Regardless of the method used to process aluminum materials and products, there will be dirt and defects on the surface to varying degrees, such as dust, metal oxides (natural or aluminum oxide films formed at high temperatures), residual oil, asphalt marks, artificial Handprints (mainly fatty acids and nitrogen-containing compounds), welding fluxes, and corrosive salts, metal burrs, minor scratches, etc. Therefore, before the oxidation treatment, the surface of the product is chemically and physically cleaned to expose the pure metal substrate so as to facilitate the oxidative coloring. Thus, the substrate is firmly bonded to the substrate, and the color and thickness are satisfactory. The best anti-corrosion, wear resistance, weather resistance and other good performance of artificial membrane.
(a) degreasing
The degreasing of aluminum and aluminum alloy surfaces includes organic solvent degreasing, surfactant degreasing, alkaline solution degreasing, acid solution degreasing, electrolytic degreasing, and emulsification degreasing. Several degreasing methods and major processes are listed in Table-1. Among these methods, degreasing with an alkaline solution, particularly a hot sodium hydroxide solution, is most effective.
Table-1 Degreasing and main processes
Degreasing method
Solution composition
Dosage g/L
Temperature/degree
Time min
Post-processing
Note
Organic solvents
Gasoline, carbon tetrachloride, trichloroethylene, etc.
Proper amount
Room temperature or steam
appropriate
No etching
Surfactant
Soap, synthetic detergent
Proper amount
Room temperature -80
appropriate.
Water cleaning
No etching
Alkaline solution
NaOH
50-200
40-80
0.5-3
After washing, use 100-500g/L nitric acid solution to neutralize and remove ash
Degreasing and corrosion remove natural oxidation, nitric acid can be replaced by dilute sulfuric acid + chromic acid
Sodium dodecanophosphate
NaOH
Sodium silicate
40-60
8-12
25-30
60-70
3-5
Water cleaning
NaOH can be replaced with 40-50g/L sodium carbonate, and the total alkalinity calculated as NaOH is 1.6%-2.5%
Sodium polyphosphate
Sodium carbonate
Sodium phosphate
Sodium borate monohydrate
Gluconic acid
Liquid wetting agent
15.6
4.8
4.8
4.8
0.3ml
0.1ml
60
12-15
Water cleaning
Stir before use for 4 hours
Sodium dodecanophosphate
Sodium silicate
Liquid soap
50-70
25-35
3-5
75-85
3-5
Water cleaning
Sodium carbonate
Sodium phosphate
25-40
25-40
75-85
appropriate
Water cleaning
Sodium phosphate
Sodium carbonate
NaOH
20
10
6
45-65
3-5
Water cleaning
Alkali resistance oil remover
40-60
70
5
Water cleaning
Degreasing the oil can prolong the processing time
Acidic solution
sulfuric acid
50-300
60-80
1-3
Water cleaning
Nitric acid
162-354
Normal temperature
3-5
Water cleaning
Loosening treatment
Phosphoric acid
sulfuric acid
Surfactant
30
7
5
50-60
5-6
Water cleaning
Phosphoric acid (85%)
Butanol
Isopropanol
water
100%
40%
30%
20%
Normal temperature
5-10
Water cleaning
Solution composition in volume
Electrolytic solution
Anodizing electrolyte
Normal temperature
appropriate
AC or cathode current electrolysis
NaOH
100-200
Normal temperature
0.5-3
Neutralization after washing with water
The aluminum product is a cathode with a current density of 4-8 A/dm2
Emulsification solution
paraffin
Triethanolamine
Oleic acid
Pine oil
water
8.0%
0.25%
0.5%
2.25%
89%
Normal temperature
appropriate
Water cleaning
Solution composition in volume
Organic solvents are degreased using the characteristics that fats and oils are easily soluble in organic solvents. Common solvents include gasoline, kerosene, ethanol, isoamyl acetate, acetone, carbon tetrachloride, and trichloroethylene. Organic solvents are only used for the degreasing treatment of small-sized or extremely dirty products. Surfactants are substances that can significantly reduce the surface tension of liquids at very low concentrations. Surfactants commonly used for degreasing include soaps, synthetic detergents, sodium lauryl sulfate, sodium dodecylbenzene sulphonate, and the like. Alkaline degreasing solution has many formulas. The traditional technology uses sodium phosphate, sodium hydroxide and sodium silicate. Among them, sodium phosphate and sodium silicate have corrosion inhibition, wetting and stabilization, and solution heating and stirring help to obtain the best. Degreasing effect. Fats and oils can also undergo hydrolysis in the presence of acid to produce glycerol and the corresponding higher fatty acids. Electrolytic degreasing can use anodic current, cathodic current or alternating current. The cathode current is degreasing in an alkaline solution, and the anode is preferably a nickel-plated steel plate. It is not commonly used in surface treatment of aluminum and aluminum alloys. The solution used for emulsification and degreasing is a two-phase or multi-phase solution composed of insoluble water and an organic solvent, and a detergent for reducing surface tension and having an affinity for each phase is added.
