1、Designation: D7091 12Standard Practice forNondestructive Measurement of Dry Film Thickness ofNonmagnetic Coatings Applied to Ferrous Metals andNonmagnetic, Nonconductive Coatings Applied to Non-Ferrous Metals1This standard is issued under the fixed designation D7091; the number immediately following
2、 the designation indicates the year oforiginal adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon () indicates an editorial change since the last revision or reapproval.1. Scope*1.1 This practice descri
3、bes the use of magnetic and eddycurrent gages for dry film thickness measurement. This prac-tice is intended to supplement the manufacturers instructionsfor the manual operation of the gages and is not intended toreplace them. It includes definitions of key terms, referencedocuments, the significanc
4、e and use of the practice, theadvantages and limitations of coating thickness gages, and adescription of test specimens. It describes the methods andrecommended frequency for verifying the accuracy of gagesand for adjusting (optimizing) the equipment and lists thereporting recommendations.1.2 These
5、procedures are not applicable to coatings that willbe readily deformed under the load of the measuring gages/probes, as the gage probe must be placed directly on thecoating surface to obtain a reading. Provisions for measuringon soft or tacky coatings are described in 5.7.1.3 Coating thickness can b
6、e measured using a variety ofgages. These gages are categorized as “magnetic pull-off” and“electronic.” They use a sensing probe or magnet to measurethe gap (distance) between the base metal and the probe. Thismeasured distance is displayed as coating thickness by thegages.1.4 Coating thickness can
7、vary widely across a surface. Asa result, obtaining single-point measurements may not accu-rately represent the actual coating system thickness. SSPC-PA2 prescribes a frequency of coating thickness measurementbased on the size of the area coated. A frequency of measure-ment for coated steel beams (g
8、irders) and coated test panels isalso provided in the appendices to SSPC-PA 2. The governingspecification is responsible for providing the user with theminimum and the maximum coating thickness for each layer,and for the total coating system.1.5 The values stated in SI units are to be regarded as th
9、estandard. The values given in parentheses are for informationonly.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 ap
10、plica-bility of regulatory limitations prior to use.2. Referenced Documents2.1 ASTM Standards:2D609 Practice for Preparation of Cold-Rolled Steel Panelsfor Testing Paint, Varnish, Conversion Coatings, andRelated Coating ProductsD823 Practices for Producing Films of Uniform Thicknessof Paint, Varnish
11、, and Related Products on Test PanelsD1730 Practices for Preparation of Aluminum andAluminum-Alloy Surfaces for Painting2.2 SSPC Standard:SSPC-PA2 Procedure for Determining Conformance to DryCoating Thickness Requirements32.3 ISO Standard:ISO 19840 Paints and varnishescorrosion protection ofsteel st
12、ructures by protective paint systemsMeasurement of, and acceptance criteria for, the thicknessof dry films on rough surfaces43. Terminology3.1 Definitions of Terms Specific to This Standard:3.1.1 accuracy, nthe measure of the magnitude of errorbetween the result of a measurement and the true thickne
13、ss ofthe item being measured.1This practice is under the jurisdiction of ASTM Committee D01 on Paint andRelated Coatings, Materials, and Applications and is the direct responsibility ofSubcommittee D01.23 on Physical Properties of Applied Paint Films.Current edition approved April 1, 2012. Published
14、 April 2012. Originallyapproved in 2005. Last previous edition approved in 2005 as D7091 - 05. DOI:10.1520/D7091-12.2For 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
15、the standards Document Summary page onthe ASTM website.3Available from Society for Protective Coatings (SSPC), 40 24th St., 6th Floor,Pittsburgh, PA 15222-4656. (see www.sspc.org)4Available from American National Standards Institute (ANSI), 25 W. 43rd St.,4th Floor, New York, NY 10036, http:/www.ans
16、i.org.1*A Summary of Changes section appears at the end of this standard.Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.3.1.1.1 DiscussionAn accuracy statement predicts theability of a coating thickness gage to measure the true thick
17、nessof a coating to be measured. Accuracy statements provide theperformance capability across the full functional measurementrange of the gage. Accuracy statements frequently include afixed portion that remains constant across the measurementrange, plus a variable portion that is related to the meas
18、urementresult for a particular thickness.3.1.2 adjustment, nthe physical act of aligning a gagesthickness readings to match those of a known thickness sample(removal of bias), in order to improve the accuracy of the gageon a specific surface or within a specific portion of itsmeasurement range.3.1.2
