ELEMENT OF THE MONTH
Each month we will explore elements of interest from the periodic table, with a brief history of discovery and development, and a review of uses and applications.
CADMIUM – ATOMIC NUMBER 48
Cadmium is a soft silvery-white metal, chemically similar to zinc and mercury. It was discovered in 1817 as an impurity of zinc carbonate by German chemist Friedrich Stromeyer. It was also discovered in that year by Karl Hermann and Johann Roloff, in a specimen of zinc oxide.
It is a rare element produced as a by-product of zinc ore refining.
Cadmium is toxic to humans and is a known carcinogen. It is classed as a hazardous substance and there are ongoing programmes to prohibit its use unless an exemption is in place, e.g. for legacy aircraft components.
As a Pure Element
In its pure state cadmium is used as a protective coating, typically deposited by a galvanic (electroplating) process to prevent corrosion of steel, copper, and aluminium alloys.
As an Alloy
Alloys of cadmium are in two broad groups
- A small addition of cadmium to alloys based on copper, tin, lead and zinc can improve features of the alloy, such as hardness, wear resistance, mechanical and fatigue strength, or electrical properties.
- Higher concentrations can be used to modify the melting point of alloys containing silver, tin, lead, indium or bismuth. Eutectic alloys with very low melting points (down to ~ 47 C), and non-eutectic high melting point alloys can be obtained.
As a Compound
There are many cadmium compounds, some of which are now prohibited because of their toxicity. Cadmium based red, yellow, and orange pigments are very durable, and are still used by artists for their brilliance and permanence.
Cadmium telluride (CdTe) is a relatively new compound, used in a 2 µm photovoltaic coating in solar cells.
Cadmium pigments can still be discovered where they are prohibited, in children’s toys, and polymeric electronic equipment housings and cable sheaths. An XRF based equipment can be used to quantify prohibited hazardous trace elements. Learn more
Where Is Cadmium Used?
Cadmium is primarily used to protect functional substrates from corrosion, typically in aircraft landing gear components. Its ability to do this is proportional to the deposit thickness, so measurement and control of this parameter is critical to ensure correct performance in a specified lifespan.
Cadmium may be found in silver jewellery alloys and 14 karat gold plated watch bezels; these uses are not permitted but some unscrupulous manufacturers still use it to reduce costs. Testing and analysis of silver should take into account the possibility of cadmium adulteration.
In solar cells, the uniformity of the active CdTe deposit is critical because the coating must meet a minimum thickness limit at any point on the surface to provide the specified conversion efficiency. Measurement of CdTe deposit thickness and uniformity is a vital tool in maximising solar cell output. Learn more
Here are some key parameters for cadmium, quantifiable using instruments from Helmut Fischer GmbH
- Thickness of cadmium coatings on steel – in the range ~ 8 to 30 µm, measurable with a contacting probe instrument such as the Deltascope FMP30
- Thickness of cadmium coatings on non-ferrous alloys – in the range ~ 8 to 30 µm, measurable using an instrument such as the Fischerscope XDL 230
- Thickness + composition of compound coatings in the range of a few micro meters, such as cadmium telluride used in solar energy modules; measurable using an instrument such as the Fischerscope X-Ray 5000
- Composition analysis and identification of solid alloys containing cadmium e.g., copper-cadmium 99:1, or silver-copper-cadmium 80:7.5:12 (‘fake’ sterling silver), measurable using an instrument such as the Fischerscope XAN 250
- Cadmium is a prohibited trace element (EU RoHS compliance >100ppm), in polymeric electronic equipment housings and cable sheaths. Verification of RoHS compliance can be confirmed to IEC 62321 using an XRF based instrument such as XDV-SDD