Note: Descriptions are shown in the official language in which they were submitted.
BACKGROUND OF THE INVENTION
The formation of chromate conversion coatings on sur-
faces of various metals, such as zinc and cadium, is presently
the most common technique of imparting increased brightness and
corrosion resistance to the metal. In a typical process, the
metal work pieces are immersed in an acidic solution containing
hexavalent chromium compounds, which react with themetal causing
the precipitation of a complex gel-like coating or film of tri-
valent chromium and entrapped soluble hexavalent chromium com-
pounds onto the metal surface. The coated work pieces are then
rinsed and dried under controlled conditions.
, There are several serious disadvantages common to all
chromate conversion coating processes. One of these is the rela-
tively short life of the process bath expressed in terms of unit
surface area coated per unit volume of bath. The main reasDn for
the short life is the continuous build-up in theba~h-of dissolved
trivalent chromium resulting from the oxidation-r~duction
reactions that occur between ~he metal and the he~n~lent chromium
Trivalent chromium is a contaminant in the process ~ffecting the
coating efficiency. Thus, when reduced coating activity iQ noted,,
or when the contaminants ha~e built up to a certain predeten~d
ievel, a process solution of this type is at least partially
replaced with freshly prepared solution, and ultimately ccmpletely
-- 1 - ' 3~
1134~;~6
discarded in favor of a fresh bath.
The disposal of the spent process solutionis ~eful,
as the solution still contains considerable quantities of hexa-
valent chromium. Not only does the loss of these values contri-
; 5 bute significantly to the overall cost of the coating process,
but disposal also adds to this cost in that the solutions present
a substantial waste treatment problem. Hexavalent chromium is
highly toxic and must be reduced to the trivalent form, e.g. by
reaction with sodium hydrosulfite or sodium bisulfite, and is
",7
thereafter precipitated from solution by addition of alkalies,such as sodium carbonate or lime. After dewatering of the pre-
cipitate by settling or filtration, the concentrated sludge of
trivalent chromium hydroxide must be disposed of in specially
designated areas, since trivalent chromium is still too toxic to
be used as landfill. Substantial waste treatment re-quirements of
spent rinse waters are also created due to dragout of toxic
chemicals from the process bath into subsequent rinse waters.
Although there are integrated processes for the reoxidation and
regeneration of spent chromate solutions and rinse water, the
small processor usually finds that the refined and sophisticated
techniques involved are neither practical nor economically feas-
ible for solving his waste treatment problems.
Applicant has developed a non-toxic conversion coating
solution which is comprise~ of sulfuric acid, hydrogen peroxide,
a soluble silicate and a primary promoter, i.e. certain
organophosphorus compounds for further enhancement of corrosion
resistance of metal surfaces treated with the solutionO
Although the acidic silicate "solution" may or may not
be a true solution by rather in the form of a hydrosol, for the
- i 113~7Z6
~purpose of this application, the term "solution" is intended to
cover a hydrosol as well as a true solution.
In addition to the formation of conversion coatings of
excellent properties, there are many other important advantages of
,the invention described in the aforementioned application. One of
these is the extremely long life of the conversion coating solu-
tion before it is discarded in favor of a fresh solution. It has
'been found that the solutions are capable of treating up to approx-
~imately 185 m2 of surface area per liter, which is far superior to
'the typical value of approximately 20 m /1 obtained with conven-
tional chromate conversion coating baths.
Another and related advantage is that, apart from some
build-up of dissolved metal in the solution, there are no detri-
! mental by-products forming and accumulating therein during use,
,as is the case with conventional chromate conversion coating solu-
tions, in which trivalent chromium ra~idly builds up.
- ' The most important advantage, however, is the non-toxic
nature of the system, which greatly facilitates waste disposal of
,spent solutions from the conversion coating process. Rinse waters
can usually be disposed of without any treatment required. Spent
conversion coating baths are merel~ treated with lime for neutral-
~ization and removal of dissolved metal ions and phosphorus as a
precipitate. After settling or other separation, the liquid phase
may be disposed of safely in common sewers, while the dewatered
sludge mainl~ composed of silicate can be dumped in municiPal
landfill areas.
