Note: Descriptions are shown in the official language in which they were submitted.
~o 94/14999 ~15 0 S ~ S PCT/US93/12044
Description
SUBSTANTIALLY NICKEL-FREE PHOSPHATE CONVERSION COATING
COMPOSITION AND PROCESS
BACKGROUND OF THE INVENTION
Field of the Invention
This invention relates to a composition and process for forming a phosphate
conversion coating on active metal surfaces in order to increase the corrosion resist-
5 ance of the s~ ces, either as treated or after subsequent conventional overcoating ofthe conversion coating layer formed by an organic based protective coating such as
a paint or lacquer. A composition according to this invention is well adapted to treat-
ing any of a variety of base metals, including at least steel and galvanized steel and
~l~lminllm and ~lnminllm based alloys.
10 Statement of Related Art
A wide variety of phosph~te. conversion coating compositions and processes
are already described in the art. Those believed to be generally considered of highest
quality at present include zinc, nickel, and at least one other divalent metal such as
m~ng~nPse in the composition.
DESCRIPrION OF THE INVENTION
In this description, except in the working ex~mples and claims and wherever
expressly in~ic~t~.d to the contrary, all mlme.ric~l specifications of amounts of
materials or conditions of reaction or use are to be understood as modified by the term
"about" in describing the broadest scope of the invention. Practice of the invention
20 within the numerical limits given is generally preferred.
Object of the Invention
Nickel has been implicated as a carcinogen in some studies, so that avoidance
of its use in metal fini~hing is desirable for health reasons. Nickel is also considered
a serious pollutant, and at the levels of 0.5 - 1.5 grams per liter (hereinafter often ab-
25 breviated "g/L") at which it is conventionally used, nickel contributes significantly tothe cost of the zinc phosphate conve,~ion coating compositions. It is an object of this
invention to minimi7~. or elimin~te the use of nickel, without wol~elling the quality
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2 ~ 5
of conversion coating obtained with conventional compositions cont~ining zinc, nickel,
and m~ng~n~se.
Summarv of the Invention
A working composition according to this invention is an aqueous liquid compo-
5 sition that compri.~çs, preferably consists essentially of, still more preferably consists
of, water and:
(A) from 3 to 50, more preferably from 5 - 20, still more preferably from 13 - 18
g/L of dissolved phosphate ions;
(B) from 0.2 to 3, or with increasing p,ere~nce in the order given, from 0.45 to
o 2.0, 0.57 to 1.24, 0.60 to 0.95, or 0.71 to 0.87, g/L of dissolved Zn+2 ions;
(C) from 1 to 200, or with increasing preference in the order given, from 20 to100, 24 to 74, or 24 to 60 milligrams per liter ("mg/L") of dissolved Cu~2 ions;(D) an effective amount of a convèntional accelerator exclusive of nitrate ions;
(E) from S to 40, or with increasing preference in the order given, from 7 to 25,
13 to 24, 16 to 22, or 18 to 21 "points" of total acid; and
(F) from -1.0 to +3.0, or with increasing p~re,ence in the order given, from -0.5
to +1.5 or -0.5 to +1.0 points of free acid; for substrates other than ~ minum
and operating temperatures above 40 C, ranges of 0.0 to 0.8, or better, 0.35
to 0.65 points of free acid are still more preferable; but if operation is at a
temperature lower than 40 C or the substrate is ~lnminllm, ranges of -0.5 to
0.0, or better, -0.5 to -0.2, points of free acid are still more preferable; and, op-
tionally,
(G) up to 2 g/L, preferably from 0.1 to 0.8 g/L, or still more preferably from 0.40
to 0.65 glL of a total of Mn+2 and Co+2 ions;
(H) up to 2 g/L, preferably from 0.5 to 1.0, or still more preferably from 0.70 to
0.80 g/L of Mg+2 ions; and, optionally,
(J) up to 5.5, preferably 1.0 to 4.0, still more preferably from 1.5 to 2.5, g/L of
free fluoride as measured by a fluoride sensitive electrode; and, optionally,
(K) not more than 40, or, with increasing p,erelellce in the order given, not more
than 20 or 11 g/L of So4-2 ions; and, optionally,
(L) not more than 4.5, or, with increasing preference in the order given, not more
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~0 94/14999 215 0 ~ ~ 5 PCTIUS93/12044
than 3.4, 2.0, 1.8, 1.3, or 1.0 g/L of NO3- ions.
It is to be understood that the specification of constituents in ionic form herein implies
the presence of some counterions of opposite charge if needed to m~int~in chargeneutrality in the composition as a whole.
