Note: Descriptions are shown in the official language in which they were submitted.
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131\CKt7R()UMD ()l? TlïF I~ NTI()I`I -
It is well ]cnown to ,form coatings on me~al surfaces
by chemical reaction between the surface and inc3reclients in
a coating solution, the resultclnt coatincJS servill~, for
example, to improve corrosion resistance and to provide a
suitable base for s~hsequently appliec,t paint coatin~s.
Various chemical coating processes are known, probahly the
most common usin~ coaling solutions such as æinc phosphate
coating solutions, alkal.i metal phosphate coatin~ solutions
and oxalate coatin~ solutions. It i5 well known that in
order to accelerate the speed of formation of the coating
and/or to modify the properties of the resul~ant coating it
is usually des,irable to include an oxidizing agent in the
solution to serve as an accelerator.
Over the years a ~ery large nul~ber OL accelerators
have been proposed as have various speci~ic combinations oi.
them. In genera] d.ifferent types o.f coating processes
re(~uire, for o~timum performance, different accelerators.
Thus, :Eor instance, as a general rule an accelerator that
~0 ~as ound to be optirnum for some particu]ar oxalat:e process
would not be expected to be optimum ~or some entirely
different process, such as a phosphate coating process.
Instead it i.s necessary to choose the oxid.i~.ing a~en-t
accele~ator having re,qard to the speciflc component-s of the
coati.n~ composition.
Pmonqst the materl.als that h-:lv~ been pro~osc~.l as .
acc~.leratoxs for Y~inc phospllate coati.nc~ so'l.llti.orls a$e~
chlorates, nitrates, orcJan,i.c nitro compoull(ls, hyclro~en
pero~ide, n.itrites, hroma-tes alld io~a1-esA Illc]u~le(~ nonqst- :
30 QJ~~anic nitrc~ coml.~o~ cls that have beell pro~c.se(l c~re so~l:ium
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m-nitrohen%elle sul3?honclte and cl:initrobenzeIle ,sulpllorlate.
Such o~id:i~in~3 a~ents are listed in, for ins-tance, Bx~l_ish
Patent Speeiication No. 828,9]6, whi.ch relates to a rather
partieular type o:E zinc phosphate coatin~ solution, namely
one eontaining ealcium.
A ~ine phosphate COatinCJ solwtion that has found
particularly wide aeceptance is one aceelerated by ehlorate~
Mo~ever it has ~een well aceepted for a long time that
ehlorate by itself is not al~A~ays adequate and when it has
been desi.red to have a second oxidisin~ agent to serve also
as an aceelerator it has become normal to use nitrate with
the chlorate. Thus zinc phosphate solutions aecelerated
by-a mi~ture of nitrate and chlorate have beeome very well
established eonunercially. Such solutions are described in,
for instanee, British Patent Speeifieations Nos. 551,261
1,296,S83, 1,2~7,715~ 1,374,963, and 1,376,309 and zinc
calcium phosphate solutions aceelerated by nitrate and
optionally also chlo~ate are deseri.bed~in British Patent
Speeifieat,ion No. 828,906.
Typieally aeeelerated zinc phosphate solutions need
to be eontaeted with the metal sl1rfaee for, say r at least
1 millute in or~ler to form the eoating. For instanee if
a solution such as is deserihed in Rritish Patent 9peeifieations
Nos. 1!296,883, 1,297,7].5, or 1,37~,963 is being ayplied to a
metal surfaee by spraying them a spraying period of at least
. 60 seeonds, and preferably 90 seeonds is needed to aehieve
opti~um eoatin~ condition.q. IIowever in some i.nstanees it
~ould be desirable to be able to obtaill optimum eoat:iny .in
a shorter period and so there i.s a nePd for a æi.ne pllosphate
eoatin~ solution that will form ~1 satlsfaetory eoatin~ qllieker
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than ~ e conventiona], an;l widely uscd zinc phosphate coating
solutions that are accelerated with c~hlorate or chlorate and
n.itrate. Of course the speed of coating can be increased
by increasing the temperature but this is inconvenient in
many in.stances.
