Unveiling the Secrets of Metal Surface Pretreatment for Corrosion Prevention

Introduction

In the realm of metal protection against corrosion, the often-overlooked hero is the pretreatment of the metal surface. Before any protective coatings are applied, ensuring that the surface is in the optimal condition is of utmost importance. This blog will take you on a journey through the significance of surface treatment, the different cleaning levels, and the concept of heavy-duty coatings in the fight against corrosion.

The Importance of Surface Treatment

Metal surfaces in their natural state are often laden with various contaminants. Dust, oil, grease, oxide scales, rust layers, pollutants, salts, and loose old paint films can all be present. Among these, oxide scales are particularly tricky. Formed during the high-temperature forging and pressing of steel, they create a dense oxide layer that adheres firmly. However, despite its seemingly protective nature, oxide scale is more brittle than the underlying steel and, being cathodic, actually accelerates the corrosion process.

If these substances are not removed prior to painting, the adhesion and overall anticorrosive capabilities of the coating will be severely compromised. In fact, statistics show that over 70% of paint-related issues stem from improper surface treatment. Therefore, for a metal anticorrosion paint system to perform at its best, appropriate surface treatment is the cornerstone.

Cleaning Levels and Standards

Sa1 Level - Manual Brush-off Cleaning Grade

This level is equivalent to the American SSPC-SP7 standard. It involves simple manual brushing and sanding methods. It is the lowest level of the four in terms of cleanliness. However, it still offers slightly better protection for the coating compared to untreated workpieces. The technical standard for Sa1 treatment dictates that the workpiece surface should be free of visible oil, grease, residual oxide scale, rust spots, and residual paint. This level is also known as the manual brush-off cleaning (or sweeping) grade.

Sa2 Level - Commercial Cleaning Grade

Equivalent to the American SSPC-SP6 standard, Sa2 utilizes a sandblasting cleaning method. It is the lowest level of sandblasting treatment but provides significantly better protection for the coating compared to manual brushing. The technical requirements state that the workpiece surface should be free of visible grease, dirt, oxide scale, rust scale, paint, oxides, corrosion products, and other foreign substances (except for allowable defects). The allowable defects are limited to no more than 33% of the surface area per square meter and may include slight shadows, minor discoloration caused by defects and rust, and oxide scale and paint defects. If the original surface of the workpiece has dents, slight rust and paint may remain at the bottom of the dents. This level is also called the commercial cleaning grade (or industrial grade).

Sa2.5 Level - Near-White Cleaning Grade

Widely used in industry and often used as an acceptance technical requirement and standard, Sa2.5 is also known as the near-white cleaning grade (near-white or white-ish grade). The technical standard is similar to that of Sa2 for the first part, but the allowable defects are limited to no more than 5% of the surface area per square meter. This can include slight shadows, minor discoloration caused by defects and rust, and oxide scale and paint defects.

Sa3 Level - White Cleaning Grade

Equivalent to the American SSPC-SP5 standard, Sa3 is the highest treatment level in industry, also known as the white cleaning grade (or white grade). The technical standard is the same as Sa2.5, but any 5% shadows, defects, rust, and other blemishes are not allowed to exist.

The Rise of Heavy-Duty Coatings

With the rapid development of modern industry, new industrial fields have emerged, and numerous modern projects have been constructed. This has led to higher demands on the environmental resistance and service life of anticorrosion coatings. Conventional anticorrosion coatings are no longer sufficient. The concept of "heavy-duty coatings" has emerged. These are generally coatings used in harsh corrosion environments, including primer and topcoat combinations. In simple terms, heavy-duty coatings are those that have a longer service life and can adapt to more severe operating environments. In chemical atmospheres and marine environments, heavy-duty coatings can typically last for 10 or 15 years or more. In acid, alkali, salt, and solvent media, and under certain temperature corrosion conditions, they can generally be used for more than 5 years.

Conclusion

In the battle against metal corrosion, proper surface pretreatment is not just a step but a crucial foundation. Understanding the different cleaning levels and the importance of heavy-duty coatings empowers us to make informed decisions in protecting our metal assets. Whether it's in industrial machinery, infrastructure, or any metal-containing application, investing in effective surface treatment and using the right coatings can significantly extend the lifespan and performance of metal components.