If your parts are exhibiting nickel show out of your hex-chrome bath, it is generally due to poor coverage, an often misapplied and misunderstood term.
Adam BlakeleyCoverage on the plating shop floor is often associated with poor thickness distribution or uniformity along the current density spectrum. When used less colloquially and more officially in surface finishing journals, coverage is defined as thickness initiation and development in the very low current density regions. Decorative hexavalent chromium (hex-chrome) solutions possess notoriously poor covering and throwing power. Throwing power in scientific journals is defined as thickness distribution along the current density spectrum. To avoid confusion, when coverage and throw are mentioned below, those terms are defined as they are in those sanctioned journals.
Coverage issues can be related to physical and chemical problems, separately or collectively. In the chemical realm, poor coverage is often due to chromic acid, catalyst and ratio disparities, organic and metallic contamination, and temperature fluctuations. Poor coverage due to physical parameters is often related to electrical anomalies (such as high ripple or residual AC, blown fuses, malfunctioning diodes), anode passivity, tight anode and cathode spacing, connectivity issues or current density disproportions, impediments to current flow and nickel deposit issues.
Define Your Problem – Not White Wash
White wash is a phenomenon that deals primarily with nickel passivity and is often difficult to pin down. A part may show both nickel show and white wash simultaneously, but the emphasis is on the lack of chrome deposit in the low current density (LCD) areas to cover the nickel deposit. Dealing with multiple issues at the same time is a source of great frustration, but defining the problem is half the battle. Consider the basic, fundamental principles of Six Sigma methodology found in the DMAIC acronym (Define, Measure, Analyze, Improve and Control). The first step is the “D,” or “Define,” stage for good reason.
Chromic Acid, Ratio And Catalyst Imbalances
Low chromic acid to sulfate (primary catalyst) ratio is the most common cause of poor coverage, and low chromic acid content is another likely cause. A hydrometer reading can quickly and easily provide a Baume reading to estimate the approximate chromic acid concentration, provided that the bath is not overly contaminated with organic and metallic pollutants. Titrimetric or AA/ICP analyses are completed to obtain exact chromic acid levels.
Regular Hull cell analysis is also essential to indicate if nickel show is due to a chemical imbalance in either the nickel or chrome plating baths. Most decorative hex-chrome electrolytes should cover 60-75% of a 3-minute, 5-amp Hull cell panel from left to right. If the chromic acid concentration is adequately high but poor coverage is still evident, then a reduction of the sulfate ion via barium carbonate precipitation might be needed. Low ratios are more common than high ones due to the fact that sulfate is reduced via dragout and is not impacted by the electrolytic reaction; however, excessively high ratios can still cause coverage issues. Low concentrations of other catalysts, especially fluoride, can impact covering power.
Electrical Issues and Current Density
Poor or intermittent contact in the plating system may increase incidences of nickel show. Voltage drops are common due to the large number of connection points running from the rectifier to the parts. Checking the voltage at each divergent point along the journey from rectifier to parts using a handheld voltmeter is key to eliminating insufficient voltage/amperage transmittance. Live entry or low-current initiation in the chrome plating tank can help to mitigate issues due to passivation. This potential for passivation can often be exacerbated by an excessive amount of organic breakdown products occluded in the nickel deposit, specifically in the LCD area, which exhibit as poor coverage or possibly white wash.
To remedy these issues, it is paramount to clean each contact point thoroughly and regularly, add conductivity grease to the saddles and other prominent contact areas, and reposition and refurbish fixtures that may be restricting or stifling current flow. Decorative hex-chrome baths often operate at a current density (CD) of approximately 90-160 ASF and will change depending on the catalyst system being employed. Operating at low CDs often increases the incidences of poor coverage.
