GB2226829A - Applying phosphate coatings to metals - Google Patents

Description

:2:2 G_. a --- 4_ 3 k 1 62/2217/01 PROCESS OF APPLYING PHOSPHATE COATINGS

TO METALS This invention relates to a process of applying phosphate coatings to surfaces consisting of aluminium and/or zinc and/or alloys thereof by means of phosphating solutions used in low-zinc technology.

Phosphating solutions used in low-zinc technology are, by definition, solutions which generally contain zinc in a concentration of about 0.4 to 1.5 g/1 as a layer-forming cation and in which the ratio of Zn to PO 4 is less than about 0.08.

It is known that thin phosphate coatings can be applied to metal surfaces consisting of steel, galvanized steel or aluminium in order to provide a suitable base for finish to be applied by a subsequent electrophoretic dip coating process. The phosphating solutions used for that process usually contain zinc, nickel, manganese, magnesium, cadmium, copper, cobalt, alkali and/or ammonium ions, also phosphate ions, accelerating additives, such as nitrite, chlorate, bromate, peroxide, m-nitrobenzene sulfonate, nitrophenol or combinations thereof. For a treatment of aluminium, its alloys and steel which has been galvanized by electrodeposition, the solutions preferably contain in addition simple and/or complex fluorides. Additional anions, such as chloride, nitrate and sulfate, serve to maintain ionic neutrality. The phosphating solutions may optionally contain grain-refining additives for improving the layer, such as hydroxycarboxylic acids, aminocarboxylic acids or condensed phosphates.

The metal surfaces consist in a major proportion of steel and in changing proportions of galvanized steel and in a small proportion consist of aluminium materials and are treated by spraying and/or dipping processes.

1 2 If surfaces consisting only of zinc and/or aluminium are contacted with the phosphating solution, it will be found that the free acid points and/or the ratio of free acid to total acid in the phosphating solution being used will increase and the phosphating solution will get out of equilibrium. As a result, the formation of the coating will be deteriorated and will finally be suppressed as the throughput increases. The surplus acid cannot be neutralized with sodium hydroxide,. ammonium hydroxide or other alkalis because this would result in a co-precipitation of a part of the bath component zinc.

It is an object of the invention to provide a continuous process for the application of phosphate coatings to surfaces consisting of zinc and/or aluminium andlor alloys thereof by means of phosphating solutions used in low-zinc technology in which increase in the free acid content and/or of the said ratio is prevented so E the known processes will be that the disadvantages o avoided and the process can be carried out in a simple manner and is economical, as the treatment solution is continuously replenished and reused in the process.

To accomplish that object ^ the process of the kind described first hereinbefore is carried out accordinq to the invention by contacting the surface with a phosphating solution which contains iron-III-ions in a concentration of at least 1 mg/1 and to which 50 to 2000 mg iron-III-ions are added per square metre of treated surface area.

The iron ions may be introduced into the phosphating solution in the form of FeiII-ions, or in the-form of iron-II-ions together with an oxidising agent oxidising iron-II to iron-III, eg with chlorate, nitrite, peroxide. Suitable compounds are, eg, the nitrates, chlorides and fluorides of dior trivalent iron as well as iron(I-T 1) 3 complexes of hydroxycarboxylic acids, aminocarboxylic acids and the like.

It is particularly desirable to introduce the iron ions in an aqueous solution into the phosphating 5 solution.

In a preferred embodiment of the invention 50 to 1000 mg iron-III-ions are added per square metre of treated surface area, introduced as iron-II or iron-III-ions.

The process in accordance with the invention can be used with all known phosphating solutions used in low-zinc technology.

In accordance with a further preferred feature, a solution is used which contains 0.4 to 1.5 g/l zinc 10 to 26 g/l P 2 0 5 and 0 to 1.3 g/l Ni to 1.3 g/l Mn and/or 0 to 1.3 g/l Mg In such solutions the weight ratio of Zn to P,)O should - 5 preferably be adjusted to a value in the rangefrom (0.075 to 0.015): I and - if applicable - the weight ratio of Ni, Mn and/or Mg to Zn should be adjusted to a value up to 1. 5: 1.

in a further desirable embodiment of the invention, phosphating solutions are used which contain as an accelerator 2 to 25 g/l NO 3 1 to 6 g/l Clo 3 0.1 to 2 g/l organic nitro compound 0.05 to 0.5 g/1 NO 2 0.02 to 0.1 g/1 peroxide (calculated as H 2 0 2 or mixtures thereof.

The phosphating solutions may be applied by conventional processes. Particularly desirable modes of 4 application are spraying and/or dipping. A temperature in the range from 30 to 70C is preferred.

During non-processing intervals, iron-III-ions must be added to maintain a concentration of at least 1 mg11.

The process in accordance with the invention will be particularly suitable for a treatment of pure aluminium and alloys, eg of the types AlMgSi, AlMg and AlMgMn.

