- Part 6 - An effective method for functional surface treatment / Continued from Galvanotechnik 3/2024
Aluminum is a versatile metal with high strength, lightness and corrosion resistance. Nevertheless, the metal can corrode under certain conditions, which can lead to a deterioration of its mechanical properties and aesthetics. Chemical passivation or chromating is a proven method of improving the corrosion resistance of aluminum while creating a surface that is functional in several respects. In the last part of the aluminum series, the basics of chromium(III)-based chemical passivation of aluminum as well as its applications and advantages are presented.
The process of chromating or passivating gives aluminum a uniform surface and significantly improves its properties: it forms a corrosion-resistant, solderable and conductive layer with low abrasion resistance. In the past, there have been many efforts by various manufacturers to promote chromium(VI)-free chromating. As hexavalent chromium is classified as carcinogenic and toxic and, according to the REACh Regulation (Regulation (EC) No. 1907/2006 concerning the Registration, Evaluation, Authorization and Restriction of Chemicals), only a limited period of use remains, new possibilities had to be created to replace chromium(VI) chromate coatings with an almost equivalent chemistry. After many years of research and development to find an equivalent replacement, chromium(III)-valent passivations, such as SurTec 650, finally prevailed.
Fig. 1: Aluminum sheet passivated with SurTec 650. Large area with SurTech 650, small area without SurTec 650
Chromium(III) passivation, also known as Trivalent chromating, is a form of surface treatment of aluminum based on the use of chromium(III) compounds. Chemical passivation is a process in which the surface of a metal is transformed into a less reactive form in order to increase its corrosion resistance. In the case of aluminum, this process takes place by forming a thin, nanometer-scale protective layer on the surface.
Process sequence Surface preparation
First, the aluminum components to be treated are thoroughly cleaned to remove any impurities, grease films or oxide layers. This is done using various cleaning stages such as degreasing, pickling and activation. The rinsing steps between the individual process stages must be strictly adhered to. Degreasing is usually carried out in an alkaline aqueous surfactant solution. For further steps such as pickling or activation, it is advisable to use the process chemicals specified by the manufacturer for the passivation solution.
Passivation / chromating Cr-III-based
A mixture of chromium(III) compounds and other chemical components is used. This solution is often supplied as a concentrate and must be diluted according to the manufacturer's instructions.
The cleaned metal parts are immersed in the passivation solution or wetted with the solution by spraying or other methods. The duration of the process varies depending on the specific requirements and the desired result. During the passivation process, the chromium(III) compounds react with the aluminum surface and form a thin protective layer of chromium(III) compounds.
Rinsing
After the required process time has elapsed in the passivation solution, the parts are thoroughly rinsed with water to remove excess chemicals.
Drying
The treated parts are carefully dried to remove water residues and prevent the formation of water stains. This can be done by air drying or using drying ovens.
Post-treatment (optional)
In some cases, additional surface treatment may be required to further improve certain properties or to meet specific requirements. This may include the application of a sealer or a further protective coating.
Properties of chromium(III) passivation Optics
The protective layer formed is usually very thin (nanometer range) and transparent, which means that it does not significantly change the surface of the aluminum. The conversion layers are visually perceived as slightly iridescent (red-yellow-blue) (Fig. 1).
Corrosion resistance
Chromium(III) passivation offers good corrosion resistance and improves the service life of aluminum components in various environments. The layer formed provides effective protection against corrosion and contributes to extending the service life of aluminum components (Fig. 2).
Electrical conductivity
The coating is electrically conductive, which makes it suitable for applications in the electronics industry. The properties of corrosion protection and electrical conductivity are combined here. The conductivity of these coatings depends on various factors, including the density, crystal structure and chemical composition of the coating. The electrical conductivity of passivation layers can vary depending on the manufacturing process, the thickness of the layer and other process parameters. A thinner layer can generally have a higher conductivity than a thicker layer. Furthermore, additives or additives in the passivation solution can influence the conductivity of the layer.
