*3.2.5. Adhesion and diffusion*

Adhesion of the coatings can be identified both qualitatively and quantitatively by a specific testing method called scratch test. This test is applied according to the procedure of mechanical failure modes and adhesion strength of ceramic materials. It is appropriate for adhesion measurement and possible failure modes prediction of metallic and ceramic substrates coated by thin ceramic films. This method does not give a characteristic value of the material. Instead, it reveals a practical engineering approach for substrate-coating system because the results are obtained depending on the several test parameters. Details of the test method can be reached from ASTM-C1624 standard [26].

#### *3.2.6. Corrosion*

Corrosion is known to be influenced by a variety of factors. It is not possible to separate corrosion from many of the other property issues related to coatings. For the proper selection of a coating, it is necessary to take its position, with respect to its substrate in the galvanic series for the intended application, into consideration [27]. Decorative nickel-chromium coatings developed for automotive industry applications are a suitable example of the application of materials science and electrochemistry for the corrosion protection of materials.

There are several examples of electro codepositited composite coatings for corrosion protection [28-30]. According to these results, it is supported that corrosion resistance of composite coatings increase significantly with respect to the pure metallic or alloy coatings [15, 21]. This phenomenon is suggested to be the effect of the inert properties of the reinforcements.

For the Ni-Co alloy matrix SiC reinforced composite coatings, Bahkit & Akbari [21] expressed the corrosion behaviour of these coatings through potentiodynamic polarization curves (Figure 7). Calculated corrosion current densities and measured corrosion potentials are summerized in Table 1.

**Figure 7.** The potentiodynamic polarization curves of alloy coating and composite coatings


**Table 1.** Calculated and measured corrosion data for alloy and composite coatings

Based on the data, Ni-Co/SiC nanocomposite coating has the highest corrosion potential whereas the pure alloy coating (Ni-Co) has the lowest. Additionally, the corrosion current density of nanocomposite coating is lower than the micro scale SiC reinforced composite coating and shows the higher corrosion resistance (Rp) [21].

Additional examples belong to the main topic of the present chapter, in-situ codeposition, and the relation between metal matrix and inert reinforcements are given in Section 4.3.
