**8. Concluding remarks**

Nanocrystalline materials are being investigated due to their unique properties. More generally, development of materials resistant to environmental degradation is not the main focus of nanocrystalline metals research to date, but it seems possible that nanocrystalline metallic materials may lead to a substantial increment in oxidation resistance; caused by promoted oxide scale formation, improved adherence and reduced spallation tendency of the oxide scale. Nanocrystalline Fe-Cr and M-Cr-Al alloys have demonstrated improved oxidation resistance and present potential to be used for high temperature applications in future. More fundamental investigations are required to fully characterise the oxidation phenomenon and underlying principles for nanocrystalline materials.

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**11** 

*Bulgaria* 

**Corrosion Behavior of Stainless Steels** 

The modifying of the surface, which involves altering only the surface layers of a material, is becoming increasingly important with the aim to enhance the corrosion resistance of many kinds of materials. The advantage of this approach lies in the fact that the natural physical and mechanical properties of the material are retained, while at the same time the corrosion resistance is increased. It is well known that electroplated zinc coating is employed as active galvanic protection for low and middle-content alloyed steels (Almeida et al.,1998; Hagans & Hass, 1994; Kudryavtsev, 1979; Lainer, 1984; Zaki, 1988). However, zinc is a highly reactive element, and therefore high corrosion rates of this coating are observed in cases of indoor and outdoor exposures. For this reason a post-treatment is needed to increase the lifetime of zinc coatings. This kind of treatment is applied in the current industrial practice to prolong the lifetime of zinc coatings and the steel substrates, respectively. This treatment consists of immersion in a chemical bath, which forms a conversion layer over the plated zinc. The so formed layer is a dielectric passive film with high corrosion resistance and it is also a better surface for paint adherence (Zaki, 1988). The main problem with the traditionally applied post-treatment procedures is the presence of Cr6+ salts that are considered to be carcinogenic substances, which are known to be very harmful to human health and environment (Schafer & Stock, 2005) and whose use is forbidden by European

Molybdates, tungstates, permanganates and vanadates, including chromium-like components, were the first chemical elements to be tested as hexavalent chromium substitutes (Almeida et al., 1998a, 1998b; Korobov et al, 1998; Schafer & Stock, 2005; Wilcox & Gabe, 1987; Wilcox et al., 1988). Recently many alternative coatings have been developed, based on zirconium and titanium salts (Barbucci et al., 1998; Hinton, 1991), cobalt salts (Barbucci et al., 1998; Gonzalez et al., 2001) and organic conductive polymers (Gonzalez et al., 2001; Hosseini et al., 2006). The use of salts of rare-earth metals as the main component in the electrolytes, developed for the formation of cerium, lanthanum and other oxide protective films is also a very promising alternative to the chromate films and it is one of the advanced contemporary methods for corrosion protection of metals and alloys (Bethencourt et al., 1998; Crossland et al., 1998; Davenport et al., 1991; Fahrenholtz et al., 2002; Forsyth et al., 2002; Hinton, 1983, 1992; Hosseini et al., 2007; Liu & Li , 2000; Montemor et al., 2002; Montemor & Ferreira, 2008; Pardo et al., 2006; Wang et al., 1997). However, some aspects of

**1. Introduction** 

regulations (Hagans & Hass, 1994).

**Modified by Cerium Oxides Layers** 

*Institute of Physical Chemistry, Bulgarian Academy of Sciences* 

Emilia Stoyanova and Dimitar Stoychev

