CA1104039A - Phosphating method - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

A continuous process of producing a phosphate coating on a ferrous metal substrate which comprises treating the ferrous metal substrate with an acidic solution of zinc phosphate in the presence of chlorate ions and adding to the solution as coating proceeds a proportion of a rapid-acting secondary oxidant for ferrous ion selected from alkali metal nitrites, ammonium nitrate, hydrogen peroxide, compounds which liberate hydrogen peroxide under acidic conditions, sodium hypochlorite and peroxy diacid salts, which proportion is sufficient to maintain the concentration of ferrous ion at less than 112 parts per million parts of the solution, there being present in the acidic solution when in the steady state a proportion of the said secondary oxidant of from 0 to 0.6 millimoles per litre of the solution.

Description

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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 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 . -4-3~

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 ~ _5_ :' '~ , 35~

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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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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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~

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(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, ~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 ~-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 -15- -;

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 - `
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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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Claims (8)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A continuous process of producing a phosphate coating on a ferrous metal substrate which comprises treating the ferrous metal substrate with an acidic solution of zinc phosphate in the presence of chlorate ions and adding to the solution as coating proceeds a proportion of a rapid-acting secondary oxidant for ferrous ion selected from alkali metal nitrites, ammonium nitrate, hydrogen peroxide, compounds which liberate hydrogen peroxide under acidic conditions, sodium hypochlorite and peroxy diacid salts, which proportion is sufficient to maintain the concentration of ferrous ion at less than 112 parts per million parts of the solution, there being present in the acidic solution when in the steady state a proportion of the said secondary oxidant of from 0 to 0.6 millimoles per litre of the solution.

2, A method according to Claim 1 wherein the acidic solution of zinc phosphate further comprises nitrate ions.

3. A method according to Claim 2 wherein the concentration of ferrous ion is maintained at less than 56 parts per million parts of the solution.

4. A method according to Claim 2 or Claim 3 wherein the secondary oxidant is an alkali metal nitrite or hydrogen peroxide.

5, A process according to Claim 1 wherein there is added to the acidic solution of zinc phosphate as the process proceeds a replenishment material (a) which comprises zinc, phosphate and chlorate and a replenishment material (b) which comprises alkali metal and a rapid-acting oxidant which is an alkali metal nitrite or hydrogen peroxide,

6. A method according to Claim 1 wherein the acidic phosphating solution comprises 0.5-5.0g/l of zinc as Zn, 3-50g/1 of phosphate as PO4, 0,5-5.0g/1 of chlorate as C103 and 0-15g/1 of nitrate as NO3.

7. A method according to any one of Claims 1-3 wherein the acidic phosphating solution has a total acid content of not greater than 30 points and a ratio of free acid to total acid in the range 0.02-0.1.

8. A method according to any one of Claims 1-3 wherein the metal surface is treated by spray application of the acidic phosphating solution.