What Aluminum Alloys Can Be Anodized?

In architectural decoration aluminum processing, anodic oxidation is a mainstream and high-quality surface treatment process. However, not all aluminum alloys can be adapted to high-quality anodic oxidation process, the flatness of different grades of aluminum after oxidation varies greatly, and improper selection of materials will easily lead to chromatic aberration, and other defects, resulting in product scrapping.
In this article, we will answer the question of which aluminum alloys are suitable for anodic oxidation from the principle of anodic oxidation, classification of oxidizable aluminum alloy grades and other dimensions, so as to help you make accurate selection and avoid production risks.

What is Aluminum Anodizing?

Aluminum alloy anodic oxidation is an electrochemical surface modification process, it is through an electrochemical reaction, so that the surface of the aluminum material itself to generate a layer of dense and stable alumina film, the film layer and the substrate into one, the adhesion is extremely strong, and is not easy to fall off.

The specific process is: the aluminum alloy workpiece as the anode, into the acidic electrolyte tank, through the direct current, the oxygen ions in the electrolyte and the aluminum surface of the aluminum atoms combined to generate alumina layer, the cathode is released hydrogen. The whole process can precisely control the thickness, density and structure of the oxide film.

Anodizing Type

Type I chromic acid oxidation: This process uses chromic acid as the electrolyte, and the thickness of the formed oxide film is controlled between 2.5-7.6μm. The biggest advantage is that the deformation of the workpiece during processing is extremely small, which will not change the precision size of aluminum, and at the same time, it has excellent anticorrosion and protection performance, which is able to meet the protection needs of high-precision parts. However, limited by the environmental control requirements of hexavalent chromium, this process has been phased out in most areas, and only a small number of applications in high-end precision fields such as aerospace.

Type II sulfuric acid oxidation: This is the most versatile anodic oxidation process in the industrial and civil fields, adopting conventional sulfuric acid electrolyte, with a standard film thickness of 5-25 μm. The process is stable, with a uniform and flat formed oxide film, which not only has good anti-corrosion ability, but also has excellent dye adsorption performance, and can realize a variety of colors and textures of decorative coloring effects, which is a perfect balance of utility and aesthetics, and is widely used in architectural aluminum profiles, daily necessities and other industries.

Type III low-temperature hard oxidation: belongs to the high-intensity functional oxidation process, using low-temperature sulfuric acid electrolyte processing, the thickness of the oxide film can reach 25-100μm or even higher, much higher than the conventional oxidation process. The hardness of the molded film layer is extremely high, which can adapt to the use of harsh working conditions, and the protection performance is far more than that of the ordinary oxidation process.

Which Aluminum Alloys can be Anodized?

The anodic oxidation of high-purity aluminum, aluminum-magnesium, aluminum-magnesium-silicon alloys has the best adaptability, the finished product has a clean and uniform appearance, and the performance is stable; while the oxidation of aluminum alloys containing copper, high-silicon, and high-zinc has a very low tolerance rate, and is prone to color difference, darkening, and other kinds of process defects after processing, making it difficult to make high-quality finished products.

High-purity aluminum, magnesium, magnesium-silicon aluminum alloy: fewer alloy interference elements, translucent and dense oxidation film, even and full coloring, able to achieve high-definition translucent decorative effect, suitable for color oxidation and other processes, is the first choice of high-end decorative anodic oxidation of the base material.

Manganese aluminum alloy: strong compatibility of oxidation process, high stability of finished products, no obvious appearance defects, taking into account the formability and cost-effectiveness, without complex process control can produce qualified products, suitable for all kinds of general industrial scenarios of anodic oxidation processing.

Copper, zinc and high silicon casting aluminum alloy: the alloying elements will seriously interfere with the normal molding of the oxide film, and it is very easy to have problems such as yellowing, etc. The decorative oxidation effect is very poor, and it can only be used in functional anodic oxidation scenarios with no appearance requirements.

Best Aluminum Alloy Series for Anodizing

1000 Series Pure Aluminum Alloy

The 1000 series is a pure aluminum alloy with a purity of 99% or more, with no interference from unnecessary alloying elements, and is the aluminum alloy series with the best anodic oxidation effect. Mainstream grades include 1050, 1060, 1100.

Oxidation characteristics: clear and transparent oxide film, clean surface without impurities, color uniformity pull full, no blackening, yellowing defects, insulation, corrosion resistance is excellent.

Applicable scenes: high-end decorative accessories, reflective lighting fixtures, electronic shell, semiconductor accessories, kitchenware interior, nameplate decorative parts.

Short board: low mechanical strength, not suitable for load-bearing structural parts.

