US3116177A - Process for degreasing and oxalating in a single spray stage - Google Patents

Description

United States Patent 3,116,177 PROCESS FGR DEGREASING AND OXALATING IN A SINGLE SPRAY STAGE Paolo de Cerrna and Roberto Vironda, Milan, Italy,

assignors to Montecatini Societa Geuerale per lIndustria Mineraria e Chimica, Milan, Italy No Drawing. Filed (Bet. 17, 196i), Ser. No. 62,903 Claims priority, application Italy Oct. 19, 1959 3 Claims. (Cl. 148-614) This invention relates to an improvement in a process for pre-treating a metal surface, particularly iron, or steel, or other ferrous metal or alloy, prior to applying paint thereto.

Conventional continuous or cyclic processes of this nature require preliminary degreasing of the surface of the metal, prior to treatment with a spray to form the coating which comprises either a phosphate or oxalate layer, or both.

The first step of such prior processes comprises a spray degreasing by means of alkali, or an organic solvent emulsion. This is followed by a washing step, and successively, in a third step, by treatment with phosphate or oxalate bath. The steps of washing and passivation then follow. This multistage process requires a complicated plant, and involves high operating costs.

Unsuccessful attempts have been made to combine degreasing and phosphatation in a single stage, by adding an emulsion of organic solvents to the phosphate baths. However, the results have never been satisfactory because the phosphate layer formation is rather irregular, especially after a certain period of pollution of the bath. This disadvantage is due to the crystalline structure of the phosphate layer, which absorbs oils and fats with great facility. In consequence, the surface is more or less spotted after the treatment, due to the traces of oil or fat absorbed.

Reports are found in the technical literature of the theoretical possibility of combining the degreasing and oxalation in a single stage or phase. Nevertheless, the reports clearly specify that the results are inferior to those obtained by a cyclic process in which degreasing and oxalation are separate operations.

Surprisingly, the applicants have found that by adding to the oxalic acid baths a degreasing substance consisting of an aqueous emulsion of organic solvents, for example kerosene, stable in acid medium, perfect results are constantly obtained. This surprising result is due to the amorphous character of the layer of oxalate, which has a tendency to absorb Water and to reject oils and fats.

Even after advanced ageing of such baths, and even when a considerable layer of oil or fat removed from the plate floats at the surface, the oxalate layer that results is in perfect formation. The layers are uniform, and free of spots usually originating from absorption of oils or fats.

Another great advantage of the present invention is the obtaining of a surface which is especially suitable for anchorage of the after-applied coat of paint, by the removal of the layer of excess oxalate after treating with a diluted chromic acid solution.

In the first phase or stage, the material is degreased and coated with a layer of oxalate, then it is washed and subjected to the action of a diluted chromic acid solution whose concentration is between 0.05 and 5%, preferably between 0.5 and 2%.

The chromic acid solution removes a large part of the oxalate layer, leaving a metallic, dark grey colored, surface that is extraordinarily effective for anchorage of paint.

After treating with the chromic acid solution, the material is rinsed to remove the chromic acid excess.

3,116,177 Patented Dec. 31, 1963 The invention will be now illustrated by two examples, which are preferred embodiments, but are not intended to be limitative.

Example I A bath is prepared containing in 1 liter water:

Grams Oxalic acid 15 Sodium tripolyphosphate 1 Ferrous sulphate 1.5 Sodium nitrite 1.5

This mixture is sprayed upon a degreased iron plate, to produce oxalate layers.

By adding, to such bath, 15 grams per liter kerosene emulsion in water kerosene), oxalate layers are obtained of the same quality as above on sheet iron not previously degreased. The bath can be used until it has treated 4 m plate per liter of bath, with unvarying results. In this way, three operating phases are suflicient (degreasing-oxalation, +washing +passivation) to obtain a normal oxalate layer.

Example II Upon employing a bath identical with that described in Example I, but adding 15 grams per liter of an aqueous kerosene solution, amorphous oxalate layers of the normal type are obtained, in a single stage treatment. The materials so treated are rinsed in running water, subjected to the action of a chromic acid solution, for example for one minute and finally again quickly rinsed to remove the excess chromic acid. The surface so obtained is free of any apparent surface layer, is uniformly grey colored, and is especially suitable for the anchorage of the successively applied paint. The material treated and painted in such a manner may even be deformed, without any detachment of the paint occurring.

It is noted, that the conventional phosphatation and oxalation slightly impair the mechanical anchorage of the paint, so that, in tests on the painted material, better resistance to deformation is found with untreated and directly painted material. In contrast, in the case of material treated according to the herein claimed process, i.e. degreasing-oxalation +washing +treatment with chromic acid -|-summary washing, the deformation tests always show an advantage over untreated material. For example, in the embossing test carried out according to Erichson, such advantage ranges between 1 and 2 mm, according to painting cycle employed.

We claim:

1. A process of preparing a ferrous metal surface for painting by simultaneously degreasing and forming an amorphous oxalate layer on said metal surface by means of an oxalating bath consisting of oxalic acid in a concentration of about 15 grams per liter, sodium nitrite in a concentration of about 1.5 grams per liter, a ferrous compound having an Fe++ concentration of about 0.6 gram per liter and an acid stable aqueous emulsion of an organic solvent having a concentration of about 15 grams per liter, rinsing the amorphous oxalate layer so formed with water and then subjecting said surface to the action of an aqueous chromic acid solution having a chromic acid concentration between 0.5 and 50 grams per liter, and then quickly rinsing oif any excess chromic acid.

2. A process of preparing a ferrous metal surface for painting by simultaneously degreasing and forming an amorphous oxalate layer on said metal surface by means of an oxalating bath consisting of oxalic acid, sodium nitrite, a ferrous compound and an acid stable aqueous emulsion of a hydrocarbon oil, rinsing the amorphous oxalate layer so formed with water and then subjecting References Cited in the file of this patent UNITED STATES PATENTS 2,744,555 Nicholson et al May 8, 1956 4 Piccinelli July 23, 1957 Cerma Dec. 9, 1958 Heller et a1. June 6, 1961 FOREIGN PATENTS Great Britain of 1913 Germany May 9, 1956

Claims (1)

1. A PROCESS OF PREPARING A FEROUS METAL SURFACE FOR PAINTING BY SIMULTANEOUSLY DEGREASING AND FORMING AN AMORPHOUS OXALATE LAYER ON SAID METAL SURFACE BY MEANS OF AN OXALATING BATH CONSISTING OF OXALIC ACID IN A CONCENTRATION OF ABOUT 15 GRAMS PER LITER, SODIUM NITRITE IN A CONCENTRATION OF ABOUT 1.5 GRAMS PER LITER, A FERROUS COMPOUND HAVING A FE++ CONCENTRATION OF ABOUT 0.6 GRAM PER LITER AND AN ACID STABLE AQUEOUS EMULSION OF AN ORGANIC SOLVENT HAVING A CONCENTRATION OF ABOUT 15 GRAMS PER LITER, RINSING THE AMORPHOUS OXALATE LAYER SO FORMED WITH WATER AND THEN SUBJECTING SAID SURFACE TO THE ACTION OF AN AQUEOUS CHROMIC ACID SOLUTION HAVING A CHROMIC ACID CONCENTRATION BETWEEN 0.5 AND 50 GRAMS PER LITER, AND THEN QUICKLY RINSING OFF ANY EXCESS CHROMIC ACID.