Optimum metal plating process performance cannot be obtained without careful control of the plating bath chemistry
Electroplating of different precious metals is a modern day necessity in a plethora of different industries that include semiconductor, aerospace, automotive, jewellery, chemical, and food to name but a few.
No matter which metal plating technique is employed, the optimum process performance cannot be obtained without careful control of the plating bath chemistry.
The quality and efficiency of the plating processes depend on the bath composition, pH and temperature.
Previous forms of analysis of the bath chemistry tended to be labour intensive employing wet chemistry techniques such as titration and colorimetric tests and often gave imprecise results.
The world of analytical chemistry today affords the user different techniques for the purpose of routine plating bath analysis.
However much of the analytical instrumentation is prohibitively expensive and can be too complex to operate for all but the most experienced analytical user, often requiring time consuming sample preparation prior to analysis.
The technique of polarography is accepted as being a natural means to solve many of the analytical problems of the plating industry since electrochemistry is a fundamental part of both.
Preparation for electroplating.
Many of the materials that are metal plated are often surface finished by means of mechanical polishing before the metal substrate is activated to allow proper adhesion with a defect free appearance.
The cleaning, degreasing and activation are carried out in aqueous cleaning solutions before receiving their noble metal topcoat.
An acid dip (10% sulphuric acid) is used to remove any oxides that may have developed on the substrate surface (often steel).
Over time, copper and organic contamination can accumulate in the acid bath.
If the copper exceeds a certain level then the bath can cause adhesion problems for the steel substrate.
Furthermore, copper contamination also impacts nickel-electroplating solution. Voltammetry can monitor the level of copper contamination present in the acid bath.
Symptoms that become prevalent during the electroless nickel plating procedure such as a slow plating rate, no deposit, blistering of surface, poor adhesion or streaking effects, dullness or pitting can often be as a result of metallic or organic impurities.
Through the use of voltammetry it is possible to quantify many of these metallic and organic contaminants and then take appropriate control steps.
Types and applications of nickel plating finishing.
Nickel plating is one of the more diverse metal finishing processes available.
The end uses of nickel plating range anywhere from decorative to functional.
Decorative platings tend to be obtained from solutions containing organic additives and tend to be smooth, bright and protective.
Functional nickel plating exhibits excellent corrosion and wear resistance properties.
Hexavalent chromium contamination in a decorative nickel-plating bath can cause discoloration of the nickel deposit and loss of plating efficiency.
Previous studies have shown that contamination of as little as 10mg l-1 chromium can lead to efficiency losses of up to 10%.
Electroless nickel produces an alloy (85 to 98% nickel and 15 to 2% phosphorous) with unusual properties that make it very useful in a broad range of functional applications.
Unlike standard nickel plating that relies on current density in a bath for deposit thickness, the thickness of electroless nickel plating depends solely on the deposition rate and the length of time in the solution.
This ensures that the entire part of the material is covered with a uniform thickness coating, including seams, sharp edges and the interior of deep drawn parts.
The use of electroless nickel applications have grown rapidly in popularity over recent years and cover a wide range of industries such as electronics, aerospace, petroleum, textile and printing. Its popularity in part is down to its excellent resistance to corrosion and wear as well as the ability to coat precision components uniformly.
One of the first commercial applications of electroless nickel was to coat the inside of vessels used to transport corrosive liquids.
It is almost completely resistant to attack from almost all chemicals except some strongly oxidising acids and salts.
The petrochemical market is a rapidly expanding market for electroless nickel because of its superior corrosion and abrasion resistance compared with the alternatives.
This has resulted in prolonged equipment life and reliability when replacing more exotic alloys reducing the investment and operating cost.
In aggressive petroleum environments electroless nickel has been found to outlast steel by more than ten times. The majority of electroless nickel applications are performed on aluminium, iron or steel substrates.
The aluminium is often given a zinc strike prior to deposition and zinc contamination can occur while plating aluminium, whilst iron contamination can occur when plating iron or steel.
These species have been shown to have an adverse effect on the plating process at concentrations above 10 mg l-1 in the bath solution.
Other metal contaminants such as cadmium and lead have been found to influence the adhesion of the coat even at trace levels.
An electroless nickel-plating bath contains nickel as its major component, with molybdenum, cobalt and iron as additives.
In theory it is possible to quantify all these metals, with the appropriate sample preparation, as well as other impurities that may be present such as cadmium, copper and lead.
Electroless nickel plating processes are based on the electrochemical reaction between nickel ions in solution and a chemical reducing agent such as sodium hypophosphite.
The reaction requires the use of a catalyst and the baths are generally operated at elevated temperatures to increase the deposition rate.
The nickel in the bath is depleted during the plating procedure and requires sporadic top up; the monitoring of nickel concentration using voltammetry allows such replenishment when required.
Metal finishing applications using voltammetry.
Plating bath compositions vary substantially throughout the industry and normally consist of the major components, some trace metals and organic additives.
For example in a brass bath, copper and zinc will be present as major components and lead and arsenic as trace metals.
