Journal of Surface Engineered Materials and Advanced Technology, 2013, 3, 94-105
http://dx.doi.org/10.4236/jsemat.2013.31A014 Published Online February 2013 (http://www.scirp.org/journal/jsemat)
Importance of Surface Preparation for Corrosion
Protection of Automobiles
Narayan Chandra Debnath
Department of Physics, Institute of Chemical Technology (ICT), Mumbai, India.
Received December 5th, 2012; revised January 7th, 2013; accepted January 14th, 2013
ABSTRACT
An overview of science and technology of pretreatment process suitable for automotive finishing with cathodic electro-
deposition primer is presented in details in this paper. Both the theoretical principles and practical aspects of tricationic
phosphating process that are used in automotive industry are discussed in details. The characteristic features of phos-
phate coatings of both conventional high zinc phosphating formulations and modern tricationic phosphating formula-
tions on steel surface are compared in details by SEM, EDX and XRD techniques. The corrosion protections of the
phosphated and painted steel panels were evaluated by both salt spray test and electrochemical impedance spectroscopy
(EIS). The analysis of impedance data in terms of pore resistance (Rpo), coating capacitance (Cc) and breakpoint fre-
quency (fb) as a function of salt spray exposure time provides a clear insight into the mechanism of superior corrosion
resistance provided by the modern tricationic phosphating formulations compared with conventional high zinc phos-
phating formulations.
Keywords: Pretreatment Process; Tricationic Phosphating Formulations; SEM; XRD; EDX; Electrochemical
Impedance Spectroscopy; Corrosion Protection
1. Introduction
The importance of surface preparation for corrosion pro-
tection of automobiles need not be over emphasized
because the durability of the phosphated and painted
metal surface depends quite critically on the quality of
cleaning, stabilization of cleaned surface and physico-
chemical characteristics of the phosphate coating that is
deposited on clean surface by chemical conversion pro-
cess prior to painting of the car body. Industrial surface
preparation process generally consists of five processing
zones viz: degreasing, derusting, surface activation, phos-
phating and passivation and these pretreatment chemicals
may be used either in spray mode, dip mode or in spray
cum dip mode. Modern car manufacturing plants mostly
use spray cum full dip mode for its obvious advantage
for ensuring satisfactory cleaning and deposition of uni-
form phosphate coating in the areas of car body which
are not normally accessible by spray mode of application.
If the car body consists of mixed metal combination for
different parts of auto body like mild steel and coated
steel, then the in-line derusting stage is eliminated from
the pretreatment line. Since the phosphate coating is de-
posited on a metals surface as a result of interfacial reac-
tion between the metal surface and the phosphating
solution, the surface composition of the steel and the
method of cleaning will have considerable effect on the
structure, composition and morphology of phosphate
coating which in turn will affect the final corrosion resi-
stance of the phosphated and painted systems. The stru-
cture and composition of the phosphate coating and also
its rate of growth depends broadly on the three following
factors:
 Structure of the clean metal surface i.e. microstruc-
ture and chemical composition of the surface.
 Design of the phosphating and other chemicals used
in different pretreatment stages.
 Parameters of the processing baths viz: temperature,
concentration, pressure and time of reaction etc.
 The quality of water used for bath preparation and in
the rinsing stages after different stages of processing.
The automotive finishing technology has undergone
significant changes in the past decades because of de-
mand for car with higher corrosion resistance and better
quality of surface finishes [1-16]. The key factors that
contributed significantly to the improvement of higher
corrosion resistance are the development of new sub-
strates with better inherent corrosion resistance and
higher strength, introduction of cathodic electrophoretic
paints for priming the car body and development of low
temperature tri-cationic phosphating formulation (45˚C

Importance of Surface Preparation for Corrosion Protection of Automobiles 95
to 50˚C) which are suitable for depositing excellent phos-
phate coating on multi metal autobody system containing
steel, coated steel and aluminium alloys. The main char-
acteristic features of modern tri-cationic phosphating for-
mulations containing ions of Zn, Mn, Ni is the superior
alkali resistance of the resulting phosphate coating which
make these formulations highly suitable for operation in
cathodic ED bath. This superior alkali resistance of phos-
phate coating results from the development of additional
crystal phases like Phosphophyllite (Zn2Fe(PO4)2·4H2O),
Phosphomagnellite (Zn2Mn(PO4)2·4H 2O) and Phospho-
nicolite (Zn2Ni (PO4)2·4H2O) in the coating besides the
Hopeite ( Zn3(PO4)2·4H2O) phase. The higher the value
of P/P + H ratio, better is the alkali resistance of the
phosphate coating in CED bath leading to superior corro-
sion resistance of the phosphated and electropainted auto
body system [1]. Here, “P” stands for the total Phospho-
phyllite phases and “H” stands for Hopeite phase present
in the deposited coating. The basic chemical reactions on
steel surface in a phosphating bath are described below:


