The Basic Principle of Laser Rust Removal:
The key to laser rust removal's effectiveness lies not in "burning" rust, but in its "photoacoustic effect."
Laser absorption by the rust layer: Rust (Fe₂O₃, Fe₃O₄) has a high absorption rate for lasers of specific wavelengths (such as 1064nm infrared light), while the clean metal substrate has a high reflectivity.
Rapid Energy Conversion: After instantly absorbing high energy, the rust layer experiences a rapid temperature increase, vibrating, vaporizing, and transforming into a plasma.
Instantaneous Exfoliation: These heated, expanding materials generate shock waves that "blast away" or exfoliate the surface rust layer. The substrate, however, reflects most of the energy, remaining largely undamaged.
In-depth Analysis: Rust Types and Laser Compatibility
1. Red Rust (Fe₂O₃)
Characteristics: Loose and porous, typically located on the surface of the material ("floating rust"). It has a weak bond with the substrate.
Cleaning Difficulty: Easy.
Reason: Its loose structure allows laser energy to easily penetrate and quickly remove it. Even lasers with low energy density can effectively remove it.
2. Black Rust (Fe₃O₄)
Characteristics: Dense, hard, and stable, typically indicating that oxidation has penetrated deep into the substrate and is strongly bonded to it. It is often the product of further oxidation or long-term corrosion of red rust under specific conditions.
Cleaning Difficulty: Extremely difficult.
Reason:
Density: The dense structure makes it difficult for the laser to penetrate and completely remove it in one go.
Adhesion: Strongly bonded to the substrate, requiring higher energy densities to break this bond.
Depth Issue: Black rust often has "rooted" deep into the substrate. As a surface treatment technology, laser treatment has a limited depth of action. Even if the surface is cleaned, the iron oxide inside may reappear on the surface soon after, forming new rust.
Choosing a Laser Type: Why "Continuous" Lasers Are Inferior to "Pulsed" Lasers
Continuous Laser Operation: Continuously outputs energy, like a continuously heated "soldering iron."
Disadvantages: Heat accumulates and conducts continuously. While removing rust, a large amount of heat is transferred to the metal substrate, causing overheating, deformation, and even damage. It lacks the instantaneous impact force to remove firmly bonded black rust.
Applications: Rarely used for precision rust removal, but may be used for large-area, thick, and loose rust removal where substrate damage is not a concern.
Pulsed Laser (Mainstream Choice)
Operation: Instantaneously releases high energy in extremely short pulses (nanosecond, picosecond, or even femtosecond), like a high-frequency "hammer" striking.
Advantages: Each pulse lasts only a short time, preventing heat from transferring to the substrate, achieving "cold processing" and protecting the substrate. Furthermore, the instantaneous high power density generates a powerful shock wave, effectively removing contaminants.
In pulsed lasers, the spot pattern (single-mode/multi-mode) is crucial:
Multimode pulsed lasers：
Spot: The energy distribution is uneven, with a "saddle" or "flat-top" shape, and the spot is typically large.
Energy density: Relatively low.
Applications: Ideal for cleaning large, weakly adherent contaminants such as surface rust (red rust), paint, coatings, and oil stains. Highly efficient and relatively low cost.
Single-mode pulsed lasers
Spot: The energy has a perfect Gaussian distribution (brightest at the center, decreasing toward the edges), allowing it to be focused to an extremely small spot.
Energy density: Extremely high ! This is because the energy is concentrated into a very small spot.
Applications: Ideal for cleaning stubborn contaminants.
Red rust: Easily removed.
Black rust: With its extremely high energy density and powerful penetrating power, single-mode pulsed lasers are the only type of laser capable of removing black rust. They can break down dense black rust layers.
However, it has limitations: even a high-power single-mode laser may not be able to completely remove black rust that has penetrated deep into the substrate. It can clean the surface very cleanly and beautifully, but it cannot change the oxidized state of the substrate inside.