3000 Series Aluminum-Manganese Alloy

The core alloy element is manganese, mainstream grades 3003, 3004, 3105, with good molding and stability.

Oxidation characteristics: uniform and flat oxide film, low defect rate, translucent silver-gray color after oxidation, excellent anti-corrosion performance, and strong process stability.

Applicable scenes: architectural decorative panels, roof profiles, HVAC heat exchange equipment, general-purpose sheet metal parts, storage tanks.

5000 Series Aluminum-Magnesium Alloy

With magnesium as the core alloying element, mainstream grades 5005, 5052, 5083, 5754, high strength, good toughness, natural anticorrosion ability is outstanding.

Oxidation characteristics: dense and hard oxide film, good gloss, excellent color consistency, not easy to change color and corrosion in harsh environments.

Applicable scenes: marine ship fittings, coastal building profiles, outdoor equipment frames, rail transportation fittings, chemical pressure vessels.

6000 Series Aluminum-Magnesium-Silicon Alloy

6000 series is the benchmark series of industrial aluminum profile anodic oxidation, taking into account the strength, processability and oxidation effect, mainstream grades 6063, 6061, 6082, 6463.

Oxidation characteristics: uniform and translucent oxide film, excellent color performance, can be adapted to a variety of color oxidation, frosted, glossy process, stable anti-corrosion and wear-resistant performance after sealing holes. 6463 higher purity, suitable for high-end glossy oxidation; 6063 is the standard grade for decorative profiles; 6061, 6082 is suitable for structural parts oxidation.

Applicable scenarios: industrial assembly line profiles, automated equipment frames, doors and windows curtain wall, furniture aluminum accessories, automotive decorative parts, precision machinery structure parts.

Aluminum Alloys That are More Difficult to Anodize

In the commonly used aluminum alloy system, some alloys have poor anodic oxidation molding stability, many appearance defects and low yield due to the special alloy ratio and microstructure. These alloys generally have excellent mechanical properties, but the decorative anodic oxidation adaptability is extremely poor, only to meet the basic functional anti-corrosion needs, is the appearance of the oxidized parts of the production of the material needs to be avoided.

2000 Series Aluminum-Copper Alloy

2000 series is a high-strength aluminum alloy with copper as the core alloying element, and the mainstream grades include 2024, 2014 and 2017, which are widely used in the field of aviation and military high-strength structure by virtue of the anti-fatigue performance of ultra-high strength.

Oxidation pain point: During the oxidation process, the copper phase inside the aluminum material cannot be oxidized synchronously, and will continue to dissolve and re-precipitate on the surface of the film layer, which directly destroys the dense structure of the alumina film, resulting in a loose and soft molding film layer, and poor abrasion resistance.

Limitations: This series is only suitable for functional protection processes such as type I chromic acid thin oxidation, and is only used to enhance the basic anticorrosive ability. The majority of high-strength aerospace and military structural components are based on functional protection, and are not oxidized for appearance.

7000 Series Aluminum-Zinc Alloy

7000 series of zinc as the main alloying element, supplemented by magnesium, copper elements to strengthen the performance of the representative grades for the 7075, 7050, is currently the industrial application of the highest strength of the aluminum alloy series, mostly used for ultra-high strength load-bearing, wear-resistant structural components.

Oxidation pain points: zinc elements will interfere with the uniform growth of the oxide film, change the micro-pore structure of the film layer, resulting in uneven thickness of the film layer. Conventional oxidation of the finished product is easy to show dark blue-gray tone, accompanied by local mottling and other issues, can not produce bright colored decorative effects. Especially in type III hard oxidation process, the film layer stress is too large, it is very easy to crack and other process defects, greatly increasing the scrap rate.

Applicable limitations: Only suitable for functional hard anodizing without appearance requirements, mainly used in aviation structures, high-end outdoor sports equipment, industrial high-strength wear-resistant parts and other scenes.

Casting Aluminum Alloy

Restricted by the casting process, this kind of aluminum generally contains high silicon impurities, internal porous, uneven distribution of elements, inherent material defects lead to its oxidation stability is far lower than all kinds of deformed aluminum alloy, decorative oxidation yield is extremely low.

Oxidation pain point: high silicon impurities can not participate in the oxidation reaction, will form a large number of black inclusions in the film layer, so that the finished product overall black and dark, a complete loss of translucent texture and color ability. At the same time, the tiny pores inside the casting will adsorb the residual electrolyte, and it is very easy for the after-effects such as seepage stains to appear in the later stage, and the color uniformity can not be guaranteed at all, and the mass production scrap rate remains high.

