US3819385A

US3819385A - Method for applying a phosphate coating to iron and steel

Info

Publication number: US3819385A
Application number: US00283446A
Authority: US
Inventors: H Schumichen; G Mueller
Original assignee: Oxy Metal Finishing Corp
Current assignee: Henkel Corp
Priority date: 1971-09-02
Filing date: 1972-08-31
Publication date: 1974-06-25
Anticipated expiration: 1991-06-25
Also published as: FR2150659B1; BR7205983D0; GB1355739A; AR196082A1; NL7211797A; FR2150659A1; ES401733A1; AU4577672A; BE780333A; ZA725433B; AT314932B; SE376772B; CH580168A5; DE2143957A1; IT964406B

Abstract

Disclosed is a method and composition useful for applying a phosphate coating to an iron or steel surface. The surface is contacted with an aqueous solution containing acidic zinc phosphate, chlorate ion and between 0.1 and 10 mg/1 vanadium, calculated as V2O5, at a temperature of less than 65* C.

Description

United States Patent 1191 Schumichen et a1.

METHOD FOR APPLYING A PHOSPHATE COATING TO IRON AND STEEL Inventors: Helmut Schumichen, Frankfurt;

Gerhard Muller, Hanau, both of Germany Oxy Metal Finishing Corporation, Warren, Mich.

Filed: Aug. 31, 1972 Appl. No.: 283,446

Assignee:

Foreign Application Priority Data Sept. 2, 1971 Germany 2143957 US. Cl. 106/14, 148/615 Z Int. Cl C09d 5/12 Field of Search 106/14; l48/6.15 R, 6.15 Z

References Cited UNITED STATES PATENTS 9/1958 Kronstein 148/615 R FOREIGN PATENTS OR APPLICATlONS 495,098 2/1937 Great Britain 148/6.l5 Z

Primary ExaminerLorenzo B. Hayes Attorney, Agent, or FirmArthur E. Kluegel; Marvin Trimas; B. F. Claeboe 5 7 ABSTRACT Disclosed is a method and composition useful for applying a phosphate coating to an iron or steel surface. The surface is contacted with an aqueous solution containing acidic zinc phosphate, chlorate ion and between 0.1 and 10 mg/ l vanadium, calculated as V 0 at a temperature of less than 65 C.

9 Claims, No Drawings METHOD FOR APPLYING A PHOSPHATE COATING TO IRON AND STEEL BACKGROUND AND SUMMARY OF THE INVENTION The invention pertains to an improved method for applying a phosphate coating to iron and steel using an aqueous zinc phosphate solution containing acid chlorate.

It has long been known that acid zinc phosphate solutions containing chlorate as accelerator can be used for applying phosphate c'oatin gs to iron and steel. Without the use of additional oxidizing agents, .solutions of this type lead to thin, hard, dark coatings, which provide very good protection against corrosion when combined with organic covering layers. At operating temperatures below 65 C, however, the chlorate alone is not sufficiently reactive to oxidize the divalent iron entering into the solution immediately and quantitatively to trivalent iron, which is then immediately deposited as insoluble iron l'lI phosphate. In the absence of other oxidizers which are capable of oxidizing the divalent iron with sufficient speed even at bath temperatures below 65 C, the divalent iron concentration, especially at high throughputs, increases rapidly. For example, during the treatment of morethan 0.04 m surface/liter/day, the concentration of divalent iron in the bath solution rapidly increases to several 100 mg/l. However, this produces disadvantages. In particular, these considerable quantities of divalent iron present in the solution, upon cessation of the passage of iron surfaces through the bath solution due to a temporary production stoppage or during standing overnight, are slowly oxidized to trivalent iron by the oxidizers in the solution such as chlorate and oxygen from the air, and precipitated as insoluble iron lll phosphate. This reaction greatly increases the acidity of the solution, since the phosphate anions are precipitated as iron (Ill) phosphate, but the hydrogen cations are left behind in the phosphoric acid solution. When iron surfaces are again treated in this phosphate treatment solution after such stoppages in operation, only thin or passive layers are produced at short phosphation times because of the excessively high acidity. Tobe sure, it is possible to obdesirable phosphate coatings again be precipitatedwithin short times. This measure is an additional, troublesome process, which can only be performed by trained personnel.

In order to avoid this disadvantageous accumulation of divalent iron solution, it has long been known that it is possible to use other oxidizing agents, in addition to the chlorate, which are capable of immediately and quantitatively oxidizing the divalent iron which enters into solution to trivalent iron even at temperatures below 65 C, e.g., nitrite (see, for example, British Pat. No. 551,261).