34 24
2
Fe2H POFeH POH 
2
4



2424 2
22
24234
2
2ZnH POFeHPO4HO
ZnFePO4HO4H PO
Coating-Phospho phyllite





24 2
2
34 23
2
3ZnH PO4H O
ZnPO4H O4H PO
Coating-Hopeite



24
2
434 2
2FeHPOO
2FePO2HPOSludge HO


The coating deposited on steel surface consists of two
phases viz: Phosphophyllite and Hopeite as described
above. And the sludge, which is a byproduct of the phos-
phating reaction, settles down on the bottom of the phos-
phating bath [10-13].
In this work, we discuss in the details the science and
technological aspects of a modern tri-cationic phosphate-
ing process which is suitable for deposition of excellent
phosphate coating on multi-metal auto body assembly
consisting of steel and coated steel and also compatible
with cathodic electrodeposition (CED) primers. A large
number of experimental techniques like SEM/EDX, XPS,
XRD and AAS, have been used in this work to charac-
terize the chemical composition of steel surface used by
automotive manufacturers and also to characterize the
morphology, chemical composition, phase composition
and coating weight of the phosphate coating deposited on
steel surface. Electro chemical impedance spectroscopy
(EIS) and Salts spray tests (ASTM B117) have been used
for evaluation of overall corrosion resistance of the
phosphated and painted steel surface as a function of
time and for understanding the underlying mechanism of
protection and degradation of the coating system on steel
surface over extended period of exposure to corrosive
atmosphere.
2. Pretreatment Process Sequence Used in a
Modern Automotive Finishing Plant
The outline of a 14 stages pretreatment line used in auto-
tive finishing plant is shown in Figure 1. It may be noted
that the derusting stage along with post derusting rinse
stages have been eliminated from this line because the
car body processed in this PT line has a mixed metal
combination of steel and electrogalvanised steel in dif-
frent parts. It may be noted that a combination of spray
and dip rinse stages makes effective cleaning between
different stages and minimizes the carry over of chemi-
cals to the next stage. The processing parameters of dif-
ferent stages of pretreatment plant are summarized in
Table 1. However, under laboratory conditions, the pre-
treatment process can be implemented in five litre baths
and generally the steel panels of 6” × 4” are used for
depositing phosphate coating which can be used for
different physico-chemical characterization viz: morpho-
logy, phase analysis, chemical analysis and coating wei-
ght determination and evaluation of corrosion resistance
(ASTM B117) after depositing paint coating of appro-
priate thickness.
The design of the chemicals used at prephosphating
stages viz. degreasing, derusting and surface activation
and the corresponding bath parameters all will have con-
siderable effect on the uniformity, morphology, coating
weight and quality of phosphate coating deposited during
the phosphating stage [17-19]. The physical structure and
chemical composition of the phosphate coating, in turn,
will affect the corrosion resistance of the phosphated and
painted system. Thus it is very important to maintain the
bath parameters at the recommended values at every
stage of processing to get the right quality of phosphate
coating.
Figure 1. Pretreatment process sequence used in a modern
utomotive finishing plant. a

