Anodize

Types of Anodize

Clear coat anodizing produces a transparent layer that adds minimal dimensional change. It provides surfaces with moderate protection, but is primarily used for cosmetics. The result is a sleek silver finish, popularly utilized on products like the MacBook. 

Hard coat anodize produces a thick layer with exceptional toughness. You can expect high dielectric strength, corrosion resistance, and part lifespan. This coating is identified by it’s signature matte black finish on 6061.

Oxalic is a less commonly used electrolyte that produces thin coatings similar to Type II. However, the coating performance dramatically exceeds that of type II. Coatings are typically light yellow in color.

Teflon is used throughout the industry for the benefits it provides parts. We can apply Teflon over the anodize layer to give wear and lubrication properties, often used for mechanical applications where there is friction. 

Selective anodize is our proprietary refurbishment process that re-anodizes damaged surfaces without having any impact on the rest of the part. This service is ideal for tiny nicks/dings that break through anodize.

AX200 is the strongest of our anodize processes. This coating is built to endure in the harshest environments. Its properties dramatically exceed all other anodize coatings. The finish ranges from dark grey to black. 

Why do we Anodize Aluminum?

As the most abundant metal on earth, aluminum’s unique capabilities have promoted growth and expansion within industries around the world.

Raw aluminum is uniquely corrosion-resistant due to its naturally occurring oxide which forms on the surface during a process called self-passivation. Anodizing mimics this phenomenon through an electrolytic passivation process that similarly stimulates oxide growth. As a result, this synthetically formed oxide coating is electrically insulated and can be modified to withstand significant corrosion, wear, and dielectric attack. Anodizing also increases surface hardness and provides significant strength and protection from outside sources of attack. The oxide is composed of a tough nanoscopic pore network that doubles as an excellent surface for dye retention because these pores can absorb dye. A subsequential sealing process helps close anodize pores, improving coating quality, while also locking in the color for dyed parts.

What is the Anodize Process?

Anodizing at its core is a straightforward procedure -utilizing a dilute sulfuric bath (electrolyte), an anode (aluminum part), and a cathode. This forms an electrolytic cell inside the tank and as direct current is applied to the part electrolysis takes place – forming the aluminum oxide on the part surface. 

Before this takes place, we need to consider the alloy, incoming condition, and surface preparation – this is a critical step to avoid potential issues with the coating. Parts typically have a specified pre-anodizing clean and etch process to remove surface impurities and irregularities.

Choices such as electrolyte composition, bath temperature, voltage, and current density all affect the outcome. The resulting characteristics are how we define different processes like Type II and Type III anodize. After the initial anodizing, the part can undergo a dye process or simply be sealed with nickel acetate or DI water. A dyed anodized part is called class 2 and a non-dyed part is class 1. 

Anodize Thickness

Many engineering drawings specify that dimensions apply after anodizing. This often raises the question for machine shops on how to account for anodized buildup. A typical rule of is to assume approximately 50% of the anodize layer penetrates the aluminum, while the remaining 50% contributes to buildup. For instance, with a 2-mil anodize, the part’s dimensions will increase by approximately 1 mil. It’s important to note that this rule is not foolproof; nevertheless, it serves as a useful consideration.

While anodized thickness can reach up to 6.0 mils, it doesn’t always mean a better coating. Typical thicknesses range from 0.10 – 3.0 mils, with 6061 and 5052 being the alloys of choice. Lower thicknesses (Clear Coat) can still be effective in protecting the underlying aluminum, but they are best suited for dye acceptance in cosmetic applications. Thicker coatings (Hard Coat) have better coating properties which are more effective in demanding, corrosive environments.

Choosing a Process

Most anodize specifications aim to bring out specific qualities in the anodize, depending on customer needs. Common considerations are electrical resistance, corrosion resistance, thickness, cost, lubricity, and color. 

Many OEM specs are originally based on MIL-A-8625, which is the U.S. military’s anodizing specification. Mil-spec balances high-performance attributes at a reasonable value. Mil-spec is our recommended default for customers looking for an exceptional anodize coating.

There are a variety of different anodize processes to choose from depending on your desired application. Semano specializes in anodizing for the semiconductor manufacturing industry due to the performance-oriented properties of each coating we produce, that allow parts to withstand harsh environments.

