Corrosion damage

From figures 8.1, 8.3 and 8.4 it can be seen that on plotting the reverse cathodic polarization scans onto the forward anodic polarization curves, the exposed specimens undergo significant hysteresis. As mentioned in section 8.3.2, hysteresis is indicative of the corrosion damage during anodic cycling. Especially, in the case of the CrN coated steel specimens without a diamond coating, corrosion in the form of very localized pits is obvious. From figure 8.6 it is observed that the pits formed on the CrN coated tool steel specimen without a diamond coating are about 300 um wide while the rest of the exposed CrN surface remains unaffected. A specific mechanism for pit initiation or nucleation and pit growth is not agreed upon. However, a plausible initiation and growth mechanism is based on the so-called concentration cell effect [lun00]. Chloride anions diffuse through the passive layer to form metal chloride species at the boundary between the metal and passive layer. As the molar volume of the metal chloride is larger than that of the passive (CrN) layer, the accumulation of chloride species causes mechanical stresses, which might deteriorate the outer layer.

To obtain subsequent pit growth, a supporting cathode is required. During anodic polarization, metal cations and H+ ions are produced at the working electrode, i.e. the steel specimen, caused by metal dissolution and subsequent hydrolysis with water (M+

+ H2O ^ M(OH) + H+). The increased concentration of positive ions results in Cl-- anions diffusing into the pit, which further increases the aggressive anion and H+

concentrations again. Dissolution of the anode is then increased and a pit begins to form. The whole process is termed autocatalytic or self-propagating. The electrons, which are liberated as a result of the anodic dissolution, are consumed by a cathodic reaction such as the reduction of O2. This means that the pit is polarized, i.e. the exterior surface of the pit remains passive and functions as a local cathode (reduction of O2). The localized pits are seen to appear in all kinds of shapes reaching penetration depths of hundreds of microns.

Pitting corrosion behaviour of .

8.5 Conclusions

The type of interlayer used for CVD diamond deposition onto tool steel specimens has a strong influence on the equilibrium corrosion potential of the whole system. With increasing chloride ion concentration in the electrolyte, the corrosion potential of blank tool steel shifts towards the more active region, while that of the diamond coated specimens shifts towards nobler direction for all interlayer systems.

Though the maximum shift in Ecorr is observed for the diamond coated specimens with a silicon interlayer, anodic polarization is observed to result in selective dissolution of the carbide phases leaving the more inert diamond crystallites unaffected. The presence of even a very thin diamond film of 2-3 pm is found to be effective in obtaining a near total resistance to pitting of the CrN coated and borided tool steel specimens. From the present study it can be concluded that the use of a PVD CrN interlayer for the deposition of diamond films onto tool steel substrates results in the best corrosion resistance.

In order to benefit from the noble properties of diamond in corrosive applications, thicker and higher density diamond films are considered necessary. By changing the diamond CVD process conditions, such as the methane concentration and applied gas activation, nanocrystalline diamond films can be obtained. Though these films will have a higher number of grain boundaries, the density of these films will also be higher. Seeping of the electrolyte solution through these dense diamond films will therefore be reduced as compared to the microcrystalline diamond coatings.

Also, (partial) delamination and/or cracking of the grown diamond films has to be avoided, as a small number of open areas within these diamond films is detrimental for the corrosion resistance of the underlying substrate material. An additional conclusion that can be drawn from this study is that the choice of interlayer material should also depend on the effect of the interlayer on the corrosion behaviour of the as- modified steel. From the present study it is very clear that the type of interlayer used has a strong influence on the electrochemical behaviour of the diamond coated system. Hence, for corrosion or erosion-corrosion resistant applications, additional care needs to be taken in the selection of a suitable interlayer system. The interlayer system chosen should preferably not decrease the corrosion resistance of the blank steel itself.

149

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Chapter 9