Surface profile - a detailed comparison of measurement methods

In the industry there are no profile standards with values that can be traced back to a national metrology institute. If this were the case, devices could be checked against these standards, accuracy claims could be published, and users could compare their results. With the help of standards, the relationship between the values of impression bands and the values of depth microns, etc. could be determined.

Since there are no physical standards, the industry has opted for a reference method. NACE, ASTM and ISO describe the height of the surface profile as the distance between the highest peak and the lowest valley in the field of view of an optical microscope. A microscope is focused on the highest peak in the field of view. The distance the lens travels to focus on the deepest valley within the same field of view is a single profile height measurement. The arithmetic mean of 20 such measurements gives the mean maximum height from peak to valley. In other words: the average of the maximum peak values.

Figure 2 Computer generated image of a blasted steel surface (left) and a blasted surface (right)

Figure 4 Stylus roughness gauges (gauges shown are similar to those used in this study)

Figure 5 ASTM Comparison Chart

Figure 6: Depth micrometer overview

Figure 7, measuring points for the individual methods

Effect of surface profile on coating thickness gauges

Coating thickness probes measure the distance between their probe tip and the magnetic plane in the steel. With smooth steel, the magnetic plane coincides with the surface of the steel. For rough steel, the magnetic plane lies somewhere between the highest peak and the lowest valley of the profile, a position that can vary depending on the device type. Therefore, roughness generally causes coating thickness gauges to read a high or positive reading.

SSPC-PA 2 and other standards require the application of a correction factor to compensate for this roughness effect. A plastic pad is usually placed over the bare profile and measured with the coating thickness gauge. The gauge is adjusted so that the result matches the thickness of the washer. The underlay simulates the paint build-up over the peaks and the adjustment ensures that the paint thickness measurements are taken from the average peak height of the profile and not from the magnetic plane

To quantify the effect of profile on coating thickness gauges, Type 1 (mechanical deduction) and Type 2 (electronic) gauge measurements were taken on all panels, having previously been zero-tested on smooth, flat steel. The average result of five measurements was recorded for each panel.

The Type 1 device was the least affected by the profile, measuring a maximum of 0.3 mils on the roughest surface. The Type 2 device measured between a low of 0 on the bead blasted surface and a high of 1.2 mils on the S390 shot blasted surface. Overall, the thickness gauge provided thickness results that ranged from 1 to 26% of the surface profile heights measured with the impression tape, with an average of 13% for all panels.

Figure 8, Coating Thickness Gauges versus Replica Film Gauges

General observations on surface profile measurement

Some surface roughnesses are beyond the capability of replica film gauges and stylus roughness gauges. In practice, it is assumed that commercially available adhesive tape qualities enable the measurement of average roughness profiles between 12.7 and 127 µm. All of the depth micrometers used in the study had an extended measurement range suitable for measuring blasted steel surfaces and did not reach their "maximum" on any panel.

The measuring ranges can be found on the respective product pages for the PosiTector SPG surface profile measuring devices.

There were areas on several panels where all instrument types gave high profile values. These discrepancies could be due to the irregularities of hand blasting. It can be assumed that larger areas would show similar irregularities.

It was not possible to test each device in the exact same location on each panel (Figure 7). The impression tape (replica film) examines a relatively large area, so fewer measurements were required to adequately characterize the surface. Stylus and depth micrometer methods have fine-tipped probes that scan a much smaller surface area and therefore require more measurements to adequately characterize a surface.

This is taken into account in the ISO, ASTM, NACE and SSPC guidelines.

All methods required initial setup and accuracy verification before testing began.

Read the PosiTector SPG and PosiTector RTR H user guides for setup and accuracy verification information.

The replicafilm method required checking the accuracy of the micrometer against a known thickness (such as a plastic backing) and setting the scale back 2 mils to account for the incompressible layer of plastic. Minor adjustments had to be made during testing to compensate for micrometer drift.

The stylus roughness measuring devices required the most adjustment work. The correct gage length was entered, reporting parameters such as Rpc (peak count) and Rt (maximum peak-to-valley height in a gage length) were set, and the tool had to be carefully positioned on the blasted steel surface.

