The whole process of rust prevention treatment of galvanized steel plate

Introduction:
Galvanized steel sheet has become the material of choice for construction, automobiles, home appliances and many industrial fields due to its excellent corrosion resistance. Its core protection lies in the zinc layer on the surface, which protects the internal steel substrate through the principle of sacrificial anode. However, galvanized steel sheet is not "rust-free"! In harsh environments (such as marine atmosphere, industrial pollution areas, high humidity or mechanical damage), the zinc layer will gradually consume or be damaged, eventually causing the substrate to rust. Therefore, it is very important to implement a systematic galvanized steel sheet anti-rust treatment process for specific application scenarios. This article will deeply analyze this process, covering the basics of galvanized steel sheet process, key metal anti-rust treatment steps and the final coating inspection standards, to provide long-term anti-rust protection for your project.

1. Understanding the basis: galvanized steel sheet process and the necessity of anti-rust
The core anti-rust ability of galvanized steel sheet comes from the zinc layer covering its surface. Common galvanizing processes include:

Hot-dip galvanizing: Immerse the clean steel sheet in molten zinc liquid to form a thicker zinc-iron alloy layer and a pure zinc layer. Provides the thickest and most durable protection, commonly used in structural parts, transmission towers, highway guardrails, etc.

Electrogalvanizing: A thin and uniform zinc layer is deposited on the surface of the steel plate through an electrolytic process. The surface is smoother and easy to paint later. It is often used in automotive panels, home appliance housings, etc.

Alloy galvanizing: After hot-dip galvanizing, the zinc layer is completely converted into a zinc-iron alloy layer (such as GA, Galvannealed) by heat treatment. The surface is gray and has good coating adhesion and weldability.

Why is additional rust prevention treatment required?

Zinc layer consumption: In a corrosive environment, the zinc layer will consume and become thinner over time until it loses its protective effect.

Damage risk: Mechanical damage (scratches, bumps) during transportation, processing, and installation will damage the zinc layer and expose the substrate.

Special environmental challenges: Extreme environments such as marine salt spray, industrial acid rain, and chemical plant areas greatly accelerate the corrosion rate of the zinc layer.

Aesthetic and functional requirements: When specific color, gloss or additional functionality (such as chemical resistance and wear resistance) is required, a single zinc layer cannot meet the requirements.

Therefore, the core goal of the entire process of galvanized steel plate anti-rust treatment is to build a more durable and stronger "second line of defense" or repair damaged areas based on the "first line of defense" provided by the zinc layer.

2. Detailed explanation of the entire process of galvanized steel plate anti-rust treatment

A complete and efficient anti-rust treatment process usually includes the following key steps:

Step 1: Surface evaluation and pretreatment
Check the status: Carefully check the surface condition of the galvanized sheet to identify pollutants or defects such as oil, dust, white rust (corrosion product of zinc), black spots, scratches, welding spatter, etc. This is the basis for formulating subsequent treatment plans.

Cleaning and degreasing: Use alkaline cleaning agents or special solvents to thoroughly remove grease, oil stains and processing residues. Incomplete cleaning is one of the main reasons for coating failure.

Removal of white rust/oxide: For surfaces that have produced slight white rust, use a nylon brush, scouring pad or special weak acid cleaning agent (such as products containing phosphoric acid or citric acid) to remove it, and be careful not to damage the zinc layer excessively. Severe white rust or black spots may require more professional treatment.

Surface activation/conversion (key step): This is the core link to enhance the adhesion of subsequent coatings. Common methods include:

Chromate passivation: A traditional and effective method to form a chromium-containing conversion film, which significantly improves corrosion resistance and adhesion (but it should be noted that environmental regulations restrict the use of hexavalent chromium).

Chromium-free passivation: The mainstream choice under the environmental protection trend, such as zirconium, titanium, silane or composite passivators, can also form an effective protective film and provide a good adhesion base.

Phosphating: Suitable for heavy anti-corrosion occasions that require coating, forming a microcrystalline phosphate layer to further enhance adhesion and corrosion resistance (less application on galvanized sheet than hot-dip galvanized sheet).

Step 2: Damage repair (if necessary)
For areas where the zinc layer is damaged and the substrate is exposed due to local scratches and bumps, repairs must be performed:

Surface treatment: Grind and remove rust until clean metal is exposed (St2 or St3 grade).

Special repair primer: Using zinc-rich primer is the best choice. Its high content of zinc powder can provide cathodic protection similar to that of the galvanized layer. Ensure that the repair area is completely covered and properly extend to the outer edge to cover part of the intact zinc layer.

