A professional explanation of hot-dip galvanized coils and hot-dip galvanized zinc-iron alloy coils

Hot-dip galvanized coil (pure zinc coating) and hot-dip galvanized zinc-iron alloy coil (alloyed coating) are two key materials widely used in the field of steel corrosion protection. Although both are based on the hot-dip galvanizing process, they differ fundamentally in coating composition, microstructure, performance, and applications.

I. Core Differences: Coating Composition and Formation Process

1. Hot-Dip Galvanized Coil

- Coating Composition: Its coating is made of pure zinc with a purity of over 99%. In terms of process, after the steel strip passes through the zinc bath, controlled cooling conditions create visible crystalline patterns on the surface, known as "zinc flowers" (which can be categorized as large zinc flowers, small zinc flowers, and no zinc flowers).

- Appearance Characteristics: Bright surface with a metallic luster and zinc flower patterns.

2. Hot-dip galvanized zinc-iron alloy coil

- Coating composition: After hot-dip galvanizing, the strip steel undergoes a special "alloying" annealing treatment immediately after exiting the zinc bath and before the coating solidifies (heating the strip steel from approximately 450℃ to 550℃ within 5 seconds). In this process, the zinc in the coating diffuses with the iron in the steel substrate at high temperature, forming a stable zinc-iron alloy layer with an iron content typically between 8% and 15%.

- Appearance characteristics: Because the surface is no longer a pure zinc layer, its appearance is a uniform dark gray or grayish-white, without zinc bloom, generally without luster, and with a smoother feel.

II. Performance Comparison

1. Corrosion Resistance

- Hot-dip galvanized coil: Its corrosion resistance mechanism mainly relies on the "sacrificial anode" effect of zinc. The pure zinc layer will preferentially corrode before the internal steel substrate, thus delaying the rusting of the steel. Excellent corrosion resistance.

- Hot-dip galvanized zinc-iron alloy coil: Its zinc-iron alloy layer has a much higher chemical stability than pure zinc, and it is less likely to react with corrosive media. After appropriate passivation treatment, its corrosion resistance, especially its resistance to red rust, can typically be 2 to 3 times higher than that of pure zinc coatings of the same weight.

2. Coating Performance

- Hot-dip galvanized coil: Good coating performance, meeting most general requirements.

- Hot-dip galvanized zinc-iron alloy coil: Coating performance is its most significant advantage. Its gray alloy layer has a hard, microporous surface with excellent adhesion to paints and coatings, making it an excellent substrate for subsequent electrophoretic, spraying, and other coating processes. This makes it the preferred material for fields with extremely high requirements for coating appearance and durability.

3. Welding Performance

- Hot-dip galvanized coil: Good weldability.

- Hot-dip galvanized zinc-iron alloy coil: Due to the iron content in its coating, its resistivity is relatively high, resulting in more stable current and less spatter during welding. Therefore, its weldability is generally superior to that of ordinary hot-dip galvanized coil.

4. Processing and Formability

- Hot-dip galvanized coils: Good flexibility, suitable for various bending and forming processes.

- Hot-dip galvanized zinc-iron alloy coils: Also possess excellent formability. However, it should be noted that their coating has high hardness, and there is a risk of slight powdering of the coating during extreme deep drawing and severe deformation. This needs to be considered in mold design and process parameters.

III. Main Application Areas

1. Typical Applications of Hot-dip Galvanized Coils

- Construction Industry: Such as roofing, wall cladding systems, ventilation ducts, light steel keels, etc.

- General structural components and warehouse racking.

- Appliance casings and facilities where surface aesthetics are not critical.

2. Typical Applications of Hot-dip Galvanized Zinc-Iron Alloy Coils

- Automotive Industry: Mainly used for interior and exterior panels of automobile bodies, such as doors, hoods, and trunk lids, providing them with an excellent coating base and long-term rust protection.

- High-end Home Appliances: Such as the front panels and casings of refrigerators, washing machines, microwave ovens, etc., ensuring perfect adhesion and smoothness of the surface coating. - Other fields with stringent requirements for corrosion resistance and coating performance.

IV. Selection and Usage Recommendations

- Identify the grade: When purchasing, please always verify the material grade. According to national standards, the suffix "+Z" usually indicates a pure zinc coating, while "+ZF" indicates a zinc-iron alloy coating.

- Decision based on end-use:

- If your product does not require subsequent coating and is cost-sensitive, or is used as an exposed structure in construction, hot-dip galvanized coil is an economical and practical choice.

- If your product requires high-quality coating (such as spray painting) and has extremely high requirements for coating adhesion, gloss durability, and overall corrosion resistance (such as automotive, high-end home appliances), hot-dip galvanized zinc-iron alloy coil is the technically more correct and superior choice.

- Storage and Protection: Both materials may develop white rust and electrochemical corrosion if tightly stacked in a humid environment. Therefore, post-processing protection is necessary:

1. Isolate the steel plate from air to prevent oxidation.

A. Spray rust-preventive oil onto the cross-section of the steel plate to form a protective oil film, isolating it from air.

B. Seal the processed steel plate completely with rust-preventive paper to prevent air convection.

2. Store in a dry, well-ventilated warehouse.

3. Manage processed inventory reasonably and use it as soon as possible.