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Industrial Protective Coatings
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 November 2006
Feature
 
William Corbett is the technical services manager for KTA-Tator, Inc., a Pittsburgh-based consulting engineering firm specializing in protective coatings and linings. He has been employed by KTA for over 27 years. He is an SSPC Certified Protective Coatings Specialist, an SSPC Certified Bridge Coatings Inspector and a NACE International Certified Coatings Inspector.

Industrial Protective Coatings

Under the leadership of Lloyd Smith, chair, and Dwight Weldon, co-chair, Subcommittee D01.46 on Industrial Protective Coatings maintains a full slate of standard test methods, practices and guides, and prides itself as one of the most active subcommittees in D01 on Paint and Related Coatings, Materials and Applications. Members of Subcommittee D01.46 include representatives from coating manufacturers and raw material suppliers, facility owners, equipment manufacturers, laboratories and consulting firms, as well as government agencies. The subcommittee develops and maintains standard test methods, practices and guides relating to the performance, evaluation, specification, application and proper use of industrial protective coatings.

Currently, this subcommittee has eight task groups that are working on 13 ASTM standards, four of which are proposed standards and nine of which are existing standards that have been slated for revision or renewal, or that have been recently published and require the development of precision and bias statements through interlaboratory research initiatives.

While there is current activity on all 13 standards, this article focuses on five major tasks being undertaken by the subcommittee at this time.

Measuring Surface Roughness of Abrasive Blast Cleaned Steel
The industrial protective coatings industry has long recognized the importance of surface profile depth (generated by abrasive blast cleaning and some power tools) as a means to increase surface area and improve the adhesion of the coating to the underlying substrate. In fact, in 1993, ASTM published D 4417, Test Methods for Measuring Surface Profile of Abrasive Blast Cleaned Steel, which describes three methods of quantifying surface profile depth generated in a steel substrate (visual comparators, depth micrometers and replica tape). These methods are used to determine the average distance between the tops of the peaks and the bottoms of the valleys on a prepared steel surface.

Recently published articles1 describe the importance of other surface roughness attributes, in addition to average peak-to-valley distance, including peak count (Pc), maximum roughness depth (Rmax) and maximum profile height (Rt). Based on laboratory research conducted by Roper et. al., increased peak density improves coating adhesion and reduces corrosion undercutting compared to surfaces with correspondingly lower peak counts. Methods of generating various peak densities are described in the articles, and are based on abrasive size, shape, hardness and other factors. Facility owners with demanding service environments may consider specifying a given set of criteria for surface roughness (above and beyond the standard peak-to-valley depth), which may translate to increased performance and reduced life cycle costs. Coating manufacturers may also consider these requirements to improve the performance and the service life of the coatings they supply. (See Figure 1.)

Because of the industry’s recognition of the importance of surface roughness characteristics (beyond average surface profile depth), the need for the development of a standardized procedure for acquiring surface roughness data was evident. Through the efforts of Subcommittee D01.46, D 7127, Test Method for Measurement of Surface Roughness of Abrasive Blast Cleaned Metal Surfaces Using a Portable Stylus Instrument, was published in 2005. The standard describes the procedures for using and verifying the accuracy of portable stylus-type instruments to obtain surface characterization data, and provides guidance to the user regarding the significance of the data, once generated.

Naturally, the ability to generate reliable data between users employing different instruments, as well as the ability of one user to generate repeatable data using a single instrument, must be assessed. An interlaboratory research initiative for the purpose of generating precision and bias statements for D 7127, is planned for 2008.

Measuring Surface Profile of Abrasive Blast Cleaned Steel
ASTM D 4417 has long been recognized as the industry standard for measurement of surface profile of abrasive blast cleaned steel surfaces. This method encompasses only one surface characteristic: average peak-to-valley depth. The method describes three different procedures for quantifying surface profile, including visual comparators (Method A), depth micrometers (Method B) and replica tape (Method C). NACE International also publishes a Recommended Practice,2 which addresses the replica tape.

Method A (visual comparator) incorporates a 5X or 10X illuminated magnifier and segmented discs containing various degrees of surface roughness (expressed in mils) generated by different types of abrasive media (sand, grit/slag and shot). A segmented disc is selected to match the type of abrasive employed, then is examined beneath the illuminated magnifier; the segment(s) depicting the roughness of the blast cleaned surfaces is recorded. (See Figure 2.)

