CA2149721A1 - Process of producing phosphate coatings on metals - Google Patents

Abstract

Disclosed is a process of producing a phosphate coating on a metal having a surface which consists at least in part of iron or steel, a phosphatizing solution for use in the process and a concentrate for forming and/or replenishing the solution. The metal is contacted at a temperature in the range from 30 to 65°C
with a phosphatizing solution which contains about 5 to about 25 g/l zinc, about .5 to about 5 g/l of magnesium, about 0.1 to about 13 g/l iron(II), about 5 to about 40 g/l phosphate (calculated as P2O5), about 5 to about 50 g/l nitrate, and about 0.5 to about 5 g/l fluoroborate (calculated as BF4). The solution has been adjusted to weight ratios of Zn : P2O5 = (0.5 to 3) : 1 and of Mg : Zn = (0.04 to 0.5) : 1 and to a ratio of free acid to total acid of (0.04 to 0.2) : 1. The process is particularly suitable for preparing metals for cold working.

Description

PROCESS OF PRODUCING PHOSPHATE COATINGS ON METALS
Bach~lou~d of In~vention The pre~ent in~ention i8 in a process for producing a phosphate coating on a metal ha~ing a surface which consists at lea~t in part of iron or ~teel by a treatme~t with an aqueou~
phosphatizing solution, which contains zinc ions, P04 ion~, nitrate ions, magne~ium ions and if ~eing in working condition, iron(II) ion6 and to the use of that process for preparing metals for sub~equen~ cold working operations such a~, ~o~ example, wire drawing, tube drawing, cold heading and cold extrusion.
The use of phosphatizing solutions which contain mangane6e, iron(II) and zinc pho~phate and are at temperature~ of from 50 to 98 C to pho~phatize metals, particularly iron and steel, i~
known. The resulting phosphate coatings are useful for varioug applications such a~ ru~t prevention, priming for the applica~ion of paint, electric insulation, reducing sliding ~riction and facilitating cold working (Published German Application 30 2 479; EP 42 631), The performance of the known phosphatizing proce~ses at relatively low temperatures, e.g., between 30 and 50~ C, often gi~es rise to difficultie~ which include, among others, the formation of phosphate coatings which only incompletely cover the metal ~urface and/or which become progressi~ely more coar~e as the iron (II) content in the phosphatizing ~olution increases.

21~9721 - . ~

, Certain impro~ements can be obtained by a preliminary acti~ating rinsing ~tep, e.g., with titanium pho~phate, but the results thus produced are not en~irely sati~factory. On the other hand, low-temperature processes are urgently required because they save energy. Additio~ally, the pro~esses which involve the pre~ence of iron(II) are superior to the u~e of pho~phatizing solutions which are free of iron(II) because much less sludge is formed and the consumption rates are fa~orable.
It has been ~ound the component solution~ for the phosphatizing bath periodically become un~table when supplied in commercial quantities. The lack of sta~ y sometime~ re~ults in the formation of precipitates which can ad~er~ely affect the coating. Also, the component solutions sometime~ exhibit a gassing which i9 believed to be ~a~ed on the ~ormation of NOX in gaseous form causing the expa~sion or -~welling of the container and resulting in the emission of a browni~h ~ume. One approach to a~oid such pro~lems is to use a number of separate concentra~e or make-up solutions and to mix the sol~tions on site as the need arises. Howe~er, on site m; ~; n~ can be inconvenient and requires the expenditure of additional time, effort and resources as the number of such component 601utions required to make the bath increases.

