What is actually happening when lasers remove rust from metal?
In: Chemistry
The rough surface of rust absorbs the laser light, heats up and evaporates. The shiny surface of unblemished metal just reflects the remaining light for long enough to move the beam away to the rusted portion again and not cut through the metal.
Welding engineer here.
These types of equipment use a Fiber Optic laser. In this type of laser a glass fiber is *doped* with a small amount of the element Ytterbium. Because of the difference in refractive index between glass and air, light that travels lengthwise down the fiber reflects off the inside edges and is trapped inside of the fiber until it is emitted out the other end of the fiber. This is very efficient, the fiber can be made several hundred meters or up to several km long and allows next to no light to escape out the sides. Moreover the fiber doesn’t need to be carefully aligned and is resistant to overheating due to it’s high surface area compared yo it’s volume.
These lasers in fact use one laser to power another. A series of diode lasers that emit in the range of red are setup to shine down one end of the fiber. These are similar to diode lasers used in laser pointers, but much brighter and more powerful. The reasons for doing this should become clear in the next few paragraphs.
The red light travels down the fiber and is absorbed by the ytterbium atoms. This causes them to become energized like setting up a row of dominoes, however they don’t release their energy immediately.
In laser rust removal, the fiber laser is setup to operate in a *pulsed* mode.
At a certain point nearly all the ytterbium atoms in the fiber become energized. At that point various methods can be used to triguer the atoms in the fiber to dump their energy all at once. This is like knocking down dominoes stacked in a row. Often a weak, very short pulse of infrared light in the range of about 1060-1100 nanometers is produced from a specialized, seperate master oscillator laser.
As this passes into the main fiber, it triggers the ytterbium atoms to discharge their stored energy at the same wavelength. This amplifies the initial pulse tremendously. In the case of a rust removal laser it may be amplified inside the highly efficient fiber, by a factor of more than ten million X. This is known as light amplification by stimulated emission of Radiation.
Because of the high peak power and peak intensity in the pulse, sophisticated methods may need to be used to prevent the fiber from being damaged. The pulse has short duration of less than a billionth of a second. This means that the park intensity of the pulse is ridiculously high, because all of it’s energy is concentrated in such a short span of time.
Meanwhile the “pump laser” operates continuously, and after the ytterbium has dumped it’s stored energy, the intense red right recharges it quickly. Several hundred pulses per second can be produced this way or more.
Now, this is probably the most interesting part that you care about. As the train of pulses exits the fiber, optics are used to scan the train of pulses across the surface of the material, usually with a rotating mirror. The optics also fine tune the focus if needed.
Rust or dark paint is very absorbant towards infrared light in the range of 1160 nm. The high peak intensity converts the surface of the rust into a hot plasma. The pulse of light is so short, heat can’t go anywhere into the underlying material by conduction. In that short period the surface directly exposed to the beam just gets hotter and hotter and hotter. The speed of the expanding bubble of plasma is positively glacial by comparison to the length of time the pulse lasts.
This expands explosively like a tiny bit of TNT, which blasts off the underlying rust in the same way as sandblasting does. However the total energy in each pulse is still modest so it doesn’t cause much or any damage to the underlying metal.
Moreover, most metals are great at reflecting infrared, even better than visible light. So once the rust is gone the pulses just bounce off the clean metal surface without vaporizing any of the exposed metal, so no shock wave is produced. For the same reason, the metal is only usually warm to the touch after treatment.
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