The three aspects of the Dacromet anti-corrosion mechanism

Dacromet is an advanced non-electrolytic metal surface treatment technology based on zinc-aluminum composite coating. It originated in Europe and America in the 1950s and began to be officially applied commercially in the 1970s. Initially, it was used to solve the corrosion problem of road de-icing agents on car chassis. This technology was introduced to China in the 1990s. Its core principle is to immerse or spray metal workpieces with a water-based coating containing ultra-fine flake-like zinc powder, aluminum powder (with a thickness of micrometers or even nanometers), chromic acid and deionized water. After being baked at a temperature of 180°C - 300°C, a silver-gray inorganic anti-corrosion coating composed of oxides of zinc, aluminum and chromium with a thickness of 4 - 8 μm is formed on the metal surface. The anti-corrosion performance is 7 - 10 times that of traditional electro-galvanizing.

Dacromet technology is widely applied in various fields such as automobiles (high-strength chassis components, metal parts around engines, fasteners, etc.), aerospace (aircraft components, engine parts), power equipment (transmission towers, transformer casings, etc.), and marine engineering. According to statistics, the global market size of Dacromet coating equipment was approximately 12 billion yuan in 2023, and is expected to reach 15 billion yuan by 2025. In 2023, the market size of Dacromet treatment in China was about 18.5 billion yuan. By 2025, the penetration rate of Dacromet treatment in the metal surface treatment field will increase from 23% to 35%, and the market size is expected to exceed 23 billion yuan.

The three aspects of the Dacromet anti-corrosion mechanism1. Shielding Effect:

The coating is composed of dozens of layers of zinc-aluminum scales with a diameter typically less than 20 μm, which are interlaced and stacked together to form a physical barrier,preventing the penetration of corrosive media such as water, oxygen, and Cl-.

2. Oxidation Inhibition:In traditional dactron coatings, chromic acid (H2CrO4) reacts with zinc, aluminum and the metal substrate during the high-temperature baking process to form a dense passivation film, which inhibits electrochemical reactions. The traditional process relies on the strong oxidizing property of hexavalent chromium to achieve passivation, but hexavalent chromium is toxic and carcinogenic. Therefore, non-chromium dactron coatings now use molybdate, zirconate or organic silane systems as passivators.

3. Cathodic Protection:

Zinc, as a more reactive metal, readily reacts with corrosive media, forming a layer of corrosion product film that provides cathodic protection for the underlying metal; aluminum can form a dense protective oxide layer on the coating surface; the oxides of chromium play a role in bonding zinc powder and aluminum powder in the coating, enhancing the adhesion between the coating and the metal substrate.

Key technologies

1. Coating liquid formulation technology:

Selection of zinc/aluminum sheets: The particle size, shape, purity, etc. of zinc and aluminum sheets directly affect the density and corrosion resistance of the coating. The role of chromate: Chromate is a key component in the Dacromet coating solution, functioning as a curing agent and a passivating agent. However, modern environmentally friendly formulations have gradually adopted trivalent chromium or completely chromium-free alternatives.

Use of additives: Add some additives such as surfactants, dispersants, and thickeners. The types and quantities of these additives need to be optimized according to the specific formula and process requirements.

2.Coating Process Technology：

Immersion coating method: Suitable for workpieces with complex shapes and internal cavities. It is necessary to pay attention to controlling the immersion coating time and speed to avoid uneven coating thickness.

Spraying method: This method is suitable for large-sized workpieces or situations where high requirements are placed on the appearance of the coating. During the spraying process, it is necessary to control the pressure, distance, and angle of the spray gun to ensure the quality of the coating.

Brush coating method: Suitable for some local coating or small-sized workpieces. It is less used in industrial applications. The operation is simple but the efficiency is low, and the thickness and uniformity of the coating are difficult to control.

3. Solidification Process Technology:

Temperature control: The curing of the Dacromet coating requires to be carried out within a specific temperature range. Time control: The high-temperature curing time after a single coating application is usually 20-30 minutes (each time after the coating is completed, a separate high-temperature curing process is required, and for multi-layer coating, the total time is accumulated according to the number of layers). The specific time depends on factors such as the material of the workpiece, the thickness of the coating, and the performance of the curing equipment.

Heating methods: Common methods include hot air circulation heating (which ensures uniform heating of the workpiece but has a slower heating speed) and infrared heating (which has a fast heating speed and high energy utilization rate, but it is necessary to be cautious to avoid local overheating that may cause hydrogen embrittlement of the coating).

4.Quality Inspection Technology:

Appearance inspection: Generally, visual inspection is used. However, for some minor defects, a magnifying glass or microscope needs to be employed for observation.

Thickness measurement: Common methods for thickness measurement include the magnetic thickness gauge method (suitable for steel substrates, simple and fast to operate), the metallographic microscope method (allows observation of the microscopic structure of the coating and measurement of the coating thickness, but requires preparation of metallographic samples, and the operation is relatively complex), and the X-ray fluorescence thickness measurement method (can accurately measure the thickness of each layer in multi-layer coatings, but the equipment is relatively expensive) etc.

Corrosion resistance test: The corrosion resistance of the coating is evaluated through methods such as salt spray test and humidity test.

Adhesion test: The tests are conducted using methods such as the grid marking method, tape method, tensile test, peel test, and shear test.