The depth micrometers were zero checked before and after each set of 50 measurements on a glass plate and backing plate of known thickness. During the entire test, no instrument deviated from the zero point.

The replicafilm method required checking the accuracy of the micrometer against a known thickness (such as a plastic backing) and setting the scale back 50.8 µm to account for the incompressible layer of plastic. Minor adjustments had to be made during testing to compensate for micrometer drift.

Most of the adjustments were required on the stylus roughness gauges. The correct evaluation length was entered, reporting parameters such as Rpc (peak count) and Rt (maximum peak-to-valley height in an evaluation length) were set, and care had to be taken to position the device on the blasted steel surface.

The depth micrometers were zero checked before and after each set of 50 measurements on a glass plate and backing plate of known thickness. During the entire test, no instrument deviated from the zero point.

Circles were observed on some panels after testing with replica film impression tape. They were probably caused by microscopic particles that had become embedded in the foam and were carried away when the foam was pulled off. Scratches were found on some panels after testing with the stylus gauge. It is believed that the steel surface was slightly damaged when the diamond stud was drawn across the tips (Fig. 9).

Figure 9 A 400x magnified photograph of garnet blasted steel with a scratch

During the tests, it becomes clear that the results of individual surface profile measurements are less repeatable and show greater fluctuations than users are used to from other industrial measurement methods such as dry film thickness, temperature or gloss testing. While two dry film thickness measurements can be expected to be very close, two surface profile measurements can differ significantly. This is in the nature of a blasted surface.

A panel blasted with a mixture of coarse and fine staurolite sand measured between 45.72 and 73.66 µm with the replica film, between 45.72 and 71.12 µm with the stylus roughness tester and with the depth micrometer between 0 and 142.24 µm. Nevertheless, all three methods gave an "average of maximum peaks" of about 63.5 µm.

Just as often, however, the three methods also provided results that were not so close to each other. The results of the replica film and the stylus roughness tester sometimes differed by up to 30%. For two panels blasted with S280 abrasive and 100 grit aluminum oxide, the replica film read 68.58 µm while the stylus method averaged 55.88 µm. In contrast, for BX-40 silica sand, the replica film impression tape read 38.1 µm, while the stylus roughness tester returned a higher average of 48.26 µm. The average values for the three stylus roughness testers were higher than the replica film impression tape values for all 4 grit blasted panels and lower for all oxide and shot blasted panels. See Figure 12 for a summary of replica film impression tape and stylus roughness tester results.

Observations on the measurement with the depth micrometer

When measuring the surface profile with the depth micrometers, the following points were noted:

• Loose Surface Contaminants: Several panels yielded high outlier measurements that were not considered in the final analysis. Participants reported that the instruments "rocked" on the surface. This alerted them to the problem of surface contamination, so they avoided these areas Measurement bias: Sandblasted panels had less bias than glass bead blasted panels. From 250 readings taken with one device on a garnet blasted 4"x6"x1/8" panel, the results ranged from 0.2 to 1.9 mils. If only the highest readings were averaged, the result was flat of 1.2 mils, close to the tape and stylus results. Occasional low readings near zero were obtained. They were probably caused by a large spike pushing the probe tip near the level of the instrument base. By averaging only the maximum readings prevents these low readings from affecting the bottom line.The highest reading in the above example of 1.9 mils is also of interest.It seems to indicate a single, deep valley into which the probe tip dipped, a large peak in the profile that lifted the foot of the depth micrometer, or a surface waviness, however, it was just one result of many that were averaged to produce a meaningful profile measurement to receive a message.

• Number of Measurements for Analysis: When only 3 measurements were taken at each location on the plates, the results did not agree exactly with the results from the gages, suggesting an insufficient number of measurements. With 5 measurements per spot, the final results were closer to the gage results. Increasing the number of measurements to 10 per point (according to ASTM) eliminated the apparent randomness of the results and provided the best correlation with the results from the gage and stylus methods. More measurements hardly improved the results. Reducing the number of measurement points from 5 to 3 had little impact on the overall results. This indicates that a minimum of 10 readings at each of the 3 locations adequately characterizes a blasted profile surface.

