Real-time elemental monitoring during electrogalvanization

Introduction

Electrogalvanization is an electrochemical process to plate iron objects with a protective Zn or Zn-alloy layer. Having elemental control in the bath allows optimal and constant product quality. Out-of-spec products cause significant costs and penalties, especially if these are used for high valuable objects.

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 and the steel parts to be galvanized as a cathode, to which current is applied.

The composition of the electrolyte baths is crucial for the galvanization process since it heavily impacts product properties such as adhesion, protection and finishing.

Real-time monitoring of the Zn concentration and its alloying elements allows fast counteractions to the process to guarantee optimal product quality, minimal costs for energy and reagents and ensures sustainable manufacturing of coated metal.  

Instrumentation

Measurements are performed using a Malvern Panalytical Epsilon Xflow spectrometer, equipped with a 15W, 50 kV silver (Ag) anode X-ray tube, 6 software-selectable filters and a high-resolution SDD30 silicon drift detector. The flowcell was equipped with a 12 μm polypropylene window for chemical compatibility with the electrolyte.

Sample preparation

Five setup samples were available to calibrate the Epsilon Xflow. The solutions contained different levels of Zn and Ni, solved in caustic soda (pH 14). Further 4 unknown samples were measured to validate the calibration. All samples were analyzed at room temperature. 

Measurement procedure

For each measurement around 50 ml of solution was circulated through the flow cell. Both elements, Zn and Ni, could be measured using only one measurement condition, see table 1. The measurement time was set at 120 seconds per sample. An example spectrum is shown in figure 1. 

Table 1. Measurement conditions

Figure 1: Example spectra of one Zn / Ni solution

Calibration results

Figures 2 and 3 show the calibration graphs for Ni and Zn in caustic soda solution. Both graphs show good correlations between certified concentrations and measured intensities. Detailed calibration results are listed in Table 2. The RMS (Root Mean Square) value is equivalent to 1 sigma standard deviation.

Table 2. Calibration details (* RMS: The more accurate calibrations have the smaller RMS values).

Figure 2. Calibration graph for Ni in caustic soda.

Figure 3. Calibration graph for Zn in caustic soda

High-precision measurements for major, minor and trace elements

One standard sample and 3 unknown samples (validation samples A, B, C,) were measured multiple times to determine the accuracy of the method and the repeatability of the instrument. While analyzing samples were circulated through the flowcell continuously. The repeat measurement of validation sample A is shown in figure 4.

The reference values, the measured concentration and the relative RMS are summarized in table 3. A small offset in Zn concentration was observed, as the formulations are unstable. The aging of the samples, while the samples were transported from the customer site to Malvern Panalytical facilities led to ZnCaoO3 and Zn oxalate formation.

Figure 4. Results of running validation sample A for 21x times repeatability.

Table 3: Results of accuracy and precision measurements. 

Conclusion

The results clearly demonstrate the capability of Epsilon Xflow to monitor the elemental composition of electrogalvanizing baths. Besides providing accurate information about Zn, it can provide trustful insights into the presence of its alloying elements or the presence of impurities in the bath. The Epsilon Xflow is designed to handle concentrated caustic soda solution.

Besides the analytical instrument, Malvern Panalytical supports the full implementation in a production environment.

The combination of state-of-the-art hardware, powerful software deconvolution algorithms and expected application knowledge allows the Epsilon Xflow to provide accurate analysis every minute.

Acknowledgments

Malvern Panalytical acknowledges the “Muschert + Gierse Unternehmensgruppe” for providing samples and helping to create this application note.