QC/T 942-2021Test methods of hexavalent chromium in automotive materials (English PDF)

3.1 Homogeneous material

The material of part or assembly, that cannot be further disassembled by mechanical means (e.g., unscrewing, cutting, rolling, scraping, grinding, etc.) AND whose parts have the same composition.

3.2 Equivalent comparison standard solution

A standard solution, that can be replaced by an equivalent value, according to the requirements of the standard, AND does not affect the detection and analysis results.

3.3 Part

Parts in the initial state, that have not been subject to sample preparation AND cannot be tested.

3.4 Sample

Parts that have been subject to the processing of the sample preparation and can be used for testing, OR parts that can be directly tested, without sample preparation.

4.1 Principle

After the part is prepared into a sample, place the sample in the sample chamber of the X-ray fluorescence spectrometer. Carry out the X-ray fluorescence spectrum analysis, on the content of hexavalent chromium (calculated as total chromium) in the sample, according to the selected analysis mode. Whether the content of hexavalent chromium (calculated as total chromium) in the sample is qualified, AND whether quantitative detection of hexavalent chromium is required, are judged based on the screening limit values.

This method is suitable for screening and rapid determination of the content of hexavalent chromium (calculated as total chromium), in automotive materials, by X- ray fluorescence spectrometer. However, it is not applicable to the determination of hexavalent chromium content (calculated as total chromium), in metal anti-corrosion plating.

4.2 Reagents and materials

4.2.1 Unless otherwise stated, only reagents confirmed to be analytically pure and distilled or deionized water or water of equivalent purity, are used in the analysis.

4.2.2 Boric acid. Excellent grade pure, which is dried at 105 °C for 1 h, then stored in a desiccator.

4.2.3 Anhydrous lithium tetraborate. Excellent grade pure, which is calcined at 700 °C for 4 hours, then stored in a desiccator.

4.2.4 Liquid nitrogen. Industrial grade.

4.2.5 Standard material containing chromium element.

4.2.6 Polyester film.

4.3 Instruments and equipment

4.3.1 X-ray fluorescence spectrometer.

4.3.2 Cutting machine.

4.3.3 Liquid nitrogen cryogenic pulverizer.

4.3.4 Grinding machine. Abrasives with tungsten carbide (WC).

4.3.5 Tablet press. The working pressure is not less than 20 MPa.

4.3.6 Fusion machine. The working temperature is not lower than 1150 °C.

4.4 Samples

4.4.1 Principles of sample preparation

4.4.1.1 The sample, which is used for analysis, must cover the measurement window of the spectrometer.

4.4.1.2 The irradiated surface of the sample shall be able to represent the whole sample.

4.4.1.3 Care shall be taken, to prevent contamination during sample preparation.

4.4.2 Sample preparation method

4.4.2.1 Solid parts

Homogeneous material part, which has flat surfaces and sizes suitable for X-ray fluorescence spectrometer requirements, is subject to direct analysis. Small parts, which have flat surfaces, can be pooled together for analysis. For thin parts, that can be stacked together to be thick enough (at least 5 mm thick and maintain thickness consistency and compositional uniformity) for analysis. In order to spread the sample flat, during analysis, a lining material can be added as a support. It shall select the lining material, which has a low background, as much as possible.

For various blocks, plates, other indeterminate parts, parts that are too large, cutting machines and grinders, etc. can also be used to process them to appropriate sizes. For small parts, which have uneven surfaces (such as plastic particles, etc.), they can be frozen by liquid nitrogen, pulverized mechanically, then pressed by a tablet press, to prepare analytical samples. For fragile parts, such as glass and ceramics, they are first crushed into small pieces; then ground into powders of less than 200 mesh. After mixing evenly, use boric acid (see 4.2.2) as base, to press it into sample, which has a thickness of not less than 1 mm. OR use anhydrous lithium tetraborate (see 4.2.3), to prepare glass melt, for analysis of samples. For heterogeneous parts, which are composed of heterogeneous materials AND do not require or are difficult to further mechanically disassemble, cut the parts into pieces; freeze them in liquid nitrogen; use a grinder to grind the broken samples into powders with a particle size of not more than 1 mm. After mixing the sample, use boric acid (see 4.2.2) as base to press it, to prepare the sample, which has a thickness of not less than 1 mm. OR use anhydrous lithium tetraborate (see 4.2.3) to make a glass melt, to analyze the sample.

