NON-CHROMATE CONVERSION COATINGS

REVETEMENTS DE CONVERSION SANS CHROMATES

English Abstract

RC-1433
NON-CHROMATE CONVERSION COATINGS
ABSTRACT
Improved brightness and corrosion resistance are impart-
ed to metal surfaces such as zinc plated surface, by treatment
with a non-toxic solution comprised of sulfuric acid, hydrogen
peroxide and a silicate. Certain organophosphorus and organonitro-
gen compound additives further enhance the corrosion resistance.

Claims

WHAT IS CLAIMED IS:
1. A conversion coating solution which comprises an aqueous
solution of from about 0.2 g/1 to ahout 45 g/1 of free
H2S04, from about 1.5 g/1 to about 58 g/1 of H202, and
from about 3 g/1 to about 33 g/1 of SiO2.
2. The solution of claim 1, wherein the free H2SO4 concen-
tration is between about 1.8 g/1 and about 18 g/1.
3. The solution of claim 1, wherein the H1202 concentration
is between about 7 g/1 and about 29 g/1.
4. The solution of claim 1, wherein the SiO2 concentration
is between about 8 g/1 and ahout 18 g/1.
5. The solution of claim 1, in which the SiO2 is provided
in the form of sodium silicate or potassium silicate.
6. The solution of claim 5, wherein the molecular ratio of
SiO2 to either Na20 or K20 in the sodium silicate or
potassium silicate is maintained between about 1 and
about 4.
7. The solution of claim 6, wherein said molecular ratio is
at least about 2.2.
8. The solution of claim 1, containing from about 0.15 g/1
to about 10 g/1 of a promoter additive or mixtures of
promoter additives selected from organophosphorus com-
pounds having the general formula:
[X(R1)m]n . [R2)p . [X(R1)m]g , wherein
X is a group of the formula <IMG>
in which Z1 and Z2 independent from each other
14

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 = O, or
(b) equal to the number of avail-
able bonds provided by R2
when p = 1;
R1 is a (a) C1 -C4 alkyl or a C1 -C4 hydroxy-
substituted alkyl and p = O; and
(b) C1 -C4 alkylene or a C1 -C4 hydroxy-
substituted alkylene and p = 1;
R2 is selected from (a) N= , m = 1
(b) =N(CH2)rN= , m = 1 and r
is an integer from 2 to
(c)
IMG
m = 1, and
(d) a C1 -C4 alkylene or a
C1 -C4 hydroxy-substitu-
ted alkylene m = 0 or 1.
9. The solution of claim 8, containing from about 0.5 g/1
to about 2 g/1 of said organophosphorus compound.
10. The solution of claim 1 containing from about 0.5 g/1
to about 50 g/1 of at least one organonitrogen
promoter additive slected from the group consisting of

thioacetamide, urea, thiourea, N,N'-alkyl substituted
ureas and thioureas, cyclic N,N'-alkylene substituted
ureas and thioureas, wherein said alkyl and alkylene
groups each contain from 1 to 4 carbon atoms.
Il. The solution of claim 10 containing from about 1 g/1
to about 10 g/1 of said organonitrogen promoter
additive.
12. The solution of claim 9 containing from about 0.5 g/1
to about 50 g/1 of at least one organonitrogen
promoter additive selected from the group consisting
of thioacetamide, urea, thiourea, N,N'-alkyl
substituted ureas and thioureas, cyclic N,N'-alkylene
substituted ureas and thioureas, wherein said alkyl
and alkylene groups each contain from 1 to 4 carbon
atoms.
13. The solution of claim 8, wherein the organophosphorus
compound is an aminotri(alkylene phosphonic acid).
14. The solution of claim 8, wherein the organophosphorus
compound is a hydroxy alkylene diphosphonic acid.
15. The solution of claim 8, wherein the organophosphorus
compound is a polymethylene diaminetetra(alkylene
phosphonic acid).
16. The solution of claim 8, wherein the organophosphorus
compound is a diethylene triaminepenta(alkylene
phosphonic acid).
17. The solution of claim 10, wherein the organonitrogen
promoter additive is thioacetamide.
16

