Note: Descriptions are shown in the official language in which they were submitted.
3~
This invention relates to a method of applying
a zinc phosphate coating to a metal surface.
Phosphate coatings ara commonly applied to metal
surfaces, for example surfaces comprising iron, zinc ~`
or aluminum, by raaction of the metal surface wlth a
solution which comprises an acidic metal phosphate,
Oxidants which accelerate this reaction and other
suitahle additives may also be present as constituents
of a working phosphating solution As the coating
reaction proceeds, the working solution becomes
depleted in certain of its constituents and the rate ;~`
of depletion of these constituents may well be different
in each case. Some constituents, for example those
which act in the manner of a catalyst, may be depleted
due to drag-out on the work pieces only or due to
leakage, whareas those constituents which react with
the metal surface will be depleted in an amount which
will usually correspond with the area of metal which
is treated.
In order to maintain or to achieve that optimum
concentration of essential constituents which is
necessary in a working solution for achieving a consis-
tent and satisfactory phosphate coating it is necessary
to add to the solution one or more replenishment con-
centrates which make good the depletion of each consti-
tuent. The chemical composition and the rate of
addition of the replenishment concentrate or concentrates
must take into account a number of factors such as (a)
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the loss of constituents by leakage, drag-out or
evaporation from the coatingplant, (b) the rate of
consumption of individual ing.redients by the coating
reaction and (c) the optimum concentration of consti-
tuents which is desirable for satisfactory operation
of the coating process, bearing in mind the effect of
other variables such as the pr~vailing temperature.
~` A further factor to be taken into account in the
case of solutions comprising zinc phosphate is that,
in order to ensure that it has satisfactory storage
stability, a replenishment concentrate comprising zinc
phosphate must usually contain a higher ratio of free
acid to total acid than can be tolerated in the working
solution for satisfactory operation of the process
(The free acid and the total acid content of a
composition or concentrate are determined by titration
of an appropriate sample against alkali using methyl ~-
orange and phenolphthalein indicators respectively).
Thus it is necessary to compensate in the overall
; 20 replenishment of the working solution for this addition
of excess acid with the zinc phosphate and it is
established practice to make appropriate add.ition to
the working solution of an alkaline material (sometimes
termed "a toner") in order to maintain a level of ~ .
acidity desired for the process.
The effect of making replenishment additions such
as those mentioned above, as well as the effect of the
generation of by-products in the reaction, is generally
that the total composition of the phosphating solution
; 30 under settled working conditions is significantly
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different from the total composi-tion of the solution at
the outset of the process, i,e as first prepared and
before coating takes place,
It has.been xecognised previously that some form
: of continuous control of the concentration of the
important constituents of the working solution is
essential for satisfactory operation. Automatic control
has been practised in certain cases where the working
solution does not comprise zinc p~osphate but with zinc
phospha~-containing solutions there have been problems
associated with the control of acidity and with the .~:
; general operation of the process which have dictated the
use of manual control procedures.
Automatic or semi~automatic control procedures
which have been proposed for phosphating processes
include a step (a) such as the measurement of electrolytic ~:
conductivity, the measurement of the chemical potential
of one or more ions in solution or the direct measurement
by titration (manual or automatic) of t~e concentration
: 20 of certain specific ions; and a step (b), the subsequent
addition of a suitable raplenishment in response to any
of these measurements in order to maintain an optimum
working composition. The mresurement of conductivity
can be achieved with simple equipment and it would be
attractive as a means of controlling the replenishment
of working solutions comprising zinc phosphate were it ;~
not for the fact that the changes in composition due ~o
the necessary addition of alkali cause variations in
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conductivity which are not directly related to the useage
of essential ingredients. Thus there would be at least
an initial period, at the outset of the process, when
; the composition of the working solution could not be
controlled by conductivity measurement and the coating
applied to a metal surface would be unsatisfactory or
-~ the process economics adverse.
We have found however that the measurement of
conductivity of acidic zinc phosphate solutions can be
used to advantage under certain specific conditions.
