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Everyone knows about anodizing. It’s the deliberate formation of oxide films on the surface of some metals to make a hard, protective layer that prevents unplanned oxidation. Surprisingly few know about a great alternative that offers better dimensional precision and an easier, less costly process. This is our guide to comparing and contrasting alodining (or chromate conversion) with anodizing.
Alodining – chromate conversion coating – is a widely-used chemical treatment for Aluminum and some other metals to enhance corrosion resistance and improve adhesion for paints or other coatings. It is commonly used on parts for aerospace, automotive, electronics, and military equipment, for maintaining the longevity and reliability of components in relatively mild environments.
Anodizing is the electrochemical oxidation of a metal surface – typically Aluminum – that forms a hard and uniform protective oxide layer. This increases corrosion resilience, wear durability, and facilitates coloring while maintaining the metallic appearance.
What is alodining?
Alodine is a surface conversion coating primarily used on Aluminum but also applicable to Magnesium and Magnesium/Aluminum alloys, Zinc, Copper, and some other metals. It uses Chromium salts to effect the conversion and some forms of these are very toxic. Although used as a general term for a variety of coatings, it is technically a Henkel product. It creates a protective layer on the surface that guards against corrosion in less aggressive environments, while optimizing paint adhesion.
Unlike anodizing, which forms a relatively thick oxide layer, Alodine results in a much thinner and electrically conductive coating, so it’s ideal for higher precision and electrically integrated parts.
The treatment is part of the family of chromate conversion coatings, converting the surface of the metal to form a protective, corrosion-resistant layer. Other equivalents are Iridite from Macdermid, Chromicoat by Chemetall and TCP-HF by Chemeon. An alternative is Bonderite based on Zirconium, Zinc phosphate and Iron phosphate alternatives.This is also from Henkel, developed as a lower toxicity group of treatments with some relative benefits in some aspects
What is anodizing?
Anodizing is an electrochemical process that is employed to impose an increased thickness of oxide layer on the surface of metals, most commonly Aluminum. This process creates a uniform oxide layer that improves the native corrosion resistance, durability, and appearance of treated surfaces. Anodizing of the metal part takes place in an electrolyte bath, and an electrical current is applied to drive oxidation. The thickened oxide layer renders the surface harder and more resistant to abrasion and environment influences.
Anodizing can also be used to add color to the metal through the introduction of dyes. The porous oxide layer absorbs and retains pigments, which are then surface sealed.
Anodizing is extensively used in aerospace, automotive, architecture, and consumer products to improve component durability and aesthetics.
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The alodine process
The Alodine process typically requires various steps, with differences in the chemistry/reaction process at the surface. These are generic descriptions of the process stages:
- Cleaning – before beginning the Alodine process, the surface of the part must be thoroughly detergent cleaned to remove any grease, oils, or contaminants that could act as a barrier to the process.
- Etching – the metal is usually etched to deoxidize the surface, ready for the reaction stage that follows. The oxide would otherwise act as a barrier layer.
- Application of the solution – The Alodine solution is then applied to react with the surface of the metal via dipping, spraying, or brushing, depending on the methodologies the supplier chooses and on the geometry of the parts.
- Rinsing – the part is rinsed with deionized water to remove the solution and curtail the reaction.
- Drying – the coated and rinsed part is allowed to dry, either in air or in a warm drying chamber.
Types of alodine treatments
There are several types of Alodine branded treatments, each formulated for specific applications. The most common are Alodine 1200 and Alodine 1000 series.
- Alodine 600, 1100 and 1200 series coatings from Henkel are the most widely-used, offering excellent corrosion resistance and a yellow/brown tint. These tend towards thicker and more durable coatings, better for use in more harsh environments. They deliver a gold or yellowish finish on Aluminum and are collectively referred to as yellow chromate.
- Alodine 1000, 1001 and 1500 series are milder forms of Alodine treatment that provides good corrosion protection while maintaining electrical conductivity and a clear finish. It is often used when electrical bonding or grounding is required, as it does not significantly affect the metal’s conductivity.
- Alodine 871 is hex-free and growing in popularity.