Optimization plan: Only 514, 713 and other low silicon cast aluminum on the market can complete the simple protective oxidation, which can meet the basic anticorrosion requirements. Conventional high silicon casting aluminum alloy is only suitable for industrial general castings without appearance requirements.

Anodizing Effect Comparison of Common Aluminum Alloys

Combined with the four core indicators of surface flatness, color uniformity, oxide film stability and anti-corrosion performance, combined with the actual effect of mass production, the comprehensive performance of anodic oxidation of aluminum alloys commonly used in the industry is classified and explained, which facilitates quick and accurate selection.

1000 Series Pure Aluminum Alloy

Top anodic oxidation performance, very low impurity content, transparent and clean oxide film, uniform and stable thickness, clean surface, no heterogeneous color, color purity far exceeding other alloys, excellent anti-corrosion and insulation properties. It is very suitable for high-end color oxidation, reflective decorative parts, and it is the preferred material for scenes with very high requirements on the degree of translucency of appearance, but the only shortcoming is the low mechanical strength.

5000 Series Aluminum-Magnesium Alloy

Excellent oxidation effect, smooth surface, uniform color, dense and stable film layer. The core advantage is strong weather resistance and salt spray resistance, suitable for coastal, marine, outdoor sun exposure and other harsh conditions, long-term use is not easy to discoloration, corrosion, widely used in outdoor construction, ship fittings and anticorrosive industrial aluminum.

6000 Series Aluminum-Magnesium-Silicon Alloy

6000 series aluminum-magnesium-silicon alloy is the oxidized alloy with the strongest industrial applicability. Among them, 6063 has the best decorative performance, smooth oxidation, full and even coloring, and can be adapted to matte, glossy, colored and other full range of decorative processes. 6061, 6082 focus on structural performance, stable oxidation quality, slightly lower than the color subtlety of 6063, but with higher strength, and is suitable industrial anodizing scenarios.

3000 Series Aluminum-Manganese Alloy

The oxidation process has excellent stability, uniform grain, flat surface without obvious defects, natural transparent silver gray color after oxidation, balanced anticorrosive performance. The overall cost-effective general-purpose materials, mostly used in conventional industrial scenarios that do not have extreme requirements for appearance.

7000 Series Aluminum-Zinc Alloy

The base material is extremely strong, but the anodizing performance is medium. Influenced by zinc, the oxidized film growth is not uniform enough, and the finished product is prone to blue-gray tones and local mottled color differences, making it difficult to make bright and clean decorative effects, and there is also a risk of film cracking in hard oxidation. Only suitable for high-strength, wear-resistant functional structural parts.

2000 Series Aluminum-Copper Alloy and Cast Aluminum Alloy

2000 series aluminum copper content is high, the oxidation film is loose and soft, the finished product is easy to be yellowed and brown, corrosion and abrasion resistance is greatly reduced, and can only be used for simple functional protective oxidation.

A380, A356 as a representative of high silicon cast aluminum, due to the material porosity, after oxidation of the overall darkness, electrolyte residue is easy to cause corrosion in the later stage of mass production of scrap rate is high; only a small number of low-silicon cast aluminum can be done as a basic protective oxidation.

How Alloying Elements Affect Anodizing Quality

Differences in the electrochemical properties of different alloying elements will directly intervene in the growth rate of the oxide film, microscopic pore structure and molding stability, which is the root cause of the differences in the oxidation effect of different grades of aluminum.

Magnesium

Magnesium is the most friendly alloying element for anodic oxidation, mainly added in 5000 series and 6000 series aluminum. During the oxidation process, magnesium can have a stable electrochemical reaction with aluminum, which helps to generate a dense structure of aluminum oxide film. This structure not only has excellent color adsorption and can present transparent and full color effects, but also significantly improves the anticorrosion and weathering properties of the film layer.

Manganese

Manganese is mainly used in 3000 series aluminum-manganese alloy, the core role is to refine the internal grain structure of aluminum. In the anodic oxidation process, a stable grain structure can guarantee the uniform growth of the oxide film. After oxidation, the surface of manganese-containing aluminum is smooth and clean, with even and soft color tone, suitable for all kinds of general industrial anodic oxidation scenarios.

Silicon

Silicon cannot be oxidized simultaneously with aluminum, but remains in the oxide film in the form of single particles, forming a large number of black inclusions and shading structures. This will directly lead to the finished product surface darkening, dye adsorption ability becomes poor, can not achieve bright and uniform color effect, so high silicon aluminum is basically not suitable for decorative anodic oxidation, can only be used for functional protective oxidation without appearance requirements.