Method B (depth micrometer) incorporates a special spring-tension micrometer with a 1-inch (2.5 cm) diameter base and a conical shaped, pinpoint stylus protruding from the center of the base. With the base of the micrometer resting on the peaks of the surface profile, and the conical pin protruding to a valley of the profile, the distance is read from the gage display (in mils or micrometers). (See Figure 3.)

Method C (replica tape) employs a pressure sensitive tape (containing a compressible foam attached to a polyester backing) to replicate the peak-valley pattern generated by the abrasive blast cleaning process. The pattern of surface roughness is replicated by burnishing the tape, effectively pressing the compressible foam into the surface roughness. Once the surface profile pattern has been replicated into the tape, it is removed from the surface and placed in a special spring tension anvil micrometer. The average surface profile depth is read from the micrometer dial (in mils or microns), after the micrometer has been corrected for the thickness of the polyester backing material. (See Figures 4, 5 and 6.)

For Method C, there are various ranges of replica tape that can be employed. The tape selected is based on the anticipated (or specified) surface profile depth. Coarse replica tape is used to measure the actual roughness of surfaces containing a profile from 0.8 to 2 mils (20 to 50 micrometers). Paint grade replica tape is used to measure surfaces containing a profile from 1.3 to 3.3 mils (33 to 84 micrometers), and x-coarse is used to measure surfaces containing a profile from 1.5 to 4.5 mils (38 to 114 micrometers). Replica tape for measuring surface roughness in excess of 4.5 mils (x-coarse plus) is also available, but is not addressed by the standard. (See Figure 7.)

An interlaboratory research initiative was performed on the paint grade replica tape a couple of years ago, and an interlaboratory study for coarse and x-coarse replica tape is in the planning stages (scheduled for early 2007). Revisions to the entire test method are also in process.

Pull-Off Adhesion of Coatings
The adhesion of a protective coating is a critical attribute that is related to performance. D 4541, Test Method for Pull-off Strength of Coatings Using Portable Adhesion Testers, has been developed to uniformly assess coating adhesion in the laboratory, shop, or field. Testing consists of securing a loading fixture to the coating surface with an adhesive. A test apparatus is attached to the loading fixture and aligned to pull perpendicularly. The force is gradually increased until the loading fixture detaches. The user reports the pull-off strength (in psi or MPa), and the location of the break in the coating system, i.e., adhesive between the primer and substrate, adhesive between other coats, cohesive within a certain coating layer, etc. While this test method maximizes tensile stress on a coating, other ASTM test methods assess shear stress, such as the tape adhesion test (described in ASTM D 3359, Test Methods for Measuring Adhesion by Tape Test). The pull-off method as currently written does not distinguish between testing on steel and concrete substrates.

The pull-off method is widely used by coatings manufacturers, specifiers, inspectors and coating specialists. Some coatings manufacturers report the pull-off strength on their product data sheets, and some specifiers require minimum pull-off strength for qualifying coatings. Pull-off strength tests may also be required as part of the execution of the project, and can be performed by inspection personnel. Coatings specialists may use the method for evaluating the adhesion characteristics of an existing coating during a coating condition survey, or in a failure investigation.

The most current version of D 4541 allows for the use of any of five instruments for evaluating pull-off strength, two of which are fixed alignment and three are self-aligning. Data published in Appendix X1 of the standard (generated by an earlier interlaboratory research initiative) reveals different pull-off strengths on four test specimens when using the various adhesion testers (at that time, only four of the five testers that are in the current standard were included in the research). As confirmed by a research paper presented at the SSPC 2001 National Conference,3 the adhesion values generated by the various test devices can vary considerably. As a result, what appears to be a very straightforward specification requirement (for example, “The adhesion of the coating system shall be measured in accordance with ASTM D 4541 to confirm that a minimum of 400 psi has been achieved”) can lead to controversy at the time of enforcement because many specifications do not clarify the type of adhesion tester to use. In this case, the minimum adhesion requirement becomes variable. Further, when data are generated using different types of pull-off testers, it is difficult to compare the tensile adhesion characteristics within a given generic coating category, or to compare the adhesion quality of different coating systems. (See Figures 8, 9, 10, 11, 12.)

