Fixing Dull Parts From Alkaline Zinc-Nickel Bath
Q: I am getting duller parts than expected out of my alkaline zinc-nickel bath. What might be the issue?
A: Brightness or specular reflectivity from an alkaline zinc-nickel bath can be hindered by several different factors. Control over material, temperature, current density (CD), specific gravity (SG), primary metal content (Ni and Zn) and hydroxide concentration is essential. Proprietary additive concentrations (complexants, brighteners and carriers) are also crucial, but most applicators rely on an automatic pump system to ensure proper levels.
Material: You can’t make a silk purse out of a sow’s ear. A thorough part inspection is crucial to ensure that product received is virtually free of observable asperities that will simply be enhanced once a deposit is applied.
Temperature: High temperatures will degrade the brighteners due to their inherent volatility. Low temperatures might maintain brightness in the high current density (HCD) and mid-range current density (MCD), but might reduce the plating rate and dull the low current density (LCD).
Current Density: Higher-than-optimal CDs (often beyond 15-25 ASF) will slightly impact brightener effectiveness as their rate of decomposition increases due to increased oxidation and hydrolysis. Lower CDs are preferred for brightness, but too low can also hurt LCD brightness, alloy percentage and plating rate.
Specific Gravity: High SG values over 1.22 g/cm³ (due to an accumulation of additive breakdown products, carbonates, sulfates, and other metal and salt contaminants) compromises the functionality of the additives. An annual bath dilution can rectify this.
Zinc: Low-zinc-metal baths (~5-7 gpl) often obtain higher levels of brightness, even in the LCD and good throwing power, but are more prone to burning. These baths are used for complex geometries often sacrificing plating speed and tolerance to contaminants to save on proprietary additive usage. High-zinc metal baths (~7-9 gpl) plate faster and are less sensitive to impurities, but run bulkier on the additives, and have poorer covering and throwing power, which limits part versatility. The small number of benefits afforded these baths do not outweigh the disadvantages as these baths are prone to burning, poor thickness and alloy uniformity, incomplete LCD brightness and weak passivation.
Sodium Hydroxide (NaOH - Caustic): Low hydroxide concentration slightly hurts brightness, but can be improved with increases due to this ion’s generally positive impact on bath conductivity. Levels beyond the standard ~130-145 gpl, however, can accelerate the consumption of proprietary additives, including brighteners. The hydroxide-to-zinc ratio (130:6) is possibly more important with a higher ratio adding to brightness, particularly in the LCD.
Nickel: Higher nickel, to a point, helps with brightness, burn alleviation, throwing power and LCD brightness, but there are negative consequences to consider, including a reduction in these three dimensions — plating speed, ductility and corrosion resistance (when nickel content exceeds 18%). The absolute concentration of nickel is less important than the zinc-to-nickel ratio, which should remain around 5-7:1.
Zinc Complexant: These proprietary components add to the overall brightness when maintained within standard guidelines and specifications because more nickel tends to deposit when the zinc is less ionic. More nickel, generally, automatically improves brightness. Insufficient zinc complexant hurts every major plating parameter. Because the zinc metal is the greatest influencing force on bath performance and quality, this means its complexant is probably the second most crucial component. Excesses of this complexant can cause oil out (precipitation), increased SG (review aforementioned details) and heavy foam, which negatively affects appearance.
Primary Brightener: This component, indubitably, is primarily responsible for overall brightness, but excesses can hurt the plating rate and impact adhesion properties. Low levels will contribute to poor appearance, burning and dullness in the LCD.
Secondary Brighteners: The secondary brightener often focuses on the LCD area and supports deposit initiation, coverage and throwing power. Overdoses often lead to a yellowing of the LCD and an increase of the nickel alloy which, in turn, impacts passivate receptivity and, therefore, corrosion resistance.
Water Conditioners/Grain Refiners: Water conditioners and grain refiners are added alongside additions of NaOH and generally benefit from being on the higher side when brightness and LCD plating are factors. Underdosing this constituent is more detrimental to plating quality than overdosing, although high amounts can hurt the plating rate.
Primary Nickel Complexant: This component helps with brightness because it balances the nickel alloy and is also used for burn prevention and LCD performance. Excesses can produce a rough grain structure, but underdosing is far more inimical, causing issues with low nickel alloy, loss of brightness and poor passivate receptivity which, again, will hurt corrosion resistance.
Secondary Nickel Complexant: This component is often designed to focus on the LCD region. Overdoses will hurt brightness, cause burning, roughen the deposit grain and reduce the plating rate. Underdoses produce poor brightness in the LCD and poor nickel-alloy control.
Carriers, Starters, Base Additives (Initials): The carriers, starters or base additives are often mixed in with the automatic pump additives, as are many of the other proprietary ingredients. These initial components help with brightness, alloy and thickness distribution and burn resistance. Higher amounts can improve brightness as they often work symbiotically with the brighteners, but they also often severely reduce plating rate and nickel content. Low amounts, however, pose a greater danger to bath quality, having the opposite effect of many benefits described above.
Wetters and Surfactants: These components rarely impact brightness, but some claim improvement to throwing power, which may improve overall brightness.
Conclusion: A lack of expected brightness is a common malady affecting many alkaline zinc-nickel platers and is often due, most commonly, to low nickel alloy, high or low caustic concentrations, high specific gravity (residual contaminants) and, most obviously, low brightener or even simply working with low quality material. Beyond these more obvious considerations, an applicator should also ensure additional supplementary factors are given a scrutinizing glance, at the very least. These include validating and verifying that the temperature, current density and zinc metal are not too high, while also confirming the pump rates for the following components are at optimal levels — zinc complexant, primary and secondary brighteners, water conditioners and softeners, primary and secondary nickel complexants, and starters or base additives. Applying these methods will achieve lasting brightness.