GB2136455A

GB2136455A - Treatment of ferrous substrates

Info

Publication number: GB2136455A
Application number: GB08406784A
Authority: GB
Inventors: Donald P Murphy
Original assignee: Pyrene Chemical Services Ltd
Current assignee: Pyrene Chemical Services Ltd
Priority date: 1983-03-15
Filing date: 1984-03-15
Publication date: 1984-09-19
Also published as: EP0121155B1; DE3461653D1; ATE24206T1; JPS59177378A; GB2136455B; DE3408787A1; GB8406784D0; EP0121155A1

Abstract

A process for treating ferrous substrates for receiving a high-solids organic solvent-thinned coating by which inadvertent drippage or splashing of the dry phosphated surfaces with the iron phosphating solution does not produce blemishes, discolourations and/or surface imperfections in the final painted surface, comprises employing an aqueous iron phosphating solution devoid of any conventional inorganic oxidants or accelerating agents such as chloride or chlorate and employs an organic oxidizing agent, preferably meta nitrobenzene sulphonic acid or a salt thereof.

Description

1 GB 2 136 455 A 1

SPECIFICATION Treatment of ferrous substrates

This invention relates to processes by which improved surface finishes can be achieved when a high solids paint is applied over a ferrous substrata 70 carrying an iron phosphate coating.

It is well known to form an iron phosphate coating on a ferrous substrate in order to enhance corrosion resistance and adhesion of subsequently applied organic paints or other coatings. Such phosphate coatings are obtained by contacting the substrate with an alkali metal dihydrogen phosphate solution. In order to improve the quality and crystalline structure of the coating and to increase the rate of coating it is common to incorporate into the solution various inorganic oxidising agents such as nitrites, chlorates, bromates, molybdates and fluorides.

Although the use of such solutions has led to very satisfactory results when the paint is a conventional solvent thinned organic coating paint, so-called high-solids paints have been introduced commercially in recent years and their use over such iron phosphate coatings has been found generally to incur various problems such as 90 localized surface discolourations and/or roughness and/or loss of gloss of the organic finish.

The adoption and use of such high-solids paints has been caused by more stringent air regulations imposed by the government bodies to reduce the 95 amount of organic solvent emitted into the atmosphere. Conventional solvent-thinned paint formulations usually contain less than about 45% by volume solids whereas the high-solids (or low solvent) solvent-thinned coating formulations contain more than about 50% by volume solids, and more usually 60% or more by volume solids.

In order to provide a sprayable viscosity at such lower solvent contents, such high-solids paint formulations contain polymers which are of a substantially lower molecular weight in comparison to the molecular weight of the polymers employed in conventional solvent thinned paint systems. As a result, high-solids paints are far more sensitive to surface imperfections in the substrates to which they are applied because they thin during solvent evaporation and tend to crater around any imperfections and to flow off any high-points on the surface, magnifying any such imperfections. 115 Splashing or dripping of the iron phosphate solution on dried phosphated part surfaces such as from the racks or conveyor systems on which the parts are transported through the processing cycle results in the formation of localized white 120 salt deposits on the phosphate surface. The salt deposits have a tendency to bleed through the subsequent high-solids paint coatings to form white dull-centred rings. Such defective articles may need to be reworked but during reworking 125 any inadvertent drippage or splashing again is liable to produce white salt deposits which bleed through and are visible on the painted part when high-solids paint formulations are employed.

We have now discovered that these problems associated with high-solids paints can be minimized or avoided if the multi-stage coating operation is conducted in a particular manner and, in particular, if a particular form of phosphate coating solution is used.

In the invention an iron phosphate coating is formed on a ferrous substrate by applying an aqueous iron phosphating solution that is accelerated by a water-soluble organic nitro compound and which is substantially free of chloride or chlorate, the coating is rinsed and dried and a high-solids paint is then applied to the coated substrate and cured.

We have surprisingly found that this coating system, and in particular the avoidance of chloride or chlorate from the phosphating solution, can lead to satisfactory coating rates and to the production of improved coatings. In particular drippage or splashing of the phosphate surfaces has less tendency to produce blemishes, discolourations and other surface imperfections in the final paint coating.

The ferrous substrate is normally of steel and should be cleaned before contact with the phosphating solution. Cleaning to remove surface contamination and soils can conveniently be achieved by any one of a variety of well known alkaline cleaning solutions. The cleaned part may further be subjected to acid pickling to remove any surface rust if necessary. Following the cleaning treatment, the part is usually subjected to a water rinse followed by an iron phosphating treatment to provide a uniform phosphate coating thereover. Following the phosphating treatment, the parts are usually water rinsed and thereafter, preferably, are subjected to a dilute aqueous sealing treatment employing a dilute aqueous acidic chromium solution. Following the sealing rinse, the phosphated parts are dried and are thereafter subjected to a coating which a high-solids organic paint formulation which may include a prime coating followed by one or a plurality of top coats or a top coat may be directly applied to the phosphated surface. The organic coatings are usually subjected to an elevated temperature to effect a baking and curing thereof after which the parts are inspected prior to shipment. If there are surface imperfections, the parts are sanded or polished to remove the surface imperfections and then completely reworked.

