BS EN ISO 3613:2021 pdf download – Metallic and other inorganic coatings — Chromate conversion coatings on zinc, cadmium, aluminium-zinc alloys and zinc- aluminium alloys — Test methods
The sample to be tested should have a surface area of (50 ± 5) cm 2 . For fasteners or samples with a smaller surface area, use a suitable number of samples to obtain the total required surface area. In cases where obtaining a total surface area of (50 ± 5) cm 2 is not possible, a reduced total sample surface area may be used by reducing the water extraction volume, while maintaining the same surface area to extraction volume ratio (1 cm 2 :1 ml).
A minimum surface area of 25 cm 2 is recommended. Similarly, a higher sample surface area can be used by keeping the same ratio. The type of adjustment shall be recorded in the final report. Remove any sealing, such as paint or wax, etc., by scraping the surface. The resulting abrasion shall also be analysed. Fill 50 ml of distilled water or water of equivalent purity with anti-bumping granules into a beaker with volume graduation, cover it with a watch glass and bring it to boiling temperature. Let the water boil for at least 10 min to deoxygenate it. Then totally immerse the sample into the boiling water and cover the beaker again with a watch glass. Extract the sample for (10 ± 0,5) min once boiling is resumed. If necessary, add water to ensure that the sample is totally submerged during the extraction. Remove the sample and allow the beaker content to cool to ambient temperature.
The solution should be colourless and free of precipitate. Adjust the volume to 50 ml. If necessary, filter the solution through a 0,45 µm membrane filter into a dry beaker. Make up the solution to the mark with water (see Clause 4) or reduce it to the mark by boiling. Add 1 ml of orthophosphoric acid (4.3.2) per 50 ml volume and mix well. Pour 25 ml of the solution using a graduated cylinder into another dry beaker. Add 1 ml of diphenylcarbazide solution (4.3.2) and mix well. Allow the solution to stand for 10 min for the colour reaction to be completed. The remaining portion of the extract will serve as the blank. If there is a colour interference (e.g. from a coating dye), a correction against the blank shall be performed. Transfer a portion of the sample solution to an absorption cell and measure the absorbance at 540 nm against the blank with the spectrophotometer (5.2).
Make three measurements and take the average as the final absorbance of the sample. In some cases, depending on the type of spectrophotometer used, the correction for the blank has to be made manually in the collected data. Measure the absorbance of the comparison solution for 0,10 µg/cm 2 Cr(VI) (4.3.2) three times as described above. Take the average of three measurements as the final absorbance of that comparison solution. Measure the absorbance of the comparison solution for 0,13 µg/cm 2 Cr(VI) (4.3.2) three times as described above. Take the average of three measurements as the final absorbance of that comparison solution.
In cases where the sample colour after the colorimetric reaction is significantly more intense than the colour of the comparison solution for 0,13 µg/cm 2 Cr(VI) (4.3.2) such that the result can be clearly determined as positive for Cr(VI), measurement with the spectrophotometer is not necessary. However, the report shall state that the result is significantly above 0,13 µg/cm 2 Cr(VI) based on visual observation and no colorimetric measurement was performed. A colorimetric measurement shall always be performed if the Cr(VI) is not clearly above 0,13 µg/cm 2 .
6.5.5 Calibration of spectrophotometer
The spectrophotometer shall be calibrated using a blank and three standard solutions at a minimum. The standard solution concentrations shall bracket 0,10 µg/ml Cr(VI) and 0,13 µg/ml Cr(VI). Zero the spectrophotometer with the 0,0 µg/ml blank standard and save this solution to re-zero the instrument before reading samples and standards. Read the standard solutions. Construct a calibration curve and determine a line equation by plotting absorbance values against µg/ml of Cr(VI) for each standard including the 0,0 µg/ml standard. The calibration curve (linear fit with zero intercept) shall have a correlation coefficient ≥0,995 or a new curve shall be built.