Importance of Surface Preparation for Corrosion Protection of Automobiles
96
Table 1. A 14-stage pretreatment process for automotive finishing.
Sr. No. Process Sequence Mode of OperationTemperature (˚C) Time of Processing
(min.) Chemical Bath Parameters
1 Manual cleaning with solvent RT 5
2 Knock-off Degrease
(or High Pressure Degrease)
Spray pressure
(4 - 6 bars) RT 1
3 Low Pressure Degrease Spray pressure
(0.7 bars) 47˚C 1
4 Dip Degrease Dip 55˚C 4
5 Rinsing with mains water Spray RT 1
6 Rinsing with mains water Dip RT 4
7 Surface Activation Dip RT 4.5 pH = 7.86; Ti = 22 ppm
8 Phosphating Dip 50˚C 4.5
FA = 2.8 - 3.2; TA = 26 - 30
Toner = 1.8 - 2.5 ml
9 Rinsing with Mains water Spray RT 1
10 Rinsing with Mains water Dip RT 4
11 Passivation Dip RT 4
12 Rinsing with Fresh D. I. water Spray RT 1
13 Rinsing with Recirculated D. I. water Dip RT 4
14 Rinsing with fresh D. I. water Spray RT 1

Wet entry into Cathodic Electrocoat Bath
Apart from chemicals, the water quality used in bath
make-up as well as at rinse stages plays a very critical
role in maintaining the stability of the different baths as
well as the quality of the phosphate coating. The post
passivation rinse stage is very critical in pretreatment
process because soluble salts of chloride, sulfate and am-
monia if not removed thoroughly from car body, will
promote blistering under a paint film. In order to mini-
mize this possibility, water supply should be free from
harmful salts as far as practical. Normally, deionised
water (DI) is used for bath make up and the replenish-
ments in surface activation stage, passivation stage and
post passivation rinse stages. For other treatment stages
like decreasing, derusting, phosphating and other rinse
stages mains water may be used provided it conforms to
the specification given in Table 2 [7-12]. To ensure
minimum carryover of harmful ions to the subsequent
stage of CED bath the phosphated surface after passive-
tion stage is given two or three DI water rinses. Fresh
water is used in the last stage, whereas recirculated water
is used in first two stages. The conductivity of recircu-
lated water should not exceed 25 µS/cm [1]. The rinse
bath should be discarded once the conductivity exceeds
the limit of 25 µS/cm.
In order to ensure consistently good quality of clean-
ing and phosphating of car body the following factors in
different stages are very important.
2.1. Degreasing Stage
The degreasing zone normally consists of at least two
stages. The first stage is usually a spray stage known as
knock-off-degrease (K.O.D.) and that is followed by a
dip stage. The advantage of having two stages is that
major portion of the oil, dirt etc. will be removed by high
pressure spray impact in the first stage leaving relatively
lower load for the dip stage to clean. In Figure 1, the
degreasing zone consists of three stages viz. two spray
stages and one dip stage for efficient cleaning of car
body. In order to ensure maximum efficacy of degreasing
stage, misting spray should be provided between K.O.D.
and dip degreasing stage and also between dip degreas-
ing and next rinsing stage to prevent the drying of the car
bodies during transition from one stage to the next stage.
Continuous oil separating systems should also be in-
stalled for high volume production of pretreatment line.
2.2. Surface Activation Stage
The purpose of this stage is to refine the crystal size of
zinc phosphate coating and to control the coating weight
uring phosphating stage. Modern surface activation che- d

Importance of Surface Preparation for Corrosion Protection of Automobiles 97
Table 2. Water specification for bath make-up and rinse stages.
Water Type used in PT line Specifications Usage
(i) Conductivity 5 μs/cm
I. De-ionised (DI) water
(ii) pH 6.5-7.2
Surface activation stage passivation stage post passivation
rinse stage (spray stage)
II. Recirculated Deionised
water Conductivity ≤ 25 μs/cm Post passivation rinse stage (dip stage)
1) Total chloride and sulphate 70 ppm maximum
(calculated as 2
4
Cl SO