The iron phosphating solution contains an effective amount of dihydrogen phosphates of an alkali metal or ammonium to provide a desired iron phosphate coating on the ferrous substrata. Typically, such dihydrogen phosphates comprise sodium dihydrogen phosphate or sodium acid pyrophosphate which may be present in an amount of about 3 to about 60 g/1, with concentrations more usually ranging from about 4 to about 10 g/1. Some free phosphoric acid can be added in accordance with conventional practice to provide appropriate pH adjustment and sodium hydroxide or sodium carbonate can also be added for this same purpose and for neutralization of all 2 GB 2 136 455 A 2 or a portion of the free phosphoric acid present. The aqueous phosphating solution can operate at a pH of about 4.2 up to about 6 with a pH of about 4.5 to about 5.5 being preferred and a pH of about 5 5 being typical.

The phosphating solution must be accelerated and in the invention the accelerator that is used is a water-soluble organic nitro compound, or a mixture thereof. Various organic nitro compounds have been proposed for use as accelerators in phosphating solutions and can be used in the invention. However best results are obtained using meta nitrobenzene sulphonic acid and/or a salt thereof that is soluble in the phosphating solution and that is compatible with the other components of the solution, in particular the sodium salt. The amount of meta nitrobenzene sulphonic acid or salt that is present in the solution is generally from 0.1 to 3 gA, preferably 0.2 to 1.5 g/1.

Suitable iron phosphating solutions are 85 solutions similar to those described in U.S. Patent Specification 2,609,308 except that the inorganic chlorate oxidant described therein is replaced by, for instance, meta nitrobenzene sulphonic acid or a salt thereof to provide an equivalent acceleration of the bath as provided when employing about 0.5 to about 4% of inorganic chlorate ions as described in the aforementioned Patent. In general, the phosphating solution should be substantially free of ail inorganic accelerators.

The aqeuous iron phosphating solution can be applied to the cleaned ferrous substrate such as by spraying, immersion or flooding for a period of time ranging from about 5 seconds to several minutes or longer depending upon the desired thickness and characteristics of the resultant phosphate coating produced. The phosphating solution is applied at a temperature of from about 21 'C up to a temperature below that at which any adverse decomposition of the organic meta nitrobenzene sulphonic acid and/or salt is obtained with temperatures of from about 21 OC up to about 501C being preferred and temperatures of about 381C to about 431C being typical. Under such conditions, an adherent phosphate coating is formed on the ferrous substrate providing a coating weight of as low as about 100 mg/ml up to about 1000 Mg/M2 or even higher depending upon the specific processing conditions.

The aqueous iron phosphating solution can also contain surfactants and/or emulsifying agents of the various types conventionally employed in metal cleaning operations which are compatible with the phosphating and oxidizing agent to provide a cleaning function concurrently with the phosphating step.

Following the phosphating step, the phosphated surface is normally water rinsed which may be further followed by a deionized water rinse and preferably by a chromium sealing treatment employing hexavalent and/or trivalent 125 chromium ions in an aqueous solution of the general types as described in U.S. Patent No.

3,222,226 and No. 3,279,958.

Following the chromium rinse treatment, the phosphated substrate is usually again water rinsed and is thereafter dried. Following the drying step, the phosphated surfaces are coated with a highsolids organic paint in the form of a top coat 7C applied directly to the phosphated substrate or in the form of a high- solids primer coating followed by one or more high-solids top coats. The coated substrate is subjected to a high temperature curing step following each coating operation to effect a curing of the coating prior to the application of a succeeding coating.

In this specification, a high-solids paint means a solvent-based coating system- containing at least 50% by volume solids, and preferably at least about 60% solids by volume. Preferably the resinous binder constituents of such high-solids paints are of substantially lower molecular weight than those contained in conventional solventbased coating systems which contain less than about 50% solids by volume, and more usually about 45% or less solids by volume. By way of further background information regarding highsolids coating technology, reference is made to the following technical publications: "High-Solids -00 Coatings: Past, Present, Futureby Robert N. Price, American Paint & Coatings Journal, June 21, 1982; "High-Solids Coatings- by John C. Dean, Modern Paint and Coatings, March, 1982; '7rends in Water-Borne, High- Solids Coatings" by Donald E. Brody American Paint & Coatings Journal, July 12, 1982; and - High-Solids Acrylic Resins for Baking Enamels", by Dr. R. R. Kuhn, et al, Modern Paint and Coatings, April, 1981.