"Points" of free acid and total acid are defined for use herein as the number
of millili~er.~ (hereinafter "ml") of 0.1 N NaOH solution required to titrate a 10 ml
sample of the composition, to a phenolphth~l~.in end point (pH 9.0) for total acid and
a bromthymol blue end point (pH 3.8) for free acid, except that if the composition has
a pH greater than 3.8 initially, the points of free acid are defined as the negative of
the number of ml of 0.1 N strong acid solution required to titrate a 10 ml sample of
the composition to a pH of 3.8.
A process according to this invention comprises at a minimum a step of con-
tacting a metal surface to be treated with a composition according to the invention for
a sufficient time to form on the metal surface a detectable conversion coating. Con-
ventional metal surface cl~ning and/or activation steps before contact between the
metal to be treated and compositions according to the invention may be used if de-
sired, and are generally preferred, as part of a process according to this invention. A
process according to the invention also may, and usually preferably does, include con-
ventional steps subsequent to the contact between the metal surface to be treated and
the compositiQnC according to the invention. These subsequent steps, e.g., may in-
clude rinsing with water, any conventional reactive post treatments, e.g., with compo-
sitions according to the te~hingc of U. S. Patent 4,963,596 or with chromate contain-
ing solutions, and painting or otherwise protecting the surface with an outer coating
of an organic based solid material.
Another embodiment of the invention is a concentrate composition, from which
a working composition as deffned above can be prepared by dilution with water only,
or by dilution with water and ~ tion of an unstable accelerator component such as
nitrite ions.
Description of Preferred Embodiments of the Invention
The phosphate ions required for the compositions according to this invention
are preferably Po4-3 ions or other ions derivable from less complete ionization of ortho-
WO 94/14999 215 0 5 ~ ~ PCT/US93112044 ~
phosphoric acid (H3PO4). Any free unionized phosphoric acid that may be present is
considered part of the content of phosphate ions, to the extent of its stoichiometric cor-
respondence to PO4~3 ions. Other free phosphoric acids such as metaphosphoric acid
and condensed phosphoric acids such as pyrophosphoric acid and all anions derivable
6 from them may also be used to supply the necess~ry phosphate ions. Preferably the
phosphate ions are derived from orthophosphoric acid and/or its neutral or acid salts
of the metal cations also specified above as part of the compositions according to this
invention.
The zinc cations required as part of the compositions are preferably derived
from neutral or acid zinc salts of orthophosphoric acid, which may be formed in situ
by dissolving zinc or zinc oxide or hydroxide in a solution containing the acid.The cupric cations required as part of the compositions according to this inven-tion may be derived from salts such as cupric sulfate and/or nitrate, or may be ob-
tained by dissolving cupric oxide in part of the phosphoric acid used.
The accelerator component required in compositions according to the invention
preferably incllldes at least one of the following: (i) from 0.01 to 0.2 g/l of nitrite
ions, (ii) from 0.5 to 5 g/l of H2O2, (iii) from 0.05 to 2 g/l of m-nitroben7~.nP.sl11fonate
ions, (iv) from 0.05 to 2 g/l of m-nitrobenzoate ions, (v) from 0.05 to 2 g/l of p-nitro-
phenol, and (vi) from 0.1 to 10, more preferably from 0.5 to 6, or still more preferably
from 0.5 to 2.0, g/L, measured as it stoichiometric equivalent as hydlo~ylamine, of a
component capable of filrniching hydroxylamine in water solution. Most preferably,
the accelerator is hydroxylamine sulfate (hereinafter often abbreviated "HAS") or a
similar safe and readily available source of dissolved hydroxylamine. The secondmost preferable accelerator is nitrite ions.
The acidity of the compositions according to the invention is preferably derivedfrom phosphoric, sulfuric, and/or nitric acids.
Part or even all of the free fluori(le may be derived from complex fluoride ionssuch as fluoborate (BF4-2), fl~loh~fn~te (HflF6-2), fl~locilic~te (SiF6-2), fluotitanate (TiF6-2),
fluozirconate (ZrF6-2), and mixtures thereof; more preferably, from fluoborate and fluo-
silicate and mixtures thereof. However, simple fl~lori~les such as alkali metal fluorides
are also entirely satisfactory sources of free fluoride.
~TVO 94/14999 215 0 5 4 ~ PCTlUS93/12044
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With increasing preference in the order given and with independent preference
for each noted component, compositions according to the invention contain no more
than 0.5, 0.2, 0.10, 0.07, 0.03, 0.01, 0.005, or 0.001 g/L of each of Ni~2, any cation
with a valence of three or higher, chloride ions including complex chloride ions, and
chlorate ions. Independently, if chloride ions are present in sufficient quantity to be
readily determined analytically, it is preferred that the ratio of the concentration in glL
of complex fluoride ions to the concentration in g/L of chloride ions have a value of
at least 8:1, or more preferably a value of at least 14:1.
