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-8
Corrosion Resistance of Iron Phosphate Coiniings
and the Influence of Accelerator Choice
George G6recki,
Ardrox Inc., Lake Bluff, Ill.
T
he corrosion resistance of iron
phosphate coatings is directly related to the type of accelerator employed in the phosphating bath. Accelerators affect the kinetics of the
phosphating reaction mechanism and,
in large part, determine the composition of the coating. Three types of
accelerators, sodium molybdate, sodium chlorate, and sodium m-nitrobenzenesulfonate (SNBS) were examined.
These accelerators find a wide use in
industrial phosphating operations. Molybdate- and SNBS-accelerated baths
can be employed in three- or five-stage
phosphating lines, while chlorate-accelerated baths are normally reserved
for five-stage washers only.
Corrosion resistances were compared by means of a salt spray test.
Results were consistent over a broad
range of paints and were independent
of the choice of final rinse.
The relationship between corrosion
protection and accelerator choice is
described by how the accelerator affects the mechanism of phosphate
coating formation. The nature of the
accelerator determines the reaction
pathway for the bath. The various
pathways can then lead to different
products. As a result, the composition
of the conversion coating is dependent
on these reaction products. The constituents of the coating ultimately determine its resistance to corrosive attack.
~~~
~
EXPERIMENTAL
Testing was carried out on standard
steel test panels. The panels went
through a five-stage pretreatment process: cleaning bath, water rinse,
phosphating bath, water rinse and final
rinse. An alkaline cleaner was used in
the first stage. A water-break-free
surface was easily attained, as the
panels were not heavily soiled. The
phosphating baths were of similar
composition and differed primarily in
the choice of accelerator (see Table I).
Panels were phosphated under the
following conditions: 60°C, 3-minute
AUGUST 1992
Table I. Phosphating Bath Compositions
Molybdate Bath
k/L)
Chlorate Bath
(UU
SNBS Bath
fUU
H F 4
3.00
2.10
1.95
-
3.00
4.50
Accelerator
0.12
3.75
4.50
0.75
Comonent
NH,H,PO,
NaH,PO,
-
contact time and pH of 4.2 to 4.8. The
bath pH was set at a level which would
optimize coating formation for a particular bath. The molybdate-accelerated bath was run at pH 4.2, the
chlorate-accelerated bath at pH 4.8,
and the SNBS-accelerated bath at pH
4.6. Two different final rinses were
employed. One was a Cr(V1)-Cr(II1)
type, while the other was a chromiumfree rinse.
Panels were painted with nine different topcoats or primers, which are
common to the metal-finishing industry. The paints were applied according
to manufacturers’ instructions. In order
to insure completeness of cure, panels
were not subjected to corrosion testing
until at least 72 hours after having been
painted. Three panels of each phosphate and final rinse type were prepared, giving 18 distinct sets of pretreatments and paints.
Corrosion testing was performed by
a method similar to that described in
ASTM B 117. The edges of the painted
panels were masked with electrical
tape. Each panel received a 10-cmlong, diagonal scribe down to bare
metal. The panels remained in the salt
spray cabinet for a duration that was
determined by paint quality. All panels
coated with a particular paint did,
however, receive the same amount of
exposure to the corrosive environment.
Upon removal from the salt spray
cabinet, the panels were rinsed in tap
water and dried with paper towels.
All loose paint and corrosion products were scraped away in the direction
of the scribe with the flat end of a
spatula. Adhesive tape was applied to
the field areas of the panels and any
-
pick-off that occurred was noted.
Measurement of the total width of the
creepage surrounding the scribe after
scraping was made at eight equally
spaced locations along the scribe. The
average of these eight measurements
was recorded for each panel. The
values for each of the three panels of
each phosphate and final rinse type
were then averaged to give the final
result.
RESULTS AND DISCUSSION
The results show that panels
phosphated with the chlorate- and the
SNBS-accelerated baths give substantially better salt spray performance
than those treated with the molybdateaccelerated bath. This conclusion is
valid regardless of which final rinse
was used (see Tables 11 and 111). The
values in the tables represent the
average total width of corrosion creepage expressed in mm. The wide disparity in corrosion resistance is evident in
all nine cases for chromium-rinsed
panels and in seven cases for the panels
treated with a chromium-free final
rinse.