2 1 ~ 9 7 2 1 The In~rention It i~ an object of t~e in~ention to pro~ide a process for the produc~ion of phosphate coatings on metal~ ha~ing surfaces which conQist at least in part of iron and steel, which can be carried out at low temperatures and in the ~resence of iron(II) and which results in the formation of uniformly covering phosphate coatings but without an un~sually high proces~
expenditure.
It i~ a further object o~ the in~ention to pro~ide a process ~or the preparation of such a metal ~ur~ace for cold working.
Another object of the invention is to provide a process which requires a minimum number of ~eparate components to prepare the phosphatizing bath.
It is another object of the in~ention to provide compositions for use in ~uch a proce~ that are consi~tently stable and which remain stable o~er an extended period of time.
It is a further object of the invention to pro~ide component solutions which are not subject to gas~ing and which do not form precipitates.
The proce~ o~ the invention overcomes the above-discussed disadvantage~. In the in~ention the metal 6ur~ace is contacted with a phosphati~ing ~olution which i-~ at a temperature in the range of from about 30 to 65 C and contain~ about 5 to about 25 g/l zinc, abou~ .5 to about 5 g/1 of magnesium, about 0.1 to - 2 ~ ~9721 .

about 13 g/l iron(II), a~out 5 to about 40 g/l pho6phate (calculated as PzOs), about 5 to about 50 g/l nitrate, about 0.5 ~o about 5 g/1 fluoroborate (calculated as BF~) and which ha~
been adjusted to weight ratios of Zn : P20s of (~.5 to 3) : 1 and of Mg : Zn of (0.04 to 0.5) : 1 and to a raeio of free acid to total acid of (0.04 to ~.2) : 1. Optionally the solu~ion may contain about 0.05 to about 3 g/l of tartaric acid and/or of citric acid. The phosphatizing solution, and the conce~trate solution(~) used for make-up and repleniQhment are e~sentially free of manganeQe.
The process of the invention is especially useful to treat iron and steel which contain up to a~out 5~ alloying additions It al~o ifi possible to treat other metals, e.g., of zinc and zinc alloys, together with iron and steel, whether such other metals are in the form of compact workpiece~ or merely coatings on workpieces.
The selJeral component3 o~ the phosphatizing l;olution, such as zinc, magnesiu~, phosphate, etc., are jointly predi~sol~ed ~o form an acid phosphatizing concentrate, a~ is usual in pho~phatizing technology, and are added to the phosphatizing solution in that form. The several component~ are added in such quantities that the required con~Pntration ranges are obtained in the phosphatizing solution. If nece6sary, the required ratio of free acid to total acid may be obtained by the introduction of 214g721.

additional ions of the group con.~isting of the alkali and ammonium ions.
In a preferred emboA;~nt o~ the in~ention, the metal i~
contacted with a phosphatizing ~olution which also contains abou~
O . 05 to about 2 g/l Ni and/or about o . 001 to about 0.1 g/l Cu and/or about O . 5 to about 2 g/l Ca. A nickel content will promote layer formation, particularly on ~inc and on materials which ha~e a relatively high resistance to the attac~ by the colution. An addition of copper ions will accelerate the phosphatizing proces~. Added calciu~ ion~ will modify t}le phosphate coating and will effect ~uch a conditioning o~ the bath ~ludge ~or~ed in a small amount that the sludge is still les~
disturbing in the pho6phatizing bath and can be remo~ed f~nm the system more easily.
The phosphatizing solutions used in the proce~s in accordance with the in~ention need not initially con~ain iron(II) ions. The bath will become enriched in iron(II) durin~ the treatment of iron and steel by di~solution of iron from the sur~ace. The ph~sphatizing ~olution may also be artificially aged by addition of an iron(II) sal~ compatible with the other components of the solution, e.g., iron(II) ~ulfate. While the phosphatizi~g solution ~ay ha~e an iron(II) conten~ of up to about 13 g/l, the ~olutionæ are pre~erably ~sed in such a m~n~