Thus, in a method of applying a æinc phosphate
coating to a continuous metal surface or to a s~ries of
metal surfaces of the type wherein:
(1) the metal surface is treated with an acidic
phosphating solution which comprises zinc,
phosphate and alkali metal ions,
(2) the acidic solution is replenished as coating
proceeds by appropriate additions of a
~ material (a) comprising zinc and phosphate ions
; 20 and of another material (b) comprising alkali
metal ions, (b) having an alkaline reaction
relative to (a), and -
(3) the composition of the acidic solution when in
the steady state is at a desired optimum which ~'
can be maintained substantially constant as
coating proceeds by additions of materials (a)
and (b) in a definite ratio of addition rates,
the composition of the acidic phosphating solution is
brought to that composition which is characteristic of
the steady state at the desired optimum, a continuous
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metal surface or a series of metal surfaces is passed
through the acidic phosphating solution, and thereafter
additions are made to the acidic phosphating solution
of materials (a) and (b) so as to maintain constant its
electrolytic conductivity at a given temperature, the
addition rates of (a) and (b) made in response to any ~.
change in conductivity being in a definite ratio as
defined in (3).
We provide, therefore, an improved and
consistent method of controlling the composition of an
acidic zinc phosphate solution when used in a continuous
phosphating process, We also provide a continuous
process of coati.ng metal surfaces, which can be automat- ;
ically maintained from the outset to provide coatings
of consistent quality given a knowledge of the optimum
concentration of essential ingredients when the coating
solution is in the steady state.
By the term "steady state" of a phosphating ~ .
solution in a given process we mean that the composition
of the solution does not vary systematically with time
of operation, the criterion of systematic variation
being established over periods of the order of several
hours. Those skilled in the art will recognise the
existence of the steady state of a coating solution in
a given type of continuous phosphating process since it
exists when a coating of a desired and consistent quality
is being continuously applied to metal surfaces (or to
a continuous metal surface) which are being passed through
thP coating solution and when the addition of replenish-
ment ingredients is in balance with the loss of ing.redients
.
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from the coating solution, e.g. as ingredients are con-
sumed by the chemical reactions taking place, by leakage
and by carry-over with the coated surface etc., such
that the concentration of the essential ingredients
remains su~stantially constant
This invention is applicable to phosphating
process in which the phosphating solution has reached
the steady state and in which the steady state can be
maintained by addition of essential replenishment
ingredients in a definite ratio of addition rates. The
maintenance of a phosphating solution at the steady
state in this way is well established in the art where
the solution is conventionally monitored by analysis
for specific ingredients, replenishment ingredients
being added subsequently in a definite ratio. It is ~ ;
our discovery that the conductivity of the phosphating
solution can be employed to sense the need for
replenishmen~ addition provided that the solution is
in the steady state from the outset.
The composition of the acidic phosphating
solution at the steady state can be determined readily
by analysis of phosphating solutions which contain ~ !
ingredients desired in the process to be used and which
by conventional procedur~s have been adjusted to coat
metal surfaces in a desired satisfactory and consistent
manner, In using these conventional procedures there
is likely to have been, at least initially, some
inconsistency in coating and wasteage which can be
eliminated by use of the present process
The composition of the acidic phosphating
solution at the steady state may also be determined, at
. ~
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3~ ~
least partly, by theoretical means taking into consider-
ation the various chemical reactions involved, the
replenishment additions, and the total losses which in-
clude both liquid losses due to entrainment on the
coated metal surface and losses due to any sludge
precipitated in the solution, and any other factor.
Whilst we refer to a process in which replenish-
ment i5 effected by additions of (a) and (b) in a
definite ratio of addition rates it should be under~
stood that in certain circumstances, as coating proceeds,
it may be desirable to vary this definite ratio,
Whereas in its simplest form the phosphating
process to which our invention applies comprises the
replenishment o the phosphating solution with materials
; (a) and (b) as above defined it is envisaged that other
materials additional to (a) and (b), for example (c),
; (d) etc, may also be added where necessary. In such a
case all o these additions will be made in a definite
ratio of addition rates to maintain the steady state.
Whilst these materials (a), (b), (c) etc. are
generally addèd to the phosphating solution individually
it may be convanient to combine one or more of the
materials before addition.
The materials (a) and (b)and any urther materials
with which the phosphating solution is replenished will
together comprise the total ingredients which are
necessary to maintain the solution in the steady state
as coating proceeds. The minimum ingredients comprise
zinc, phosphate and alkalimetal ions but in general most
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phosphating processes will require replenishment with
further ingredients, for example a depolarising oxidant.
