Methods for Measuring Profile on Rough Steel Surfaces

 

Introduction

Surface profile is defined as the maximum peak-to-valley depth created by the impact of abrasives applied at high speed to the surface and the impact generated by using certain power tools. These cleaning methods effectively increase the surface area and provide an "anchor point" for the applied paints and coatings system. It creates a kind of anchoring effect. Surface profile depth should be compatible with the total coating system thickness; typically, the thicker the coating system, the deeper the surface profile. For example, a system of 3 coats at 300 microns may require a 50–75 micron profile, whereas a system at 1000 microns may require a 100–125 micron profile. Profile depth is very important. On very deep surfaces where you cannot increase the amount of paint, you will encounter pin point rusting as a coating defect, and if the profile is shallow, you will face coating adhesion failure. Therefore, compliance with the specified minimum and maximum surface profile depth is critical to the success of the paints and coatings system. Factors affecting surface profile depth include the condition of the prepared surface and the type, hardness and size of the abrasive used. Less important factors include the distance of the blasting nozzle from the surface and the angle at which the operator holds the nozzle to the surface. For power tool cleaning, the tool type and configuration of "impact tools" generally determine the surface profile depth. Steel surfaces cleaned by abrasive blasting and power tool are continuously monitored to verify that the desired/specified surface profile has been achieved. ASTM D4417, Standard Test Methods for Measurement in the Field of Surface Profile of Steel Surfaces Cleaned by Abrasive Blasting, NACE International SP0287, Standard Practice for Measurement in the Field of Surface Profile of Steel Surfaces Cleaned by Abrasive Blasting Using Replica Tape, and SSPC: Society for Protective Coatings PA 17, Procedure for Determining Conformance of Steel Profile/Surface Roughness/Peak Count Requirements, explain the procedures for application of these measurements and the recommended frequency of measurements and acceptability of values. Additionally, the standards assume properly prepared steel; they provide little information on measurement of surface profile on rough or irregular surfaces such as pitted steel, weathered steel or cast iron surfaces. This article explains several methods and considerations that may be important for measuring surface profile on these irregular surface types. Many steel structures that have served for long periods may contain irregular, rough surfaces due to corrosion. Usually this results in loss of steel thickness and may even require replacement. However, when it is concluded that sufficient loss of steel has not occurred to warrant repair to the underlying steel, the coating applicator frequently faces the requirement to comply with specification requirements for cleaning and profile acquisition on rough surfaces that often exceed the specification's surface profile requirements. Similarly, other steel surfaces such as cast iron and weathered steel (ASTM A588, A242, A606-4, A847 and A709-50W) typically have a rougher surface than ASTM A36 steel that has been abrasive blasted after exposure to atmospheric conditions, resulting in a higher surface profile acquisition than permitted by specification and consequently a non-conformance. Surface profile measurement on rough or pitted surfaces can generally result in incorrectly high readings, because measurements reveal that the depth of pits or the internal surface roughness of the steel differs from the surface profile created by the abrasive or impact type power tool itself. This raises the question, "How do you verify surface profile on these types of surfaces?" There are several alternatives that can be considered; however, these should be discussed as needed and addressed at the pre-work meeting, not during on-site measurements during the project.

