Why iron metal density matters for laser rust removal efficiency

Abstract

This paper presents an analysis of the influence of material density on laser beam machining (LBM) process efficiency in surface oxide removal from ferrous materials. Laser rust removal efficiency and safety are not only a function of the laser optical system performance but are also highly dependent on the thermophysical differences between the ferrous substrate and its surface oxide films. This includes iron metal and oxide volumetric densities that are associated with iron metal thermal diffusivity, oxide acoustic impedance, and ablation thresholds. This paper uses a data analysis approach to identify the iron-specific density and associated iron material characteristics that allow for a highly selective, self-limiting, subtractive process with strict micrometer precision for the iron substrate preservation.

Introduction to Thermophysical Material Interaction.

The common goal for any industrial surface preparation and infrastructure maintenance is the removal of corrosion byproducts from carbon steel surfaces to enable effective coating adhesion. Laser surface processing using nanosecond-pulsed near-infrared (NIR) lasers offers a thermodynamically controlled method as compared to the mechanical abrasive blasting process. Laser-based surface processing is not fully understood, and the selective removal of rust by high-power laser radiation without distorting the iron surface remains unexplained. It is therefore important to investigate the mass and structural changes to the atomic lattice of iron to enable a deeper understanding of the iron surface and its interaction with the laser radiation. The iron metal density at standard temperature and pressure (STP) is 7.874 g/cm³. Upon iron oxidation, oxygen atoms are incorporated into the iron lattice, and a contaminant iron oxide layer forms. The iron oxide contaminant layer has different thermophysical properties, and the numerical value of the iron metal density will be used to derive the heat diffusion equation, photo-thermal vaporisation threshold, and photo-acoustic spallation threshold, and hence the simulation model for laser cleaning.

Crystallography and Iron Mass Density

To understand how the huge macroscopic effects of laser ablation are produced on a relatively small scale, it is necessary to begin by more or less qualitative examination of the structure of the substrates and, consequently, of the microparameters associated with these structures. At room temperature, the crystal lattice of the most well-known pure metal (iron at room temperature is purely crystalline, and the structure at this temperature is body-centred cubic (BCC) ferrite or iron) may be conveniently specified through the equation for iron mass density Fe being a function of its atomic weight a Fe and the packing efficacy of this structure:

Where:

• n is the number of atoms per unit cell (for BCC, n = 2).
• A is the atomic mass of iron.
• V_c is the volume of the unit cell.
• N_A is Avogadro’s constant.

Iron has a density of approximately 7.87 g/cm³. High thermal mass means that the iron is able to absorb a certain amount of heat energy before undergoing a phase change. This, and high mass density, means that in laser processing, a high energy density is generally required to bring iron to a melting temperature of 1538 °C or vaporisation temperature of 2862 °C. Again, a high iron metal density acts as a thermal sink for the absorbed photons, limiting any buildup of heat in the interaction zone. This helps to prevent the formation of surface defects due to heating or melting caused by the photons directly interacting with the bare substrate.

The Oxidation Mechanism and Density Stratification

Corrosion is an electrochemical process in which the dense α-iron structure is converted into iron oxides. The iron oxides that are formed on the structural steel are three in number:
Iron Oxide

Wüstite (FeO) Density at Temperature: ~5.70 g/cm³. Wüstite forms at higher temperatures.
Magnetite(Fe3O4): A mixed Fe(II, III) oxide with a density of 5.17 g/cm³.
Hematite(Fe2O3): The most common form of environmental rust, presenting a density of 5.26 g/cm³.
Iron to Hematite Corrosion. The major observation of interest in this figure is the drop in density by a factor of nearly four. The original solid iron has a density of 7.874 g/cm3, and upon complete conversion to hematite (fluid magnesia), the value drops to 5.26 g/cm³. The increase in volume of a solid material upon reaction to form a new solid is measured by a quantity known as the Pilling-Bedworth Ratio or R_pB. The R_pB is defined as the ratio of the volume of the product oxide to the volume of the original metal consumed, which, in the case of iron converting to hematite, is given by:

The oxide density(p_oxide) is much lower than the iron metal density(p_metal), so the volume of the rust is much greater than the iron consumed to form the rust. The R_PB for iron oxidising to Fe₂O₃ is approximately 2.14.
The rust layer is porous, structural integrity is compromised, and it has a lower density than the metallic substrate. This density stratification – a low-density porous structure on top of a high-density solid crystalline lattice – is exactly what laser cleaning preys on.

