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topic 43776

Silver Plating is Blistering


A discussion started in 2007 but continuing through 2018

2007

Q. We are plating Silver .002 to .004 inches thick over nickel strike. Nothing out of the ordinary. After 10,000 hours at 400F, the silver plating shows blistering. I know this is not unexpected after such a test, but my question is: What is the mechanism that causes the silver to blister? It seems that in order to form a blister the silver must have been stretched and plastically deformed by some force. What is the is the source of this force? A colleague has suggested differential thermal expansion of the silver after the plating bond is broken. Is this plausible?

Kenneth W. Cornett
- North Haven, Connecticut, USA


simultaneous 2007

A. Blistering is normally a result of improper cleaning or the strike. It can also result from the brightener component in the silver breaking down and volatilizing. There is an outside chance that you are baking out hydrogen from hydrogen embrittlement, but I would have thought that it would have shown up a lot sooner.
With that time and temperature, you might be causing an intermetallic between the nickel and the silver with a slightly different coefficient of expansion.

James Watts
- Navarre, Florida


2007

A. Some silver brighteners are antimony, some are selenium; both cause a stressed deposit that cannot take high heat.
Heavy build-up silver plating normally uses a carbon disulfiude type brightener that can stand the heat.
Further, Cyanide platable contaminants will stress the silver deposit, migrate and cause blisters: copper, zinc, cadmium. Best way to go for heavy thickness is, no brightener, super purity electrolyte, very low carbonates (caused by insoluble anodes), and bagged 0.9999 (that is 4 nines)silver anodes.

robert probert
Robert H Probert
Robert H Probert Technical Services
supporting advertiser
Garner, North Carolina
Editor's note: Mr. Probert is the author of Aluminum How-To / Aluminio El Como
and co-author of The Sulfamate Nickel How-To Guide


2007

A. Where does one start? Your colleague could be on to something, but there are a zillion other possibilities. If your bake cycle is indeed 10,000 hours (=400 days!), all sorts of diffusion can occur - is your substrate copper? If it is some sort of brass, bronze or such, my guess is insufficient nickel to prevent copper-silver diffusion which in the right proportion form a brittle intermetallic and upon cooling -- lead to you guessed it. What does the interface between the blister and part look like - both surfaces might give all a hint!

milt stevenson jr.
Milt Stevenson, Jr.
Anoplate Corporation
supporting advertiser 
Syracuse, New York

Anoplate banner


2007

A. What is the atmosphere? If it is air,then:

Silver at elevated temperature absorb oxygen. the higher the temperature the higher the absorption rate. I guess as a function of time the air diffuses thru the silver plate and react with the Nickel strike to form Nickel oxide or whatever base metal you have. Once the chemical reaction occurs you break the metal to metal bonding.

Hamilton M. Solidum
- New Jersey


2007

A. More details (substrate, process details, exposure atmosphere) may be necessary to get a response from the silver platers. Analysis of the Ni surface under the blister would help, preferably with the blister broken open inside an SEM. Analyze the gas within a blister if possible.

>From metallurgical principles, DTE isn't a likely cause of the blistering -- the stress would be maximum during initial heating and then anneal to zero at 400 F. DTE blisters would be immediately obvious. Also, silver plating is commonly used at elevated temperatures to prevent galling & form gas-metal seals in turbine engines.

A plating stress arises if the silver epitaxially deposits onto the nickel in a strained lattice; lattice constants are 3.524 & 4.086 Angstoms for Ni & Ag, respectively. But, as the plating is quite thick (lessens the effect) and silver recrystallizes at relatively low temperatures, effects would probably occur during initial heating. Similarly, plating stresses due to solution additives can be relaxed by annealing. This occurs to some extent when cyanide contamination in silver plating is leached out in near-boiling water after plating.

W/o further info, my guess is that some impurity such as CN migrated along the Ag/Ni interface and concentrated at some defect. Possibly, oxygen or hydrogen diffused inward from the atmosphere, or hydrogen diffused out of the substrate, and reacted with the impurity, forming a gas pocket at the interface.

Ken Vlach
- Goleta, California
contributor of the year

Finishing.com honored Ken for his countless carefully
researched responses. He passed away May 14, 2015.
Rest in peace, Ken. Thank you for your hard work
which the finishing world continues to benefit from.



2007

Q. A little more info:

- Substrate is Inconel 718, a nickel-based superalloy not normally susceptible to hydrogen embrittlement

- Plating procedure is a) clean in ultrasonic tank with mild detergent, b) rinse with deionized water, c) alkaline clean 3-5 minutes (MacDermid Process 235L, Concentration 16-18 oz./gal), d) rinse with deionized water, e) acid activator (30-40% HCl), f) nickel strike (Nickel Metal 8-9 oz/gal, HCl 10-12.5%, g) rinse with deionized water, h)silver strike (Silver Metal 0.5 - 0.7 TR. oz/gal, Free Potassium Cyanide 8-12 oz/gal), i) silver plate (Silver Metal 3.0 - 5.0. oz/gal, Free Potassium Cyanide: 8-13 oz/gal)

- The silver plating tanks have Sel-rex Silver Industrial Brightener added at 4 ml / gal (nice unit mix, huh?)

- Silver anodes are certified 99.99% pure. Silver plate purity is 99.9% min (tested monthly).

