Anodizing and chemical film are both used for metal surface treatments but they are not the same. Each has its own process, characteristics and applications even though both protect metal surfaces. In this blogpost you will learn main differences between these two to help you choose the right finish for your needs.
What is Anodizing?
Anodizing is an electrolytic passivation process which strengthens the oxide layer that is naturally present on metals, specially aluminum. This improved oxide layer gives more durability, a better appearance and better corrosion resistance. Unlike coatings that are applied externally, anodizing integrates the oxide layer into the metal itself and prevents it from chipping or peeling.
Also See: Complete Guide to Aluminum Anodizing
Types of Anodizing
1. Chromic Acid Anodizing
Chromic acid anodizing (CAA) produces a thin oxide layer, typically 5‐25 microns thick, using chromic acid as an electrolyte. This is used for applications where dimensional accuracy is necessary as it produces minimal thickness changes.
2. Hard coat Anodizing
Also known as hard anodizing, hard coat anodizing produces a very thick and robust oxide layer which can be up to 150 microns. This method needs lower temperatures as well as higher current densities in comparison to other anodizing processes.
3. Sulfuric Acid Anodizing
The most common anodizing method is sulfuric acid anodizing. In this procedure, technicians immerse aluminum parts into a sulfuric acid solution and apply an electrical current. Compared to chromic acid anodizing this method produces a thicker oxide layer of 10 to 25 microns.
Anodizing Process
Anodizing process has four stages:
Cleaning
First technicians clean the aluminum thoroughly to remove contaminants, debris and oils. In most cases they use a heated non‐etching alkaline detergent at 145°F.
Etching
Next the aluminum part is etched in a sodium hydroxide (caustic soda) solution. This step chemically removes a thin aluminum surface layer which forms a matte finish and prepares it for anodizing.
Anodizing
During the main anodizing step the aluminum part acts as the anode in an electrolytic bath usually containing chromic or sulfuric acid. A conductive metal, generally stainless steel, is the cathode.
Passing an electric current through the bath oxidizes the aluminum surface and gives a porous oxide coating.
Sealing
Finally sealing closes the pores in the newly formed oxide layer. To do this technicians immerse the anodized aluminum in hot water or expose it to steam. This hydration step seals the surface and locks in any dyes used for coloring.
Also See: Anodized Steel_ Process, Benefits & Applications
Main Properties of Anodized Surfaces
Wear Resistant and Hard Surface
Anodizing greatly increases surface hardness for hard coat anodizing—up to 500 HV. This hardness gives outstanding mechanical wear resistance and abrasion.
Electrical Insulation
Anodized oxide layer does not conduct electricity so it is useful in electronic applications where electrical insulation is required.
Color Customization
As the oxide film is porous, therefore anodized surfaces can absorb dyes. You can get a variety of colors like bronze, black, gold as well as custom colors.
Chemical Film (Chromate Conversion Coating)
Chromate conversion coating or chemical film is a chemical treatment of metal surfaces. This process not only retains electrical conductivity but also gives better corrosion resistance and improves paint adhesion as well.
Anodizing uses an electrochemical process to build up a thick oxide layer but chemical film forms a thin protective coating by direct chemical reaction with your metal surface.
Chemical Film Types
1. Hexavalent Chromate (Traditional)
For many years industries have used hexavalent chromate coatings because of their extraordinary durability and corrosion protection. But environmental regulations now limit their use because of toxicity issues.
2. Trivalent Chromate (Environment Friendly)
If sustainability is important to your application then trivalent chromate coatings are the safer and environmentally compliant option. Although their protection against corrosion is slightly lower under extreme conditions, they are good enough for most applications. Industries are moving towards trivalent chromates as they prioritize environmental responsibility.
Chemical film Application Process
Chemical film application has many steps.
Surface Cleaning
First metal parts are cleaned thoroughly. This step vanishes grease, dirt and oxide layers so a proper chemical reaction can happen later.
Water Rinse
Next your cleaned parts are rinsed with water to remove any remaining cleaning residues.
Chromate Coating
Your metal is then sprayed or immersed in a chromate solution which has either trivalent or hexavalent chromium compounds. This solution chemically reacts with your metal surface to form a protective as well as thin oxide layer.
Main Properties
Maintains Electrical Conductivity
Chemical film coatings are different from anodizing because they retain your component’s electrical conductivity. This property makes it best for electronic applications.
Corrosion Protection
Chemical film gives your metal parts good resistance against environmental degradation and oxidation.
Thin and Dimensionally Stable Coating
This chemical film coating is usually under 300 nanometers thick. This low thickness lets your parts preserve their dimensions and also gives adequate protection.
Anodizing vs Chemical Film
It’s wise to know the main difference between chemical film and anodizing to select the right surface finish.
Electrochemical vs Chemical Process
Anodizing is an electrochemical process. This process uses an electric current for aluminum to form a thick and durable oxide layer. On the other hand chemical film is a purely chemical reaction. In this case aluminum reacts with a chromate solution to give a thin protective coating without electricity usage.
Coating Thickness
Coating thickness changes greatly between anodizing and chemical film.
Anodizing gives a coating thickness of 0.0002‐0.001″. This thicker layer is super resistant to wear and is more durable. Chemical film coatings are much thinner as they give a thin layer of just a few microns. This thinner coating not only protects the aluminum but preserves its original dimensions too.
