GB/T 38933-2020Cold rolled steel sheet for automobile—Test method of phosphate conversion coatings (English PDF)

4.3 Sample pretreatment

4.3.1 The oil stains, rust and scale on the surface of the sample can be cleaned by one or several methods. Oil stains are usually cleaned by degreasing and electrolytic methods; rust and scale are usually cleaned by pickling or mechanical polishing.

4.3.2 The degreased sample shall be free of grease, emulsion and other dirt; its surface shall be completely wetted by water.

4.3.3 The pickled sample shall be free of visible oxide, rust and over-corrosion.

4.4 Sample phosphating treatment

4.4.1 The main construction method of phosphating treatment can be dipping, spraying or a combination of dipping and spraying methods.

4.4.2 For slightly oily or rusty samples, in general, use degreasing, rust removal, phosphating and passivation step-by-step treatment; in special cases, combine degreasing, rust removal, phosphating and passivation in one treatment.

4.4.3 Phosphating treatment can be carried out in iron-based, zinc-based, zinc-manganese-based, zinc-manganese-nickel-based phosphate solutions; the phosphating treatment process is negotiated and determined by the supplier and the buyer.

4.4.4 The preparation, adjustment and management of the phosphating bath solution shall be carried out in accordance with the corresponding process specifications.

4.5 Sample post-processing

4.5.1 The phosphating sample shall generally be washed with water. In order to improve the cleaning quality of the surface of the workpiece, it is best to use deionized water for final washing.

4.5.2 For the final water-washed sample, according to the subsequent coating needs, choose coating after drying or direct coating without drying.

4.5.3 In order to prevent the surface of the sample from being contaminated or rusted back, it shall be coated as soon as possible after drying. The interval between phosphating and coating is generally not more than 16 h. For special processing procedures and special samples, the interval can be appropriately

5) scratches.

5.2 Crystal size of phosphate coating

Use a scanning electron microscope (SEM) to take a secondary electron image of 1 000 times. Select three fields of view; select 10 measurement areas in each field of view; place a 10 µm ruler in the selected measurement area, to measure the number of grains in the interval; calculate the crystal size of this area. Take the average of all crystal sizes as the final result; the unit is micrometer (µm).

5.3 Phosphate coating weight

Determine the coating weight according to the method that is specified in GB/T 9792.

5.4 P ratio of phosphate coating

For zinc-based phosphate conversion coating, perform it on an X-ray diffractometer. The recommended analysis conditions are Co target, voltage 40 kV, and current 35 mA. Use an X-ray diffractometer to measure the intensity of the diffraction lines of the Hopeite (020) surface and the Phosphophyllite (100) surface; calculate the P ratio according to Formula (1):

Where: P0 -- P ratio, %;

P -- the diffraction line intensity (CPS) of Phosphophyllite (100) surface;

H -- the diffraction line intensity (CPS) of Hospeite (020) surface.

5.5 Phosphate coating corrosion resistance test

Immediately immerse the phosphating sample that has been lowered to room temperature in a 3% sodium chloride (analytical reagent) aqueous solution; keep it at 15°C ~ 25°C for 1 hour; take out the sample; wash and blow dry; visually inspect the phosphorus surface to see whether there is corrosion (except for edges, holes, corners and welds).

5.6 Phosphate coating + coating corrosion resistance test

5.6.2.2 Soak all the processed samples in a fully washed container that is filled with distilled water or deionized water (the conductivity is not higher than 20 µS/cm at 25 °C ± 2 °C); control the temperature to 40 °C ± 1 °C; after performing the immersion test for 240 h, perform the coating adhesion test; the test method is the same as 5.6.1.

5.6.3 Phosphate coating + coating wet heat resistance test

5.6.3.1 For the phosphate coating + coating samples that meet the treatments of 4.3, 4.4 and 4.5, use the "100 grid method" to treat the sample; the treatment method shall be the same as 5.6.1.

5.6.3.2 Put the processed sample in a wet heat test box that complies with GB/T 2361; conduct a wet heat test at 50°C and 95% relative humidity for 240 hours; then, perform the coating adhesion test. The test method is the same as 5.6.1.

5.6.4 Phosphate coating + coating saline soak test

For the phosphate coating + coating samples that meet the treatments of 4.3, 4.4 and 4.5, use materials of stable performance, such as paraffin wax, tape, to seal the edges of the sample; the edge width is greater than 10 mm from the circumference of the sample. According to the cutting tool that is recommended by GB/T 9286, cut the coating film into 12 cm cross diagonal line, so that the blade penetrates the coating and reaches the surface of the substrate. The diagonal line does not penetrate the diagonal corner, and the diagonal end is equidistant from the diagonal corner. See Figure 2. Take three samples; make the scratch side facing up. After 240 h of immersion test in 55 °C, 5% sodium chloride (analytical reagent) solution, measure and record the size of the film bubble at the "scratch" part of the coating and the size of the rust spread at the "scratch" part. Respectively record the maximum size of bubbling and rust spread as the final result; the unit is millimeter (mm). […]