The aluminum profile is sent to the oxidation workshop after extrusion, aging heat treatment, operation and other processes, and its surface has natural oxidation film, oil film, dust, knock and scratch and bad alloy layer. The surface must be thoroughly cleaned and homogenized before oxidation. This is very important for obtaining high quality oxidized coloring material. Statistics show that more than 60% of the apparent quality problems of oxidation coloring of aluminum profiles are caused by poor pretreatment, among which the unqualified alkali corrosion quality accounts for the vast majority, indicating the importance of pretreatment.

At present, only degreasing, alkali etching and decontamination are used in the pretreatment of aluminum profiles in China. Only when the color of stainless steel is imitated and the color is golden can mechanical or chemical polishing methods be used for further refinement.

1 skim

Degrease calls divide oil again, its purpose is to remove film of natural oxidation, craft lubricants, anticorrosive oil, hand sweat and the dirt that stick together with grease, when in order to safeguard alkalic etch, the surface can be corroded even ground, protect the cleanness of alkalic etch groove. If the oil is not removed, the caustic corrosion is not uniform. After oxidation and coloring, there are dark patterns, uneven surface and other defects, which is also one of the causes of caustic corrosion. Our country mostly USES the non-lubrication extrusion technology, the surface grease and dirt and so on dust reduction, generally USES the chemical degreasing method. Chemical degreasing can be alkaline, neutral or acid degreasing fluid.

1.1 basic chemical degreasing

Alkaline chemical degreasing is widely used because of its low cost and good effect. The essence of basic chemical degreasing is the result of both saponification and emulsification. Alkaline degrease solution contains caustic sodium, detergent phosphate, buffer carbonate and emulsifiers such as sodium silicate and OP. Generally alkaline chemical degreasing USES anion and non - ionic surfactant; Acid degreasing agent is selected cationic surfactant.

Often composed of alkaline degreaser for: NaOH 10-20 g/L, Na3PO4 30-50 g/L, Na2CO3 10-20 g/L, Na2SiO3 10-20 g/L, surfactants 1 g/L, 60-70 ℃, 1 -- 3 min.

The normal temperature quick degreasing agent sold on the market is mostly neutral or weakly alkaline, the former is composed of a variety of low foam high efficiency surfactant with different properties, and the latter is composed of detergent and surfactant. No matter which one as in the below 20 ℃, degreasing time must be greater than 10 min, under 35-45 ℃ zui effect is good.

1.2 acid degreasing

Acid degreasing is only suitable for LD31 aluminum alloy, now acid degreasing method is more than alkaline method, the reason is that this method can make full use of aluminum oxidation waste acid and dirt removal light waste acid, make the best use of everything, reduce the cost. Common acid degreasing fluids are:

1) acid method - H2SO4 10% - 15% of 50-60 ℃ for 2-4 min

2) nitric acid method HNO3 15% -- 20% room temperature 2 -- 4 min

3) mixed acid HNO3 10% H2SO4 10% room temperature 1 -- 3 min

Some add small amounts of cationic or amphoteric surfactants to acidic degreasing agents to improve efficiency.

Some enterprises skimmed careless, such as sulfuric acid skimming room temperature immersion only ten seconds on the proposed, did not achieve the purpose of skimming, and washing is not clean into the alkali corrosion tank, bring a series of problems: 1) skimming is not clean, alkali corrosion is not uniform, the surface appears different luster, then oxidation coloring is not uniform. 2) sulfuric acid and other strong acids are brought into the caustic acid corrosion tank to neutralize the caustic soda. At the same time, sulfuric acid reacts with aluminum ions in the alkali tank and sodium ions to generate aluminum alum [NaAl(SO4)2], which forms granular precipitation in the alkali corrosion tank and will bond on the profile to cause cross-contamination in the subsequent process tank.

2 alkali corrosion

The quality of alkali corrosion largely determines the surface quality of profiles, and the oxidation, coloring and hole sealing processes behind will not be significantly improved. If the alkali corrosion is bad will appear long pit, spots, color material has dark patterns, uneven luster, low brightness dark gray, especially in aluminum impurities exceed the standard is more serious.

2.1 purpose of alkaline corrosion

1) remove the residues of natural oxidation film and degreasing process corroding aluminum matrix;

2) remove grease and other dirt infiltrating into the aluminum base surface layer;

3) remove the metamorphic alloy layer on the profile surface;

4) eliminate extrusion, mold marks, scratches and other surface defects, adjust and level the substrate surface to make it uniform;

5) moderate corrosion to obtain matte, fine sand surface, pitted surface and other special effects, in order to improve the decorative products.

2.2 why prolong the life of alkaline corrosion agents

During alkaline corrosion, the following chemical reactions occur between aluminum and alkaline corrosion solution:

Al2O3 naoh - > 2 naalo2 H2O (1)

2 al naoh 2 h2o - 2 naalo2 3 h2 write (2)

2 naalo2 4 h2o - 2 al (OH) 3 left 2 naoh (3)

Reaction (1) remove the natural oxide film; Reaction (2) is caustic soda corrosion of aluminum matrix dissolution; Reaction (3) is the hydrolysis of sodium metaluminate to produce Al(OH)3 precipitation. This reaction will occur when the dissolved aluminum in the alkali etch tank reaches 35 g/L and the life of the alkali etch agent is not prolonged. The solid particles generated by Al(OH)3 are also the active center to stimulate the hydrolysis of sodium metaaluminate. This reaction is irreversible, and Al(OH)3 gradually dehydrates into Al2O3 at a certain temperature. It is deposited on the bottom of the tank, the wall of the tank and the heating pipe to form hard stones, which are called duralumite. Often want to stop production to clear every months, affect production already, clear again difficult also insecurity. The addition of long-life alkali corrosion agent can make sodium metaluminate maintain high stability, will not produce hard bauxite, when the dissolved aluminum reaches 110 -- 120 g/L, the aluminum and aluminum ions out of the profile to achieve a dynamic balance, the solution can be used for a long time.

The performance of the long-life alkali corrosion agent developed in China has reached and exceeded the level of imported products, and has the following characteristics: 1) the soluble aluminum is allowed to reach 100 -- 120 g/L without hydrolysis and agglomeration, and when the temperature drops, there is occasionally hydrolysis and agglomeration, which is easy to clean; 2) the alkali corrosion mold is smoothed with ease due to the high soluble aluminum content.