As to the formation of coatings on zinc surfaces, materials consisting of solid zinc, particularly of steel which has been galvanized by hot dip or electrolytic processes, may be treated. Other suitable materials are alloys of zinc with, eg Ni, Fe or Al in the form of solid bodies or of coatings on steel.

By means of the process in accordance with the invention, phosphate coatings which are entirely uniform and continuous can be formed even for a prolonged time. Such coatings will be particularly suitable when an electrophoretic dip coating process is to be performed thereafter. 20 The invention will be explained by way of example and more in detail in the following Examples. Example Previously degreased and pickled sheet metal elements of aluminium (70%) of the grades AlMgSi and AlMg3 and of electro-galvanized steel (30%) were treated in a 10-litre phosphating container with a phosphating solution which contains 0.7 g/1 Zn 0.7 g/1 Ni 1.0 g/1 Mn 3.4 g/1 Na 11.5 g/1 P 2 0 5 3.0 g/1 NO 3 0.5 g/1 F 0.1 g/1 NO 2 j The temperature of the solution was 55 to 601'C. The treatment was performed by spraying for 3 minutes. The number of free acid points of the phosphating solution was 1.0.

By an addition of iron(III) citrate, the phosphating solution was adjusted to an initial iron-IA.II concentration of 2 mg/1.

During the processing, 250 mg iron(III) citrate (calc. as Fe) per m2 of surface area were added to the 10 litres of phosphating solution. As is apparent from the following Table, the number of free acid points was virtually constant. The resulting phosphate covering was uniform and there was continuous throughput.

Throughput (m2) 0.4 0.8 1.2 1.6 2.4 2.8 3.2 Free acid points 1.0 1.1 1.1 1.0 1.0 1.1 1.0 Control Example In a control example, sheet metal elements having the same quality were treated in the same manner with the above-mentioned phosphating solution, with the difference that said solution did not contain iron-III-ions and was not replenished by an addition of iron-III-ions. As the throughput increased, the number of free acid points increased as stated below. Whereas high-grade phosphate coatings were initially obtained, the quality deteriorated as the free acid concentration increased. No coating was formed when the concentration of free acid was 1.50 points or more.

Throughput (m2) 0.4 0.8 1.2 1.6 2.0 2.4 2.8 3.2 Free acid points 1.0 1.15 1.25 1.35 1.45 1.50 1.55 1.63 c -1 1 6

Claims (7)

1. A process of applying phosphate coatings to surfaces consisting of aluminium and/or zinc andlor alloys thereof by applying a low-zinc phosphating solution comprising zinc in an amount of 0.4-1.5 g/1 and phosphate in an amount such that the ratio of Zn:PO is less than 0.08, 4 characterised in that the surface is contacted with a phosphating solution which contains iron-III-icns in a concentration of at least 1 mg11 and to which 50 to 2000 mg iron-III-ions are added per square metre of treated surface area.

2. A process according to claim 1, characterised in that the metal surface is contacted with a phosphating solution to which 50 to 1000 mg iron-III-ions are added_ per square metre of treated surface area.

3. A process according to claim 1 or claim 2, characterised in that the surface is contacted with a phosphating solution which contains 0.

4 to 1.5 g/1 zinc 1)o 10 to 26 g/1 P 2 0 5 and 0 to 1..3 g/1 Ni 0 to 1.3 g/1 Mn and/or 0 to 1.3 g/1 Mg 4. A process according to any preceding claim, characterised in that the surfaces are contacted with a phosphating solution in which the weight ratio of Zn to p 2 0 5 has been adjusted to a value in the range from (0.075 to 0.015): 1 and - if applicable - the weight ratio of Ni, Mn and/or Mg to Zn has been adjusted to a 30 value of up to 1.5:1.

5. A process according to any of the preceding claims, characterised in that the surface is contacted with a phosphating solution which additionally contains 2 to 25 g/1 NO 3 1 to 6 g/1 C10 3 1 k 1 7 0.1 to 2 g/1 organic nitro compound 0.05 to 0.5 g/1 NO 2 0.02 to 0.1 g/1 peroxide (calculated as H 2 0 2) or mixtures thereof.

6. A process according to any preceding claim, characterised in that the surface is contacted with phosphating solution at a temperature in the range from 30 to 700C.

7. A process according to any preceding claim in which 10 the iron-IIIions are added to the phosphating solution in the form of a replenishment solution containing dissolved iron-III-ions or dissolved iron-II-ions with an appropriate amount of oxidising agent.

State House. 66'71 High 116oorn. Lon;.. W].T.4TP Further copies maybe obtainedfrom The Patent Office Wes Branch, St Mar3r Cray. Orpington. Kent BR5 3RID Printed by Mu!V;Ilv.% techniques ltd, St Mary Cray, Kent. Cc. n. 1'87 Sales brancn, in A-I.Y -.Y.