• Different results with the depth micrometers: The depth micrometers used in this study had probe tips machined at an angle of 30° and 60°. Their spring pressures ranged from 70 to 125 g of force. Devices with 30° probes often gave lower results than devices with 60° probes. Instruments with weak tactile forces generally gave lower results than instruments with strong tactile forces. This suggests that the angle of the probe tip and the force of the probe tip affect the measurement results (Fig. 10). High-resolution photos of the probe tips were examined. All of the tips correctly measured 30 or 60° as advertised, but their tip radii varied significantly. Some were really rounded. Others featured flattened or chiseled ends (Fig. 11).

Fig.11a High resolution photo of different depth micrometer tips

Fig.11b Close-ups of different depth micrometer tips

• Analysis Methods: When 50 readings of each depth micrometer were averaged according to ASTM D4417, the resulting profile height measurements were almost always lower than those of the replica film/imprint tape and the stylus roughness meter. When only the maximum values from each site were averaged, the results correlated better with those of the replica film/impression tape and the stylus roughness tester (Fig. 12).

Fig.12 Comparison of measurement methods, results from devices of one type are summarized

Conclusions and Deductions

The results of this study confirm the close relationship between tape measure and stylus measurements as previously established in the ASTM interlaboratory comparison. The results also revealed interesting information about the third type of gauge, the surface profile depth micrometer, which yielded comparable results to the tape measure and caliper when using the 'maximum peak average' analysis approach (Fig.12).

The surface of the blasted steel undergoes a random change at each point, so a series of measurements must be taken. The goal of the evaluation is to make a maximum of peak-to-valley determinations. Individual surface measurements of a blasted metal surface will vary significantly from area to area in a given area. How these measurements are combined depends on the parameter required for the task, which can be the average peak-to-valley height, its maximum, or something else. By applying the "maximum peak average" analysis approach, a depth micrometer provides reliable surface profile measurements that correlate closely with replica tape and stylus instrument results.

PosiTector SPG Advanced models feature SmartBatch mode to meet various standards and testing methods. By default, SmartBatch produces results that approximate those of the replica tape and drag pin method by automatically averaging the maximum tread depth for all points within the test area and displaying "the average of the maximum peaks".

Sources:

1. International Organization for Standardization (ISO), 1 rue de Varembé, Case postale 56, CH-1211, Geneva 20, Switzerland Preparation of steel substrates prior to the application of paints and related products - Surface roughness characteristics of blast-cleaned steel substrates - Part 1: Specifications and definitions for ISO Surface Profile Comparators for Evaluating Blasted SurfacesASTM International, 100 Barr Harbor Drive, West Conshohocken, PA 19428ASTM D7127 "Standard Test Method for Measurement of Surface Roughness of Abrasive Stray Cleaned Metal Surfaces Using an Electronic Portable Stylus Instrument" (West Conshohocken, PA: ASTM)ASTM D4417 "Standard Test Methods for Field Measurement of Surface Profile of Blasted Cleaned Steel" (West Conshohocken, PA: ASTM) From NACE Standard RP0287-2002, "Field Measurement of Surface Profile of Abrasive Stray-Cleaned Steel Surfaces Using a Replica Tape". (Houston, TX: NACE, 2002)ISO 8503-2 Preparation of steel substrates prior to the application of paints and related products - Surface roughness characteristics of blasted steel substrates - Part 2: Method for classifying the surface profile of blasted steel - Comparative methodsResults of NACE Working Group T interlaboratory tests -6G-19. Report of the Technical Committee of NACE 6G176 (withdrawn). "Cleanliness and Anchorage Patterns by Wheel Blasting of New Steel" (Houston, TX: NACE International). (Available from NACE International as a historical document only.) This statistical summary was performed using imperial units. For conversion to metric units, 1 mil = 25.4 microns (μm) is used. ISO 4287: 1997 Geometrical Product Specifications (GPS) - Surface texture: profile method - Terms, definitions and surface parameters KTA-Tator, Inc. (KTA), 115 Technology Drive , Pittsburgh, PA 15275 USA.