The use of nitrite in addition to chlorate, to be sure, represents a highly effective procedure with respect to the removal of the iron which goes into solution, and the related possibility of maintaining constant, short treatment times; however, the use of nitrite also entails 2 procedural disadvantages, e.g., the necessity of waste water processing and of removingnitrogen-containing gases from the waste air. The nitrous gases can also lead to corrosion at the transmission points before and after the treating zone. In addition, the continuous analytical monitoring of the nitrite content in the bath solution entails a certain expense. It has been found that zinc phosphate coatings which are applied from solutions containing chlorate and nitrite provide poorer corrosion protection in combination with an organic covering layer than corresponding coatings produced with a chlorate-containing but nitrite-free solution.

Since the coatings applied using chlorate-containing zinc phosphate solutions, because of their quality and low layer weight, represent a particularly suitable base for subsequent electrophoretic painting, considerable interest exists in improving this method while avoiding the disadvantages linked with the additional use of nitrite.

The object of the invention is now a method for applying a phosphate coating to iron and steel with the aid of a chlorate-accelerated zinc phosphate solution at temperatures below 65 C, which is characterized by the fact that a zinc phosphate solution containing 1-10 g/l C10 and 0.1 to 10 mg/l vanadium, calculated as V 0 is used.

' It has surprisingly been found that the addition of the small quantities of vanadium to the chlorate bath eliminates the disturbances occurring at operating temperatures below 65 C, without increasing the treatment time and without the simultaneous use of nitrite or other customary oxidizers which are capable of immediately and quantitatively oxidizing the divalent iron which goes into solution at the intended temperatures. The procedure according to the invention is therefore simpler and easier to carry out. In addition, it exhibits the advantage that the applied phosphate layers display the low layer weight desired for subsequent electrophoretic painting and advantageous hardness, without the decrease in corrosion protection observed with the use of nitrite occurring.

The vanadium can be added to the chloratecontaining zinc phosphate solution in accordance with the invention in the form of vanadium pentoxide (V 0 or in the form of other vanadium compounds. It' can also be added to the concentrate used in making up the solution. The vanadium content of the treating solution preferably amounts to 0.4-7 mg/l, calculated as V 0 The range of 0.4-2.7 mg/l is particularly suitable. The significance of the vanadium content is independent of the valence of the vanadium actually present in the solution. Contents of more than 10 mg/l can interfere with layer formation by passivation of the surface.

The zinc phosphate solution used advantageously contains about 1 to 10 g/l zinc, corresponding quantities of phosphate and l to 10 g/l chlorate. It can also contain nitrate and other customary additives, such as nickel, calcium, simple or complex fluoride, boron compounds and the like. The further use of nitrite is undesirable because of the disadvantages mentioned above.

The iron and steel surfaces to be provided with the coating can be brought into contact with the treating solution in any desired manner, e.g., by immersion in the solution or spraying with the solution. The spray method is advantageously used. The spraying time advantageously amounts to a maximum of 150 seconds. Somewhat longer contact times may be required in the immersion process.

The method according to the present invention is advantageously carried out in the temperature range of 40 to 65 C. At temperatures above 55 C, the treating bath also remains essentially free from divalent iron even at higher throughputs. At temperatures below 55 C, certain quantities of divalent iron can enter the solution, without an appreciable accumulation occurring.

TABLE 1 V Minimum Expericoncen- Temperphospha- Layer m/l Iron (11) ment No. tration ature, tion time weight treated content mg/l C g/m surface mg/l The maximum quantities of Fe (11) which occur in the solution amount to about 85 mg/l. They do not lead to the interference mentioned in the preceding.

The replenishment of vanadium in the solution used in accordance with the present invention can be easily accomplished by adding 0.044 mg vanadium, calculated as V 0 to the solution per square meter of treated surface.

Supplemental zinc, P 0 and chlorate is provided in the customary manner, to provide point constancy or corresponding to use. The vanadium compound may be added to the supplemental concentrate if desired.

