The Hull Cell: Key to Better Electroplating - Part II

This is the second article in a two-part series about Hull cells and their use in plating operations. 

In part 1 of this series, we looked at the types of Hull cells available to platers. We noted that the Hull cell is really a clear plastic miniature plating tank, set up to allow observation of what is happening in real time as electrodeposition proceeds. There are various sizes of Hull cells, available with and without accessory equipment such as heaters and agitators. Properly used, Hull cells can tell you a lot about throwing power, covering power, and what happens when bath components or operating conditions are varied. Now let’s look at how Hull cells are used to evaluate specific plating solutions.

Bright Acid Chloride Zinc

Let’s assume you are looking at bright acid chloride zinc plating solutions from a barrel plating line that is plating parts for five minutes at one amp, or from a rack line producing at three amps for five minutes. Tables I and II show what to expect from Hull cells.

Introduction into acid chloride zinc solutions of cleaners, acid inhibitors, cutting oils and other organic soils is detrimental to the appearance of the resulting finish. Surface-active agents, gluconates, silicates and various “acid inhibitors” can cause gray hazes in the mid- and HCD areas, and these do not respond to additional brightener additions. Synthetic oils can cause irreversible reactions that will destroy a brightener system.

Poor cleaning, poor rinsing, slow barrel rotation and overloaded barrels are all causes of organic contamination. Continuous carbon filtration should bring relief, but this is not a substitute for proper cleaning and rinsing.

Yet another variation on the standard Hull cell, a jiggle cell moves the panel up and down to simulate the effects of agitation.

Bright Alkaline Non-Cyanide Zinc
Suppose you are plating bright alkaline non-cyanide zinc at one amp for five minutes on a barrel line, or at three amps for five minutes on a rack line. Tables III-V show what to expect in Hull cell evaluations.

In terms of deposit thickness, a proper level of refiner will produce thickness readings in “tenths” (0.0001 inch) on the Hull cell panel. Table IV shows ratios of deposit thickness at the left, center and right sides of a panel. Please note that eddy-current-type thickness testers often give consistently low readings, compared with readings from magnetic-type testers.
A thickness ratio of 1:1 or higher between high- and LCD areas would indicate higher levels of refiner. A dark LCD deposit or dark deposit on the back side of the panel would indicate purifier is needed.

Cyanide Zinc

Assume you are plating on a barrel or rack line at two amps for five minutes. Table VI shows what you can learn by looking at Hull cell panels.

Cyanide zinc solutions exemplify the importance of chemical analyses as a partner in Hull cell testing.

Bright Nickel

On this barrel line, nickel plating is done at one amp for five minutes. Because of the high efficiency of nickel, each line will have characteristics unto itself as it relates to the parts being processed. On the Hull cell panel, asf areas (spread) will identify what produces acceptable parts. Beyond that, rack-line characteristic apply.

On a rack line, plating will be assumed to occur at two amps for five minutes. Tables VII and VIII outline the characteristics identified by Hull cell tests at these conditions.

Of course, many variations in bath chemistry exist, and each has its place in determining appearance and deposit performance. Regardless of whether the nickel metal is coming from chloride, sulfate, sulfamate, or combinations of these salts, the final deposit characteristics are largely determined by grain refiners, both low- and high-current density, wetting agents, anti-pitting agents, etc.

For these reasons, Hull cell evaluation of bright nickel processes is probably most useful for identifying contaminants. One constant in all the additives is boric acid. Its main function in the bath chemistry is to maintain the pH within the correct range.

Chromium

Assume deposition of chromium at five amps for four minutes. Table IX shows Hull cell panel effects.

Cyanide Copper Plating

Assume barrel and rack plating at two amps for five minutes. Tables X and XI show Hull cell effects.

Acid Copper

This system has several variations of bath chemistry. In general, the rule of thumb is: “Do not allow the sum of copper sulfate and sulfuric acid to exceed 40 oz/gal or 300 g/liter.” Tables XII and XIII show Hull cell results in simulation of plating conditions for rack and barrel plating at three amps for five minutes.

