Stopping Turbid Milky White Appearance in Electroless Nickel Solutions

A finisher routinely pumps out their electroless nickel (EN) solution to strip and passivate the tank. However, they recently removed the bath at the end of the production day, and when they brought it back into the tank, the solution was clear.

Adam ColburnAfter reaching the operating temperature of 190°F, the solution was a turbid milky white appearance. They wanted to know why this occurred, and how this be prevented in the future.

Electroless nickel chemistry requires replenishment of the nickel metal and sodium hypophosphite during its operation. The bath’s age is measured by the nickel replenishments made to the operating solution over time. A metal turnover (MTO) is when the original makeup concentration of nickel (typically 6 g/L) has been added back to the solution. As the bath ages, reaction byproduct concentrations build with each MTO. The primary and most important byproducts generated include sodium sulfate and sodium orthophosphite.

Whiteout is a generic term used to describe the precipitation of nickel orthophosphite complex (the byproduct of the oxidation of sodium hypophosphite). The EN solution appears turbid or “milky” white in appearance once the complex has precipitated out of the solution. This condition may ultimately lead to stability issues in the chemistry, potential roughness in the resulting deposits, clogging of the filters, and can disrupt production time.

Table 1

A whiteout condition usually occurs as a result of one (or a combination) of the following factors:

Factor	Cause	Corrective Action
Significant loss of working EN solution and subsequent dilution with water	Results in the loss of key complexing agent concentrations that effectively maintain the solubility of the nickel ions in the EN solution.	Addition of makeup component to replenish the complexing agents (refer to formula 3 below).
Operation of the chemistry at too high pH or at a high temperature (above normal range)	Nickel-orthophosphite is less soluble at higher pH and temperatures.	Operate the EN solution within the recommended technical data sheet (TDS) guidelines and contact technical representatives if other parameters are required.
Operation at high nickel metal concentration	EN solutions are formulated to operate at a given nickel metal concentration and higher concentration can lead to insufficient complexing.	Operate the EN solution within the recommended TDS guidelines and contact technical representatives if other parameters are required.
Contamination of the chemistry	Contaminants can contribute to insolubility.	Analyze the chemistry for excessive buildup of metals, such as iron, zinc or aluminum. Eliminate sources of alkaline contamination, such as blind hole drag-in.

The most common root cause for whiteout of EN chemistries is solution loss resulting in too low complexor concentration. There are numerous modes of solution loss and some of the most common and widely observed are:

Relatively high dragout of working EN solution by barrel operation or rack operation of parts that “cup” solution.
Leaks in tank or piping of EN tank that cause significant loss of solution over time.
Incomplete transfer of working EN solution during pump up or pump down during standard maintenance (stripping/passivation/cleaning).
Overflow of working EN solution while in tank due to overfill with water.

When overflow is observed, a standard Ni titration can be performed to determine the amount of solution lost if the original concentration is known and a subsequent addition of the EN “B” makeup material can be made per the following calculation:

% tank volume lost = 100 percent - [(post overflow Ni conc. (g/L) / (pre overflow or original Ni conc. (g/L)] x 100 percent]

Addition (gal) = [tank volume (gal)] x [percent tank volume lost] x MU component use by percent

Example: If original Ni concentration is 6.0 g/L and post overflow Ni concentration is 5.0 g/L, then percent tank volume loss ~17 percent and the addition for a 100 gallon tank = 2.6 gallons of electroless makeup (MU) component.

Even moderate solution loss situations (<10 percent) can cause whiteout if there is one or more other contributing factor (see Table 1). Every process line has a balance for operation, where whiteout occurs, the routine maintenance of 0.5 to 1 percent v/v addition of EN makeup component can be made to avoid the occurrence in the future.

To resolubilize the precipitated material, a series of corrective steps must be followed. If the solution is not treated properly, then reformation of the insolubility can potentially occur, causing need for additional action to be taken. The process to correct an EN bath is as follows:

Cool the bath down to room temperature; step not necessary, but offers safer conditions for addition of dilute sulfuric acid (Step 2) to lower the solution’s pH.
Decrease the pH to ~3.0 with the addition of 10 percent v/v H₂SO₄ ; 50 percent H₂SO₄ may be added to avoid solution growth in EN solutions at ages >4 MTO, but care must be taken to avoid boiling at interface upon introduction of the sulfuric acid by adding extremely slowly.
Circulate or mix the solution to ensure homogeneity and complete dissolution of the precipitated material.
Inspect the solution to ensure precipitate (Ni-Orthophosphite complex) has been resolubilized by sampling with a clear container and assessing the solution clarity. If solution clarity is confirmed, then proceed to Step 4; if solution remains turbid, then lower the pH further according to Step 2 until the precipitate has been resolubilized.
Make an addition of corresponding EN makeup component concentrate using the following formula: Addition (gal) = tank volume (gal) x 1.5 percent. Example: For 100 gallon tank, add 1.5 gallons of EN makeup component concentrate. This is equivalent to a 10 percent EN component B addition. Greater additions may be needed, depending on the severity of the whiteout.
Adjust the pH back to the normal operating level as indicated in the TDS by adding either ammonium hydroxide or potassium carbonate.
Heat the bath to recommended operating temperature as indicated in the TDS. If the solution precipitates again, the EN makeup component addition may be insufficient to maintain the solubility of the nickel. In this case, repeat Steps 1-6.