Sunday, March 23, 2008

Hard Nickel Baths

A bath recommended for producing hard nickel deposits has been used for applying nickel on the surface of rolls, the salvaging of worn on mismachined parts, and coating machines part from abrasion resistance. Close control of pH, temperature, and current density is necessary in this bath to maintain the desired hardness value. The tensile strength increases and the ductility decreases with an increase of pH and a decrease in temperature. The deposit has a rather low annealing temperature and will not retain full hardness above 232oC. Disadvantages of this hard nickel bath are a greater tendency to form nodules and trees than the chloride or Watts bath and a high internal stress in the deposits which can cause sporadic cracking.

In almost all cases, it is much better and much simpler to use the class I addition agents, the stress reducers, in Watts or sulfamate baths to obtain the desired hardness reducers, in Watts or sulfamate baths to obtain the desired hardness without tensile stress. The stress reducers such as saccharin, p-toluene sulfonamide, sodium m-benzene disulfonate, sodium 1,3,5-naphthalene trisulfonamide, o-sulfobenzaldehyde, which introduce about 0.03% sulfur as sulfide in the plate, are most generally used. In no case just the stress reducer endow the plate from a Watts solution with a compressive stress of the same order of magnitude as the original tensile stress. Found that o-formyl benzene sulfonate produced deposits with a hardness of 440 to 710 and a compressive stress of 170 to 365 kg/cm2, such deposits have been used successfully on steel airplane propellers where hard, abrasion-resistant, ductile and compressively stressed coating must be employed.

Electrodeposits of nickel containing phosphorous offer another means for increasing hardness. This type of deposit has the advantage that suitable heat treatment increases the hardness. Hot hardness, however, is still low. The range of phosphorous co deposited is form 2 to 15% depending on the amount of phosphorous acid in the plating bath.

Addition of oxalate to a nickel bath has been reported to give a hard deposit with a high percentage of carbon.