The four most commonly used accelexat,ors are
chlorates, nitrates, hydrogen peroxlde and ni.l:rites.
Peroxldes do not help solve the particula,r problem since they
tend to cause even slower coating formation than chlorates
with or without nitrates. Nitrites will give ~uicker
coating formation, either by themselves or with chlora.te,
but unfortunately suffer from the disadvantage that their
; use generates toxic sases, this being a particularly serious
problem in a spxay process. Accordingly they are not
desirable either. '
, A combination of chlora~e and aromatic nitro compolmd
is disclosed as an accelerator in an oxalate solution in
British Patent Specification No. 823,373 but the requirements
for such accelerators are entirely different from those ~or
zinc phosphate solutions and, as disclosed therein, the
purpose of adding the aromatic nitro compound is to overcome
the'd.isadvantage that chlorate alone, ln an oxalate solutiont
~id not ~ive a ~ood coating with a freshly prepared solution, ',:
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and there was no su~gestion in that specificat.ion that the
combination of chlorate and nitrate would have any desirable
' : or other effect either on coating weigh,t or on the rate oE
coating formation in oxalate coatings. : :
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SUMMAR~ OF THE INVENTION
We have now found the coating formation using a zinc
phosphate coating solution accelerated with chlorate can be
accelerated still further by including in it a water soluble
aromatic nitro compound as additional accelerator. It is par-
ticularly surprising that the combination of chlorate and water
soluble aromatic nitro compound results in increased coating
formation since the aromatic nitro compounds themselves are by
themselves less effective accelerators than both chlorate and
nitrate and so would not be expected to increase -the rate of
coating formation above that when chlorate alone or chlorate
and nitrate are used. Thus the combination of chlorate and
aromatic nitro compound is, in a zinc phosphate solution, syner-
gistic as regards rate of coating formation. It also has the
advantage that it tends to give a lower coating weight than when
either chlorate or aromatic nitro compound is used alone, and
this is desired in many instances.
The aqueous treating solution according to the inven-
tion consists essentially o~: an a~ueous treating composition
consisting essentially o~:
Zinc 0.15 - 0.4 wt%
Phosphate 0.4 - 1.6 wt%
Chlorate 0.3 - 0O5 wt%
Aromatic
Nitro Compound ~,O~ - o.l~ W t ~
wherein the weight ratio of chlorate to aromatic nitro compound
is 3 - 5 : 1.
A phosphate coating is formed on a metal surface by
contacting the surface with the aqueous treating solution accord-
ing to the invention.
DETAILED DESCRIPTION OF THE INVENTION
The aromatic nitro compounds pre~erably contain not
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more than two nitro groups. The preferred compound for use inthe invention is sodium meta~nitro-benzenesulphonate, but other
compounds including dinitro aromatic compounds such as those
disclosed in sritish Patent Specification No. 970,366 and
nitro phenols may be used.
The composition of the invention may also advantage-
ously contain nitrate. We also find that the maximum coating
speed is achieved where zinc is present in an amount greater
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than tha~ stoiclli.ometrically required to form ~inc dihyclrogen
phospllate. The excess zinc i~ preferabl~ aclded in zinc nitrate.
The e~sential ingredien-cs of the coatin~ solution
are zinc, phospllate r chloratc and the aromatic ni~.ro compound.
Preferred amounts fox the various ingredlents are Zn 0.15
O . 4 0 % w/~r; P 0 4 0 ~ 4 0 - 1 0 6; C 10 3 0~3 ~ Q.5; NO3 0 - 1.2;
aromatic nitro compound 0~05 - 0O15; and most preferably
Zn 0.3 - 0 35, P0~1 O.A5 - 1~2; Cl03 0.33 - 0.4; .N03 0.3 -
0.7; arol~atic nit.ro compound 0.06 - 0.09. In these formulations
the aromatic nitro compound is preferably sodium nitro benzene
sulphonate (SNBS). The ration by wei.ght of chlorate to
aroma~ic nitro compouncl is preferably 3 : l 5 : l.
Although the advantages are less marked, if desired
calcium can be included in the coating solution, for instance
in the proportions based on zinc phosphate suygested in
British Patent Specification No. 828,916.
