Summary for cadmium plating types Discussion
Cadmium plating is used for corrosion resistance, cathodic protection, lubricity, and compatibility with aluminum, and it is usually followed by a chromate conversion coating. In this thread, Grant Leersnyder describes aircraft landing gear parts with LHE cad and chromate, bright cad, ti-cad, and silver-colored high-strength bolts, and asks for a way to tell the types apart. Ajay Raina explains that chromates are often yellow, but can also be green for greater corrosion resistance or clear when less protection is acceptable. He also notes that cadmium plating can cause hydrogen embrittlement in hardened parts, so high-strength components must be baked soon after plating, and low-hydrogen-embrittlement processes may be specified. He believes LHE means low hydrogen embrittlement and that ti-cad is one such process, but cautions that visual identification is not a practical or safe way to determine coating type or whether baking was done correctly. He suggests consulting specifications such as AMSQQP416 and reference books like the Canning Handbook or Electroplating Engineering Handboo…
Q. I am involved in commercial aircraft landing gear products and most of these parts are a cad finish called LHE cad with chromate conversion (with 4 to 23 hr bakes) which is a dull gold finish, there is also bright cad which is a bright gold color but no bake. I understand there is also ti-cad but recently I came across some HSS AN bolts that are a silver color. Can you help and do you have a color chart to show the various cads and there purpose please.
Grant Leersnyder
aircraft mechanic – Annandale, Virginia
March 3, 2009
A. Hi, Grant. Cadmium plating is done to give a surface corrosion resistance, cathodic protection, lubricity, compatibility with aluminum, freedom from stick-slip, and non-sticky corrosion products. Almost always there will be a chromate conversion coating after the cadmium plating; this is usually yellow, but could be green (for greatest corrosion resistance) or clear (if less corrosion resistance is okay).
Cadmium plating, like many other surface treatments, generates molecular hydrogen which can cause hydrogen embrittlement of hardened components. To battle this embrittlement, high strength parts must be baked very shortly after plating; further, cadmium plating processes may be specified which have a lower potential for hydrogen embrittlement. I believe that “LHE” stands for “low hydrogen embrittlement”. And further, that ti-cad is one type of LHE coating (I believe that the codeposition of tiny particles of titanium produces porosity that is believed to allow the hydrogen to escape easier in the baking cycle.
But after this introduction, I can’t really answer your question because I don’t think visual identification is a practical or safe way to determine whether the plating is an LHE type, nor whether the parts have been properly baked.
Regards,
Ted Mooney, P.E.
Striving to live Aloha
finishing.com – Pine Beach, New Jersey
Need quick confidential answers? $25
Need project assistance? $100/hr.
March 3, 2009
A. Grant,
Further to Ted’s comments there is only one satisfactory way to tell if the embrittlement has been done and that is by embrittlement testing in accordance with ASTM F519. The majority of MIL and AMS specifications call this up as an obligatory test.
Ted is also right that you cannot rely on color standards. There are various forms of LHE (You’re right Ted, it does stand for Low Hydrogen Embrittlement), some are plated at very high current density to form a porous coating, some are doped, it will all depend on the specification that they have been plated to.
Brian Terry
Aerospace – Yeovil, Somerset, UK
March 6, 2009
Q. Thanks guys for the feedback. Perhaps what I need is an idiots guide to cad-plating if there is such a thing. Do you have any suggestions?
Grant Leersnyder
Aircraft Mechanic – Annandale, Virginia
March 10, 2009
March 11, 2009
TO ME IT SOUNDS LIKE THE PERSON HAS SEEN DIFFERENT TYPE OF CHROMATE CONVERSION COATINGS ON CADMIUM PLATING.
ONE IS YELLOW CHROMATE(GOLDEN)
SECOND SILVER TYPE.
I WILL SUGGEST TO CONSULT A GOOD CHEMICAL SUPPLIER TO SOLVE THE PROBLEM.
REGARDS,
Ajay Raina
Ludhiana, Punjab, India
Grant,
A good starting place would be some of the national specifications, such as AMSQQP416.
Books wise (English bias) you should read something like the Canning Handbook or the
Electroplating Engineering Handbook ⇦ this on
eBay,
AbeBooks, or
Amazon [affil link]
, both of which give some general descriptions about the process. Personally I think that the Canning Handbook is better.
Alternatively, do a Google search for Cadmium plating and LHE Cadmium, you will come across literally hundreds of websites that will help you understand the process and what you can expect from the coatings.
Brian Terry
Aerospace – Yeovil, Somerset, UK
March 12, 2009
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