5.1 Principle

Under acidic conditions, the hexavalent chromium in the metal anti-corrosion plating reacts with diphenylcarbazide. The diphenylcarbazide is oxidized to diphenylcarbazone, whilst the hexavalent chromium is reduced to trivalent chromium. The trivalent chromium further reacts with diphenylcarbazone, to form a red or purple complex, which can be used to qualitatively judge whether there is hexavalent chromium in the metal anti-corrosion plating.

This method is suitable for qualitative determination of hexavalent chromium in automotive metal anti-corrosion plating.

5.2 Reagents and materials

5.2.1 Unless otherwise stated, only reagents confirmed to be analytically pure and distilled or deionized water or water of equivalent purity are used, in the analysis.

5.2.3 Potassium dichromate.

5.2.6 Orthophosphoric acid solution (mass fraction 75%).

5.2.7 Hexavalent chromium stock solution. Weigh 0.1414 g of potassium dichromate (see 5.2.3), which was dried to constant weight. Dissolve it in water. Transfer it into a 100 mL volumetric flask. Dilute to the mark. The hexavalent chromium content of this solution is 500 µg/mL. Cap the container tightly. This solution is valid for 12 months. Or use the national standard solution.

5.2.2 Diphenylcarbazide.

5.2.5 Ethanol.

6.1 Principle

The hexavalent chromium in the metal anti-corrosion plating is extracted, by boiling water extraction. Under acidic conditions, the hexavalent chromium reacts with diphenylcarbazide; the diphenylcarbazide is oxidized to diphenylcarbazone, whilst the hexavalent chromium is reduced to trivalent chromium; the trivalent chromium and diphenylcarbazone further react, to form a red or purple complex, which can be quantitatively determined by a colorimeter at 540 nm; then the content of hexavalent chromium in the metal plating is quantitatively determined.

This method is suitable for quantitative determination of hexavalent chromium content, in automotive metal anti-corrosion plating, including no additional covering layers (e.g., oil films, water-based or solvent-based polymers or wax films).

Note. For the new off-line metal anti-corrosion plating part, it shall be sealed for at least 5 days, to make the plating stable, before testing.

6.2 Reagents and materials

6.2.1 Unless otherwise stated, only reagents confirmed to be analytically pure and distilled or deionized water or water of equivalent purity are used, in the analysis.

6.2.3 Potassium dichromate.

6.2.5 Orthophosphoric acid solution (mass fraction 75%).

6.2.6 Hexavalent chromium stock solution. Weigh 0.113 g of potassium dichromate (see 6.2.3, dry at 100 °C for 1 h before use) in a glass vessel. Add water to make the volume reach to 1000 mL. The hexavalent chromium concentration, in this solution, is 40 µg/mL. Cap the container tightly. This solution is valid for 12 months. OR use national standard solution.

6.2.7 Equivalent standard solution of hexavalent chromium. Pipette 3.3 mL of hexavalent chromium stock solution (see 6.2.6), into a volumetric flask. Dilute it to 1000 mL. The concentration of this equivalent standard solution of hexavalent chromium is 0.13 µg/mL, which is equivalent to 0.13 µg/cm2. Cap the container tightly.

6.2.8 Color developing solution B. Weigh 0.5 g of diphenylcarbazide (see 6.2.2).

6.2.2 Diphenylcarbazide.

6.2.4 Acetone.

This solution is valid for 24 hours. OR use the national standard solution.

Dissolve it in 50 mL of acetone (see 6.2.4). Use 50 mL of water to dilute it, under stirring (quick mixing may produce diphenylcarbazide precipitates). This solution shall be refrigerated at 7 °C ± 2 °C in brown glass bottles. It shall not be used, if discolored.

7.1 Principle

After dissolving or swelling samples of various matrices, by organic solvents, use an alkaline digestion solution, to extract the hexavalent chromium in the samples. Then under acidic conditions, diphenylcarbazide reacts with hexavalent chromium; diphenylcarbazide is oxidized to diphenylcarbazone; hexavalent chromium is reduced to trivalent chromium. Trivalent chromium and diphenylcarbazone further react, to form a red or purple complex, which can be quantitatively determined, by a colorimeter, at 540 nm.

