Part One of this blog described some of the difficulties associated with accelerated corrosion testing. The chemistry is complex. There are many microclimates to consider. And the list goes on. There is good news, however. We are not alone. Extensive amounts of research across all coatings areas is done and reported routinely.

As a Part Two blog on accelerated corrosion testing, here is a sampling of work done by the French Corrosion Institute, an organization that has done a great deal of work in the past with the European Coil Coating Association. The bullet points following the titles and attributions are my own comments from reading the documents:

“Hydrolysis of interfacial bonds in a metal/polymer electrical double layer,” Nazarov, A.P., Thierry, D. Protection of Metals and Physical Chemistry of Surfaces (2005)

• In this work, the bond between the paint and metal substrate are probed using Electrochemical Impedance Spectroscopy (EIS). This technique is used to monitor the moisture as it permeates the coating and reaches the polymer-metal interface. The study found that hydrolysis at this interface is a source of a corrosion defect area.
• Observation: Studying the interface between paint and metal is very complex. Not everyone has the technology and expertise available to do this kind of analysis. EIS is a particularly powerful tool to perform this kind of experiment.

“A model for the release of chromate from organic coatings,” T. Prosek, D. Thierry, Progress in Organic Coatings, April 2004

• Here the well-known chromate-leaching phenomenon is studied, again using EIS. The researchers found that leaching is independent of temperature, pH and chloride concentration and is necessary for corrosion prevention.
• Observation: Nothing surprising, but FCI demonstrates an ability to probe and understand complex ion transport mechanisms. While EIS is a simple tool, it requires a real expert to understand the data generated. Anyone can generate the data, but few can make sense of it.

“In situ studies of the corrosion during drying of confined zinc surfaces,” D. Persson, A. Mikhailov, D. Thierry, Materials and Corrosion, Volume 58, Issue 6, June 2007, Pages 452–462

• This study is applicable to automotive and construction applications. In Coil, we call the failure Wet Stack Storage corrosion. The work demonstrates that localized corrosion in a confined area can be worse than corrosion in an unconstrained condition (an observation that many of us have made over the years). Transport properties of the moisture (studied by FCI earlier) help to explain the effect. This paper is an excellent example of FCI striving to understand the mechanisms that drive the defect.
• Observation: Understanding mechanisms in this study is dependent upon other work from earlier studies. This is a typical approach: Use past learnings to continually push the level of understanding. In a typical industry laboratory, there is little capability—nor time—to get to the point of understanding mechanisms.

“Composition of corrosion products formed on Zn–Mg, Zn–Al and Zn–Al–Mg coatings in model atmospheric conditions,” T. Prosek, D. Persson, J. Stoulilc, D. Thierry, Corrosion Science, Volume 86, September 2014, Pages 231–238

• In this work, the corrosion product chemistry was studied to make certain that the right corrosion chemistry was occurring. Accelerated (“model atmospheric”) testing results are being studied. X-ray Diffraction and Fourier Transform Infrared spectroscopy used to determine the chemistry. Although the Mg-modified metallic layers showed less corrosion than HDG or Galvalume, no firm conclusions could be drawn from this study about the mechanisms that explain the improved performance where Mg is present.
• Observation: In a scientific environment, careful testing of hypotheses is paramount, and drawing proper conclusions can be a frustratingly slow process, requiring multiple studies to find confirmation.

“New cyclic test for building coil coated materials,” Conference Paper, September 2013, Galvatech, Beijing, China, N. Lebozec, D. Thierry, P. E. Augustsson, L. Opdam

• The objective of this work was to define reliable testing conditions for coil-coated materials with respect to cut-edge performance. It carries forward work done in a similar project for the automotive industry. Good correlation was found with an accelerated test that also incorporated an SO₂ exposure phase. Working with SO₂ is very difficult; this project strives therefore to eliminate the need for an SO₂ cycle. The works suggests that this electrolyte solution be used for accelerated testing:
  • 3% NaCl
  • 2% Na2SO4
  • pH 3.0
  • One-hour exposure to electrolyte solution, three times a week.
  • Alternating high and low humidity (90% RH and 35% RH, respectively) for 4 and 8 hours, respectively
  • 84-day test cycle (2000-hour test)
• Observation: Good correlation to seaside exposures, but does it predict field performance of real buildings in a variety of environments? More work is needed. Understanding corrosion is more than simply measuring scribe and edge creep. Understanding mechanisms is never simple, and agreeing on the chemical mechanism is even more difficult.

If you would like to learn more about the French Corrosion Institute, please visit their website.

Future blogs will discuss the difficulties with accelerated corrosion from other perspectives.

-David A. Cocuzzi, NCCA Technical Director