The following are some Examples of the invention.

EXAMPLE 1

In order to simulate conditions prevalent in many production facilities employing inorganic accelerated iron phosphate processing solutions in which parts are splashed or dripped upon with such solutions following the formation of the phosphate coating and/or painted parts, two 4inch by 6-inch steel test panels were employed which were cleaned and thereafter subjected to a conventional iron phosphate treatment. The operating bath as prepared contained about 12 g/1 of 75% phosphoric acid, about 4.7 g/1 of sodium carbonate for neutralizing the phosphoric acid forming the acid phosphate, and about 5.2 g/] of sodium chlorate as the oxidant. The solution was spray applied to the panels at a pH of about 5 and at a temperature of about 700C for a period of about 1 minute and were tap water rinsed. The panels were air-blown dry and thereafter a drop of the same operating solution was applied to the dry iron phosphate surface. The panels were placed flat in a recirculating air oven and baked for 10 minutes at 500C.

A high-solids white solvent-thinned top coat paint was applied to one of the panels comprising a coating formulation sold under the designation Duracron 100 (Trade Mark) available from PPG, Inc. The second panel was spray painted also employing a high-solids organic solvent-thinned t 3 GB 2 136 455 A 3 top coat of a black colour commercially available under the designation 4653 from SherwinWilliams. Both panels after spray painting were placed in a recirculating air oven at 163 'C fora period of 15 minutes to effect a curing of the organic finish.

In both instances, the test panels evidenced the presence of a white ring visible through the paint having a dull appearance in the centre of the ring.

EXAMPLE 2

Employing the same procedure as described in 50 Example 1, two steel test panels were phosphated but this, time an iron phosphating solution was employed devoid of any inorganic oxidants and employing an organic oxidant or accelerator. The operating iron phosphating solution contained about 12.5 g/1 of 75% phosphoric acid, about 4.9 g/1 of sodium carbonate and about 1.8 gi of meta nitrobenzene sulphonic acid. The operating bath was spray applied to the test panels under the same conditions as described in Example 1 with the exception that the temperature of the solution was at 500C.

After the phosphated panels had been dried, a drop of this phosphating solution was applied in accordance with the procedure of Example 1 and thereafter baked, followed by the application of the two high-solids top coat paints under the same conditions as previously described in Example 1. A visual inspection of the test panels indicated no discernible ring or discolouration of the paint surface.

EXAMPLE 3

A test similar to that described in Example 2 was conducted but employing an alternative iron phosphating solution devoid of any inorganic oxidants and further containing about.7 g/1 of a surfactant. In addition to the surfactant, the operating solution contained about 3.6 9/1 of 75% phosphoric acid, about 3.6 g/1 of sodium dihydrogen phosphate and about.3 g/1 of metal nitrobenzene sulphonic acid.

The same procedure and conditions as described in Example 2 were repeated and the final painted test panel surfaces were visually inspected and found to be substantially free of any surface discolourations visible to the naked dye.

Claims

1. A process in which an iron phosphate coating is formed on a ferrous substrate by applying to the substrate an aqueous iron phosphating solution that is accelerated by a water-soluble organic nitro compound and that is substantially free of chloride or chlorate, the coating is rinsed and dried, and a high-solids paint is then applied to the coated substrate and cured.

2. A process according to claim 1, in which the iron phosphating solution is accelerated by meta nitrobenzene sulphonic acid or a soluble salt thereof.

3. A process according to claim 1, in which the phosphating solution is accelerated by sodium nitrobenzene sulphonic acid in an amount of 0.1 to 3 g/L

4. A process according to any preceding claim, in which the iron phosphating solution has a pH of from 4.2 to 6.

5. A process according to any preceding claim, in which the iron phosphating solution contains from 3 to 60 g/1 dihydrogen phosphates.

6. A process according to any preceding claim, in which the iron phosphating solution has a pH of 4.5 to 5.5, contains 4 to 10 9/1 dihydrogen phosphates and 0.1 to 1.5 g/1 sodium meta nitrobenzene sulphonate.

7. A process according to any preceding claim, in which an aqueous chromium containing rinse solution is applied to the rinsed iron phosphate coating prior to the step of drying the coating.

Printed in the United Kingdom for Her Majesty's Stationery Office, Demand No. 8818935, 911984. Contractor's Code No. 6378. Published by the Patent Office, 25 Southampton Buildings, London, WC2A lAY, from which copies may be obtained.