Let’s Talk About Surface Profile: Understanding Angularity and Measuring Peak Count/Density Using Stylus Instruments and Optical Grade Replica Tape

KTA’s Certified Coating Inspector Forum Volume 2, Issue No. 12 – December 2023

William Corbett, COO

AMPP Senior Certified Coating Inspector & Certified Protective Coating Specialist

KTA’s Certified Coating Inspector Forum is designed to provide professional development/continuing education on standards, inspection practices, new instruments, and other topics to help keep certified AMPP and FROSIO coating inspectors current. It represents the views of the author and KTA-Tator, Inc. It may or may not represent the views of AMPP: The Association for Materials Protection & Performance, even though SSPC, NACE, and AMPP standards are frequently referenced in the content.

Introduction

Many coating manufacturer product data sheets and project specifications that invoke abrasive blast cleaning or power tool cleaning according to SSPC-SP 11[1] or SSPC-SP 15[2] will invoke a minimum/maximum surface profile depth and require measurement to determine conformance. Coating inspector training and certification courses frequently address surface profile depth but focus primarily on its importance and various methods of measurement. There are two additional attributes that can be equally important to helping ensure long-term coating system performance, which include angularity and frequency (density) of the peaks of the generated surface profile. These attributes are not often taught in training courses, and may not be well-recognized in the industry, but depending on the coating system and service environment may be critical and may be invoked by specification. Let’s explore these terms and learn how they are assessed/measured.

Peak Count/Density

ASTM D4417-21[3] defines peak count or Rpc as, “the number of peak/valley pairs, per unit of length, extending outside a “deadband” centered on the mean line of a stylus trace,” and peak density or Pd as, “the number of peaks per unit area obtained from burnished replica tape.” Theoretically, the greater the Rpc or Pd, the greater the surface area, which equates to better adhesion of the coating (i.e., more surface for the coating to adhere to). However, according to SSPC Surface Preparation Commentary for Metal Substrates (SSPC-SP COM), if the valleys of the surface profile are too deep and narrow, the applied coating may not be able to penetrate to the bottom of the valleys (due to viscosity or reduced wet-out time) leaving a void at the bottom of the “valley.” So, maximizing peak count/density may not be advantageous for all coatings.

Angularity

Unlike peak count and peak density, peak angularity is not defined in ASTM D4417, or for that matter in any known surface preparation standard that addresses surface profile. Angular is defined by Merriam-Webster as, “forming an angle, sharp-cornered, having one or more angles.” Depending on abrasive size, the peak count/density produced on an abrasive blast-cleaned surface (expressed in peaks/cm or peaks/mm²) may be similar for carbon steel and stainless steel shot (rounded) and mineral, slag or steel/stainless steel grit (angular) abrasives, but the angularity will be vastly different. This is illustrated in Figure 1, where the peak count is identical, but the angularity is substantially different. Therefore, obtaining peak count/density measurements and using that data to determine whether the peak/valley pattern of a prepared surface possesses angular characteristics may be misleading.

One parameter for measuring angularity is Sdq, as described by ISO 25178-2[4] (“root mean square at the surface gradient”). Put more simply, this parameter is calculated by finding the angle of each point on the surface, and generating a weighted average that represents the ‘average angle.’ However, at the current time, this parameter can only be measured using costly laboratory equipment.

Figure 1: Angular and rounded surface profile with same peak count

Frequently, coating specifications and/or coating manufacturer PDS will require the production of an “angular” surface profile of a given depth (typically a range). Determining angularity of a surface profile is at best subjective. Determining angularity has largely been a qualitative (visual) assessment, perhaps done in conjunction with a reference disc such as the Keane-Tator Surface Profile Comparator (Grit/Slag [G/S] Disc) or the ISO Comparator (Grit) under 5x or 10x illuminated magnification. While we may rely on the natural or manufactured shape of an abrasive to determine whether the profile generated is angular, what happens when a recyclable abrasive such as steel/stainless steel grit becomes sub-angular due to repeated use? To complicate things even further, there are no equivalent reference discs/comparators for surfaces prepared by power tool cleaning, so determining angularity of the profile on surfaces prepared by power tools is even more challenging.

Instrumentation for Determining Peak Count/Density

There are two recognized methods for determining peak count/density, including drag stylus-type instruments and optical-grade replica tape in conjunction with a replica tape reader.

Drag stylus surface profile measurement is far from new technology. These devices, sometimes called profilometers have been used for decades in the machine surface finishing industry. In June 2005 a technical article published in the Journal of Protective Coatings & Linings (JPCL), titled, The Effect of Peak Count of Surface Roughness on Coating Performance brought the stylus-type instruments to the forefront in the protective coatings industry. Written by Hugh J. Roper, Raymond E.F. Weaver, and Joseph H. Brandon, it was one of the first published studies in the protective coatings/linings industry that demonstrated that an increase in peak count greatly influences adhesion to steel and resistance to corrosion undercutting when the coating gets damaged in service. The article encouraged owners to consider specifying a minimum peak count in addition to surface profile depth (as a range). However, traditional methods of measuring surface profile, including visual comparators, depth micrometers, and replica tape described in ASTM D4417 were not capable of determining peak count. So ASTM Subcommittee D01.46 embarked on developing a new standard that was published in 2005 titled, ASTM D7127, Standard Test Method for Measurement of Surface Roughness of Abrasive Blast Cleaned Metal Surfaces Using a Portable Stylus Instrument. Unfortunately, specifying peak count was not widely adopted. Further, the drag stylus instruments were not user-friendly, were expensive, and the 5 µm stylus was fragile.