The coatin~ solutions may be used ~or orming
coating on iron, steel, zinc and alumi.nium surfaces, but
i the solution is to be used for coating aluminium surfaces
then fluoride should be included in the solution. Fluoride
containinc..~ ~.inc phosphate so].utlons preferably contain from
0.03 - 0.1~- F. . ~ :
The total acid content of the soluti.ons is
preferably from 7 to 2A point:s and the :Eree acid content is
~5 preferabl~ from 0.2 to 22 points.
: ~he 7.inc~, phosphate, ch].orate and, ie present,
fluoride can~all }~e introduced in conventi.onal form. ~.
~ It i~ a cO~i~Oll practice to ma]-~c a liquid conc~entrate
and to dilute this to orm a work:lnq soluti.on. If ~ hic~hl.y
concent.xated composition is l:o be Inacle ancl stored :Eox use
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without forminy a precip.i.tate dur.ing storaye, it mu-;t con~air
too much free acicl i.n proportion to thetotal acid, ancl ~hcn
it is diluted some neutralising substance must thel-e~ore bc
added to reduce the free aeidity~ Suitable sub.st~lnces for
this purpose include zinc carbonate, zinc ox.ide, c~ustic soda
and sodium carbonate, but care must he taken not to ~Idd
anything which might be deleterious to the phosphate coatinc3.
Solutions according to the invention ma~ contain
other ingredients such as metal accelerators and coating
refining additives. Specifically, the solution may also contain
from 0-001 to 0.025% w/w niekel, eobalt or copper as a
metal aecelerator, and preferably the metal is niclcel in
an amount from 0.005 to 0.02% w~w. Examples of coat refining
additives are tartarie and eitric acids, an~ calcium in the
form of any suitable compound as described in British
Specifications Nos. 828,916 and 866,377.
The solutions of the invention can be applied i.n
any eonvenLent manner, for example by immersion or spray,
but are preferably applied by spray or combinecl spray and
2() immersion. The temperature is pref~rably from 40 - 55C.
rrhe time of contact with immersion may be S - 10 minutes
and with spray .it is preferably 45 - 90 seconds.
` During use the bath of coating solution will need
to be replenished and preferably it is repleni.shecl usi.ny
two repleni~hment solutions, as descri.bed in Brl tish Patent
Speeifieation No. 1,376,30~. Thus one .';OlUtiOII eon-ta.i.ns
: the zinc and phosphate and optionally ni.trat.e a~d ~luoride
and the other contains a].kali, usually sod.ium hydroxicle,
ehlorate, usually sodium chlorate, and the aromatic nitro
~O compouncl.
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The following are Examples. - ~
6" x 4" panels were treated by 60 second spray at
45C. In all 445m2 of cleaned mild steel were processed.
Sample panels were taken every lOOm2 electropainteci in a
grey electroprimer and salt spray tested accordlng to
ASTM B117. After 240 hours all gave excellent~ corros.ion
results. Adhesion results were also good.
Finely crystalline uniform phosphate coatings
were obtained throughout wlth coating weights in the range
1.4 -- 1.8 g/~
I.M.U. Main Replenishment
.
ZnO = 125 pts. wt~ ZnO = 125 pts. wt.
~3PO,l = 173 " " El3PO~ = 230 1!
HN03 = 128 " " ~IN03 = 80 " "
HF -~ 14 " " HF = 3 " "
Tartaric acid = 27 " " Tartaric acid = 27 " "
H20 to ma]ce 1000 H20 to make 1000
ACCI~I.ERATO_PLENISI-I~IENT
NaO~I ~ 101 pts. wt.
2~ Na~103 = 16 " "
SNBS -- 8 ''
II20 to ma]ce 1000
3Q Litres of working solution ~Jere prepared from:
30 ~/1 I.M.U. ~ 5 g/l NaC103 -~ 1 cJ/l SNr,S -~ 2g/1 NaOH.
During use the zinc content was ]cept constant by freclLIent
additions of the main replenishment and the re;3 acidity of
-~ the solution ~r!aS recju1ated by additiorls of accelerator
replenishment.
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