Galvanized Steel Pipe Decarburization: Principles And Countermeasures

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Decarburization, simply put, is the removal of carbon from the surface of steel.

Steel is essentially an iron-carbon alloy, and the carbon content determines the strength and hardness of the steel. When steel pipes are exposed to a high-temperature environment, the carbon atoms on the surface react with oxygen, water vapor, carbon dioxide, etc. in the surrounding atmosphere, turning into gases and escaping, resulting in a significant reduction in the carbon content on the surface layer compared to the internal matrix. This layer of low-carbon area is called the decarburization layer.

For example: It's like baking bread; the outside gets burnt and loses moisture, while the inside is still soft - decarburization is when the "surface layer of the steel is burned and loses its carbon".

The decarburization of galvanized steel pipes mainly occurs in two high-temperature stages:

Stage 1: Black pipe production stage (welding pipe/heat treatment)：

If the steel strip undergoes high-temperature heating before welding, or undergoes annealing/normalizing treatment after welding, in an oxidative atmosphere, decarburization will occur.

Stage 2: Annealing stage before hot galvanizing (most critical)：
In continuous hot galvanizing production lines, steel strips/steel pipes enter the zinc pot after passing through an annealing furnace (usually at 800-900°C), with the purpose of softening the steel, eliminating stress, and restoring the surface oxide film. If the atmosphere inside the furnace is improperly controlled, decarburization occurs here.

Four chemical reaction principles of decarburization：

Core logic:

The higher the temperature, the stronger the oxidizing atmosphere (more O₂/CO₂/H₂O), and the longer the holding time, the more severe the decarburization. Carbon diffuses from the interior of the base material to the surface and is then "eaten up" by the atmosphere, forming a decarburization gradient that gradually decreases from the surface to the interior.

Additional note:

The temperature in the galvanizing process itself (immersion zinc, approximately 450°C) is relatively low, and generally does not directly cause significant decarburization. Most decarburization problems in galvanized pipes are rooted in the annealing or black pipe stages before galvanizing.

1. Abnormal appearance of the coating

• The surface appears dark, grayish, with uneven luster, and "gray spots" may occur
• The coating may be too thick and rough because the ferrite in the decarburized layer is abundant and the carbon content is low, leading to uncontrolled growth of the iron-zinc alloy layer (Fe-Zn intermetallic)
• In severe cases, the coating has poor adhesion and peels off

2. Decreased mechanical properties

• The surface hardness decreases (the decarburized layer is mainly ferrite, which is soft and tough)
• The wear resistance decreases, and it is prone to scratching during use
• The tensile strength and yield strength at the surface decrease, affecting the overall load-bearing capacity

3. Characteristics of metallographic structure

• The pearlite in the decarburized layer decreases or disappears, almost all being ferrite
• From the surface to the core, the carbon content increases in a gradient
• Complete decarburized layer + Partial decarburized layer (transition zone)

4. Reduced corrosion resistance

• The zinc layer protection in abnormal areas (gray spots, peeling areas) fails
• The electrochemical activity of the decarburized layer itself is different from the base material, resulting in abnormal corrosion behavior

5. Post-processing problems

• The decarburized layer is prone to wrinkling and cracking during bending and expansion
• The surface hardness is insufficient during thread processing, and the tooth shape is irregular

1. Control the atmosphere of the annealing furnace (most crucial)

• Use a nitrogen-hydrogen protective atmosphere (N₂ + 5%~15% H₂), maintaining a slightly reducing environment
• Strictly control the dew point (usually required below -30°C), reducing water vapor
• Control the oxygen content and CO₂ partial pressure to avoid an oxidative atmosphere,

2. Control heating temperature and time

• While meeting the annealing requirements, try to lower the temperature (the decarburization rate increases exponentially with temperature)
• Shorten the holding time in the high-temperature section to reduce carbon diffusion and reaction opportunities

3. Control raw materials and surface condition

• When purchasing steel strips, require the steel mill to control the depth of the decarburized layer (can be stipulated in the contract, such as ≤ 0.05mm)
• Reduce the oxide scale on the steel strip, as the oxide scale will catalyze the decarburization reaction
• Avoid excessive pickling, as excessive surface activation will also exacerbate subsequent decarburization

4. Optimize pre-treatment before galvanizing

• Use a reasonable composition of the flux (flux), usually ZnCl₂ + NH₄Cl, control pH and concentration
• Use an appropriate drying temperature to avoid surface oxidation before the steel pipe enters the zinc pot
   

5. Process monitoring and inspection

• Regularly use metallographic methods to detect the depth of the decarburized layer (in accordance with GB/T 224 standard)
• Monitor the atmosphere parameters of the annealing furnace (dew point, oxygen content, hydrogen content) and record them
• Focus on controlling high-carbon steel (the higher the carbon content, the easier it decarburizes) with greater attention

In summary, decarburization is when the carbon on the surface of steel is "eaten away" by the atmosphere at high temperatures. The root cause lies in the atmosphere and temperature control of the annealing furnace; it manifests as gray coating, poor adhesion, and soft surface; the countermeasures are a three-pronged approach of protecting atmosphere + temperature control + raw material control.