(B) The alkaline solution
An alkaline solution is a process of surface cleaning of an aluminum product with or without addition of other substances in a sodium hydroxide solution, which is also commonly referred to as alkali etching or caustic washing. Its role is as a product after certain degreasing methods degreasing after the supplementary treatment, in order to further clean up the surface of the attached oil sludge; remove the surface of the natural oxide film and minor scratches. Thus, the product is exposed to a pure metal matrix, which facilitates the formation of an anodic film and obtains a higher-quality film layer. In addition, by changing the composition of the solution, temperature, treatment time, and other operating conditions, it is possible to obtain a smooth or satin-like matte or lustrous etched surface in different states. The basic composition of the etching solution is sodium hydroxide, in addition to addition of regulators (NaF, sodium nitrate), scaling inhibitors, (gluconate, heptanoate, tartrate, acacia, dextrin, etc.), multivalent Chelating agents (polyphosphates), detergents. Alkali wash solution composition is often determined by the use of aluminum products Several alkaline wash methods are listed in Table-2.
Table-2 composition and process of alkaline etching solution
Solution composition
Dosage %
Temperature/degree
Time min
Note
NaOH
3.5-9
50-70
3-10
Corrosion amount 10-55g/m2, aluminum ion content >30-80g/L
NaOH
Sodium gluconate or sodium citrate
4.5-5.5
0.15
55±1
8-10
Corrosion amount 20μm, aluminum content 5-25g/L
NaOH
Sodium heptanoate
D
5
0.1
0.2
55-70
1-10
Usually used for cleaning
NaOH
NaF
Sodium polyphosphate
D
10
2
0.2
0.2
55-70
1-10
No surface etch
NaOH
Sodium nitrate
Sodium heptanoate
5
4
0.1
55-60
1-10
Smooth surface cleaning
NaOH
Sodium phosphate
3-7.5
5-10
Normal temperature
appropriate
Sodium fluoride added to a shiny white surface
(III) Neutralization and water cleaning
The gray or black ash attached to the surface of the aluminum product after etching is not dissolved in the cold or hot water washing, but it can be dissolved in the acidic solution, so the products washed by the hot alkali solution have to be removed in order to remove The ash and residual lye are soaked to expose the surface of the shiny base metal, which is called neutralization, gloss, or light treatment. The process is the product in the 300-400g/L nitric acid (1420kg/m3) solution, dipping at room temperature, the leaching time varies with the metal composition, the general immersion time 3-5 minutes. The ash on aluminum alloy products containing silicon or manganese can be treated with a 3:1 volume ratio of nitric acid and hydrofluoric acid at room temperature for 5-15 seconds. The neutralization treatment can also be performed at room temperature in a solution containing 300-400 g/L of nitric acid and 5-15 g/L of chromium oxide or a solution of chromium oxide 100 g/L plus sulfuric acid (1840 kg/m<3> ) in 10 ml/L. The purpose of water washing between processes is to thoroughly remove the residual liquid and water-soluble reaction products on the surface of the product, so as to prevent contamination of the next process bath and to ensure the treatment efficiency and quality. Most cleaning uses a cold water cleaning. However, the alkaline-etched products generally use double cleaning of hot water followed by cold water. The hot water temperature is 40-60 degrees. The neutralized product can be oxidized after being washed with water, so this cleaning should be performed with special care to prevent the clean surface from being contaminated. Otherwise, the effective treatment of the first few processes may result in failure due to improper cleaning. The neutralized and washed products should be treated with oxidation. The time spent in the air should not be too long. If it stays for 30-40 minutes, the product will need to be washed and neutralized again.