Acid Fluoborate Copper Plating

Assume production rack plating at three amps for five minutes. Table XIV shows the appearance of Hull cell panels.

In considering contaminants of fluoborate copper solutions, lead is the major bad actor. It causes a loss of ductility in the deposit.

Acid Pyrophosphate Copper Plating

Consider an installation that is plating pyrophosphate copper at two amps for five minutes. Table XV shows what to expect from Hull-Cell panels.

In considering contaminants of pyrophosphate copper solutions, zinc contamination is the most frequent problem. It produces brassy looking deposits. When making up a new bath, perform a “break-in” LCD plate-out to eliminate trace metallic impurities.

Electropolishing

An electropolishing operation running at three amps for five to 15 minutes is considered here. Table XVI shows what one might learn from a Hull cell. Note that many of these bath chemistries have notorious water absorbability.

Table I: Hull Cell Panel Appearance – Bright Acid Chloride Zinc
Plating Solution Variable	Panel Appearance & Recommendations
Optimum Composition	Full bright panel with no burning on high-current-density (HCD) edge.
Low Zinc	Burning and/or extreme roughness on HCD edge.
High Zinc	Poor throwing power in low-current-density (LCD) end of panel.
Low Chloride	LCD efficiency is poor.
High Chloride	Not detectable.
Brightening Agents	Follow addition rates recommended by technical data sheet.
Low Brightener	Measure width of roughness or burn from HCD edge of the panel on an “as-is” sample of the bath. A brightener addition that gives a measurable reduction in the rough or burned width indicates lower-than-optimum brightener level in the production tank.
High Brightener	Very bright appearance of mid- to HCD areas. In extreme cases, the LCD area may show a skip plate and HCD areas will show blistering.
Covering Power	Plating at 0.2 amp for one minute will yield a cathode panel that represents approximately 0.4 to 12 asf (267-ml Hull cell). You now have a much broader visual indication of the need for LCD additives.
Throwing Power	By running a 3-amp panel, effectiveness of the high-current- density inhibitor portion of the brightener additive will be more pronounced (degree of roughness, darkness, and width of these visual indications).

 

Table II: Contaminants of Bright Acid Chloride Zinc Solutions
Contaminant	Panel Appearance
Iron	Yellow deposit; blue to black stain in HCD end of panel after post-dip in 0.25% by volume nitric acid or clear chromate.
Copper	Brown to black stain in LCD end of the panel after post-dip in 0.25% nitric or clear chromate.
Cadmium	Overall dullness, black stain in LCD area after 0.25% nitric acid or clear chromate.
Chromium	Overall dullness, first apparent in the LCD area. Progressive degree of chromium contamination will show skip plate in LCD area and blistering in HCD area.

 

 

Table III: Hull Cell Appearance – Bright Alkaline Non-Cyanide Zinc
Plating Solution Variable	Panel Appearance and Recommendations
Optimum Composition	Uniform bright surface.
High Zinc	High efficiency and brightness in HCD end of panel and a resultant decrease in efficiency, throw and brightness in LCD end. In non-cyanide zinc baths, increasing the zinc metal content is similar to reducing the ratio in zinc cyanide.
Low Zinc	Opposite of conditions observed above: LCD end of the panel is uniformly brighter; HCD end exhibits decreased efficiency and brightness. Reducing zinc metal content produces results similar to raising the zinc/cyanide ratio in a cyanide bath.
Low Caustic	Excessive anode polarization (not to be confused with the normal discoloration of the zinc anode that is characteristic of this type of process). More voltage is required to obtain the testing amperage initially, and during the five-minute plating time several adjustment increases will be required to maintain amperage; bath efficiency is poor. Metal content may decrease.
High Caustic	Not detectable for the short running time of a Hull-cell panel test. The production bath will show unusual metal-concentration increase.
Brighteners	Best controlled by observation of production parts. Follow addition rates recommended by solution supplier’s technical data sheet. Low brightener usually evident as an overly dull deposit and burning at the HCD end of panel. High brightener content manifests itself as an overly brilliant deposit, sometimes accompanied by flaking.

