I received a call from a contractor to do failure analysis on a bridge painting project. The contractor was being blamed for the failure, and a considerable sum of the money still owed him on the project was being held up until the issue was resolved.

The bridge was part of the interstate highway system in a state in the midwestern United States. The bridge was originally painted before lead laws were enacted and had been primed with a red lead primer. That was used frequently as a primer on steel because of its excellent passivation qualities. There are still steel structures out there that have red lead primers on them that have very little corrosion.

I have seen specifications for repaint projects where red lead was used as the original primer and the specification calls for finish coats to be removed but the red lead primer is to be left on the steel, as it is still protecting the steel from corrosion. On the bridge in question, this was the case.

Coating Compatibility

The specification on this bridge called for the red lead primer to be left on the steel, with repainting to consist of an intermediate coat and a topcoat. The contractor had removed the existing topcoat and intermediate coat but had left the red lead primer, which was in good condition, on the substrate.

The supplier of the coatings to be used had recommended applying a 100-percent solids clear epoxy sealer over the red lead before applying a coat of urethane as the topcoat. The sealer was chosen because, being solvent free, it should have been compatible with the existing red lead primer regardless of what generic type it happened to be. The sealer would also help bind the red lead and provide a good base for the urethane topcoat.

Everything seemed to be going well on the project, as the removal of the existing intermediate and topcoats was completed according to specifications and without any major problems. The 100-percent solids clear epoxy sealer was applied by spray and allowed to dry for the recommended recoat time before the urethane was applied — by spray application as well. There were no problems with the application of the sealer and urethane topcoat. It appeared as if the project was completed, so all equipment was demobilized and removed from the job.

Shortly after the contractor left the site, though, the coatings started to delaminate, and the owner withheld the final payment on the project — a considerable sum.

The owner’s representative determined that the surface preparation or application of the coatings had been faulty, resulting in the delamination. The contractor was notified that he would be held responsible for repairing the affected areas to bring them into conformance with specifications and the contractual documents. But after an investigation, the contractor decided to bring in a consultant to do a failure analysis to determine the exact cause of delamination before any repair work was started.

The contractor was not ready to accept responsibility for the failure and pay for the repairs until it was proven that the work his crew had done was faulty. I had done some consulting for the contractor in the past and he was familiar with my company, so I got the call.

Analysis Investigation

I traveled to the bridge and proceeded with the failure analysis. I collected samples of the delaminating coatings and found that there was red lead primer on the back of the delaminating sealer as well as the substrate. This was a cohesive failure within the layer of red lead primer; it took all succeeding coats off with it. But why was it happening?

When I enquired if any testing had been done before this coating system was applied, I was informed that none had been done. The owners had relied on the material supplier’s recommendation to use the 100-percent solids clear epoxy with the urethane topcoat.

It was not possible to determine what generic type of binder had been used in the red lead primer without having laboratory testing performed. Samples of the delaminating coatings were collected and sent off to a testing laboratory. The binder in the red lead primer turned out to be an alkyd type. Once I learned this and considered all the facts that I had gathered regarding this delamination, I was able to determine the cause of the failure: The cohesive failure of the red lead primer was being caused by the cross linking of the 100-percent solids epoxy sealer.

In the curing process of the epoxy, cross linking occurs and creates surface tension. In this case, the surface tension created by the cross linking was too much for the old alkyd to withstand, and the cohesive failure happened. The material supplier recommended the 100-percent solids epoxy sealer because a solvent-free coating should be compatible with most generic type coatings. And he was correct in that it was compatible with the alkyd binder in the red lead primer. However, he did not take into consideration that the surface tension created by the cross linking of the epoxy could cause the cohesive failure. The situation was exacerbated by the fact that the alkyd red lead was several years old, and it did not take a lot of surface tension to cause it to fail.

Key Takeaways

I provided a written report including copies of the laboratory findings with a full explanation of the cause of the failure and my recommendations for remedial action to the contractor. The contractor, in turn, sent my report to the bridge owner with the expectation that he would no longer be held responsible for the delamination and should receive full payment for his work on the project. However, this was not to be the case.

The bridge owner informed the contractor that my company was unknown to them and they would not accept my conclusion as to the cause of the delamination of the coatings on the bridge. The contractor challenged them to get a second opinion from a coatings consulting company of their choice. The owner agreed to do so and contracted with a company that was known to them to do a failure analysis of their own. This delayed things a bit, but when the owner’s consulting firm presented their report, they came to the same conclusion that I had as to the cause of the failure. This resolved the issue and the material supplier ended up being held responsible for the delamination of the coatings on the bridge. The contractor was relieved of any responsibility for the coatings failure and was paid in full for his work — and I had another happy client.

This failure could have been prevented if some testing had been performed before the whole bridge was painted. Preparing the surface and applying the coating system on some small test patches and then doing some testing on those patches would have revealed the surface tension problem with the 100-percent solids epoxy sealer, and a different system could have been used. It’s also helpful to keep in mind that you can’t rush these kinds of discussions. Although the owner’s investigation revealed the same results, had the contractor pushed his point, he may have been on the hook for some of the solution.

As the expression goes, hindsight is 20/20!

About the Author:

Malcolm McNeil has 65 years of experience in the coatings industry. He is a NACE International Certified Level 3 Coating Inspector, a Lead Instructor of the NACE Coating Inspector Program, a Society of Protective Coatings (SSPC) Protective Coatings Specialist, and the owner and operator of McNeil Coatings Consultants, Inc. McNeil has been teaching the NACE Coating Inspector Program, the NACE Bridge Inspection Program, and the NACE Protective Coating Specialist program for 20+ years. His company, McNeil Coatings Consultants, Inc., offers coating consulting services, coatings failure analysis, coating inspection, coating specification writing, training, lead testing, and expert witness. He is also the Technical Editor for CoatingsPro Magazine. For more information, contact: Malcolm McNeil, www.mcneilcoatingsconsulting.com