Electroplating with Alkaline and Cyanide-Containing Electrolytes

Alkaline and Cyanide-Containing Electrolytes in Electroplating

Zinc ranks as the second most important coating metal after nickel, accounting for approximately 20% of the total electroplated area in Europe. Today the market is divided into 50% acidic electrolyte, 30% alkaline cyanide-free, and 20% alkaline cyanide. This demonstrates that economical and environmentally friendly use of alkaline electrolytes remains highly important. For electrolytic deposition of zinc and zinc alloys from alkaline media, electrolytes are available containing cyanide, cyanide-free alkaline, pyrophosphate, and amino-complex systems. However, for various reasons, operational practice essentially uses only cyanide-containing and cyanide-free alkaline electrolytes. Pyrophosphate-containing electrolytes with amino and possibly other complexes, and so-called neutral electrolytes, are used only in exceptional cases. The most important components of alkaline electrolytes are a zinc salt, sodium hydroxide, and in cyanide variants, sodium cyanide. For alloy deposition, the electrolyte contains compounds of the relevant alloy components to be added. Additionally, organic compounds are typically included in small quantities with a brightening function. Commercially supplied brighteners are usually mixtures and may contain compounds from the aldehyde group, polymers such as polyepoxy amines, but also in small quantities nickel, selenium and other metals that promote gloss formation in part.

Tables 1 and 2 contain typical basic electrolyte compositions for the deposition of zinc and zinc alloys. The most important deposition parameters are also listed in each case for better understanding.

Alkaline cyanide electrolytes have long been used for electroplating on an industrial scale, and their development in this respect is thus particularly mature. Beyond the frequently desired high gloss, more controllable layer properties are reliably guaranteed at high quality. The relevant technologies – processes and systems – have reached a very high level and have enabled rational processing in both rack and barrel plating. However, although efforts are being made to replace cyanide-containing processes with alkaline cyanide-free or weakly acidic processes, these efforts are based on achieving a less costly operation. Of course, cyanide-containing wastewater can also be prepared to comply with regulations related to wastewater discharge. However, in addition to special cyanide treatment facilities, measures must be taken to ensure that insoluble cyanides are not formed in any way in reaction with any metallic contamination present. During cyanide oxidation, known difficulties may also arise, including in some cases AOX formation. Therefore, to fully benefit from the advantageous properties of an alkaline cyanide process and simultaneously ensure maximum environmental friendliness – that is, compliance with all legal requirements – certain criteria must be considered. The most important are: • Selection of the best available technique, • Quality assurance during electroplating, • Bath management, • Regeneration, • Recirculation of electrolytes (circulation control).

1. Selection of the Best Available Technique

As previously mentioned, approximately 50% of electroplating today is performed with acidic electrolytes. This is due, to a minor extent, to the increase in strip plating, which until now has only been used with acidic electrolytes. However, the primary reason is that rack and barrel plating, which previously worked only with cyanide electrolytes, has been converted to acidic electrolytes. This became possible thanks to the excellent properties of weakly acidic electroplating electrolytes developed in the 1970s and subsequently refined. Nevertheless, there are still many situations where alkaline electrolytes cannot be replaced, whether during electroplating or alloy deposition. The reasons lie in the electrolyte properties, particularly in the applicable current density range and, generally in the case of geometrically complex parts and in mass barrel plating, in the high scattering power of cyanide electrolytes which is often indispensable. As a determining factor, in alkaline cyanide processes, the necessity for less intensive pre-treatment compared to acidic processes may be cited in certain cases. In some cases, zinc coatings from alkaline electrolytes can be better passivated, and sometimes hydrogen degassing may be more effective. Frequently, the alkaline character is required due to the base material, but there are also cases where conversion of a system is very complex or integration of a process flow does not permit acidic solutions. In such cases, the optimal solution must be sought from among the available alkaline electrolytes for the problem at hand. The electrolyte in question is usually one with the lowest possible cyanide content or a cyanide-free electrolyte.