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that the concentration of iron(II) ions does not exceed 10 g/l and is preferably in the range o~ from 4 to 7 g/l.
In order to prevent an undesirable increase of the iron(II) concentration, a portion of the iron(II) ions which have entered the solution a~ a re~ of the pickling action are preferably oxidized to form irontIII) ~ons. The iron(III) ion~ are precipitated as difficultly soluble iron(III) phosphate sludge In a preferred embodiment of the ~nvention, ~urplu~ iron~II) is transformed to tri~alent iron by contact with an oxygen-containing gas or by mean-~ of chlorate and i9 precipitated as iron(III) phosphate ~ludge. In the first case, iron kI) can be removed, e.g., in a separate aerating container, which i5 s~cceeded by a filter.
Sodiu~ carbonate, zinc oxide and/or zinc carbonate are preferably used to adjust the ratio of free acid to total acid to (0.04 to 0.2) : 1. The~e co~ponent-~ are added to the pho~phatizing 601ution ag a powder or in an aqueous ~uspen~lon.
To determine the contents of free acid and of total acid, bath sample~ of 10 ml are titrated with NJ10 ~aOH to the fir~t and ~econd tran~itions of pho~phoric acid a~ indicated by a color change, e.g., with dimethyl yellow (free acid test) and phenolphthalein ~total acid test) ~sed a~ indicator~. The consumption of Nt10 NaOH in milliliters corresponds to the points of free acid or total acid.

-: 2 1 4 9 7 2 1 : ;

It ha~ also been found desirable to contact the metals with a phosphatizing solution which contain~ a nitrite-destroying su~tance, such as urea or amidosulfonic acid. This will inhibit the autocatalytic formation of nitrite from the nitrate and will preclude a turrlover of the bath from the iron side to the nitrite ide .
In prior art ~igh zinc phosphatizing processes, it ~as been ~ound that the addition of tartaric or citric acid to the pho~phatizing solution re~ulted in a coating having a finer crystal ~ructure than obtained by the u~e of the same solution without those acid addition~. Howe~er, in the process of the invention, such substances are not necessary to obtain a fine crystal structure and their presence, whic~ is optional, ha~
li~tle or no ef~ect on the finenes~ of the crystal 6truc~ure.
The addition of the tartaric or citric acid can result in a lower coating weight. The fine coatings produced by ehe proces~ o~ the invention have a ~ood drawing ability, exhibit good coating retention during drawing and consume less drawing lubricants, i.e., drawing soap~.
During the phosphatizing proces~, co~ponents o~ the ~olution are con~umed by the formation of the coating, the formation of ~ludge, and by the mechanical discharge of pho~phatizing solution. The pho~phatizing solution must be repleni~hed wieh the con~umed components. The replenishing i~ preferably by means of a replenishing concentrate containing a number of, or all of, the individual component~. The phosphatizing solution can be replenished i~ a particularly desirable manner by an addition of zinc ion~, magnesium ions, phosphate ion~ and nitrate ions in the form of at least one concentrate which has been adjus~ed to weight ratios of Zn : P2O5 = (0.3 to 0.8) : l, o~ Mg : 2n =
~ 05 to 0.2) : 1 and of NO3 : P2Os , (0.2 to 1) : 1. Any other components of the phosphatizing solution to be replenished should be added in weight ratios of Ca : Zn - (o . 00~ to 0.1) : l, of Ni : Zn - (0.005 to 0.05) : l, of Cu : Zn = (0.001 to 0.03~ : 1, and of BF4 : P205 - ~0.008 to 0.04) : l. When the solution is to contain tartaric and/or citric acid, the ratio of tartaric and/or citric acid to P2O5 is (0.05 to 0.3) : 1. The concentrates of the invention need not contain stabilizing agents ~ut, if de~ired, the ~sual stabilizers may be added.
The metal surface may be contacted with the pho~phatizing solution by dipping, flooding or spraying. In mo~t cases the 501ution is applied by dipping with treatment ti~es of, e.g., 5 to 15 minutes. If the flooding technique is used the trea~ng time i~ the ~ame as with the dipping treatment. In case of ~praying the treatment time is in general in the range o~ 0.5 to 3 minutes.
In dependence on the ~pecific compo~ition of the solution, the treating temperature, the treating time and the sequence of ~14-9721 ~ `