Optical Thermodynamics and Thermal Diffusivity

Since we are talking about the oxidised surface of a catalyst and are using a nanosecond pulsed 1064nm laser, energy must be absorbed in order to cause the ablation process that we are interested in. A quantity that is often mentioned in relation to the optical absorption of a material is the optical absorption coefficient (this term is often used interchangeably with reflectivity vs. absorptivity). However, energy must then be dissipated within the material before it can cause a physical effect, and this is controlled by the thermal diffusivity(α).
Thermal diffusivity is the rate at which heat diffuses from a hot spot to a cold spot in the material. It is related to the material density through the equation:

Where:
k is the thermal conductivity(W/m⋅K).
ρ is the material density(kg/m3).
C_p is the specific heat capacity(J/kg⋅K).
The low density and poor thermal conductivity of the iron oxide film (k of iron oxide is less than 3 times that of air, versus 50 for steel) give a low thermal diffusivity (α), which means that the rust has very poor heat dissipation properties. Thus, when the laser pulse is absorbed, there is little heat removal, and the heat remains in the low-density oxide structure, causing a very rapid peak in the local temperature in excess of the vaporization threshold.
On one hand, the rust is easy to remove while the laser is still on the surface. Once the rust is removed, the equation changes. The high iron content and high thermal conductivity of the metal result in a higher thermal diffusivity. The heat generated by the laser is then quickly dissipated into the bulk of the metal, keeping the substrate temperature beneath the melting point.

The Ablation Threshold and Fluence

The useful physical properties relate to the threshold fluence needed to remove material. The threshold fluence, F_th, is defined as the laser fluence that is required to remove material, and is measured in J/cm² (Joules per square centimetre).
The theoretical ablation threshold for a given material is proportional to its density and the latent heat of vaporisation(Lv):

Where τ is the laser pulse duration.
This equation shows that density is important in determining the interaction of a laser with a target. The ablation threshold fluence F_th is a quantity that describes the minimum amount of energy that needs to be absorbed from a laser in order to vapourise the target material. The equation shows that it is directly proportional to the mass density of the material.
The higher density of Fe (iron) as compared to hematite means that much higher fluence is needed to ablate bare steel as compared to the rust layer. By metering the laser energy to operate at a well-defined fluence between the low fluence threshold of the rust and the high fluence threshold of the dense iron, a strictly self-limiting process is then created. The laser will then vaporise any low-density contaminant, while the laser energy is under specified conditions (i.e., too low to vaporise the dense iron of the bulk steel).

Acoustic Impedance and Spallation Dynamics

In addition to thermal vaporisation, laser rust removal also uses the principle of photo-acoustic spallation. When a laser pulse hits the surface, the material rapidly heats up, and this causes pressure buildup in the form of a shockwave.
The behaviour of this shockwave is determined by the acoustic impedance Z of the materials it is passing through. Acoustic impedance is given by the formula v.

The large difference in acoustic impedance at the solid steel interface is a result of the huge difference in iron metal density compared to the low density of the oxide layer.
Physics of the interaction. The wave in the low impedance of the gas (air) hits the high impedance of the iron lattice and bounces back as a wave in the iron. This compressive acoustic shockwave mechanically interacts with the thin oxide layer, causing the tensile wave generated by the reflectivity of the iron lattice to “spall” or fracture the oxide, causing it to eject from the surface. The iron acts as a dense “acoustic reflector” necessary for this effect to occur; without it, the laser interaction would be far less effective.