1.1 Different Cyanide-Containing Electrolytes

Alkaline cyanide electrolytes are highly variable and can be applied at almost any concentration to obtain different properties. However, a permanent prerequisite for function is that the components of the electrolytes maintain certain ratios with one another. The ratios of zinc to cyanide and zinc to hydroxide – that is, the separation result values – are particularly important. The separation result MCN affects temperature sensitivity and the depth distribution of deposition. The separation result MOH affects anode solubility and the electrical conductivity of the electrolyte, that is, the amount of applicable current density. The variability of cyanide electrolytes also makes it possible to reduce cyanide content to achieve cost reductions in wastewater treatment through reduced cyanide transport. This advantage is achieved particularly at low cyanide concentrations, through reduction of cyanide content in bath rinses or by extending residence time in drag-out baths with drag reduction. However, it is not always possible, or only possible to a limited extent, to adjust the concentrations of other bath components to achieve optimal conditions with certain limitations expected. If these are accepted, it is possible to continue working with cyanide baths and avoid costly conversion compared to another process (different system design, plating line technology, etc.).

Generally, the system, auxiliary equipment, analytical monitoring, controls, etc., are either not changed at all or only slightly modified, and most importantly, recourse can also be made to existing operational experience.

As shown previously in Table 1, a distinction is made in practice: • High-cyanide electrolytes, • Medium-cyanide electrolytes, • Low-cyanide electrolytes, and • Very low-cyanide electrolytes. The decision as to which of these electrolytes should be used without compromising quality and economics is individual and depends solely on the specific task at hand. The best way to do this is to start from the existing electrolyte, that is, to generally make a comparison between the high-cyanide version and the very low-cyanide versions. From this, conclusions are drawn as to which bath or layer properties possible limitations may occur and how they relate to the expected benefits. The high-cyanide version is the most robust and most variable electrolyte. It is used where good metal distribution at high current is required. The high-cyanide electrolyte permits application of high current densities with a wide optimal current density range, good metal distribution, and excellent gloss and depth distribution. Temperature sensitivity is very low, as are maintenance and analytical costs. Its disadvantages are high concentrations of electrolyte components and consequently high discharge of active components. A medium-cyanide electrolyte is in principle a high-cyanide mixture diluted in a 1:1 ratio; it is the most commonly used in practice. If there is no excessive requirement for depth distribution and somewhat higher costs are factored in, it can essentially always replace the high-cyanide electrolyte.

Maximum current densities, current density range, metal distribution, and depth distribution of gloss in the medium-cyanide electrolyte are quite comparable to the high-cyanide version when taking into account its lower temperature sensitivity. Therefore, temperature must be maintained in a narrower range.

Additionally, the electrolyte is more sensitive to changes in the concentration ratios of bath components compared to the high-cyanide version; that is, maintenance and analytical costs are higher. In medium-cyanide electrolytes, it is advantageous that generally the same brightener additives as in high-cyanide electrolytes are typically used, where the degree of gloss is partly higher. In the case of low-cyanide electrolyte, lower maximum current density, smaller current density range, lower gloss depth distribution, and weaker metal distribution should be expected. The electrolyte is essentially used when simpler parts are electroplated, when there are no special expectations regarding coatings, when deposition rate is secondary, and when chemical savings are a priority. Compared to high-cyanide electrolyte, approximately 30% savings in chemical costs can be achieved. However, low-cyanide electrolyte requires different brighteners, which may be partly more expensive. Very low-cyanide electrolyte is actually a compromise solution in cases where cyanide-free electrolytes cannot be used for certain reasons. Due to the applicable low current densities, these electrolytes are essentially used for barrel plating. Their main advantages are based on low usage, maintenance, and detoxification costs. Savings of up to 90% in chemical costs compared to high-cyanide electrolyte can be achieved, but generally higher costs must be expected for the required brightener additives. Comparison of the performance and efficiency of individual electrolytes can be seen from the data in Table 3. Table 3: Performance (average current density im, average deposition rate VA) and efficiency (relative operating and wastewater treatment costs) of cyanide electrolytes at different concentrations In the event of conversion of an existing electrolyte to a low-cyanide version, conversion costs will naturally also be incurred. Particularly if larger electrolyte volumes are involved and the conversion must take place without operational interruption, implementation can take weeks or even months. Since quality must be maintained in the interim, additional analytical effort must be expected. Therefore, there will certainly be cases where a new installation could be more economical.

