What causes incomplete galvanizing of seamless steel pipes?

Incomplete or uneven galvanizing not only affects the appearance of seamless (SMLS) steel pipes but also reduces their corrosion resistance and service life in demanding environments. Understanding the root causes and applying the correct corrective measures is essential for ensuring a stable and high-quality galvanizing process.

1. Unclean Surface of Seamless Steel Pipe

A contaminated or improperly treated surface prevents the zinc layer from forming evenly.

Common Issues

Paint, grease, welding slag
These residues hinder zinc adhesion.

Iron oxide scale / insufficient pickling
Remaining oxide film acts as a barrier during galvanizing.

Solutions

Remove surface contaminants by grinding or using appropriate solvents.

If pickling is incomplete, extend pickling time until the surface is clean.

2. Issues During Pickling and Handlingof Seamless Steel Pipe

Improper pickling directly affects the surface reactivity of steel pipes.

Common Issues

Insufficient pickling
→Leaves rust or oxide layers.

Over-pickling
→Damages the steel surface; forms refractory oxides such as Si compounds.
→May cause hydrogen absorption, leading to hydrogen escape during galvanizing and missed areas.

Incorrect stacking or insufficient agitation
→Causes vortex zones and“missed pickling.”

Solutions

Strictly control pickling duration and concentration.

Spread out the workpieces during pickling and agitate frequently to ensure full exposure.

3. Fluxing Problems of Seamless Steel Pipe

Flux quality determines how well the zinc adheres during dipping.

Common Issues

Low flux concentration
→Poor fluxing effect, leading to incomplete galvanizing.

Incorrect flux ratio / excessive zinc salts
→Flux film absorbs moisture and becomes deliquescent, producing gas during dipping and causing“missed plating.”

Solutions

Adjust the flux concentration to the proper range.

Correct the flux formulation to avoid excessive zinc salt content.

4. Zinc Bath Composition & Furnace Operation Issues

The zinc bath environment directly determines coating quality.

Common Issues

High aluminum content
→Prevents proper iron–zinc reaction.

Slow feeding into the zinc bath
→Heat decomposes the flux film before immersion.

Insufficient immersion time
→Iron–zinc alloy layers do not form completely.

Unremoved zinc ash
→Overheated ash burns additives, causing missed plating.

Gas escape from internal cavities / structural gaps
→High-temperature gas burns additives, interrupting coating formation.

Solutions

Reduce aluminum content by adding zinc or ammonium chloride.

Increase loading speed to prevent premature flux decomposition.

Ensure adequate immersion time.

Clean zinc ash from the bath surface before operation.

Add an appropriate amount of ammonium chloride when high-temperature gas is present.

5. Material-Related Causes 

Certain material properties can inherently resist proper galvanizing.

Common Issues

Steel containing refractory oxides (Si, Al) slows the iron-zinc reaction.

High carbon content or excessive iron carbide reduces bonding ability.

Forming defects such as cracks allow acid and additives to penetrate and damage the coating.

Solutions

Currently, ammonium chloride treatment remains the primary effective method to mitigate material-related plating issues.