Sound technical content, curated with aloha by
Ted Mooney, P.E. RET
Pine Beach, NJ

finishing.com -- The Home Page of the Finishing Industry

The authoritative public forum
for Metal Finishing since 1989
-----

Silver plating: do we need a silver strike or live entry?

Q. Hi,

Can anyone give me an explanation about the necessity and the function of a silver strike bath (for example 60 g/l KCN, 5 g/l Ag) before the actual silver bath?

We use such strike bath here in a plating shop after electrodepositing bright nickel on decorative pieces (barrel plating).

We use the strike at about 0.07 A/dm2 (0.7 A/ft2) for 15 min. with satisfying results. On internet I have read higher figures about 2.2 A/dm2 as a minimum with a time of 5 min. So what is the right way to use that bath? I think using higher current densities will cover up the bright (decorative pieces!) nickel layer too much, correct?

In the Hull cell the strike bath looks awful to me, I even wonder if you need to do a Hull test cell for that bath?

I also have a question about the low content of Ag in the strike. Being so low, it is difficult to have it always correct at 5 g/l? I assume all current is going in the electrodepositing of Ag, so the concentration much fluctuate a lot.

Marc [last name deleted for privacy by Editor]
Plating shop - Quer´taro, Mexico
2005

A. Hi, Marc. The purpose of a silver strike (and they are necessary) is to prevent an immersion deposit and poor adhesion, which is why the concentration of silver is typically 5 g/l or less. I think you may have a typo in your question, as 0.7 A/dm2 sounds more correct than the 10x lower number you say you are using are using or the 3x higher number you saw on the net. It does sound like you are spending more time than necessary in striking though. Your 5-minute value sounds achievable.

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
2005

A. Just a reply to some questions you had. Ag strike baths also help to minimize contamination of the higher Ag concentrated bath (Ag plating bath)especially in barrel plating with the dangers of drag-in. It is a lot cheaper to replace a strike bath than a plating bath. Running Hull cells with strike baths won't really reveal too much because there is no grain refiner present in the strike bath. The normal deposit thickness for a strike is about 5-10 microinches. You could run a test load in the barrel and measure Ag thickness after the strike, then adjust your strike time accordingly. Using Ag anodes in the strike bath would keep metal concentrations more stable and increase anode efficiency but you would deplete some metal due to drag-out. The current density could be raised slightly, but remember you are barrel plating in a strike bath - lower solution transfer and lower metal concentrations.

Good Luck,

Mark Baker
process engineer - Malone, New York
2005

A. Silver strike is an inefficient process that puts down a silver coating whilst also providing much better adhesion. Normal silver baths suffer from immersion deposition that results in poor adhesion, but a strike does not do this. Once you have a good adhesive silver layer you can put down thicker silver without running the risk of loss of adhesion. It may be a chore to do it, but it is much better than re-working the job.

Trevor Crichton
R&D practical scientist
Chesham, Bucks, UK

Silver Entry - Live or Not?

Q. Hello,

We have a small silver electroplating process with cleaning, strike, then silver-cyanide. Parts are transferred and dipped by rack. New to plating myself, I've seen texts say that "recommend" the parts be live before they enter the strike and silver solutions without much explanation why. The silver is for industrial contacts, so cosmetic appearance is not a customer requirement.

Over the years, some of our operators have been dipping parts live, while others have decided not to. Those who have stopped claim to have observed the parts "looking burnt," so they just plug in after submersion instead.

For the sake of standardization, can anyone explain the benefits of live entry? Are there any drawbacks, like the reported burning? Should both the strike and silver be live, or just the strike? We're trying some small-scale experiments, but very little noticeable difference so far.

Any advice is much appreciated. We're fighting the uphill battle to make our process more science than art.

Brad Eibs
- Pittsburgh, Pennsylvania, USA
June 20, 2013

A. Live entry helps to prevent immersion plating. Immersion plating leads to bad adhesion and blistering. The live entry also prevents contamination of the Ag solution with the base metal.

When using live entry it is important to have the rectifier set to constant voltage control rather than constant current control. This way the current will increase as the surface are of the part increases while being immersed and the parts will not burn.

Pat Mentone
St Paul, Minnesota
June 29, 2013

A. Hello Brad,
The problem lies around preventing an immersion deposit of silver forming before electroplating begins. These immersion coatings usually have poor adhesion to the substrate and the subsequent electrodeposit may form blisters or even peel off. The more silver and less cyanide in the bath the more likely is an immersion deposit. But these conditions are best for efficient electrodeposition. So to lessen the risk of immersion deposition the work may be loaded live so that electrodeposition begins immediately. But, ideally the work should be thoroughly wetted with the plating solution before plating commences. Clearly this cannot happen with live loading. One consequence may be a burnt, powdery deposit. Also, freedom from immersion deposit is far from assured. So, we use a strike bath which has low silver and high free cyanide. Whilst being useless for producing thick deposits it should give a thin but adherent initial coating on which the main plating bath can build. If the strike is properly maintained, there is little risk of immersion plating. In strike baths, if you are using silver anodes, the silver content tends to rise and cyanide to fall. My advice is keep the strike bath properly maintained and not to load live.
Platers being platers may decide "better to be sure" and live dip! We platers are known as being free from any sort of superstition but we would never start work without first rubbing our magic rabbit's foot. Also, walking on the lines of paving blocks is strictly taboo.

Harry Parkes
- Birmingham, UK
June 29, 2013

A. Silver strike is a must.
When using silver strike there is no need to enter live.
Silver strike concentration should be 1-2 g/l silver plus 90-100 g/l potassium cyanide.
Plating time approx. 90 sec.
Don't forget to make up a new strike whenever the solution is turning dark.
The Hull Cell test is to observe that you have covering on the low current density as well.

Sara Michaeli
Tel-Aviv-Yafo, Israel
August 14, 2013

(No "dead threads" here! If this page isn't currently on the Hotline your Q, A, or Comment will restore it)

Q, A, or Comment on THIS thread -or- Start a NEW Thread

Disclaimer: It's not possible to fully diagnose a finishing problem or the hazards of an operation via these pages. All information presented is for general reference and does not represent a professional opinion nor the policy of an author's employer. The internet is largely anonymous & unvetted; some names may be fictitious and some recommendations might be harmful.

If you are seeking a product or service related to metal finishing, please check these Directories: