Note: Descriptions are shown in the official language in which they were submitted.
~469~Z
This invention relates to a method for treating
ferrous metal and galvanized articles, and to articles thus
treated~ More particularly, it relates to a method for
producing a:conversion coating on a ferrous metal or galvanized
article which comprises contacting said article with an .
aqueous solution of a trivalent chromium compound,
It has been the practice for many years to form
conversion coatings on meta~ sur~aces by treatment with
solutions of various chemicals whieh react with the surface
to form a coating which protects the metal against corrosion
and also serves as a paint base. Among the commonly used
conversion coating compositions are aqueous phosphate and
chromate solutions, Many of these are known in the art;
I among the simplest of the phosphate compositions are the
- 15 so-called "iron phosphates" which comprise solutions of
alka-li metal phosphates or the like, and which react with
the iron of the ferrous metal surface to form an iron
phosphate coa~ing~ Most iron phosphate solutions must be
contacted with the metal at elevated temperatures, typically
50~C, or above. In view of the current need to conserve
energy, it is of interest to develop conversion coating
solutions which may be used at lower temperatures, especially ~`
room temperature,
Other conversion coating compositions in general
use are the chromium-containing solutions known in the art.
.
.. .
These generally contain hexavalent chromium in the form of
chromate or dichromate, frequently in combination with
trivalent chromium or in forms such that partial reduction
to trivalent chromium takes place during treatment of the
ferrous metal surface, Recent emphasis on water pollution
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problems has drawn attention to the fact that chromates are
serious pollutants To meet water quality standards, it is
frequently necessary to go through a multi-sta~e purification
sequence in order to remove chromates from metal treatment
process effluents, ~ypical steps in this sequence include
reduction ~f the hexavalent chromium to trivalent chromium
and precipitation with lime or some similar chemlcal. The
result is that the chromate content of the effluent water is
decreased, frequently to zero or near zero, but the expense
~ of the treatment process to the user is quite high
A principal object of the present invention,
therefore, is to develop new methods and compositions for
the treatment of ferrous metal surfaces to improve adhesion
of siccative coatings and to increase corrosion resistance,
especially when the surfaces are further coated with such
; siccative coatings
~ A further object is to provide a treatment system ;~
.
for ferrous and galvanized metals which provides results
similar to those of iron phosphates and other phosphate
conversion coatings, but which can be applied at low tem-
peratures,
` Another object is to provide a treatment system
which can be used without a resulting discharge of hexavalent
,
chromium or other pollutants as park of the effluent
Other objects will in part be obvious and will in
~part appear hereinafter.
The present invention is based on the discovery
i
~ j that aqueous solutions containing chromium entirely in the
e
~ , trivalent stage are advantageously used to produce conver-
,,
30 ~ sion coatings on ferrous and galvanized metals, especially
steel and galvanized steel.
~111469~Z
Any aqueous solution of a trivalent chromium com-
pound may be used in the method of this invention. Thus,
solutions of chromium sulfate, chromium nitrate and the like
are useful. The preferred trivalent chromium solutions,
however, are those prepared by reduction of an aqueous
hexavalent chromium-containing solution. Many suitable
reducing agents, both organic and inorganic, are knownO The
organic ones in~lude such materials as methanol, ethanol,
ethylene glycol, formaldehyde, hydroquinone and the like.
The methods of reducing hexavalent chromium with -~
organic reducing agents are ~nerally known in the art.
For example, U.S. patents 3,063,877 and 3,404,045 describe
methods or reducing chromium trioxlde with formaldehyde
and methanol, respectively. ~owever, the amounts of the
reducing agents used according to those patents are insuf-
ficient fGr complete reduction of hexavalent to trivalent
chromium. According to the present invention, the amount
of reducing agent is increased to at least the amount
required for complete reduction.
AmOng the suitable inorganic redu~ing agents are
alkali metal iodides, ferrous salts, sulfur dioxide, alkali
`i metal`sulfites or bisulfites and the like. The alkali metal
sulfites and bisulfite~, notably the bisulfites and especially
~,~ sodium and potassium bisulfite, are preferred. As with the
. - i
~ ~5 organic reducing agents previously described the inorganic
.
