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What is Surface Profile and Why is it Necessary?
Surface profile is defined as a measurement of the maximum peak-to-valley depth created by abrasive impingement against a surface during abrasive blast cleaning operations, or by an impact-type power tool. During abrasive blast cleaning, the mass of the abrasive and the velocity of the abrasive movement created by compressed air generates kinetic energy (the abrasive can reach speeds of over 600 miles per hour as it exits the blast nozzle). When the abrasive impacts the surface, it cuts into the surface (angular abrasives) or peens the surface (round abrasives) and creates a series of peaks and valleys in the surface.
The creation of this peak-valley pattern in the surface effectively increases the surface area, providing an anchor for the coating system. Both the structure and the coating system protecting the structure will move while in service. This movement may be caused by expansion and contraction of the substrate due to temperature fluctuation, or live loads placed onto a structure; for example, traffic crossing a bridge. The surface profile must be compatible with the coating system. Typically, the higher the coating thickness the greater the surface profile depth. Peak density (the number of peaks per unit area) also plays a key role in maintaining adhesion of the coating system and provides greater resistance to corrosion undercutting when the coating system gets damaged while in service.
Standards for Measurement of Surface Profile
There are currently four primary standards for measurement of surface profile in steel surfaces. Note that ASTM D4417 and SSPC-PA 17 also address measurement of surface profile generated by impact-type power tools. The four standards are:
- ASTM D4417, Standard Test Methods for Field Measurement of Surface Profile of Blast Cleaned Steel;
- ASTM D7127, Standard Test Method for Measurement of Surface Roughness of Abrasive Blast Cleaned Metal Surfaces Using a Portable Stylus Instrument;
- NACE SP0287, Field Measurement of Surface Profile of Abrasive Blast-Cleaned Steel Surfaces Using a Replica Tape; and
- SSPC-PA 17, Procedure for Determining Conformance to Steel Profile/Surface Roughness/Peak Count Requirements.
How is Surface Profile Depth Quantified?
ASTM D4417 contains three methods of measuring surface profile depth: Method A describes the proper use of a comparator; Method B describes the use of a depth micrometer, and Method C addresses the use of replica tape (as does NACE SP0287). Today, Method B and Method C are the most commonly used, so that is what we will be focusing on.
ASTM D4417, Method B
Method B in the ASTM standard describes the use of a depth micrometer. The surface profile depth micrometer measures the depth of the valleys of the surface profile relative to the height of the peaks using a 60° cone-shaped point protruding from the base of the gage. The base of the instrument rests on the peaks of the surface profile while the cone-shaped point projects into the valley. The depth is displayed on the gage in mils (0.001”) or micrometers (0.001mm; there are 25.4 micrometers [µm] in 1 mil). This method can be used to measure the surface profile depth that is created by abrasive blast cleaning or impact-type power tools. Models from a few gage manufacturers are available that conform to this standard.
According to ASTM D4417, a minimum of 10 readings is obtained per area; the maximum surface profile is reported (discarding obvious outliers). SSPC-PA 17 states that a minimum of three 6” x 6” areas are measured per work shift or 12 hours of preparation time, whichever is shorter. The measurements must conform to the minimum and maximum surface profile requirements listed in the project specification.
ASTM D4417, Method C
Method C in the ASTM standard describes the use of replica tape. A mirror image of the peak-valley pattern generated by abrasive blast cleaning is created in an emulsion foam applied to the underside of a 2-mil polyester film (Mylar®) by pressing the Mylar using a burnishing tool using medium pressure. Once the burnishing process is complete, the replica tape is removed from the surface and the image is measured using a spring-loaded micrometer.
The Mylar thickness (2 mils) is deducted from the measurement, revealing the depth of the surface profile within the measured area (approximately 3/8” diameter). Alternatively, a Replica Tape Reader (RTR) can be used to read the replica tape.
According to ASTM D4417, a minimum of 2 readings is obtained per area; the average of the two readings is reported. SSPC-PA 17 states that a minimum of three 6” x 6” areas are measured per work shift or 12 hours of preparation time, whichever is shorter. The measurements must conform to the minimum and maximum surface profile requirements listed in the project specification.
Same Surface, Different Results?