 

Table IV: Hull Cell Deposit Thickness Ratios: Bright Alkaline Non-Cyanide Zinc
Current/Time	3 cm from left edge (40 asf)	Center (15 asf)	3 cm from right edge (3 asf)
1 amp/15min	0.20 – 0.28	0.15 – 0.30	0.10 – 0.15
1 amp/30 min	0.40 – 0.56	0.30 – 0.60	0.20 – 0.30

 

Table V: Contaminants of Bright Alkaline Non-Cyanide Zinc Baths
Contaminant	Panel Appearance
Organics	Blistered dull or dark deposits that stain readily.
Iron	Dull deposits in LCD area turn blue-black after post-dip.
Copper	Deposit darkens in LCD area after post-plate dip in ¼% by volume nitric acid.
Lead	Dull deposit that neither darkens nor brightens with a post dip in ¼% by volume nitric acid.
Cadmium	Darkening of the deposits after a ¼% by volume nitric acid post dip. Mid-current-density area also will be cloudy.
Chromium	Blistering of the panel in the LCD area of the panel. If severe enough, there will be skip plate in this same area.

 

 

Table VI: Hull Cell Panel Appearance—Cyanide Zinc
Plating Solution Variable	Panel Appearance & Recommendations
Optimum Conditions	Full bright panel with little or no loss of reflectivity across the panel.
Low Cyanide	LCD dullness.
Cyanide: Zinc Ratio	Since cyanide zinc systems range from full- to mid- to low- cyanide, the amount of zinc is rarely critical. The NaCn to Zn ratio influences throwing power and cathode efficiency. These ratios are as follows: full cyanide, 2.5-3.1:1, mid, 2.0-2.8:1, and low, 1.0-2.0:1.
High Ratio	Low cathode efficiency and good throwing power.
Low Ratio	The opposite occurs—high cathode efficiency and loss of throw.
Caustic Content	High and low amounts affect anode and cathode gassing, which in turn affect throwing and covering power, bath efficiency (thickness). Hull cell anode darkening or high polish.

 

Table VII: Hull Cell Panel Appearance – Bright Nickel
Plating Solution Variable	Panel Appearance & Recommendations
Optimum Conditions	Full bright panel. The extreme high-current end of the panel may show a light burn or roughness.
Covering Power	At 0.2 amp for 1 minute, panel reads approximately 0.4 to 12.0 asf.
Low Metal	Poor panel coverage and lack of good throwing power. Rough deposit.
High Metal	Hardness increases, LCD thickness builds faster.

 

Table VIII: Contaminants of Bright Nickel Baths
Contaminant	Panel Appearance
Copper and Brass (usually from brazed joints)	Darkening of LCD area and, if severe enough, poor adhesion.
Zinc	Dull to dark LCD areas.
Iron	Roughness anywhere, such as a shelf area. Easily confirmed with a right-angle bend of the cathode panel at the low- to high- current-density end of the cathode panel. Roughness may be accelerated by pH below 3.5.
Hexavalent Chromium	Skip plate; by poor adhesion.
Trivalent Chromium and Aluminum	Difficulty getting plating into recessed areas, as shown in LCD area.
Lead	Brittle deposits and loss of adhesion.
Tin	Darkness in LCD area.
Phosphorus	Cloudy, irregular areas. Often associated with incomplete precleaning, resulting in phosphate drawing compound remaining on formed parts.

 

 

Table IX: Hull Cell Panel Appearance—Chromium Plating
Plating Solution Variable	Panel Appearance & Recommendations
Optimum Conditions	Deposit does not extend to the LCD end of the cathode panel; plate “starts” at 15-20 asf.
Low Catalyst	“Rainbow” lines.
High Catalyst	Extended limit is less, with a brownish dark band.
Low Sulfate	Dull, blotchy, rough, burned deposits.
High Sulfate	Lesser covering and throwing power, irregular gassing pattern.

