1、Designation: A 754/A 754M 08Standard Test Method forCoating Weight (Mass) of Metallic Coatings on Steel byX-Ray Fluorescence1This standard is issued under the fixed designationA 754/A 754M; the number immediately following the designation indicates the yearof original adoption or, in the case of rev
2、ision, the year of last revision. A number in parentheses indicates the year of last reapproval.A superscript epsilon () indicates an editorial change since the last revision or reapproval.1. Scope*1.1 This test method covers the use of X-ray fluorescence(XRF) for determining the coating weight (mas
3、s) of metalliccoatings on steel sheet. The test method is intended to be usedfor “on-line” measurements of coating on continuous produc-tion lines.1.2 This test method is applicable to the coatings covered bythe following ASTM specifications: A 599/A 599M, A 623,A 623M, A 653/A 653M, A 792/A 792M, A
4、 875/A 875M,A 879/A 879M, A 918, and A 924/A 924M. It may be appli-cable to other coatings, providing that the elemental nature ofthe coating and substrate are compatible with the technicalaspects of XRF such as the absorption coefficient of thesystem, primary radiation, fluorescent radiation, type
5、of detec-tion.1.3 This test method includes the procedure for developinga single standard determination of coating weight (mass).1.4 This test method includes procedures for both X-raytube and isotope coating weight (mass) measuring instruments.1.5 The values stated in either inch-pound units or SI
6、unitsare to be regarded separately as standard. Within the text, theSI units are shown in brackets. The values stated in eachsystem are not exact equivalents; therefore, each system shallbe used independently of the other. Combining values from thetwo systems may result in nonconformance with the sp
7、ecifi-cation.1.6 This standard does not purport to address all of thesafety concerns, if any, associated with its use. It is theresponsibility of the user of this standard to establish appro-priate safety and health practices and determine the applica-bility of regulatory limitations prior to use.2.
8、 Referenced Documents2.1 ASTM Standards:2A 599/A 599M Specification for Tin Mill Products, Electro-lytic Tin-Coated, Cold-Rolled SheetA 623 Specification for Tin Mill Products, General Require-mentsA 623M Specification for Tin Mill Products, General Re-quirements MetricA 653/A 653M Specification for
9、 Steel Sheet, Zinc-Coated(Galvanized) or Zinc-Iron Alloy-Coated (Galvannealed)by the Hot-Dip ProcessA 792/A 792M Specification for Steel Sheet, 55 %Aluminum-Zinc Alloy-Coated by the Hot-Dip ProcessA 875/A 875M Specification for Steel Sheet, Zinc-5 %Aluminum Alloy-Coated by the Hot-Dip ProcessA 879/A
10、 879M Specification for Steel Sheet, Zinc Coatedby the Electrolytic Process for Applications RequiringDesignation of the Coating Mass on Each SurfaceA 902 Terminology Relating to Metallic Coated Steel Prod-uctsA 918 Specification for Steel Sheet, Zinc-Nickel AlloyCoated by the Electrolytic Process f
11、or Applications Re-quiring Designation of the Coating Mass on Each SurfaceA 924/A 924M Specification for General Requirements forSteel Sheet, Metallic-Coated by the Hot-Dip Process3. Terminology3.1 DefinitionsFor general definitions of terms relating tometallic-coated steel products, see Terminology
12、 A 902.3.2 Definitions of Terms Specific to This Standard:3.2.1 averaging time, nthe period over which an elec-tronic measuring instrument acquires samples or “counts” priorto each update of coating weight (mass) output; refer to X1.2for a more detailed explanation.1This test method is under the jur
13、isdiction of ASTM Committee A05 onMetallic-Coated Iron and Steel Products and is the direct responsibility ofSubcommittee A05.07 on Methods of Testing.Current edition approved Nov. 1, 2008. Published November 2008. Originallyapproved in 1979. Last previous edition approved in 2006 asA 754/A 754M 06.