Comparative Substrate Densities in Laser Processing

The principles we have derived from iron cannot be directly applicable to other industrial alloys. It remains to be seen how easy it will be to control the laser parameters in accordance with the different densities of other alloys.

Density of Aluminum

The density of aluminum is 2.70 g/cm³ or 0.0975 lb/in³. The low density, about one-third of iron, results in a low thermal mass. Aluminum also has a very high thermal conductivity and high optical reflectivity at the 1064 nm wavelength.
It is important to take into account the low density of aluminum when setting parameters for laser cleaning and welding applications. The high thermal conductivity of aluminum causes the heat to be dissipated very quickly, but due to the low mass density, this effect can be easily overcompensated with too high energy input. This leads to aluminum reaching the melting point (660^∘C) at a much faster pace than with steel. Generally, for cleaning and welding aluminium, one needs higher peak powers with shorter pulses than for iron.

Density 6061 Aluminum

Certain aerospace and automotive alloys, such as 6061 aluminium alloy, which consists of aluminium with magnesium and silicon, have a density of approximately 2.70 g/cm³. The slightly differing specific heat capacity and thermal diffusivity of the alloys mean that very minor adjustments are required in the laser pulse overlap factor to avoid surface micro-melting.

303 Stainless Steel Density

The most common grades used in precision machining are 300-series stainless steels. The density of 303 stainless steel is estimated to be 8.00 g/cm³, which is close to that of iron but slightly increased due to chromium, nickel, and sulfur, which are introduced for better machinability.
Compared to standard carbon steel, dense stainless steel has a higher density and lower thermal conductivity. Therefore, when heated by laser energy, the surface of the material stays hotter for a longer time. During laser cleaning of oxidation or heat tint from 303 stainless steel, the frequency must sometimes be decreased to allow for thermal relaxation of the material to prevent HAZ expansion.

Dimensional Tolerance and the Self-Limiting Effect

With such a high iron mass density, the threshold of ablation is quite high. In the design of many systems, components have specific dimensional tolerances that must be met. An example of this would be the slip-critical joints that are often used on bridges. The faying surfaces of these joints must have no more than ± 0.05 mm tolerance.

The method of abrasive blasting also uses energy, but in a vastly different manner. The rust and a small portion of the bulk material are broken down because of the indiscriminate kinetic energy of the abrasive (such as steel grit or garnet) on the surface. Although much care is taken to ensure a minimum amount of substrate profiling,

The tolerances that are required to ensure the proper fit of components are almost always exceeded.

Laser beam machining, however, is based on much different principles. The phase change that occurs in the material due to the laser is restricted to a very discrete area. The laser fluence is always controlled so that the material does not go into an ablating phase. The material is only removed up to the a-iron/diffusion oxide interface, and therefore the dimensional integrity of the bulk material is ensured.

Empirical Validation and Surface Profilometry

To verify the results concerning density and ablation that have been derived theoretically, the engineers make use of strictly metrological post-processing of diagnostics.
White Light Interferometry
The 3D profile of the laser-cleaned surface has been obtained by non-contact optical profilometry (Aeroscope), confirming that the surface parameter S_a is related only to the initial profile of the steel produced by the manufacturing process, indicating that the high iron metal density has allowed the surface to avoid removal by laser ablation.
Energy-Dispersive X-ray Spectroscopy (EDX)
After ultrasonic cleaning and rinsing, the sample surface was analyzed using a Scanning Electron Microscope (SEM) and Energy-Dispersive X-ray spectroscopy (EDX). The oxide removal process being carried out based on density differences should result in a surface with near zero Oxygen weight percent, whilst maintaining the original metallurgical iron alloy iron ratio.
Carefully controlling design parameters based on the density of Fe ensures that the components’ structure, chemistry, and dimensions are not compromised in any way.