Fig. 2: Upper half uncoated, lower half SurTec 650 passivated. After 168 hours NSS (neutral salt spray test, ASTM B-117 or DIN EN ISO 9227)
Bonding primer
Chromium(III) passivation can also be used as a primer for subsequent paintwork or coatings, as it improves adhesion. They also provide additional corrosion protection. The adhesion of paints, adhesives and sealants is optimized. Chromium(III) passivations form a thin protective layer of chromium(III) compounds on the surface of the aluminum. This layer significantly improves the adhesion of subsequent coatings, such as paints or powder coatings. Chromium(III) passivation serves as an effective method of preparing the surface by creating an even and clean protective layer. This layer helps to remove impurities, oxides and other contaminants from the surface and creates an ideal base for the subsequent coating.
By forming a chemical bond between the passivation layer and the coating applied to it, a robust bond is created. Chromium(III) passivations are compatible with a wide range of coating materials, including paints, powder coatings and other coating types. This allows them to be used in a variety of industries and applications, regardless of the specific coating requirements. Using chromium(III) passivations as an adhesion primer not only improves the adhesion of the coating, but also the overall quality of the coating. Good adhesion means that the coating adheres evenly to the surface, resulting in improved resistance to peeling, cracking and other damage. This helps to improve the aesthetic and functional properties of the coated aluminum parts.
Environment
Compared to the use of chromium(VI) compounds, chromium(III) passivations are environmentally friendly and much less harmful to human health and the environment. Of course, as with most chemical substances, certain safety measures must be observed, which can be found in the relevant data sheets.
Areas of application
Chromium(III) passivation can be used in various industries and applications. They are frequently used in the automotive, aerospace, construction and electronics industries.
Automotive
Chromium(III) passivations are used in the automotive industry, particularly for components such as car bodies, wheel rims and engine components. Passivated surfaces are also frequently used in the field of electric motors. The properties of corrosion resistance, electrical conductivity or adhesion for subsequent coatings are decisive here.
Aviation
In the aerospace industry, the properties of electrical conductivity, corrosion resistance and low influence on flexural fatigue strength are utilized. For structural components, it is generally necessary for the surface finish to have no negative influence on the alternating bending loads. For this reason, either conversion coatings or very thin oxide layers are used as an adhesive base for subsequent painting or powder coating. The electrical conductivity is used to generate the necessary electrical paths on the missile to dissipate electrostatic charges, for example.
The chromium(III) passivation SurTec 650, for example, fulfills the corrosion protection requirements according to MIL DTL 5541F and MIL-DTL-81706B.
Electrical industry
Chromium(III) passivation is also used in the electronics industry for components such as printed circuit boards and housings. Here, the functional properties of electrical conductivity, low surface resistance and good corrosion resistance are combined.
Combination layers
Conversion coatings are also ideal for the post-treatment of damaged anodized coatings. In certain areas, anodized coatings and passivations are combined. This allows the electrically insulating properties of the anodized layers and the conductive passivation layers to be combined. In most cases, the areas to be passivated on the component are masked, the parts are anodized, unmasked after the anodizing process and then the previously masked areas are passivated.
Conclusion
Overall, chromium(III) passivation offers an effective and economically viable solution for improving the performance and durability of aluminum products. Their versatility, combined with their superior protective properties and compatibility with various coating materials, makes them a preferred choice for companies looking to produce high-quality and sustainable products.
In industries such as automotive, aerospace, construction and many others where aluminum is widely used, protection against corrosion while maintaining electrical conductivity is crucial to ensure the reliability and functionality of components. For subsequent coatings, they provide an excellent bonding base that significantly improves the functional properties and aesthetics of components.
In the meantime, the majority of chromium(III)-containing passivations, such as SurTec 650, have replaced chromium(VI)-containing processes in many areas of application. Through years of intensive development of chromium(VI)-free alternative products, a real alternative has been created with the chromium(III) variants. This development marks an important step towards more sustainable and safer solutions in the field of functional surfaces for industry.
