Selection of zinc layer thickness for galvanized welded carbon steel pipe

When choosing the zinc layer thickness for galvanized welded carbon steel pipes, it is essential to consider the environment in which the pipe will be used and the corresponding standards. The thickness of the zinc layer significantly influences the pipe’s corrosion resistance, durability, and overall performance. Below are the international and industry-specific standards, recommended thickness based on usage environments, and considerations for selecting the appropriate zinc layer thickness.

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International Mainstream Standards for Zinc Layer Thickness Requirements

International Standard: ISO 1461

Zinc Layer Requirements:

≥45μm (Normal Environment)

≥55μm (Harsh Environment)

Applicable Scenarios: General industrial use.

American Standard: ASTM A123

Zinc Layer Requirements:

≥43μm (1.5mil)

≥85μm (3.0mil)

Applicable Scenarios: Heavy-duty anti-corrosion projects.

Chinese National Standard: GB/T 13912

Zinc Layer Requirements:

≥45μm-55μm (Normal Grade)

≥80μm (Thickened Grade)

Applicable Scenarios: Construction and municipal applications.

EU Standard: EN ISO 1461

Zinc Layer Requirements:

≥50μm

≥70μm

Applicable Scenarios: Marine environments.

Zinc Layer Thickness Selection Basis

Select Based on the Use Environment

Indoor Dry Environment:

Recommended Zinc Layer Thickness: 20-45μm (Electrogalvanizing or Thin Hot-Dip Galvanizing)

Examples: Furniture brackets, wire casings, indoor steel structures of buildings.

Ordinary Outdoor Environment:

Recommended Zinc Layer Thickness: 45-80μm (Hot-Dip Galvanizing)

Examples: Building guardrails, municipal water pipes.

High Humidity/Coastal/Industrial Areas:

Recommended Zinc Layer Thickness: ≥80μm (Thick Hot-Dip Galvanizing)

Examples: Port facilities, chemical plant pipelines.

Extremely Corrosive Environment:

Recommended Zinc Layer Thickness: 80-150μm (Special Process Required, Double Galvanizing or Alloy Plating)

Examples: Offshore platforms, acid rain-prone areas.

Select Based on Industry Specifications

Oil/Gas Pipelines (API 5L):

Buried Pipelines: ≥80μm + Anti-Corrosion Coating.

Construction Industry (GB/T 13912):

Ordinary Structural Parts: ≥45μm

Key Load-Bearing Parts (e.g., Steel Structures): ≥80μm.

Electric Power Communication (YD/T 322):

Wire Casings: ≥20μm (Electrogalvanizing) or ≥45μm (Hot-Dip Galvanizing).

Risks of Insufficient or Excessive Zinc Layer Thickness

Thin Zinc Layer:

Cause: Short galvanizing time or low temperature.

Consequence: Increased rusting and reduced lifespan.

Uneven Zinc Layer:

Cause: Incomplete pretreatment (e.g., inadequate pickling).

Consequence: Localized rust spots.

Thick Zinc Layer:

Cause: Excessive immersion time in the galvanizing bath.

Consequence: Increased brittleness and peeling.

Trade-off Between Cost and Zinc Layer Thickness

Hot-Dip Galvanizing Cost:
For every 10μm increase in zinc layer thickness, the cost increases by approximately 5%-8%.

Recommended Strategy:

General Environment: Choose 45-60μm (best cost-performance balance).

Harsh Environment: Choose 80-120μm (lower long-term maintenance costs).

Calculation of Zinc Layer Corrosion Rate

The corrosion rate of the zinc layer varies with the environment:

Rural Environment: 1-2μm/year

Urban Environment: 2-4μm/year

Industrial Area: 4-7μm/year

Coastal Area: 5-10μm/year

Design Life Formula:
Zinc Layer Thickness=Annual Corrosion Rate×Design Life×Safety Factor (1.5-2)

Methods for Measuring Zinc Layer Thickness

Magnetic Thickness Gauge:
Measures the thickness of non-magnetic zinc layers using the magnetoresistance method.

Metallurgical Microscope:
Provides high accuracy through microscopic observation after sample cutting, though it may damage the sample.

Weight Method (ASTM A90):
Measures zinc per unit area (g/m²) after dissolving the zinc layer by pickling.

Conversion Formula: 1μm ≈ 7.1g/m².

How to Choose the Zinc Layer Thickness?

Clarify the Use Environment: Consider factors such as humidity and corrosive media.

Refer to Industry Standards: For instance, building pipes should have ≥45μm, and buried pipes should have ≥80μm.

Balance Cost and Lifespan: Opt for a thicker coating for harsh environments to reduce long-term maintenance costs.

Test Zinc Layer Quality: Ensure uniformity and avoid defects for optimal corrosion protection.

By selecting the appropriate zinc layer thickness based on these guidelines, you can maximize both corrosion resistance and economic efficiency for galvanized welded carbon steel pipes.