9 minute read
ALUMINIUM ANODIZING
Chemetall’s Anodizing Technology
From architecture to construction to packaging to automotive – there are almost no limits to the possible industrial applications of aluminum. The processing of this light metal also leaves room for new ideas. Due to the growing competitive pressure, companies are looking for cutting-edge ways to stay ahead of their global competitors with innovative and cost-saving technologies. The solutions to this are closer than you may think: “standard technologies” offer opportunities for saving on costs without compromising on high quality.
Finding potential for improvement in established and proven technologies and processes may not seem worthwhile at first glance. In fact, however, a closer examination and evaluation of the individual process steps reveals possibilities which not only reduce costs but also boost productivity. Chemetall’s anodizing technology demonstrates how this can be achieved.
The anodizing technology
Anodizing is much more than just an optical enhancement of aluminum. The anodized layer that forms using standard technologies reaches at about two thirds into the treated aluminum part and grows at about one third on its surface, thereby affording an attractive, smooth surface and longterm corrosion protection. In addition to these excellent properties, it is also easy to process, making anodized aluminum extremely versatile and suitable for a wide range of applications. A reevaluation of the anodizing process and of the technologies commonly used in the industry is worth the effort since modified processes have proven to be very good alternatives in practice. This paper presents ideas for optimizing the anodizing process. Experiences and case studies from customer applications around the world have demonstrated the successful implementation of these new solutions in practice. However, it should be decided on a case-bycase basis which technologies are best suited for the respective application, as the applicability should be evaluated individually for each application. In this context, a particular focus is placed on the possibilities of saving energy or increasing productivity by reducing the time of the anodizing process and also in the final sealing step.
Efficiency improvements and costs
Over the years, almost any anodizer has tried to enhance his productivity by increasing the total number of workpieces to be anodized per day. Frequently, batches are simply increased in size without adjusting other important factors such as cooling. However, an increase of the batch size in many cases also means a drop in current density, which is crucial for surface quality. Yet, the process temperature, current density and anodizing voltage have a great impact on quality, productivity and costs. As a general rule, higher temperatures result in a larger pore diameter, which in turn leads to less aluminum oxide and thus a softer anodic coating. The same effect can usually be observed at lower current density or (too) high acid concentrations. Coatings with larger pores are generally easier to dye, but in many cases show poorer sealing and corrosion resistance and are generally less durable because the anodic coating may become too soft. Conversely, lower temperatures and/or higher current density usually result in smaller pores that are more durable and thus of better quality. So how can efficiency be improved while at the same time reducing the direct costs of anodizing? Is there a way to speed up the anodizing process to increase productivity, but without compromising the structure and quality of the anodic coating? Theoretically, it is easy to raise productivity by increasing the current density provided that sufficient rectifier capacity is available. An (excessively) high current density and/or the higher anodizing voltage needed to this effect, would generate a lot of heat that can lead to soft coatings and burns. Additional cooling would thus be necessary.
Additives from Chemetall Nowadays, it is possible to work with modern additives to the sulphuric acid in the anodizing bath. To this effect, the Surface Treatment global business unit of BASF’s Coatings division, operating under the Chemetall brand, offers innovative additives from its Gardobond range. As a further development of the well-known oxalic acid, it reduces the redissolution rate in the anodizing process and increases the tolerance to dissolved aluminum. This allows for anodizing at a higher temperature, a higher aluminum content and a higher current density Additives from Chemetall at unchanged voltage and/or at a reduced anodizing Nowadays, it is possible to work with modern additives to the voltage with the same current density. sulphuric acid in the anodizing bath. To this effect, the Surface Treatment global business unit of BASF’s Coatings division, operating Chemetall offers a new generation of highly efficient under the Chemetall brand, offers innovative additives from its additives. Added to the anodizing bath, they afford a Gardobond range. very consistent surface and reduce the aluminum and As a further development of the well-known oxalic acid, it reduces the temperature sensitivity of the anodizing process. redissolution rate in the anodizing process and increases the tolerance to dissolved aluminum. This allows for anodizing at a higher Gardobond® Additive H 7526 thus yields coatings of temperature, a perfect quality with a high aluminum content (max. 30 g/l), higher aluminum content and a higher current density at unchanged at higher temperatures (max. 30°C) while at the same voltage and/or at a reduced anodizing voltage with the same current time requiring a lower sulphuric acid concentration. density. Chemetall offers a new generation of highly efficient additives. Added to the anodizing bath, they afford a very consistent surface and
Chemetall’s Anodizing Technology.
reduce the aluminum and temperature sensitivity of the anodizing process. Gardobond® Additive H 7526 thus yields coatings of perfect quality with a high aluminum content (max. 30 g/l), at higher temperatures (max. 30°C) while at the same time requiring a lower sulphuric acid concentration.
