PAINTSQUARE BLOG

As many of my readers are aware, I have always been intrigued by standards. I recently carried out a review and blogged about the ISO 8501-1 visual assessment of surface cleanliness standards, which raised a great deal of interest and discussion within the industry.

For this blog, I turn my attention to the ISO 8502-3 Standard, Preparation of Steel Substrates Before Application of Paints and Related Products – Tests for the Assessments of Surface Cleanliness, Part 3: Assessment of Dust on Steel Surfaces Prepared for Painting (Pressure-Sensitive Tape Method).

There are many positives about this standard, but there are also many questions that come to mind when attempting to apply it. We should always strive for more clear standards, and I think there's room for improvement here.

Surface Prep and Adhesion

It is well known and accepted throughout the coatings industry that the performance of any protective coating system applied to steel is greatly influenced by the amount of surface cleanliness carried out and the remainder of surface contaminants prior to coating or painting applications.

© iStock.com / nightman1965

Promoting the adhesion of a coating system is the primary objective of surface preparation.

Coatings generally fail in direct proportion to their lack of adhesion. It’s rather simple, really, as the stronger the bond between a coating system and a substrate, the longer the coating will continue to create a barrier against corrosion, eliminating the requirements needed for a complete corrosion cell. Promoting the adhesion of a coating system is the primary objective of surface preparation, including the establishment of surface cleanliness and the creation of a surface profile.

Regardless of whether you are applying the latest and greatest protective coating, if you have not cleaned the substrate, you can expect premature failure of your protective coating, or a reduced service life at the very least.

This is causing chaos across the industry: I have personally attended arbitration cases where adhesion-related failure has ultimately cost hundreds of millions of dollars. It’s no small issue, particularly when someone has to pick up the tab.

There are many standards currently in circulation intending to assess the factors which may lead to adhesion-related failure; these include ISO8501-1, ISO8502 and ISO 8503. ISO8502-3 was developed in order to ascertain the levels of dust contamination that remains upon the substrate after abrasive blasting.

It is well known that excessive levels of dust on blast-cleaned surfaces may significantly reduce the adhesion of any subsequently applied protective coating systems and, by absorbing moisture, may also promote and accelerate corrosion of freshly blast-cleaned surfaces.

It is clear that there is a requirement for testing of the levels of dust remaining on the substrate after dry abrasive blasting. However, I pose the question: Does the standard ISO8502-3 do this?

Subjective Measures

Because of subjective factors in the test procedure, the test does not indicate the precise determination of dust retained on blast-cleaned steel surfaces. This is clearly stated within the standard. The standard states that nevertheless, when the test is carried out by experienced operators, and particularly when used to compare the performance of surfaces under test with agreed standard specimens, it gives very useful information.

As an inspector, I find this statement highly ambiguous, and this leaves the standard wide open to misinterpretation, with a potential to cause widespread chaos in the field.

The principles of this test method are rather simple: A pressure-sensitive adhesive tape is pressed onto the steel surface that has been dry abrasive-blasted. The tape, with any adhering dust, is then removed from the substrate and placed onto a display board of a color that contrasts with that of the dust. The inspector then examines it visually, estimating the quantity of dust adhering to the tape and the dust particle size.

© iStock.com / GlenJ

It is well known that excessive levels of dust on blast-cleaned surfaces may significantly reduce the adhesion of any subsequently applied protective coating systems.

This brings me back to subjective factors: The test procedure does not allow nor indicate the precise determination of dust retained, and is highly dependent upon the experience of the inspector carrying out the pressure-sensitive testing.

Another major drawback of this test method and the standard requirement is that there are many variables in conditions at a site, and complexities in the substrates that have been abrasive blasted. The standard states that agreements made between interested parties, where appropriate, should include the number or frequency of tests.

Here is my problem: Should the standard not include the number or frequency of tests, as well as test locations? Many other standards do this as a bare minimum, and this saves major confusion and ambiguity on projects.

For example, it is well known and widely accepted that dust accumulates more naturally on horizontal surfaces, the internals of pipework and structural cavities. The standard states that special inspection should be carried out to ensure that such areas are adequately cleaned and free from dust before painting. If the standard recognizes that these areas are potential hot spots for dust accumulation, should a minimum frequency be specified within these areas?

