Electrolytic painting of anodized aluminum
Variety of color anodizing aluminum - aluminum electrolytic coloration - a two-stage process, in which the pores of the pre anodized aluminum, located in electrolyte on the basis of a metal salt, via the electric current of the precipitated particles of metal. Most often, the inorganic salt is used tin-based, cobalt, nickel and copper. Below is a description of an electrolytic coloring process based on a tin salt, namely, tin sulfate.
The electrolyte is based on stannous sulfate
Electrolyte for electro-painting of aluminum based on tin sulfate has the following composition:
- 14-18 grams per liter of tin sulfate;
- 15-20 grams per liter of free sulfuric acid;
- organic and inorganic additives.
Colors, which can be produced with this electrolyte, They are a series of light and dark bronze to black until. The color depends on the duration of treatment - from 30 seconds to 20 minutes.
Equipment for color anodizing of aluminum
The electric equipment includes an electric current converter with programmable voltage increasing from 0 to 18 V within 25-40 seconds. At the end of each working cycle the current source is automatically disconnected.
Figure 1 - an electric current converter
the electrolyte temperature - bath temperature - can be from 15 to 25 degrees Celcius, but it is usually about the nominal value thermoregulatory 20 degrees Celcius. For this bath elektrookrashivaniya connected to a small heat exchange unit. Using a special pump electrolyte bath is continuously circulated through the heat exchanger and the filter.
Figure 2 - Heat exchanger, Pump and filter
The color intensity depends on the duration of supply of electric current to the bath. Modern equipment for elektrookrashivaniya aluminum uses cycles of AC and DC. This equipment is more difficult to manage, but compared with a conventional rectifier provides higher quality color anodizing. This is achieved due to more effective "pushing in" the metal into the pores of the anodized aluminum and reduce the shadow effect between adjacent aluminum profiles, that is characteristic for a conventional electrolytic colored aluminum anodizing.
The central electrode is often used to increase performance eletrookrashivaniya - center cathode, wherein, often as an electrolytic bath coloring, and the anodizing bath prior colorless.
Figure 3 - Cathodes anodizing bath
Figure 4 - The cathodes of the electrolytic bath dyeing with a central cathode
Parameters aluminum elektrookrashivaniya
Differences in thickness anode layer in different profiles of one sample does not significantly affect the resulting color, as is usually the case with adsorption staining. The thing is, that in the adsorption staining just penetrates into the pores of the entrance into the pores, and the electrolytic - first settles to the bottom of the anode pores and then moves to its entry.
Figure 5 - absorption and difference of electrolytic coloring of aluminum
The following factors are important to achieve good color repeatability:
- constant chemical composition of the bath;
- constant temperature;
- constant number of profiles, which are subject to elektrookrashivaniyu.
obviously, that differences in the chemical composition of the bath in excess of certain limits will give different coloring results.
Similar, temperature differences, by which the electric conductivity of the electrolyte depends, may lead to differences in color, even, If the treatment time was the same.
The number of profiles in the bath is also very important. If, for example, in the bath load only one aluminum profile and black, supposing, achieved 5 minutes. Then, if the same bath download, let us say, 30 same profiles, the black color on them will be through 15 minutes. Therefore staining length within a certain range depending on the number of profiles, have been added to the electrolytic bath coloring.
Therefore, it is recommended to load samples with the same surface area (and the same current density) into the bath., to eliminate the influence on the quantity of material parameters staining.
For each bath is substantial experience to determine the duration of dyeing at different operating parameters of the processing.
If the color is too bright as a result of processing, then it can be made darker by increasing the duration of the staining.
If the color is too dark, it can be "lighten" by exposing the sample to elektrookrashivaniya profiles in the bath for some time - from 30 seconds to 1 minutes - when electric current is switched off.
Bath electrolytic coloring filter must be periodically - at least once a day, to remove the insoluble materials, which are formed during the decomposition of tin sulfate (see. Figure 2). Slight movement of the electrolyte in the bath does not adversely affect the quality of staining.
Fast extraction painted profiles
Painted aluminum profiles need to be removed from the bath without delay, to avoid, more pale profile sections will become even paler in the part of the sample, which is the longest in the bath during the sample recovery.
In the electrolytic coloring bath electrodes - cathode - operate normally made of stainless steel AISI 316 in the form of plates or round pipes (see. Figure 4).
The surface of the cathodes of the electrodes need to be cleaned periodically and monitor the reliability of the electrical contacts. The current density in the electrolytic coloring bath should be below 0,8 amperes per square decimeter.
The electrolyte level in the working baths
The level of electrolyte in the anodizing bath to be always higher, than the level of the electrolyte in the electrolytic coloring bath. This is to ensure, to batches of aluminum rod, which are immersed in an electrolyte bath electrolytic coloring, proanodirovannymi always been at the previous stage of the working cycle. Otherwise the hydrogen will be allocated for the electrolytic coloring of aluminum neanodirovannom, which will lead to defects staining.
Bath life time based on tin sulfate
Electrolytic coloring bath - except in cases of accident or pollution - is a "long-playing" bathroom. In general, it does not require periodic replacement of full reset and. Normal bath of tin sulfate may work for several years on the condition of its periodic filtration and addition working preparation for filling consumables. It should be taken into account, based electrolyte that tin sulfate has a natural tendency to lose its efficiency at spontaneous transition of the ions to stannous sulfate II IV stannous sulfate.
Features hanging profiles
It should be noted, that titanium sample or material can not be used in the electrolytic coloring.
Mounting profiles for electrolytic coloring should be done carefully. If any profiles have shifted after anodizing, the dielectric layer of the anode in contact with the profile of the rod will not provide the desired electrical contact and, Consequently, electrolytic coloring.
The amount of aluminum profiles, which is hinged to an electrolytic coloring, usually less, than other methods of dyeing. This is due to the fact, that in order to achieve a uniform color throughout the hitch profiles should be placed on it so, so they do not shade each other from the cathodes.
Electrolytic coloring through the use of low current density is almost athermal reaction. Therefore, the temperature of the bath is not subject to any noticeable changes. However, if during the day there is a large number of electrolytic staining profiles, the bath should be provided with a corresponding cooling system. In the electrolytic coloring solution is mixed with air, as it exerts a harmful influence. The air promotes precipitation of sludge in the bath as stannic oxide, which is formed during the decay of stannous sulfate.
Source: Company materials Monti Engineering (Italy)