These further ingredients may be included in materials
(a) or (b) or in further replenishment materials,
depending for example upon their relative reactivity
and their solubility in concentrated solutions.
In a preferred process according to the
invention, the acidic phosphating solution comprises as
essential ingredients zinc, phosphate, chlorate and
optionally nitrate ions, and in such a case, for example,
material (a) comprises zinc, phosphate, nitrate and
chlorate ions and material (b) comprises sodium ions
However, other suitable depolarising oxidants may be
used in the process, for example, nitrite perchlorate,
persulphate, perborate and hydrogen peroxide. Another
suitable alkali metal ion for use in material (b) is
potassium ion.
The process may be applied to ferrous or non~
ferrous metal. ;~
Preferably the process of the present invention
is applied to ferrous metal surfaces. EIowever, in a
conventional phosphating process wherein the preferred
essential ingredients comprise zinc, phosphate chlorate
and, optionally, nitrate ions there is an accumulation
in the working solution of iron as ferrous ion which is
liberated in the process reaction but which is only
slowly oxidised to ferric ion by chlorate or chlorate/
nitrate ions. Two disadvantages of -this accumulation
of ferrous ion are recognised in the art: (1) the quality
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of the coating may be adversely affected, and (2) during
any interruption of continuous working, for example due --
to inadvertent or overnight stoppage, the slow oxidation
of ferrous ion and the precipitation of tha thus
formed ferric ion as ferric phosphate causes an uncom-
pensated rise in the acidity of the working solution
with a resultant adverse effect on its coating
performance when operation is resumed.
It has previously been proposed to alleviate the
above disadvantages by, for example, raising the
temperature of the working solution or by adding a
catalyst to accelerate the rate at which ferrous ion
is oxidised by the combination of chlorate and nitrate
ions. A rise in temperature of the working solution
is costly to produce and to maintain and may have an
adverse effect on the coating characteristics of the
working solution and its general performance,
particularly in a spray process Catalytic means of
accelerating the rate of oxidation of ferrous ion may
alleviate the problems mentioned above but they by no
means eliminate them.
We have found that much improved results are
obtained, for example in respect of uniformity of
coating characteristics, by employing a rapid-acting
secondary oxidant for ferrous ion, the proportion of
said oxidant being chosen such that the oxidation of
ferrous ion is accomplished but such that the oxidant
itself does not accumulate to such an extent that it
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4~3gl ~
is able to play a substantial part as a depolariser in
the principal process of coating formation, i,e. its
concentration as a depolariser is not allowed to reach
those levels in the working solution, e.g. of 1 milli-
mole/litre and above, at which it is known from the
prior art that such rapid-acting oxidants may
function as primary oxidants.
According to a further aspect of this invention
wherein a ferrous metal ~urface is treated with an
acidic solution which comprises zinc, phosphate, chlorate
ions, optionally nitrate ions, and alkali metal ions
there is added to the solution as coating proceeds a
proportion of a rapid-acting secondary oxidant for
ferrous ion (as herein defined) which is sufficient to
maintain the concentration of ferrous ion at less than
112 parts per million (ppm) parts of the solution,
!:
there ~eing present in the working solution when in the
steady state a proportion of the said secondary oxidant
of from 0 to 0,6 millimoles per litre of the solutionO
Preferably the concentration of ferrous ion is
maintained at less than 56 ppm parts of the solution.
Preferably the acidic phosphating solution
contains 0.5-5.0g/1 o~ zinc as Zn, 3~50g/1 of phosphate
as P04, 0,5-5.0g/1 of chlorate as C103, and 0-15g/1 of
ni trate as N03. Preferably the total acid content of ~ ;
the solution is not greater than 30 points and the ratio ;,~
`~ of free acid to total acid is in the range 0,02-1
3~
:
(Pointage = mls of N~lo sodium hydroxide required to
titrate a lOml sample of the solution using phenol-
phthalein as indicator for total acid and methyl orange
for free acid), Preferably the temperature of the
solution does not exceed 65C.