Coating Quality

Measuring coating properties is the most vital key to ensuring quality service. Few anodizers have the capabilities to evaluate their coatings which often requires they get outside evaluation with slower results. With quality being our top priority, Semano not only develops internal testing protocols but has also contributed to writing OEM Specs. As a result, we can provide our customers with key insights.

Key Measurements Standards and How We Test

Anodize properties can be heavily dependent on your choice of process, part material, and machining quality.

Thickness

  • Our standard measurement is mils (0.001″) or microns (μm). We can achieve tolerances as tight as +/- 0.20 mils.
  • Our equipment uses eddy currents to pinpoint coating thickness to the 0.000001″ place accuracy

Corrosion Resistance

  • Measured by HCL bubble test time (hours/minutes). 
  • The test fails once acid breaks through the coating surface, signaled by tiny bubble streams.  

Dielectric Voltage Breakdown

  •  Generally evaluated per mil thickness of coating (DBV/mil). Most standards require 350+ DBV/mil.
  • Determines insulative strength by applying increasing voltage until current leaks through and signals coating breakdown

Seal value

  • Our equipment  forms a capacitor on the coating that measures admittance in microSiemens (μS)
  • Seal value is calculated using the admittance and it evaluates anodized seal quality

Surface Roughness

  • Roughness average (Ra), evaluates coating roughness in micro inches (μin)
  • We use custom solutions to measure Ra on a variety of different part geometries with different levels of roughness 

Color

  • A spectrometer measures light values to maintain coating color consistency. Color depends on alloy and process type.
  • We follow CIELAB color space measurement consistent with OEM specifications and industry standards

Type II - Clear Coat Anodize

Thinner than Type III – Clear Coat is also anodized in a sulfuric electrolyte and still provides the same benefits as bare aluminum, just on a lesser scale. Type II is often a more affordable option which makes it ideal for less demanding applications. It also is the preferred substrate for dye coloring – a signature of anodizing. 

General properties of our Type II Coating:

  • 0.07 – 1.2 mils Thickness
  • Sleek Silver Finish – Colorless/Clear
  • Increased Corrosion/Wear/Dielectric Strength
  • Excellent surface for Dye Coloring
Read more about Clear Coat Anodize.

Type III - Hard Coat Anodize

Hard Coat Anodize is a preferred option for harsh corrosive environments. The Type III standard is derived from the military anodizing spec Mil-A-8625 which has a category for Type III coatings. 

General properties of our Type III Coating:

Read more about Hard Coat Anodize.

Oxalic Anodize

Although it has all but been replaced by Sulfuric anodizing, Oxalic acid Anodizing is a legacy process that still suits some select applications. It provides a very smooth coating that is a shade of yellow. At low thicknesses the performance rivals that of Type II and Type III. 

General properties of Oxalic Anodize:

  • 0.10 – 2.0 mils
  • Smooth yellow finish
  • High performance for a thin coating
  • Alternative to Sulfuric based coatings

Dye and Teflon Anodize

Dye is a popular cosmetic option that fills anodized pores giving deep uniform color to parts. Semano offers both black dye and gold dye options that are rich in color. Black dye is a common choice as it offers deeper blacks than hard coat.

Our PTFE Teflon finish helps to improve many of the properties of the anodized coating. It helps increase corrosion resistance, dielectric strength, and wear resistance while giving parts dry lubricity.

Selective Anodizing

Selective anodizing is our in-house process that re-anodizes damaged coatings. Rather than stripping an entire part, which can break dimensional tolerances and could affect coating quality, we can spot-anodize a location to more than match the OEM spec.

This provides huge cost savings to machine shops, OEMs, and IDMs that must scrap parts with even small amounts of damage. This service is just one of the offerings we provide in our refurbishment program.

Learn more about part refurbishment.

AX200 Anodize Coating

AX200 is our patent-pending, Type III coating process which produces the strongest and most durable anodize. Semano’s efforts in R&D are a big factor that sets us apart, and AX200 is a breakthrough in an industry that rarely sees change. This natural coating is built to outperform and outlast all other anodized offerings.

General Properties of AX200:

  • 1.0 – 3.0 mils
  • Dark Matte Finish
  • 48+ hour HCL bubble time
  • 1700+ DBV/mil

Read more about AX200 here.

How do we Manage Quality?

Approved Vendor for

milspec

For more details, please refer to our Specifications Page which lists our most common specs.