14、2For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at serviceastm.org. For Annual Book of ASTMStandards volume information, refer to the standards Document Summary page onthe ASTM website.1*A Summary of Changes section appears at the end of this sta
15、ndard.Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.3.2.2 response time, nthe time required for a coatingweight (mass) gauge to detect 90 % of a 10 % step change incoating weight (mass).3.2.3 sample, nthe area of moving sheet that m
16、ust bemeasured under standardized conditions to develop a singledetermination of coating weight (mass).3.2.4 standards, nthe physical standards, either externalor internal, that are used to calibrate the measuring instrument.3.2.5 substrate, nthe steel sheet upon which the metalliccoating is applied
17、.3.2.6 time constant, nan electronic filtering term, uniqueto the design of each type of measuring instrument, that definesthe time taken to respond to a step change in coating thickness;refer to X1.3 for a more detailed explanation.3.2.7 X-ray fluorescence, nthe X-rays emitted by an atomwhen excite
18、d to a higher energy state.4. Basic Principle4.1 The measurement of coating thickness by XRF methodsis based on the combined interaction of the coating andsubstrate, with an intense beam of primary radiation from anX-ray or isotope source. This interaction results in the genera-tion of X-rays of wel
19、l-defined energy. These fluorescent X-raysare detected by a radiation detector that can discriminatebetween selected energy levels in the secondary beam.4.1.1 The radiation detector can discriminate between spe-cific fluorescent X-rays because the X-rays generated by theinteraction between the prima
20、ry beam and the surface beingfluoresced have energy levels that are unique to each elementin the targeted material. Each element fluoresces at an energythat is characteristic of that element alone. Thus the fluorescedradiation can be detected separately for either the elements ina coating or the sub
21、strate material.4.1.2 The detection system includes the radiation detector inconjunction with suitable electronic discriminating circuitry.4.1.3 The thickness of a coating can be determined becausea quantitative relationship exists between the intensity of thesecondary radiation captured by the dete
22、ctor and the thicknessof the coating material. The thickness of a sample can beestablished by comparing the measured intensity and that of aseries of standards.4.1.4 The coating weight (mass) can be calculated from themeasured coating thickness for a specific coating type. Inpractice, the electronic
23、s are established to report the coatingweight (mass) in commonly used units such as oz/ft2g/m2.4.2 Measurement Techniques:4.2.1 Two measurement techniques are used. The firsttechnique involves direct measurement of the intensity of thefluorescent X-rays emitted by the coating itself. With thismethod
24、, the coating weight (mass) is correlated with theintensity of the fluorescent X-rays emitted by the coating.4.2.2 The second technique involves the measurement ofthe attenuation of the fluorescent X-rays emitted by thesubstrate as they pass through the coating whose weight (mass)is being determined
25、. The correlation in this case is based on theprinciple that the intensity of the X-rays from the fluorescedsubstrate is a function of the weight (mass) of the coating fora specific coating type.4.2.3 Appendix X2 and Appendix X3 contain a moredetailed discussion of these two methods of measuring coa
26、tingweight (mass).5. Factors Affecting Accuracy5.1 The equipment used to make a coating weight (mass)measurement using XRF typically consists of a radiationsource, a detector, and an electronic system to process thedetected signal. The sample absorbs radiation from the sourceand produces fluorescent
27、 radiation. The detector detects thisradiation, and the electronic system converts it into coatingweight (mass) information. Since an X-ray measurement isbasically an accumulation of random events, the accumulationtime must be long enough to produce statistically acceptabledata. The precision of a c
28、oating weight (mass) measurement isdetermined by the equipment and the data collection time.Without a good calibration curve, however, highly preciseequipment cannot produce an accurate result. For example, avery thick coating may produce a very precise X-ray fluores-cent signal, but it may be outsi
29、de the range of the equipment.Therefore, the measurement accuracy depends on the equip-ment, data collection time, and calibration of the instrument.The environment may also influence the measurement accu-racy. Since equipment and coating each have unique charac-teristics, equipment specifications s
30、hould be reviewed carefullyprior to purchase and installation.5.2 In order to measure coating weight (mass) accurately,the source must have enough strength to produce fluorescentradiation from the entire sample volume of interest. Thesample volume of interest varies, depending on the XRFmethod used.