The major components can simply be diluted into the polarographic cell.
The control of trace metals in plating baths is often critical to the integrity of the final product, with non-electrochemical techniques the high concentration of the background matrix often prevents determination of the minor constituents.
As almost every functional group exhibits some degree of electrochemical activity, polarography can often be applied to the analysis of a large variety of organic species. Many different types of plating bath applications have been completed in-house by Metrohm in the Swiss Applications Laboratory.
Germanium from an electroplating bath containing high levels of aluminium, iron, manganese and zinc was determined directly using a catechol and acetate buffer electrolyte system with no sample preparation of the plating bath required.
Chromium and selenium were determined separately in a silver-plating bath after wet digestion with nitric acid.
For chromium, the supporting electrolyte was sodium acetate/nitrate and diethylenetriaminepentaacetic acid and for selenium; ethylenediaminetetraacetic acid and ammonium sulphate were used. Nickel, iron and copper were also determined in a silver-plating bath.
Nickel was quantified by formation of a dimethylglyoxime complex with ammonium chloride buffer and iron and copper determined simultaneously using catechol with piperazinebis (ethanesulphonic acid) buffer.
Lead was determined directly in a nickel plating bath with no additional electrolyte required, the only sample preparation step was to ensure that the pH of the cell solution was weakly acidic.
Tin and lead have been determined in an organo-plating bath after wet digestion of the sample with sulphuric acid and hydrogen peroxide using a hydrochloric acid, ammonium oxalate/chloride and methylene blue based electrolyte.
Thiourea in a nickel plating bath has been determined using cathodic stripping voltammetry with no sample preparation using acetate, ammonia and perchlorate based electrolyte system.
The high levels of chloride present in the sample did not interfere with the determination.
Thiourea and its derivatives are very nearly universal additives added to many plating baths to improve the quality and finish of the metal plate.
Being able to quantify using voltammetry, usually in the presence of high concentrations of other species, is of great use and importance as its analysis using traditional methods has not been entirely successful.
Formaldehyde was determined in a reductive copper-electroplating bath with no sample preparation using a lithium hydroxide and ethylenediaminetetraacetic acid based electrolyte.
With voltammetry, it was possible to conduct speciation studies on iron (II) and iron (III) in a galvanic zinc sulphate bath using a tetrasodium pyrophosphate electrolyte.
10ml of deionised water and 5ml of electroless nickel bath were added to the reaction vessel in the Metrohm 757 VA Computrace to give a weakly acidic solution pH.
Many determinations are pH dependent; so the electrolyte (where present) can increase the conductivity and selectivity of the solution as well as helping to avoid interferences.
The solution was then degassed with nitrogen for a period of 5 minutes to remove the electrochemically active oxygen, before the lead content was determined using two standard additions with the Dropping Mercury Electrode (DME).
The DME is the classical mercury electrode where the mercury flows freely from the glass capillary until the mercury drop is knocked off by a tapping mechanism after each voltage step time set in the measurement mode.
An advantage of the DME compared to other electrode modes is that the MME capillary is subjected to less mechanical stress.
The Metrohm 757 VA Computrace is PC controlled and is well suited to routine analysis.
With the MME, it is possible to vary the sensitivity of the electrode mode by a simple click of the mouse within the software.
The parameters required for quantitative determination are displayed within a series of windows contained within the operating interface and the signal evaluation and concentration calculation is done automatically.
The analysis can be facilitated by the addition of up to five 765 Dosimats to automate the addition of standard additions or dispensation of the electrolyte.
The analysis produced a result of the order 1 mg l-1 of lead present in electroless nickel bath.
Conclusion of Voltammetry for Metal Finishing Applications The chemistry of plating baths can become a major analytical problem that could be difficult and expensive to manage if not properly controlled.
In addition to continuously monitoring the concentration of the metal being plated one also has to consider the levels of other metals (and organics) that may be present within the plating bath to ensure an optimum manufacturing process.
Voltammetry is an increasingly popular technique that in many instances offers unrivalled detection limits even when compared to vastly more expensive analytical techniques.
Voltammetry is a cost-effective analytical solution for those interested in the control of plating bath chemistry. The initial outlay of the instrument is relatively cheap when compared with spectroscopic alternatives and the running costs considerably lower.
No specialist laboratory infrastructure such as expensive fume extraction is needed.
All that is required is a sturdy bench top on which to mount the instrument and a regulated flow of an inert gas.
Quite often voltammetry requires no sample preparation and the result, usually determined by standard addition, obtained in less than ten minutes.
It is often possible to do simultaneous analyses, for example cadmium and lead, in a single voltammetric sweep.
The advantage of using standard addition as a means of calibration is that any matrix effects present in the sample are taken into account.
In terms of its advantages over spectroscopic techniques, voltammetry can be used for speciation studies as well as quantifying the organo, anionic and metallic species present in the plating solutions.
This provides a powerful case for the use of voltammetry in the control of bath chemistry where all three types are active constituents present in the plating baths.
