Galvanized Coil Processing: Analysis of Performance Differences Between Passivated and Unpassivated Galvanized Coil

In the steel processing and procurement world, galvanized coil is one of the most widely used anti-corrosion steel materials. However, many users face a key question when selecting this material: What are the differences between passivated (oiled) galvanized coil and non-passivated (bright) galvanized coil? How should one choose? This article will delve into the processing and performance differences between these two products to help you make the most informed decision.

1. The Core Difference: Passivation Treatment
To understand the performance differences, it's important to first understand what "passivation" means.

Non-passivated galvanized coil (bright galvanized coil) refers to coiled products directly after the hot-dip or electroplating galvanizing process. Its surface is a pure zinc layer, exhibiting zinc's characteristic silvery-white metallic luster.

Passivated galvanized coil (oiled/passivated galvanized coil) refers to products that undergo a critical "passivation" process after galvanizing. Chromate passivation (or chromium-free, environmentally friendly passivation) is typically used to form an extremely thin and dense passivation film on the zinc surface. A thin layer of rust-proof oil may then be applied as needed.

This invisible "passivation film" is the fundamental reason for the stark difference in performance between the two.

II. A Comprehensive Analysis of Performance Differences
Performance Indicators
Passivated Galvanized Coil
Non-passivated Galvanized Coil
Corrosion Resistance: Excellent
White Rust Resistance: Excellent
Poor
Storage and Transportation: Low
High
Surface Appearance: May be slightly darker, with a passivation film. Bright, with a strong metallic luster.
Post-processing: May affect painting and welding. Better compatibility.
Cost: Slightly higher
Lower
Let's take a closer look:

1. Corrosion Resistance and White Rust Resistance

Passivated Galvanized Coil: The primary function of the passivation film is to "passivate" the active zinc surface, significantly reducing its chemical activity, thereby effectively isolating corrosive media such as moisture and oxygen from direct contact with the zinc layer. This not only prevents "black rust" (base corrosion) but also effectively prevents the formation of "white rust" (basic zinc carbonate, a corrosion product of zinc). Passivation is particularly crucial for ensuring rust-free storage and transportation, especially in humid, rainy, or coastal areas.

Unpassivated galvanized coil: The zinc layer is directly exposed to air. High humidity easily reacts with water and carbon dioxide in the air, rapidly forming white rust, which affects the product's appearance and subsequent use.

2. Storage and Transportation Requirements

Passivated galvanized coil: Due to its excellent rust resistance, its ventilation and humidity requirements are relatively relaxed, allowing for longer inventory turnover and significantly reducing the risks of long-distance ocean transport or transportation in humid environments.

Unpassivated galvanized coil: Must be stored in a dry, well-ventilated warehouse and must not be stored outdoors. It should generally be used as soon as possible (e.g., within 30 days) to prevent rust. Strict moisture-proof measures must also be taken during transportation.

3. Surface Appearance and Post-Processing

Passivated Galvanized Coil: The passivation film slightly dims the zinc layer's originally bright metallic luster. Depending on the passivation solution, it may have a slightly bluish, yellowish, or iridescent hue. This film may slightly affect paint adhesion (requiring appropriate surface preparation) and may also impose specific requirements on welding processes.

Unpassivated Galvanized Coil: The clean, bright surface is more suitable for direct painting, phosphating, and other subsequent treatments, and paint adhesion is generally better. Welding is also less affected by the passivation film.

4. Cost and Environmental Protection

Passivation treatment increases production process steps and chemical raw material costs, so the price of passivated galvanized coil is generally slightly higher than that of unpassivated products. Furthermore, traditional chromate passivation involves the heavy metal chromium, and environmental requirements are becoming increasingly stringent. More environmentally friendly chromium-free passivation technologies are now being promoted.

III. How to Choose: Application Scenario Guide
Based on the above differences, you can choose:

Scenarios for Choosing Passivated Galvanized Coil:

Products with high corrosion resistance requirements.

Materials requiring long-term storage or long-distance shipping or export.

Projects operating in high-humidity environments (such as the Jiangnan rainy season and coastal areas).

Subsequent processing primarily involves sheet metal forming and shearing, rather than immediate painting.

Optimal storage conditions (constant temperature and humidity).

Subsequent processes requiring direct painting or painting require extremely high substrate cleanliness.

Cost-sensitive projects requiring the ability to complete processing before rusting.

Conclusion
In summary, passivation and non-passivation are two distinct finishing steps for galvanized coil. Passivated galvanized coil sacrifices some brightness and processing compatibility in exchange for superior rust resistance and flexible storage and transportation conditions. Non-passivated galvanized coil, on the other hand, offers a smooth surface and excellent workability at the lowest cost, but is extremely demanding in terms of storage and transportation.

Your choice shouldn't be limited to price; instead, you should make a comprehensive assessment based on the end use, storage life, environmental conditions, and subsequent processing. We hope this article will help you gain a deeper understanding of these two materials and provide solid technical support for your purchasing and production decisions.