By the term "secondary oxidant" we mean an
oxidant the function of which in the process is solely
to oxidise the ferrous ion without taking part to any
significant extent in the prLmary coating formation
process. Any rapid acting oxidant will fulfil the
function of the secondary oxidant in the present ~ '
invention, By a rapid acting oxidant we mean an oxidant
which, when added to an acidic zinc phosphate solution
containing ferrous ion, will within 10 minu-tes at the
normal operating temperature of the solution reduce
the concentration of ferrous ion by at least one half
. ,:
of the extent theoretically possible.
Suitable rapid acting secondary oxidants include '~
alkali metal nitrites or ammonium nitrite, hydrogen
peroxide, compounds containing combined hydrogen perioxiae
~'. :
which liberate hydrogen peroxide under acidic conditions,
sodium hypochlorite, peroxydiacid salts such as perphos-
phates and perborates, Particularly suitable oxidants
are sodium nitrite and hydrogen peroxide,
Because the rapid-acting oxidant required for
this further aspect of the present invention supplies
only part of the total oxidant requirement of the
process, i,e. to oxidise ferrous ion as opposed to the
depolarising action of the chlorate and optionally
' ~
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nitrate ions in the main coating process, the quantity
of, for example, sodium nitrite required to be fed is
smaller than in those typical processes of the prior
art in which, for example, sodium nitrite is the sole
oxidant replenishment material apart from the replenish-
ment material comprising zinc phosp~ate. When nitrite
ion is effective as a depolarising oxidant there will
be present at least 2 millimoles nitxite/litre of the
solution. A substantial quantity of an alkali e,g,
sodium hydroxide, is xequired however to neutralise
excess free acidity in the replenishment material
which comprises phosphate. The necessary alkali and
the rapid acting oxidant may or may not be combined
in a single replenishment material depending on their
compatibility. If they are incompatible, the bath
would be re~uired to be replenished by 2 separate
replenishment materials in addition to the replenish-
ment materials comprising æinc phosphate but if
compatible only one replenishment material would be
needed. In either case, the two or more replenishment
materials are required to be delivered to the bath in
a fixed ratio of feed rates, e.g. by volume, and can
for example be delivered by multiplepumps driven off
a common shaft or by pumps actuated by a common power
supply. Such an arrangement represents an advance on ;~
the typical processes of the prior art in which the
bath is fed from two or more sources at rates re~uiring
independent adjustment,
;The present process is applicable to spray
application to dip application of zinc phosphate
coatings, The procsss is particularly useful in spray
application,
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39
~xample 1
This Example describes the coating of steel ;
panels with zinc phosphate according to -the method of
the present invention, using a phosphating solution
which comprised zinc, phosphate, chlorate, nitrate and
sodium ions. The optimum composition of the solution
at the steady skate was determined by analysis of
prior phosphating baths of this type which were known
to be in the steady state and which give satisfactory
coatings at that steady state,
Replenishment materials (a~ and (b) according to
~; the invention were as ~ollows:
- (a) Zinc/Phosphate/~itrate/Chlorate
: Zinc Oxide 122 parts ~ ~`
5~/0 nitric acid 102 parts - :
81% phosphoric acid 338 parts
Sodium Chlorate 79 parts
were dissolved in water to give a total weight
of 1,000 parts.
(b) Sodium/Oxidant ("Toner")
Sodium EIydroxide 84 parts
Sodium Mitrite 25 parts
were dissolved in water to give a total weight
of 1,000 parts.
An initial acidic phosphating solution was
prepared by mixing 102 parts of the solution of
replenishment material (a) with 50 parts of an
intimately mixed solid starter powder ~consisting of ~ :
145 parts sodium dihydrogen phosphate, 67 parts sodium
chlorate, 213 parts sodium nitrate and 76 parts sodium
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chloride) the mixture being dissolved in further water
to a total weight of 5,000 parts. This initial solution
(also containing a small proportion of sodium carbonate)
had a total acid pointage of 10~5 and a free acid
pointage of 0,5 tPointage ~ mls of ~/10 sodium
hydroxide required to titrate a 10 ml sample of the
solution using methyl orang~ as indicator for free acid
and phenolphthalein as indicator for total acid). The
conductivity of the solution was 2.32 x 10 2 ohms 1
cm 1 at 50C,
Rolled mild steel panels measuring 30,5 cm x 22,9
cm x 0.9 mm thick were treated by spray application ~ !