The results also show that the chlorate and SNBS coatings provided comparable corrosion resistance. The excellent performance of the SNBS coating means that high levels of corrosion
protection can be attained by users of
three-stage phosphating operations.
Such users had previously been restricted to molybdate-acceleratedbaths
because cleaning and coating formation need to occur simultaneously in a
three-stage system. Chlorate-accelerated baths have traditionally provided
27
I
Table 11. Salt Spray Results, Chromium Final Rinse
Paint ID
Exposure Time
fhr)
Molybdate Bath
f")
Chlorate Bath
("1
SNBS Bath
48
48
265
287
72
624
124
119
48
14.2
5.9
24.3
26.6
11.1
4.7
9.0
6.9
17.6
2.6
0.8
3.7
1.6
2.6
1.3
0.5
0.8
2.9
4.9
0.8
5.3
1.4
2.8
0.8
0.3
0.7
2.5
A
B
C
D
E
F
G
H
I
poor performance in such systems.
This poor performance has been attributed to the incompatibility of chlorate
and surfactants that are typically used
in three-stage operations.
Examination of the fundamental
_chemicalreactionssfph~sphatingprovide insight to the observed differences
in salt spray performance. When steel
is treated with a phosphating bath,
metallic iron is dissolved, giving ferrous iron. Ferrous iron can then react
with the various phosphate ions in
solution, producing ferrous salts (Eq.
2), like tertiary ferrous phosphate.
Ferrous iron can also react with accelerator (Eq. 3) in the bath, leading to the
formation of ferric iron, which also
reacts with phosphate ions to produce
ferric salts (Eq.4). (Note that in Eq. 3,
Acc,, refers to the original, oxidized
form of the accelerator and Acc,~ is
the reduced form.) The mechanism of
coating formation comprises an intricate network of chemical reactions.
The above discussion picks out the
essential reactions to illustrate some of
the roles played by the accelerator.
This simplified scenario is summarized
in Eqs. 1-4:
H3P04 + Fe +
Fez")
+ H.2p042-(aq>+ H,(g) (1)
3 Fe2 (aq) + 2 P04"(aq) +
Fe3(P04>2(s>
(2)
Fe2+(aq)+ ACC,, +
Fe3+(aq)+ AccEd
(3)
f")
determined by the nature of the accelerator employed.
A competition between phosphate
ion and accelerator exists in a
phosphating bath, as both species are
intent on reacting with ferrous ion. Any
preference that ferrous ion may show
to react with either phosphate or accelerator depends on the oxidizing
strength of the accelerator. As the
oxidizing power of an accelerator increases, the fraction of ferrous ions
reacting with accelerator increases at
the expense of phosphate ion. As more
ferrous ions are then converted to the
ferric form, the amount of ferric phosphate salts found in the conversion
coating increases.
Previous work involving iron phosphate coatings' has shown that corrosion resistance improves as the amount
of ferric salts in the coating increases.
Coatings produced by chlorate- and
SNBS-accelerated phosphating baths
containabout 30% FeP04.2H20, while
the coating produced by a molybdateaccelerated bath contains about 14%.
The molybdate coating also contains
about 23% Fe3(P04),.8H,0. This ferrous salt is not present in either the
chlorate or the SNBS coatings.
The differences in the coating compositions can be explained by examining the oxidizing strengths of the
..
accelerators. The reduction potentials
(EO) of both chlorate2 (E0=1.451 V)
and of SNBS3 (E0=0.60 V) are relatively large, which allows for easy
conversion of ferrous ion to the ferric
state, leading to the formation of ferric
phosphate. That chlorate and SNBS
have such high reduction potentials
accounts for the fact that these two
coatings are so strikingly similar in
composition. It is then no surprise that
chlorate and SNBS provide approximately equal levels of corrosion resistance. Molybdate, on the other hand, is
the weakest oxidize# (E0=0.48 V) of
the three and its coating contains less
than half of the ferric salts found in the
chlorate and SNBS coatings.
