Myth or Fact: Higher Surface Profile Increases Coating Adhesion (Part 1)
As the corrosion control industry adopts new coating technologies, the debate continues regarding best practices in steel surface preparation requirements and profile height for new, thicker coatings.
Driven by a need for increased coating life where new ultra-high-build (UHB) coating technologies are used, surface profile is a regular topic of conversation with contractors and inspectors. There are many inconsistencies in the industry with profile requirements for these super-thick coatings.
The traditional thought has been: “The thicker the coating, the higher the profile required.” On the other hand, the rule of thumb is to have no more than one-third surface profile height compared to the total coating thickness. Coating data sheets and customer specifications often conflict, and there seems to be little understanding of what is best practice. (For more on this topic, see Questioning Current Methods in Defining Proper Coating Surface Profile.)
In an attempt to understand the effect of surface profile height on the adhesion of UHB epoxies, Blast-One International conducted testing of one coating on steel panels with different profile heights and different total coating thicknesses. This research is described below.
By looking at the findings of this and other studies, and comparing them with traditions in the industry, it is easy to see where misunderstandings can occur with protective coatings contractors. Coatings contractors have much on their minds while on project sites. Surface profile and surface cleanliness are so important; the old saying goes that “a coating is only as good as the surface preparation.” With that in mind, it is time to pay attention to the surface preparation as a whole.
Surface Profile Measurement
We have some evidence to clarify the effect of high or low surface profile and to clear up the myth that a higher surface profile is necessary to increase coating adhesion. This study is a start to increase our understanding and reduce confusion on best practices for surface profile.
Many coating product data sheets call for surface profiles of 75–100µm. Others call for 38–50µm surface profile for a very similar coating type. Why should this be? To start to understand this, we need to first look at why we create and measure surface profile on steel surfaces. It comes down to two main reasons:
- Abrasive blasting creates peaks and valleys, thereby increasing the surface area to provide a better bond between the coating and steel substrate. We have always thought that an acceptable increase in surface area was about 33%. More recent studies have shown that it is more likely to be 16–18%, but this is still being verified by independent testing.
- The assumption is that by blasting the surface, some cleaning will take place, which will remove contaminants that would impede adhesion between the coating and the substrate.
Surface profile is also known by other names, such as anchor pattern and surface roughness. Classifications of surface roughness include maximum roughness depth (R-max); roughness average (R-a); and total peak to valley profile height (R-t). For the technically minded, the coatings industry has typically measured R-t, which is the total height from the lowest valley to the highest peak in a given area.
The R-t measurement has some limitations because it is affected by rogue peaks, but it is a generally accepted method of measuring the surface profile in the coatings industry.
Figure 1 – Surface Profile:
The author experienced these issues firsthand, early in his career, on a new-build project for an unmanned offshore gas platform. The specification called for a 75–100 micron profile based on a specification of a 3,000 micron UHB epoxy coating to be applied to the splash zone. Their client’s representative said, “If the range in the specification is 75–100 microns, I want closer to 100 microns.” In other words, more is better. Of course the contractor said, “Yes, sir!”
Incidentally, the inspector had his own measurement challenges and insisted that the profile was only 33 microns. I witnessed the client spending tens of thousands of dollars to achieve a profile that appeared to be unnecessary to achieve the right surface for the coating to be applied. (Discover one way to measure surface profile in the article Replica Tape: A Source of New Surface Profile Information.)
Another confusing aspect of surface profile is units of measurement. For example, some coatings suppliers in Australia ask for a profile of 88 microns. Eighty-eight microns is an unusual number, which appeared simply from the conversion from mils or thous (thousands of an inch) to microns. In other words, 3.5 mils equals 88 microns.
Imperial vs. Metric
Many of the specifications and data sheets that we use today are converted from the United States, where Imperial units like mils/thous (1/1000 of an inch = 25.4 microns) are commonly used. When we measure profile and the specification has been converted from an Imperial standard (for example 2.5 mils) to the equivalent metric standard (i.e. 63 microns), it’s a little bit like comparing analogue time with digital time.
Figure 2 – Lost in Translation?
If we say, “I’ll see you at half past 2,” and in reality the meeting started at 2:26 or 2:34, no one would really mind. But if it is digital and you say the meeting starts at 2:30, in that case 2:29 is early, and 2:31 is late.
Applying that rationale, one of the most common methods for measuring surface profile is Testex replica tape. If the specification calls for 3 mils and the actual reading is 2.75 mils, it is only 1 increment away on the gauge face and the inspector may say, “It is close enough; I’ll pass it.”
But if you have the same reading in microns, and the specification says it must be 75 microns (3 mils is about 75 microns), and a reading of only 69 microns is achieved (the equivalent to 2.75 mils), that reading hasn’t even got a 7 on the front and the inspector may be likely to say, “No, it’s a fail. It needs to be re-blasted to get a higher profile.”
These challenges that confront the industry are merely pointed out as areas that leave clients, contractors and inspectors in a position where they feel that they need to make their own interpretation. They are making on-the-spot decisions, all with good intent, but these decisions may have the hidden effect of unnecessarily costing someone, often the contractor, a lot of money.
In Part 2 of this article series, the author explores five independent studies with relevance to whether higher surface profile increases coating adhesion. In addition, results of a test conducted by Blast-One are presented along with interpretations that have bearing on determining best practices for surface preparation.