This method is suitable for the determination of hexavalent chromium content, in automotive polymer materials and electronic materials. The content of hexavalent chromium in other automotive materials, such as glass, ceramics, fabrics, paints, is determined with reference to this method.

7.2 Reagents and materials

7.2.1 Unless otherwise stated, only reagents confirmed to be analytically pure and distilled or deionized water or water of equivalent purity are used, in the analysis.

7.2.2 N-Methylpyrrolidone (NMP). Add 10 g of active molecular sieve, to each 100 mL of newly opened NMP. Seal it. Store it in a brown bottle. Store it in the dark, at 20 °C ~ 25 °C. Shake it occasionally. It shall be stored for more than 12 hours before first use. The validity period after opening is 28 days.

7.2.3 Nitric acid (35% by volume fraction). Store it in the dark at 20 °C ~ 25 °C. Do not use yellow concentrated nitric acid when preparing it, because the contained NO3- will decompose into NO2- in light, leading to the reduction of hexavalent chromium.

7.2.6 Anhydrous magnesium chloride. 200 mg of MgCl2 is equivalent to 50 mg of Mg2+.

7.2.7 Phosphate buffer solution. Prepare pH = 7 buffer. Use 700 mL of water, to dissolve 87.09 g K2HPO4 and 68.04 g KH2PO4. Transfer it into a 1 L volumetric flask. Dilute it to the mark. The prepared solution contains K2HPO4 of 0.5 mol/L and KH2PO4 of 0.5 mol/L.

7.2.8 Lead chromate. Excellent grade pure, which is sealed and stored at 20 °C ~ 25 °C.

7.2.9 Digestion solution. Use water to dissolve 20.0 g ± 0.05 g of sodium hydroxide and 30.0 g ± 0.05 g of anhydrous sodium carbonate. Place it in a 1 L volumetric flask.

7.2.11 Potassium dichromate stock solution. Dissolve 0.1414 g of K2Cr2O7, which was dried at 105 °C, in water. Dilute it into a 1 L volumetric flask. The hexavalent chromium content of this solution is 50 µg/mL. OR use the national standard solution.

7.2.12 Potassium dichromate standard solution. Use water to dilute 10 mL of potassium dichromate stock solution (see 7.2.11), in a 100 mL volumetric flask. The hexavalent chromium content of this solution is 5 µg/mL. OR use the national standard solution.

7.2.13 Sulfuric acid (10% by volume fraction). Use water to dilute 10 mL of sulfuric acid, in a 100 mL volumetric flask.

7.2.14 Color developing solution C. Weigh 0.25 g of diphenylcarbazide (see 7.2.4).

7.2.15 1000 µg/mL hexavalent chromium standard solution. Dissolve 2.829 g of K2Cr2O7, which was dried at 105 °C, in water. Dilute it into a 1 L volumetric flask. Seal and store it under the conditions of 20 ° C ~ 25 ° C, which is valid for 6 months. OR use the national standard solution.

7.2.16 100 µg/mL hexavalent chromium substrate standard solution. Take 10 mL of 1000 µg/mL hexavalent chromium standard solution (see 7.2.15), in a 100 mL volumetric flask. Use water to dilute it to the mark.

7.2.17 Molecular sieve (4A). CAS. 70955-01-0, desiccant.

7.2.5 Acetone.

Dilute it to the mark. Seal it in polyethylene bottles at 20 °C ~ 25 °C, which is valid for 30 days. Check the pH value of the digestion solution, before use. If the pH value is less than 11.5, it requires preparing the digestion solution again.

7.2.10 Toluene.

Dissolve it in 50 mL of acetone (see 7.2.5). This solution shall be stored in a brown glass bottle, which is valid for 14 days. It shall not be used, if discolored.

