CHEM02200704003 Nilamadhab Pandhy - Homi Bhabha National ...

Chapter 6
Covalent aspect of lattice cohesion such as bond energy also influences the overall dissolution rate
because, on dissolution bonds between constituents of a crystal have to be broken. Thus, higher
energy is needed to breakdown the bond and lattice disintegration to initiate.
Although, thermodynamical and electrical properties determines the overall stability of the
passive film, structural defects present on the coated surface also play a significant role in
accelerating corrosion rate by increasing the accessible areas of base material to the corrosive
environment. Local defects such as pinholes and continuous boundaries in between the columnar
grains on the coated surface are the preferable sites for the initiation of sub-coating corrosion due
to absorption of electrolyte as a result of capillary action [205]. These surface defects create
solution paths, and facilitates diffusion of oxygen within the coating leading to crevice as well as
galvanic corrosion between the coating and the underlying substrate. Once solution reaches the
bottom of the coated surface, the exposed areas of the substrate will subject to accelerated anodic
dissolution resulting increase in passive current density and corrosion current density. Moreover,
unfavourable area ratio such as, large cathodic area of coating and small anodic area of the
pinholes reduces the corrosion protection of the coated surface due to galvanic effect. As galvanic
corrosion becomes localized, local active dissolution of the steel surface increases and
consequently propagates laterally along the interface between the coating and the substrate.
However, presence of porosity in the coating is inevitable, and is inversely proportional to the
coating thickness. Thus by increasing the thickness and carefully selecting the deposition
parameters the extent of pore formation can be reduced and consequently corrosion resistance of
the coated materials can be increased. Nevertheless, by increasing thickness stresses are also
generated in the coated surface leading to peeling off and flaking of the coating and thus optimum
coating thickness of 1 µ is generally preferred for corrosion protection.