The Important Role of Skinpass in Hot Dip Galvanized Coils

In the production of hot-dip galvanized (HDG) coils, the skinpass process—a light cold rolling—serves as a critical finishing step that improves surface quality, refines mechanical properties, and ensures the final product meets industry standards. This process is essential across sectors such as automotive, construction, and household appliances, where high-performance galvanized steel is critical for both functional and aesthetic purposes.

What is Skinpass?

Skinpass is a light cold-rolling process performed after hot-dip galvanizing, where the galvanized steel passes between smooth or lightly textured rolls under controlled pressure. This process typically involves an elongation of 0.5% to 2%, just enough to refine the steel's surface and mechanical properties without significantly altering its thickness.

Surface Finish and Appearance

One of the key benefits of the skinpass process is the improvement in surface finish. The hot-dip galvanizing process often produces a crystalline pattern called "spangle" on the steel surface, which may be undesirable in industries requiring a smooth, uniform finish—especially when the material will be painted or coated.

Skinpass minimizes or eliminates spangles, creating a smoother surface that enhances aesthetic appeal and reduces the likelihood of defects during downstream coating processes. This improvement in surface uniformity ensures better adhesion of paints and coatings, which is crucial for applications requiring corrosion resistance and durability.

Flatness: A Positive Side Effect

Although skinpass is mainly used to improve surface characteristics and mechanical properties, it also has a beneficial side effect: better flatness. The light compression applied during the process helps reduce minor surface undulations and irregularities, improving the overall flatness of the steel strip.

However, in modern galvanizing lines, a tension leveller is typically installed downstream from the skinpass mill. The tension leveller applies controlled tension to the steel strip, further improving flatness by relieving internal stresses and ensuring geometric consistency. While skinpass provides some flatness correction, the tension leveller is the key device responsible for achieving the precise flatness required for applications with strict dimensional tolerances.

Mechanical Properties: Stability, Linearity, and Ageing Resistance

The skinpass process also plays a critical role in refining the mechanical properties of galvanized steel. A common issue in low-carbon steel, especially in grades that are not interstitial-free (IF) or microalloyed, is the occurrence of yield point elongation, which can lead to defects such as stretcher strains during forming operations. This phenomenon is particularly problematic in industries requiring high-quality, defect-free surfaces, such as automotive manufacturing.

By applying light compression, skinpass releases low-energy activated dislocations—structural irregularities within the steel’s atomic structure. These dislocations, if left untreated, can cause inconsistent mechanical behavior during further processing. Skinpass relieves these dislocations, ensuring the linearity of mechanical properties, which makes the material more predictable during forming or bending operations.

Another key benefit of skinpass is its ability to prevent ageing, a phenomenon where dislocations within the steel become locked over time, leading to increased yield strength and reduced ductility. By stabilizing the dislocations, skinpass ensures that the steel’s mechanical properties remain stable and consistent over time, preventing unwanted brittleness and ensuring reliable performance in long-term applications.

Coating Adhesion and Durability

Skinpass also improves the adhesion of protective coatings, such as paints and lacquers. By producing a smooth and uniform surface, skinpass allows coatings to bond more effectively, which is crucial in applications where corrosion protection is a priority.

In environments such as construction or automotive, where galvanized steel is exposed to moisture, chemicals, or harsh weather conditions, coating adhesion directly impacts product longevity. The smooth surface achieved through skinpass ensures that coatings adhere properly, reducing the risk of premature coating failure and extending the product’s operational life.

Post-Galvanization Surface Defects

Hot-dip galvanizing can leave various surface defects, such as roughness, waviness, or visible zinc flow marks. Skinpass smooths the surface and improves both appearance and performance. This is especially important for applications where galvanized steel will be visible or exposed, such as in roofing or architectural components.

By improving surface quality, skinpass ensures that the steel meets high aesthetic and mechanical standards, making it suitable for demanding applications in both structural and cosmetic roles.

Wet Skinpass: Importance of Fluid Application

A crucial aspect of modern skinpass operations is that they are typically conducted wet, meaning that a skinpass fluid is sprayed during the rolling process. This fluid serves multiple functions, including optimizing friction in the roll-bite and keeping the rolls free from debris or coating material that could stick to the rolls and cause surface defects, such as unwanted marks on the finished product.

The correct setup of the fluid application—including the right quantity and distribution—is essential for preventing coating damage and ensuring the desired surface finish. The fluid also helps maintain roll cleanliness, preventing the buildup of zinc or other contaminants that can degrade the quality of the coil surface. Poor control of the fluid can lead to aesthetic issues and reduced product quality.

Moreover, the fluid used in the skinpass process must be carefully selected so that it does not negatively impact downstream treatments, such as chemical passivation or oil coatings applied for temporary corrosion protection. Additionally, the skinpass fluid should be easily removable during reprocessing, such as in common degreasing operations, to ensure perfect adhesion of subsequent coatings, especially in post-painting operations. Any residue left behind could hinder coating adhesion, leading to defects in the final product’s appearance and performance.

Conclusion

The skinpass process is a vital finishing step in the production of hot-dip galvanized coils, providing essential improvements in surface quality, mechanical property stability, and coating adhesion. While the process positively affects flatness, the downstream tension leveller is the key device for achieving the high flatness required for modern applications.

Given the complex impact of skinpass on the final product, proper setup of the skinpass process is essential. For each application, the correct elongation must be carefully defined based on the combination of steel thickness and grade. Additionally, parameters such as tension, roll roughness, and the transfer coefficient must be optimized to control the final surface roughness and ensure stable mechanical properties.

Moreover, the process fluid used in the typically wet skinpass must be carefully controlled in terms of quantity and application. This ensures that friction is optimized, rolls remain free from debris, and surface defects are avoided. Additionally, the fluid must not interfere with downstream protection treatments, such as oiling or chemical passivation, and should be easily removable during degreasing to guarantee perfect adhesion during subsequent coating operations, such as painting.

When all these factors are correctly controlled, the skinpass process delivers stable, high-quality galvanized steel that meets the demanding requirements of industries ranging from automotive to construction.

Luigi Villani is the owner of GTG Consulting and specializes in analyzing industrial trends in materials science. For more insights, visit www.gtgcons.com.