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All About Adhesion

FRIDAY, AUGUST 7, 2015

By Lee Wilson

As I have stated time and time again, if a protective coating does not stick to the substrate it is intended to protect, then it is of little use as a method of corrosion control. You see, my colleagues, it’s all about adhesion.

There have been many debates on social media sites recently regarding the main causes of protective coating failures, as well as many theories on strategies to prevent the same. Ultimately, however, adhesion can be considered the key to a protective coating’s effectiveness.

On the Surface

Put simply, and regardless of any theories and strategies, coatings generally fail in direct proportion to their lack of adhesion. It’s rather simple, really, as the stronger the bond is 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.

© iStock.com / Rufous52

Inadequate surface preparation usually ranks as the main culprit of adhesion-related coating failure.

For this very reason, promoting adhesion of a coating system is the primary objective of surface preparation in terms of surface cleanliness and the creation of a surface profile.

Regardless of social media debates, the reality is that system failure due to inadequate surface preparation (the main cause of adhesion failure) makes up approximately 80 percent of all protective coating failures.

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. As we can see, it’s not a small issue—particularly when someone has to pick up the tab.

What’s Behind Coatings Failures?

There are many causes of adhesion failure, and a coating inspector has to be very diligent and open-minded during any coating failure analysis. It’s very easy to say the paint is falling off; however, understanding why is a complete different story.

What we have to remember is that there are many causes and many different factors that can and do contribute to an adhesion-related failure of a protective coating system.

These factors include but are not limited to:

  • Inadequate surface preparation and the presence of surface contaminants;
  • The lack of a surface profile or failure to provide an anchor pattern or key;
  • Coating when the environmental conditions are inappropriate for the coating system;
  • The presence of soluble salts, i.e., chlorides, sulphates and nitrates;
  • Cathodic disbondment;
  • Insufficient wetting of the surface during application;
  • And, last and not least, poor coating formulation.

On that last point, although our friends in the coating manufacturing industry will swear to the quality of their coating formulas, trust me: mistakes are often made.

In my experience during failure analysis, the main causes are predominantly tied the top four on the list above. However, nothing can be ruled out.

© iStock.com / mollypix

Any foreign matter contamination present at the surface will greatly affect the forces at play, reducing adhesion in these critical areas.

Inadequate surface preparation usually ranks as the main culprit behind adhesion-related failure, though, because most failures can usually be directly attributed to insufficient surface cleanliness. This can be due to a number of factors, for example:

  • The surface cleanliness standard not being achieved.
  • Oil, grease, dirt, mill scale, etc., not being removed.
  • Dust and grit not being removed from the blast.
  • Moisture/dew point being reached before coating.

These are just a few; however, the list goes on.

With that said a focus on implementing measures to avoid these failures should be first and foremost on the minds of any project execution team (see my post on inadequate specifications).

Correct engineering and planning in regard to inspections—with a focus on eliminating or at the very least reducing the likelihood of known causes of adhesion-related failure—prior to preparation and application can greatly influence the success of any protective coating project.

Substantial problems can result from ignoring the importance of preplanning, compiling adequate specifications, and sufficient inspection and testing. It is the inspection detail and care taken before coating application that is paramount to the success of the system.

We have to remember adhesion testing will only calculate the adhesive strength of a coating, usually in random areas, whether or not the results are desirable.

The emphasis needs to be on prevention, ensuring that every precaution has been taken to promote adhesion between substrate and primer coat and intercoat adhesion.

© iStock.com / zarinmedia

The coating needs a key to take hold of or bite into to achieve the physical bond.

We have to remember that adhesion is created by both physical and chemical forces which interact at the interface of the coating film and the substrate. Simply put, any foreign matter or contamination present at this interface will greatly affect the relevant forces at play, reducing adhesion in these critical areas.

Failure to produce a surface profile during the surface preparation process also has an impact, as this greatly affects and reduces the primer-to-substrate adhesion. A key is vital for the coating to take hold of or bite into in order to achieve the physical (mechanical) bond—hence the term “anchor pattern,” a common term to denote surface profile.

An Ounce of Prevention

The sad truth is that adhesion-related failures are easily preventable, yet they remain the most common form of protective coating failure. As I have stated in many JPCL articles, the most common cause of these failures are simple ignorance, a lack of knowledge and a lack of inspection.

Because surface preparation is essential to establish a tightly adhered coating, the protective coating inspector can play a key role in ensuring that the necessary preparation has been achieved, monitored and accepted or rejected according to the specification requirements.

We also have to understand that many things can change during a coating project. I have personally witnessed huge fluctuations in temperature during a working shift, which results in condensation on the surface of the coating which minutes before was bone dry. This is an extreme example, but it happens.

And, as I stated before, poor formulation by protective coating manufacturers, although rare, should never be ruled out as a potential risk.

The list is endless, but it all comes back to the same thing: Adhesion getting that coating material to stick to the substrate it is intended to protect.

The Physical and the Chemical at Work

So far in this blog, and my JPCL articles, I have focused on the physical mechanisms of adhesion primarily due to surface preparation. We must remember, of course, that adhesion is created by physical and chemical forces interacting at the substrate surface and subsequent coating interface.

The physical is merely a simple bond between two materials. However, bonds attributed to chemical forces have a lot more to do with chemical compatibility and the ability of the coating to wet the surface and flow.

© iStock.com / PelicanKate

The emphasis needs to be on prevention, ensuring that every precaution has been taken to promote adhesion between substrate and primer coat and intercoat adhesion.

