So far in 2019 Gold is roughly up by 3% and if the stock market loses pace, investors looking for safety tend to re-direct their attention to the precious metal. This is not a new phenomenon as Gold plays an important part of a diversified investment portfolio, as it’s price increases when the value of paper investments, such as stocks and bonds, may start to decline. Although the price of gold can be volatile in the short term, it has always maintained its value over the long term.
Alongsidethe attractive feature of Gold being a reliable investment as part of a diversified portfolio,runs the endemic problem of Gold (Au) bar and ingot forgery. The production of fake gold bars has become more common-place as criminals use this precious metal’s popularity to their advantage.
A common forgery method used is to plate Tungsten (W) with a thin layer of gold (about 0.1mm). Tungsten is used as it has a very similar density to gold but worth 1% of the value. With this approach the result is that bars can be created weighing the same as a genuine gold bar. A standard gold bar held in reserves by central banks and traded among bullion dealers is 400-troy-ounce , equivalent to 12.4kg or 438.9 ounces.
NDT methods are often used to test whether a Gold bar is legitimate or not, but NDT inspectors have discovered that these methods of forgery defeat the current favoured NDT inspection method of X-Ray Fluorescence (XRF), as this only tests the material surface.
Drilling a hole to take a sample for an assay analysis is not permitted as this would remove valuable material. These challenges have led to the enquiry and exploration to find another NDT method which could be used, with reliable results, to test for this condition.
This is where Ether NDE come in, as we were approached to examine whether Eddy Current inspection could be used, as an alternative to the X-Ray methods, to test Gold (Au) bars, reliably establishing authenticity.
“Sigmacheck 2 success…”
In response John Hansen, Ether NDE’s General Manager, took up this challenge and established that the Gold bars could indeed be tested effectively using Eddy Current. John states "I established in theory this was possible at 60kHz using our SigmaCheck 2conductivity meter, as the conductivity of Gold is 45 Mega Siemens per m (MSm-1) (or if you prefer 77.6% IACS or resistivity 2.2 ƒÝƒà.cm) whereas Tungsten is 18 Mega Siemens per m (MSm-1) (or if you prefer 31.3% IACS or resistivity 5.5 ƒÝƒà.cm).”
“The Skin Effect”
John continued … “However there is of course the skin effect to overcome (this is a phenomena that limits the eddy current concentration to the material surface). The good news here is that at 60kHz the traditional frequency for conductivity measurement the skin depth (average depth of penetration) in gold is 0.3mm and the gold plate is no more than 0.2mm, so this works very well."
Following this cycle of in-house research and test, our enquirer came back to us reporting that they had conducted further tests tests on forged and real gold bars and that using the Eddy Current NDT with the SigmaCheckconductivity meter performend very well.
The SigmaCheck 2 now carries this capability as standard, with enhanced software enabling the penetration of thicker plating.
As well as forged or materials plating detection the SigmaCheck Conductivity meters are also ideal for the following test scenarios:
Material Verification and Sorting
Heat Treatment Verification
Heat or Fire damage investigation
Lightning damage inspection
Material ageing assessment
Measurement of phosphor content of CU Alloys
Condition assessment of highly stressed parts monitoring of deposition process for non ferrous materials on non-ferrous substrates.
If you would to know about the SigmaCheck Conductivity meter performance and it’s specifications you can download the brochure here https://www.ethernde.com/images/brochures/sigmacheck-2-brochure-issue-1.pdf
Or feel free to contact me directly with any application challenge you may have.
Please Note : ""Au" Image courtesy of Aaron Munoz".