-, , ,, , operations, the pho~phate layer~ produced by the process in accordance with the invention will have a weight of about 3 to 18 g/m2 ~

sefore ~he pho~phatizing proces~, ~he ~etal6 are pretreated in ~he conventional manner. ~or in~tance, the metal~ may be degreased by means of an aqueou~ alkali~e cleaning solution which contains suita~le surfactants. Any ~cale or rust which may be present should be removed by a pi~kling treatment, e.g., with ~ulfuric acid or hydrochloric acid.
Before the workpieces are phosphatized, they may be prerin~ed in a manner kno~n per se, e.g., with an acti~ating bath which contains titanium phosphate, in order to promote the formation of finely cry6talline phosphate coa~ing~. Such a treatment is not compulsory.
The phosphatizing treatment i~ usually ~ollowed by a water rinsing and by an optional aftertreatment and drying, if required. The re~istance to corrosion can be increased by an aftertreatment, e.g., wi~h chromic acid and/or corrosion protecti~e emulsions. Workpieces to be cold-worked may be aftertreated, e.g., with a ~oap-applying bath.
The pho~phate coatings produced by the proces~ in accordance with t~e in~ention can be used to advantage in all fields in which phosphate coatings are u~ed and are particularly suitable for preparing metals for a sub~equent cold working.
g The invention will be explained more in detail by the following E~ample.
ExamDle Steel wire having a carbon content of from 0.5 to 0.9~ i9 acid treated to remo~e ruQt and ~cale and is then thoroughly rinsed with water. The rinsed wire is pho~phatized by dipping for 6 to 10 minutes in a phosphatizing ~olution which is at 45C
and compo~ed as follows:
15.0 g/l Zn, 1.6 g/l Mg, 7.0 g/1 Fe(II), 0.5 g/l Ca, g/l ~i, 0.01 g/l Cu, 0.3 g~1 Na, 30.0 g/l NO3, 12.0 g/l P20s 1.6 g/l BF"
0.5 ~/l Urea, 5.4 points free acid, and points total acid.
The pho~phatizing treatment is followed by another rin~ing with water, which i~ followed by neutralization in a hot a~ueous borax solu~ion and by o~en-drying. After that ~equence o~ Qteps, the phosphate coating ha~ a weight of 8 to 12 g/m2. Although there had been no activating prerinse with titanium pho~phate, the phosphate coating i6 ~inely cry~talline and pro~ides a uniform coverage.
After that treatment the wires could ~e fiatisfactorily ~haped in up to lO to 13 drawing passes to a ~ximl~m reduction in cro~ ection by 93.5~ with a very low wear of the drawing dies.
After drawing, the qurface o~ the wire continues to have a uniform re~idual phosphate layer and is free of groove~.
Air is blo~n into the phosphatizing bath during the treatment to maintain the iron(I~) ion content in the range of from 2 to 7 g/l under the existing condition~. ~o maintain as constant the point~ of total acid in the bath, the solution i6 replenished with a replenishing concentrate containing, on a weight percent ba~
10.1 ~ Zn, ,163 ~ Mg, o.o9 ~ Ni, O.021 ~ Cu, 19.5 ~ P2Os, 10.6 ~ NO3, 0.~ ~ BF" and 0.15 ~ Ca.

21497`21 secause the bath i~ operated on the iron side, only a small amount of bath sludge forms. Repleni~hing concentrate i9 consumed at the very low rate of about 27 g per m2 of metal surface area.
It ~ill be understood that the ~pecification and examples are illustrati~e but not limitati~e of the present invention and that other embodiments within the Qpirit and scope of the invention will suggest themselve~ to those skilled in the art.

Claims (16)

1. A process for producing a phosphate coating on a metal having a surface which consists at least in part of iron or steel comprising: contacting said metal surface with an aqueous phosphatizing solution which is at a temperature in the range from 30 to 65°C, the solution comprising about 5 to about 25 g/l zinc, about .5 to about 5 g/l of magnesium, about 0.1 to about 13 g/l iron(II), about 5 to about 40 g/l phosphate (calculated as P2O5), about 5 to about 50 g/l nitrate, about 0.5 to about 5 g/l fluoroborate (calculated as BF4), the phosphatizing solution having weight ratios of Zn : P2O5 = (0.5 to 3) : 1, of Mg : Zn =
(0.04 to 0.5) : 1 and a ratio of free acid to total acid of (0.04 to 0.2) : 1.

