Understanding Cadmium

Why is it important for us to understand the properties of cadmium?

Cadmium - was known as the best solution against atmospheric corrosion. Why is that so? We know more about why they got rid of it than we do why it was so great to begin with.

Properties of Cadmium

Cadmium Oxide, forms much like aluminum oxide or titanium oxide, very quickly and multiples faster than iron oxide can, so it has a sort of self healing ability. Unlike zinc oxide which is a little slower to form, cadmium oxide forms a solid barrier and not a flaky dust oxide.

Cadmium's open circuit potential is(OCP) = -.825mV in 3% salt water. This means that if plated on a 4130 (B7) stud. The galvanic indifference would be [-.675mV to -.825mV = -.150mV] This .150 mV is well below the .300mV limit set by the Navy. What it means is that the speed of corrosion is very slow, the bigger the difference between the OCP, the more corrosion occurs quickly. Many times zinc was plated a top of the cadmium, this was because the zinc had a higher negativity in (OCP) -1.0mV, so it was more anodic. The cadmium would slow down the anodic negative charge and sustain it, with impressed currents the cadmium would also block the high negativity from reaching the substrate and in stead would use the current as an anode to the cadmium first. Giving extended periods of protection.

Cadmium has one of the lowest thermal diffusivity temperatures of 321ºC/ 609ºF, with halid salts used to plate the cadmium and other catalyst in the plating chemistry, we see the cadmium accually diffusing into a substrate vacancies at much lower temperatures, high 290ºC s/ 560ºF s. If thermal diffusion properties were present you would have seen a hydrogen barrier, a H2S barrier, increased hardness in the cadmium and wear protection. Cadmium-Nickel is two platings that are applied to the surface of a metal substrate and then thermally diffused together to emped hydrogen from reaching the substrates surface.

What Properties can we replace and how

• The self-healing and quick to oxide properties can be found in aluminum oxide spin dips at 5µms. (Because the oxidation under the spin dip is so slow the aluminum has time to form a oxide layer faster than thermal diffused zinc would.)
• The OCP of (-.825mV) same as cadmium can be found in thermal diffused zinc. The TDZ being applied first, would act as a stair step of OCP between the aluminum spin dip and the substrate. Giving well over the 3500 salt spray hours, seen by Cadmium plating + zinc plating top coat + PTFE = 1 mil thickness
• The thermal diffused properties can be found in any thermally diffused alloy, zinc, just happens to be diffusable at below 800ºF, we usually run at 707ºF.

Thermally diffusing zinc into nickel would be the best solution, for replacing (Cadmium-Nickel), but it may be a small price jump for replacing cadmium.

Why Nots

• Many people have been discussing nano-plating systems as a replacement to cadmium. Nano-plating only covers the top of the pores. Once it is eaten away as an anode plating, the pores are then open for hydrogen to enter once again.
• Zinc-nickel plating - has a -.900mV OCP. It has a harder surface to deal with wear (12% nickel). It still has to be post-heat treated and is subjected to plating chemistry, alkaline or acid plating.
• Open for discuss , since there are so many types of coating and plating to discuss on fastener threads.The fact that cadmium is toxic to handle and can not be used in thermal applications, is important, but is well known. I am not condoning the return of cadmium in this form. Instead I am pointing out the many characteristics and properties that made it great. The reasons we have looked at for its previous success are a misnomer.