Two standards address the use of non-destructive dry film thickness gages: ASTM D7091, Standard Practice for Nondestructive Measurement of Dry Film Thickness of Nonmagnetic Coatings Applied to Ferrous Metals and Nonmagnetic, Nonconductive Coatings Applied to Non-Ferrous Metals and SSPC-PA 2, Procedure for Determining Conformance to Dry Coating Thickness Requirements. Both address the need to compensate for substrate/surface conditions including curvature, metallurgy, thinness, roughness, and other properties that can affect (or influence) dry coating thickness measurements. This article is limited to a discussion of roughness (surface profile). While coating specifications may not explicitly state it, specified dry coating thickness requirements are typically recognized as the thickness “above the peaks of the surface profile.”
While paints and coatings are applied to smooth metal surfaces in some industries, most industrial and marine coatings are applied to metal surfaces prepared by abrasive blast cleaning or impact-type power tools that generate a surface roughness known as surface texture, surface profile, or anchor pattern. Coating thickness readings taken using traditional SSPC-PA 2 Type 1 magnetic pull-off gages and Type 2 electronic coating thickness gages are affected by the surface roughness. When either type of gage is placed on bare, prepared (roughened) metal, the gage displays a value even though no coating is present. This is because the gage is reading to a plane part way into the peak/valley pattern, rather than just to the tops of the peaks. In effect, the surface texture creates a “background value” that must be accounted for in the subsequent gage readings, so the measurements represent the thickness of the coating above the peaks of the surface profile.
Base metal effect versus surface profile depth. Before going further, it is important to not confuse the base metal effect (the background values) and surface profile depth. Surface profile is defined as a measurement of the maximum peak to valley depth generated by abrasive blast cleaning (dry or wet) or impact type power tools. It is measured according to ASTM D4417, methods A, B, or C; the frequency and acceptability of the measurements is described in SSPC-PA 17. Conversely, base metal readings are the effect of surface profile/roughness on a coating thickness gage. The effect of a 3-mil surface profile on a coating thickness gage will likely be less than 1-mil. There is no direct correlation between surface profile depth and the net effect on a coating thickness gage. It must be measured using a coating thickness gage to determine the value. If one were to deduct the actual surface profile (rather than the thickness gage reading) from the coating thickness readings, the actual thickness of the coating would be significantly understated.
Potential Consequences of Ignoring the Effect of Surface Roughness. If the gage user elects to ignore the effect of the roughened base metal, the actual thickness of the applied coating will be overstated. The degree to which it is overstated, and the significance of the difference, is based on the depth of the surface profile and the thickness of the applied coating. We’ll explore each.
A 1-mil (25 µm) surface profile will have little influence on the gage reading, since the readings on a bare shallow surface profile are likely a few tenths of a mil at most. However, a bare surface with a 3-4 mil (75-100 µm) profile can register 1 mil or greater on the gage. The significance of these background values on coating thickness readings depends on the thickness of the applied coating. For example, for a 4-mil primer, a 1 mil background value would represent 25% of the primer thickness, while a 1 mil background value on a 25-mil coating would be negligible.
Options for compensating for the background value can be dependent on gage type and manufacturer. The information below presents options for addressing the background value and the inherent advantages and limitations of each. The options, procedures, advantages and limitations are based on the assumption that the gage has a current calibration status and has been verified for accuracy (in the intended range of use) using traceable coated standards or certified shim(s) on smooth metal prior to executing one of the options. The procedures for verifying accuracy are described in the above standards. And as always, it is important to: (1) comply with the project specifications and (2) conform to the gage manufacturer’s instructions.
Measure, Average, and Deduct a Base Metal Reading (applicable to Type 1 and Type 2 Gages)
Procedure: Section 7.3.4 in ASTM D7091-13 and Section 6.2 in SSPC-PA 2 both describe a process wherein a minimum of 10 readings (arbitrarily spaced) are obtained using a coating thickness gage on the prepared, uncoated metal and averaged to determine the “base metal effect” and generate a base metal reading (BMR). This value represents the depth into the surface profile that the gage is reading and is subtracted from the gage readings in coated areas to determine the thickness of the coating above the peaks of the surface profile. This procedure is followed for both single coat and multi-coat systems. For multi-coats systems the same BMR is deducted for each coating thickness measurement, whether the measurements involve the first coat alone, or all coats combined.