When a project specification simply invokes ASTM D4417 and not a specific method, the results of the surface profile measurements may differ when two different methods are used on the same project, even on the same surface and within the same area (i.e., the contractor’s quality control inspector is using replica tape and the facility owner’s quality assurance inspector is using the depth micrometer). While both the depth micrometer and replica tape methods conform to ASTM D4417, the measurement acquisition principles are quite different. The depth micrometer is measuring a single valley depth in relationship to potentially hundreds of “peaks” beneath the base of the instrument. Conversely, the replica tape image represents many peaks/valleys, and the micrometer is measuring a portion of those obtained (the test area on the replica tape is approximately 3/8” diameter and the anvils of the micrometer are approximately 1/8” in diameter). So, in effect the reading on the micrometer or the RTR from the replica tape represents several peaks and valleys, while the depth micrometer does not. Therefore, differences are inevitable, particularly with deeper surface profiles, and the results may or may not fall within the specified range for one of the two methods. To avoid these discrepancies, it is recommended that a single method be employed on a project. This can be discussed and agreed upon at the pre-construction conference.
For many in the health care/fitness industry, BMR is an acronym for basal metabolic rate. Sorry to disappoint if you thought this would be a health science article about expending energy. Rather, this article is about a different BMR: Base Metal Reading. We’ll describe what it is, its significance, how to obtain it, and how it impacts coating thickness.
Introduction to Coating Thickness Standards
There are two common industry standards that govern measurement of coating dry film thickness on metal substrates, including 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 use of Type 1 (magnetic pull-off) and Type 2 (electronic) gages as well BMR acquisition. SSPC-PA 2 also addresses measurement frequency and the acceptability of the measurements.
What is BMR?
BMR is the effect of substrate roughness on a coating thickness gage. The roughness is created by preparation of the substrate (e.g., abrasive blast cleaning or power tool cleaning), which generates a surface texture or “profile,” or by a manufacturing process that imparts roughness into the substrate. Instruments that measure the dry film thickness of the applied coating reach part way down into the roughened metal surface to operate properly (illustrated by the red line). However, specifications list the required coating thickness as measured from the tops of the peaks of the surface profile (illustrated by the blue bar). This inherent delta is known as the base metal effect. It is deducted from the coating thickness measurements to eliminate any effect of surface roughness. If the BMR is ignored, the thickness of the coating from the tops of the peaks of the surface profile may be overstated.
Acquisition of a BMR is not predicated on the gage type (Type 1 magnetic pull-off verses Type 2 electronic), but rather the way the gage is set-up by the operator to compensate for surface roughness. For both Type 1 (see photo, left) and Type 2 gages a BMR may be acquired and deducted from the coating thickness.
As an alternative, for Type 2 gages one or more measured shims (one shim is considered a one-point adjustment while the use of two shims spanning the range of intended use is considered a two-point adjustment) may be placed onto the prepared (roughened) metal surface and the gage adjusted to correspond to the shim thickness, effectively removing any need to measure and deduct a BMR. According to SSPC-PA 2, these measured shims are not permitted to be used with Type 1 gages unless explicitly allowed by the gage manufacturer, so in most cases a BMR will be required when using a Type 1 gage.
Obtaining Base Metal Readings
Section 6.2 in SSPC-PA 2 states, “To compensate for any effect of the substrate itself and surface roughness, obtain measurements from the bare, prepared substrate at a minimum of ten locations (arbitrarily spaced) and calculate the average value. This average value is the base metal reading.” Here are the steps:
- Verify the accuracy of the coating thickness gage before use. Traceable coated standards are required for both Type 1 and Type 2 coating thickness gages.
- Obtain a minimum of ten readings on the prepared, uncoated substrate in random locations. To avoid forgetting to acquire a BMR, it is best to take the measurements at the same time surface profile measurements are obtained.
- Measure the coating thickness.
- Deduct the average BMR.
The BMR is not only deducted from the primer thickness, but the cumulative layer thickness measurements as they are obtained. This is illustrated below:
Measured primer thickness: ———————————————————- 4.9 mils
BMR: ———————————————————————————— (0.6 mil)
Actual primer thickness from the top of the peaks of the surface profile: —— 4.3 mils
Cumulative primer & topcoat thickness: ——————————————— 9.2 mils
BMR: ————————————————————————————- (0.6 mil)
Actual cumulative thickness from the top of the peaks of the surface profile: — 8.6 mils
It is important to recognize that BMR and surface profile are related, but they are not the same. Surface profile is a measurement of the maximum peak-to-valley depth created by abrasive blast cleaning or some type of impact power tool. It is measured using one of three methods described in ASTM D4417, Standard Test Methods for Field Measurement of Surface Profile of Blast Cleaned Steel and SSPC-PA 17, Procedure for Determining Conformance to Steel Profile/Surface Roughness/Peak Count Requirements. BMR is the effect of this surface profile on a coating thickness gage. A 3-mil surface profile may have an associated BMR of 0.7 mil. Deducting surface profile from the coating thickness instead of the BMR will result in a significant under-recognition of the actual coating thickness.