 

Table X: Hull Cell Panel Appearance—Cyanide Copper Plating
Plating Solution Variable	Panel Appearance & Recommendations
Optimum Conditions	Small dull area in HCD end of panel.
Potassium Salts	The presence of potassium (potassium hydroxide is used to maintain chemical balance) improves plating speed and improves brightness.
Free Cyanide	Low: reduces plating efficiency and brightness. High: produces lower plating “range” and duller deposits, especially in high- to mid-current-density areas.

 

Table XI: Contaminants of Cyanide Copper Solutions
Contaminant	Panel Appearance
Hexavalent Chromium	Blotchy, dull, blistering (non-adherent) deposits anywhere on the cathode panel.
Organic Compounds Such as Buffing Residues	Dull deposits in medium- to HCD areas.
Sulfur	Dull “bands,” especially with selenium brighteners.
Zinc	Brassy-looking deposit. The bath is trying to be a brass solution!

 

 

Table XII: Hull Cell Panel Appearance —Acid Copper Plating
Plating Solution Variable	Panel Appearance & Recommendations
Optimum Conditions	Full bright panel with excellent throw, covering and leveling characteristics.
Low Copper Sulfate	Burning or dull deposits in HCD areas.
High Copper Sulfate	Dullness in LCD areas and low deposition rates.
Low Sulfuric Acid	HCD burning and uncharacteristic drop in copper content. (An example of chemical analysis playing an important role in interpreting a condition). Slow plating rate, LCD dullness and poor leveling.
High Sulfuric Acid	HCD burning, passivation of anodes. Conditions are much like those of low acid.
Low Chloride Ion	Lack of brightness, especially in medium-current-density range; rough deposits.
High Chloride Ion	Same characteristics as seen with high sulfuric concentration, but a noticeable loss of leveling.

 

Table XIII: Contaminants of Acid Copper Solutions
Contaminant	Panel Appearance
Iron	Lower plating rate, hazy appearance. This is a rarity in actual operating baths.
Zinc	High enough quantity in the presence of other metals may cause precipitation, showing as roughness.
Calcium	Roughness; usual source is hard water.
Aluminum	LCD dullness.
Hexavalent Chromium	Poor adhesion, negatively affects brighteners.
Tin	Darkening of deposits, roughness in LCD areas.
Antimony	Brittle deposit, but a rarity: it is usually being dummied out in LCD areas.

 

Table XIV: Hull Cell Panel Appearance—Fluoborate Copper Plating
Plating Solution Variable	Panel Appearance & Recommendations
Optimum Conditions	Very similar to sulfate system.
Low Fluoboric Acid	Brittle, dull to dark (at a pH above 1.7).

 

 

Table XV: Hull Cell Panel Appearance — Pyrophosphate Copper Plating
Plating Solution Variable	Panel Appearance & Recommendations
Optimum Conditions	Bright, ductile panel, but additives are critical. Follow supplier recommendations closely.
High Orthophosphate	Above 13.5 oz/gal bright range decreases and the deposit takes on alternate dull-bright bands. For printed-circuit applications, the limit is 5.3 to 8.0 oz/gal, because throwing power and ductility are lost.
Lead	Dull, streaky deposit. Current density range for good operation is greatly curtailed.

 

Table XVI: Hull-Cell Panel Appearance — Electropolishing
Solution Variable	Panel Appearance & Recommendations
Optimum Conditions	Electropolishing chemistries run at temperatures above 180ºF would melt or badly distort the plastic Hull cell. Aluminum, copper and alloys, steel, and one or two stainless steel formulations have operating temperatures that are within the scope of Hull cell evaluations. Sulfuric, phosphoric, and chromic acid appear in many formulations. When used in conjunction with the correct hydrometer, valuable information as to the bath make-up and status of the solution will be apparent.
Low Phosphoric Acid	Non-uniform patterns appearing as “clouds” or etching.
High Sulfuric	Stock removal is rapid and pitting occurs.
Low Wetter	Streaking in the vertical plane.
Low Chromic	Poor and irregular etching rates.