19、.1 DiscussionAn adjustment will affect the outcomeof subsequent readings. Also known as “optimization.”3.1.3 base metal reading (BMR), na measurement ob-tained on the uncoated substrate using a coating thicknessgage.3.1.3.1 DiscussionThe BMR is the determined effect ofsubstrate roughness on a coatin
20、g thickness gage that is causedby the manufacturing process (for example, castings) orsurface profile (roughness)-producing operations (for example,power tool cleaning, abrasive blast cleaning, etc.). Non-compensation for the base metal effect can result in anoverstatement of the true thickness of t
21、he coating.3.1.4 calibration, nthe high-level, controlled and docu-mented process of obtaining measurements on traceable cali-bration standards over the full operating range of the gage, thenmaking the necessary gage adjustments (as required) to correctany out-of-tolerance conditions.3.1.4.1 Discuss
22、ionCalibration of coating thickness gagesis performed by the equipment manufacturer, an authorizedagent, or by an authorized, trained calibration laboratory in acontrolled environment using a documented process. Theoutcome of the calibration process is to restore/realign the gageto meet/exceed the m
23、anufacturers stated accuracy.3.1.5 certification, ndocumentation of the state of condi-tion of the gage, which can (but not required by definition) beaccompanied by corrective action (such as adjustment orcalibration, or both, or the replacement of components) neces-sary to correct any out-of-tolera
24、nce conditions.3.1.6 coating thickness standard, ncoated or plated metalplates, or uncoated shims of flat sheet, with assigned valuestraceable to a National Metrology Institution.3.1.6.1 DiscussionIn the case of the eddy current prin-ciple, the coating and shim material must be non-metallic,whereas
25、in the case of the magnetic induction and the Hall-effect methods the material must be nonmagnetic.3.1.7 compensation value, ngenerating a verifiable value,which is deducted from a measured value read from the gage,to correct for any surface conditions (that is, base metal effect).3.1.8 dry film thi
26、ckness, nthe thickness of a coating (orcoating layers) as measured from the surface of the substrate.3.1.8.1 DiscussionIf the surface is roughened, the dryfilm thickness is considered the thickness of the coating orcoating layers above the peaks of a surface profile.3.1.9 ferrous, ncontaining iron.3
27、.1.9.1 DiscussionDescribes a magnetic material such ascarbon steel. That material may also be known as ferromag-netic.3.1.10 gage (gauge), nan instrument for measuring quan-tity, or an instrument for testing.3.1.10.1 DiscussionIn this practice, the term “gage” re-fers to an instrument for quantifyin
28、g coating thickness.3.1.11 manufacturers specifications, na statement or setof statements that describes the performance characteristics ofthe gage under a given set of conditions.3.1.11.1 DiscussionManufacturers specifications typi-cally include the range of measurement, accuracy statement,operatin
29、g temperature range, power source, dimensions andweight, and conformance to industry standards.3.1.12 measurement, nthe value obtained when placingthe probe of a thickness gage in contact with a surface.3.1.13 micrometer (micron), none one-thousandth of amillimeter (0.001 mm); 25.4 microns = 1 mil.3
30、.1.14 mil, nan imperial unit of measure; one one-thousandth of an inch (0.001 in.); 1 mil = 25.4 microns.3.1.15 nonconductive, na material that is unable to con-duct electricity.3.1.16 non-ferrous metal, na nonmagnetic metal or metalalloy (for example, copper, aluminum or brass).3.1.17 reference sam
31、ple, na coated or uncoated metalspecimen of the same material and geometry as the specificmeasuring application used to adjust and/or verify the accuracyof a coating thickness measuring gage for a specific project.3.1.17.1 DiscussionA coated reference sample may ormay not have thickness values trace
32、able to a National Metrol-ogy Institution. However, the reference sample should bemarked with the stated value and the degree of accuracy. Thecoating thickness of the sample should be close to the userscoating thickness measurement requirement.3.1.18 shims, nstrips of flat sheet, with the thicknesss
33、tated or referenced in some form, that can be used to adjust acoating thickness gage in the intended range of use over thesurface of the representative substrate material.3.1.18.1 DiscussionOther uses include: placement oversoft coatings to obtain thickness measurements without thegage probe depress
34、ing the coating film, and verification of gageoperation. Also known as “foils.”3.1.19 substrate, nthe base material, the type of surface,or the component that is being coated.NOTE 1This practice addresses only metal substrates.3.1.20 surface profile, nsurface texture generated duringthe manufacturin
