Q) We detail spirals for a PT anchorage zone and specified epoxy coating for ASTM A227 1/4" diameter high carbon spring wire for corrosion protection (182-210 ksi material). Due to scheduling the fabricator requested to be allowed to hot-dip galvanize or electroplate. Because of the potential for hydrogen embrittlement we will deny the request for hot-dip galvanizing. I am wondering if electroplating is a viable alternative. Are there any issues electroplating this high strength material like hot-dip galvanizing?
Thanks,
James Nelson
A) Thank-you for the question you posted to the Ask the Experts column on the Metal Finishing website.
Both hot dip galvanizing and electroplate have the potential to produce Hydrogen Embrittlement in hardened parts. The hydrogen is generally introduced in the cleaning part of the process which is critical to both HD and Electroplate. If handled correctly, minimal pickling, inhibited acids etc., the potential can be kept to a minimum. The parts can also be baked @ 400oF for 4 or more hours, depending on the hardness, to insure the release of any trapped hydrogen.
Another process, Mechanical Plating, is considered to be a non embrittling process that cold welds the zinc to the steel. Even with this care must be used, selecting inhibited cleaners and acids.
Q) I am a project manager for a company that installs high tension shade sails - especially in school and commercial environments.
We have been using stainless steel fittings, (eye-bolts and chains etc) but
as some sails are large and new regulations have come into effect, the
commercial SS fittings are NOT load-rated, and we have had to increase our
sized from 12mm to 16mm as some have failed. Even so, there are no
engineering safe loads for these commercial fittings.
Commercial galvanized steel fittings are available, but not load rated
either.
One solution is to go to grade 8.8 load rated steel fittings, but they do
NOT come galvanized, only cadmium plated.
As these are outdoor exposure fittings, and are subject to some wear and
abrasion under strong wind, I'm not sure how good such plating finishes
are, compared to hot-dipped gal.
If we were to have these fittings galvanized, their load ratings will be
affected.
Can you please advise what is the best solution. Is cad plating OK?
Thanks,
Steve Huppert.
A) Cadmium will perform well in most corrosive outdoor environments,
especially marine environments. However, Cadmium is considered a very toxic
metal and has been eliminated from virtually all applications outside of
the military and aerospace industries. Given that your application is in
school and commercial environments, I would look toward other coatings for
your fasteners.
I suspect that the heat involved in a hot dip galvanizing process is
affecting the load rating of the 8.8 fasteners. I would work with your
supplier to provide a coating, other than Cad, that meets your requirements.
There is a process called Mechanical Galvanizing or Mechanical Plating,
which is a low temperature process that will not produce hydrogen
embrittlement problems. This process is capable of applying zinc
thicknesses, equivalent to hot dip, so the corrosion protection will be as
good or better. Other alloys of zinc can also be applied using this
process. Your plating contractor for this process can provide details.
The parts also are generally easier to work with because there is no
excessive zinc build-up on the edges to blind threads or heads, making
installation easier with less scrapped fasteners. Additionally, you can
specify the use of enviornmentally friendly passivates and top-coats,
elimating the use of hex chrome. These coatings will deliver addtional
corrosion protection, empart specific friction characteristics and are
available in a number of colors. The fasteners will not have an
environmental impact, meet your specific needs and be aesthetically pleasing.
Sincerely,
Bob Griffin
Q) I am running a barrel line using chloride zinc. My brightener consumption has been steadily increasing since the bath was installed 3 months ago. It has become harder and harder to maintain the brightness on my parts, especially in the low current density areas.
The chemistry is:
Zinc Metal 5.1 oz/gal
Total Chloride 17.5 oz/gal.
Ammonium Chloride 4.0 oz/gal
pH 5.0
Temp. 95 oF
What would cause the brightener consumption to increase?
A) Acid chloride zinc plating systems for barrel applications typically run with the zinc metal concentration at a level of 3.0 oz./gal. As the metal rises you will shift the bright range on your parts toward the high current density areas, at the expense of brightness in the lows. This has resulted in your needing significantly more brightener in the bath to keep the low current density areas bright. You will need to get the metal back into the 2.5 oz./gal. - 3.5 oz./gal. range. You might also monitor the temperature of your operating bath. Brightener consumption is typically lowest when the baths operate under 90 oF. Lowering the bath temperature by chilling will lower your consumption. To moderate metal concentration increases moving forward, you should measure the amount of anode area in your tank. The anode current density should not exceed 20 amps per square foot. Maintaining the anode current density below the 20 ASF will minimize the amount of zinc anode dissolution needed for the job. You could also explore the use of inert anodes in your chloride zinc bath if this remains a long term problem. The cost associated with using and maintaining inert anodes would typically be more than off-set by your savings for not having to dilute and cut back your bath and waste treating the solution growth.
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