18. The solution of claim 10, wherein the organonitrogen
promoter additive is selected from urea and thiourea.
19. The solution of claim 10, wherein the organonitrogen
promoter additive is selected from N,N'-alkyl
substituted ureas and thioureas.
20. The solution of claim 10, wherein the organonitrogen
promoter additive is selected from cyclic N,N'-alkylene
substituted ureas and thioureas.
21. The solution of claim 12, wherein the organonitrogen
promoter additive is thiourea.
22. The solution of claim 12, wherein the organophosphorus
compound is 1-hydroxyethylidene-1,1-diphosphonic acid.
23. The solution of claim 22, wherein the organonitrogen
promoter additive is thiourea.
24. In a process for the formation of corrosion resistant
conversion coating onto metallic surfaces selected
from zinc, cadmium, silver, copper, aluminum, magnesium
and zinc alloys, wherein the metallic surfaces are
immersed in a conversion coating solution, and subse-
quently rinsed and dried, the improvement which
comprises:
immersing the metallic surfaces into the con-
version coating solution of claim 1.
25. The process of claim 24, in which the metallic surfaces
are immersed into the conversion coating solution of
claim 8.

26. The process of claim 24, in which the metallic surfaces
are immersed into the conversion coating solution of
claim 10.
27. The process of claim 24, in which the metallic surface
are immersed into the conversion coating solution of
claim 12.
28. The process of claim 24, in which prior to rinse, the
metallic surfaces are immersed for a period of about 5
to about 300 seconds into a second aqueous treatment
solution containing from about 0.5 g/1 to about 50 g/1
of at least one organonitrogen compound selected from
the group consisting of thioacetamide, urea, thiourea,
N,N'-alkyl substituted ureas and thioureas, cyclic
N,N'-alkyl substituted ureas and thioureas, cyclic
N,N'-alkylene substituted ureas and thioureas, wherein
said alkyl and alkylene groups each contain from 1 to 4
carbon atoms.
29. The process of claim 25, in which prior to rinse, the
metallic surfaces are immersed for a period of about
5 to about 300 second into a second aqueous treatment
solution containing from about 0.5 9/1 to about 50 g/1
of at least one organonitrogen compound selected from
the group consisting of thioacetamide, urea, thiourea,
N,N'-alkyl substituted ureas and thioureas, cyclic
N,N'-alkyl substituted ureas and thioureas, cyclic
N,N'-alkylene substituted ureas and thioureas, wherein
said alkyl and alkylene groups each contain from 1 to
4 carbon atoms.
18

30. The process of claim 28 wherein the second aqueous
treatment solution also contains from about 0.15 g/1
to about 10 g/1 of at least one organophosphorus
compound having the general formula:
[X(R1)m]n . [R2]p . [X(R1)m]q , wherein
X is a group of the formula <IMG>
in which Z1 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 z 1;
R1 is a (a) C1-C4 alkyl or a C1-C4 hydroxy-substituted
alkyl and p = 0; and
(b) C1-C4 alkylene or a C1-C4 hydroxy-
substituted alkylene and p - 1;
R2 is selected from (a) N= , m = 1
(b) =N(CH2)rN= , m = 1 and r is
an integer from 2 to 6
(c) <IMG> ,
m = 1, and
(d) a C1-C4 alkylene or a C1-C4
hydroxy-substituted alkylene,
m = 0 or 1.
19

31. A metallic surface coated by the process of claim 24.
32. A metallic surface coated by the process of claim 25.
33. A metallic surface coated by the process of claim 26.
34. A metallic surface coated by the process of claim 27.
35. A metallic surface coated by the process of claim 28.
36. A metallic surface coated by the process of claim 29.
37. A metallic surface coated by the process of claim 30.
38. The metallic surface of claim 31, wherein the metal is
zinc plate.