An object of the present invention is to ~rovide a novel
conversion coating and a method of its forming, which coating
exhibits brightness and further improved corrosion resistance.
Another object is to provide a non-toxic, conversion
coating solution which imparts a superior corrosion resistance to
metal surfaces.
Still another object is to provide bright, decorative
'work nieces of superior corrosion resistance.
!l ~hese and other objects will become apparent from the
,Ifollowing specification, examples and claims.
1134~7Z6
THE INVENTION
:
In accordance with the present invention there is pro-
vided a novel conversion coating solution which comprises an
aqueous solution of from about 0.2 g/l to about 45 g/l of free
" 5 ~2SO4, from about 1.5 g/l to about 58 g/l of H2O2, from about
: 3 g/l to about 33 g/l of SiO2, from about 0.15 g/l to about 10 g/l
of at least one of the organophosphorus compound promoters speci-
fied below and from about 2 g/l to about 20 g/l of at least one
secondary ~romoter selected from the group consisting of ascorbic
acid, boric acid, gluconic acid, glycolic acid, tartaric acid and
salts of said acids, wherein the organic phosphorus compound is on~
: ,.having the general formula:
[X(Rl)m]n [R2]p [X(Rl)m]q
.wherein
X is a group of the formula Z1 ~ P ~ Z2 in which
O
Zl and Z2 independent from each other are hydrogen,
sodium or potassium;
m is either 0 or 1;
p is either 0 or 1;
n + q is either (a) 1 when p = 0, or
. (b) equal to the number of available bonds
provided by R2 when p = 1;
Rl is a (a) Cl-C4 alkyl or a Cl-C4 hydroxy-substituted al~.yl
and p = 0; and
(b) Cl-C4 alkylene or a Cl-C4 hydroxy-substituted
alkylene and p = l;
R~ is selected from (a~ N- , m = 1
( H2)rN= , m = 1 and r is an inte-
ger from 2 to 6
_ y
11347Z6
"
(c) =N(CH2)2 N ~CH2)21
( Rl ) m
il X
!l and
(d) a Cl-C4 alkylene or a Cl-C4 hydroxy-
substituted alkylene, m = 0 or 1.
The SiO2 component is conveniently provided in the form
of a soluble silicate, e.g. sodium silicate or potassium silicate,
I!of predetermined contents of SiO2 and Na2O or K2O. The mole
'ratios of SiO2 to either Na2O or K2O generally range betweer. 1 and,
4, and it is preferred to use those silicates wherein the mole
ratio is at least about 1.8 and most preferably at least about
,2.2. Ammonium or lithium silicates are also useful in providin~
~the SiO2 component.
j Examples of the organophosphorus compounds include
ICl-C4 alkyl phosphonic acids, C]-C4 hydroxyalkalenephosphonic
;acids, amino tri-Cl-C4 alkylene phosphonic acids, C2-C8 alkylene
diamine-tetra (C1 C4 alkylene ~hosphonic acid), diethylenetriamine-
~penta (Cl-C4 alkylene phosphonic acid) as well as the acid or
'neutral sodium or potassium salts of any of the above-listed phos-
phonic acids. l-hydroxvethylidene-l,l-diphosphonic acid is a
preferred compound.
The secondary additives can either be provided in the
acid form or as a sait, e.g. of sodium, potassium, zinc, etc.
~, The solution is easily prepared, e.g. by first adding
sufficient sulfuric acid to at least a major portion of the ~akeup
water under agitation to provide the desired free H2SO4 content
,and taking into account that some of the free acid will be subse-
~uently neutralized by the Na2O or K2O portions introduced with
~'the silicate. The silicate is added under agitation to the cooled
acidic solution until it is completely dispersed. The remaining
components are then added. Preferably the peroxide is added last,
however, the sequence of addition can be changed without any
detrimental effect, provided that the silicate is acidified with
, _ r_ !
1~347Z6
sulfuric acid prior to mixing with the hydroqen ~eroxide, or
peroxide decomposition will occur.
The preferred concentrations of the components in the
aqueous solution are from about 1.8 g/l to about 18 gJl of free
l~2SO4, from about 7 g/l to about 29 g/l of H2O2, from about 8 g/l
to about 18 g/l of SiO2, from about O.S to about 2 g/l of the pri-
mary organophosphorus promoter and from about 3 to about 10 g/l of
the aforementioned secondary promoters.
