[China Aluminum Industry Net] The purpose of removing ash after alkali etching is to remove the 'stain' remaining on the surface of the product after alkali erosion to obtain a bright and clean metal surface. These 'spots' are mainly formed by the deposition of elements such as silicon, iron, magnesium, and copper in aluminum alloys. It can be removed with nitric acid or sulfuric acid solution. In addition to ash and the role of neutralizing alkali, it can also be called neutralization and light.
(A) Nitric acid ash removal
Usually use 10% - 25% (volume fraction) of nitric acid at room temperature for immersed 1-3min for ash; also use 30% (volume fraction) of nitric acid; there are 25% - 50% (use The volume fraction of nitric acid was performed after chemical polishing. In nitric acid solution, when the concentration is about 30%, the corrosion rate of aluminum is greater. If the temperature of the solution increases, the corrosion rate increases.
Nitric acid ash removal process can meet the requirements of a variety of aluminum alloy materials. For aluminum alloys with high silicon content, nitric acid alone does not meet the requirements and fluoride needs to be added. In order to solve the hazard of releasing nitrogen oxides and acid mists due to nitric acid decomposition, some commercially available ash removal formulations use a combination of nitrates, which also contain persulfates and hydrogensulfates.
In order to improve the ash removal effect and enhance the light extraction efficiency, there are some special auxiliaries for sale, which are added to a 100-150 g/L nitric acid solution, and the light emission speed is fast, and the hard-to-remove aluminum alloy surface with high copper content can be removed. The black-grey layer generally contains hydrofluoric acid, hydrogen peroxide, corrosion inhibitors, sodium sulfate, and surfactants.
(B) Ash Removal with Sulfuric Acid
The content of sulfuric acid in the sulfur removal of sulfuric acid is approximately the same as the content of sulfuric acid in the anodization, and it is usually 15% to 25% (by volume) of sulfuric acid. For 6063 aluminum alloy building profiles can be more satisfactory ash removal effect, but other alloying components with higher aluminum alloy is not necessarily suitable, that is, 6063 aluminum alloy has to control the content of impurities. The operation temperature of sulfuric acid ash removal is room temperature, and the operation time is longer than that of nitric acid, generally 3 to 5 minutes.
When the concentration of aluminum in the sulfuric acid solution exceeds 40% (volume fraction), the corrosion rate rapidly increases, and the corrosion rate is approximately 85% (volume fraction).
Some commercially available ash removal additives based on sulfuric acid are mostly added with one or more additives such as oxidants.
(III) Practical Formulas and Processes
In addition to the above-mentioned nitric acid and sulfuric acid-based processes, the ash removal process also often uses ash-containing processes containing chromic acid, phosphoric acid, and fluoride, which are not separately described here. The collected ash removal processes and formulations are listed below. In the following table, the water must be washed after treatment.
In the table, process 2 is used for die-casting aluminum, and process 8 is used for high-silicon die-cast aluminum. The product Top Desmut S-10 is a non-nitric acid ash remover and has strong ash removal effect by adding sulfuric acid. Top Desmut N-10 has a very low nitric acid content and is particularly effective in removing ash from the surface of eroded aluminum.
Top ADD-320 liquid used, temperature ≤ 40 °C, time 15 - 60s, can evenly activate the cast aluminum surface, remove stains and hang ash. Top ADD-350 10 - 50g/L action is to restore TopADD-320 detergency. Top ADD-400 30mL/L can be exempted from corrosion after washing in the Top ADD-320 treated material. Specicalty 982 is a chromium-free neutralizer for the 6000 series aluminum; Specicalty 985 is a chromium-free neutralizer for the 2000 series aluminum.
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