Description

1~347~7
BACKGROUND OF- ~HE INVENTION
The formation of chromate conversion coatings on
surfaces of various metals, such as zinc and cadmium, 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
the metal causing the precipitation of a complex gel-like coating
or film or trivalent chromium and entrapped soluble hexavalent
chromium compounds 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
relatively short life of the process bath expressed in terms of
unit surface area coated per unit volume of bath. The main
reason for the short life is the continuous build-up in the bath
of dissolved trivalent chromium resulting from the oxidation-
reduction reactions that occur between the metal and the
--1--
: `

~ 113~7Z7
hexavalent chromium. Trivalent chromium is a contaminant in the
process affecting the coating efficiency. Thus, when reduced
coating activity is noted, or when the contaminants have built up
to a certain predetermined level, a process solution of this
type is at least partially replaced with freshly prepared
solution, and ultimately completely discarded in favor of a fresh
bath.
The disposal of the spent process solution is wasteful,
as the solution still contains considerable quantities of hexa-
valent chromium. ~ot only does the loss of these values
contribute significantly to the overall cost of the coating
process, but disposal also adds to this cost in that the
~olutions present a substantial waste treatment problem. Hexa-
valent chromium is highly toxic and must be reduced to the tri-
valent form, e.g. by reaction with sodium hydro8ulfite or sodium
bisulfite, and is thereafter precipitated from ~olution by
addition of alkalies, such as sodium carbon~te or lime. After
dewatering of the precipitate by settling or filtration, the
concentrated sludge of trivalent chromium hydroxide mu8t be
disposed of in specially designated areas, since trivalent
chromium is still too toxic to be used as landfill. Substantial
waste treatment requirements 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 feasible for solving his
l waste treatment problems.
¦ It is, therefore, an object of the present invention
¦ to provide a novel conversion coating and a method of its

-` ,` 1~ 3~7Z7
jforming, which coating is as bright and corrosion resistant as con~
ventional chromate conversion coatin~s. Ii
Another object is to provide a novel conversion coating i
solution, which has a longer service life.
A further object is to provide a novel conversion coat- ¦
¦ling solution, which, after use, requires minimal after-treatment
¦¦and can be disposed of as municipal landfill.
I These and other objects ~?ill become apparent from the
¦¦following specification, examE,les, and claims.
j~ THE INVENTION
,,
` In accordance with the present invention there is pro-
~vided ~ novel conversion coating solution which com~rises an
¦aaueous solution of from about n.2 g/l to about 45 g/l of free
IH2SO4, from about 1.5 g/l to about 58 g/l of E~202 and from about
¦~3 ~/1 to about 33 g/l of SiO2. The last com~onent is conveniently
rovided in the form of a soluble silicate, e.g. sodium silicate on
potassium silicate, of predetermined contents of SiO2 and Na2O or
K2O. Ammonium or lithium are also useful in providing the SiO2
j,co~ponent.
'l ~lthough the acidic silicate 'solution" may or mav not be
a true solution but rather in the form of a hydrosol, for the pur
pose of this application, the term "solution" is intended to cover
a hydrosol as well as a true solution.
, The mole ratios of SiO2 to either Na2O or K2O generally
ran~e between 1 and 4, and it is preferred to ~se those silicates
wherein the mole ratio is at least about 1.8 and most preferablv
at least about 2.2 The solution is easily prepared, e.g. by
first addin~ sufficient sulfuric acid to the water under a~itation
;to Drovide the desired free H2SO4 content an~ takin~ into account
j,that SQ~e of the free acid will be subseauentlv neutralized bv the
Na2O or K2O portions introduced with the silicate. The silicate
is added under agitation to the cooled acidic solution until it
is ccm~letely dispersed.
t'
1il! _3_

1134727
,
;The peroxide addition is made last, preferably just prior to use.
The se~uence of addition can be changed, ho~ever, without any
detrimental effect, provided that the silicate is acidified with
~Isulfuric acid prior to mixing with the hydrogen peroxide, or
jlperoxide decomposition will occur.
il The preferred concentrations of the com~onents in the
¦¦aqueous solution are from about 1.8 g/l to about 18 9/1 of free
IH2SO4, from about 7 g/l to about 29 g/l of H2O2 and from about
j! 8 g/l to about 18 g/l of SiO2.
ll The solution is useful for forming conversion coatings
Ijon various metallic surfaces, such as those of zinc, cadmium,
¦¦silver, copper, aluminum, mganesium, and zinc alloys.
The most common application is, however, i~ the forma-
~! tion of convexsion coatings on zinc plated articles such as zinc
~plated steel articles, and the invention will be discussed here- I
linafter with respect to such application. The zinc ~late provides
the steel with cathodic protection against corrosion, and the
conversion coating further improves the corrosion resistance,
;reduces the susceptibility to finger markings and enhances the
1 appearance by chemical polishing of the article. It is important
that the zinc plate deposit is relatively smooth and fine-grained
prior t~ coating, and that the thickness of the plate deposit is
at least 0.005 mm since some metal removal occurs when the film is
~iformed. The preferred ~late thickness is between about o.no5 mm
; and about 0.02 mm.
,l Usually the formation of the conversion coating follows
immediately after the last rinse in the plating cycle. Thus,
;the freshly plated articles ar~ immersed for a period of from
j about 5 seconds to about 300 seconds into the solution which
¦lis maintained at ambient temperatures. For best results,
I the immersion treatment is carried out fcr a duration of from
Il l
il i