)
2) Total alkalinity-200 ppm max. (calculated as CaCO3)
III. Mains water
3) Both together should not exceed 225 ppm
Degreasing, derusting stage phosphating stage all other
rinse stages
micals are weakly alkaline colloidal dispersion of tita-
nium complex. This treatment leads to the formation of
large number of finer crystallites of titanium compound
on the metal surface which act as crystal nuclei for the
growth of fine zinc phosphate crystals during the phos-
phating stage. Greater is the number of nucleating centres
on the surface, more will be the inter-crystalline colli-
sions and consequently finer will be the crystals and
more compact will be the phosphate coating and better
will be the paint adhesion and corrosion resistance of the
phosphated and painted system [14,19]. The efficacy of
surface activation bath is critically dependent upon:
 pH of the bath.
 The concentration of titanium.
 For better colloidal stability of the surface activation
bath, it must be made with deionised (DI) water and
the activation bath should have a circulation rate of 3
to 4 tank turn-over/hour.
 According to the results published by Yoshihara [1].
The ideal pH of the activation bath should be in the
range of 8.5 to 9.5 and Ti concentration should be
minimum 10 ppm. We have observed that within the
range of 10 to 30 ppm, the coating weight remains es-
sentially constant, Table 1 shows the Ti concentration
in the bath as 22 ppm.
 The grain refining action depends on amount of Ti
adsorbed on the metal surface and for steel surface
the adsorption is inversely proportional to the amount
of segregated carbon on the surface.
2.3. Phosphating Stage
For most effective functioning of modern tricationic zinc
phosphating formulation the following points are very
important:
 The coating formation reaction is largely dependent
upon the free acid (FA), total acid (TA), concentra-
tion of oxidizing agents or toners, temperature, depo-
sition time etc. In Table 1 the phosphating bath pa-
rameters are given. The phosphate coating weight on
mild steel substrates generally lies in the range of 2.8
to 3.2 g/m2.
 The circulation of phosphating bath solution is an
essential requirement for ensuring the deposition of
uniform phosphate coating and normally a circulation
rate of 3 to 4 tank turn-over/hour is recommended.
The direction of solution flow will be opposite to that
of the moving car body to be phosphated. In order to
ensure consistent good quality of phosphate coating
on car body the phosphating bath should be provided
with a continuous sludge removal system like filter
press to minimize the accumulation of sludge in the
phosphating bath. Usually, the sludge containing
phosphating bath solution is pumped to Tilted Plate
Separator (TPS) where a major portion of the sludge
will be separated and collected at the bottom of the
separator. The comparatively clear supernatant liquid
from the TPS is then pumped through the filter press
(containing a series of filters) where the phosphating
bath solution will be completely free from sludge and
then pumped back to the main phosphating tank. The
total sludge separating unit will be under continuous
operation as long as the phosphating plant is running
and at least 60 to 70 percentages of the filters should
always be in working condition for effective sludge
removal. Yoshihara et al. [1] recommends a sludge
concentration of about 300 ppm maximum at any
stage in the phosphating bath.
 The total surface area processed per hour in a given
volume of bath solution is very important for main-
taining the chemical equilibrium of the phosphating
bath. For light and medium coating weight zinc phos-
phating bath, the optimum recommended rate of pro-
cessing is about 2 sq. ft/hour/4.5 liters of bath solu-
tion.
 Phosphating solution should be heated indirectly by
external plate heat exchanger by using low pressure
hot water. The temperature differential between the
phosphating solution and hot water should not exceed
10˚C. This will prevent the formation of scale on the
heat exchanger plate.
 For high production volume automotive plant, the