31、 When the coating weight (mass) is measuredusing fluorescence from the coating, the sample volume is theentire layer of the coating. When fluorescence from thesubstrate is used, the sample volume of interest is the lesser ofthe entire substrate or 5/ ( is the absorption coefficient of thesubstrate f
32、or the primary beam energy) thickness of thesubstrate under the coating. The radiated spot size must belarge enough to cover a sample area as described in theprocedure (refer to Table 1). The range of coating weight(mass) for which the measuring instrument can be useddepends on the strength of the s
33、ource and the coating compo-sition. If a coating is thicker than 5/ ( of the coating for thefluorescent beam energy), XRF produced underneath the 5/TABLE 1 Control Variables to Define a Single Data Point (SingleSpot)Variable ValueAType of Gauge X-ray Tube IsotopeArea of fluorescence 1.5 to 5 in.2970
34、 to 3200 mm25to14in.23200 to 9000 mm2Traverse scan speed(traverse mode)1 in./s 25 mm/s min 1 in./s 25 mm/s minDwell time (dwell mode) 4 s max 4 s maxTime constant (X-ray) oraveraging time (isotope)min 2.5 in. 65 mm travelto allow 3 or more “timeconstants” to elapse1to4sABoth X-ray tube and isotope c
35、oating weight gauges, when used for determin-ing conformance to coating weight (mass) specifications, have diminished accu-racy above 1.3 oz/ft2/side 400 g/m2/side for zinc coatings, 0.066 oz/ft2/side 20g/m2/side for tin coatings, and 0.82 oz/ft2/side 250 g/m2/side for aluminumcoatings.A 754/A 754M
36、082thickness cannot emerge from the coating due to absorption. Acoating thickness of 5/ is defined as the critical thickness. If acoating is very thin, there may not be enough signal from thecoating.5.3 The detector must be able to discriminate betweensignals originating from the coating and the sub
37、strate. Whenthe sample contains elements having similar atomic mass orsimilar X-ray characteristics, detected signals are difficult todiscriminate and the measurement accuracy is affected ad-versely. The measurement accuracy may also be affectedadversely when fluorescence from one element influences
38、fluorescence from another. Equipment capable of measuringXRF from several elements simultaneously, including compen-sating for variations in coating composition, is required whenthe coating composition is unknown (for example, % Zn inZn-Al or Zn-Ni coating), or the coating contains elements thatare
39、present in the substrate (for example, Zn-Fe coating on Fe),or the coating consists of multiple layers of metal or alloy.5.4 The required data collection time is determined by thestrength of the source, sensitivity of the detector, and coatingweight (mass). A stronger source and a more sensitive det
40、ectortypically require a shorter data collection time. The datacollection time shall be long enough to achieve the requiredprecision. For example, if N is the number of counts detectedby a counter in a given time interval, the inherent error inradiation detection is equal to =N. As a guideline, the
41、datacollection time should be long enough to record 10 000 countsfor a desired precision of 61%.5.5 The calibration of the equipment has a very significantimpact on the accuracy of the measurement. The coatingcomposition of the material to be measured must be similar tothat of the calibration standa
42、rd. If the substrate has anyinfluence on the X-ray signals, then both substrates must besimilar. Significant differences in surface roughness and coat-ing component segregation may also affect the accuracy of themeasurement adversely. The coating weight (mass) range ofthe standards must exceed that
43、of the material to be measuredand must be within the useful range of the equipment.5.6 Additional precautions are necessary for measurementsmade on-line or in a mill environment.5.6.1 CleanlinessThe measurement instrument windowmust be kept clean to avoid any interference with the X-raysignal. A fil
44、m of mill dust containing metal powder is normallymore deleterious than that of oil and moisture.5.6.2 StabilityThe equipment should be maintained at asteady temperature to avoid any instability due to temperature.The influence of variations in air temperature in the gapbetween the instrument and th
45、e material on X-ray measure-ments must be compensated. The gap between the instrumentand the sheet must be uniform and within the specifications ofthe equipment. Excessive variations in coating weight (mass)readings may be the result of variability in the strip pass-linedue to such conditions as str
46、ip off-flatness (for example, wavyedges).5.6.3 Averaging TimeDuring an on-line measurement, theequipment must be operated using an averaging time suitablefor detecting variations in the coating weight (mass) withoutaffecting measurement accuracy adversely. A very long aver-aging time will mask varia
47、tions in the coating, resulting in amisleading indication of average coating weight (mass).Averyshort averaging time will yield unreliable results. (Refer toTable 1 for acceptable combinations.)6. Calibration6.1 GeneralWhen taking instrument readings for thepurpose of establishing an instrument cali
48、bration, exactly thesame instrumental conditions should be used as those that willbe used on material being measured. The measuring time forcalibration standards may be longer than that on material beingmeasured in order to reduce the effect of statistical fluctuations.6.2 StandardsReliable standard
49、s must be used in thecalibration of any type of X-ray equipment if accurate resultsare to be obtained. It should be understood that prolongedcounting periods will not compensate for unreliable standards.Calibration standards that are certified for weight (mass) perunit area are reliable for coatings that have the same compo-sition. The same density is not necessary for weight (mass) perunit area measurement. Calibration standards should be pro-duced using the same material for coatings and substrates andthe same coating technique as the materia
