Alodine coating is the term used for a group of Aluminum coating processes that apply a conductive chemical alteration to the surface of parts that affords them protection from corrosion.
These processes work on most Aluminum alloys and can also generally be applied to Magnesium alloys.
Alodining is widely used as a surface preparation for painting or clear coating, but in particular when its conductivity is a desired characteristic then it is a final surface treatment.
The broad use of the term alodine (uncapitalized, in general) refers to chromate conversion coatings, chem-film and chemfilm coatings.
These are a type of surface treatments that are commonly used in the aerospace industry to improve the corrosion resistance and adhesion of Aluminum components, as well as some other metals.
Collectively, they are chemical processes that involve the conversion of the metal surface into a thin, protective and electrically conductive film – often of chromate or related chemistry.
The word Alodine® is a trademark of the Henkel Corporation and refers to type I alodine coating. There are various companies offering similar products and surface treatments. Other examples are Iridite 14-2®, TCP-HF, Bonderite®, and Chromicoat®.
Because of its first-in-market position, the term alodining is used interchangeably with newer and less toxic coating processes in the Aluminum manufacturing sector.
Henkel, the originators of the type I hexavalent Chromium based Alodine® process now markets Bonderite® in this sector, based on the considerably less toxic type II alodine trivalent Chromium process.
This product does not rely on the extremely toxic hexavalent Chromium chemistry – and this is still sold under the Alodine brand.
Alodine coatings, of all forms, offer some excellent benefits in surface finishing Aluminum:
- Good corrosion resistance is achieved, despite the simplicity of the process. For high corrosion environments, anodizing is more effective, though more costly/complex to achieve.
- Chemfilm processes make good surfaces for painting.
- Conductivity of the base metal is not isolated, in equipment requiring static dispersal.
Chromate conversion coatings are generally very thin (1 µm), meaning they do not interfere in precision fit of parts.
These treatments are used for automotive (Magnesium and Aluminum alloy) wheels, interiors of electronics enclosures, aerospace applications, such as aircraft frame and skin parts, landing gear and flight control system actuators and aerodynamic surfaces.
Low abrasion environments, where static electricity dissipation is valued benefit particularly from this family of surface coatings.
The original alodine processes were based on hexavalent Chromium chemistry, though these are being displaced by trivalent Chromium approaches that are proving equally effective but considerably less toxic – and in many cases operationally simpler.
What is Alodining?
The alodine surface treatment process requires the degreased and deoxidized Aluminum or Magnesium alloy part be immersed in a chemical bath containing a chromate solution.
The chromate solution contains hexavalent (type I alodine) or trivalent (type II alodine) Chromium, which reacts with the metal surface resulting in a well coupled film of metal chromate. This film acts as an oxygen barrier, preventing the surface from reacting with atmosphere.
Alodine (type I) coatings naturally stain, ranging from clear to gold/yellow/tan, but they are easily dyed to any color.
The coating thickness is adjusted via the concentration of the chromate solution and the duration of the immersion time, but the thickest coating is rarely more than 1 µm.
In addition to improving corrosion resistance and adhesion, Alodine coatings can maintain the electrical conductivity of metal components.
They are commonly used in the civil and military aerospace sectors for a variety of applications, including aircraft frames/skins and control surface actuators, engine components, and electronic enclosures.
It is worth noting that hexavalent Chromium, which is used in the type I alodine coating process, is a highly toxic substance that causes health problems in exposed persons and when present in environment areas such as ground water.
Considerable safety measures must be taken when applying such coatings to protect operators and the environment.
Principle of the Alodine Finish?
While alodining can be achieved by a number of different proprietary methods, there are great commonalities between them and various standards that alodining, as a class of coatings, must comply with.
Alodine Coating Types
A range of standards apply to alodine, chromate conversion and chem-film processes used on Aluminum and Magnesium alloy components:
- MIL-DTL-5541 Type 1 covers military standards for hexavalent Chromium processes
- MIL-DTL-5541 Type 2 covers military standards for trivalent Chromium processes, but includes the commercial variants TCP-HF and eTCP which are considered interchangeable to some degree
- ISO 8081:2021 Aerospace process — Chemical conversion coating for Aluminum alloys — General purpose is the ISO all-purpose standard that encompasses all published versions of the technology.
The Type I and Type II standards also encompass two classes (within each standard):
- Class 1A offers maximum corrosion protection
- Class 3 offers improved electrical conductivity, with moderate corrosion protection
MIL-DTL-5541 Type 1
MIL-DTL-5541 Type 1 utilizes hexavalent chromium in the chromate conversion. This was the sole option in alodining for many years, but it is being displaced by the easier and less toxic type II trivalent Chromium based process.
The toxicity of hexavalent chromium, both for operators exposed to it and in the wider environment when leaks occur, is so high that it is considered a highly controlled substance and banned in many areas.
Its use is tightly restricted in the US by the EPA and OSHA, as a powerful carcinogen. If it is necessary to specify type 1 alodining, it CAN be done. But it is expensive, dangerous and best avoided.
