How to Restore an Obliterated VIN: Chemical Etching vs. Magneto-Optical Imaging

Just like every human’s fingerprints are unique, no two cars have the same vehicle identification number (VIN). And just like criminals try to destroy all fingerprints around the crime scene, obliterating original VINs is a common method used for vehicle machinations.

The method usually involves grinding off the existing VIN so that the digits can’t be read. As a result, the actual parts might be genuine, but the identity of the car is changed.

The good news is that the original VIN can be restored.

When manufacturers stamp a VIN into the chassis of a car, the structure of the metal around the number is also compressed due to displacing of the atoms from their position in a crystal lattice. The displacing of the atoms creates stress in the structure which remains even if an original stamp has been scratched, ground, chiseled, or otherwise mechanically obliterated.

It doesn’t matter which technology was used to apply the number. Internal stress in metals arises with any other marking technologies, be it engraving, laser marking, or mechanical drawing.

Examination methods in this area are similar to the techniques applied in industrial defectoscopy, technical condition diagnostics, analysis of disaster root causes (in transport, energetics, building constructions, etc.), as well as forensic examination of firearms or even artworks. They can be split into two groups: 

• Destructive: Chemical etching, electrochemical etching, thermal plasticization, mechanical microhardness testing;

• Non-destructive testing (NDT): Visual and optical inspection, magneto-optical imaging, magnetic particle inspection, ultrasonic flaw detection, eddy-current testing, and X-rays.

Nowadays, the most popular and effective methods of restoration are chemical etching and magneto-optical imaging. Let’s have a closer look at both.

The chemical and electro-chemical etching procedure typically requires intensive polishing of the surface to get a clear contrast image. That may seem counterintuitive, but scratches make the etching results much less readable. Still, the expert must be careful not to peel away too much material. 

In the next step, an etchant is applied to the surface. The etchant is a chemical reagent that reacts differently depending on the amount of work-hardening the VIN plate material was subjected to. Different materials require different chemicals. If the material is unknown, you’ll have to analyze it first.

Due to the internal stress, compressed and non-compressed areas of metal have different rates of reaction with chemicals so their etching speed differs. Thanks to this, a new surface relief is formed: the outlines of digits will appear where they were previously stamped. At this point, the reaction should be stopped and the results must be documented. Otherwise, the reaction in the non-compressed areas will catch up, and the information will be lost. 

You can see how it works in this video by Carlo Frezzotti:

Although the technology has been a proven method for decades, several caveats exist.

First of all, the contrast of the observed characters is always weak: gray outlines on a gray background. Hence, there’s a chance of wrong or incomplete VIN recognition.

Second, capturing a photograph of the result can be difficult. As seen in the video above, characters are visible alternately: in the glare, and at certain angles of illumination or observation. Sometimes an expert can see the digits but can’t take a photo of them.

Another caveat is that strong acids used for etching damage coating layers and metal surfaces. This means that in some cases, you may not only fail to reveal the VIN, but also possibly destroy it completely.

For example, that’s often the case when the object is corroded. Surface corrosion must be removed before etching. But if the corrosion is deep, all the information might be hiding within this layer. Chemical etching in this situation will lead to complete information loss.

Working with acids requires strict safety measures. Contact of etchants with skin and mucosae is strictly prohibited. Acid vapors are also harmful to health, so the expert should wear protection, because the etching process can take hours. 

Last but not least, VIN plates on fully assembled vehicles aren’t as easily accessible as a small steel block. And, of course, you have to be careful not to damage the paintwork on the car.

Magneto-optical imaging (MOI) is a non-destructive method that visualizes magnetic field anomalies of the object under scrutiny. To do this, the object is magnetized using a magnetostatic field generator and an eddy current inductor.

The examination is performed in three stages:

1. Magnetography, or recording the object’s magnetic field data on a magnetic tape;

2. Visualization of a magnetogram;

3. Magneto-optical (MO) image analysis.

In this video, an expert demonstrates real-time restoration of the eliminated number with magneto-optical imaging. The test object is similar to the one shown in the etching video above.

A magnetic tape, which serves as a signal source for a magneto-optical sensor, is demagnetized. Then the expert places the tape over the area where the destroyed number is supposed to be. 

Magnetography is performed by drawing an electromagnetic scanner across the tape. Meanwhile, the object is simultaneously magnetized using magnetostatics and eddy current. As a result, a magnetogram is produced—a recording of data on the object’s magnetic field, showing the pattern of residual metal stress in the surface layer.

In the second stage, the expert visualizes the magnetogram by scanning it with a magneto-optical sensor. It’s an automatic process. The magnetogram is fed in small steps, with each step tightly pressing against the surface of the MO sensor. It creates separate frames of the MO image, a video camera reads these frames, and then they are panoramically stitched in a computer.

