Hello John63,
Thank you for your question.
My first discovery was a discussion of the value of pre vs post
plating for power supply trays at
http://www.finishing.com/1000-1199/1027.html .
"My vendor is proposing a cost reduction of ~ $ 8.00 to the power
supply tray used in our unit. This will consist of changing the
material from a post-plated material to a pre-plated material.
What difference that will make in term of material quality and
performance? "
Reply:
Normally, the cost advantages of preplating accrue from the fact that
the raw material is plated as a continuous strip. The problem is that
when you shear pieces out of this, there will be no plating on some or
all of the edges.
Rarely will an unplated steel edge be a satisfactory finish unless you
can roll it under to keep it unexposed. If you are zinc plating,
maybe, since zinc is sacrificial to steel and can protect it to some
extent even where the steel is not covered. But nickel or tin
preplating? I don't think so. "
Another member replies:
I have spoken with mechanical engineers and designing for many
electronics OEM's, and there is definitely a trend toward using
preplated material.........PCs and other chassis are now frequently
made from preplate.......the exposed steel on the sheared edges is not
a problem to them. I am talking about companies like HP etc. "
So it seems that the major issues are the lack of plating that will
occur on edges after the steel has been sheared or trimmed vs. cost
savings that are available from plating entire sheets before the
material is formed.
Another thread at this site entitled "Computer Chassis Pre-Plating"
says:
"Steel is zinc plated because zinc is a sacrificial material that will
prevent the steel from rusting. Preplating is not quite as good as
plating after assembly because the sheared edges of preplate will have
no zinc on them, nor will any spot welds, screw holes, etc. But it is
more economical to plate strip than a fabricated chassis.
Zinc itself will corrode (white rust) almost immediately unless it too
is protected by painting or chromating. If the parts are not to be
painted they must be chromated. "
Yet another thread asks "Should Galvanic Corrosion be a concern in
computer chassis? "
"We manufacture equipment used in the computer industry. The question
that I have relates to plating sheet metal parts with Electrolysis
Nickel verses using a Zinc (ASTM-B-633 Type 3) plating. We have an
engineer that has specified Electrolysis Nickel on a chasis with
complex bends because he feels that the process used to plate Zinc
does not yield acceptable coverage (special electrodes would be
required). Because of a concern about galvanic corrosion parts that
come in contact with the above part are also specified as electrolysis
nickel rather than zinc. Some venders seem to prefer to work with
Zinc. In some cases the plating is then covered with paint.
Where can I find more information on galvanic corrosion and its
relative importance in our market. (I can understand that concern to
equipment that has a long life cycle exposed to the weather but I need
help assessing the issue as it relates to computer assemblies.
Is Galvanic corrosion a concern when designing a computer chasis
(typical office environment) using parts in contact with each other
that have been plated with Electoless Nickel and Zinc? Is there any
reason to specify that all of the parts be either one or the other
type of plating? "
The reply:
"Zinc plating is the usual approach, and I am not familiar with any
computer chassis being electroless nickel plated, although I can't say
it isn't done.
There are some environments where electroless nickel exhibits superior
corrosion resistance to zinc, and some where it doesn't. It is
important to recognize that nickel is cathodic to steel and
accelerates its corrosion if the coating is breached. Zinc is anodic
and protects steel even if there are scratches or pores. "
And ...
"Have you considered using precoated sheet steel? Coverage is
guaranteed and you can avoid plating costs entirely. Galvanneal and
electrogalvanized sheet stock work well in many cases for computer
frames and chassis. Many computer manufacturers are now using
precoated steels for some or all of their sheet metal parts. Pop the
top on your PC and have a look inside. Chances are it's precoated
steel inside. In a benign office atmosphere, precoated steels can
provide adequate corrosion protection, even on cut edges. It may not
work for everyone, but it's well worth considering. "
And another reply...
"Galvanic corrosion should be taken into consideration for the future
computer design because computers are not only used in offices but
also on production lines where the serious corrosion environment is
often encountered. Although plated sheets will be further painted,
scratching and partially mechanical damaging are inavoidable during
use and transportation. Both nickel-plating and zinc plating have
advantages and disadvantages. Of course, zinc plating is cheaper than
nickel plating. More importantly, zinc as a sacrificial anode can
provide protection for other parts while nickel as a cathode could
accelerate the corrosion of other parts in a computer. Using
galvanized sheet as computer chassis material is a better choice. As I
know, IBM is now thinking about zinc-cobalt alloy plating with blue
chromating post-treatment in computer chassis design."
Sequent notes problems it has encountered when switching to zinc
plated components:
"Sequent manufactures computer systems which are enclosed in sheet
metal housings. We recently began a new project which included some
cost-reductions. One of the proposed cost reductions was to change our
electroless Ni plated aluminum and steel chassis parts to a pre-plated
cold-rolled steel design. The previous design which used the Ni
plating was chosen for cosmetic appearance, compatibility with the Au
flash plating on the circuit boards, and for the best conductivity
with the gaskets used for sealing the enclosure for electromagnetic
compatibility (EMC). Anyway, the logistics of the chassis change to
pre-plated cold-rolled steel (GALXC material) was such that the
proposal included changing only the outside chassis, while the inner
chassis parts (brackets, several circuit-board holders, etc) were to
remain as Ni plated parts. The 1st test of this in our humidity
chamber was a disaster. Several of the newer Zn pre-plated parts were
covered in a white residue, which we suspect is galvanic corrosion.
The EMC testing showed a significant degradation in sheilding
effectiveness."
The above are interesting discussions from industry experts and worth
reading. The site describes itself as:
"Finishing.com is an open web site designed to be the home page of the
finishing industry -- a place for people around the world who have an
interest in metal finishing and the allied surface finishing arts to
gather around and exchange information. Although most of the site is
in English, we welcome communications in any language.
Our home page serves as a table of contents of the site. Our most
popular area is a very active 'Letters / Hotline' page. We also have
chat rooms, help wanteds, situation wanteds, a calendar of events for
the finishing industry, a technical library, FAQs about metal
finishing, and links to kindred websites.
On the commercial side, we have Worldlink Directories of chemical &
equipment suppliers to finishing shops, consultants, environmental
services & supplies, jobshops, software & training services, and
testing services & test equipment. Those firms who buy a Worldlink
listing or a banner ad on this site, optionally linked to external
sites or to home pages here, provide the income that makes the site
possible."
You may wish to post specific questions to the specialists that
frequent their message boards and forums.
The American Iron and Steel Institute has a very informative pdf file
on Zinc Coating, and while it does not specifically address pre vs
post plating, it has a wealth of background material about variables
in zinc plating processes.
www.steel.org/images/Zinc20Coatings.pdf
I was unable to uncover a comparitive study of pre vs post plating,
and it may well be that one has not been undertaken. In fact, there is
precious little information on the web comparing these techniques.
Finishing.com is a fabulous resource for reaching experts online that
specialize in this area and may be your best venue for posing more
specfic questions of this nature.
It would seem that there is little risk in using pre plated material
in computer applications unless they are to be subjected to unusual
conditions, perhaps in a harsh manufacturing environment, for example,
where chemical vapors may be excessive. On the other hand, as you
mention that your products are "high end", the fit and finish, even on
interior components, may outweigh the cost savings that pre-plating
will offer.
I hope the above information has helped you to weigh this decision and
will help to lead you to additional answers. If anything above is
unclear, please do ask for clarification.
Regards,
-=clouseau=-
Search strategy:
pre plated vs post plated steel comparison OR study OR report
pre vs OR versus post plated OR plating steel computer parts |
