Update on Quality Control of Electro Polishing Acid

Electro-polishing has become one of the leading surface preparation processes to reach high gradients in superconducting cavities. For industrialization and for reproducible results in the electro-polishing process a study on acid quality control and quality management was launched under the scope of the European investigations under the CARE. First results of this acid management studies were presented at the TTC meeting 2006 and the JRI-SRF summary reports 2008 A second study with experimental results on the quality control methods proposed was launched where the different methods and instrumentations for acid quality control are studied in detail. We present the results of this following up investigations made on acid quality control for the electro-polishing mixture applied at DESY. INTRODUCTION Since start up of electro polishing (EP) at DESY and during the start up of industrial electro polishing [1] it is found that variations of the acid components result in variation of the polishing process. The consumption (Aging) or the evaporation of the hydro fluidic component during the mixing process is origin of variation of the mixture [2] Acid mixture variations of about 5 % of the basic mixture [3] prescription result in process variations of material removal per time unit (μm/min) and hence in process duration. For industrial application this variation and the need of an associated subsequent cavity treatment is not acceptable. To set up an acid quality management for the EP acid, an industrial study was started. Basing on this analysis of analytical methods [4] a second part of the study is launched, where the applicability of the methods is studied. In order to evaluate the potential of each analysis method a calibration acid mixture as well as defined variations of this mixture and resetting sample from industrial production are in use [Table 1]. In a fist step the reproducibility of the methods is studied on the reference mixture (Sample 1 to 3). To investigate the sensitivity of the methods, they are applied on composition with well defined variations in the amount of Hydro Fluoride (HF) and variations in the mixing process where the temperature of the acid during mixing is kept stable in different regimes (sample 1to 10) Table 1: specification of the acid mixtures in use for evaluation of analysis methods Sample Nr. Notes min. temp. max. temp. remarks 1 Cold 14 21 2 Cold 12 19 3 Cold 12 20 4 -10% HF 11 19 5 -20% HF 11 18 6 +10% HF 10 18 7 +20% HF 9 18 8 Warm 20 45 fume evap. 9 Warm 21 47 fume evap. 10 Warm 20 48 fume evap. 11 Used (main EP) 12 Used (main EP) 13 Used (main EP) 14 Used + silicon oil (main EP) 15 Used + silicon oil (main EP) 16 Used + silicon oil (main EP) 17 Warm 19 46 fume evap. 18 Warm 18 47 fume evap. 19 Used (2main ep) 20 Cold + polish 9 19 21 Cold + sulfur 8 18 ICP-OES AND TOC MEASUREMENTS Inductively coupled plasma optical emission spectrometry (ICP-OES), is a commonly in use analytical technique used for the detection of trace metals. It is a type of emission spectroscopy that makes use of the inductively coupled plasma to produce excited atoms and ions that emit electromagnetic radiation at wavelengths characteristic of a particular element. Total organic carbon (TOC) is the amount of carbon bound in an organic compound which can be oxidized and measured. Measurements on Different Acid Sample Mixtures For all sample mixtures the amount of metallic impurities and the TOC value are determined. It is found, that the metallic contaminations of the mixtures which are made form pure basic components (pro analysis) do not vary (Table 2). For used acids which have been in contact with softeners imbedded in PVC tubes or contaminated with silicon oil, increased TOC value are measured. Proceedings of SRF2009, Berlin, Germany THPPO081 09 Cavity preparation and production 811 Evaluation of the Methods ICP-OES is a suitable analysis method to analyze impurities in very small quantities (ppm). Several, mostly metallic elements can be analyzed in the same run. One major draw back is that the chemical compounds can’t be analyzed directly, they will be cracked into elements by the plasma (4000-5000 K). A high dilution is necessary and can only be done in well equipped laboratories. TOC is suitable to investigate organic impurities like softener from material in use for pipes of the installations or organic oils. A high dilution of the acid is necessary and the measurements can only verify that there are oxidizable substances. This method does not allow determining the kind of impurities. Both analysis methods are limited in applicability on the quality control of the EP acid, but allow as statically analysis of the basic components in use and the purity of the process. FTIR-ATR (FOURIER TRANSFORM INFRARED SPECTROSCOPY ATTENUATED TOTAL REFLECTANCE) Attenuated total reflectance (ATR) is a sampling technique used in conjunction with infrared spectroscopy which enables samples to be examined directly in the solid or liquid state without further preparation ATR uses a property of total internal reflection called the evanescent wave. A beam of infrared light is passed through the ATR crystal in such a way that it reflects at least once off the internal surface in contact with the sample. This reflection forms the evanescent wave which extends into the sample, typically by a few micrometers. This beam is collected by a detector as it exits the crystal. This evanescent effect works best if the crystal is made of an optical material with a higher refractive index than the sample being studied. Allocation of the Chemicals In order to assign the peaks to the chemical compound, a calibration of the wave length is made by pure and well knows chemical compounds (Fig.1). A problem introduces by adding components is shifting the peaks by changing the acid composition. Figure 1: Measurement to allocate the chemicals. Reproducibility of the FTIR-ATR Method For the FTIR-TAR probe in use the peak height reproducibility in the absorption unit (absu) range from 1,42 to 1,50 (absu) of ± 0,012 (<1%) is found for the three samples of identical composition (Fig.2). For sample 1 the HF peak is determined at 1,5 absu. An increase of HF by 10 percent shifts this peak to 1,483 absu, the increase of 10 % of HF show that peak 1,530 absu. Figure 2: Reproducibility measurement for three different samples. Bottom 1/ 4.25 magnification. Measurements of the Sample Mixtures Only in the case of a deviation of the hydrofluoric acid concentration of more than 10% reproducible variation in signal height is found (Fig.3). A distinction between cold and warm mixed acid was not possible within the sensitivity range of the probe. Evaluation of the Method The advantage of this analysis method is the easy handling and the fast response of data in undiluted Table 2: Results of the ICP-OES and TOC Measurement F:\Desy\Messungen\Henkel Probe 1.0 Henkel Probe 1 MATRIX-MF, IN350-T F:\Desy\Messungen\Henkel Probe 2.0 Henkel Probe 2 MATRIX-MF, IN350-T F:\Desy\Messungen\Henkel Probe 3.0 Henkel Probe 3 MATRIX-MF, IN350-T 26/01/2009