2. The process of claim 1 wherein the metal is contacted with a phosphatizing solution which also contains about 0.05 to about 3 g/l of tartaric acid and/or of citric acid.

3. The process of claim 1 wherein the metal is contacted with a phosphatizing solution which also contains about .5 to about 2 g/l Ni and/or about 0.001 to about 0.1 g/l Cu and/or about 0.5 to about 2 g/l Ca.

4. The process of claim 1 wherein the metal is contacted with a phosphatizing solution which has been adjusted to an Fe(II) concentration not in excess of 10 g/l.

5. The process of claim 1 wherein the phosphatizing solution is contacted with an oxygen containing gas to transform surplus iron(II) to Fe(III) and to precipitate the latter as iron(III) phosphate sludge.

6. The process of claim 1 wherein the metal is contacted with a phosphatizing solution in which surplus iron(II) is reacted with chlorate to form iron(III) and to precipitate the latter as iron(III) phosphate sludge.

7. The process of claim 6 wherein the iron(III) is precipitated as iron(III) phosphate sludge.

8. The process of claim 1 wherein the phosphatizing solution is replenished by a concentrate of zinc ions, magnesium ions, phosphate ions and nitrate ions having weight ratios of Zn : P2O5 = (0.3 to 0.8) 1, of Mg : Zn - (.005 to .2) : 1 and of NO3;
: P2O5 = (0.2 to 1) : 1.

9. The process of claim 8 wherein the phosphatizing solution is replenished by a concentrate also containing Ni, Cu, Ca, as well as fluoborate in weight ratios of Ca : Zn = (0.005 to 0.1) : 1, of Ni : Zn = (0.005 to 0.05) : 1, of Cu ; Zn = (0.001 to 0.03) : 1 and of BF4 : P2O5 = (0.008 to 0.04) : 1.

10. The process of claim 9 wherein the phosphatizing solution is replenished with tartaric acid and/or citric acid :
in a weight ratio of tartaric acid and/or citric acid to P2O5 =
(0.05 to 0.3) : 1.

11. The process of claim 1 wherein the metal is contacted with a phosphatizing solution having a free acid content that has been adjusted by an addition of at least one of sodium carbonate, zinc oxide and/or zinc carbonate.

12. The process of claim 1 wherein the metal is contacted with a phosphatizing solution which contains a nitrite-destroying substance.

13. The process of claim 12 wherein the nitrite-destroying substance is at least one selected from the group consisting of urea and amidosulfonic acid.

14. A process of preparing a metal for cold working comprising: contacting said metal surface with an aqueous phosphatizing solution which is at a temperature in the range from 30 to 65°C comprising about 5 to about 25 g/l zinc, about .5 to about 5 g/l of magnesium, about 0.1 to about 13 g/l iron(II), about 5 to about 40 g/l phosphate (calculated as P2O5), about 5 to about 50 g/l nitrate, about 0.5 to about 5 g/l fluoroborate (calculated as BF4), the phosphatizing solution having weight ratios of Zn : P2O5 = (0.5 to 3) : 1 and of Mg : Zn = (.05 to .2) : 1 and a ratio of free acid to total acid of (0.04 to 0.2) : 1.

15. The process of claim 14 wherein the metal surface is contacted with an aqueous phosphatizing solution which also contains about 0.05 to about 3 g/l tartaric acid and/or citric acid.

16. The process of claim 14 wherein the metal is contacted with a phosphatizing solution which also contains about .5 to about 2 g/l Ni and/or about 0.001 to about 0.1 g/l Cu and/or about 0.5 to about 2 g/l Ca.