In 2005, a plan was developed to conduct a comprehensive interlaboratory study involving six coatings, six laboratories and five types of adhesion testing equipment. The study was conducted in 2006 and is close to completion. The results of the interlaboratory study and the content of the research report will be reviewed in early 2007. The data will reveal the variations in testers and operators, and will be used to formulate new precision and bias statements. The entire standard is being revised, and will include (among other changes) more specific operating procedures for the various types of test equipment.

Pull-Off Adhesion of Coatings on Concrete
As stated above, the original version of D 4541 published in 1995 did not distinguish between testing the pull-off strength of coatings applied to steel versus concrete surfaces. While the test method is designed to quantify the adhesion strength of the applied coating system, the substrate beneath the coating cannot be ignored. Steel is a fairly homogeneous substrate and its tensile strength does not vary from test site to test site. Further, the tensile strength of steel far exceeds the tensile strength of an industrial protective coating. The coating system’s inherent tensile strength would yield to the rigors of the pull-off test long before the steel.

Conversely, concrete is a heterogeneous substrate. Portland cement concrete is a mixture of aggregate and cement paste. While concrete has an impressive compressive strength (often in excess of 28 Mpa (4000 psi)), the inherent tensile strength is only about 2.1 MPa (300 psi), which is less than the tensile strength of most coatings. As a result, there are differences in testing procedures that should be considered when evaluating coatings applied to concrete, versus coatings applied to steel. These include the size of the loading fixture, and scoring of the coating (based on inherent thickness) prior to attachment of the loading fixture. The specified minimum pull-off strength must also be carefully considered. For example, requiring a 5.2 MPa (750 psi) coating pull-off strength for a coating on concrete may be meaningless, as the tensile strength of the concrete is likely to yield at pull-off pressures between 2.1 and 2.8 MPa (300 and 400 psi).

Size of the Loading Fixture Contact Surface
When a loading fixture with a small contact area is used, the pull-off strength achieved may be significantly influenced by the composition of the surface of the concrete directly beneath the loading fixture. For example, a loading fixture inadvertently attached over a large piece of aggregate in the concrete will reduce the stress applied to the cement paste (the weaker constituent in concrete) and generate a highly variable pull-off strength tensile value that exceeds the inherent tensile strength of the concrete. Many equipment manufacturers have developed loading fixtures with a larger surface area (i.e., 50 mm or 2-inch diameter) for testing coatings applied to cementitous substrates.

Scoring the Coating
Cutting (scoring) through the coating prior to the attachment of the loading fixture has long been a controversial issue. As acknowledged in D 4541, scoring can cause micro-cracking in the coating, which may cause diminished pull-off strengths. The decision whether to score has been left to the parties requiring the testing and those conducting it. Generally, if it was not addressed in the specification, then scoring was not to be done. However, thick film coatings (i.e., greater than 500 micrometers (20 mils), like those often applied to concrete surfaces, must be considered differently. Some of the coatings are applied up to 3.2-6.4 mm (125-250 mils) in thickness, and the pull-off values obtained may be misleading due to the lateral adhesion properties of the coating (higher values obtained by pulling on an area of the coating beyond the area beneath the pull stub). So scoring is likely necessary on thick-film systems applied to concrete when trying to determine if the coating is firmly bonded to the substrate.

In response to these and other issues, Subcommittee D01.46 elected to prepare a standard test method (separate from D 4541) for testing the pull-off strength of coatings on concrete using portable adhesion testers. The new standard, D 7234, Test Method for Pull-Off Adhesion Strength of Coatings on Concrete Using Portable Pull-Off Adhesion Testers, was published in 2005 but without precision and bias statements. The data to support these statements will be generated via an interlaboratory study, which is planned for 2007. Conceptually, the study will include two types of concrete (normal and high strength), two coatings (thick film with scoring and thin-film without scoring), five types of adhesion testers and up to seven laboratories. Information relating to testing the tensile adhesion strength of coatings applied to surfaces other than steel will be removed from D 4541 during revisions that are also planned for 2007.