Importance of Surface Preparation for Corrosion Protection of Automobiles
98
auto dosing system of the chemicals and accelerators
(toners) to the phosphating bath is usually recom-
mended to ensure consistently satisfactory phosphate
coating on car bodies.
2.4. Passivating Rinse
In order to improve the corrosion resistance of phosphate
coating on steel surface, it is useful to give a final rinse
with chromium containing solutions. This treatment which
is known as passivation process provides additional sta-
bility to the phosphated surface by partial sealing of the
pores in zinc phosphate coating. The trend, so far has
been to use passivating solutions containing a mixture of
hexavalent and trivalent chromium ions for best results.
The latest trend, however, is to use Chrome free formula-
tions for passivation purpose and a number of Zirconia
based products are now available for use in automotive
finishing industry.
3. Characterization of Phosphate Coating
In order to establish the structure-property-performance
correlation of the phosphate coating with its protective
value and also to carry out failure analysis of the Me-
tal/Phosphate/ED primers interfaces, it is very important
to characterize the phosphate coating at microscopic
level [6-27]. The key physico-chemical characteristics
parameters of phosphate coating on a metal surface are:
 Coatings morphology (crystal size, shape, orientation
and coating compactness).
 Crystal phases of the coating.
 Coating weight.
 Coating composition.
 Chemical stability.
Usually coatings morphology and crystal size are de-
termined by Scanning Electron Microscopy (SEM) and
the crystal phases are determined by X-ray Diffraction
technique (XRD)and elemental analysis on the surface
coating may be done by Energy Dispersive X-ray analy-
sis (EDX). Coating weight can be determined by chemi-
cal methods by dissolving the coating in dilute chromic
acid solution and the phase composition (“P” ratio) may
be determined by chemical methods like Atomic Absorp-
tion Spectroscopy (AAS) or by XRD techniques [12-21].
The chemical stability of the phosphate coatings may be
determined by exposing the coatings in dilute solution of
sodium hydroxide and checking the extent of solubility
[19,21]. The final corrosion performance of phosphated
and painted surface maybe evaluated by accelerated
tests like salts spray (ASTM-B117) and also by Electro-
chemical impedance Spectroscopy (EIS] [22-27]. In auto-
motive industry, salts spray tests are widely used to
evaluate the performance of phosphated (PT) and Elec-
tro-deposited primer (ED) coating. For mild steel surface,
the normal specification for anodic electrocoat (AED)
process is that the coating system (PT + AED) should
pass 600 hrs of salts spray tests (with 20 µm primer
thickness) while for cathodic electrocoat system (PT +
CED) with similar thickness of primer film, the coating
system should pass a minimum of 1000 hrs of salts spray
test.
The structure, composition and coating weight of phos-
phate coating deposited on a metal substrate is a function
of several factors:
1) Structure and chemical composition of the metal
surface.
2) Design of the phosphating chemical.
3) Mode of application i.e. dip or spray.
4) Bath parameters like free acid, total acid, concentra-
tion of oxidizing accelerators or toners, temperature, time
of coating deposition and loading rate i. e. total surface
area processed per hour in a given volume of phosphate-
ing solution.
The effect of some of these parameters on the structure,
morphology and performance of phosphate coating on
steel surface have been reported in details in some of our
earlier works [14,16-19]. In the following section we
highlight some key results which are important for the
both the development of new phosphating formulations
as well as for solving the quality problems encountered
during their application to industrial metal finishing pro-
cess.
3.1. Morphology and Chemical Composition of
Phosphate Coating
The application of SEM, EDX and XRD techniques pro-
vides a comprehensive idea about the physical structure,
Chemical composition and nature of the coating de-
posited on metal surface. The uniformity of crystal size
and compactness of the coating is very important for ad-
hesion of the phosphate coating to the metal surface as
well as the adhesion of the paint coating or organic coat-
ing to the phosphated surface. In Figures 2(a)-(c), the
morphology of three different types of phosphate coating
deposited on steel surface from three phosphating for-
mulations 1, 2, 3 are shown. The formulations 1 and 2
are conventional high temperature immersion type phos-
phating formulations (70˚C) with relatively high zinc
content 1) and calcium modified zinc phosphating for-
mulations; 2) leading to uniform compact coating with
nodular shaped and spherical shaped crystals respectively.
In contrast, formulation; 3) which is a tricationic low
temperature phosphating formulation (45˚C - 50˚C) de-
posits a highly uniform compact coating with cubic
shaped crystals. The most important aspect of all these
coating is that all three formulations provide highly uni-
form, thin and compact coating on steel surface but av-
erage crystal size varies in the range of from 4 - 10 mi-

Importance of Surface Preparation for Corrosion Protection of Automobiles 99
(a)
(b)
(c)
Figure 2. (a) SEM micrograph of phosphate coating depos-
ited on steel surface (formulation I); (b) SEM micrograph
of calcium modified phosphate coating deposited on steel
surface (formulation II); (c) SEM micrograph of phosphate
coating deposited on steel surface from tricationic phos-
phating formulation III.
crons for different formulations. Further, the coating
composition of all these formulations varies because of
difference in the formulations. The corrosion perform-
ance of these three phosphating formulations are dis-
cussed in the last section. Figure 3 provides an example
of poor phosphate coating on steel surface which was not
properly cleaned at degreasing stage from formulation III
and hence undesirable in production line.
The morphology of phosphate coating on zinc coated
steel and aluminium substrates from formulation III are
shown in Figures 4 and 5 respectively. It is quite evident
that the coating morphology is very compact on the
former but not so satisfactory on aluminium substrate.
The XRD diffractograms of phosphate coating on steel
and zinc coated steel surface are shown in Figures 6(a)
and (b) respectively. It is quite evident that the coating
on steel surface consists of both Phosphophyllite and
Hopeite phases whereas on zinc coated steel surface the
it consists of only Hopeite phase as expected. Similary
EDX spectra of phosphate coatings on steel surface from
all three formulations are shown in Figure 7. The differ-
ent elements present in the coating are quite evident from
the spectra.
Figure 3. SEM micrograph of poor phosphate coating from
formulation III on steel surface which is not properly
cleaned.
Figure 4. SEM micrograph of zinc phosphate coating de-
posited on zinc coated steel surface from tricationic phos-
phating formulation III.