Fast forward to 2021. ASTM Subcommittee D01.46 opted to withdraw ASTM D7127 and incorporate the stylus method into ASTM D4417, which was much more widely accepted as the surface profile measurement standard. It became Method “D” in the 2021 version. It requires the use of an instrument capable of reporting Rt (defined as the vertical distance between the highest peak and the lowest valley within a given stylus evaluation length) and Rpc (peak count), the number of peak/valley pairs per unit of length (mm) using a stylus instrument. There are many other parameters that can be measured, but according to Method D these are the two most critical. The drag stylus instrument contains a self-retracting arm (image below) with a 5 µm stylus that traverses (drags) across the abrasive blast-cleaned surface for 2.5 mm to 12.5 mm (set by the operator) and measures Rt (up to about 6 mils) and Rpc (up to about 180/cm). Five sampling lengths are measured (an initial plus 4 more in each compass direction, about 1” apart). Each trace for each parameter is reported, along with the average of 5 traces for each parameter.

Figure 2: Portable Drag Stylus Instruments from Mahr

Testex optical grade replica tape used in conjunction with the PosiTector RTR – 3D probe can measure peak density (Pd), which is expressed in mm². As part of the updates to ASTM D4417 in 2021, the ability to measure peak count using replica tape was added as an optional additional measurement within Method “C.”

In his article, “Replica Tape – A Source of New Surface Profile Information,” David Beamish with DeFelsko Corporation described how replica tape and 3D surface mapping may be used to better characterize a roughened metallic surface. Mr. Beamish explains that a property of the tape related to its capacity to replicate surfaces is its increase in optical transmission where it is compressed during the burnishing process. Transmission of light is proportional to the degree of compression. A photograph of a back-lit piece of replica tape reveals light areas of higher compression (peaks) and dark areas of lower compression (valleys). See Figure 3, image (a).

Using this transparency principle, peak counts are determined by simply counting bright spots on the impression using a digital image sensor. These brightness measurements (each comparable in size to the 5 µm probe of a stylus profile measurement device) correspond to thickness measurements, which, in turn, mirror the profile of the original surface. An instrument with a processor running a suitable algorithm (like the RTR 3D) can identify peaks and determine areal peak density; that is, how many peaks are present per unit area, or Spd.

In contrast to stylus profilers, replica tape peak counters, like delicate and expensive laboratory-grade interferometric optical profilers, calculate true two-dimensional peak densities. Stylus profilers measure only a single line on a roughened surface and most of the characteristics it records as “peaks” are actually “peak shoulders” where the stylus traced over the side of the peak rather than over the top of the peak.

Another advantage of these images is that more data is used to derive each measurement (1,000,000 points for a single replica tape measurement vs. 5,000 points for a single 2.5 cm stylus scan).

(a)                                                                             (b)                                  

Figure 3: 2D (a) and 3D (b) Images Derived from Optical Grade Replica Tape, courtesy of DeFelsko Corporation

Complications Associated with Specifying Minimum Peak Count/Density Requirements

Given all we know or have learned about the value of peak count/density data, why is it rarely specified? And if the characteristic of “angular profile” is so important (and commonly specified), then why hasn’t the industry developed a way to determine angularity of power tool-cleaned or abrasive blast-cleaned steel in the shop/field? The answer? It’s complicated…

1. Abrasive manufacturers and power tool manufacturers do not publish peak count/density values, so specifiers do not have a technical source they can reference.
2. Peak count/density values vary with power tool design, as well as abrasive type, shape, and size.
3. If specified, what happens when the surface profile depth conforms to the project specification, but the peak count/density values do not, and vice-versa?
4. The SSPC Abrasive Standards for mineral/slag and ferrous metallic abrasives (SSPC-AB 1 and AB 3, respectively) do not require peak count/density values to be reported by the abrasive manufacturer, nor measured by the independent laboratory.
5. The SSPC power tool cleaning standards that invoke surface profile requirements (SSPC-SP 15 and SP 11) do not require peak count/density measurements, only profile depth. Further, angularity requirements are typically associated with abrasive blast cleaning not power tool cleaning.
6. Since “angular” is a relative characteristic, tools for determining angularity need to be developed beyond visual assessments with comparators.

Summary

Surface preparation is typically regarded as one of the most critical steps in ensuring the long-term performance of an industrial protective coating system. Yet despite research conducted nearly 20 years ago we are still primarily specifying only one characteristic of surface roughness – profile depth, requiring angularity without defining it/having a way to determine it, and disregarding the characteristic of angularity when power tools are used.

This final edition of the Certified Coating Inspector Forum for 2023 focused on the differences between surface profile depth and peak density, introduced the subject of angularity, described the methods of measuring peak count/density using stylus instruments and optical grade replica tape, and presented the challenges associated with specifying peak count/density requirements. It is the opinion of the author that the industry could benefit from the development of a guide on selection of surface profile/roughness requirements for specifications so that specifiers can better understand (and consider the potential value) of surface roughness characteristics beyond profile depth.

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[1] Power Tool Cleaning to Bare Metal, AMPP

[2] Commercial Grade Power Tool Cleaning, AMPP

[3] Standard Test Methods for Field Measurement of Surface Profile of Blast Cleaned Steel, ASTM International

[4] Geometrical Product Specifications; Surface Texture: Areal, Part 2: Terms, Definitions, and Surface Texture Parameters