Conclusion

Removing surface oxides using high-power lasers is a thermophysical process that is quite complex. It is not simply a question of optical power, but more so of the physical differences between the contaminant and the base material. The iron metal density of 7.874 g/cm³ serves as a reference value for thermal mass, acoustic impedance, and the energy required to change the state of the material.
The iron corrosion process and its associated iron oxide products are well characterized. Understanding the mathematical relationship between the dense metallic iron lattice and the low-density, highly porous structure of the iron oxides has made it possible to precisely control laser pulse length and energy fluence. Thus, allowing for the selective vaporisation of corrosion products without exceeding the thermodynamic limits of the base material. Thus, enabling stringent dimensional tolerances, which are a prerequisite for many of the applications of laser surface processing in advanced manufacturing and maintenance of heavy infrastructure.

References

1. Bäuerle, D. (2011). Laser Processing and Chemistry (4th ed.). Springer-Verlag Berlin Heidelberg.
2. Steen, W. M., & Mazumder, J. (2010). Laser Material Processing (4th ed.). Springer-Verlag London.
3. Callister, W. D., & Rethwisch, D. G. (2014). Materials Science and Engineering: An Introduction (9th ed.). John Wiley & Sons.
4. Ready, J. F. (1997). LIA Handbook of Laser Materials Processing. Laser Institute of America, Magnolia Publishing.
5. Kou, S. (2003). Welding Metallurgy (2nd ed.). John Wiley & Sons.

Frequently Asked Questions (FAQs)

1. What is the iron metal density, and how does it compare with the rust?
   The theoretical density of pure iron is 7.874 g/cm³ at standard temperature and pressure. Environmental rust or brus is formed from iron oxide or hematite (Fe₂O₃), and the average density of the rust (iron oxide) is about 5.26 g/cm³. This large difference in density is a major reason that the oxide can be laser vaporised and the base metal remains intact.
2. What is the effect of iron (Fe) density on thermal diffusivity when laser processing FeNi alloys?
   Thermal diffusivity is the ratio of the square of the distance that heat diffuses in that distance. High thermal diffusivity values mean low material density. Iron has a high density and a high thermal conductivity. It is an excellent heat sink that removes heat from the surface area very quickly. The rust has a lower density and an extremely low thermal conductivity. The heat of the laser is trapped, and the temperature of the rusted area is rapidly increased. The rust vaporises very quickly.
3. Why is it more difficult to laser clean aluminum when the density of aluminum is less than that of iron?
   The density of aluminum is approximately 2.70 g/cm3, which is lower thanthat of iron. In fact, the optical reflectivity of aluminum is high at this wavelength, and the thermal conductivity is also very large. If the mass density is low and no effective heat dissipation occurs, it will lead to instant melting. Hence, it is crucial to create very well-controlled, short pulses while laser processing the aluminum to prevent damage to the substrate.
4. Are the density differences between 303 stainless steel and standard carbon steel materials relevant for the adjustment of laser cleaning parameters?
   Yes. The density of 303 stainless steel is approximately 8.00 g/cm³. This higher density is caused by the addition of elements such as Chromium and Nickel. Stainless steel generally has a lower thermal conductivity compared to carbon steel. The surface heat takes longer to dissipate, which means that laser parameters may need to be adjusted by reducing the pulse frequency or increasing the scan speed to ensure that the metal has time to cool and expand in a minimal area known as the heat-affected zone (HAZ).
   A) Attraction between iron and a magnet
   B) Relationship between acoustic impedance and iron mass density during cleaning
   C) Difficulty cleaning iron powder particles that stick to the core
   D) Connection between acoustic impedance and iron mass density that affects cleaning efficacy
   
5. The acoustic impedance is the combination of the material’s density and the speed of sound in the material. Iron has high density and thus the steel substrate has high acoustic impedance, which acts as a boundary. The laser excites a shock in the low-density rust, and a sound wave is reflected towards the steel by the higher acoustic impedance between the steel and the rust, which has very different properties. This reflected sound wave then develops sufficient tensile stress in the rust to spall it off the surface of the steel via photo-acoustic spallation.