~` reducing agents are employed in amounts sufficient to com-
pletely reduce hexavalent to trivalent chromium. HOwever, a
substantial excess of sulfite or bisulfite over this amount
should generally not be present since the presence of
sulfite or bisulfite in the final treatment solution sometimes
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1~916~.z
results in the formation of "blush rust" on the metal surace
being treated. In general, the amount of sulfite or bisulfite
employed should be less than about a 1~ excess (by weight)
over the stoichiometric amount required for complete reduc-
tion of hexavalent to trivalent chromium. If, because of
incom~lete reaction, further reducing agent is necessary,
methanol or a similar organic reducing agent, or an inorganic -
reducing agent other than suifite or bisulfite, should be
used to complete the reduction.
The preparation of trivalent chromium solutions
Nseful (usually after dilution with water as described ~ ~ ;
hereinafter) in the method of this invention is illustrated
by the following examples. All parts are b~ weight. Equiva- ;
lents of acid are calculated on the basis of acidic hydrogens.
An equivalent of a material to be oxidized or reduced is its
molecular weight divided by the number of units of valence
change in the oxidation~reduction reaction.
Example 1 ;~
: . -- . .
A solution of 51 parts (9.55 e~uivalents) of
` methanol in 144 parts of water is added dropwise, with
external cooling, to a solution of 300 ~arts ~8.96 equiva-
lents) of 99.5% assay chromium trioxide in 204 ~arts of `
water. After methanol addition is com~lete, 900 parts (9.25
. .
equivalents) of concentrated hydrochloric acid is added
., . ~ , .
over several hours, with stirring, at a temperature of
a4_~8C The product is the desired trivalent chromium
solution. ~ -
!
The procedure of Example 1 is repeated, except
that 850 parts (9.04 equivalents) of concentrated nitric
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acid i9 used, A similar product is obtained,
Example 3
The procedure of Exam~le 1 is repeated, except
that 550 parts (9.17 equivalents) of glacial acetic acid is
used, A similar product is obtained,
Example 4
The procedure of Example 1 is repeated, except
that 465 parts (9.12 equivalents) bf concentrated sul~uric
acid is used, A similar product is obtained,
-
Example 5
Solid sodium bisulfite, 12,2 parts (0,2~5 equiva-
lent), is added gradually, with stirring~ to a solution
of 7~8 parts (0,2~3 equivalent) of 99.5~ assay chromium
trioxide in 80 parts of water. The addition rate of the
sodium bisulfite is such that the temperature does not
' exceed 65C, The product is the desired trivalent chromium
solution,
In the metal treatment operation in which the
~ " method of this invention is used, the metal surface is usually
k
first cleaned by chemical and/or physical means to remove
any grease, dirt and oxides, When the trivalent chrornium
solution contains a surfactant as described hereLnafter,
the initial chernical cleaning stage may be unnecessary,
The surface (or any portion thereof for which
25 ~ treatment is desired) is then rinsed-with water and treated
` ~ w1th the tri~alent chromium solution, Treatment may be by
any of the commonly used techniques such as spraying,
brushing, dipping, roller-coating, reverse roller-coat-Lng,
and~flow-coating, ~he solutions of the present invention
5-
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469~2
are particularly useful in a spray system. The concentra- -
tion of chromium in the metal treatment solution is ~enerall~
about 0.01-0.~% b~ weight and may be somewhat lower for
spray applications (e.g., 0.05-0.11%) than for immersion
applications (e.g., about 0.11-0.2%).
The pH of the trivalent chromium solution during
application is about 3.5-6.0, usually a . 0-5Ø Since a con-
centrate prepared from an already formed trivalent chromium
salt (e.g., chromium chloride or nitrate~, or by reduction
of hexavalent chromium with certain reducing agents such
as sulfur dioxide, may be too acidic to produce a metal
treatment solution having a pH within this range, it may be
necessary to adjust the pH of the concentxate or treatment
solution by adding an alkaline reagent thereto. Such
reagent (usually ammonium hydroxide, sodium hydroxide or
potassium hydroxide~ and preferably one of the latter two) r,
is most preferably added to the concentrate.
On the other hand, when bisulfîte or the like is
the reducing agent it may sometimes be necessar~ to acidify
the water used for diluting the concentrate to form the
treatment solution, so as to avoid the formation of a pre-
cipitate or a colloid during dilution. When this is done,
the ~ater should have a pH after adjustment below 7.0, pre-
; ferably about 5.0-5.5. The preferred acid for adjusting pH
is sulfuric acid.
` ( The temperature of application of the trivalent
chromium solution to the metal surface is usuallv about
` 0-50C., although it may be higher (up to about 75C.) if
, . .. . .. .
l desired, and is preferably about 10-30C~ Thus, it will be
.~ j .
appreciated that the method of this invention requires the
use of temperatures much lower than are generally required
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10~6912
for the application of an iron phosphate coating, and con-
sequently that a lower energy input is necessary. This low
energy input is another advantage of the method of this
invention.
