Monitoring an Electrogalvanizing Process

Based in Salzburg Austria, MACO is a leading international manufacturer of window and door fittings. The company was established in 1947 and now has a international workforce of around 2300 personnel, working at three locations in Austria and various other international sites.

MACO’s window and door fittings are manufactured in sophisticated facilities in Austria and are marketed worldwide. A large number of in-house production includes numerous surface-finishing processes like anodizing (electrolytic oxidation of aluminum), electrogalvanizing, and chrome-free powder coating. In order to reduce environmental impact, MACO uses only modern technologies for wastewater treatment and processing operations.

The Electrogalvanizing Process

Taken for granted in our daily life, window and door fittings are typically produced in huge quantities. Based on where they are utilized, window and door fittings should be able to fulfill different types of needs, and at the same time they should be able to endure adverse weather conditions. As a result, manufacturers usually add an additional coating to these fittings to realize these product properties. For instance, when steel components are covered with a thin zinc layer, they develop more resistance against corrosion.

In the electrogalvanizing process, a zinc layer is applied using an electric current. A zinc electrolyte bath equipped with two electrodes is utilized - an anode (positive pole) and the steel parts to be galvanized as a cathode (negative pole), to which current is applied.

Dissolved zinc is also included in the bath, and is introduced as a concentrate through metering pumps and a conducting salt, typically caustic soda (sodium hydroxide/NaOH) is also added to boost conductivity. Under the influence of the electric current, the zinc that is dissolved in the bath (Zn²⁺) is reduced on the cathode and then gradually deposited on the steel component surface (Figure 1).

Figure 1. Electrogalvanizing in an alkaline zinc bath.

During the electrogalvanizing process, the zinc ions present on the cathode collect electrons and are subsequently deposited on the component as elemental zinc:

    Zn²⁺ + 2e^¯ → Zn

In the current scenario the anode does not contain zinc, but includes an inert material. During the galvanizing process, zinc ion concentration reduces in the zinc bath and zinc ions are added again as zinc concentrate. Zinc coating quality relies on the whole manufacturing process being executed properly. In this context, downstream and upstream process steps are equally significant as the galvanizing itself with respect to creating high-quality and durable coatings.

The steel components are cleaned and prepared for the coating process with the help of pickling baths and upstream degreasing. Since process work is performed with a cyanide-free and eco-friendly zinc bath, it must be ensured that the components to be coated are free of impurities, like surface scale and rust. Subsequent to coating, the zinc’s sensitive layer is further protected and sealed through passivation.

The numerous baths are:

• Acid degreasing bath (1)
• Rinsing baths (2 + 3)
• Acid pickling bath (4)
• Rinsing baths (5 + 6)
• Alkaline zinc bath (7)
• Rinsing baths (8 + 9)
• Passivating bath (10)
• Rinsing and cleaning baths (11 + 12)

Therefore, the electrogalvanization process includes four active baths as well as complementary cleaning and rinsing baths (Figure 2). In order to realize excellent product quality, it is important to ensure that the composition of the numerous baths remains within the given range. Even a slight difference from the process window can significantly reduce the quality and lead to increased number of rejects.

Figure 2. Schematic diagram of the various process stages in electrogalvanization.

Different coating methods can be utilized based on the size of the metal parts to be coated. A good current flow is required during the galvanizing process, so smaller parts are galvanized in a barrel while larger parts are allowed to travel via the baths in a rack.

This is referred to as barrel and rack galvanizing. Here, the adjustment has to be made to the current density in the zinc bath to suit particular conditions. Electrogalvanizing is one of the most commonly used processes in metal finishing and is also the most cost-effective technique for ensuring consistent protection against corrosion.

Demands Placed on a New Process Monitoring System

The fabrication of window and door fittings followed by electrogalvanizing is a time-intensive process.

To obtain a highly reliable product quality, each step of the process should be closely observed and performed within the specified limits. Earlier, the bath analysis were performed by personnel who were responsible for the process, in addition to carrying out their usual process duties. However, precious time was lost because the analysis were not only difficult, but were also carried out manually to a large extent.

In response to this scenario, automation of the bath analysis was eventually made. Nevertheless, an ideal solution would need to meet the process needs and should make it viable to measure the numerous bath parameters in a direct and rapid way within the process, and without taking up any extra amount of time. The idea is to aim for simple operation.

Process Monitoring at a Glance

Metrohm has developed a new ProcessLab analysis system that meets MACO’s needs in the most suitable way. The system has a modular design, which in this case, is custom-made for the galvanization process. As a result, it is designed to meet the customer‘s needs and helps to make fast and easy measurements at the process line. The ProcessLab system designed for MACO’s bath analysis includes two analysis modules and a TFT monitor to regulate the overall system (Figure 3).

Figure 3. ProcessLab is characterized by its ability to carry out analysis directly at the process line (atline analysis). At MACO the analysis system is located directly by the galvanizing baths.

