1、Designation: A754/A754M 11 (Reapproved 2016)Standard Test Method forCoating Weight (Mass) of Metallic Coatings on Steel byX-Ray Fluorescence1This standard is issued under the fixed designation A754/A754M; the number immediately following the designation indicates the yearof original adoption or, in
2、the case of revision, 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. Scope1.1 This test method covers the use of X-ray fluorescence(XRF) for determining the coati
3、ng weight (mass) 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: A599/A599M, A623,A623M, A653/A653M, A792/A7
4、92M, A875/A875M, A879/A879M, A918, A924/A924M, A1046/A1046M, and A1063/A1063M. It may be applicable to other coatings, providing thatthe elemental nature of the coating and substrate are compat-ible with the technical aspects of XRF such as the absorptioncoefficient of the system, primary radiation,
5、 fluorescentradiation, type of detection.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 eit
6、her inch-pound units or SI 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
7、 nonconformance with the specifi-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
8、limitations prior to use.2. Referenced Documents2.1 ASTM Standards:2A599/A599M Specification for Tin Mill Products, Electro-lytic Tin-Coated, Cold-Rolled SheetA623 Specification for Tin Mill Products, General Require-mentsA623M Specification for Tin Mill Products, General Re-quirements MetricA653/A6
9、53M Specification for Steel Sheet, Zinc-Coated(Galvanized) or Zinc-Iron Alloy-Coated (Galvannealed)by the Hot-Dip ProcessA792/A792M Specification for Steel Sheet, 55 %Aluminum-Zinc Alloy-Coated by the Hot-Dip ProcessA875/A875M Specification for Steel Sheet, Zinc-5 % Alu-minum Alloy-Coated by the Hot
10、-Dip ProcessA879/A879M Specification for Steel Sheet, Zinc Coated bythe Electrolytic Process for Applications Requiring Des-ignation of the Coating Mass on Each SurfaceA902 Terminology Relating to Metallic Coated Steel Prod-uctsA918 Specification for Steel Sheet, Zinc-Nickel AlloyCoated by the Elect
11、rolytic Process for Applications Re-quiring Designation of the Coating Mass on Each SurfaceA924/A924M Specification for General Requirements forSteel Sheet, Metallic-Coated by the Hot-Dip ProcessA1046/A1046M Specification for Steel Sheet, Zinc-Aluminum-Magnesium Alloy-Coated by the Hot-Dip Pro-cessA
12、1063/A1063M Specification for Steel Sheet, Twin-RollCast, Zinc-Coated (Galvanized) by the Hot-Dip Process3. Terminology3.1 DefinitionsFor general definitions of terms relating tometallic-coated steel products, see Terminology A902.1This test method is under the jurisdiction of ASTM Committee A05 onM
13、etallic-Coated Iron and Steel Products and is the direct responsibility ofSubcommittee A05.07 on Methods of Testing.Current edition approved May 1, 2016. Published June 2016. Originallyapproved in 1979. Last previous edition approved in 2011 as A754/A754M 11.DOI: 10.1520/A0754_A0754M-11R16.2For refe
14、renced 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.Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West
15、 Conshohocken, PA 19428-2959. United States13.2 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 explanati
16、on.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 must bemeasured under standardized conditions to develop a singledetermination of coating weight (mass).3.2.4 standard
17、s, 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.3.2.6 time constant, nan electronic filtering term, uniqueto the design of each type of measuring instrument, that d
18、efinesthe 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 excited to a higher energy state.4. Basic Principle4.1 The measurement of coating thickness by XRF methodsis based on the c
19、ombined 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 well-defined energy. These fluorescent X-raysare detected by a radiation detector that can discriminatebetween selected
20、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 primary beam and the surface beingfluoresced have energy levels that are unique to each elementin the targeted material. E
21、ach 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 substrate material.4.1.2 The detection system includes the radiation detector inconjunction with suitable electronic dis
22、criminating 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 detector and the thicknessof the coating material. The thickness of a sample can beestablished by comparing the measured
23、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 electronics are established to report the coatingweight (mass) in commonly used units such as oz/ft2g/m2.4.2 Measurement Techni
24、ques: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, the coating weight (mass) is correlated with theintensity of the fluorescent X-rays emitted by the coating.4.2.2 Th
25、e 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. The correlation in this case is based on theprinciple that the intensity of the X-rays from the fluorescedsubstrate
26、 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 coatingweight (mass).5. Factors Affecting Accuracy5.1 The equipment used to make a coating weight (mass)measurement usin
27、g 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 radiation. The detector detects thisradiation, and the electronic system converts it into coatingweight (mass) infor
28、mation. 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 coating weight (mass) measurement isdetermined by the equipment and the data collection time.Without a good calibratio
29、n 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 outside the range of the equipment.Therefore, the measurement accuracy depends on theequipment, data collection time, and
30、calibration of the instru-ment. The environment may also influence the measurementaccuracy. Since equipment and coating each have uniquecharacteristics, equipment specifications should be reviewedcarefully prior to purchase and installation.5.2 In order to measure coating weight (mass) accurately,th
31、e 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. When the coating weight (mass) is measuredusing fluorescence from the coating, the sample volume is theentire layer of t
32、he 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 for the primary beam energy) thickness of thesubstrate under the coating. The radiated spot size must belarge enough to co
33、ver 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 source and the coating compo-sition. If a coating is thicker than 5/ ( of the coating for thefluorescent beam energy), XRF
34、 produced underneath the 5/thickness 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
35、 the coating and the substrate. Whenthe sample contains elements having similar atomic mass orsimilar X-ray characteristics, detected signals are difficult toA754/A754M 11 (2016)2discriminate and the measurement accuracy is affected ad-versely. The measurement accuracy may also be affectedadversely
36、when fluorescence from one element influencesfluorescence 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 coatin
37、g), or the coating contains elements thatare 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
38、). A stronger source and a more sensitive detectortypically 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 d
39、etection is equal to N. As a guideline, the 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
40、 be similar tothat of the calibration standard. 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 (m
41、ass) range ofthe standards must exceed that 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
42、any interference with the X-raysignal. A film 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 tempera
43、ture in the gapbetween the instrument and the 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
44、strip pass-linedue to such conditions as strip 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.
45、 A very long aver-aging time will mask variations 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 t
46、he pur-pose of establishing an instrument calibration, exactly the sameinstrumental conditions should be used as those that will beused on material being measured. The measuring time forcalibration standards may be longer than that on material beingmeasured in order to reduce the effect of statistic
47、al fluctuations.6.2 StandardsReliable standards 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
48、 (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 material be
49、ing measured.When correlating to standard weigh-strip-weigh techniques,great care must be exercised in selecting the sample becausethe coating is destroyed in the weigh-strip-weigh test proce-dure. Recommended sampling is to choose a uniform areaapproximately 9 by 9 in. 230 by 230 mm. This can bemeasured by using an XRF instrument to find areas of uniformsignal, from which five weigh-strip-weigh samples are cut in across-like pattern, wherein the center sample is in line with twoother samples in the longitudinal direction and with two othersamples in the cr
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