Copper

During the electrochemical oxidation process, the copper phase inside the aluminum material cannot be oxidized stably, and it will continue to dissolve and re-precipitate on the surface of the film layer, completely destroying the dense structure of the aluminum oxide film. The final formed film layer is loose and soft, and the finished product is very easy to appear overall yellowing and other defects, and at the same time, it greatly reduces the corrosion resistance of aluminum, and is prone to pitting, which is only suitable for simple thin oxidation protection treatment.

Zinc

Zinc will change the electrochemical potential of aluminum, resulting in pore distribution disorder of the oxide film, and the finished product generally shows a dull blue-gray tone, accompanied by local mottling and other color problems. Especially in the hard thick film oxidation process, the film layer internal stress imbalance, very easy to crack and other process defects, can not meet the appearance of decorative oxidation requirements.

Iron

When iron is unevenly distributed, it will disturb the local oxidation reaction rate, resulting in irregular flow marks and other appearance defects on the surface of the finished product, and destroying the overall color uniformity and finish. In order to ensure the quality of anodized products, high-end decorative aluminum will strictly control the iron impurity content.

Aluminum Alloy Anodizing Precise Selection Scheme

Decorative Color Oxidation

Core requirements: smooth and flawless surface, even and bright color, not easy to fade and darken in long-term use, high requirements on aluminum oxidation transparency and color stability.

Applicable grades: 6063, 5005, 1100, 6463

Applicable scenes: electronic product shells, precision signs and other parts with strict requirements on appearance and texture.

Hard Anodizing

Hard anodized main thick film layer, high hardness, strong wear resistance, suitable for high-frequency friction, pressure impact of the harsh working conditions, the requirements of aluminum molding of the oxide film dense and hard, strong adhesion, not easy to crack and fall off.

Applicable grades: 6061, 6082, process-controlled 7075.

Applicable scenes: automation wear parts, firearms parts and other high-strength, high wear-resistant industrial parts.

Outdoor Building Oxidation

The core demand is excellent anti-ultraviolet ray, anti-aging, anti-weather erosion performance, and large area construction needs to protect the overall color unity, long-term color change without fading.

Applicable grades: 6063, 5005

Applicable scenes: building doors and windows, glass curtain walls, open-air landscape aluminum components and other long-term outdoor use of construction aluminum.

Oxidation of Industrial Structural Profiles

The requirements of aluminum is not easy to deform, oxidation defects, high yield, suitable for batch standardized production and long-term stable use.

Applicable grades: 6063, 6061, 6082

Applicable scenarios: industrial assembly line frames, general industrial load-bearing aluminum profiles and sheet metal structural components.

Marine Corrosive Environment Oxidation

Need to use magnesium content, passivation and anticorrosion ability of 5000 series alloy, its oxide film density and salt spray resistance is far more than conventional alloys, can withstand harsh corrosive environments for a long time.

Applicable grades: 5005, 5052, 5083

Scenario: Ship fittings, aluminum components in wet and corrosive chemical environments.

Factors to Consider When Choosing an Alloy for Anodizing

Aluminum alloy anodic oxidation selection does not need to be complicated to determine, just combine the appearance requirements, mechanical strength, use of the environment and processing technology of the four core dimensions of matching materials, you can accurately avoid oxidation defects, taking into account the effect of the finished product and production stability.

Appearance Requirements

In pursuit of high quality appearance, priority is given to 1000 pure aluminum, 6063, 5005, conventional texture, 6063 alloy stability is optimal, the strongest suitability; functional workpieces with no appearance requirements can be selected from the 2000, 7000 series and ordinary cast aluminum.

Mechanical Properties

6063 is preferred for general lightweight components, taking into account the formability and basic strength; 6061 and 6082 are suitable for industrial high-strength structural components with strong structural stability; 7075 can be chosen for ultra-high-strength special accessories; lightweight decorative parts are preferred to 1000 and 3000 series.

Usage Environment

All high-quality oxidized aluminum alloys can be used for indoor routine working conditions; 6063 and 5005 are preferred for outdoor open-air scenes; 5000 series are preferred for coastal and high salt spray humid environments; and high-copper and high-silicon alloys need to be avoided in acidic and alkaline corrosive conditions, so as to prevent the film layer from being broken and the base material from being corroded.

Processing Technology

Complex thin-walled extruded profiles are suitable for plasticity 6063; CNC precision machining components preferred 6061, 6082; need to be welded fittings priority 6000, 5000 series; die-casting aluminum parts can only be used for basic functional protection oxidation.

Common Anodizing Defects and Their Alloy-Related Causes

Most of the common defects of aluminum alloy anodic oxidation are rooted in the uneven composition of the base material alloy, interference of special elements, and improper control of the matching process.

