GB/T 38933-2020 English PDF (GBT38933-2020)

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GB/T 38933-2020: Cold rolled steel sheet for automobile--Test method of phosphate conversion coatings
GB/T 38933-2020
GB
NATIONAL STANDARD OF THE
PEOPLE’S REPUBLIC OF CHINA
ICS 77.140.50
H 46
Cold rolled steel sheet for automobile - Test method of
phosphate conversion coatings
ISSUED ON: JUNE 02, 2020
IMPLEMENTED ON: DECEMBER 01, 2020
Issued by: State Administration for Market Regulation;
Standardization Administration of the People’s Republic of
China.
Table of Contents
Foreword ... 3 
1 Scope ... 4 
2 Normative references ... 4 
3 Terms and definitions ... 4 
4 Sample processing ... 5 
5 Test method ... 7 
6 Test report ... 12 
Cold rolled steel sheet for automobile - Test method of
phosphate conversion coatings
1 Scope
This Standard specifies the terms and definitions, sample treatment, test
methods and test reports for phosphate conversion coatings (hereinafter
referred to as "phosphate coatings") of cold rolled steel sheet for automobile.
This Standard applies to the determination of iron-based, zinc-based, zinc-
manganese-based, and zinc-manganese-nickel-based phosphate conversion
coatings of cold rolled steel sheet for automobile before coating.
2 Normative references
The following documents are indispensable for the application of this document.
For dated references, only the dated version applies to this document. For
undated references, the latest edition (including all amendments) applies to this
document.
GB/T 1765, Method of producing of paint films for testing heat and humidity
resistance, salt-fog resistance and accelerated weathering
GB/T 1771, Paints and Varnishes - Determination of resistance to neutral salt
spray(fog)
GB/T 2361, Rust preventing oils and greases - Test method of wet heat
GB/T 9286, Paints and varnishes - Cross cut test for films
GB/T 9792, Conversion coatings on metallic materials - Determination of
coating mass per unit area - Gravimetric methods
GB/T 10125, Corrosion tests in artificial atmospheres - Salt spray tests
GB/T 20854, Corrosion of metals and alloys - Accelerated testing involving
cyclic exposure to salt mist, "dry” and "wet" condition
3 Terms and definitions
The following terms and definitions are applicable to this document.
4.2.3 For samples of special purpose, stress shall be eliminated before
phosphating treatment and hydrogen embrittlement shall be eliminated after
phosphating treatment.
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.
5.1.5 When there are no obvious defects or allowable defects in the appearance
of the phosphate coating, it can be used as the to-be-tested sample according
to the provisions of 5.2~5.6.
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 surf...