Following the trivalent chromium treatment, the
metal surface is usually rinsed with water, usually also at
a temperature below about 50C. and advantageously at room
temperature, and is then dried. Drying may be by simply
air-blowing at room temperature or may be at higher temp-
eratures, usually up to about 65C.
~ The conversion coating produced on a ferrous
metal surface by the method of this invention is usually
deep blue in color~ In addition to furnishing an attrac-
tive appearance to the metal, it improves corrosion
resistance and paint adhesion.
After a metal article has been treated in accor-
dance with the method of this invention, it is preferred to
apply an organic coating composition which may be a siccative
coating such as paint, lacquer, varnish, synthetic resin,
enamel or the like (which are preferred), an electrostat-
ically deposited powder coating, or any other suitable type.
s Examples of siccative coatings which may be used are the
acrylic, alkyd, epoxy, phenolic, melamine and polyvinyl
alcohol resins and paints.
` 25 Application of a siccative coating composition
can be effected by any of the ordinary techni~ues such as
brushing, spraying, dipping, roller-coating, flow-coating,
electrostatic or electrophoretic attraction, etc. I The
coated article is dried in the manner best suited for the
siccative coating composition employed, e.g., by air-
drylng at ambient or elevated temperature, baking in an oven,
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or baking under infra-red lamps. In most instances, the
thickness of the dried ~ilm of the siccative organic coating
composition will be about 0.1 10.0 mils, more often about
0.3-5.0 mils.
From the above description, it will be apparent
that the advantages of this invention include the avoidance
of hexavalent chromium as a pollutant in the process
effluent, lower energy requirement for formation of a con-
version coating, improved paint adhesion of the treated
metal, and improved corrosion resistance, especially of the
painted metal surface.
It is within the scope of this invention to include
in the trivalent chromium solution a minor amount of a
sur~actant, usually a non-ionic surfactant. When the sur-
factant is present, the chromium solution mà~ be used as a
cleaner as well as a conversion coating~solution, thus
' eliminating the initial chemical cleaning step described
hereinabove.
The effectiveness of the method of this invention
for forming conversion coatings on ferrous metal surfaces
., .
is demonstrated by a procedure in which panels of cold-
rolled steel are cleaned by spraying with a sodium phosphate-
based cleaner at 66-72C. and rinsed with water at room
temperature. They are then sprayed at 18-22C. with a
solution of Example 5. The treated panels are rinsed again
with water, dried under infrared lamps and painted with a
while alkyd-melamine baking enamel. Six sets of test panels
are processed at the end of consecutive half-days of working
of the spray line.
A set of control panels is similarly cleaned and
~ rinsed, treated at 50-60C. with a commercial chlorate- ;
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i9~2
accelerated iron phosphate conversion coating solution
having a p~l of 5-~, and rinsed with water and with a
commercial hexavalent-trivalent chromium post-rinse solu-
tion. These panels are also painted as described above.
The paint film on each panel is ruptured down to
-~ the bare metal by scoring a six-inch line on the surface
of the panel. The scored panel is placed in a cabinet
containing a 5% aqueous sodium chloride solution at 95F.
Air is bubbled through the solution to produce a corrosive
salt atmosphere which acts on the surface of the test panel
suspended above the level of the salt solution. The panels
remain in this atmosphere for 120 hours after which they
are removed, washed with water and dried with a cloth. A
pressure-sensitive tape is then applied to each panel and
removed suddenly. This ~rocedure is repeated until no more
paint can be removed in this manner. The loss o~ adhesion
. , .
; caused by corrosion from the scribed line is measured in
thirty-seconds of an inch. -~
' When measured in this manner, the average loss of
adhesion for the panels treated by the method of this inven-
tion is 0.7S thirty-seconds of an inch. The average for the`
control panels is 0.~4 thirty-seconds of an inch. Thus, the
method of this invention gives results when the chromium
olution is applied at room temperature or below, which are
~' ~5 comparable to those provided by a commercial iron phosphatesolution applied at elevate~ temperatures and ~ollowed by a
commexcial hexavalent chromium-containing rinse.
The trivalent chromium solutions used in the
method of this invention may also be used as post-rinses
for phosphated metals, especially ferrous metals. The
phosphating treatment used prior to this post-rinse may he
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1~46912
iron phosphate, zinc phosphate, calcium- or magnesium-
modified zinc phosphate, or the like.
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