Manufacturers have to take into consideration a wide variation of resins, pigmentations and their combinations for the service life of a particular coating material. This requires a great deal of research and development on their end.

Research has come a long way. For example, take the early vinyl coatings based upon chloride and acetate copolymers. These materials had high cohesive strength but were extremely susceptible to adhesion-related failure until the addition of a third monomer, which resulted in the presence of polar reactive groups.

To put it simply, the groups of atoms (molecules) that make up coatings are held together by chemical bonds. They can be primary valence bonds or, in the case of materials with polar molecules, secondary valence bonds. Coatings adhere by both primary and secondary valence bonds. (More info on this can be found in Chapter 2 of Clive Hare’s Protective Coatings: Fundamentals of Chemistry and Compositions).

It’s clear that adhesion is a very complex subject, and one could spend a lifetime studying this one field. We inspectors unfortunately do not have that luxury of time. However, a grasp on the fundamentals always helps.

So to put it simply, without sufficient adhesion, coatings are a waste of a precious commodity. Any single factor or combination of factors that reduce adhesion will most probably result in an adhesion-related failure.  

Don’t get me wrong, the average coating inspector is not expected to have a degree in chemistry nor should they; they are there simply to ensure that the specification requirements are fulfilled. However, it is my belief that reading any literature in regard to the basics of primary and secondary valence bonding can only add to their field knowledge.

Let me close this post with an interesting fact taken from “Corrosion and the Preparation of Metallic Surfaces” (Unit 26 of the Federation Series on Coating Technology, FSCT 1978):

The strength of secondary valence attraction increases at a rate proportional to the sixth power of the intermolecular distance but does not become effective until the distance is under 5A. As this distance is no more than 3x the diameter of an oxygen atom the importance of clean well prepared surfaces in obtaining good adhesion can readily be appreciated. To simplify this guys grains of dirt or even mono molecular films of oil are considerably thicker than 5 A and will effectively nullify all adhesion.

The quote emphasises not only how crucial surface preparation is but also highlights the important connection between the physical and chemical bonds.

© iStock.com/Kozmoat98

Correct engineering and planning in regard to inspections prior to preparation and application can greatly influence the success of any protective coating project.

I am a firm believer that adhesion failures can be greatly reduced as long as there is a basic understanding of the predominant failure modes.

Investing in good project engineering, following a good specification, and proactive inspection carried out by competent inspection personnel who understand the field will certainly reduce the risk of adhesion failures and potentially save millions of dollars of rework.

ABOUT 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.

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Tagged categories: Adhesion; Coating failure; Surface preparation

Comment from Tom Schwerdt, (8/10/2015, 8:35 AM)

Failure in coatings is extremely complex. Allow me to provide a counterexample to the hypothesis that adhesion rules: The classic red lead primer. Typically applied over a minimally prepared surface (no blasting, apply directly to millscale) and topcoated. These coating systems typically have quite low adhesion values by modern industrial coating standards, yet regularly lasted 40+ years protecting steel structures outdoors as long as direct physical damage is minimized and the structure is at least somewhat inland. Lasting 60+ years with a single overcoat is not uncommon.


Comment from Clyde Morgan, (8/10/2015, 8:41 AM)

One of my biggest challenges regarding "Moisture/dew point being reached before coating". It seems every other month or so there is a paint system report comes through that states that either the dew point, temperature or humidity is out of the manufacturer's recommened range substantially. The manufactuer makes the call "it visually look good", throwing out thier own guidelines.


Comment from Lee Wilson, (8/11/2015, 9:45 AM)

Hi Tom Schwerdt, Thanks for your comments. I have to agree that Red Lead / Calcium Plumbate primers had good adhesion properties and were remarkably surface tolerant; however, you have no chance of seeing red lead specified in UK specifications due to the safety implications of using such products, although red lead was not banned as many people commonly believe. The 1992 European Union legislation stopped the sale of these types of paint to the general public. Rightly so, I may add; due to the risk of lead dust and subsequent health problems associated with exposure, red lead was phased out of UK specifications. I would agree as stated within the article that coating failure is extremely complex; however, I am sure that we all have to agree that a lack of adhesion is predominantly the main cause of most coating failures. I could name several major failures in the US and Canada which have resulted in huge financial claims. Pretty much all adhesion related failures whether that be attributed to surface preparation or one of the many other causes of adhesion related failure. I would imagine that any system lasting 40 to 60+ years must surely be located in a C1 or maybes a C2 environment in accordance with ISO 12944. For example you certainly wouldn't see that kind of system performance in a C5M environment regardless of the coatings generic type. I agree some coatings have better adhesion properties than others; however, ultimately, and as I have stated, if the paint don't stick it is of little use as a method of corrosion control for that simple purpose adhesion rules. Best regards Lee


Comment from Tom Schwerdt, (8/12/2015, 8:33 AM)

Hi, Lee! Good points - environments would be C1 through C3. I don't use the ISO standards, though I did mention "at least somewhat inland." Lead paint has been long discontinued here as well (and rightly so!) but given the lifespan I end up seeing it at end of life pretty frequently. Another example of "low adhesion, excellent corrosion protection" would be some of the wax tapes. Decades of performance, virtually no adhesive strength - they're mostly held in place by the tension of the tape wrapped around a pipe. As a bonus, they're still being used today with great success. Adhesion is critical for certain types of coating - epoxy would be the most common example.


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