/a> the DC voltage and the concentration of the acid, while maintaining the bath at lower temperatures.<\/p>\n

Hard anodizing creates a superior corrosion resistant<\/a> layer that is hard as well as abrasion resistant<\/a>. For example, aluminum anodized in a solution of sulfuric acid<\/a> at a temperature of 5°C (41°F) creates a thick layer of hard anodic oxide with a dull grey color, whereas at a bath temperature of 20°C (68°F) the sulfuric acid solution creates a soft and thin anodic film.<\/p>\n

Sulfuric acid anodizing<\/h3>\n

The process of sulfuric acid anodizing enables the formation of precisely controlled thicknesses of anodic oxide films<\/a> on the metallic substrate. The desired color finish is obtained through an additional color treatment. The precise thickness is achieved through the choice of voltage, bath temperature and the composition of the acid solution. For aluminum anodization, a sulfuric acid bath solution is commonly used.<\/p>\n

Chromic acid anodizing<\/h3>\n

Chromic acid anodizing is the main choice if there is a requirement that the overall fatigue strength<\/a> of the product not decrease due to the process. The anodization layer's thickness is between one to 10 micrometers. This is not a preferred method if the color consistency of the outer surface is important, because a very thin oxide film cannot be the base for consistent coloring.<\/p>\n

Aluminum alloys that are used for their high strength in aircraft are often anodized by chromic acid anodizing. However, the process is not environmentally friendly because it contains chromium (VI), whose usage is restricted by regulations<\/a> due to its toxicity<\/a>.<\/p>\n

White anodizing<\/h3>\n

The white anodizing process has been studied<\/a> for its suitability in space applications because it produces an oxide film with a low solar absorptance value. In this type of anodizing, the solution consists of sodium molybdate, glycerol, lactic acid and sulfuric acid. Optimum film thickness and optical color consistency are achieved by studying the impact of alternative formulations of bath solutions, DC voltage, current density<\/a>, temperature of the bath and the duration of anodizing.<\/p>\n

Silicon anodizing<\/h3>\n

When the alloy contains silicon, the resulting layer is more resistant to wear and corrosion, athough it has a characteristic grey and opaque color. This type of anodizing is not used for decoration, but rather for parts that will not be visible.<\/p>\n

Titanium anodizing<\/h3>\n

Titanium <\/a> anodization is carried out in a diluted bath of sulfuric acid while applying a fixed value of DC voltage. The impact of process variables such as process duration, acid solution formulation, bath temperature and current density on the anodic oxide film thickness and color properties have been studied<\/a> to optimize the titanium anodization process. (Related reading: 5 Things to Know and Understand About Titanium Corrosion<\/a>.)<\/p>\n

Orthopedic titanium alloy anodization<\/h3>\n

Anodizing is one method of forming a nano-structure<\/a> anodic oxide film on the surfaces of titanium alloys used for biomedical implant applications. In this process, the fine adjustment of oxide layer thickness and other characteristics such as topography of the pores constituting the layer is possible.<\/p>\n

Titanium alloy based orthopedic implants are color coded through anodization in a sulfuric acid solution. One study indicated<\/a> that if the color-coded implant is anodized again in a hydrofluoric acid<\/a> solution, the implant would potentially facilitate increased bone growth in the patient.<\/p>\n

Anodized standard color coding of implants and devices used in dental, orthopedic and other applications facilitates quick recognition, accurate and fast assembly of components and eases medical procedures. <\/a> This advantage is applicable to the assembly<\/a> of anodized (color-coded) titanium alloy components used for aerospace applications as well.<\/p>\n

Magnesium anodizing<\/h3>\n

Magnesium anodization is carried out in an alkali rich electrolyte<\/a>. The bath formulation ensures that the film formed<\/a> on the surface has high corrosion resistance, saltwater resistance, wear resistance and an aesthetic finish.<\/p>\n

The formation of the anodic oxide film of magnesium is directly affected by the voltage. Anodizing magnesium at a low applied voltage does not enable an oxide film with adequate corrosion protection, thus higher DC voltages are necessary. New anodizing processes<\/a> have adopted spark discharge energy to produce a wear-resistant ceramic oxide film on magnesium substrates. (Ceramic coatings are discussed in the article Top 5 Applications for Ceramic Coatings<\/a>.)<\/p>\n

The Chemistry of Anodizing<\/h2>\n

During the process of anodizing, a DC voltage is applied between a metal workpiece (e.g., aluminum) and a metallic cathode<\/a> (often zinc is used as a cathode). The water particles of the acid solution break down near the anode, generating oxygen that is collected<\/a> at the anode. Rich oxygen reacts with the aluminum to produce aluminum oxide (Al2<\/sub>O3<\/sub>).<\/p>\n

2Al + 3H2<\/sub>O <\/span>à<\/span> Al2<\/sub>O3<\/sub> + 6H+<\/sup> + 6e–<\/strong><\/sup> <\/span><\/p>\n

A thin layer of aluminum oxide is quickly formed on the substrate and a thicker oxide layer of porous<\/a> structure is formed at a slower pace. On the surface of aluminum a thin layer of anodic oxide may already be present, but this thin layer is susceptible to damage and cannot ensure strong corrosion and abrasion resistance.<\/p>\n

Anodizing enhances the thickness and other characteristics of the anodic oxide film according to the requirements. These anodic oxide film parameters can be tailored to the specific service conditions (such as found in the chemical industry or in proximity to coastal areas). Whenever the oxide film's porosity is not acceptable, a non-porous film can be created by anodizing in a non-acidic neutral bath.<\/p>\n