Electrical Conductivity
Electrical conductivity is another big difference. Anodized aluminum becomes nonconductive because of the insulating oxide layer formed during the process. In contrast, chemical film keeps the aluminum’s natural conductivity. So chemical film is better for applications that need EMI shielding or electrical grounding.
Wear Resistance and Durability
Anodized surfaces are super durable and wear resistant. Hard coat anodizing can deliver abrasion resistance up to 100 times more compared to untreated aluminum. On the other hand, chemical film is softer, thinner as well as wears more easily. So chemical film is better for low stress applications.
Corrosion Protection
Anodizing has better corrosion protection due to the thicker and more robust oxide layer. It’s good for harsh or extreme environments. On the other hand, chemical film self‐heals minor scratches and has medium corrosion protection. But it is less effective in severe conditions.
Visual Appearance
Anodizing has a broad range of finishes and colors. This is good for aesthetic or decorative purposes. Compared to anodizing, chemical film coatings are only gold or clear. So chemical film is less useful visually.
Ease of Repair and Rework
Chemical film is easy to repair. You can do localized touch ups without stripping the entire surface. Compared to chemical film, anodized coatings are thicker and harder so repairing is more difficult. You usually have to re‐anodize the entire surface to fix this type of issue.
Environmental Impact
From an environmental standpoint anodizing is generally safer since it’s nontoxic and has no VOC emissions. But anodizing consumes more energy and needs careful handling of chemical waste．
In comparison with anodizing, traditional chemical film coatings, particularly those with hexavalent chromium, have some environmental concerns. But newer trivalent chromate chemical film options are available and they are more environment friendly.
Cost and Application Time
Anodizing is more expensive and time consuming since it’s a multi step process. So anodizing is better for large production runs. On the other hand, chemical film is quicker and simpler to use so it’s affordable for prototypes or small batches.
Paint Adhesion and Primer Suitability
Anodizing and chemical film both improve paint adhesion. But chemical film works better as a primer. Their chemically active and thin surface bonds better with adhesives and paints. Compared to chemical film, anodized surfaces give good adhesion too but works well when your application needs more durability than primer performance.
See Also: Powder Coating vs. Anodizing
Application’s Comparison
Anodizing
If you need strong visual appeal and durability then anodizing is better. Aerospace parts such as fasteners and skin panels use anodizing because it gives a hard coating which can withstand severe conditions.
In automotive industry, manufacturers use anodizing for engine parts and wheel covers because of dye acceptance and corrosion protection. Consumer electronics also benefit from anodized aluminum as it gives a protective surface and color variety for device enclosures.
Chemical Film
Chemical film works best when you need corrosion resistance and electrical conductivity without changing part dimensions. Aerospace applications use it for landing gear, aircraft hulls and flight control systems. In electronics, chemical film is applied to electrical connectors and heat sinks for protection.
The automotive industry uses chemical film on wheels and other parts exposed to harsh environmental conditions. Military and defense applications prefer chemical film because it gives a thin conductive layer. This makes it perfect for electrical properties as well as where you need reduced weight.
Choice of Right Finish: Decision Matrix
When deciding between chemical film and anodizing you have to take into account their conductivity, durability, environmental impact and cost.
Select anodizing if your application needs long lasting wear resistance, visual appeal and corrosion protection. However if you need electrical conductivity, fast turnaround or compatibility with paints and adhesives then chemical film is the better choice.
Keep in mind production volumes and environmental factors as they are also important. Chemical film is faster and cheaper for smaller production runs. Anodizing is more expensive for small batches but more environment friendly. Eventually your budget, performance needs and material compatibility will guide you towards the final decision.
To Sum Up
Chemical film and anodizing are two different aluminum finishing options. Both are used for different applications. Anodizing is better for visual appeal and durability and chemical film for cost and electrical conductivity. So the choice is yours.
If you want to know which surface finish will be best for your particular need, then our experts can help you choose. You can contact us anytime.
Related Questions
Can I apply chemical film onto anodized aluminum or anodize chemically filmed aluminum?
Generally you can’t combine anodizing and chemical film on the same aluminum part. A chemical film can fix minor issues on anodized surfaces. So using one coating over the other isn’t recommended.
Does chemical film hold up in marine or outdoor environments?
Yes chemical film coatings do provide protection against moisture and saltwater. But if you plan for long term exposure outdoors then your parts may need extra protective layers.
What happens to EMI shielding and electrical grounding with chemical film and anodizing?
Anodizing forms insulating surfaces which affects electrical grounding. Compared to anodizing, chemical film coatings are conductive so they are best for electrical grounding and EMI shielding.
Can chemically filmed or anodized aluminum parts be recycled?
Yes you can recycle aluminum even after anodizing or chemical filming. Anodizing doesn’t affect aluminum’s recyclability at all and the thin chemical film coating doesn’t compromise recycling either.
Are chemical film coatings compatible with sealants and adhesives?
Yes chemical film increases bonding for sealants and paints. Just make sure the adhesives you choose don’t have chemicals or defoamers as they can cause defects or issues.
What tests are used to check chemical film or anodizing quality?
Mostly used tests are HCl Bubble Test to evaluate corrosion resistance, Eddy Current testing to measure coating thickness and ASTM B136‐84 Modified Stain Test to check sealing.
Can anodizing be combined with chemical film or other surface treatments as hybrid coatings?
Yes you can find hybrid ways today. These combined processes use anodizing with other treatments such as electroplating to get particular performance.
