Nickel plating is an important electro deposition process for preserving steel, brass and other basis metals from corrosion. Plated as a bright deposit often combined with chromium, nickel is the most effective electroplated coating for preserving a decorative appearance for extended periods of time in corrosive environments. Nickel electro deposition is also popular for engineering applications as a non-decorative functional plate.
Bright nickel electroplated for decorative uses differs appreciably from non-decorative nickel deposits. The high sulfur content (>0.05%) of the bright deposit reduces its ductility and corrosion resistance. Bright deposits are typically deposited from the watts formulation (see below) with the addition of organic-sulfur brightening systems. Functional deposits are typically deposited from the watts bath without the addition of brighteners (commonly referred to as watts non-bright) or from sulfamate nickel chemistries (see below).
Brightness in nickel deposits is induced with organic-sulfur compounds that decompose at cathode surfaces forming very small particles of nickel sulfide which refine the grain size of the deposit at the cathode surface (plated part) by at least two orders of magnitude. The selection of brightener additions to the plating bath affects ductility, internal stress, electrical conductivity and corrosion resistance of the deposit, all in a negative manner. Nickel coatings stressed in tension reduce the fatigue strength of steel. Nickel deposits lose corrosion resistance, ductility and electrical conductivity as the amount of co-deposited sulfur or other impurities increases from the brightening system. The change of the above characteristics is a rather complex subject which is primarily dependent on the bath chemistry, but also to a lesser extent on the operational parameters of that chemistry. The following charts will provide general trends/properties of nickel as deposited from the three (3) traditional chemical systems: watts non-bright, watts bright (watts with organic brighteners) and sulfamate.
|Type of Bath||Resistivity, microhm-cm|
|Watts, Organic brightener||10.00|
|Type of Bath||Elongation %|
|Watts, Organic brightener||4-5|
|INTERNAL DEPOSIT TENSILE STRESS|
|Type of Bath||Stress, PSI|
|Watts, Organic brightener||30,000||0-60,000|
The corrosion resistance of nickel deposits is very complex and the object of much study. In general, as any impurity is added to the deposit, the nickel plate loses its ability to fight off corrosion. The most common deposit impurities are hydrogen, oxygen, carbon, sulfur, chloride and metallic impurities. Because the watts non-bright and sulfamate formulations do not co-deposit sulfur and carbon from a brightening system, they are more corrosion resistant.
In summary, for engineering applications where ductility and corrosion resistance of the deposit take precedence over a decorative finish, a watts non-bright or sulfamate formulation is the deposit of choice. If low internal stress is also a concern for parts that will be significantly deflected or bent, sulfamate nickel processes are superior. However, bear in mind that these deposits are not bright and as such do not have as high of an aesthetic appeal. The old adage “bright is right” does not always apply for engineered coatings!