11347Z7
about 20 seconds to about 50 seconds in a bath maintained at
temperatures not less than about 20C and not more than about
35C. The coated articles 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 about 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
resistance to corrosion as determined by the accepted accelerated
corrosion test ASTM B-117-64. By the use of one or more of
certain organic promoters, either as additives to the solution
of sulfuric acid-hydrogen peroxide-silicate or employed in a
subsequent treatment, the corrosion resistance of the coatings
can be further enhanced. The group I organop~osphorus compoundS
and the group II organic nitrogen compounds ~ cified hereinafter
have been found to be especially useful in this respect.
The group I promoters are organic phosphorus
compounds having the general formula:
[ ~ l)m]n [R2]p [X(Rl)m]q , wherein
X is a group of the formula Z1 ~ 1l ~ Z2 in which
Zl and Z2 independent from each other is hydrogen,
sodium or potassium;

~ 113~727
m is either o or l;
p is either 0 or 1;
n + q is either (a) 1 when p = 0, or
(b) equal to the number of available bond
S provided by R2 when p ~ 1;
Rl is a (a) Cl-C4 alkyl or a Cl-C4 hydroxy-substituted
alkyl and p = 0; and
(b) Cl-C4 alkylene or a Cl-C4 hydroxy-~ubstituted
alkylene and p = l;
R2 is selected from (a) N_ , m = 1
(b) =N(CH2)rN= , m = 1 and r is an
integer from 2 to 6
(c) =N(CH2)2 N (CH2~2N
(Rl)m
X
and
(d) a Cl-C4 alkylene or a Cl-C4
I hydroxy-substituted alkylene,
¦ m = 0 or 1.
l . I
¦ Examples of these organophosphorus compounds include
Cl-C4 alkyl phosphonic acids, Cl-C4 hydroxyalkalenephosphonic
acids, amino tri-Cl-C4 alkylene phosphonic acids, C2-C8 alkylene
diamine-tetra (Cl-C4 alkylene phosphonic acid), diethylenetri-
amine-penta (Cl-C4 alkylene phosphonic acid) a~ well as the acid
or neutral sodium or potassium salts of any of the above-listed
phosphonic acids. l-hydroxyethylidene-l,l-diphosphonic acid is a
preferred compound.

113'}'7'~7
The oryanophosphorus compound or mixture of such com-
pounds is added either to the conversion coating solution or to
a subsequent aqueous bath to provide a concentration therein of
¦Ifrom about 0.15 g/l to about 10 g/l, preferably from about 0.5 g/l~
I!to about 2 g/l.
The group II promoters are organonitrogen additives
l~selected from thioacetamide, urea, thiourea, N-N'-alkyl substi-
¦ltuted ureas or thioureas and cyclic N-N'-alkylene substituted
j ureas and thioureas, wherein said alkyl and alkylene groups each
¦Icontain frorn 1 to 4 carbon atoms.
' Particular examples of suitable promoters belonging to
group II include tetramethyl urea, tetramethyl thiourea,
~dimethylthiourea, di-n-butyl thiourea, di-t-butyl thiourea,
,~'ethylene thiourea, etc. Thiourea is one preferred ~roup II
compound.
The organonitrogen compound or mixture of such com-
pounds is either added to the conversion coating solution or to a
separate aqueous solution to provide a concentration in either
case of from about 0.5 g/l to about 50 g/l, preferably from
about 1 g/l to about 10 g/l.
Mixtures of promoters from groups I and II can be used
with advantage in the invention, e.g. a mixture of thiourea
and l-hydroxy-ethylidene-l,l,diphosphonic acid.
Good results can also be obtained with group II
promoters by employing a two-step technique, in which the
articles to be coated are first dipped into the solution of
SO4 - ll2O2 - SiO2 under the conditions d~scribed hereinbefore
and then into a second solution of the group II additive in
water, which solution is maintained in the same range of tempera-
,, ture, i.e. from about 20 to about 30C. The immersion time in
the second step should be from about 5 to about 300 seconds
. .
l -7-