Surface Chloride Extraction and Analysis
The protective coatings industry has long recognized the potential consequences of coating over soluble salts (i.e., chloride), in terms of reduced coating performance and early deterioration of the steel substrate. Chloride contamination is the result of exposure to marine or coastal environments, chemical processes, or exposure to deicing materials (i.e., on bridge and highway structures). Soluble salts like chloride, when trapped beneath a newly applied coating, can draw water through the coating film (by a process known as osmosis) and cause blistering and underfilm corrosion. Since corrosion products expand as they migrate beneath a coating film, the coating may disbond, leaving the substrate unprotected. Elevated levels of chloride can also lead to section loss and pitting of the steel substrate, as chloride contamination can accelerate the corrosion process.

Early detection and removal of appreciable amounts of surface chloride contamination prior to coating installation can prolong the service life of a coating, reducing the frequency of maintenance painting and reducing life cycle costs.

Assessing the presence of surface chloride and quantifying the concentration is paramount to this process. Without reliable methods for extracting a sample from the surface and analysis of the collected sample, the degree and type of surface cleaning required prior to coating installation can be uncertain.

ASTM WK3049, Draft Practice for Assessing the Concentration of Soluble Chlorides on Metallic Substrates, provides procedures for four methods of extracting a surface sample, four procedures for analyzing the collected sample for soluble chlorides, and one procedure for analyzing a collected sample for total dissolved salts (conductivity). Other trade organizations (NACE International and the Society for Protective Coatings) are jointly developing a guidance document on acceptable levels of soluble surface chloride, based on the coating type and prevailing service environment. (See Figures 13, 14, 15.)

It is envisioned that the draft will be published in 2006 as a test method, but without precision and bias statements. A plan is currently under development for an interlaboratory study on the various methods used to extract and analyze samples, as described in the method.

Resistance to Graffiti
The resistance of coatings to graffiti and the ability to remove graffiti from coated surfaces is of interest to owners of buildings and other structures, as graffiti artists continue to deface public and private property. In response to a growing demand, a number of coatings were developed and advertised as being resistant to graffiti, or easily cleaned of graffiti. Coating manufacturers needed a standardized test method to demonstrate that their products worked, and owners needed a method that could be used to select products.

ASTM D 6578, Practice for Determination of Graffiti Resistance, was developed by D01.46 in 2000 to meet these needs. The practice identified four commonly used graffiti marking materials, five cleaners, and up to two removal methods (hand or automated). The cleaning agents are used in a particular order of increasing aggressiveness to determine which one can remove the graffiti, with limits on level of gloss reduction and color difference to the affected coating. Tests are done on smooth surfaces, and are not designed to address rough or porous surfaces like concrete block or brick, which would be substantially more difficult to clean without special tools and methods.

Coating manufacturers and others in the user community found the practice to be cumbersome and time consuming, and rarely did the facility owners require the level of testing dictated by the standard. As a result, a major revision to the standard was initiated in 2004 and continues today. There are many revisions being considered by the subcommittee. Some of the major ones include:

1. Elimination of the automated (washability tester) method of graffiti removal.
2. Total of seven possible cleaning materials rather than five.
3. Adjustment of the maximum color difference (of the original painted surface) after graffiti removal from ∆E1 to ∆E2, which is a slight relaxation in the amount of color difference in the coating caused by cleaning efforts.
4. Evaluating graffiti resistance on freshly applied coating as well as coating that has been artificially weathered in the laboratory.

As illustrated by this article, the dedicated leadership, committee member and interlaboratory study participation, and the knowledge and experience of those working in the task groups allows Subcommittee D01.46 to effectively and efficiently manage a full load of new and existing standards, in support of the industrial protective coatings industry. //

References
1 Roper, Hugh J.; Brandon, Joseph H; Weaver, Raymond E.F.; The Effect of Peak Count or Surface Roughness on Coating Performance, Journal of Protective Coatings and Linings, Vol. 22, No. 6
Roper, Hugh J.; Brandon, Joseph H; Weaver, Raymond E.F.; Peak Performance from Abrasives, Journal of Protective Coatings and Linings, Vol. 23, No. 6
2 NACE Standard RP0287, “Field Measurement of Surface Profile of Abrasive Blast Cleaned Steel Surfaces Using a Replica Tape”
3 Corbett, William D.; Tensile (Pull-off) Adhesion: Specifying the Test? Specify the Test Instrument! Proceedings of the SSPC 2001
National Conference

 
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