Importance of Surface Preparation for Corrosion Protection of Automobiles
100
Figure 5. SEM micrograph of phosphate coating on alu-
minium substrate from tricationic phosphating formulation
III.
(a)
(b)
Figure 6. (a) X-ray diffractogram of phosphate coating on
steel surface and (b) X-ray diffractogram of phosphate
coating on zinc coated steel surface.
(a)
(b)
(c)
Figure 7. (a) EDX spectra of phosphate coating from for-
mulation I (b) from formulation II and (c) formulation III.
It may be noted that the phosphate coating layer is the
most critical link in the chain of multiple coating layers
that are deposited on car body during autobody finishing
and thus the integrity of metal/phosphate interface is ex-

Importance of Surface Preparation for Corrosion Protection of Automobiles 101
tremely important for better corrosion resistance of car
body. An example of excellent steel/phosphate interface
is provided by the SEM micrograph shown in Figure 8.
The dark part in the micrograph is steel substrate and the
bright part is zinc phosphate coating.
3.2. Effect of Surface Composition on Quality of
Phosphate Coating on Steel Surface
In order to address the problem of variation of coating
quality viz. coating morphology and coating weight on
steel panels supplied by different steel manufacturers, a
systematic work was done on a set of 12 panels procured
from different suppliers and phosphated under laboratory
conditions and their coating quality was evaluated by
SEM technique. The results were classified into four
grades A, B, C, D depending on the quality of phosphate
coating. Both bulk and surface composition of these
samples were determined by Vacuum Emission Spec-
troscopy and XPS technique respectively and the data
revealed that even though the bulk chemical composition
of all the panels is essentially quite similar as shown in
Table 3, there is substantial difference in the surface
composition of the four panels classified under different
grades. Figure 9(a) and (b) shows the XPS results (Fe
2p3/2 and C1s spectra) of steel surface for four samples S2,
S4, S6 and S8. The surface Fe/C ratio decreases sys-
tematiccally form 0.41 to 0.15 as the coating morphology
degrades systematically from the best (A) to the worst (D)
as shown in Figures 10(a)-(c) and summarized in Table
4. The SEM picture corresponding to the D-grade steel is
not shown here as it is completely amorphous coating
without any structure. It was thus established that surface
Fe/C ratio is a very important index and can be used as a
reliable criterion for grading the steel panels and to dis-
tinguish the good steel from bad steel as far as the phos-
phatibility is concerned. More details of this were pub-
lished in an earlier publication [16].
Figure 8. SEM micrograph of steel/phosphate coating in-
terface. Dark part is metal and bright portion s phosphate
coating.
(a)
(b)
Figure 9. XPS results of (a) Fe2p3/2 (b) C1s spectra of steel
surface of the four steel samples.
3.3. Evaluation of Corrosion Performance of
Phosphated and Painted Steel Surface
In order to evaluate the comparative corrosion perfor-
mance of tricationic phosphating formulation(III) with
two conventional immersion type zinc phosphating for-
mulations (I and II), three sets of mild steel panels (6” ×
4”) were cleaned, phosphated by immersion process at
the recommended parameters of the each phosphating
formulations in the laboratory, keeping the degreasing,
derusting and passivation stages identical. The details of
phosphating process parameters for formulations I and II
are already reported in earlier work [17,18]. The phos-
phate panels were subsequently coated with an alkyd

