Metal detectors have been common and useful devices to detect metal objects for a long time. Most people have very likely been tested by metal detectors many times in their life. In most cases, metal detectors are used for security and safety purposes to allow security staff to prevent people from carrying weapons and/or dangerous objects into sensitive locations like airports, government buildings, hospitals, schools, or shopping centers. This makes it especially important for metal detectors to be able to detect a great variety and number of different metals.
Metal detectors are excellent tools for finding hidden metal objects like coins, artifacts, items of archeological interest, and similar things. They may also be used by police to detect either belonging of a lost person or objects that need to be recovered and/or could pose a threat to anyone. So what metals cannot be detected by a metal detector?
Usually, metal detectors can recognize any metal object, but there are some rare exceptions. This also depends a lot on the quality standards of a device. Low quality or older detectors may have more issues with detecting certain metal objects that consist of a material that may not be detected as easily, for example, metals like iron. Most metal detectors also can be adjusted and programmed so that there is a higher chance of detecting metals they would usually struggle to find. These adjustments, however also may make it more difficult to detect other metals with different characteristics in return.
Categories Of Metals
Generally, one can speak of two categories of metals that are relevant to detecting metal objects. There are ferrous metals and non-ferrous metals. Ferrous metal is any sort of metal that can be attracted to a magnet. Ferrous metals have the characteristic of easily and quickly rusting when exposed to air and water. Ferrous metal is typically the easiest metal to detect and usually the most common contaminant in industrial environments. Examples include paperclips, thumbtacks, pins, staples, most screws, nails, washers, welding slag, rust, abrasions from metal to metal contact, and tools dropped into the conveyor.
Non-Ferrous metals are all those metals we can call non-magnetic metals (copper, aluminum, brass, lead, etc.) It will take approximately 50% more mass of a non-ferrous metal to be as detectable as a ferrous metal. Manganese is also a non-ferrous metal and is difficult for most metal detectors to detect.
How Do Metal Detectors Work?
Metal detector struggles with metals that have a low level of electric conductivity. The device works by sending and receiving electromagnetic fields. This is why metals need to be proper conductors so that they can be recognized by a metal detector.
Metal detectors send electromagnetic fields, for example, to discover items that are hidden in the ground or covered by other materials such as stone, sand, leather, wood, or plastic. If the device locates an object that has conductive characteristics, the constitution of these electromagnetic fields sent by the device change. This change signals, that a metal object has been detected. The lower the conductive quality of the metal is, the lesser the chance that it would change the behavior of the sent electromagnetic fields and thereby would be recognized. If the electromagnetic field sent by a metal is very low, the object needs to be larger to give a higher chance of being detected by a metal detecting device.
Stainless Steel
The metal that detectors have the most difficulties in terms of detecting is stainless steel. Stainless Steel is always the most difficult metal to detect due to its poor electrical conductive qualities. By definition stainless steel has low magnetic permeability. A stainless steel sphere would have to be 50% larger than a ferrous sphere to produce the same signal strength on the metal detector Stainless steel is an alloy of iron and chrome (alongside some other materials such as carbon, nitrogen, aluminum, silicon, sulfur, titanium, nickel, copper, selenium, niobium, and molybdenum). The chromium content, which is usually around 10-12%, produces a very thin layer of chrome oxide on the surface of the steel. This layer is called the passivation layer. This passivation layer prevents the stainless steel to corrode. However, this also leads to a strong reduction of the electromagnetic conductivity of the material, thereby making it a lot more difficult for some metal detectors to detect objects made from stainless steel. If there is any metal that may not be detected by a device at all, then it would by any means be stainless steel.
Titanium
Another metal that may not be detected by all metal detectors is Titanium. Titanium is a fairly strong material with a very low density that is often used for dental implants or in the manufacturing of jewelry. It also falls under the category of non-ferrous metals. Just like stainless steel, it has excellent corrosion resistance but also a very low grade of electromagnetic conductivity. These characteristics also make it very difficult for metal detectors to find objects made of Titanium.
Conclusion
It should be added that it would be wrong to say that any of these metals outlined as difficult to detect by metal detectors would be entirely impossible to detect. This is not correct. As said above, there are differences in terms of the quality and characteristics of metal detectors. Some devices may have stronger problems with finding a metal like stainless steel or Titanium. Some devices may indeed be unable to locate those altogether. This is especially the case is an object is very small and therefore emits a more insignificant electromagnetic field.
But due to technical progress and also a higher demand, especially in regards to security issues, manufacturers of metal detecting devices keep constantly improving their products and the technology behind them. Sooner or later, it can be expected that devices that are entirely unable to detect critical metals or which aren´t reliable in detecting those will disappear from the market and will be replaced by such devices that would make it possible to also detect metals with very low emission of electromagnetic fields.