In order to impart pleasing and lasting colors to the
conversion coated work pieces without detrimentally affecting the
corrosion resistance of the coating or the stability of the
coating solution, it has been found necessary to employ cationic
triarylmethane dyes which heretofore predominantly have been used
in the dyeing of natural fibers such as paper, cotton, wool, silk,
etc. Conventional metal dyes or conversion coating dyes either
affect the stability of the system or do not impart any color to
the coatings.
The triarylmethane dyes used in this invention are well
known in the art and are recognized as a separate generic group
of dyes having a Colour Index (C.I.) in the range from 42,000 to
44,999. They are commercially available in a wide varietv of
colors both in solid form or as aqueous solution concentrates with
solids contents typically in the 40-50% range. The effective
amount of d~le to be added to the conversion coating solution de-
~ends obviously on the desired depth of color. Tvpically, this
amount ranges between about 0.05 and about 2 g/l.
The solution is useful for forming conversion coatin~s
on various metallic surfaces, such as those of zinc, cadmium,
silver, copper, aluminum, magnesium, and zinc alloys.
The most common application is, however, in the forma-
tion of conversion coatings on zinc plated articles such as zinc
plated steel articles. The zinc plate provides the steel with
cathodic protection against corrosion, and the conversion coating
further improves the corrosion resistance, reduces the suscepti-
113~
bility to finger markings and enhances the appearance by chemical
polishing of the article and by the color imparted by the dye.
It is important that the zinc plate deposit is relatively smooth
and fine-grained prior to coating, and that the thickness of the
;plate deposit is at least 0.005 mm since some metal removal occurs
~when the film is formed. The preferred plate thickness is between
~;about 0.005 mm and about 0.02 mm.
Usually the formation of the conversion coating follows
~immediately after the last rinse in the plating cycle. Thus, the
freshly plated articles are immersed for a period of from about
'j5 seconds to about 300 seconds into the solution which is main-
' tained at ambient temperatures. For best results, the immersion
~treatment is carried out for a duration of from about 20 seconds
, to about 50 seconds in a bath maintained at temperatures not less
1l than about 20C and not more than about 35C. The coated articles
j,are subsequently rinsed, first in cold water and then briefly in
¦ warm water to aid drying of the films. The hot water rinse
~typically has a temperature in the range of from about 60 to
,labout 70C. The final step of the coating process is a drying
~ step, which is carried out by any means that will neither abrade
the soft and then rather fragile film, nor expose it to excessive
,temperatures, i.e. temperatures higher than about 70C. The use
~of circulating warm air or an airblast are examples of suitable
means in the drying operation. After drying, the conversion
coatings are quite resistant to damage from abrasion and generally
do not require the 12-24 hour aging necessary with conventional
chromate conversion coatings.
The resulting conversion coatings have very good resis-
; tance to corrosion as determined by the accepted accelerated
~ corrosion test ASTM ~-117-64.
During the course of the coating process, the coating
solution becomes depleted in both free sulfuric acid and hydrogen
peroxide values and must be replenished. Therefore, monitoring
of these ~alues should be carried out Oll a regular basis to assure
-` 113~726
: ' !
that the respective concentrations have not fallen below their
minima and to assess the amounts needed for replenishment. Free
~sulfuric acid can be determined by conventional titration methods
llusing sodium hydroxide or by pH determinations. In order to main-
jltain the free sulfuric acid within the broad ranges of about 0.~ 1
to about 45 g/l the pH should be controlled between about 0.5 and '
I about 3.5 and preferably between about 1.0 and about 3.0 which
approximately corresponds to a free sulfuric acid concentration
llof from about 1.8 to about 18 g/l. The hydrogen peroxide concen-
lltration levels are advantageously monitored by conventional titra-
tion with ceric ammonium sulfate. The silicate (SiO2) consumption
is relatively small compared to the consumptions of either the
free sulfuric acid or the hydrogen peroxide, and generally neither
''monitoring (which can be carried out using e.g. colorimetric
Ijprinciples involving the reaction of silicate with ammonium molyb-
date to form a yellow-colored molybdo silicate solution) nor
llreplenishment is required during the practical life of the con-
'Iversion coating bath. The rates of consumption (i.e. percent
decrease in concentration per unit time) of the primary and
' secondary additives have been found to be approximately of the
same order as that of the hydrogen peroxide consumption. There-
fore~ replenishments of the solutions with these additives are
suitably carried out at the time of hydrogen peroxide replenish-
I ment in amounts proportional to the hydrogen peroxide addition.