Importance of Surface Preparation for Corrosion Protection of Automobiles
102
Table 3. Bulk composition of steel samples used in this study. Table 3. Bulk composition of steel samples used in this study.
Sample No. Sample No. Elements  Fe Elements  Fe C C Mn Mn S S Si Si Ni Ni Cr Cr Al Al
S2 99.18 0.07 <0.1 0.024 0.01 0.04 0.11 <0.012
S4 98.89 0.08 0.14 0.024 <0.01 0.01 0.12 >0.111
S6 98.77 0.059 <0.10 0.023 <0.01 0.01 0.15 >0.111
S8 99.37 0.153 0.26 0.022 0.05 0.01 0.16 >0.111
Table 4. Surface analysis of steel samples.
Sample No. Rating of phosphate coating quality Surface Fe/C ratio
S6 A 0.41
S4 B 0.37
S2 C 0.22
S8 D 0.15
based stoving clear to 20 micron thickness by spray
process and then baked at 150˚C for 30 min. The mild
steel panels used in this study were procured from an
automobile manufacturer and were cut out from a single
sheet to minimize the variation on the substrate quality.
(
RΩ = Solution Resistance.
Rpo = Pore Resistance.
Rct = Charge Transfer Resistance.
Cc = Coating Capacitance.
Cdl = Double Layer Capacitance.
Zω = Warburg Impedance.
(a) The corrosion performance of the phosphated and al-
kyd coated panels with20 micron thickness in salts spray
test (ASTM-B117) were monitored periodically both
visually as well as by Electro-chemical Impedance Spec-
troscopy (EIS),over a period of 600 hrs. The impedance
measurements were carried out on these panels at differ-
ent interval of exposure time over a frequency range of
10−2 Hz to 105 Hz. The amplitude of the signal was 5 mV.
The impedance measurements were carried out at open
circuit potential using a “Schlumberger 1255 Frequency
Response Analyzer” (FRA) operated under computer
control. The FRA was connected to the electro chemical
cell through “EG&G potentiostat/Galvanostat 273” More
details of the experimental set up for impedance meas-
urement have been reported in an earlier publication [26].
(b)
In order to interpret the impedance data we have used
an equivalent circuit model of painted metal/solution inter-
face as shown in Figure 11. The impedance data were
analyzed in terms of three coating parameters viz. Pore
resistance (Rpo), coating capacitance (Cc) and break-
point frequency (fb) [26]. The results of variation of Rpo,
Cc and fb as a function of salt spray exposure time are
shown in Figures 12-14 respectively. The numerical
values of Rpo, Cc and (fb) at 0 hr, 100 hrs and 300 hrs of
salts spray exposure are tabulated in Table 5. A com-
parison of Rpo values after 300 hrs of salts spray expo-
(c)
Figure 10. SEM micrographs of phosphate coating on steel
sample (a) S6; (b) S4; (c) S2.

Importance of Surface Preparation for Corrosion Protection of Automobiles 103
Table 5. Variation of Rpo, Cc and fb values of different phosphate coatings on steel surface coated with 20 μm thick alkyd
coating with salts spray exposure time.
Phosphating Salt Spray Exposure Time
System 0 hrs 100 hrs 300 hrs
Pore Resistance, Rpo (Ohm·cm2)
I 2.39 × 107 7.42 × 105 8.9 × 104
II 2.08 × 107 4.07 × 105 3.6 × 104
III 2.08 × 108 4.16 × 106 1.62 × 106
Coating Capacitance, Cc (F·cm−2)
I 6.42 × 10−10 3.16 × 10−8 3.23 × 10−7
II 1.48 × 10−9 4.2 × 10−8 8.12 × 10−7
III 6.9 × 10−11 9.3 × 10−10 2.95 × 10−8
Break Point frequency, fb (in Hz)
I 19.3 2706 8447
II 94.3 4965 27,994
III 7.5 120 1345
Figure 11. Equivalence circuit model for painted metal/solu-
tion interface.
Figure 12. Pore resistance (Rpo) as a function of salt spray
exposure time.
sure clearly shows the superior performance of tricationic
phosphating formulation III (Rpo—1.62 × 106 Ohm·cm2)
compared with formulation I (Rpo—8.9 × 104 Ohm·cm2)
and formulation II (Rpo—3.6 × 104 Ohm·cm2). As shown
in Figure 12, the Rpo values for formulation III remain
quite steady at this high value even after 500 hrs of salts
Figure 13. Coating capacitance (Cc) as a function of salt
spray exposure time.
Figure 14. Break point frequency (fb) as a function of salt
spray exposure time.
spray exposure, while for other two formulations, Rpo
values fall sharply indicating the rapid degradation of the
protective value of those phosphate coatings.

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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