Mottled Black Surface

Core cause: mainly caused by poor quality alloy base material, 2000 series of high copper, 7000 series of high zinc and high silicon cast aluminum alloy elements distribution disorder.

Improvement plan: Decorative parts priority replacement of 6063, 5005 and other oxidation stability alloy; unified batch of raw materials, to avoid mixing production, while strengthening the pre-expurgation, etching pretreatment.

Surface Pitting

Core causes: high copper aluminum alloy in the copper phase is easy to prioritize the dissolution, destroying the integrity of the film layer; cast aluminum comes with internal pores and a large number of impurities, coupled with the aluminum iron content exceeds the standard, will be formed in the oxidation process of pinholes, pitting defects.

Improvement plan: choose high purity and high quality aluminum; increase the processing of dust removal, multiple pure water cleaning process.

Uneven Coloring

Core causes: cast aluminum, silicon monomers in high silicon alloy will block the microporous oxide film, high copper, high zinc alloy will destroy the microporous regular structure.

Improvement plan: Choose magnesium-silicon and pure aluminum alloys with excellent coloring performance; accurately regulate the oxidation process parameters to avoid excessive etching damage to the pore structure of the film layer, and to ensure the uniformity of dye adsorption.

Oxide Film Cracking

Core cause: mostly seen in 7000 series aluminum-zinc alloy, the special electrochemical properties of the alloy, hard thick film oxidation of excessive internal stress, coupled with uneven conductivity of the substrate.

Improvement plan: give priority to 6000 series stable alloy for hard oxidation; strictly control the low temperature environment and current voltage when processing 7000 series, reduce the current density and relieve the internal stress of the film layer.

Surface Water Flow Marks

Core causes: aluminum substrate impurity segregation, or pre-heat treatment process is not standard, resulting in inconsistent oxidation reaction rate, and ultimately the formation of regular stripes and irregular flow marks.

Improvement plan: Choose high quality aluminum billet, unified base material heat treatment and extrusion process parameters; reasonable optimization of etching time, balanced surface reaction rate, eliminating grain defects.

Tips for Achieving Better Anodizing Results

Base Material Selection

Decorative appearance parts give priority to 6063, 5005, 1100; industrial load-bearing structural parts suitable for 6061, 6082; outdoor, corrosive environments preferred 5000 series to enhance weather resistance; high-strength non-appearance functional parts can be used with caution 7000 series. At the same time, the same project needs to unify the alloy grade and heat treatment status, and resolutely avoid high silicon cast aluminum to do colorful decorative oxidation.

Strictly Control the Surface Pretreatment

Pretreatment directly determines the oxidation yield rate, need to follow a standardized process: degreasing cleaning, alkaline etching, ash removal and neutralization, pure water multi-stage washing. For high-end glossy appearance parts, chemical polishing process can be added to further improve the surface finish and color uniformity, laying a good foundation for high-quality oxide film molding.

Stabilization of Core Process Parameters

The oxidation process requires precise temperature control, flow control and pressure control to ensure process stability. Conventional type II sulfuric acid oxidation, suitable temperature 18-22 ℃, current density 1-2A / dm²; type III hard oxidation needs to maintain a low temperature environment of 0-5 ℃, uniform voltage control, to avoid local ablation of the workpiece. At the same time to maintain a stable electrolyte concentration, continuous cycle filtration, to avoid the temperature difference in the tank, concentration difference caused by various types of process defects.

Unified Molding and Heat Treatment Standards

It is necessary to fix the extrusion temperature, speed and cooling parameters in production, and unify the T5 and T6 heat treatment status of the workpiece to ensure the uniformity of the base material’s organization and structure, so as to reduce the oxidation appearance defects from the process end.

Sample in Advance

The oxidation suitability of different alloys varies greatly, new products must be carried out before mass production of small batch test samples, according to the characteristics of the base material fine-tuning process parameters. Special high-strength alloys and modified aluminum need to be adapted to the process in advance, strictly control the key processes of sealing and quality inspection, and rely on mature processing technology to ensure the stability and consistency of the finished product.

Conclusion

From a comprehensive point of view, 6000 series, 5000 series, 1000 series and 3000 series are the high-quality choices for anodic oxidation, which are suitable for most of the industrial, decorative and outdoor scenes; while 2000 series, 7000 series and high-silicon casting aluminum alloy are only suitable for functional oxidation.

In actual production, only according to the appearance needs, strength requirements, the use of the environment and accurate selection of alloy, with standardized pretreatment and oxidation process, you can completely output of stable, beautiful, durable anodic oxidation of aluminum products.