We have to take into consideration that the scope of ISO 8502 describes a method for the assessment of dust remaining on cleaned surfaces prepared for painting. It provides pictorial ratings for the assessment of the average quantity of dust. (Again, this average quantity is not precise as previously stated in the standard, and highly operative dependent.) The standard also provides descriptive classes for the assessment of the average size of the dust particles.

The method may be carried out as a pass/fail, accept/reject test by assessing the quantity of dust present on a test surface, and the average dust particle size, in comparison with specific limits. But what are these specific limits?

Test procedure

Despite the issues I have provided above, which are clearly problematic areas with the standard, it is the test procedure that I personally find creates the most problematic field-testing concerns and issues. There are two procedures available to the inspector: methods A and B of Section 6.2 of the standard.

The standard states that the pressure-sensitive tape about 150mm long should be placed firmly onto the surface and one of the following alternative methods given in Method A and Method B applied:

Method A states: “Place the thumb across one end of the tape and move the thumb, while maintaining a firm pressure, at a constant speed along the tape three times in each direction so that each stroke takes between 5 and 6 seconds. Remove the tape from the test surface, place it on an appropriate display board and cause it to adhere to the board by rubbing with the thumb.”

Now I ask the questions:

• What is a firm pressure?
• What is a constant speed?
• How do I remove the tape?
• How do I rub the tape onto the display board?

Method B states: Apply a calibrated spring-loaded roller centrally across one end of the tape and move the roller, while maintaining a downward load between 39.2 Newtons and 49.0 Newtons at a constant speed along the tape, three times in each direction, so that each stroke takes between 5 and 6 seconds. Remove the tape from the test surface and place it on an appropriate display board causing it to adhere by rubbing with the thumb.”

At least with this method, I have a force for applying pressure to the adhesive tape, so I have to ask: If I have a specified force for pressure with one method then why do I not have a specified force for the other? We know the answer to this, of course: We can’t measure the force when using only a thumb in the test. So why have Method A as an option when this can be and is seriously abused in the field?

Supplied by author

The standard contains a reference table that the tape tests are then compared against.

The standard contains a reference table which the tape tests are then compared against. The predominating dust particle quantity and size are compared to the standard chart and designated from 1 to 5.

Discoloration and Sufficiency

Another major issue with regard to the standard is Section 6.4, which states: “Report any overall discoloration as size class 1.” This then refers the reader to Note 5.

Note 5 goes on to state: “It is not unusual after carrying out this test, to find that the tape displays an overall discolouration, usually reddish brown or black, sometimes with the presence of discrete visible particles, depending on the type of abrasive used. The discoloration is caused by microscopic dust from the test surface that can cause serious interference with paint adhesion.”

If, in the words of the standard. this discoloration can cause serious interference with paint adhesion, then why is this not labeled as a fail/rejection as per the standard?

Section 6.5 of the standard states: “Carry out a sufficient number of tests to characterize the surface under test.” Again I have to ask, what is sufficient? This has the potential to cause chaos whilst in the field. The same section then goes on to say that for every surface of one particular type and aspect, carry out not less than three separate tests. If the results do not have a spread of one or less quantity rating, carry out at least two additional tests to establish the mean!

And that’s all it is: a mean average with no acceptance or rejection criteria, and again, wide open to interpretation.

I could go on with regard to this standard; however, I will save it for another blog. There are many other issues with this standard; however, there are many positives. I am not saying the standard is not adequate, but I do believe that it could use a serious revamp.

ABOUT THE THE BLOGGER Lee Wilson Lee Wilson, CEng, FICorr, is a NACE Level 3-certified CIP Instructor, NACE Corrosion Specialist, NACE Protective Coating Specialist and Senior Corrosion Technologist, as well as an ICorr Level 3 Painting Inspector and Level 2 Insulation Inspector. The author of the best-selling Paint Inspector’s Field Guide, Lee was named one of JPCL Top Thinkers: The Clive Hare Honors in 2012. Contact Lee. SEE ALL CONTENT FROM THIS CONTRIBUTOR