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Prevention of silver change of color

Ag plating surface very easily occur discoloration by the reaction of the surface and the corrosive medium (H2S, SO2 gas, etc.) in the activity. Light giving extra energy, I promote the reaction of ionization and Ag and corrosive media of Ag.
The table below -1 influence of light wavelength and exposure time on the color change of Ag, showed a color change and chemical composition by the exposure time in the following table 2.

Table 1 Influence of light wavelength and exposure time on the silver discoloration

Lignt Exposure time(ｈ)
6 12 18 24   48
2537Å
3650Å
Sunlight		 No discoloration
No discoloration
No discoloration		 Yellow spots
No discoloration
No discoloration		 Yellowish brown
No change
No change		 Brownish black
Yellow
Yellow spots		 color
-
-

2537ÅLight of exposure time 6 12 18 24 48
color Silver Yellow Yellowish brown Brownish black Black
The main chemical composition Ag(Metal) Ag2O+AgO Ag2O+AgO gO+Ag
(Ultrafine particle)		 Ag
(Ultrafine particle)

Discoloration products from these results sulfides of Ag, a chloride or Ag particles.
The color of discolored Ag different by chemical composition of the product,Such discoloration also degrade the electrical characteristics increases the contact resistance not only impairs the decorative appearance.
From the above mechanism, to prevent discoloration of the Ag plating the Ag plating surface O2, light, isolated from other corrosive media is achieved by blocking.
How organic vapor compounds with an inorganic compound as a discoloration of the Ag plating conventionally method, a method of combination thereof are used with a surfactant.

The treatment with an inorganic compound

Is immersed for a predetermined time sample test piece was subjected to Ag plated 2 ~ 3μm to compare the effects of various anti-tarnish method 0.2% ammonium polysulfide solution was determined visually (※).


Ag as a discoloration preventing, the same color as Ag is In, Zn, Cd, Pd, Rh, Sn, Be, Al, Th, Zr attempted use of a metal film or a metal oxide material coating is found among them chromate electrolysis process is used most widely in excellent discoloration resistance, then the Rh,Be etc are used.

Treatment with precious metals

Treatment with tin and tin alloys
Treatment with beryllium
Treatment with aluminum
Treatment with chromate
Treatment with organic compounds and surfactants



Organic compounds which have been effective in preventing discoloration in the 2-heptadecylimidazole, 2-mercaptobenzothiazole, Chionarido, the organic compounds of these anti-tarnish effect has the following in common.

  1. PH7 and is treated with an alkaline region from the vicinity.
  2. Hydrophobic 其 (long-chain alkyl 其, phenyl 其) have.
  3. >NH 其 or -SH 其 have, also a double bond.
  4. Insoluble in water
  5. Individual close to colorless

But 2-mercaptobenzoxazole as no effective but has been true for the above conditions to the contrary, there is a 2-mercaptobenzimidazole interaction with these are not clear.

※However, in recent studies it has been reported to be optimal as a test mix gas NO2 and H2S are reflected in the field.

Plating process to a connector

Connector : Are classified as connected components with switch mechanism parts and terminals for electrical connection between each other such as a circuit or device, round the shape, square, Coaxial, is divided into for example, for a printed circuit board.

Japan and Russia Nar, Inc. provides process example