All the user has to do is load the bath sample onto the system and begin the analysis, and the ProcessLab system will automatically perform all the additional steps. The system determines the amount of sample required, moves it to the measuring vessel, and introduces the auxiliary substances and reagents. Following this, the analyte concentration is determined by subjecting it either to photometric measurement in the analysis module on the right, or to titration in the analysis module on the left.

A fully integrated photometer is used for making photometric measurement. Evaluations and calculations are automatically carried out with the built-in tiamo™ for ProcessLab software. With the help of this control and measurement software, all the analysis results are saved in a central database which can be accessed at a later time.

Status signals can be easily forwarded using the input/output controller integrated within the ProcessLab system. For instance, if there is a potential system fault or if a value exceeds or falls below a limit, these data can be sent. The quantified bath concentrations can also be relayed to the control room as an analog signal of 4…20mA.

Based on these quantified values, all other process steps are started and automatically controlled. ProcessLab is used by MACO as an atline system to track the overall process. All parameters related to the processes in the active baths are rapidly and directly examined with just a single system, and more process chemicals can also be added if required.

Since seven parallel process lines are being operated at the Trieben site, Applikon’s ADI 2040 online analyzer was also installed because of the high frequency of analysis. Applikon is part of Metrohm AG, which has been manufacturing process analyzers for over 25 years. The ADI 2040 is used to monitor the NaOH and zinc content in the zinc baths and the same are automatically adjusted, while the ProcessLab system is used to observe the overall process within all the active baths.

At the Salzburg site there are fewer parallel lines being operated, and hence only the ProcessLab system is being used there to monitor the processes. This case shows how atline and online analyzers ideally complement each other, particularly if the solutions are delivered from the same supplier.

Detailed Description of the Method

An automated analysis system is able to measure all the pertinent parameters of the electrogalvanizing procedure and tracks all of the active baths.

Acid Concentration in the Degreasing Bath

Determination of acid concentration is done using a basic titration with a pH electrode (Profitrode) and caustic soda, and based on the amount of NaOH consumed the acid concentration can be measured.

Acid Concentration in the Pickling Bath

In the pickling bath, the acid concentration is established using titration with a pH electrode and NaOH. Using the ProcessLab system, the acid concentration is automatically determined from the consumed NaOH.

Zinc Concentration in the Zinc Bath

A complexometric titration is used to determine zinc concentration. When a Cu-EDTA buffer solution is added to a bath aliquot, the Cu²⁺ ions that are discharged are titrated with an ion-selective copper electrode (Cu-ISE) and an EDTA solution. The zinc concentration in the zinc bath can be determined from the volume of EDTA consumed. Figure 4 illustrates a standard titration curve for the titrimetric Zn²⁺ determination, as an example of the numerous titrations.

Figure 4. Typical titration curve for complexometric Zn²⁺ determination.

NaOH Concentration in the Zinc Bath

NaOH concentration in the alkaline zinc bath is measured by means of titration with HCl and a pH electrode. The concentration of NaOH in the zinc bath is determined from the quantity of HCl consumed.

Bath Concentrate in the Passivating Bath

A bath concentrate can be dissolved to achieve the passivating bath. Due to the presence of additives in the bath the bath features an intrinsic color, which is utilized for photometric tracking of the passivating bath.

Determination of Metals in the Wastewater

Heavy metals in the wastewater should also be inspected, in addition to tracking the parameters pertinent to the process. This helps in complying with stringent statutory requirements. Due to the modular design of the ProcessLab system, the photometric analysis module can also be utilized to make these measurements, and metals like iron, chromium, copper or zinc can also be measured based on the specific requirement. Using photometry, chromium is automatically measured with the aid of 1,5-diphenylcarbazide, copper with 2,2‘-biquinoline, zinc with zincon, and iron with 5-sulfosalicylic acid.

Conclusion

The ProcessLab system designed by Metrohm fulfills all the process demands of MACO, makes it easy and simple to perform routine atline analysis, and offers reproducible and reliable readings of all relevant bath parameters in the galvanizing process. The system can also be used to measure the concentrations of heavy metals in the wastewater. While choosing the system, an important factor is to opt for an automated system that removes pressure from the personnel responsible for the process, and provides them with additional time to track the actual process. MACO also selected the Process Lab system for its modular design as well as for the prospect of expanding it in the future.

Further literature

• Monograph: Practical Titration, 2005, 164 pages, Metrohm AG, Switzerland.
• T. W. Jelinek, Prozessbegleitende Analytik in der Galvanotechnik, 1999, 440 pages, Eugen G. Leuze Verlag, Saulgau.
• Mordechay Schlesinger, Modern Electroplating, Fourth Edition, 2000, 870 pages, John Wiley & Sons, New York.
• N. Kanani, Electroplating: Basic Principles, Processes and Practice, 2005, 354 pages, Elsevier, Oxford, Amsterdam.

This information has been sourced, reviewed and adapted from materials provided by Metrohm AG.

For more information on this source, please visit Metrohm AG.