Temperature factor
A higher treatment temperature means that the anodizing voltage can be reduced to achieve a specific current density (e.g., for 1.5 A/dm² the following was observed in a reference lab test: 19°C - 18 V, 25°C - 15,5 V, 30°C - 14 V). This already provides a possibility for significant energy savings. Moreover, basically the same amount of cooling energy will be needed. Thus, the use of an adequate anodizing additive will allow for energy savings of ca. 40%. Further potential effects can also be achieved with regard to the speed of the anodizing process. The following applies for the growth of the layer thickness during anodizing with standard parameters, for example those for architectural anodizing: the thicker the layer, the slower its growth until it stops growing because layer growth and acid attack have come to equilibrium. The empirical formula to calculate this is: thickness (μm) = 0.3 x current density (A/dm²) x time (minutes) For the standard 1.5 A/dm², practical experience has produced the following approximate values: 10 μm anodizing layer = approx. 20 - 22 minutes, 20 μm anodizing layer = approx. 40 - 44 minutes. This calculation formula clearly shows the effect a higher current density can have on productivity enhancement (due to shorter treating time).
Gardobond® Additive H 7526 19°C without additive 25°C without additive 25°C with Gardobond® Additive H 7526 Positive side benefits Another possible “side benefit” are less frequent bath Gardobond® Additive H 7526 can thus significantly renewals. By increasing the maximum tolerance value improve the surface quality. The chart below shows for aluminum in the anodizing bath, bath renewals need the options for improving the surface quality taking to be conducted less frequently, thereby reducing the the example of the loss of mass as per ISO 3210 in an volume of acidic effluents and the volume of fresh water anodizing process with and without additive: required for new bath make-up. Furthermore, a reduced consumption of sulphuric acid is possible, also combined with a reduction of the waste water flow due to reduced acid carryover into the rinsing baths. The most important technical effect of the modified anodizing process, however, lies in the significant improvement of the surface and sealing quality. Higher anodizing temperatures also mean higher layer dissolution caused by acid attack, leading to an increase Loss of mass measurement [mg/dm²] in anodic pore diameter while the oxide content in the layer decreases which causes the overall surface quality to drop considerably. This in turn affects the sealing quality as well as the abrasion and corrosion resistance. The effect can be countered by using the appropriate additive. The pictures show the surface structure at different temperatures and aluminum contents: with Gardobond® Additive H 7526, the same pore sizes can be achieved at 25°C as in a bath at 19°C without any additive.
120 100 80 60 40 20 0 5 g/l / 5 g/l / 5 g/l / 15 g/l / 15 g/l / 15 g/l / 25 g/l / 25 g/l / 25 g/l / 18°C 25°C 30°C 18°C 25°C 30°C 18°C 25°C 30°C AI content / temperature
Gardobond® Additive H 7526 Positive side benefits
Another possible “side benefit” are less frequent bath renewals. By increasing the maximum tolerance value for aluminum in the anodizing bath, bath renewals need to be conducted less frequently, thereby reducing the volume of acidic effluents and the volume of fresh water required for new bath make-up. Furthermore, a reduced consumption of sulphuric acid is possible, also combined with a reduction of the waste water flow due to reduced acid carryover into the rinsing baths. The most important technical effect of the modified anodizing process, however, lies in the significant improvement of the surface and sealing quality. Higher anodizing temperatures also mean higher layer dissolution caused by acid attack, leading to an increase in anodic pore diameter while the oxide content in the layer decreases which causes the overall surface quality to drop considerably. This in turn affects the sealing quality as well as the abrasion and corrosion resistance. The effect can be countered by using the appropriate additive. The pictures show the surface structure at different temperatures and aluminum contents: with Gardobond® Additive H 7526, the same pore sizes can be achieved at 25°C as in a bath at 19°C without any additive. Gardobond® Additive H 7526 can thus significantly improve the surface quality. The chart below shows the options for improving the surface quality taking the example of the loss of mass as per ISO 3210 in an anodizing process with and without additive:
19°C without additive 25°C without additive 25°C with Gardobond® Additive H 7526 Gardobond® Additive without additive
Possible benefits with Gardobond® Additive H 7526:Possible benefits with Gardobond® Additive H 7526:
Lower overall process costs for energy Lower overall process costs for energy (power and cooling) (power and cooling)
Filter cake disposal: less sulphuric acid Filter cake disposal: less sulphuric acid in the effluent in the effluent Productivity: reduced anodizing times
Productivity: reduced anodizing times Anodization with the technology developed by Chemetall does not only improve the treatment processes, but also the characteristics and benefits of the aluminum proper. As a tried and tested technology supplier, Chemetall develops and optimizes customized technologies for all anodization applications – worldwide. Contact and further information:
Anodization with the technology developed by Chemetall does not only improve the treatment processes, but also the characteristics and benefits of the aluminum proper. As a tried and tested technology supplier, Chemetall develops and optimizes customized technologies for all anodization applications – worldwide.
Gary Mothersole UK Sales Manager Aluminium Finishing Telephone: 07825 319214 Email: gary.mothersole@basf.com Visit: www.chemetall.com