1 113~7~7
preferably from dboue 15 to about 50 seconds. If a group I
promoter is used in combination with a group II promoter, it can
be added to either of tne solutions.
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 values should be carried out on a regular basis to
assure that the respective concentration~ have not fallen below
their minima and to assess the amounts needed for replenishment.
Free sulfuric acid can be determined by conventional titration
methods using sodium hydroxide or by pH determinations. In order
to maintain the free sulfuric acid within the broad ranges of
about 0.2 to about 45 g/l the pH should be controlled between
about 0.5 and about 3.5 and preferably between about 1.0 and
about 3.0 which approximately corresponds to a free sulfuric
acid concentration of from about 1.8 to about 18 9/1. The
hydrogen peroxide concentration levels are advantageously
monitored by conventional titration 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 principles involving the reaction of
silicate with ammonium molybdate to form a yellow-colored molybdo
silicate solution) nor replenishment is required during the
practical life of the conversion coating bath. The rate of
consumption (i.e. percent decrease in concentration per unit
time) of either of the group I and group II additives has been
found to be approximately of the same order as that of the
hydrogen peroxide consumption. Therefore, replenishments of the
solutions with these additives are suitably carried out at the

113~727
time of hydrogen peroxide replenishment in amounts proportiona
to the hydrogen peroxide addition.
In addition to the formation of conversion coatings of
excellent properties, there are many other important advantages
of the present invention. One of these is the extremely long
life of the conversion solution before it is discarded in favor
of a fresh ~olution. It has been found that the solutions are
capable of treating up to approximately 185 m2 of surface area
per liter, which is far superior to the typical value of
approximately 20 m2/1 obtained with conventional chromate con-
version coating baths.
Another and related advantage is that, apart from some
build-up of dissolved metal in the solution, there are no
detrimental by-products forming and accumulating therein during
use, as is the case with conventional chromate conversion coating~
solutions, in which trivalent chromium rapidly 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 hny treatment required.
Spent conversion coating baths are merely treated with lime for
neutralization and removal of dissolved metal ions and
phosphorus (when organophosphorus promoters are used) as a
l precipitate. After settling or other separation, the liquid
¦ phase may be disposed of safely in common sewers, while the
dewatered sludge mainly composed of silicate can be dumped in
municipal landfill areas.
The following examples are provided to illustrate and
not to limit this invention.
_g_

113~7;~7
EXAMPLE 1
The general procedures used in preparing the conversion
¦ coating solutions and test specimens, forming the conversion
¦ coatings and testing the corrosion resistance of the coatings
¦ are described below.
¦ The aqueous conversion coating solution was prepared
¦ to contain 2.4 g/l free H2SO4 , 16.2 g/l SiO2 and 11.7 g/l
H2O2. The SiO2 ingredient was added in the form of sodium
l silicate ~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 Na20 in the
sodium silicate.
Standard Hull cell steel panels (10 cm x 6.8 cm x
l 0.03 cm) were plated with zinc using a cyanide elect~olyte.
After thorough rinsing and drying, the samples were then
immersed for 20 seconds in the conversion coating solution main-
tained at room temperature. The treated samples were then
rinsed in water and then dried with a hot air gun.
The dried, coated test specimen~, which had a bright
luster, were then subjected to the accelerated salt spray
corrosion test in accordance with the ASTM test B-117-64. The
tests were carried out for 6 hours and 24 hours and showed only
traces of any corrosion after 6 hours and medium corrosion after
1 24 hours on a rating scale from No = No corrosion ~ Tr/S =
¦ trace (scattered) ~ Tr = trace ~ Mi/S - mild (scattered) ~
¦ Mi = mild ~ Me/S = medium (scattered)< Me = medium ~ H/S =
hea~y (scattered) < H = heavy.
EXAMPLES 2-10
The additional beneficial effects of organophosphorus
compo d additives are demon trated in these examples. The