- I peeled off some of the silver around one of the blisters. The nickel layer underneath was clearly oxidized (very dark grey in color)

- I don't think out-gassing is the culprit. I just can't see where the gas would be coming from. The parts were examined every 1000 hours. No blisters were seen until the 6000 hour mark. If caused by out-gassing, wouldn't this have occurred much sooner?

- Identical seals (processed at the same time and in the same tanks) showed no blisters after 10,000 hours at 150 °C.

- Identical seals (processed at the same time and in the same tanks) but with gold strike before the silver strike show no blisters after 12,000 hours at 250 °C.

- All tests were conducted in standard atmosphere

Kenneth W. Cornett [returning]
- North Haven, CT,USA



January 18, 2012

Q. I am having a problem getting silver to not blister in a 900 degree bake. Material is ASTM A276-10 [stainless steel]. We have tried all kinds of activations. Hydrochloric acid, reverse sulfuric, actane 340, Powered direct HCl, Combined with a range of strikes. Woods nickel, sulfamate nickel, copper. Every time parts completely blister after 900 deg bake. At a loss.

Ian M [last name deleted for privacy by Editor]
plater - Westfield, Massachusetts, USA

January 19, 2012

A. Hi, Ian.

I hate to respond with something as general as "read this book", Ian, but anybody who is plating onto stainless steel and hasn't yet seen Jack Dini's "Electrodeposition - The Materials Science of Coating and Substrates" may be overlooking a vital tool. Jack tested many different plating conditions, and offers actual adhesion values in his "Adhesion" chapter.

Regards,

Ted Mooney, finishing.com
Teds signature
Ted Mooney, P.E.
finishing.com
Pine Beach, New Jersey



Silver Plated Copper Blisters when Brazed

February 9, 2018

Q. Hi everyone!

We have a brazing process of terminals. They are made out of copper (substrate) and have a silver plating, which is made by a 3rd party. The silver plating has to conform to ASTM B700 [link by ed. to spec at TechStreet].
These pieces are brazed to an electrical motor, but we are having two specific problems:
1) Little blisters (bubbles) after brazing.
2) After brazing, the silver plating comes off in some other pieces.

Our client is rejecting all the pieces with these problems and it's causing serious problems for us.

Do you have any ideas on how to fix this?

Antonio Bonilla
- El Paso, Texas

Blistering of silver electroplated parts after high temp. bake (760 °C)

October 5, 2018

Q. Hello my dear friends,

I am working in company which is producing fasteners. I am responsible for special processes department (Heat treatment, Plating, NDT).

My problem is that we are trying to plate some parts with silver electroplating. Specification AMS2411 [link by ed. to spec at TechStreet] wants us to make adhesion test at 760 °C for minimum 2 hours. All the time I see blisters all around the parts.

43776-1b

43776-1a

My process is as below (we are doing barrel plating):

1) Alkaline cleaning
2) Sandblast
3) Alkaline cleaning
4) Stress relief
5) Surface activation
6) Nickel plating
7) Depassivation
8) Pre-Silver Plating
9) Silver Plating
10) Hydrogen embrittlement treatment (190 °C, 23 hours)

When I examine the parts by using microscope, there is a gap between nickel and silver. My opinion is that problem is about somewhere between nickel and final silver plating. From my experience most critical parameter about adhesion is cleaning of the surface. I changed all of the rinse bath water. But problem is still there. The funny thing is when I did the same test at 650 °C for different raw-materials, I didn't see any blisters on the parts. This is the all information that I remember. If you need additional information to help me, just let me know. Thanks in advance for all.

Baris Irmak
- Izmir, Turkey


October 2018

? Hi Baris. Apologies for the delay in posting.

That is indeed a very high temperature and I would suspect that you are right about the issue being between nickel plating and silver plating. Is this sulfamate nickel plating?

Regards,

pic of Ted Mooney
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey
Striving to live "Aloha"


December 4, 2018

Q. Yes Mr. Mooney our nickel bath contains nickel sulphamate and also nickel chloride and boric acid. But the funny thing is i am not still sure that are these bubbles really blisters ? Because when I scratch it with something sharp like a needle, it doesn't flake.

Baris Irmak [returning]
- Izmir, Turkey


December 2018

A. Hi Baris. Those are indeed interesting "blisters", being very small, and with many on the bolt head exhibiting a "deflated" look, like a mylar helium balloon that got too old :-)

You might see if you can pull any off with strong adhesive tape to study their backside. Your issue is not within my experience set, and again that temperature strikes me as very hot, and perhaps this looks more like melted silver than blistered silver? Hopefully a reader with more experience in this will comment.

Regards,

pic of Ted Mooney
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey
Striving to live "Aloha"


December 5, 2018

A. Come back and let us know: (1) What do you mean, in detail, about "depassivation" -- is it a "strike", what kind of anodes, has the metal concentration increased? (2) Do you have metallic brightener in the silver solution, like antimony or selenium?

robert probert
Robert H Probert
Robert H Probert Technical Services
Garner, North Carolina


December 11, 2018

Q. Actually de-passivation bath is used to prepare suitable surface for pre-silver plating bath. It is used to clean surface from nickel oxide. Bath include potassium cyanide and potassium carbonate. We use stainless steel anodes for de-passivation bath.

No we don't have any brightener in our silver bath. Actually we had it (antimony based brightener) but we didn't add brightener in more than 1 year.

BARIS IRMAK [returning]
- Izmir, Turkey



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