- Alodine 5200 and 5700 are non-Chromium based conversion coatings based on poly-hydroxy aromatics that avoid the Chromium toxicity issues.
There are other variations of chromate and non chromate conversion treatments designed for different materials, application methods, or environmental standards:
- Iridite 14-2, 15 and NCP from Macdermid.
- Bonderite 1020, 1030, NT-1 and 3410 are Iron, Zirconium or Zinc phosphate conversion materials from Henkel, for use on steels.
- Chromicoat L25 from Chemetall is a hexavalent Chromium conversion system.
- TCP-HF and eTCP from Chemeon are trivalent Chromium based solutions for various alloys
Applications of alodine treatment
Alodine treatment is typically used where corrosion resistance is required in relatively benign environments AND part precision is critical;
- Aircraft structures and components are often made from Aluminum alloys, which are lightweight but prone to corrosion. Alodine treatment enhances corrosion resistance without adding significant weight to the parts and maintains dimensional precision.
- Alodine is used on Aluminum car parts, such as engine components and body panels, to prevent corrosion and improve paint adhesion while maintaining electrical conduction.
- Military equipment receives reliable corrosion protection to withstand harsh environments fromAlodine-treated parts.
- Alodine 1000, in particular, is used in the electronics industry due to its relatively greater ability to maintain the electrical conductivity of metals for chassis, housings, and connectors.
- Marine environments are highly corrosive due to the presence of saltwater and humidity. Alodine treatment is resistant to chloride corrosion (unlike acids or alkalis).
The anodizing process
The anodizing process involves several steps that are ostensibly similar to alodining;
1. Cleaning and pre-treatment: The aluminum surface is thoroughly cleaned using a degreaser or alkaline solution to remove dirt, oil, and contaminants.
2. Etching: The part is immersed in an acid or caustic solution to remove imperfections and leave a smooth and uniform surface.
3. Anodizing: The metal is immersed in sulfuric acid and an electrical current is applied, which rapidly oxidizes the Aluminum.
4. Coloring: Dyes can be introduced into the porous oxide layer for coloration. The oxide absorbs the dye making very uniform coloration.
5. Sealing: To close the pores of the oxide layer, the part is immersed in hot water or steam, which seals the surface and enhances corrosion resilience.
Type of anodizing
There are several types of anodizing, offering unique properties that increase the options in product quality/durability;
- Type II – sulfuric acid anodizing: Most common, producing moderate thickness, and ideal for color dyeing and general corrosion resistance.
- Type III – hardcoat or hard-anodizing: Creates a thicker, hard, and very wear-resistant surface, suitable for high-friction or heavy-duty applications.
- Pore-sealed anodizing: Involves sealing the porous surface for enhanced corrosion resistance – applicable to Type II and Type III.
- Phosphoric acid anodizing: Used primarily as a surface preparation method for adhesive bonding, providing a microporous keying surface.
Each type offers different benefits, depending on the application and performance requirements.
Applications of anodizing
Anodizing is widely employed in any industry that uses Aluminum and its alloys, due to its ability to enhance the durability, corrosion resistance, and aesthetics of components.
- In aerospace, anodized components are used for lightweight, corrosion-resistant structures.
- In automotive parts, it is applied to parts like Magnesium alloy wheels and Aluminum trims for improved wear resistance and aesthetics and cooling system components for reduced corrosion.
- Consumer products use anodizing for enclosures for laptops, smartphones, and tablets, providing scratch resistance and appealing surface finish.
- Architectural components use anodizing in window/door frames and accessories, siding panels, and facades,to increase weather resistance and reduce maintenance.
- In marine applications, anodized parts better resist saltwater corrosion and handle abrasion.
- Industrial machinery uses anodized surfaces for reduced friction and improved durability in harsh environments.
Relative advantages of alodining and anodizing
There are various benefits that Alodine treated parts offer:
- Corrosion resistance: Alodine treatment protects metal surfaces from corrosion. The thin chromate layer serves as a barrier against moisture, salt and mild acid/alkali attack.
- Enhanced paint adhesion: Alodine creates a priming layer that improves the durability and longevity of painted surfaces.
