Aluminum anodizing by QUALANOD: Part 1
Below is an overview of the basic requirements of QUALANOD for manufacturers of anodized aluminum products by technology aluminum anodizing. To put these requirements into practice, you need to refer to the latest edition of the QUALANOD Specifications document at www.qualanod.net.
1. General requirements
1.1. Anodic oxide coating manufacturers (hereinafter – anodizers) must follow the instructions of the chemical suppliers for the anodizing and dyeing process.
1.2. Before and after anodizing, aluminum products must be stored outside the anodizing line.. After anodizing, the product must be protected against condensation of water on its surface., as well as dust and other contaminants. Each anodized product must be marked with an indication of the class of coating thickness..
2. Rinses after working baths
2.1. After each stage of processing (surface preparation, anodizing, staining) must be performed, at least, one flushing.
2.2. Some processing steps require several washes.. it, in particular, concerns anodizing. Since the first wash is usually very acidic, then before staining or filling it is necessary to perform a second wash.
2.3. Anodized products should never be left in acid wash for more than 1-2 min. On products, which are left for some time in an acid wash, usually visible signs of corrosion of the anode coating.
3. alkaline etching
3.1. The anodizer must strictly follow the instructions of the aluminum etching chemical supplier..
3.2. It is necessary to control within a narrow range the concentration of free sodium hydroxide, aluminum and various additives, as well as temperature. The chemical composition of the alkaline solution must be constantly updated using a special “crystallizer” – alkaline solution cone. It is also possible to use the so-called “eternal” alkaline etching bath. In such a bath, the incoming and outgoing mass flows of chemicals and materials are approximately equal and the bath has an approximately constant composition.
3.3. During alkaline etching aluminum is vented at an approximately constant rate, and the gloss reduction rate of the treated surface is gradually slowing down. Therefore, after a certain etching thickness, a further decrease in surface gloss no longer occurs.. Each anodizer must determine the optimum etching time interval for their particular conditions..
4.1. Typical technology is aluminum anodizing in sulfuric acid electrolyte without additives or with additives of oxalic acid.
4.2. Sulfate Electrolytes
Concentration free sulfuric acid: no more 200 g / l, deviation from the set value ± 10 g / l.
Aluminum content: no more 20 g / l, preferably – from 5 to 15 g / l.
Chloride content: no more 100 mg / l.
Acid concentration is critical only at high anodizing temperatures.. A high acid concentration reduces the required anode voltage (approx. 0,04 In the 1 g / l H2SO4), but also leads to increased acid removal and to an increase in its consumption. Low aluminum content increases coating sensitivity to elevated bath temperature. Higher aluminum content, the higher the voltage required for anodizing (about 0,2 In the 1 g / l aluminum). The presence of chlorides in the anode electrolyte can lead to pitting corrosion during anodizing, and also adversely affects the resistance of the coating to climatic influences.
bath temperature should be kept in the range of ± 1,5 ºС from the set value irrespective of the size of the charge. The temperature difference of the electrolyte near the surface of the product should be no more than 2 oC. Bath temperature for coating thickness classes 5-10 microns should be no higher 21 oC, and for classes 15-20-25 μm - not higher 20 oC.
4.3. Oxalic acid sulfate electrolytes
Concentration free H2SO4 should be no more 200 g / l, deviation from the set value ± 10 g / l.
Oxalic acid concentration should be no less 7 g / l. Higher concentration 10 g / l has almost no effect on the process. Concentration 5 g / l is too low, to influence the quality of the anode coating. Oxalic acid concentration more 15 g / l has no positive effect, but increases production costs.
Aluminum content: no more 20 g / l, preferably from 5 to 15 g / l.
4.4. Current Density for Anodizing Aluminum
For sulfate anodizing, the average current density should be:
1,2 – 2,0 A / EP2 for classes 5 and 10 m
1,4 – 2,0 A / EP2 for class 15 m
1,5 – 2,0 A / EP2 for classes 20 and 25 m.
The use of low current density to obtain a thick coating (grades 20 and 25 μm) is risky for coating quality. High current density requires good contact and good electrolyte mixing, but has fewer quality issues.
4.5. Cathode Area
The ratio of the area of the working surfaces of the cathode to the anode should be in the range from 1 : 1,5 to 1 : 2,5.
Aluminum cathodes recommended. At the cathodes, bathtubs, only one side is taken into account, at the central cathodes - both sides.
A high ratio of cathode area to anode area can lead to problems with uniform coating thickness. Aluminum electrodes require the lowest operating voltage.
The distance between the cathode and the anode must be at least 150 mm.
4.6. Product transfer after anodizing
After completing the anodizing cycle, the products should be transferred from the anodizing bath to the washing bath as soon as possible.. Products must never remain in the anodizing bath without current. This is one of the reasons., which can lead to corrosion of the anode coating and a decrease in its quality.