Plating Methods Cost Comparison
A discussion started in 2003 but continuing through 20182003
I am a technical writer at an engineering company. My assignment is to write a comparison of plating methods, specifically the following: electro-deposited, plasma spray coating, electro-less, hard anodizing, carburizing, and PVD/CVD. I have found plenty of background material on the web, including this site. However, the last criteria requested was a relative cost comparison of these techniques, for example, anodizing is less expensive than electro-less. I realize there are many factors that affect the cost of plating, but could anyone propose a simple hierarchy of cost for these six methods?
- Foothill Ranch, California, USA
A. Hi Susan. As you probably realize, Susan, these processes not only perform different functions but apply to different substrates. For example, anodizing applies to aluminum and not steel whereas carburizing applies to steel and not aluminum.
Further, often the processes go together. A part that is to be carburized--for example, a gear--is first selectively copper electroplated to prevent hardening in the bore area. 'Lifetime' finishes and some 'gold' finishes employ a PVD topcoat on top of electro-deposited platings. Plasma spray coating may be cost competitive with the highest quality heavy chrome plating it competes against, but could be 100 times the cost of zinc plating.
I suppose I would rank them, cheapest first, as electroplating, hard anodizing, carburizing, electroless plating, PVD/CVD, plasma spray coating. But again, there are electroplatings that cost a lot more than hard anodizing, carburizing, or electroless plating. And there are PVD/CVD coatings that cost more than plasma spray. And probably another half-dozen "Buts", too :-)
Best of luck,
Ted Mooney, P.E.
Pine Beach, New Jersey
November 15, 2012
Q. Within electroless nickel, cad, and any other similar plating for the sole purpose of protection against the elements, how would the cost comparison match up?Loren Coffin
- Elmira, Oregon, US
November 16, 2012
A. Hi Loren.
Years ago, before it became an environmental albatross, cadmium plating could be cost competitive with zinc plating in some circumstances, but no longer. Electroless nickel is much more expensive than zinc plating.
Zinc plating is the least expensive plating for corrosion protection. Some of the zinc alloy platings like zinc-iron, zinc-cobalt, and zinc-nickel may not be too much more expensive than plain zinc plating, and may exhibit sufficient extra corrosion protection as to be as inexpensive on a functional scale. Good luck.
Ted Mooney, P.E.
Pine Beach, New Jersey
July 19, 2018
"Within electroless nickel, cad, and any other similar plating for the sole purpose of protection against the elements, how would the cost comparison match up?"
A. Good question. It does not depend only on the coating type, but also whether the coating is uniformly sealed or microcracked, etc.
For thin coatings a 2-3% Phosphorous Nickel could be the best, but it needs to be without any pinholes and such. The cost is higher. It can, however, be used on microminiature components, where it is very uniformly applied. It can also cover non-conductive surfaces if pretreated with a catalyst.
If you can afford adding 10-20 microns, then Zn-Ni plating, sealed with trivalent chromium and some varnish on top. Much cheaper too.
Cadmium is (WAS, luckily, cancer cad) a uniform type of coating, much like copper, it is malleable, plastic and wears off with friction in many cases.
Zinc plating or dipping in molten zinc is the classic, cheap and easy. If covered with special paint layers afterwards, provides good protection for many years, but compared to Zn-Ni is much softer and less corrosion resistant. (150HV hardness compared to 400HV hardness)
There is also phosphating in boiling alkaline bath which provides an even, black surface, after which the steel or iron component is oiled and thus corrosion protected. A very simple and very, very cheap method.
Thus, it depends on the aggressivity of the environment in question. If you expect salt, see the blue zincate or even gold-plated air conditioning exchangers. Zinc alone is good, but put it into seawater and see it vanish. Thus, any plating will use further modifiers, sealers, varnishes, etc. that assure the plating wasn't in vain. And that cost more. 10% more or 400% more. Depends. Thicker coatings: more protection. More sealers: more protection. More passivators: more protection.
Very thick Zinc-Nickel plating, plus a lot of post-processing can exceed the cost of a very thin nickel (on some substrates), but then Zinc plating can do 20-ton blocks, while electroless nickel is best suited for tabletop usage due to bath costs and limited life of the bath.
Thus, for huge volume applications, coatings and plating with the cheapest and lowest cost (and lowest risk during operation) raw materials will be used, but in very small parts, even some expensive coats or plating may be very cost effective due to some other properties (Aesthetic, etc.)
If you really need to be protected against the elements, you can even apply many layers of metals (if you must avoid paints), which could be NiP (electroless Nickel-phosphorus) as the base layer, then sealed and then over plated with 5 or 10 or even 15 micrometers of gold, for example. When properly executed and with Cobalt as the additive in gold plating, such corrosion protection would be amazing, even if wasteful financially.
Many times it is not just the nominal metals plated, it is the whole process and the resulting surface, plus organic or inorganic passivating compound over that surface.
Imagine you have a 10-micrometer thick copper plating, that is absolutely even and 100% copper. Copper alone can corrode, but slowly and very uniformly. This coating would be in some applications superior to cracked surface of untreated electroless nickel, which, by itself does not corrode or dissolve in water, but the unsealed surface would let water and air to the steel surface below and slowly start pitting corrosion holes. The copper, after wearing off, would more likely start the corrosion over the whole surface at once.
Many factors, such as, corrosion in what environment are you protecting against, or, if there are any other design constraints (protecting for 10 years, 20, 100, 1000 or only 5?) need to be considered, as the protections work in a different manner.
Zinc coating is "sacrificial", that is, any attempted corrosion will corrode the zinc. Gold plating (or nickel, etc.) is purely a barrier coating, aiming to prevent oxygen and other factors from the environment to the surface you are trying to protect.
Thus, cost-effective depends on: desired result, design environment, yearly volume, availability, planned lifetime, risk, etc.
Metal blackening by phosphating is the very cheapest. Very simple, allows huge parts, cars, etc.
Zinc is cheapest. But a zinc plating that protects better will cost 3x as much.
Zinc-Nickel great. Better hardness and anti-corrosion properties. Also added cost. If you plan on long term and rough conditions, it will be more effective than Zinc.
The rest is likely more cost effective for marine environments, where the required thickness of everything is much higher.
The "cost effective" also changes with the atmosphere: Hydrogen sulfide? Sulphur dioxide? Risk of abrasive damage? Frequent changing temperatures? Exhaust products? Manipulation of the plated surface after expedition? Any other mechanical strain? Is the environment alkaline, acidic, neutral? What kind of dissolved salts? Any electric potential driving the corrosion?
Each of the corrosion protection mechanism has different weak points upon which it fails to protect, and each of the plating and sealing processes has a slightly (or completely) different properties in this manner.
That all affects the calculation of what will be cost effective, sometimes disabling some of the options.
- Vienna, (The Coffee City), Austria
July 22, 2018
A. Jane D. Stanton has written a comprehensive overview, but even that may not answer your question. If you ask a specific question, and provide some facts yourself, you will get a simpler and more easily understood answer.
What items do your wish to protect?
What are they made of?
What are the service conditions?
How long do you want them to last?
How many are there?
There's a world of difference between a million of an item which will be used indoors, in a clean environment and needs to last for two years against one which:
Is a zinc die casting, made fifty at a time, and which must withstand twenty years on the front of a truck which is driven 100K miles a year on salted roads.
Item #1 might cost pennies to finish, and item #2 tens of dollars.
Your question is akin to asking a physician: "I have a pain in my belly. How long do I have to live?"
Jeffrey Holmes, CEF
Spartanburg, South Carolina
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