Alternative Methods

The first method is to obtain measurements in a desired area (non-rough) taking into account which method has been selected/specified (depth micrometer or replica tape). However, this may not be possible when pitted or rough steel is uniform. Of the three methods listed in the consulted standards, the depth micrometer (Method B in ASTM D4417) is generally considered optimal in these situations because a single valley measurement can be obtained and the upper range of the tool, which can be reasonably measured at maximum values of 500 microns using replica tape (125 microns), is higher. A minimum of ten measurements is made in an area and the average surface profile is calculated. Another method relies on a visual comparator and a reference disc. The comparator is a lighted magnifier (typically 5–10x magnification) that allows the user to closely examine surface roughness and compare it with master discs containing different degrees of roughness (5 segments per disc). An appropriate reference disc representing the abrasive used (sand/slag or shot) is placed on the prepared steel and the user selects the disc and segment that most closely matches the surface profile of the steel. The third method is to measure the surface profile on a test plate, which is a supplementary steel piece cleaned with abrasive blasting using the same abrasive and pressure as used on the rough steel. This procedure has been accepted in the nuclear energy industry for many years for coating cast iron motor housings. Finally, the abrasive manufacturer can be consulted regarding typical surface profile values produced by the type and size of abrasive used. Some abrasive manufacturers can provide a Certificate of Conformance stating the range measured under laboratory conditions for a particular batch. It should be noted that surface hardness greatly affects surface profile depth, and therefore the abrasive manufacturer's data may be misleading. In general, ASTM D 4417 Method B and Method C are preferred to obtain more numerical values on a surface, but using both on the same project can create some issues. Problems occur when the paint contractor's inspector uses Method B while a third-party inspector uses replica tape and they both find different results. Which method should be selected as the correct one?

ASTM D 4417 Method B or Method C?

In the industrial and protective coatings industry, metal surfaces are chemically stripped, mechanically ground or blasted, among other things, creating surface lines or profiles. Surface profile increases the surface area that plays a key role in coating adhesion. The required surface profile is determined by the coating manufacturer or by all accepted coating specifications. Surface profile is determined using the difference between valley and peak. If surface profile does not conform to specifications and is too high or too low, it causes coating defects and consequently corrosion. This is why surface profile is so important. There are many ways to measure surface profile and they all work differently. According to ASTM D 4417 standards, there are three methods specified. These are: • Method A – Visual comparator, • Method B – Depth micrometer, • Method C – Replica tape. Method B and Method C are the two commonly used methods for surface profile measurement.

Method B

The depth micrometer's profile measurement principle measures the distance between valley and peak using a spring needle. Ten readings are taken in an area according to ASTM D 4417-B standard and their statistical average is calculated.

Method C

The replica tape method is applied with a replication tape together with a foam pad that can be compressed onto the surface and is rubbed with a polishing tool. As the replica tape rubs against the surface, the foam is pushed downward, creating a reverse or mirror profile. The replication tape is then removed and measured using a thickness gauge. Unfortunately, this method is less accurate because the tape replicating the valley peak depth may have taken some measurement points not from the bottom of the valley but somewhere on the slope, or you may need to test the same area multiple times for linear spacing intervals. Still, completely accurate measurements cannot be obtained and it has the characteristic of being somewhat more subjective compared to Method B. The depth micrometer measures a single valley depth in relation to potentially hundreds of "peak points" below the base of the device. Conversely, the replica tape image represents many peaks/valleys, and the micrometer measures only some of those obtained (the test area on the replication tape is approximately 3/8" in diameter and the micrometer's needles are approximately 1/8" in diameter). Therefore, while reading from the micrometer or replica tape represents several peaks and valleys, the depth micrometer's reading does not. For this reason, especially with deeper surface profiles, differences are unavoidable and results may or may not be within the specified range for one or the other method. To avoid these inconsistencies, it should be agreed that a single method be used on a project.

Conclusion

The important point to remember is that when the surface is rough or irregular, one or more of these profiles can be used to more accurately determine the surface profile depth. Additionally, rough surfaces may require application of a thicker coat or additional coating layers to help ensure corrosion protection. The coating manufacturer should be involved in making these decisions. Seda Ömercikoğlu Yüce - MICorr / Director - STM Coatech

References: 1) KTA University, Surface Profile Measurement Options on Rough/Pitted Steel Surfaces, Accessed: 22 November 2018, https://ktauniversity.com/surface-profile-pitted-steel-surfaces/ 2) https://www.elcometerusa.com/Methods-For-The-Measurement-of-Surface-Profile-in-the-Field_2.pdf (last accessed: 28.11.2019 – 16:45) 3) https://kta.com/kta-university/surface-profile-astm-d4417/ (last accessed: 28.11.2019 – 16:45)