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SIMULATION-                                                                                   èSMALL GRAINS WITH BIG
LED STRATEGY                                                                                  IMPACT
PREVENTION                                                                                    Pitting corrosion occurs in a metal when
                                                                                              electrochemical reactions and mass
Costing billions of dollars annually, corrosion is everyone’s                                 transport in an adjacent electrolyte
problem. Fundamental research in corrosion science at the                                     solution result in localized loss of material,
Naval Research Laboratory will enable scientists to design                                    as shown in Figure 1. “The pit keeps
materials that inherently prevent corrosion.                                                  growing and eventually the material or
                                                                                              component will fail under load,” says
by JENNIFER A. SEGUI                                                                          Qidwai, emphasizing the effect that
                                                                                              corrosion can have on the strength or
FIGURE 1. Example of pitting corrosion (top-down view) in an aluminum alloy clearly           integrity of a material.
demonstrates the characteristic localized loss of material. The formation of pits can reduce  	 Qidwai and his colleagues have come
the strength of a material. Image courtesy of C. Feng and S. Policastro, NRL.                 up with an innovative and comprehensive
                                                                                              approach to better understand pitting
Corrosion is a complex multiphysics          and its application to practice is directly      corrosion. “We are modeling the
problem that is currently under              reflected in the high societal cost of           growth of corrosion pits in metals in a
investigation by Siddiq Qidwai, a            corrosion.” Based on figures reported in         seawater environment,” he explains.
mechanical engineer, and his colleagues      December 2010, as much as 600 billion            “The microstructure of the metal has not
at the Naval Research Laboratory (NRL)       dollars, that is 2 to 4 percent 
of the U.S.     been the focus of attention in previous
in Washington, D.C. “In the long run,        gross national product, were spent to            work and consequently the challenges
the success of our research will result      repair or prevent corrosion damage.              associated with irregular growth due to
in microstructure-corrosion correlations     	 The transportation industry including          microstructure have not been considered.
that will enable material designers to       sea, air, and ground transport is                Our goal is to perform fully-coupled
include or preclude certain features in the  particularly affected by corrosion               multiphysics modeling of pit growth
development of new corrosion-resistant       where maintenance costs to preserve              under the application of mechanical
materials,” explains Qidwai.                 passenger safety and vehicle longevity are       forces to quantify the overall effect
	 A 2011 National Academy of Sciences        extremely high. “For the Navy specifically,      on structural integrity with material
report from the National Research            corrosion is the number one maintenance          microstructure taken into account.”
Council in the U.S. states that a “lack of   problem,” 
says Qidwai.                          	 The irregular corrosion growth due
fundamental knowledge about corrosion                                                         to the metal microstructure is shown
                                                                                              schematically in Figure 2 and arises
                                                                                              because of the unique size and shape
                                                                                              of each individual grain. Each grain can
                                                                                              also have a particular crystallographic
                                                                                              orientation that can affect the corrosion
                                                                                              rate or front movement locally. Secondary
                                                                                              phases, precipitates, and twin boundaries
                                                                                              are additional features of a metal that
                                                                                              can affect the initiation and growth of
                                                                                              corrosion pits.

                                                                                              SIMULATIONS WITH METAL

                                                                                              “A complete description of pit growth,”
                                                                                              explains Qidwai, “requires the coupling
                                                                                              of electrochemical and mass transport
                                                                                              equations for multiple ionic species and
                                                                                              constitutive descriptions of reaction rates
                                                                                              and species diffusion in the electrolyte,
                                                                                              while tracking the metal-electrolyte
                                                                                              interface or corrosion front whose
                                                                                              movement depends upon the history
                                                                                              of the solution.” Figure 2 depicts the
                                                                                              complex corrosion mechanism simulated
                                                                                              by Qidwai in COMSOL Multiphysics®
                                                                                              software and Figure 3 shows the
                                                                                              corresponding model geometry used to
                                                                                              evaluate pit growth in metals.

                                                                                              COMSOL NEWS  19
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