To be sure, it was already known from German Pat. No. 1,078,845 that oxidizing agent-containing solutions of layer-forming phosphates could be supplemented with up to 1 g/l vanadium, calculated as V0 in order to achieve a considerable increase in the weight of the layer in comparison to vanadium-free baths. The weight of the layer was supposed to practically double at an identical treatment time. This effect was described, based on the example of nitrateand nitrate-nitrite-containing zinc phosphate baths at temperatures of 98 C in the immersion process or 65 to 70 C in the spray process. It should also be noted that the effect of the desired layer increase was taught to be practically independent of the operating temperature, and that chlorate, among others, could also be used as accelerators. However, thin, hard phosphate coatings with layer weights of only 1.6 to 2.5 g/m are specifically desired as pretreatments prior to electrophoretic painting. Therefore, a considerable prejudice existed against the use of the vanadium additive which was known to increase the layer thickness. It is entirely unexpected for the addition of small quantities of vanadium to a zinc phosphate solution containing 1-10 g/l of chlorate as accelerator and at treating temperatures below 65 C, to be suitable for eliminating the difficulties mentioned above, without increasing the layer weight.

DESCRIPTION OF THE PREFERRED EMBODIMENTS A treating solution was prepared, which contained 3.1 g/l Zn, 8.3 g/l P 0 5.2 g/l C10 13.8 g/l N0 and 0.1 g/l Ni. Sheets of steel were treated by spraying at temperatures of 53 C and 60 C. In accordance with It is obvious that the addition of vanadium has resulted in a marked reduction in the iron (11) content of the solution, and that the layer weight of the coating was scarcely affected by this addition.

The corrosion behavior of the coatings produced at 60 C in accordance with the invention (Experiment No. 4) was likewise compared with coatings obtained from vanadium-free solutions, containing only chlorate, and containing iron (11) (Experiment No. 2) and the coatings produced at 60 C from solutions containing chlorate and nitrite (Experiment No. 5). The solution used in Experiment No. 5 contained 3.1 g/l Zn, 8.3 g/l P 0 5.2 g/lC10 13.8 g/l NO;,, 0.1 g/l Ni and 0.11

g/l NaNO The phosphated sample panels were first electrophoretically painted, then provided with a dual-layer lacquer coating. The salt spray test was carried out according to ASTM B 117-64. The results obtained are summarized in Table 2.

It is obvious that the use of the nitrite-free, vanadium-containing solution in accordance with the invention leads to coatings which offer corrosion protection at least percent better than coatings produced with the aid of the nitrite-containing solution.

What is claimed is:

l. A method for applying a phosphate coating to an iron or steel surface comprising contacting said surface with an aqueous solution comprising acidic zinc phosphate, chlorate ion and from 0.1 to 10 mg/l of vanadium, calculated as V 0 at a temperature of less than 65 C.

2. The method of claim 1 wherein the chlorate ion content of the solution is between 1 and 10 g/l.

3. The method of claim 2 wherein the zinc content of the solution is between 1 and 10 g/l.

4. The method of claim 3 wherein the vanadium is present in an amount of from 0.4 to 7 mg/l, calculated as V205.

5. The method of claim 4 wherein the vanadium is present in an amount of from 0.4 to 2.7 mg/l, calculated as V205- 6 g/l chlorate and from 0.1 to 10 mg/l vanadium, calculated as V 0 8. A method of maintaining the ferrous ion content of a chlorate accelerated acidic zinc phosphate solution for the treatment of iron or steel below a maximum of about 85 mg/l when the solution is below 65 C comprising including from 0.1 to 10 mg/l of vanadium, calculated as V 0 9. The method of claim 8 wherein the chlorate content is from ll0 g/l and the zinc content is from l-lO g/l.

Claims

2. The method of claim 1 wherein the chlorate ion content of the solution is between 1 and 10 g/l.
3. The method of claim 2 wherein the zinc content of the solution is between 1 and 10 g/l.
4. The method of claim 3 wherein the vanadium is present in an amount of from 0.4 to 7 mg/l, calculated as V2O5.
5. The method of claim 4 wherein the vanadium is present in an amount of from 0.4 to 2.7 mg/l, calculated as V2O5.
6. The method of claim 1 wherein said surface is contacted with said solution for a period of time sufficient to produce a coating of layer weight between 1.6 and 2.5 g/m2.
7. An aqueous composition useful for pretreating an iron or steel surface prior to electrophoretic paint deposition comprising acidic zinc phosphate, from 1 to 10 g/l chlorate and from 0.1 to 10 mg/l vanadium, calculated as V2O5.
8. A method of maintaining the ferrous ion content of a chlorate accelerated acidic zinc phosphate solution for the treatment of iron or steel below a maximum of about 85 mg/l when the solution is below 65* C comprising including from 0.1 to 10 mg/l of vanadium, calculated as V2O5.
9. The method of claim 8 wherein the chlorate content is from 1-10 g/l and the zinc content is from 1-10 g/l.