Passive or Polarized Anodes
The lead or lead alloy (Sb and Sn most commonly for rigidity and corrosion resistance) anodes in a hex-chrome tank develop a chocolate-brown, lead-peroxide film when they are actively completing the circuit and helping to oxidize tri-chrome ions back into hex-chrome. Idle anodes, those with low ACD, those that have poor connection to the bus bar or those that are too far from the cathode itself might develop a sickly, yellow lead-chromate film and scale. Ensure anodes are inspected regularly to avoid this malady.
Shields, Baffles or Thieves
Areas of parts may be blocked from current transmission due to unintentionally placed shields, baffles or thieves that block or steal current from the part. Inspect your equipment regularly to ensure there is nothing obstructing or hindering current delivery.
Anode/Cathode Spacing, Type and Orientation
Often times, anodes are too large or too long for proper coverage. If possible, create or design anodes that are smaller and shorter than the cathode pieces to avoid a dog-boning effect. Ensure that anodes are spaced correctly and parts on the rack are oriented properly so that current is being transmitted to all areas of the part. The use of auxiliary anodes may be necessary for some LCD areas and may require new rack tooling, part adjustment or other more innovative solutions.
High Temperature
Most decorative hex-chrome plating solutions operate at temperatures between 105 and 115ºF. Often times, poorly placed probes may only reveal the temperature in a localized area. Poor circulation, especially with long and/or wide tanks, leads to stratification, or the creation of layered temperature gradients due to poor heat dispersion. Multiple probes are often needed to ensure agreement and accuracy. Regular manual temperature checks in multiple tank sections should be employed to ensure validity and compliance.
Contamination – Organics or Metallics
Electrolysis or dummying the solution at high CCD’s (100-200 ASF) with an ACD of 20-100 ASF will initiate metallic removal. Vigorous agitation and high temperatures (145ºF) — provided that the equipment can handle it — can increase the opportunities for oxidation and thus metal removal. Utilizing porous pot technology via ceramic membranes is a more controlled version of the electrolysis method. Electrodialysis is similar in that it uses a heavy-wall polyester membrane often immersed in the plating solution or in an auxiliary tank.
Confirming that parts are secure on the racks and have not fallen to the bottom of the tank where they will dissolve should be a regular course of action to avoid heavy metal accumulation. Chloride contamination is caused by poor rinsing between the nickel and chrome tanks and behaves like a powerful catalyst, causing coverage issues. Chloride is removed via high ACD (~200 ASF). Organic contamination can produce excess trichrome which sometimes manifests as incomplete coverage.
Nickel Deposit and Bath Issues
As mentioned previously, coverage can be due to chemistry outside the chrome bath. Often times, nickel baths precede a chrome bath, so it’s essential to ensure that the nickel bath is clean by optimizing filtration and confirm proprietary organic additives are being metered in at the proper replenishment rates. The suitable implementation of a nickel activation tank prior to chrome plating will act as added insurance should the nickel bath be populated with excessive organics.
Conclusion
Nickel show is a phenomenon reserved mainly for baths that fail to cover well. The reasons for poor coverage can be multifaceted, varied and ultimately exasperating, but some basic preventative maintenance protocol can be initiated to preempt these possibilities. Poor coverage is sometimes due to chemical imbalances and other times to equipment malfunctions or both concurrently. Regular analysis of the hex-chrome bath for chromic acid and catalyst content is non-negotiable as is a consistent monitoring of the temperature and other parameters, including cleanliness.
Regular analysis of other baths in the process, such as nickel plating baths, is also integral to the proper functioning of the whole system. A fully-functioning rectifier and anodes that are secured and distributing current properly without impediment is vital and indispensable to quality production. A basic checklist to address each of these concerns can be manufactured and executed without incurring a significant or dramatic increase in labor and time concerns and constraints. It may also save you from losing money due to lost production or failed attempts to rectify the issue, but, more importantly, it will secure your peace of mind.
Adam Blakeley, MSF, and CEF, is Technical Manager of the Eastern Region at MacDermid Enthone Industrial Solutions. Visit www.macdermidenthone.com
