Which is More Important for Pre-painted Steel Sheets: Coating or Plating?

Introduction to Pre-painted Steel Sheets

Pre-painted steel sheets are characterized by their attractive appearance, excellent corrosion resistance, ease of processing, long service life, low pollution, and practical functionality. They find extensive applications in various fields such as construction, transportation, home appliances, furniture, and office equipment. In particular, within the construction industry, pre-painted profiled steel sheets offer significant advantages in meeting building requirements, shortening construction periods, and reducing maintenance costs.These steel sheets possess “plating” and “coating” suitable for use under different environmental conditions. The diversity of profiled sheet types, convenient construction techniques, and a wide range of colors make pre-painted steel sheets highly flexible and adaptable.So, which factor—coating or plating—is more important for pre-painted steel sheets? Today, let’s explore this topic from three aspects.

1. Impact on Corrosion Resistance

The corrosion protection of architectural pre-painted products results from the combined effect of plating, pre-treatment, and coating (primer, topcoat, and back-coat). This combination directly influences the lifespan of pre-painted steel sheets.

• Coating: It acts as an isolating material, akin to a colorful “outer layer” that separates the base plate from corrosive media, thereby achieving anti-corrosion effects.However, from a microscopic perspective, coatings contain many pinholes invisible to the naked eye. These pinholes allow external corrosive media (such as water, oxygen, chloride ions) to penetrate the substrate, leading to filamentous corrosion under certain relative humidity levels. This can cause blistering and peeling of the entire paint film. When the coating thickness exceeds 20 microns, the corrosion resistance becomes relatively stable.
• Plating: Similar to a “skin” on the base plate, it forms a barrier using hot-dip or electroplating technologies, isolating the base plate from external media and preventing corrosion.Specifically, when zinc is used as the plating element, its different electrical potential compared to iron means that once the zinc layer is damaged and exposes the steel base plate, zinc, acting as the anode, corrodes preferentially, protecting the relatively higher potential iron as the cathode. This process is known as “cathodic protection.”

2. Influence of Different Coatings and Platings on Pre-painted Steel Sheets

The lifespan of pre-painted steel sheets includes two concepts: aesthetic lifespan and service life.

• Aesthetic Lifespan: This refers to the period during which pre-painted steel sheets maintain their appearance. It is measured by whether the surface shows no peeling, cracking, or crazing, and if the color difference value and color fading rate remain within acceptable limits. This mainly depends on the color, paint type, and film thickness of the coating.
• Service Life: This refers to the time materials can be safely used without external force influence. The standard for measuring service life is that pre-painted steel sheets do not develop corrosion holes within a certain period. Factors influencing service life include not only paint coatings but also the type of plating, plating thickness, and usage environment.

3. Differences Between Various Coatings and Platings

After comparing the corrosion principles, aesthetics, and service life between coatings and platings, let’s discuss the differences among them.

• Coating Types: Typically, the types of coatings for pre-painted steel sheets are named after the front finish paint, including polyester (PE), silicone-modified polyester (SMP), PVDF resin (polyvinylidene fluoride), high-durability polyester (HDP), and specially reinforced polyester (SRP).
• Plating Types: Currently, the most common types of plating for architectural pre-painted steel sheets are hot-dip galvanized, hot-dip aluminum-zinc, and zinc-aluminum-magnesium. Depending on the aluminum content, zinc-aluminum-magnesium can be divided into high aluminum zinc aluminum magnesium (55% aluminum) and medium aluminum zinc aluminum magnesium (5%-11% aluminum). Different types of plating exhibit varying degrees of corrosion resistance, edge protection, and acid-alkali resistance.

Conclusion

Both coatings and platings are crucial for pre-painted steel sheets. Coatings play a larger role in aesthetics and functionality, while platings contribute more significantly to performance protection in areas like edges and bends. Selecting appropriate types of coatings and platings can better enhance the durability and corrosion resistance of pre-painted steel sheets.