Electroplated Cadmium

Electroplated cadmium is a robust and versitile metallic coating. Cadmium is a soft white metal that, when plated onto steel, cast iron, malleable iron, copper, and powdered metal, functions as a “sacrificial coating,” corroding before the substrate material. To enhance the corrosion protection of cadmium plating, chromate conversion coatings can be applied over the plated metal, yielding the familiar gold color as seen in the above picture. Other colors, such as olive drab and black are also available.

Cadmium plating offers an exceptional bonding surface for adhesives, such as those increasingly used in aircraft manufacturing, and is the preferred coating for salt-water environments. Additional advantages of cadmium plating include: low electrical resistance; outstanding conductivity; superior solder-ability; favorable galvanic coupling with aluminum; and excellent natural lubricity, which results in prevention of galling and a low coefficient of friction. Furthermore, the corrosion products of cadmium are less significant that those of other plated coatings such as zinc. These characteristics are especially useful in applications where components will be repeatedly disassembled and reassembled, such as in scheduled maintenance of aircraft. Thus cadmium plating continues to be critical to the aerospace industry. Cadmium plated surfaces resist mold or bacteria growth.

Uses of Cadmium Plating

Cadmium coatings are used principally to impart corrosion resistance to steels, and in a great variety of applications which call for other engineering properties of cadmium. The aerospace industries specify cadmium plating to prevent bimetallic corrosion between high tensile steel fasteners and aluminium alloys.

Aerospace engineers regard cadmium plating as important for bolts used with engines, major structural members and landing gear, and for fasteners for aluminium sheet. Nevertheless, cadmium is not the widely used “cure for all applications” coating it used to be, primarily due to regulatory actions restricting its use on applications where an alternative coating is not appropriate.

Two coating weights are commonly used to meet the automobile industryÂ’s requirements :
<strong>Grade A :</strong> coating weight &gt; 24 g/m², average thickness 5 to 7 µm
<strong>Grade B :</strong> coating weight &gt; 36 g/m², average thickness 8 to 10 µm
Greater coating weights may be applied to increase the duration of the corrosion protection

Applicable Specifications :

AMS 2400 ASTM B766 AMS 2401 ASTM A165 ISO 4521 QQ-P-416

High Tensile Steels

Most steels are readily electroplated with cadmium and require no heat- treatment, either for stress-relief or for avoidance of embrittlement due to hydrogen entrapped during the process. Base metals of tensile strength above 1100 MPa should not be electroplated with cadmium by conventional methods. Instead, specialized pre-treatment and coating procedures have to be used (Apticote 900L) along with stress-relief and de-embrittlement by way of specific heat-treatment cycles.

Coating Properties

Cadmium as an electroplated metallic coating has the following advantages :

  • It provides sacrificial protection to the underlying steel (as described under “background”)
  • The atmospheric corrosion protection of cadmium is predictable and is proportional to the thickness of the coating. Cadmium lectroplating is normally specified with minimum thickness between 5 and 25 microns, depending on the severity of atmosphere. Cadmium has good resistance to rural and marine atmospheres, in alkaline conditions and detergents.
  • It proves an effective barrier to prevent the galvanic/bimetallic reaction between steel and aluminium, such as where aircraft undercarriage and weaponry mechanisms are fixed to aluminium framework.
  • Undercutting of threads on nuts and bolts is not necessary. The coating has a low coefficient of friction, which reduces the tightening torque and allows repetitive dismantling.
  • Cadmium corrosion products have small particle volume and are adherent, so valves and delicate mechanisms will not likely to be jammed with debris.
  • Thin Cadmium coatings are appropriate on threaded components where dimensional tolerances must be maintained.
  • Cadmium can be formed as easily as the substrate.
  • It can easily be soldered without the use of corrosive fluxes, and has a lower electrical contact resistance than zinc coated steel. These are important properties for the electrical and electronics industry.
  • Cadmium can have an attractive polish-able silvery finish.
  • The Cadmium plating process can be applied to all ferrous materials, including malleable iron, and to brass and aluminium. The process can enable a high efficiency throwing power, i.e. the recesses are more readily coated with a reasonably even deposit.
  • Cadmium plated steel is readily adhesive-bonded.
  • Chromating directly after electroplating can increase the corrosion resistance of the coating, and greatly extend the coating life by preventing the sacrificial process from commencing until in