35、g process (for example, casting), or thepeak-to-valley depth generated by some power tools and byabrasive blast cleaning operations.3.1.21 Type 1 gage, na magnetic pull-off instrument thatmeasures the dry film thickness of nonmagnetic coatings overa ferrous metal base.3.1.21.1 DiscussionFor Type 1 g
36、ages, a probe containinga permanent magnet is brought into direct contact with thecoated surface. The force necessary to pull the magnet from thesurface is measured and interpreted as the coating thicknessvalue on a scale or display on the gage. Less force is requiredD7091 122to remove the magnet fr
37、om a thick coating. The scale isnonlinear. Also known as “pull-off gage.”3.1.22 Type 2 gage, nan electronic instrument that useselectronic circuitry and (but not limited to) the magneticinduction, Hall-effect or eddy current principles, or a combi-nation of a magnetic and eddy current principles, to
38、 convert areference signal into a coating thickness reading.3.1.22.1 DiscussionThe probe of a Type 2 gage remainson the surface during the measurement process. Also known as“constant pressure probe” or “electronic” gages.3.1.23 verification of accuracy, nobtaining measurementson coating thickness st
39、andards, comprising of at least onethickness value close to the expected coating thickness, prior togage use for the purpose of determining the ability of thecoating thickness gage to produce thickness results within thegage manufacturers stated accuracy.4. Significance and Use4.1 This practice desc
40、ribes three operational steps necessaryto ensure accurate coating thickness measurement calibra-tion, verification and adjustment of coating thickness measur-ing gages, as well as proper methods for obtaining coatingthickness measurements on both ferrous and non-ferrous metalsubstrates.4.2 Many spec
41、ifications for commercial and industrial coat-ings projects stipulate a minimum and a maximum dry filmthickness for each layer in a coating system.Additionally, mostmanufacturers of high performance coatings will warrantycoating systems based upon, in part, achieving the properthickness of each laye
42、r and the total coating system. Even if aproject specification is not provided, the coating manufactur-ers recommendations published on product data sheets canbecome the governing document(s). Equipment manufacturersproduce nondestructive coating thickness testing gages that areused to measure the c
43、umulative or individual thickness of thecoating layers, after they are dry. The manufacturers provideinformation for the adjustment and use of these gages, nor-mally in the form of operating instructions. The user of thisequipment must be knowledgeable in the proper operation ofthese devices, includ
44、ing methods for verifying the accuracy ofthe equipment prior to, during and after use as well asmeasurement procedures.5. Principles, Advantages, and Limitations of Gages5.1 Type 1 magnetic pull-off gages employ an attractionprinciple and a static (non-time varying) magnetic field. Thesemechanical i
45、nstruments measure the force required to pull apermanent magnet from a coated ferrous metal substrate. Themagnetic force of attraction to the steel substrate beneath thecoating is opposed by a spring or coil. Tension is applied to thespring/coil until the magnetic attraction to the steel is over-com
46、e. The gage must be placed directly on the coated surfaceto obtain a measurement. The force holding the permanentmagnet to the ferrous base is inversely proportional to thethickness of the coating layer(s) between the magnet and theferrous substrate. For example, a thin coating applied to aferrous s
47、ubstrate will require greater spring tension to pull themagnet off than will a thicker coating, since the magnet iscloser to the ferrous substrate with the thinner coating. Thisinverse relationship is reflected on the nonlinear gage scale.Most Type 1 magnetic pull-off gages do not require a powersou
48、rce (for example, batteries). The manufacturers statedaccuracy is typically 5 to 10 % of the reading.5.2 Type 1 magnetic pull-off gages are susceptible tovibrations, which may cause the magnet to release from thecoated substrate prematurely, yielding a false high value. Themanually operated gages ma
49、y be susceptible to human errorcaused by inadvertently turning the dial wheel past the point atwhich the magnet pulls from the surface, yielding a false lowmeasurement. Type 1 gages should not be used on soft or tackycoatings, as the magnet may adhere to the coating causing falselow measurements, or coating materials may dry on the magnetcausing false high measurements. The exposed magnet mayattract metal filings, which can contaminate the magnet andcause false high measurements. Type 1 gages cannot be used tomeasure the thickness of
