If we talk about the simple stainless steel, which is inherently corrosion resistant due to a passive layer of Chromium Oxide, but coloured stainless steel is a bit different. The coloring means a very thin protective layer is applied in different colors. The common processes used include Physical Vapor Deposition (PVD) and Electrochemical Coloring. There are different purposes to coloring stainless steel: enhanced corrosion resistance and aesthetic look. These colored stainless steels are used in many applications, such as in architectural facades, interior decorations, and public installations. Let’s discuss the corrosion resistance and longevity of coloured stainless steel in this blog.
Understanding the Passive Layer
A stainless steel contains at least 10.5% Chromium (Cr) content. The role of this alloying element is very crucial in the world of stainless steel because this element makes it corrosion-resistant. When 10.5% or more Cr element is added to the steel, it spontaneously reacts with oxygen and forms a protective layer of Chromium Oxide Cr2O3, called as passive layer. This protective layer covers the metal’s surface and makes it resistant to moisture, so, corrosion corrosion-resistant.
Environmental Factors
The environment for stainless steel is very important, though it is protected with a passive layer of Cr2O3. A simple, uncoated stainless steel can be used for decades in a simple environment, but may be prone to corrosion in harsh environments, such as in acidic rain, chloride-rich environments, etc. In chloride-rich environments, stainless steel easily catches pitting corrosion.
Mechanical Damage
The passive layer of Cr2O3 can reform, but once it is disrupted, stainless steel can be prone to corrosion. Mechanical damage, such as scratches, welding heat, or contamination, breaks the passive layer. For instance, during the welding process, due to the heat of welding, Cr reacts with C and forms Chromium Carbide along the grain boundaries. This compound formation leads to a lack of Cr within the grains and weak grain boundaries. Due to this phenomenon, stainless steel components fail due to intergranular corrosion. It is common in stainless steel 316.
How Coloring Affects Corrosion Resistance?
In this section, it is discussed how the coloring affects the core property of stainless steel, i.e., corrosion resistance. How will coloring affect, depending on the technology used for the coloring process? Two major processes of coloring stainless steel are discussed below.
PVD Coating Layer and Its Protective Mechanism
The physical vapor deposition method is carried out in a vacuum chamber. In this method, stainless steel is coated with the vapors of TiN4, ZrN, etc. This ceramic coating is highly dense and acts as a barrier, preventing oxygen, moisture, and chloride salts from reaching the surface. It enhances surface hardness and abrasion resistance greatly. A PVD colored stainless steel can maintain its color and gloss for decades, even in difficult conditions.
Electrochemical (Oxide Layer) Coloring
Electrochemical coloring is a process of applying an oxide layer to the stainless steel. This oxide film is applied through an electrical process. The different colors can range from bronze and blue to black. This change in color of the oxide film comes from the light interference within the oxide film. It is often more suitable for indoor decorative applications. In the case of outdoor applications, due to very low thickness, the color or integrity of the film may fade away during harsh conditions earlier than PVD coating because PVD coating is thicker than an oxide film.
Comparison Table: PVD vs Chemical Coloring vs Painted Finishes
The following table covers the effects of three different coatings on stainless steel properties.
Property | PVD Coating | Electrochemical Coloring | Painted Finish |
Color Formation | Metallic compound layer (TiN, ZrN) | Optical interference in oxide film | Pigment-based |
Corrosion Resistance | Excellent | Moderate | Variable |
UV Resistance | Outstanding | Limited | Depends on paint quality |
Hardness & Wear | High | Moderate | Low |
Typical Application | Façades, elevators, architecture | Interior panels | Industrial or temporary use |
Importance of Proper Surface Finish
The level of surface finish has a great impact on corrosion resistance. A general thumb rule is that the higher the surface finish, the higher the corrosion resistance. Such as,
- Mirror Finish: Smooth, reflective surface; highest corrosion resistance and easy cleaning.
- Brushed Finish: Linear texture; moderate corrosion resistance, needs regular maintenance.
- Bead-Blasted Finish: Matte, rough texture; lower corrosion resistance, traps contaminants.
Passivation and Protective Film Maintenance
Post-fabrication passivation processes, such as using citric or nitric acid, remove Fe-free ions from the surface, thereby recovering the Cr2O3 protective layer. Further, gentle cleaning and environmental monitoring can hence the life of passivated and protective films.
Real-World Performance and Testing Standards
Various accelerated aging and durability tests are carried out to evaluate the corrosion resistance of colored stainless steels.
Some standard tests include:
Salt Spray Test
It is performed according to ASTM B117. In the process, stainless steel is exposed to simulated marine salts to evaluate its resistance against pitting corrosion and surface degradation.
Humidity Test
This test is performed by exposing colored stainless steel to high-humidity cycles to check its oxidation resistance.
UV Exposure Test
It is done under UV radiation to check the color stability and gloss of colored stainless steel.
Abrasion and Adhesion Tests
These tests are carried out to determine the resistance coating to mechanical wear.
The case study shows that properly maintained PVD-coated stainless steel facades retained 90% of their original color and gloss even after 10 years of outdoor use.
Maintenance Tips to Extend Service Life
- For the extended lifetime of colored stainless steel, it is important to frequently clean the stainless steel completely, for instance, every 3-6 months indoors and 6-12 months outdoors. Use the detergent with neutral pH, which removes dirt, salts, and organic deposits that can trap moisture.
- Do not use abrasive or acid-based cleaners because they will break the coating, thereby reducing the coating’s lifetime.
- Try to inspect the stainless-steel components regularly to find scratches and coating damage on time. An inspection every 6-12 months of use is essential to use colored stainless steel panels for decades.
Conclusion
The colored stainless steel is used for two main reasons: aesthetic appearance and enhanced corrosion resistance. To color stainless steel, the most common techniques include physical vapor deposition (PVD) and electrochemical coloring. This colored stainless steel is used in architectural structures, indoor decorations, and public installations.
The coating applied through PVD is ceramic-like and dense, which becomes a barrier against moisture, salt, oxygen, etc., protecting the underneath surface. PVD-coated stainless steel can be used in harsh environments, such as in a chloride-rich environment. Electrochemical coloring provides a very thin layer and is used for aesthetic purposes. Regular cleaning and inspection can lead to using the colored stainless-steel panels for decades, as the case study shows.
Of course, superior processing and techniques will extend the lifespan significantly. Jianglin has a professional team, so please contact us right away.
