Introduction

In numerous industrial applications, a material's corrosion resistance is a pivotal factor determining its service life and performance. As two widely used metal sheets, galvanized and galvalume sheets enhance the corrosion resistance of steel substrates to a certain extent through their protective coatings. This test aims to conduct a comparative study on the corrosion resistance of galvanized and galvalume sheets made of the same substrate material (51D) and with the same coating weight (80 g/㎡) using the standard Neutral Salt Spray Test (NSS). The findings are intended to provide a scientific basis for material selection in practical engineering applications.

Test Principles

The Neutral Salt Spray Test is an accelerated testing method that simulates salt spray corrosion found in natural environments. By atomizing a 5% sodium chloride solution within a specialized test chamber, a continuous salt fog environment is created, accelerating the corrosion process on the material's surface. Under these conditions, the coating and substrate of the metal undergo electrochemical reactions when exposed to the salt mist, leading to corrosion. By recording the time at which white rust (coating corrosion products), red rust (substrate corrosion products), and extensive red rust (covering over 30% of the surface area) first appear on the specimen, the corrosion resistance of the materials can be evaluated.

Test Materials

• Galvanized Sheet: Substrate material 51D, coating weight 80 g/㎡.
• Galvalume Sheet: Substrate material 51D, coating weight 80 g/㎡.

Test Equipment

• Salt Spray Test Chamber: Model [Specific Model], compliant with relevant standards, capable of precisely controlling test temperature (35°C ± 2°C), humidity, and salt fog deposition rate.

Test Procedure

1. Galvanized and galvalume sheet specimens were cut into standard specimens of size [Length × Width × Thickness, unit mm]. Three parallel specimens were prepared for each group to ensure the reliability of the test results.
2. Specimens underwent cleaning and degreasing pretreatment to remove surface oil and impurities, ensuring test accuracy.
3. After pretreatment, specimens were suspended inside the salt spray test chamber. An appropriate distance was maintained between specimens to avoid mutual shielding and ensure uniform salt fog deposition.
4. A 5% sodium chloride solution was added to the chamber. The equipment was started, and the test temperature was set to 35°C for continuous spraying.
5. Specimens were inspected at regular intervals (e.g., every 24 hours). The time of first appearance of white rust, red rust, and the time when red rust covered over 30% of the surface area was recorded. For each group, if 2 out of the 3 parallel specimens exhibited the corresponding corrosion phenomenon, that time was recorded as the time for the group to exhibit that phenomenon.

Test Results

6.1 Galvanized Sheet Test Results

表格

6.2 Galvalume Sheet Test Results

表格

Result Analysis

• Time of White Rust Appearance: The galvanized sheet exhibited white rust at an average of 125 hours, whereas the galvalume sheet showed it at an average of 205 hours. This indicates that the aluminum coating on the galvalume sheet performs better in resisting initial corrosion. Aluminum rapidly forms a dense aluminum oxide protective film in air, delaying the onset of corrosion. In comparison, the zinc coating on the galvanized sheet demonstrates slightly weaker initial protection in a salt spray environment.
• Time of Red Rust Appearance: The average time for red rust appearance was 310 hours for the galvanized sheet and 510 hours for the galvalume sheet. The appearance of red rust signifies the onset of substrate corrosion. The galvalume sheet protects the substrate from corrosion for a significantly longer duration, further demonstrating its superior corrosion resistance compared to the galvanized sheet. This is because the aluminum oxide film is not only dense but also possesses a certain self-healing ability when slightly scratched, continuously protecting the substrate. Conversely, once the zinc layer of the galvanized sheet is compromised, corrosion gradually spreads to the substrate.
• Time of Red Rust Covering >30% Area: The galvanized sheet reached this stage at approximately 510 hours, while the galvalume sheet reached it at approximately 810 hours. This result further confirms that under long-term salt spray corrosion, the galvalume sheet more effectively prevents the spread of corrosion, maintaining the material's integrity and performance.

Conclusion

Based on the results of this Neutral Salt Spray Test, under conditions of identical substrate material (51D) and coating weight (80 g/㎡), the galvalume sheet demonstrates superior corrosion resistance across the board compared to the galvanized sheet. Whether it is the time of white rust appearance, red rust appearance, or extensive red rust coverage, the galvalume sheet significantly outperforms the galvanized sheet.

In practical applications, if the environment demands high corrosion resistance—such as marine or chemical industrial settings—the galvalume sheet is a more suitable choice. Conversely, if cost sensitivity is high and the operating environment is less corrosive, the galvanized sheet remains an economical and practical option. However, overall, the galvalume sheet exhibits a clear advantage in terms of corrosion resistance.