Guest Editorial -For Plateworld.com                                                 

 Don Baudrand, Don Baudrand Consulting,   e-mail:donwb@tscnet.com

 

Doís and Doníts of Sulfamate Nickel Plating

Why is sulfamate nickel plating used? The deposits are ductile, yet strong. The deposits have good thermal and electrical conductivity, favorable modulus of elasticity; low coefficient of expansion, the formula is simple. It does not cause hydrogen embrittlement per se. Embrittlement, is often due to pre-treatment such as the use of hydrochloric acid, or sulfuric acid. A mild sulfamic acid or mild phosphoric acid is suggested for a very short time when treating high strength steels. The sulfamate radical is very activating of steels. There is a sulfamate nickel strike formula that will activate some stainless alloys that are difficult to activate using a Woods strike.

Sulfamate nickel solutions can plate at very high current density using mild agitation. Over 100 A/sq.ft (10.8A/dm2) is possible. With moderate agitation deposit the can be up to 400 ASF (43A/dm2 ) With high agitation, 600 or more ASF (65A/sq dm) or more is possible and with "hone plating" where agitator is in contact with the deposit surface moving at relatively high speed, 6000 ASF (646 A/sq dm) is achievable. Plating rate is about a mil per minute.

Trace metals in the plating solution can influence some of the characteristics of the sulfamate nickel deposit. For example, a small amount of manganese (Mn) decreases stress and more important, Mn can inhibit sulfur migration to grain boundaries of the nickel deposit. Also, Mn in the deposit will stop cracking at weld sites by preventing sulfur from reaching weld areas. Sulfur is a common impurity in sulfamate nickel deposits. Sources of sulfur are carbon filters, over flow from anode baskets because of the sulfur depolarized (SD) anodes required for sulfamate nickel. The particulate sulfur can co-deposit in the nickel. On heating of the deposit, sulfur migrates to grain boundaries.

Small amounts of magnesium have little or no influence on the physical characteristics of the nickel deposit.

Small amounts of chromium copper, zinc and cadmium change the properties of the nickel deposit drastically.

Cobalt additions will harden the deposit and enhance the hard magnetics.

Iron will also increase the magnet magnetic characteristics.

Sulfamate nickel deposits make a good base for bright nickel plating. It is used with diamonds particles used in cutting tools. Sulfamate nickel deposits are useful for many different applications.

DOíS

    1. Keep undesirable impurities out of the sulfamate plating solution. If impurities are indicated, remove those as soon as possible using correct methods for purifying the SN plating solution.
    2. Take advantage of the high plating rate using high agitation.
    3. Use "educators" properly placed to increase solution agitation, that aid in high speed plating.
    4. Test for metallic and organic impurities frequently and remove them as soon as possible to maintain the desirable deposit characteristics.
    5. Use a moderate solution temperature. High temperature can slowly hydrolyze the sulfamate resulting in ammonia. Ammonia is an undesirable impurity.
    6. Keep the pH at 4.0 to 4.4 at pH 4 or above keeps iron insoluble to be picked up in the filter. A pH below 4 allows iron to stay in the solution causing adverse effect in the deposit. At pH 5 or above hardness increases and ductility decreases due to the co-deposition of oxygen.
    7. Use "sulfur depolarized anodes" only. Any other anode will cause compressive stress in the deposit. Keep bags full of anode chips. Keep the top of anode bags above the SD nickel anode squares.

DONíTS

    1. Over heat the plating solution. Temperature over 120F (49C) causes hydrolysis of the sulfamate producing ammonia that has adverse effects on the deposit. Ammonia is difficult to remove.
    2. Use mineral acids such as hydrochloric acid or sulfuric acid in preparation for sulfamate nickel plating. 4-5 oz/gal sulfamic aid or dilute phosphoric acid and using short immersion time should be used to minimize hydrogen embrittlement of the basis metal. Even weak sulfamic acid has good deoxidizing ability (activation power).
    3. Donít use rolled depolarized anodes, or any insoluble anodes. The intrinsic stress increases rapidly.
    4. Ignore the stress. If internal anodes are required such that sulfur depolarized anodes cannot be used. Be aware of the increase in compressive stress of the deposit.
    5. Donít allow particles from anode bags to get into the solution. Agitation can sometimes cause particles to come over the top of the anode bad.

You may download this article FREE in .pdf form, save it or share it with a colleague. Click here.