In the areas where the digits were previously punched, you can notice magnetic field anomalies caused by the different metal structures. This is known as the magneto-optic Faraday effect, the rotation of the light polarization plane depending on the magnitude and direction of the magnetogram’s magnetic field.

The expert sees the MO image of the magnetogram on the computer screen and performs an analysis. In the example in the video above, the value of the destroyed (grinded off) original marking is confidently recognized: it’s 29165. The high sensitivity of the MOI device provides a good level of detail, which allows experts to identify the shape and specific attributes of the used stamp.

Unlike chemical etching, where there’s always a risk of destroying evidence, you can run a magneto-optical examination as many times as you need. 

The method doesn’t require any preparation, depending on the material of the examined object. It can be any alloy possessing magnetic properties (steels, cast irons) or electrically conductive characteristics (aluminum alloys, stainless steel).

Surface corrosion is not a problem either. On the contrary, it’s an important source of information which is susceptible to a magnetic field.

The result of the examination is available almost immediately. The high contrast of the MO image (black and white outlines of digits on a gray background) provides high reliability of original VIN recognition. If necessary, you can then save the examination result as a graphical file of a microscope-level quality image to attach to a report or forward to stakeholders.

More often than not, altered VINs look pretty normal. Now, let’s put ourselves in the shoes of an officer who is examining a vehicle. 

Do we really have grounds for removing the paintwork? 

Or maybe this is the original VIN? 

The thing is, when a slight suspicion is all we have, checking the VIN quickly, convincingly, and without creating unnecessary troubles is only possible by means of non-destructive methods, such as MOI.

Below is an example of complete VIN removal along with a surface layer of metal. MOI allows you to catch the distortion of the magnetic field in the area of the former relief markings and visualize the outline.

Corrosion, which may be natural or artificially stimulated to hide the signs of alteration, makes it impossible to recognize the VIN optically. 

However, rust is an excellent magnetic medium, and the structural heterogeneity of rust is the carrier of VIN data. There are no alternatives to the MOI method among non-destructive examinations of a corrosion layer and slag.

We’ve carried out a simple experiment to find out what can see deeper: chemical etching or magneto-optical imaging.

For the experiment, we used 2 mm thick steel coupons with stamped numbers at a distance of 10 mm from each other. The specimens then were 1:100 cone tapered. A 1:100 ratio means that, at the end of the 100 mm segment of the coupon, 1 mm of material was removed from the surface of the sample.

Thus, we got metal coupons where 0.4 mm of material was removed in the area of the number four—that’s the average depth of a VIN stamp. From there, we can no longer see the numbers with optical instruments. The place of the number 8 corresponds to the double depth of the VIN stamp, and the second 2 to the triple depth of the stamp.

For the chemical (acid) examination, we used the standard acid kit forensic experts usually have on hand. At the time of the experiment, we were able to see numbers all the way to number 9. Numbers 8 and 9 later faded and were no longer visible by optical tools.

We examined both acid-treated and non-treated test coupons, using the eddy current testing method. Here’s what we got:

Being a destructive method, acid-based examination removed portions of the information layer. Such removal results in a shorter area of definitive character recognition for the acid-treated specimen.

We can clearly see the first twelve digits (up to the second numbers 1 and 2) and traces of 3 and 4 on the specimen, previously treated with acid. On the non-treated coupon, there are fifteen clear digits, all the way to the second 4. 

In some experiments, we were able to recover all sixteen digits from the specimen. These results enable us to confidently state that non-destructive MOI methods under certain examination conditions can reach up to the depth of 3–4 times of the original stamping.

Regula’s line of forensic hardware includes two devices designed for the examination of metal surfaces and detection of VIN falsification.

Regula 7505M. This is a multifunctional device used by the police, insurance companies, car rental and leasing companies, and court experts. They use it for visualization of structural inhomogeneity of ferromagnetic metal surfaces, such as vehicles or firearms registration numbers. 

The set comprises several tools: a magnetic copying toolkit, a USB device for magneto-optical imaging, and the Spectral Luminescent Magnifier Regula 4177. The latter is used for express document authenticity examination. 

Given the difficulty of accessing certain areas of a vehicle, the MOI kits include extension pieces that increase the reach and the angle.

The package also includes an auto document template database, so you get a multi-station solution for complex vehicle identification, from VIN checks to driver's license verification.

Regula 7515M, an eddy current magnetographing device.

This compact device serves as an extension to the Regula 7505M by offering the capability to examine the surface of nonferromagnetic materials, such as alloys of zinc, aluminum, copper, or stainless steel. Being sensitive to internal stresses of metal, rather than physical deformation, it can also work with weak signals in surface layers of ferromagnetic objects.

We hope our overview of magneto-optical imaging shed light on this useful method of revealing the true identity of a car. 

Now, we’d like to hear from you.

Was there anything that we missed? Or maybe you’d like to get a consultation on using Regula’s tools for MOI? 

Either way, we are always glad to hear from you in our LinkedIn group, as well as via the form below.