Rapid Plating Solution Analysis using X-Ray Fluorescence (XRF)
The X-Ray Fluorescence (XRF) method is so versatile, that in addition to measuring coating thickness, it offers a fast, simple and unskilled analysis of plating solutions. Following extensive research by the Helmut Fischer Group's physicist Dr Simone Dill, Ray Scruby, MD of Fischer Instrumentation (GB) Ltd, explains the difference between XRF and more traditional methods of analysing plating solutions.
The quality of the solution in a plating bath plays a vital role in the deposition of a plated coating. The solution is depleted with use and in some processes it is contaminated by the items being plated, especially if the solution is acid based. In Zn-Ni baths, for example, Fe is dissolved from the steel components and contaminates the solution.
The concentration of the metal ions is indicative of the solution's condition and obviously plays a vital role and must be monitored. The traditional analysis method is titration, where a reagent is slowly added to a sample of the bath solution until a physical chemical change is observed or electrochemical potential change measured. This technique requires some experience with quantitative analysis techniques and the careful preparation of samples. It requires skill and takes time to achieve good results, however, the investment in the equipment is relatively low, which must be offset against the time each analysis takes.
The other traditional method is AAS (Atomic Absorption Spectroscopy). The technique makes use of absorption spectroscopy to assess the concentration of an analyte in a sample. This requires the investment in a quite expensive instrument and each measurement involves a good deal of preparation.
The system operates by forcing electrons into higher orbitals for an instant by introducing energy, in the form of a flame or light source. The electrons give off a characteristic energy pulse which can be analysed. The wavelength depends on the metallic ion and the amount depends on the ion concentration. The process is complicated for liquids, as a sample has to be evaporated firstly, then vaporised into a gas before being atomised.
The principle also requires the preparation of samples and the various processes obviously take a good deal of time.
The XRF method uses the same X-Ray system which is used for measuring coating thickness in most plating companies. The technique requires the measurement of a sample of solution of known thickness. This is achieved using a simple measuring cell which is filled with a few drops of solution and then covered with a clear foil. Fig 1 shows the process. Measurement times are typically 30 seconds for an analysis for all the metallic ions in the solution; separate tests are not needed for each component.
The advantages are that the process is simple and unskilled and takes such a short time that it becomes convenient to monitor the solution more frequently and make corrections before the quality of plating suffers.
The table shows comparisons of the various methods. The X-ray system was used "standards free", that is without the use of known standards to improve the results. The results compare very closely with the other methods and the technique can accommodate complex solution matrixes which are present in most baths, for example brighteners, hydroxides, surfactants etc.
Table 1:Comparison between AAS measurements, Titration and XRF
Bath Type Metal ion AAS[g/l] XRF[g/l]Meas.Time 5*20(s) Titration [g/l]
Nickelsulphate Ni
New Batch 30g/l 30,8 30,3 (0,2) -
Nickel sulphate/Nickel chloride Ni
New Batch 15g/l 15,3 15,3 (0.2) -
New Batch 30g/l 30,3 30,5 (0.3) -
New Batch 60g/l 61,2 60 (0,2) -
Chem.Nickel Ni 5,3 4,9 (0.2) 5,3
Production Bath 6,1 5,3 (0.1) 6,1
Alkaline Zinc Zn
Production Bath 17,9 16,5 (0,3) 17,0
Acid Zinc Zn
Production Bath 30,0 27,1 (0,7) 30,1
Gold Au
Production Bath 1,5 1,8 (0,1) -
Gold Au
Production Bath 5,8 6,5 (0.2) -
(s) = Standard Deviation
All Fischer X-Ray systems can perform the plating solution analysis. However, the XDL is the most universal and most sold XRF instrument, its popularity being due to its usefulness and flexibility and subsequent cost effectiveness. Elements with atomic numbers from Z = 13 (Al) to Z = 92 (U) can be measured in a normal environment, without elaborate, expensive apparatus for generating a vacuum. Solids, pastes and even liquids can be measured, as this plating solution application clearly demonstrates. Measurements are fast, contact-free and can be made even on the smallest test areas be they on steps, in recesses or with irregular contours.
The capability of these measuring instruments are driven by the WinFTM control software, which means that specific user knowledge is not necessary and there is no requirement for sample preparation.
Energy dispersive X-ray fluorescence (EDXRF) is a versatile technology for quick, non-destructive qualitative and quantitative multi-element analysis and coating thickness measurement. Due to the large range of elements that can be detected, the range of applications for X-RAY instruments reaches from industrial applications to research and science. The Helmut Fischer Group has been developing and manufacturing coating thickness measurement, materials analysis and general materials testing technology for over 50 years.