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Blistering on ETG100 zinc-plated pins
We use ETG100 steel (DIN C45S20; SAE 1144; composition: C 0.4-0.48%, Si 0.15-0.30, Mn 1.35-1.65, P 0.04, S 0.24-0.33) for turned pins, that are zinc-plated and Cr3-passivated.
Often we have problems of delayed blistering (i.e. delamination of the zinc layer). It seems the problem has get tougher after we switched to Cr3 from Cr6.
Some people suggested the cause may be the sulfur content(in the same way as lead in lead-bearing steels). For others the current density in plating is too strong, resulting in a stressed zinc layer. Finally other people says it is a hydrogen corrosion problem.
Anybody can propose a way to avoid the problem?
Thanks a lot from Italy.
- Bergamo, Italy
Dear Mr Antonello;
Yes it could be a combination of all three factors or only one; maybe after you switched to chrome 3, the plating also was changed from chloride base to alkaline base; which is prone to delayed blistering if some of the additives are not balanced in the system properly.
Bengaluru, Karnataka, India
Mr Antonello Antioniazzi
Delamination of Zinc from the base metal indicates poor activation. Poor cleaning.
You have not stated the cleaning and pretreatment cycle.
Try this cycle for Alkaline zinc plating..
Soak clean in High caustic cleaner for 3 to 6 minutes
Pickle in an inhibited Hydrochloric acid pickle, diluted to 40-50 % + a Dry acid salt at 60 Grams per liter, for VERY short time ..2 minutes to 3 minutes. Check all rust is actually removed and establish timing here first.
Rinse well X 2 times..
Anodic clean in caustic soda solution 150 GMS / liter for 60 to 120 secs at 50 Centigrade, 50 ASF. Use a fume suppressant.
Dip in Caustic soda 100 grams per liter for 30 secs.
Alkaline Zinc Plate. Start at 5 to 10 ASF and ramp up to usual current density after 5 minutes.... slowly...slowly...
Rinse 2 times in flowing water
Nitric acid dip twice.. In 2 dips
a) 20 mls / liter 20 - 30 secs..
b) 5 mls per liter 20 secs..
Passivate directly without rinsing. In trivalent passivate.
Rinse well x 2 times
If you still run acid zinc then the above cycle up to
Anodic clean AND RINSES is followed by a 10 percent HCl dip for 30 seconds before Acid Zinc Plating...
Post plate is similar as above.
The sensitive materials lead and sulfur should be effectively removed off the surface and not allowed to RE-DEPOSIT.
Changing the acid pickle daily is necessary to achieve this.
... And the slow rise in Plating current will help..
Asif Nurie [dec.]
- New Delhi, India
With deep regret we sadly advise that Asif passed away on Jan 24, 2016