7.3 Instruments and equipment

7.3.1 All reusable containers (glass containers, quartz containers, polyethylene, polytetrafluoroethylene (PTFE), etc.), including sample containers, shall be immersed overnight in a laboratory-grade detergent solution. Use water to wash it. Then use HNO3 (the volume fraction is 20%) or mixed acid (volume ratio HNO3.HCl. H2O = 1.2.9), to soak it in 4 h. Use water to wash it. The cleaning effect can be proved by the method blank test, OR other cleaning methods can also be used.

7.3.2 Vacuum filtration device.

7.3.3 Heating or microwave digestion device. It can keep the digestion temperature at 150 °C ~ 160 °C.

7.3.4 Ultrasonic water bath device. The temperature can be maintained at 60 °C ~ 65 °C.

7.3.5 Calibrated pH meter. The accuracy is ±0.03; the pH measurement range is 0 ~ 14.

7.3.6 Analytical balance. The accuracy is 0.1 mg.

7.3.7 Thermometers, thermistor thermometers or other temperature testers. The measurement temperature range is greater than 160 °C.

8.1 Principle

The phosphate buffer solution, which has a pH value of 7.0 ~ 8.0, is used as the extraction solution, to extract hexavalent chromium in the leather sample. When necessary, the substance that interferes with the test can be removed by solid-phase extraction. The hexavalent chromium in the extract reacts with diphenylcarbazide, under acidic conditions; the diphenylcarbazide is oxidized to diphenylcarbazone, whilst hexavalent chromium is reduced to trivalent chromium. Trivalent chromium and diphenylcarbazone further react, to form a red or purple complex, which can be quantitatively determined by a colorimeter, at 540 nm.

This method is suitable for the determination of hexavalent chromium content, in automotive leather materials.

8.2 Reagents and materials

8.2.1 Unless otherwise stated, only reagents confirmed to be analytically pure and distilled or deionized water or water of equivalent purity are used, in the analysis.

8.2.4 Methanol. HPLC grade.

8.2.5 Phosphoric acid solution. Take 700 mL of phosphoric acid (rho = 1.71 g/mL). Dilute it to 1000 mL.

8.2.6 Phosphate buffer solution (extract). Dissolve 22.8 g of dipotassium hydrogen phosphate, in 1000 mL of water. Use phosphoric acid solution (see 8.2.5), to adjust the pH to 8.0 ± 0.1. Then use argon or nitrogen (see 8.2.10), to expel air, OR use ultrasonic bath. Extracts can be stored at 4 °C ± 3 °C, for 7 days; however, it shall be brought to room temperature and degassed, before use.

8.2.7 Color developing solution D. Take 1.0 g of diphenylcarbazide (see 8.2.2).

8.2.8 Hexavalent chromium stock solution. Dissolve 2.829 g of potassium dichromate in water. Use a volumetric flask, to dilute it to 1000 mL. The concentration of hexavalent chromium in this solution is 1000 µg/mL. OR use the national standard solution.

8.2.9 Hexavalent chromium standard solution. Pipette 0.5 mL of hexavalent chromium stock solution (see 8.2.8), into a 100 mL volumetric flask. Use phosphate buffer (see 8.2.6), to make its volume reach to the mark. The valence chromium's concentration of this solution is 5µg/mL. The solution is stored in a refrigerator at 4 °C ± 3 °C, which has a valid period of 7 days. However, it shall be brought to room temperature, before use.

8.2.3 Acetone.

Dissolve it in 100 mL of acetone (see 8.2.3). Add 1 drop of acetic acid to acidify it. This solution is refrigerated in a brown glass bottle and stored in the dark at 4 °C ± 3 °C. The validity period is 14 days.

9.1 Principle

Extractable hexavalent chromium is extracted from the sample, by phosphate buffer at pH 7.0 ~ 8.0. An aliquot of the filtrate extract is analyzed for hexavalent chromium, using ion exchange chromatography or high performance liquid chromatography.

This method is suitable for the determination of hexavalent chromium content, in automotive leather materials.

9.2 Reagents and materials

Prepare reagents and materials, as required in 8.2.

9.3 Instruments and equipment

Use the equipment required in 8.3. Meanwhile use an ion exchange chromatograph with a UV detector OR a high performance liquid chromatograph (HPLC) with an anion exchange column and a UV detector. A photodiode array detector (DAD) can also be used.