1.2 Alkaline Cyanide-Free Electrolyte

In zinc deposition from alkaline cyanide-free electrolytes consisting of zinc hydroxyl complexes, two process types applicable in operations are distinguished. Traditional zinc baths, in addition to brighteners, also require partly soft complexing agents as additives for deposition of compact coatings. However, these can only be used to a limited extent on an operational scale, as they must be maintained in very narrow ranges for satisfactory separation of electrolyte composition and working parameters. As long as they are still used today due to good covering properties, application is limited to galvanizing operations with a fixed program and where there is no variety of parts. A transition from cyanide electrolytes to those in this group generally does not occur.

New-generation electrolytes, known for several years, are based on the addition of synthetic polymers that correspondingly modify the cathode reaction.

Additionally, as they permit the use of more concentrated electrolytes (according to data in Table 2, 14 to 26 g/l zinc and 105 to 165 g/l sodium hydroxide), high-gloss coatings with good depth distribution can be deposited at high speeds over a relatively wide current density range. The electrolytes have good covering properties and the coatings are sufficiently soft. Therefore, new-generation electrolytes are quite suitable for replacing cyanide electrolytes under average conditions. For conversion, cyanide is processed down to the very low-cyanide electrolyte level. Then, under continuous control, conversion to the cyanide-free process can be achieved by adding suitable additives. The rinse waters will become cyanide-free as time progresses so that cyanide treatment in wastewater is no longer necessary; the duration depends on the conditions and can be very long. For this reason, in most cases it is easier, safer, and much less costly to set up a new system.

2. Quality Assurance During Electroplating

Quality assurance holds special significance in galvanic coating technology. On one hand, defective coatings, particularly in functional applications, can result in extremely extensive consequences. On the other hand, this is also a prerequisite for economical and environmentally friendly operation. For evaluation, the most important criterion here is avoiding defective coatings to prevent high stripping and replating costs. Prevention of defective deposition includes a series of technical and organizational measures to comply with all parameters affecting deposition. What these measures are and how they should be implemented and monitored is today determined by many standards and comprehensive monitoring systems. While quality assurance measures are intended to ensure defect-free coating deposition, quality control during implementation verifies whether the achieved depositions comply with previously specified conditions. However, repeatable quality is guaranteed only when feedback between quality control and quality assurance measures occurs.

In galvanic processing, the most important technical quality assurance measures consist of monitoring and maintaining constant electrolyte composition and controlling deposition conditions (current density, temperature, bath circulation, etc.). On the other hand, quality control essentially involves control of electrolyte properties (scattering, current density range, gloss, etc.) through

Hull cell testing and important coating properties such as coating thickness, gloss, or hardness. Therefore, to effectively ensure repeatable quality, correlation must be established on one hand between electrolyte or coating properties and on the other hand between electrolyte composition or deposition parameters. Such correlations have been researched for most methods and are published in literature in the form of error tables or as provided by process suppliers. In galvanic process application, such tables are used essentially to measure quality assurance measures, along with information on the working conditions of the electrolytes. For example, to determine within which limits temperature can fluctuate, in what quantities brightener additives should be dosed, and much more. Automatic feedback, for instance through control of immersion time or measured coating thickness values on current density, is found only in rare cases. These and therefore especially the conversion of correlations in tables is one of the fundamental objectives of technology, particularly in functional electroplating technology. İzzet Aydın - General Manager - Hillebrand Chemicals Kimyasal Pazarlama Ltd. Şti