I The dye, if present, generally does not need to be replenished
during the practical lifetime of the conversion coating bath.
Monitoring of the color depth quality of the coating is easily
carried out by visual inspection of the coated article and compar-
j'ison against a reference color.
I The following examples are provided to illustrate but
not to limit the invention.
The general procedures used in the examples for pre-
paring the conversion coating solutions, test specimens and form-
ing the conversion coatings are described below.
, .
-`` ; 11:3~7Z6
The aqueous conversion coating solutions were each pre-
pared to contain 2.4 g/l free ~l2SO4, 16.2 g/l SiO2, 11.7 g/l
~22 and 0.85 g/l of l-hydroxyethylidene-l,l,-diphosphonic acid.
~ The SiO2 ingredient was added in the form of sodium silicate
I~(SiO2 = 33.2% w/w; Na2O = 13.85~ w/w) and a sufficient excess of
',sulfuric acid was provided to result in the indicated free H2SO4
content after neutralization of the l~a2O in the sodium silicate.
Standard l~ull cell steel panels (10 cm x 6.8 cm x
li0.03 cm) were plated with zinc using a cyanide electrolyte. After
'Ithorough rinsing and drying, the samples were then immersed for
40 seconds in the converslon coating solution maintained at room
i temperature. The treated samples were then rinsed in water and
then dried with a hot air gun.
~ The dried coated test specimens were then subjected to
lS ¦~the accelerated salt spray corrosion tests in accordance with the
jlASTM test ~-117-64. The tests were carried out for various
~periods of time, i.e. 6, 16, 24 and 30 hours. After each test the
~specimens were examined for evidence of corrosion on a rating
,scale from 1 (heavy corrosion) through 10 (no corrosion).
EXAMPL~S 1-3
The beneficial effects of boric acid and zinc gluconate
as secondary additives are demonstrated in these examples. ~lhe
general procedures described above were followed except that the
solutions of ~xamples 2 and 3 also contained the additives indi-
cated in Table 1, which includes the results of the corrosion
tests performed on the bright, coated test samples.
TA~L~ 1
Add.
,ILx. Conc. Extent of corrosion after
INo. Additive g/l 6 hrs. 24 hrs.
. '
Control 1 ~ione ~ 9 7
! ~oric Acid 5 10 8
3 ~n Gluconate 5 9 8
!' ~1347Z6
.
EXAMPLES 4-12
In this series of experiments all the conversion coating
solutions contained ~riarylmethane dyes in addition to the second-
, ary additives s~lown in Table 2. These dyes used were a mixture
of E.I. DuPont de l~emours' liquid dyes Victoria Pure Blue BOP
solution (0.2 ml/l, Basic Blue 7, C.I. 42,595) and Paper Blue R
Liquid (0.1 ml/l, Basic Violet 3, C.I. 42,555).
The results of corrosion tests on the bright, colored,
coated test specimens are shown in Table 2.
11
I TABLE 2
Add.
Conc. Extent of corrosion after ;
~IEX. No. Additive g/l 16 hrs. 24 hrs. 30 hrs. i
~l Control 4 None - 7 6
¦~5 Boric Acid 5 9 8 7
6 Boric Acid 20 9 8 7
,j7 Ascorbic Acid 5 9 8 7
~l8 Potassium Sodium
~, Tartrate 5 10 8
9 Glycolic Acid 5 9 9
Zn Gluconate 5 9 8 7
11 Na Gluconate 5 9 7
12 Na Gluconate + (1
Zn Sulfate ) 3.2 9 8
_ _ .
The amount of Zn in 3.2 g ZnSO4 7H2O is equivalent to
that in 0.5~ Zn gluconate.
.; . ~ I