- Electrical conductivity: Unlike anodizing, which can reduce/eliminate electrical conductivity, Alodine treatment (especially Alodine 1000) maintains the metal’s conductive properties.
- Low weight impact: Alodine forms a very thin coating with near zero mass gain, which is particularly important in aerospace applications
Anodizing offers several key benefits:
- Corrosion resistance: Anodizing typically offers greater corrosion resilience than alodining, due to the thicker, tougher film.
- Improved durability: The anodized surface is harder and more scratch-resistant than alodining.
- Aesthetic customization: Anodized metals can be dyed in various colors, offering aesthetic versatility.
- Environmentally friendly: Anodizing is a safe, non-toxic process that doesn’t produce harmful byproducts, unlike other coatings.
- Increased adhesion: The porous nature of anodized surfaces makes them excellent for bonding with adhesives, paints, and primers.
- Low maintenance: Anodized surfaces are easy to clean and maintain, making them practical for long-term use in harsh environments.
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Limitations and considerations drive selection
Alodine treatment offers many advantages but it also carries some issues that cannot be ignored:
- Environmental concerns: Hexavalent Chromium is a highly toxic substance regulated by all environmental agencies due to its harmful effects on health and the environment. Workers handling hexavalent Chromium compounds must take fairly extreme precautions, and disposal of waste is highly restricted by environmental guidelines. Trivalent Chromate alternatives provide a less hazardous option while still offering good corrosion resistance, though typically with more impact of conductivity.
- Limited durability: Compared to more robust coatings like anodizing or electroplating, Alodine provides a relatively thin layer of protection. While it is effective for preventing corrosion, it may not be suitable for applications requiring abrasion or impact resistance.
- Aesthetic finish: Alodine-treated surfaces typically have a yellow, gold, or clear iridescent appearance, which may not be visually appealing in applications where aesthetics are important. For such applications, additional coatings like paint or powder coating may be applied over the Alodine layer.
- Regulatory restrictions: Due to ever increasing environmental and health concerns surrounding hexavalent chromium, its use has been heavily restricted in many regions. Generally, companies are transitioning to trivalent chromate systems, which are more environmentally friendly but have slightly different performance characteristics.
The limitations of anodizing are also well understood and must be balanced against the strengths and benefits it offers:
- Dimensional accuracy is somewhat lost, as the minimum coating is typically 25µm and thicker for hard anodizing – up to 20 times the thickness of Alodining.
- Greater thickness carries greater weight so anodizing is not well suited to large area applications in aerospace.
- Anodizing is a more costly and time consuming process than alodining.
While it offers good abrasion resilience, its thickness is such that the coating will fracture and flake when parts flex, as it lacks all elasticity and has very low tensile strength.
Conclusion
Alodine treatments are vital processes for industries requiring corrosion resistance and surface preparation that does not impact precision – or even weight, in flight components. Chromate conversion’s ability to provide a thin, efficacious protective layer while maintaining electrical conductivity delivers a versatile and cost-effective option for metal protection. Growing environmental concerns and regulatory restrictions are driving the industry toward safer alternatives like trivalent chromium coatings and Iron or Zirconium based processes.
Anodizing is more widely used in small parts, but less dominant in the critical aerospace sector due to its weight addition and dimensional alteration.
As manufacturers and industries seek to balance performance and environmental responsibility, the future of Alodine treatment will likely involve continued innovation in more eco-friendly processes while maintaining the protective properties that have made chromate conversion coatings indispensable in precision engineering and metal treatment. However, the performance bar set by hexavalent Chromium solutions is high, making alternatives challenging to adopt.
Anodizing is a very stable technology that is undergoing only relatively slow development. This is in part because it works so well in meeting the needs of the market and also because it is an optimized process with limited scope for improvement. Developments such as self colored white anodizing are influential and he implications of anodizing on nanoporous metallic structures may be significant
The selection between the two processes of alodining and anodizing is often straightforward – surface treatment types for Aluminum and other metal parts are a well established sector where the appropriateness of each has been well established. This does not mean that the decision is obvious in every case, so the range of considerations we have discussed here should be analyzed.
