7.2 Composite Deposition from Particle Suspensions
247
140, 141, 164, 189], SiO 2 [86, 102, 190], Si 3 N 4 [101], CeO 2 [128, 138, 169], TiO 2
[143], ZrO 2 [191], TiN [79], diamond [192] and PTFE [100]. As an evidence for the
physical origin of the diminished corrosion rate, it can be brought up that the particle
effect works for essentially all metal matrices employed in these studies (Ni, Ni–Co,
Cu, Zn etc.).
The incorporation of the inert particles usually leads to the ennoblement of the
deposits, i.e., the corrosion potential is more positive than that of the metal matrix
plated without particles. The estimation of the corrosion rate of the composite coatings is based on two major methods; namely, the potentiodynamic curves recorded
with a small scan rate and the impedance spectra recorded at the corrosion potential.
Although the Tafel-representation of the potentiodynamic curves offers an opportunity for the quantitative estimation of the corrosion rate, the linear regions usually
cannot be observed clearly; hence, the evaluation of the measurements becomes
somewhat arbitrary. It can also be seen that the corrosion rates tabulated appear
to be overestimated as compared to the Tafel plots of the potentiodynamic curves.
The electrochemical impedance (EIS) spectra make it possible to read the estimated
charge transfer resistance which cannot be a priori associated with the corrosion
rate; rather, this parameter can be used to express the estimated ratio of the corrosion
rates of the samples. Whichever method is used, the decrease in corrosion rate as a
result of the composite formation is reported to fall between a factor of 2–20.
The origin of the diminished corrosion rate is multifold: (i) The particle incorporation leads to grain refinement, which itself can decrease the corrosion rate of the metal
matrix. The grain refinement is often coupled with a more compact deposit structure
with lower roughness. (ii) The particles cover a part of the composite surface. Since
the particles are inert, the particle-covered areas essentially do not corrode. This is
a simple physical exclusion effect. (iii) The particles impede the penetration of the
corrosion cracks in the composite.
While the surface coverage by the particles is usually rated to be the least important among the effects listed above, the hindrance of the corrosion crack penetration
is strongly related to the advantage of the application of nanoparticles as opposed to
microparticles. Since the numerical density of the nanoparticles can be much larger
than that of the microparticles, their application can drastically enhance the corrosion inhibition efficiency because the growing corrosion cavities run into a blocking
inert particle within a very short distance, which locally stops the corrosion cavity
penetration. The importance of the grain size effect is evidenced by the observations
where the minimum in the grain size in a deposit series can be related to the minimum
of the corrosion rate [79, 124, 141, 189].
Exceptionally, the increase of the corrosion rate as a result of inert particle incorporation can also be observed [193]. In this case, the metal matrix of the Ni–W(Al 2 O 3 )
coating has fundamentally different properties than the blank ones; namely, the
particle incorporation leads to the crystallization of the amorphous matrix. The impact
of the particle-induced crystallization overwrites all other advantageous effects of
the particles incorporation concerning the corrosion rate of the composite.
When the particles in the MMCs are electrically conducting ones, the change in the
corrosion rate can also be associated with the so-called local cell effects; or, in other
247
140, 141, 164, 189], SiO 2 [86, 102, 190], Si 3 N 4 [101], CeO 2 [128, 138, 169], TiO 2
[143], ZrO 2 [191], TiN [79], diamond [192] and PTFE [100]. As an evidence for the
physical origin of the diminished corrosion rate, it can be brought up that the particle
effect works for essentially all metal matrices employed in these studies (Ni, Ni–Co,
Cu, Zn etc.).
The incorporation of the inert particles usually leads to the ennoblement of the
deposits, i.e., the corrosion potential is more positive than that of the metal matrix
plated without particles. The estimation of the corrosion rate of the composite coatings is based on two major methods; namely, the potentiodynamic curves recorded
with a small scan rate and the impedance spectra recorded at the corrosion potential.
Although the Tafel-representation of the potentiodynamic curves offers an opportunity for the quantitative estimation of the corrosion rate, the linear regions usually
cannot be observed clearly; hence, the evaluation of the measurements becomes
somewhat arbitrary. It can also be seen that the corrosion rates tabulated appear
to be overestimated as compared to the Tafel plots of the potentiodynamic curves.
The electrochemical impedance (EIS) spectra make it possible to read the estimated
charge transfer resistance which cannot be a priori associated with the corrosion
rate; rather, this parameter can be used to express the estimated ratio of the corrosion
rates of the samples. Whichever method is used, the decrease in corrosion rate as a
result of the composite formation is reported to fall between a factor of 2–20.
The origin of the diminished corrosion rate is multifold: (i) The particle incorporation leads to grain refinement, which itself can decrease the corrosion rate of the metal
matrix. The grain refinement is often coupled with a more compact deposit structure
with lower roughness. (ii) The particles cover a part of the composite surface. Since
the particles are inert, the particle-covered areas essentially do not corrode. This is
a simple physical exclusion effect. (iii) The particles impede the penetration of the
corrosion cracks in the composite.
While the surface coverage by the particles is usually rated to be the least important among the effects listed above, the hindrance of the corrosion crack penetration
is strongly related to the advantage of the application of nanoparticles as opposed to
microparticles. Since the numerical density of the nanoparticles can be much larger
than that of the microparticles, their application can drastically enhance the corrosion inhibition efficiency because the growing corrosion cavities run into a blocking
inert particle within a very short distance, which locally stops the corrosion cavity
penetration. The importance of the grain size effect is evidenced by the observations
where the minimum in the grain size in a deposit series can be related to the minimum
of the corrosion rate [79, 124, 141, 189].
Exceptionally, the increase of the corrosion rate as a result of inert particle incorporation can also be observed [193]. In this case, the metal matrix of the Ni–W(Al 2 O 3 )
coating has fundamentally different properties than the blank ones; namely, the
particle incorporation leads to the crystallization of the amorphous matrix. The impact
of the particle-induced crystallization overwrites all other advantageous effects of
the particles incorporation concerning the corrosion rate of the composite.
When the particles in the MMCs are electrically conducting ones, the change in the
corrosion rate can also be associated with the so-called local cell effects; or, in other