11347'~7
1 '.
general procedures of Example 1 were followed exCept that the
, conversion coating solutions contained the organophosphorus addi-
tives in the amounts specified in Table 1, which also includes the
Il result of the corrosion tests performed on the bright, coated test
i, samples.
Il TABLE 1
¦l Add.
,I Ex. Conc. Extent of corrosion after
il No. Additive g/l 6 hrs. 24 hrs.
Il
'~ 1 None ~ Tr/S Me/S
2 Aminotri(methylene 0.75( ) Tr/S Me/S
phosphonic acid)
, 3 Aminotri(methylene 7.50(1) -No Me
j,phosphonic acid)
!l4 l-l~ydroxyethylidene 0 75(2) _IJo Mi/S
l,l-diphosp~lonic acid
5 l-l~ydroxyethylidene 1.50(2) ~No Tr
~~l,l-diphosphonic acid
¦i 6 l-Hydroxyethylidene 7.50(2) ~~O Tr/S
! l,l-diphosphonic acid
7 ~thylenediarnine tetra 0.50(3) ~l~o Me/S
(methylene phosphonic
, acid)
I, 8 Hexamethylene diamine 0.50(4) ~ No Mi
tetra (methylene-
phosohonic acid)
9 Diethylene triamine 0.75( ) ~No Mi
penta (metllylene-
phosphonic acid)
Diethylene triamine 7 50(1) ~_~Jo Me/S
penta (methylene-
phosphonic acid)
(1) Active content about 50~
(2) ~ctive content about 60%
~ (3) Active content about 90%
(4) Active content about 97%
; EXAMPLES 11-18
,,
The procedures followed in these examples were essen-
~i tially those described in Example 1 except that the H2O2 concen-

11347~7
? tration of the coating bath was 23.4 g/l and 5 g/l of the various
organonitrogen promoters listed in Table 2 were included in the
solutions. The results of corrosion tests on the bright, coated
test specimens are shown in the table.
~' ,
~j TABLE 2
j~ Ex. Extent of corrosion after
!~ No. Additive 6 hrs. 24 hrs.
I'
11 None Tr/S Me/S
~, 12 Thioacetamide ~No Tr/S
1~ 13 Urea ` ~No ~li/S
~ 14 1,1,3,3-Tetramethyl urea l~r/S Tr/S
i~ 15 Ethylenethiourea ~No Mi
il 16 N,N'-Di-n-butylthiourea ~No Tr
~ 17 N,N'-Di-t-butylthiourea ~No Tr
! 18 N,N~-Dimethylthiourea ~No Mi
1'
j EXAMPLES 19-22
~,,
The procedures of Example 1 were followed except that
the various promoters shown in Table 3 were added to the coating
; bath. The results of the testing on the bright, coated samples
are shown in the table.
TA~LE 3
Add.
Ex. Conc. ~xtent of corrosion after
Wo. Additive g/l 6 hrs. 24 hrs.
_ .
19 None ~ Tr/S Me/S
20 Thiourea 5 ~-~o Mi
21 l-~ydroxyethylidene-
l,l-diphosphonic (1)
acid 1.5 No Tr
22 Thiourea + l-~ydroxy-
ethylidene-l,l,-di- (1)
phosphonic acid 5 + 1.5 No Tr/S
(1) Active content about 60%
, ~
! -12-

Il 11347~7
EXAMPLE 23
Zinc plated test specimens were first dipped for 2~
seconds in a coating solution of the composition and temperature
of Example 1 and then immediately into a second solution contain-
ing 5 g/l of thiourea in water for another 20 second period. The
samples were then rinsed, dried and tested according to the
procedures outlined in Example 1. The resulting coating was
found to give a slightly better corrosion protection than one
obtained in a one-step process involving the same compounds in
th- s a ~ne c O~e~t ra e i S .