MIL-DTL-5541 Type 2
MIL-DTL-5541 Type 2 is differentiated as hex-free chromating. The hexavalent chromium is substituted with a trivalent chromium based process and in some limited number of cases with Titanium or Zirconium based chemistry that is essentially non-toxic. These processes are increasingly accepted across most industries.
Chemeon is a leader in the sector of hex-free trivalent chromate conversion chemistries, offering a product branded TCP-HF (hexavalent free) with general approvals for the majority of civil and military aerospace applications.
Most of the trivalent Chromium based processes produce a slight blue tinge that is hard to differentiate from the bare material.
Thai has resulted in development of eTCP by Chemeon, which is a colored version of TCP-HF that usually shows a violet or blue tinge, for easy identification.
ISO 8081:2021 Aerospace Process
The ISO 8081:2021 standard specifies the requirements for applying and evaluating the entire range of chemical conversion coatings used on Aluminum and Magnesium alloys.
It specifies the applications for these coatings, in military and civil aerospace components and systems in terms of thickness, corrosion resistance, hardness, application methods and test procedures/standards to be met.
This is a single standard that applies to all chromate and related surface coating conversion methods, including optimization for either corrosion protection or conductivity.
The Alodine Finish Process
The Type 1 (hexavalent) Alodining process requires typically 9 steps:
1. Cleaning: This removes dirt, oil etc from the parts
2. Rinse: Generally uses deionized water and absence of droplet formation at with drawl is used to assess cleaning (the water-break-free test)
3. Etch: This uses alkaline etch compounds to finely texture the areas to be chromated, ensuring good adhesion. Masking can be used to selectively exclude areas from the process.
4. Rinse: Deionized water ensures any etching chemistry is rinsed to end the etching.
5. Deox: The part undergoes oxide removal to prepare for conversion coating.
6. Rinse: Deionized water removes the oxide stripper solution.
7. Chem-film or alodining: The part is immersed in hexavalent Chromium solution, where solution concentration and exposure time define coating thickness.
8. Rinse: This removes the alodining solution and ends the reaction
9. Warm rinse: This ensures all hexavalent Chromium residues are removed, as a final safety precaution
The Type 2 (hex-free) trivalent chem film surface conversion TCP-HF comes in two forms, below are the Acid Clean process steps:
- Acid Clean
- Trivalent chromium solution coat immersion
- Second Rinse
The Alkali cleaning version of the type II process includes the etch stage that is included in acid cleaning in the acid process above:
- Alkaline Clean
- Trivalent chromium solution coat immersion
- Second Rinse
Main Characteristics of Alodine Finish
Alodining has several attractive properties that make it the go-to surface protection for light metals in a range of markets:
- Thin film – alodining and the related chromate conversion methods leave a maximum film thickness of 1 µm, allowing this treatment to not alter part precision
- Application at room temperature – no hot processes are required for chromate conversion
- Alodine type I solution is highly toxic and carcinogenic, but the type II solutions have eased this problem considerably
- The application process is fast, lasting just a few minutes for the actual conversion stage
- Great conductive surface – alodined parts (from all methods) have a low resistance surface that benefits many applications
Alodine vs. Anodizing
Alodining and anodizing are both surface treatment processes used to protect light metals from corrosion, while improving their cosmetic appearance. The processes differ entirely in their chemical composition and application methods.
As discussed, alodining converts the surface to a chromate film of 1 µm thickness that offers moderate corrosion protection and high electrical conductivity. The process is lower cost in materials, labor and time than anodizing.
Anodizing involves the electrochemical oxidation of the surface, which creates a thicker and more durable ceramic oxide layer on the surface of metal parts. Anodized surfaces are hard, abrasion-resistant, and corrosion-resistant and can be process-coloured or dyed in a wide range of colors.
Anodizing is commonly used in the manufacturing of automotive parts, architectural structures, and consumer products.
It leaves a tougher, more corrosion resistant surface than alodining, but it is slower and more costly to process and creates films up to 5 mils (0.125 mm) thick, affecting precision at times.
Applications of Alodine Finish
The alodine family of processes are used in:
- Military and civil airframe parts
- Aircraft landing gear
- Wing parts, control surfaces, actuators
- Heat sinks, for better IR emission properties
- Automotive wheels
- General light metal hardware and CNC machined parts are often alodined, where environmental and handling exposure are limited
Other metals like Copper and zinc-plated steel/iron can also benefit from chromate coating.
Pros and Cons
- Low cost
- Fast processing
- Good corrosion resistance
- Good electrical conductivity
- Precise, thin films
- Alodining makes a weak and non self repairing film
- Generally moderate corrosion resistance compared with anodizing
- Key materials are toxic, though this is improving
Kemal’s Alodine Surface Finish Services
Kemal has a team that has huge experience in delivering high quality outcomes for clients in CNC machined Aluminum and Magnesium parts.
A critical skill within this process is knowing which surface treatments to use for which applications. Our experience with the range of chromate conversion technologies is extensive, so whatever your project needs, we have the skills.
Kemal is a great partner, committed to the quality of your product and we are eager to hear from you, so we can see how best to assist with your manufacturing needs.