In a process according to the invention, contact between the metal surface to
be treated and a composition according to the invention may be accomplished by
spraying, dipping, or any other convenient method or combination of methods. Thetemperature during contact between the metal treated and the composition according
to the invention preferably is, with increasing preference in the order given, in the
range from 21 to 85, 25 to 55, or 31 to 44, C. The total time of contact between
the composition according to the invention and the metal surface to be conversion
coated yreferably is, with increasing preference in the order given, in the range from
5 sec to 15 mimlt~s (hereinafter often abbreviated "min"), 15 seconds (hereinafter
often abbreviated "sec") to 10 min, 30 sec to 5 min, or 90 to 120 sec. The add-on
mass of the phosphate coating formed preferably is, with increasing preference in the
order given, in the range from 1.1 to 5.4, 1.6 to 4.3, or 2.2 to 3.8, grams per square
meter (hereinafter "g/m2) of surface treated. The weight percent of copper in the coat-
ing formed preferably is, with increasing preference in the order given, from 0.50 to
10, 1.0 to 8.0, 2.0 to 6.0, 3.0 to 4.1.
Further appreciation of the present invention may be had from considering the
following ex~mp]çs and comparative ex~mplçs which are intended to illustrate, but not
limit, the invention.
Composition Examples and Comparison Examples Group I
These compositions, except for one comparison çx~mple, illustrate the use of
HAS accelerator. Specific compositions are shown in Table 1 below. These composi-
tions, except for comparison example 13 which was prepared from a commercially
compounded concentrate, were prepared according to the following general procedure:
wo 94/14999 ~ Q ~ 4 5 PCT/US93/12044~
An amount of water equal to about three-quarters of the volume eventually desired for
the composition was initially used, and an amount of zinc dihydrogen phosphate suffi-
cient to provide the desired zinc ion concentration in the final desired volume was dis-
solved in this water. An amount of 75 % aqueous solution of orthophosphoric acidthat was sufficient, together with the previously added zinc dihydrogen phosphate, to
provide the desired final concentration of phosphate ions in the desired final volume
was then added. Sufficient amounts of sodium fluoride to provide the desired fluoride
ion content, of HAS to provide the desired hydroxylamine content and some sulfate,
of cupric sulfate pentahydrate to provide the cupric ion and part of the sulfate ion con-
tents, of m~n~nous nitrate to provide any desired m~ng~nese cation content, and of
m~gnPcillm hydroxide or m~gnPsillm nitrate to provide any desired m~nesillm ion
content were then added to the previously prepared solution in any order. Finally, the
composition was adjusted with additions of sodium hydroxide solution and additional
water as needed to produce the specified free acid and total acid concentrations.
Process Examples and Comparison Examples with Compositions of Group I
Test panels of cold rolled steel, galvanized steel, and/or conventional ~lllminllm
alloys used in automobile and appliance m~nnf~ture were used and subjected to the
following general sequence of process steps: (1) Conventional ~lk~line cleaner for
120 sec at 43 C, (2~ Water rinse at 38 C for 60 sec; (3) Conventional colloidal
tit~ninm phosphate activator for 30 sec at 38 C; (4) Contact with one of Composi-
tions 1 - 12 from Table 1 below for 120 sec at 35 C or at 43 C; (5) Cold water rinse
at 20 - 25 C for 60 sec; (6) Post treatment for 30 sec with a conventional commer-
cial post treatment composition cont~ining hexavalent and trivalent chromium; and (7)
Rinse with deionized water for 30 sec at 20 - 25 C.
~VO 94/14999 2 15 ~ 5 4 5 PCT/US93/12044
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Composition and Process Examples Group II
These compositions were prepared in the same general manner as for Group
I, except that sodium nitrite instead of HAS was used as the accelerator and cupric ni-
trate with an average of 2.5 molecules of water of hydration per cupric ion was used
5 as the copper source. Compositions are as follows:
Component ~/L of Component in:
Example 14 Example 15
Zn(NO3)2 4.74 3.6
75 % by weight aqueous H3PO4 12.4 20.4
~o Cu(NO3)2 2% H2O 0.29
NaF , 1.1 1.5
NaNO2 0.12 0.11
Mn(NO3)2 2.3
Total Acid Points 17.0 18
Free Acid Points -0.3 0.4
Example 14 was used for spray phosphating at 33 C in a process sequence other-
wise like those of Group I above. Example 14 was used for immersion phosphating
at 57 C in a process sequence otherwise like those of Group I above.
20 Benefits of the Invention
Not only were the cost reduction and pollution reduction objectives of the in-
vention achieved, but also there were several unexpected and surprising benefits from
the invention, at least in its most preferred embo~limentc Improved corrosion resist-
ance, particularly to ~lk~lin~ corrosion; operation at a lower temperature than for con-
25 ventional Zn-Ni-Mn phosphating compositions, with at least comp~r~ble and some-
times better quality; better adhesion to paint applied over the conversion co~ting~c; and
improved coatings on zinciferous surfaces such as galvanized steel.
