3D Corrosion Modeling of Coating Defects

Almost every corrosion engineer is aware of the immense cost for which corrosion is responsible.  A frequently quoted reference is that corrosion costs the US roughly 3% of their GDP annually [1].  This makes corrosion a major concern for many industries: ranging from Oil & Gas to Utilities to Automotive and Aerospace.

Two important ways to prevent corrosion are cathodic protection and protective coatings.  As an example, the automotive industry combines these strategies by applying galvanized zinc coatings followed by layers of polymer coatings to protect car bodies.

New polymer coatings are vetted with the use of accelerated corrosion testing.  In these tests, extreme temperature and humidity cycling is performed to accelerate the corrosion process.  However, many questions exist as to how these changes may effectively change the underlying corrosion mechanisms.  If the underlying mechanism goes through an intrinsic change, applicability of accelerated test results to the field is suspect.  In addition to this uncertainty, accelerated corrosion tests are very expensive and can take months to perform.

Physics-based computer modeling provides an important complement to accelerated testing that can be performed in a matter of hours rather than months. Physics-based computer modeling in the form of computation fluid dynamics (CFD) has a strong tradition in the Automotive Industry that allows engineers to gain understanding, drive innovation, and efficiently evaluate prototypes.  Corrosion modeling is a relatively new concept to many of these engineers, but the motivations are similar: to identify key factors in order to control and mitigate corrosion damage and cost. For such models to be effective, they must accurately capture the key physical processes and geometry, but not necessarily every mechanistic nuance.  As long as key physics are included, result trends can provide insight into effects of geometry or chemistry changes from known base-case conditions.  This is similar to the strategies used with accelerated lab testing.

In this work, 3D numerical simulations are used to examine the mechanisms responsible for delamination of coatings on galvanized and non-galvanized substrates.  Delamination of coatings on non-galvanized steel is driven by a cathodic reaction and diffusion of salts underneath the coating [2], [3], [4]. In contrast, delamination rates of galvanized coatings can be affected by both anodic and cathodic processes, and may be controlled by anodic dissolution of the sacrificial zinc coating [5], [6], [7], [8].

A combination of parametric studies and scaling analysis is used to better understand and predict the influence of kinetic, material, and geometric properties. Applying mechanistic models to realistic 3D geometries provide unprecedented insight into these important processes. References

[1]	G. H. Koch, M. P. Brongers, N. G. Thompson, Y. P. Virmani and J. Payer, "Corrosion Costs and Preventive Strategies in the United States," NACE International, 2002.
[2]	A. Leng, H. Streckel and M. Stratmann, Corrosion Science, vol. 41, no. 3, pp. 547-578, 1998.
[3]	A. Leng, H. Streckel and M. Stratmann, Corrosion Science, vol. 41, no. 3, pp. 579-597, 1998.
[4]	A. Leng, H. Streckel and M. Stratmann, Corrosion Science, vol. 41, no. 3, pp. 599-620, 1998.
[5]	W. Furbeth and a. M. Stratmann, Corrosion Science, vol. 43, no. 2, pp. 207-227, 2001.
[6]	T. M. Amorim, C. Allély and J. P. Caire, in COMSOL Users Conference, Grenoble, 2007.
[7]	W. Furbeth and a. M. Stratmann, Corrosion Science, vol. 43, no. 2, pp. 229-241, 2001.
[8]	W. Furbeth and a. M. Stratmann, Corrosion Science, vol. 43, no. 2, pp. 243-254, 2001.
[9]	K. N. Allahar, M. E. Orazem and K. Ogle, Corrosion Science, vol. 49, no. 9, pp. 3638-3658, 2007.