Importance of Surface Preparation for Corrosion Protection of Automobiles
104
Similarly, Figure 13, where log coating capacitance is
plotted against exposure time, indicates that for trica-
tionic formulation (III) the increase in Cc is relatively
much less compared with formulations I and II. The in-
crease in Cc with exposure time can be attributed to the
formation of blisters due to water ingress underneath the
film. After a certain exposure time, there was no further
increase in the coating capacitance values, either it re-
mained constant or started decreasing. This may be at-
tributed to simultaneous occurrence of two opposing
phenomena:
At long exposure times, the ingress of water and ac-
cumulation of corrosion products underneath the paint
film exert pressure from inside the blister and the blister
breaks. This process decreases the capacitance.
2) The nucleation and growth of some blisters con-
tinue even at long exposure times. This process increases
the capacitance.
The break-point frequency versus exposure time plots
(Figure 14) clearly show three distinct stages in coating
failure process: water ingress, coating disbonding and
blister growth.
Since the break-point frequency is proportional to the
area of delamination, the performance of various coat-
ings system could be assessed by comparing the “fb” val-
ues at a particular exposure time to salts spray environ-
ment. For example as shown in Table 5, fb value for
formulation III after 300 hrs. of exposure is 1345 Hz
which is much lower compared with corresponding val-
ues 8447 Hz and 27,994 Hz for formulation I and II re-
spectively, indicating clearly that formulation III offers
much superior corrosion resistance (minimum area of
delamination) followed by phosphating formulation I and
II, which was also corroborated by visual observation of
the panels from salts spray test [26].
The other point to note is the induction times for steep
increase in break-point frequency values for this particu-
lar coating system (Figure 14) which are approximately
300, 150 and 400 hours for phosphating formulations I, II
and III respectively which is again a clear indication of
the superior adhesion and corrosion performance of phos-
phating formulation III.
Thus, superior performance of formulation III may be
attributed primarily to the difference between chemical
composition, compactness and superior alkali resistance
of the phosphate coating compared with formulation I
and II. The superior alkaline resistance of tricationic
phosphating formulation is attributed to the presence of
higher level of additional crystal phases like phospho-
phyllite, phosphomangallite and phosphonicollite besides
Hopeite phase in phosphate coating on steel surface.
4. Acknowledgements
I would like to thank my collegues Mr. G. N. Bhar and
Mr. P. K. Roy of ICI India, R and D Center for Paints,
Kolkata, India for their contribution to this work. I would
also like to thank Mr. Nikhilesh Chaudhary of Material
Science Department, Indian Association for the Cultiva-
tion of Science, Kolkatta, India for all the SEM micro-
graphs and Dr. S. Badrinarayan of National Chemical
Laboratory, Pune, India for surface analysis of steel pan-
els and finally to Ms. Gayatri Devi and Prof. V. S. Raja
of IIT Mumbai, India for evaluation of phosphate and
painted coatings by EIS spectroscopy. Finally, I would
also like to thank Miss Vaishali Shinde and my research
students, Dr. Shilpa Vaidya, Dr. Priyanka Bhat and Dr.
Rohan Jadhav of ICT for putting this paper together in
the present form.
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