Drying and curing: Strictly follow the instructions of the repair primer for drying and curing.

Step 3: Coating application (core anti-rust layer)
Coating selection: Select the appropriate coating system based on the final use environment, life requirements, aesthetic requirements, and cost budget:

Primer: Epoxy zinc-rich primer (provides cathodic protection and excellent adhesion), epoxy zinc phosphate primer (good adhesion and chemical resistance), two-component epoxy primer (strong versatility and good adhesion).

Intermediate paint: Epoxy micaceous iron intermediate paint (increase film thickness, good shielding, enhanced weather resistance), polyurethane intermediate paint.

Topcoat: Acrylic polyurethane topcoat (excellent weather resistance, gloss and color retention, decorative), fluorocarbon topcoat (super long weather resistance, self-cleaning), epoxy topcoat (high chemical resistance, but easy to powder, often used indoors or on top of intermediate paint).

Coating method: Airless spray (most commonly used, high efficiency, uniform film thickness), brush coating (small area repair), roller coating (specific occasions).

Film thickness control: Strictly follow the dry film thickness (DFT) required by the design for construction and testing. Sufficient film thickness is the key to long-term rust prevention. Generally, the total dry film thickness of a complete heavy-duty anti-corrosion coating system is required to be 120μm - 300μm or even higher.

Step 4: Curing and post-treatment
Curing conditions: Ensure that the coating is fully cured under the specified temperature and humidity conditions. When the environmental conditions are not good, heating, ventilation or the use of low-temperature curing coatings may be required.

Protection and inspection: During and after curing, protect the coating from pollution, rain or mechanical damage. Perform a preliminary visual inspection.

III. Quality Assurance: Coating Inspection Standards and Acceptance
The final judgment of the anti-rust treatment effect depends on strict inspection. Key inspection items and standards include:

Appearance inspection:

Standards: ISO 12944-7, ASTM D7091

Requirements: The coating surface should be uniform and continuous, without defects such as sagging, pinholes, bubbles, cracks, missing coating, obvious impurities, etc.

Dry film thickness measurement:

Standards: ISO 19840, SSPC-PA2, ASTM D7091, ASTM D6132 (non-destructive magnetic method, most commonly used)

Requirements: Measure multiple points (usually 5 points per 10㎡), the average value must be ≥ the specified minimum DFT, and the lowest point value must be ≥ 80% of the specified minimum DFT (or as required by the specific contract). Insufficient film thickness is the primary risk of premature failure of the coating.

Adhesion test:

Standards: ISO 4624 (Pull-off method), ASTM D4541 (Pull-off method), ISO 2409 (Cross-cut method), ASTM D3359 (Cross-cut method)

Requirements: The pull-off test value should meet the design requirements (usually ≥5MPa or higher); the cross-cut method should reach level 0 or level 1 (highest level). Excellent adhesion is the basis for long-term service of the coating.

Pinhole detection (continuity detection):

Standards: NACE SP0188, ASTM D5162

Method: Use low-pressure wet sponge method or high-voltage electric spark method (selected according to coating thickness) to detect penetrating defects (pinholes) in the coating. Pinholes are not allowed.

Corrosion resistance evaluation (optional/laboratory):

Salt spray test: ASTM B117, ISO 9227 - Evaluate the coating's resistance to salt spray corrosion, usually requiring hundreds or even thousands of hours without substrate corrosion.

Cyclic corrosion test: such as ASTM D5894, ISO 11997 - simulates more realistic dry-wet alternation, UV irradiation and other comprehensive environments, and the results are more valuable for reference.

IV. Summary: The key to long-term rust prevention
The entire process of rust prevention of galvanized steel sheets is an interlocking system engineering:

Start with accurate assessment: Understanding the substrate status (galvanized steel sheet process type, surface condition) is the first step to success.

Focus on scientific treatment: Thorough surface pretreatment (cleaning, activation/conversion) and necessary damage repair are the cornerstones of long-term adhesion of the coating.

The core is high-quality coating: Selecting materials according to environmental science and strictly controlling the coating process (especially film thickness) are the core of building a metal anti-rust treatment barrier.

Finally, strict testing: Multi-dimensional acceptance according to authoritative coating testing standards is the ultimate guarantee to ensure that the treatment effect reaches the design life and achieves long-term rust prevention.

Following this full-process specification can not only significantly extend the service life of galvanized steel plate structures or equipment and reduce maintenance costs, but also ensure their safety and aesthetics, providing maximum value return for your investment.