Cadmium Electroplated coatings play an important role in a number of
key manufacturing areas, Poeton expect this to continue well into the
21st century. We are therefore committed to maintaining our substantial
facility to offer Cadmium Electroplating as a key part of our complete
metal finishing service. Poeton is registered with the appropriate authorities
and is licensed to this effect, having invested to ensure better than
full compliance to statutory obligations.
Cadmium Electroplated coatings offer a unique range of properties for
which no single alternative exists now or in the foreseeable future. Along
with the National Centre of Tribology, leading academic bodies and aerospace
contractors, Poeton was a member of the DTI sponsored "RECAP" project,
set up to seek substitutes for Cadmium Electroplating.
Certainly, careful evaluation of applications can often permit use of
other coatings that the metal finishing industry offers, indeed as are
also carried out by Poeton. However, this is very selective and depends
much on the priority property for which Cadmium Electroplating became
the original selection in the first place. So, Poeton continues with confidence
to provide certified Cadmium Electroplating to the highest standards on
many exacting applications including weaponry, civil aviation, offshore
and safety related applications.
Some Important Points About Cadmium
Cadmium, a metal currently under scrutiny, has many wide and varied uses
from batteries (Nickel/Cadmium) to plastic pigments. Electroplated Cadmium
coatings represent approximately 9% of Cadmium's total use (December 1992)
and as a process of significance to Poeton (Apticote 900L & 900N) we are
certainly motivated to ensure our customers have the best information
available regarding the properties and uses of Cadmium.
Background
Corrosion destroys one-fifth of the world production of ferrous metals
annually.
Most methods of protection, involve putting an extra layer of some corrosion-resistant
material between steel and the environment. Paints and plastics for example,
prevent contact between the atmosphere and the steel, however if there
is damage to such a protective film corrosion will occur at the break
and spread outwards to lift and destroy the film as it goes. Similar effects
will also occur from the inherent pores in paint films.
Metallic coatings on the other hand, isolate the steel from the atmosphere
by a truly impermeable layer. Cadmium, like other anodic metals such as
Zinc, still protects the steel at small gaps in the coating. Slight damage
to the coating therefore causes no loss in protection and while Cadmium
is subject to corrosion, it is "sacrificing" itself at a predictably slow
rate - up to one-fiftieth that of bare steel, less in some severe marine
environments.
Cadmium coatings also have good lubricity, easy solderability and other
properties which are essential for reliable service life of many engineering
components. Cadmium is normally applied by electro-deposition from cyanide
or acid solutions in barrels or vats. Cadmium can also be applied by mechanical
plating, vacuum deposition and metal spraying, but generally only electroplated
Cadmium is of commercial importance.
Cadmium is in fact a by-product of Zinc production and it was not until
the 1920's that electroplating of Cadmium became widely used as a protective
coating on iron and steel. Today, the Western World annual consumption
of Cadmium for coatings is about 1500 tones which is a very small proportion
of total Cadmium consumption. Over 90 per cent of all Cadmium coatings
are deposited electroplating.
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 electroplating
is normally specified with minimum local thickness between 5 and 25
micron, 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, for example.
- Undercutting of threads on nuts and bolts is not necessary. The coating
has a low coefficient of friction, which reduces the tightening torque
required 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 polishable 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 service.
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, specialised 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.
Chromate Coatings
After electroplating, and heat treatment if required, a chromate conversion
coating is usually applied, giving the coating its well known iridescent
green/brown appearance. A chromate conversion coating adds corrosion resistance
and the protective value increases with the mass of the coating. A thin
conversion coating can maintain a bright as-plated finish and does not
appreciably affect the electrical conductivity of the surface nor its
solderability. A full chromate conversion coating can double the life
of a typical Cadmium coating in most atmospheres. Additional protection
can be provided by a clear lacquer coating.
Uses
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 to applications where an
alternative coating is not appropriate.
Health Warnings
Cadmium and Cadmium electroplated products are safe to deposit, use and
handle normally, however under certain conditions Cadmium can present
a health hazard. Effective respiratory equipment and exhaust ventilation
must be arranged when welding or otherwise heating Cadmium electroplated
products over 250 oC, as the Cadmium oxide fumes produced are highly toxic.
Adequate exhaust ventilation must also be arranged when handling Cadmium
metal in the form of powders or dusts. Routine precautions taken under
the UK COSHH regulations will ensure risks are properly controlled.
Cadmium - Why all the fuss..?
Why all the fuss? That's a hard one to answer. Certainly Cadmium, like
all metals, needs to be monitored and controlled, however we have not
been able to clearly establish why Cadmium in particular has been highlighted,
in particular preference over other metals of significance to the welfare
of our planet.
What's Wrong with Cadmium?
Nothing. We have plenty of details regarding the many uses of Cadmium,
from batteries to pigments, to of course electroplated coatings for corrosion
resistance. The environmental goal is to prevent Cadmium from entering
the food chain, and there is an increasing volume of evidence to suggest
that the levels are not significant and decreasing rapidly.
Is it Harmful?
No. Cadmium can be handled just like any other metal. It will only prove
to have harmful effects if absorbed into the bloodstream though, for example
smoking (there is significant cadmium in tobacco). Welding Cadmium or
Cadmium electroplated products, gives off a highly toxic Cadmium Oxide
fume, which if breathed in can enter the bloodstream where it is readily
absorbed and in severe cases will cause kidney problems, failure and death.
Breathing apparatus must always be worn should such welding be necessary,
or when handling Cadmium in a fine powder form.
- Contact us for complete information regarding EEC directives on the
use of Cadmium Electroplating, Health & Safety information and reputable
sources of independent advice.
Can I Specify Cadmium?
Yes, if the use of Cadmium is for aircraft, defence, offshore, safety,
nuclear or electrical contacts. There are many areas in which use of Cadmium
is or will be restricted, such as food handling, sanitary and household
goods.
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