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Silver Blistering Mechanism
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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
First of five simultaneous responses -- +++++++
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.
- Navarre, Florida
Second of five simultaneous responses -- +++++++
Some silver brighteners are antimony, some are selenium; both casue 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 electrolite, very low carbonates (caused by insoluble anodes), and bagged 0.9999 (that is 4 nines)silver anodes.
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Robert H Probert Robert H Probert Technical Services  Garner, North Carolina
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Third of five simultaneous responses -- +++++++
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!
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Milt Stevenson, Jr. Anoplate Corp Syracuse, New York |
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Fourth of five simultaneous responses -- +++++++
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.
- New Jersey
Fifth of five simultaneous responses -- +++++++
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.
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Ken Vlach - Goleta, California |
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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 Selrex 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 150C.
- 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 250C.
- All tests were conducted in standard atmosphere
- 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 Mplater - Westfield, Massachusetts, USA
January 19, 2012
Hi, Ian.
I hate to respond with something as general as "read a book", but anybody who is plating on 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 tests many different plating conditions, and offers actual adhesion values in his "Adhesion" chapter.
Regards,
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 Ted Mooney, P.E. RET finishing.com Brick, New Jersey |