Measured primer thickness: 4 mils
Average BMR: 0.5 mil
Actual primer thickness: 3.5 mils
Measured cumulative primer & topcoat thickness: 7 mils
Average BMR: 0.5 mil
Actual cumulative primer & topcoat thickness 6.5 mils
This is the only option that can be exercised to compensate for surface roughness when using Type 1 (magnetic pull-off gages) since they cannot be adjusted. This method, and the others listed below can be used for Type 2 gages.
Advantages: The BMR is measured and recorded prior to application of the coating system. Once obtained and recorded, it remains constant throughout the project provided the same size abrasive is being used and the resulting surface profile is relatively consistent. Access to prepared, uncoated metal throughout the project is not required.
Limitations: The actual surface profile across elements being prepared can vary by 1-2 mils when measured with a depth micrometer or replica tape and likely still fall within the specified range for surface preparation. With such a possible variation in actual profile depth, the effect of the varying depths on the gage readings (the BMR) will likewise vary at any given location, and not precisely match the the average that is being deducted. For example, assume the surface profile ranges from 2-4 mils, and the BMR ranges from a few tenths of a mil to 1 mil. When the thickness readings are taken, the average BMR (say 0.5 mils) is deducted from each of the spots even though the actual effect of the BMR in the precise spot being measured may be less than or greater than 0.5 mils. While this is a limitation, taking many readings across the surface as stipulated in SSPC-PA 2 should normalize these differences.
Adjust the gage using a certified or measured shim on the prepared, uncoated surface (applicable to most Type 2 Gages that can be adjusted by the user)
Procedure: Section 7.4.1 in ASTM D7091-13 and Appendix 8 in SSPC-PA 2 both describe a process wherein certified or measured shims of a known thickness are placed onto the prepared, uncoated metal and a minimum of 10 measurements are taken with the Type 2 (electronic) gage. The thickness of the selected shim(s) should be reasonably close to the intended range of use (i.e., cover the range of coating thickness that will be measured). If necessary, the gage is adjusted so that the displayed reading matches the shim thickness. A one-point or two-point adjustment can be made. A one-point adjustment encompasses the use of a single shim (middle of the range of intended use), while a two-point adjustment encompasses the use of two shims, one below and one above the intended range of use.
Advantages: This procedure effectively removes all effect of the underlying surface, since the shim(s) represent the applied coating from the tops of the peaks of the surface profile. No BMR measurement or deduction from the measured coating thickness is required. Measured shims are provided with the gage at no charge from most manufacturers, and replacements are available for a few dollars. Certified shims may be used but are not required, unless stipulated by contract.
Limitations: Verification of gage accuracy using traceable coated standards must be performed prior to each period of use, followed by gage adjustment using the measured or certified shims as described above. Therefore, access to prepared, uncoated metal throughout the project is required, which may be problematic. Alternatively, small “companion” steel plates can be prepared using the same blast media or power tool, then preserved and used with the shims to adjust the gage prior to each period of use.
Use the “zero function” to remove the effect of the roughened surface (applicable to most Type 2 Gages that have a “zero” option under Calibration Settings)
Procedure: Section 18.104.22.168 in ASTM D7091-13 states that “adjusting to zero on an uncoated sample of the test specimen is the simplest form of a one-point adjustment.” The specific steps to follow are based on the manufacturer’s instructions. If available, select “zero” from the calibration settings menu on the gage and obtain a minimum of 10 readings on the prepared but uncoated metal. Depending the gage being used, the actual readings may not show on the display while being taking, but after acquiring the desired number of readings, the gage will display 0. Theoretically this removes any effect of the roughened metal.
Advantages: This procedure effectively removes all effect of the underlying surface, since the gage has been “programmed” to treat the roughened surface as “zero.” No BMR measurement or deduction from the measured coating thickness is required and use of measured or certified shims may be eliminated, unless a two-point adjustment is undertaken (one-point is the zero-set and the second point is a certified or measured shim at the high end of the expected range of use). The reason for doing a two-point adjustment is to overcome the limitation discussed below.
