What is chromated?

Definition

Chromate treatment is a process to transform the metal surface into a film mainly composed of chromate.

The medium used to achieve this conversion is generally a solution with chromic acid, alkali metal chromate, or dichromate as the basic component.

Chromating process

Add some activators, such as fluoride, phosphoric acid, or sulfuric acid to the passivation solution, so that a thicker chromate film can be obtained after passivation.

When there is fluoride in the passivation solution, the surface tension of the steel strip can be reduced, the film-forming reaction can be accelerated, and the chemical polishing effect can be increased to make the passivation film fine and bright.

What color is chromate?

The passivation solution is made of chromate added with different color chemicals, and the appearance of different colors is obtained after passivation.

Luster: black, blue, yellow, army green, color zinc, water red, golden yellow, dark green, etc.

Generally, there are two colors of galvanizing, white-galvanized passivation white, yellow-galvanized passivation color

In the GB/T13911-92 standard, there are four types of post-treatments for electro-galvanizing:

Bright chromate treatment-the first type belongs to the gloss galvanized, which is also white zinc; the symbol for the subsequent treatment is c1A; it is bright white!

Bleaching chromate treatment-the second type is also white zinc, which is often referred to as blue-white zinc; the symbol for the subsequent treatment is c1B; blue-white!

Rainbow chromate treatment-the third type is color zinc; the symbol for the post-treatment is: c2C;

Dark chromate treatment-the fourth is black zinc, army green, olive green. The symbol for subsequent processing is c2D.

Physical properties of chromate

1. Chromate is oxidizing, and the reduction of CrVI often results in blue-green CrIII.

2. In the aqueous solution, chromate ion (yellow) and dichromate ion (orange) are in equilibrium. Adding acid promotes the formation of dichromate ions, making the solution red; adding alkali makes the balance shift to the left, and the solution turns yellow:
2 CrO42− + 2 H+ ⇌ Cr2O72− + H2O

3. Chromate can be used to determine the chemical oxygen demand (COD) in water bodies.

4. The chromates of heavy metals, lanthanides, and alkaline earth metals are mostly slightly soluble or hardly soluble in water and have few uses. The solubility of alkali metal chromates is relatively large.

5. Chromate conversion coating can be used to prevent metal corrosion and enhance the adhesion of paint.

6. Pyridine chlorochromate is used to oxidize alcohol to the aldehyde in organic synthesis.

Advantages

  • Improve the corrosion resistance of metals or metal coatings.
  • Improve the adhesion of metal to paint or other organic coatings.
  • Avoid contamination of metal surfaces.
  • Get a colored decorative appearance.

Chromate conversion coating

Chromate conversion coating is generally used as an intermediate layer between aluminum profiles and coatings.

With the extensive promotion and application of sprayed aluminum profiles, chromate conversion coatings are increasing. The quality of chromate conversion coating directly affects the quality of sprayed aluminum profiles.

Specifically, the chromate conversion coating is a chromium-free metal surface conversion solution that imparts superior corrosion resistance to the aluminum alloy surface, and a metal surface treatment method using the chemical conversion solution.

At present, in the chromate chemical conversion membrane technology, the conversion solution contains harmful potassium ferricyanate and the highly toxic carcinogen hexavalent chromium, which pollutes the environment.

The production environment has high requirements, and wastewater treatment is complicated and expensive. The invention is made from an aqueous solution containing silicate, titanium salt, peroxide, fluoride, and sulfuric acid.

When the solvent is used, the temperature is 50-99°C, the pH is 4-9, and the contact time with the aluminum alloy is 2-20 minutes.

Anodizing vs. Chromating

Chromate treatment chromate is reduced, and chromate is oxidized in chromate anodization.

Chromate oxidation is where the chromium element has a high valence, such as potassium dichromate, which has a strong oxidizing property. The power can be understood as that the chromium nucleus attracts electrons, and its atoms have high valence in this valence state and are unstable (relatively unstable, such as perchloric acid positive 7-valent chlorine, but because of its stability, the oxidation is not as strong as chlorine).

Anodizing is a power source that controls the direction of current and forces electrons to migrate. Even if an inactive metal like Cu is connected to the anode, it still loses electrons.

Chromate passivation vs Chemical conversion coating

Both refer to passivation, the difference is: chromate contains trivalent or hexavalent chromium, and chemical conversion coating mostly refers to chromium-free passivation.

Chromate passivation is a surface containing chromium ions, and chromium-free passivation is a molybdate component. There are differences in surface products and their respective corrosion resistance.

In general, chromate passivation has better corrosion resistance than chromium-free passivation.

Aluminum Chromating 

There are many traditional surface treatment processes for aluminum and aluminum alloys, and they are relatively mature, including anodizing, chromate conversion, chromate-phosphate conversion, and tannate conversion processes.

The reason why the chemical treatment of aluminum takes chromate treatment as the mainstream is that the chromate film has high corrosion resistance, low cost, and good bonding force.

But it is easy to cause the pollution of Cr+6 to the environment, so in recent years, the chemical treatment of non-chromate systems has been studied, but its appearance and corrosion resistance can not be compared with chromate film.

The process of aluminum chromate treatment is divided into three parts: surface preparation (pre-treatment) of the workpiece-chromate treatment-post-treatment of the conversion coating.

Aluminum chromate chemical conversion process

Oil removing

Immerse the parts to be processed in a strong cleaning solution for about 0.5~3min at 25~45°C. When there are dense small bubbles, take them out immediately to avoid “blackening”, and scrub them with a nylon brush in running water.

Degreasing

The parts are immersed in the P3-almeco18 solution for 5 to 12 minutes, if there are blind holes, the hole position is upward, 40 to 70 °C.

Light out

Put the parts into deoxidizer395H, room temperature, 0.5-8min, remove the surface oxide layer.

Chromate conversion coating treatment

Put the parts into the chromating tank so that the parts are 20-40mm below the liquid surface. Pay attention that the parts cannot overlap each other, and the blind holes are positioned upward. The parts should be placed so that the chemical reaction is easy to proceed, and the gas is easy to escape. Out.

Shake the part 1 to 2 times in the solution. The immersion time is 2~5min, the temperature is around 40°C, and the pH value is controlled at 1.5~1.9. The pH value can be adjusted with nitric acid or sodium hydroxide.

Drying

Put it into a constant temperature blast drying oven (oven) for drying and sealing, the temperature is 49~60°C, not more than 66°C, and the time is 5~15min.

Inspection of finished products

  • Aluminum and aluminum alloy base materials are different, and the color of the chromate film is also different. Generally bright light yellow, golden yellow, dark brown, or rainbow colors of different shades.
  • The film needs to be continuous, uniform, complete, and good in bonding. 
  • Allowable defects: the film is slightly dark, the small blind hole is slightly lacking in the depth, and the fixture contact point has no film.
  • Defects are not allowed: the film is loose, rough, and can be wiped off with fingers or cotton cloth; the film is obviously scratched, scratched, and white spots without oxide film.

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