GB/T 4957-1985 Eddy current method for measuring thickness of non-conductive coatings on non-magnetic metal substrates GB/T4957-1985 standard download decompression password: www.bzxz.net

Non-conductive coatings on non-magnetic metal substrates-Measurement of coating thickness-Eddy current method UDC 621.793: 531.717.1: GB4957—B5 This standard specifies the method for non-destructive measurement of the thickness of non-conductive coatings on non-magnetic metal substrates using eddy current instrument. This method is suitable for measuring the thickness of most anodized coatings, but not for measuring all thin conversion coatings: This standard is equivalent to the international standard IS023601982 "Full-current measurement method for the thickness of non-conductive coatings on non-magnetic metal substrates" 1 Principle
The high-frequency electromagnetic field generated by the probe device of the eddy current thickness meter causes the conductor placed under the probe to generate eddy currents. Its amplitude and phase are functions of the thickness of the non-conductive coating between the conductor and the probe. 2 Factors affecting measurement accuracy
2.1 Coating thickness
This method is carried out with the maximum measurement error. For thin coatings, the error is a constant and has nothing to do with the coating thickness. The absolute error for a single measurement is at least 0.5 microns. When the coating thickness is greater than 25 microns, the error is approximately proportional to the coating thickness. When measuring the thickness of coatings of 5 microns or less, the half-mean of several readings should be taken. When measuring the thickness of coatings below 3 microns, the accuracy requirements specified in Chapter 5 may not be met. 2.2 Electrical properties of base metal
The conductivity of the base metal has an effect on the measurement, and the conductivity of the base metal is related to its material composition and heat treatment method. The influence of conductivity is closely related to the manufacture and type of the instrument. 2.3 Thickness of base metal
Each instrument has a critical thickness of base metal. Due to this thickness, the measurement is most affected by the thickness of the base metal. The critical thickness of the base metal is related to the measurement frequency of the probe device and the conductivity of the base metal. If the instrument manufacturer does not provide this critical thickness value, it should be determined through experiments.
In general, for a given measurement frequency, the higher the conductivity of the base metal, the smaller the critical thickness. When the conductivity of the base metal remains unchanged, the higher the measurement frequency, the smaller the critical thickness.
2.4 Edge effect
Eddy current source instrument is sensitive to abrupt changes in the surface shape of the sample. Flash ratio is unreliable when measuring at an inner corner near the edge of the sample unless the instrument is calibrated for this purpose.
The curvature of the sample has an effect on the measurement. The effect of curvature is related to the manufacture and shaping of the instrument. However, this effect always increases significantly with the decrease of the curvature diameter. Therefore, it is impossible to measure on the surface of the sample unless the instrument is specially calibrated for this. National Bureau of Standards 1985-02-14 Issued in
19A5-12-01 Practice
2.6 Surface Roughness
GB 4967-85
The surface roughness of the base metal and the coating has an effect on the measurement. The effect increases with the increase of the roughness. Rough surfaces will cause systematic errors and accidental errors. Each time the measurement is made, the number of measurements should be increased at different positions to overcome this accidental error. If the base metal is rough, it is also necessary to take several positions on the base metal sample with similar roughness without coating to calibrate the zero point of the instrument. After removing the coating with a solution that does not corrode the base metal, the zero point of the instrument should be calibrated. 2.7 Adherent substances
Since the eddy current instrument is sensitive to adhering substances that prevent the probe from making close contact with the coating surface, the instrument probe must be in direct contact with the test surface.
2.8 Probe pressure
The pressure applied to the specimen will affect the measured readings. Therefore, the pressure should be kept constant. 2.9 Probe placement
The placement of the probe has an impact on the measurement. During the measurement, the probe should be kept perpendicular to the specimen surface. 2.10 Deformation of the specimen
The probe can deform soft coating specimens or thin specimens. Therefore, it is difficult to measure reliable data on these specimens. In this case, special probes or auxiliary devices are required for measurement. 2.11 Temperature
Significant temperature changes have an impact on the characteristics of the probe. Therefore, calibration and measurement should be carried out under conditions of roughly similar temperatures. 3 Calibration of Instruments
3.1 Overview
Before measuring, each instrument should be calibrated according to the manufacturer's instructions, and appropriate calibration standards should be selected for calibration. Attention should be paid to the influencing factors listed in Chapter 2 and the operating procedures described in Chapter 4. 3.2 Calibration Standards www.bzxz.net
Foils of known thickness or specimens with known coating thickness can be used as calibration standards. 3.2.1 Calibration Foil
3.2.1.1 Calibration foils used for eddy current instrument calibration are usually made of suitable plastics. They are more suitable for calibration on curved surfaces than standards with coatings.
3.2.1.2 To prevent measurement errors, it is necessary to ensure that the calibration box is in close contact with the substrate, and the use of flexible foils should be avoided. Calibration foils are prone to indentation and should therefore be replaced frequently. 3.2.2 Standards with Coatings
Use non-conductive coatings of known thickness, uniformity, and firm bonding to the substrate as standards. 3.3 Verification
3.8.1 The electrical properties of the base metal of the calibration standard shall be similar to those of the base metal of the specimen. To verify the suitability of the standard, the readings measured on the base metal of the calibration standard may be compared with the readings measured on the base metal of the specimen. 3.3.2 If the base metal of the specimen and calibration plate does not exceed the critical thickness specified in 2.3, the following two methods may be used for verification: calibration on a metal standard of the same thickness as the base metal of the specimen, or calibration of the standard or specimen with a sufficiently thick metal backing of similar electrical properties, but without any gap between the base metal and the backing metal. The backing potential method cannot be used for specimens with coatings on both sides.
3.3.3 If the curvature of the coating to be measured is so great that it cannot be calibrated on the surface, the curvature of the coating standard or the curvature of the other metal under calibration shall be the same as the curvature of the specimen. 4 Operating Procedures
4. Overview
Each instrument shall be operated in accordance with the manufacturer's instructions and the influencing factors listed in Section 2 shall be noted. GB4967-65
Before each measurement, the instrument shall be calibrated at the test site. During intermittent use, the instrument shall also be calibrated frequently (at least once every hour) to ensure that the instrument is in normal working condition. The provisions of the following ten clauses shall be observed during operation. 4.2 Base Metal Thickness
Check whether the base metal thickness exceeds the critical thickness. If not, use one of the two methods in 3.3.2. .3 Edge Effects
Measurements shall not be made at sudden changes in the specimen, such as edges, holes and inner corners, unless it has been verified that the calibration for this measurement is reliable.
4. Curvature
Measurements shall not be made on the curved surface of the specimen unless it has been verified that the calibration for this measurement is reliable. .5 Number of readings
Usually, several readings must be taken within a given area because the readings of the instrument are not exactly the same each time. Local differences in coating thickness also require multiple measurements within any given area, especially when the surface is rough. 4.6 Surface cleanliness
Before measurement, any adhering materials on the surface, such as dust, grease and corrosion products, should be removed, but any covering materials should not be removed.
5 Accuracy requirements
The measurement accuracy depends on the performance, operation and calibration of the instrument. The measurement error must be within ±10%. When measuring the coating thickness of 5 microns or less, the average of several readings should be taken. When the coating thickness is less than 3 microns, this accuracy requirement may not be met. Additional remarks:
This standard was proposed by the Ministry of Machinery Industry of the People's Republic of China and is under the jurisdiction of the Wuhan Institute of Materials Protection of the Ministry of Machinery Industry. This standard was drafted by the Wuhan Institute of Materials Protection of the Ministry of Machinery Industry. The main drafters of this standard are Hu Tieqi and Wang Yu.
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