Limitations: “Programming” the gage to recognize a roughened surface as zero can be problematic if the surface texture is highly variable. Further, since “zero” coating thickness is never measured, the reliability of the gage in the range of use is unknown. This can be eliminated by employing the two-point procedure described in “advantages” above. Finally, access to prepared, uncoated metal throughout the project is required (to verify “zero” daily). Similar to Option 2, small “companion” steel plates can be prepared using the same blast media or power tool, then preserved and used to verify zero prior to each period of use.
Smooth surface adjustment (applicable to Type 1 and Type 2 Gages)
Procedure: SSPC-PA 2, Appendix 8, section A8.3 states that if access to the bare blast cleaned substrate is not available because the coating has already been applied, that a “correction value” may be deducted from the measured coating thickness. For fine profiles, the correction value will be lower than that associated with a coarse profile. A table extracted from ISO 19840 gives approximate correction values to be used when a blast cleaned surface is not available to adjust the gage using options 1, 2 or 3, above.
TYPICAL GAGE CORRECTION VALUES USING ISO 8503 PROFILE GRADES
(SOURCE: ISO 19840)1
|ISO 8503 Profile Grade||Correction Value (mil)||Correction Value (μm)|
Advantages: This procedure removes the theoretical effect of the underlying surface without having to acquire an average BMR value or use measured or certified shims on bare prepared substrate.
Limitations: A significant limitation to this method is determining whether the surface profile beneath the coating should be categorized as “fine, medium, or coarse.” Further, these terms aren’t quantified; that is, the associated surface profile depth (in mils/microns) associated with these three terms is undefined and the information is not available from ISO. Because Option 4 is based on assumptions it is the least precise and desirable and should only be exercised as a last resort.
Practical Approach to Surface Roughness Compensation
The fifth and final option presents a practical approach that is more reliable than Option 4 when the coating has already been applied. It may be useful during a coating failure investigation when destructive means of establishing the effect of surface roughness (described below) is of little concern.
Procedure: A Type 2 (electronic gage) is first verified for accuracy using traceable coated standards, then coating thickness readings are quickly obtained (discriminately) in several locations to find both low and high coating thicknesses. Once located, a small (i.e., ¼-1/2-inch) circle is drawn and an incision made using a Tooke Gage in accordance with the procedure described in ASTM D4138 and the total thickness measured through the ocular. The total thickness of the coating immediately adjacent to the incision is subsequently measured using the Type 2 gage, then the gage is adjusted to match the coating thickness values (low and high) obtained with the Tooke gage, effectively eliminating any effect of the prepared metal surfaces. That is, the coating itself becomes the low and high “measured shims” described in Option 2 resting on the peaks of the surface profile.
Advantages: This procedure removes the theoretical effect of the underlying surface without having to acquire an average BMR value, use measured or certified shims on bare prepared substrate, or apply a correction factor based on qualitative references to roughness (fine, medium, coarse) and the associated uncertainty of Option 4.
Limitations: The procedure is destructive to the coating film (albeit in a limited number of locations) and is not referenced in any industry standard.
The ASTM and SSPC standards for measurement of dry coating thickness on ferrous and non-ferrous metal substrates acknowledge the importance of compensating for substrate characteristics when obtaining coating thickness measurements. One characteristic that must be considered is surface roughness created by abrasive blast cleaning and impact or rotary impact-type power tools. This brief article presented the advantages and limitations of five options for compensating for the effect that surface profile has on coating thickness measurements. The options are based largely on the type of gage, manufacturer’s instructions, and accessibility of the prepared but uncoated substrate. Independent of the method selected, implementing a formal gage calibration program, verifying the accuracy of the coating thickness gage prior to and after each period of use, and compensating for surface roughness and other substrate characteristics are paramount to obtaining reliable, representative coating thickness data.
 Standard Test Methods for Field Measurement of Surface Profile of Blast Cleaned Steel
 Procedure for Determining Conformance to Steel Profile/Surface Roughness/Peak Count Requirements
 ASTM D4138, Standard Practices for Measurement of Dry Film Thickness of Protective Coating Systems by Destructive, Cross-Sectioning Means