with the above solution at a temperature of 50OC and
at a rate of 4 panels/ hour, The rate of metal
treatment was thus 0.112 sq,m/litre of bath/hour and ;~
at this rate of treatment after 12 hours total running
there had been a complete turnover of the zinc content
of the bath,
Coating was continued for a total time of 24 hours
~0 but in four separate periods of 6 hours each,
The replenishment of the phosphating solu-tion ~
was effected by simultaneous additions of the above ~ '
solutions (a) and (b) in a constank ratio of feed rates, ;
0.43g of (b) being added for every lg of (a), in response
to changes in the electrical conductivity of the
phosphating solution, The electrical conductivity was
measured by conventional means there being provided ,~
means for preventing insulation of the conductivity
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sensor by precipitated materials. 50 part by volume
portions of the bath were rejected at 1/2 hour
intervals and the original ~olume restored in order
to simulate the carry-over in an operational plant.
No additions were made to the bath other than those
mentioned. At no time did the concentration of
ferrous ion in the phosphating solution exceed 56 ppm
and the concentration of nitrite ion did not exceed
0.3 millimoles/litre.
~ high standard of coating was maintained
throughout the experiment, the coating weight being
approximately l,9g/sq, m. The final free acid pointage
was 0,5, the final total acid pointage 10.4 and the
conductivity 2,23 x 10 2ohm cm 1, The analysis of
the bath remained substantially as it was at the
beginning of the experiment when it was as follows:-
2g/1 of zinc as Zn, 7.7g/1 of phosphate as P04;
2~3g/1 of chlorate as C103; 4,3g/1 of nitrate as ~03
3,2g/1 of sodium as ~a; and 0,93g/1 of chloride as Cl.
The phosphated panels were subsequently satisfactorily
painted by electro-deposition or by spraying and the
finisheA panels were consistent in appearance and
; corrosion resistance,
Example 2
This Example describes the coating of steel
articles on a plant scale by the spray application
; of a working solution which comprised zinc, phosphate,
chlorate, nitrate and sodium ions,
'$~ '
A phosphating tank of 5,400 litres capacity was
charged w.ith an initial ("start-up") phosphating
solution prepared by mixing 102 parts of a r~plenishment
concentrate (a~ which was compounded from the ingredients:-
Zinc oxide 122 parts
59~/O nitric acid102 parts
81% phosphoric acid338 parts
Sodium chlorate 79 parts
:
these ingredients being dissolved in water to give a - `
~ .
total weight of 1000 parts, and 50 parts o~ an
intImately mixed solid starter powder consisting of~
Sodium dihydrogen phosphate 145 parts
Sodium chlorate 67 parts
~.
Sodium nitrate 213 parts
Sodiwm chloride 76 parts
the mixture being dissolved in further water to a total
weight of 5,000 parts. The initial solution had a
total acid pointage of 10.5 and a free acid pointaye
;
o~ 0.5.
Steel articles were sprayed with the solution ~`
prepared as described above at a temperature o~
110-115F to give a coating weight on the steel of
- 1,3g/square metre. The replenishment concentrate ~a)
` described above and a toner concentrate (a) des~ribed
above and a toner concentrate (b), which comprised~
Sodium hydroxide 44 parts
Sodium nitrite 44 parts
(these incredients being dissolved in water to give a ~ -
total weight of 1,000 parts,) were fed concurrently so
.
that they were added to the working solution in equal `~
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volumes. Additions were initiated by an automatic
controller so as to hold the conductivity of the
solution constant. The chemical analysis of the
solution was maintained substantially constant at:
Zinc as Zn, 2.00g/1; phosphate as P04, 7,04g/1;
chlorate as C103, 2.10g/1 and nitrate as ~03, 3.95g/1.
The concentration of nitrite ion in the solution under
these conditions was substantially zero and that of
ferrous ion was less than 20 ppm,
10The process was continued for 12 hours a day
over 20 working days and a total of 1.5 x 105 square
meters of steel was coated. It was found by scanning
electron microscopy that the deposited phosphate
coating completely covered the steel surface and was
of fine grain, A coating of paint, applied subsequently ~;
by electro-deposition, gave excellent per~ormance when
subjected to accelerated tests for corrosion resistance
and mechanical properties.
.
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