CORROSION IN THE NANOSTRUCTERED REGIME

Nanostructured materials are an emegin class of materials that show some interesting properties unfamiliar to those working in the larger scale of microns. Nanostructured materials are typically defined as those materials that are, or are composed of particles less than 100nm in size.

Here the corrosion behavior of some of these materials is discussed. Not much work has been done in this area, the examinations have been investigated chiefly by electrochemical methods. A refresher of the kinetics of corrosion might be useful.

Nanocrystalline Nickel

Rofagha et al.¹ Investigated the electrochemically behavior of nanocrystalline nickel (32nm grain size) compared to cold rolled annealed nickel with a grain size of 100microns in 2N H₂SO₄ by observing the behavior as the potenital was varied on the sample ( see kinetics ).

The resulting potentiodynamic curves are shown in figure 1.

comparison of the electrochemical behavior of Ni in the nanocrystalline and annealed stages. Rofagha et. al.¹

This group concluded that the high defect denisty catalyzed the hydrogen reduction ( increasing the cathodic current), reduced the kinetics of passivation, compromised the stability of the passive film. This can be seen by the anodic shift of E_corr and the higher current denisty in the nanocrystalline case.

Basically, because of the small size of the nickel grains in the nanocrystalline case, there were more sites ( grain boundaries and triple juntions) for reduction to occur- thus increasing the corrosion rate of the nanocrystalline nickel over the annealed nickel.

304 Stainless Steel

Inturi et. al.² preformed similar investigations into the electrochemical behavior of 304 stainless steel. Conventional 304SS ( grain size 30microns) was compared with nanocrystalline 304SS with grains 25nm in diameter. Anodic polarization was carried out in a 0,3wt% NaCl solution. Cl ions can cause problems by breaking down the passive film that naturally forms on stainless steel leading to localized corrosion.

Figure 2. Anodic polarization curves comparing 304SS in the nanocrystalline (sputtered) state and with conventional grain size.²

Both the nanocrystalline and conventional materials showed pitting and crevice corrosion after polarization. Figure 2. shows that there is a wide difference in the breakdown potential and current denisty between the nanocrystalline adn conventional stainless steel. But, in this case it was found that the nanocrystalline materials outperformed the conventional grain size materials.

This was accounted for by the high degree of larger number of defects on the surface allowing the Cl^- to be spread more evenly over the surface which causes a decrease in Cl^- at any on epoint on the surface. With a decreased concentration of Cl^-ions on the surface the necessary driving force of localized corrosion is increased.

References

¹Rofagha et. al. "The Corrosion Behavior of Nanocrystalline Nickel",Scripta Metallurgica et Materilia, Vol.25, 1991, pp.2867-2872.
²Inturi R.B., Szklarska-Smialowski Z.,"Localized Corrosion of Nanocrystalline 304 Type Stainless Steel Films"Corrosion, May 1992, pp. 398-403.

Other References

El Kedim O. et. al.,"Corrosion Behavior of Nickel Coating Obtained by Ball Milling",Materials Science Forum, Vol. 225-227, 1996, pp.825-830.

Gonzalez, F. et. al.,"Electrodeposited Nanostructered Nickel for in-Situ Nuclear Stream Generator Repair",Materials Science Forum, Vol. 225-227, 1996, pp.831-836.

Leth-Olsen, Hakon, et.al.,"Formation of Nanocrystalline Surface Layers by Annealing and Their Role in Filiform Corrosion of Aluminum Sheet",Jouranl of the Electrochemical Society, Vol.144, No.7, July 1997, L196-7.

Phaneuf, M.W., et.al., "Direct Observation of Amorphous and nanocrystalline Phases in Commercial Galvannealed Steel Sheet",Scripta Metallurgica et Materialia, Vol. 31, No 1., 1994, pp. 1-4.

Schneider, M., et.al.,"Electrochemical Behavior of Nanocrystalline FeAl8 and FeCr10 Alloys"Materials Science Forum, Vol. 225-227, 1996, pp.819-824.

Zeiger W. et. al., "Corrosion Behavior of a Nanocrystalline FeAl18 Alloy",Nanostructered Materials, Vol.6, 1995, pp.1013-1016.

Obviously, there is plenty of room for more work in this area. In the two papers presented thus far we have seen two different behavior for two different reasons based on the same fact (that there is an increase in the defect density in nanocrystalline materials). Hopefully, in the future, more careful studies can be made to sperate out grain boundary effects from environment and material effects so that the corrosion behavior of nanocrystalline materials can be understood. The information presented here are an indication of the type of things that need to be condsidered in these type of tests.

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