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Measuring the surface profile of blast-cleaned steel and operating procedures for salt spray (fog) apparatus are the subjects of two standards recently revised by ASTM International.
D4417: Measuring Surface Profile
ASTM D4417: Standard Test Methods for Field Measurement of Surface Profile of Blast Cleaned Steel describes techniques for measuring the profile of abrasive-blast-cleaned surfaces in the laboratory, field or fabricating shop.
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Sui Generis International Ltd. |
| ASTM D4417 describes techniques for measuring the profile of blast-cleaned steel in the laboratory, field or fabricating shop. |
As ASTM notes, the height of surface profile has been shown to be a factor in the performance of coatings applied to steel. Therefore, surface profile should be measured before coating application to ensure that it meets specifications.
Methods, Instruments
Optical microscope methods serve as a referee method for surface-profile measurement. Profile depth designations are based on the concept of mean maximum profile, which is determined by averaging a given number (usually 20) of measurements (from the highest peak to the lowest valley) made in the field of view of a standard measuring microscope.
This is done, ASTM says, because of evidence that coatings performance in any one small area is primarily influenced by that area’s highest surface features and not by its average roughness.
The instruments described in the standard are portable and sturdy enough for field use.
ASTM also notes that there are additional techniques suitable for laboratory use not covered by these test methods.
The standard was revised by Committee D01.46 on Industrial Protective Coatings and published in ASTM Volume 06.02 Paint -- Products and Applications; Protective Coatings; Pipeline Coatings.
B 117- 11: Salt Spray (Fog) Apparatus
ASTM B117 - 11 Standard Practice for Operating Salt Spray (Fog) Apparatus covers the apparatus, procedure and conditions required to create and maintain the salt spray (fog) test environment.
This practice provides a controlled corrosive environment used to produce relative corrosion resistance information for specimens of metals and coated metals exposed in a given test chamber.
ASTM notes that prediction of performance in natural environments has seldom been correlated with salt spray results when used as stand-alone data.
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NACE |
| ASTM B117 - 11 covers the apparatus, procedure and conditions required to create and maintain the salt spray (fog) test environment. |
It also warns that correlation and extrapolation of corrosion performance based on exposure to the test environment provided by this practice are not always predictable. Therefore, the organization advises, correlation and extrapolation should be considered only in cases where appropriate corroborating long-term atmospheric exposures have been conducted.
The reproducibility of results in the salt spray exposure is highly dependent on the type of specimens tested, the evaluation criteria selected, and the control of the operating variables. Variability has been observed when similar specimens are tested in different fog chambers, even though the testing conditions are nominally similar and within the ranges specified in this practice, ASTM says.
In any testing program, sufficient replicates should be included to establish the variability of the results.
The standard was revised by Committee G01.05 on Laboratory Corrosion Tests and published in ASTM Volume 03.02 Corrosion of Metals; Wear and Erosion.
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