Clearly, the ability to form ferric
ions is an important factor when a
high-quality conversion coating is desired. When chlorate or SNBS are
present in the phosphating bath, the
reaction mechanism is better able to
direct more ferrous ions through Eq. 3
than when molybdate is present. The
ferric ions produced in Eq. 3 are
eventually found as ferric phosphate.
Molybdate's relatively poor oxidizing
ability prevents it from furnishing
ferric ions as efficiently as chlorate or
SNBS can. Thus, fewer ferric ions are
available for the formation of ferric
phosphate. This also means that phosphate ions in a molybdate-accelerated
bath can remain competitive for reaction with ferrous ions, which leads to
the production of ferrous salts. As the
fraction of ferric salts in the conversion
coating decreases, so does its corrosion
resistance.
The superior corrosion resistance
displayed by the chlorate and SNBS
coatings in the salt spray tests indicates
that these coatings, rich in ferric salts,
are less susceptible to corrosive attack
than is the molybdate coating. The first
step in the corrosion process is dissolu-
+
Fe3+(aq)+ PO?-(aq)
+ FeP04(s)
(4)
Once iron has been dissolved in the
phosphating bath, the mechanism can
branch into different paths via Eqs. 2
and 3. Ferrous iron reacts with both
phosphate and accelerator, the end
result being a conversion coating composed of a mixture of ferrous and ferric
phosphate salts. The bath kinetics are
28
Table 111. Salt Spray Results, Chromium-Free Final Rinse
Paint ID
A
B
C
D
E
F
G
H
I
Exposure Time
fhr)
48
48
265
287
72
624
124
119
48
Molybdate Bath
I")
22.1
14.7
>go
44.1
43.7
>90
42.3
10.9
59.4
Chlorate Bath
f")
8.5
12.1
10.0
14.4
12.8
11.0
9.3
9.5
16.2
SNBS Bath
f")
11.1
14.4
9.3
10.1
12.3
9.4
16.0
-
METAL FINISHING
tion. Naturally, corrosion slows down
in the presence of a phosphate coating.
As dissolution of the phosphate coating becomes more difficult, the more
slowly corrosion proceeds. This probably means that ferric phosphate salts
are more insoluble than their ferrous
counterparts. The salt spray results
have shown that increasing the fraction
of ferric salts in the coating improves
corrosion protection. Future developments in iron phosphate technology
must keep this fact in mind.
CONCLUSION
The corrosion resistance afforded by
iron phosphate coatings increases
when the amount of ferric salts in the
conversion coating is maximized. Bath
conditions determine the composition
of the coating. Accelerators that have
high reduction potentials are most
capable of producing the amount of
ferric ions necessary for optimum
Biography
corrosion resistance. Chlorate and
SNBS are good examples of accelerators that can lead to phosphate coatings
containing a large fraction of ferric
salts. The relatively low reduction
potential of molybdate permits the
formation of an appreciable amount of
ferrous salts. The presence of the
ferrous salts diminishes the coating’s
corrosion resistance.
MF
George Gorecki is a research chemist for Ardrox, Inc. He earned his M.S.
in chemistry in 1989 from DePaul
University. He has studied metal-finishing processes for seven years. His
research interests include the chemistry
of phosphating and the related corrosion phenomena.
References
Want to write for
Metal Finishing?
1. G6recki, G., “Iron Phosphate Coatings: Morphology and Corrosion Resistance,” presented at CORROSION ’91, paper 381,
Cincinnati; NACE, Houston; March 1991
2. Weast, R.C. (Ed.), CRC Handbook of Chemistry and Physics, 67th Ed., p. B-207; CRC
Press, Boca Raton, Ha; 1986
3. Filipiak, K. and P. Matyjewski, “Electroreduction of m-Nitrobenzenesulfonic Acid,”
Zeszenie Naukowe-Politechnika
Lodzka,
Chemia,41:220 1987
4. Dean, J.A. (Ed.), Lunge’s Handbook of
Chemistry, 12th ed., pp. 6-12; McGraw-Hill,
New York; 1979
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