US3578510A - Conditioning ferrous metal substrates - Google Patents

Abstract

TO PREPARE A FERROUS METAL SURFACE FOR RECEIVING A HEXAVALENT-CHROMIUM-CONTAINING COATING COMPOSITION THERE IS FIRST ESTABLISHED ON THE SURFACE A PARTIALLY AUTOGENOUS, LOOSE DRY POWDER. THIS POWDER IS FORMED BY CONTACTING THE SURFACE WITH AN AQUEOUS SOLUTION HAVING A PH BELOW ABOUT 2.6 AND CONTAINING UP TO ABOUT 30 WEIGHT PERCENT OF AN ACID OF PHOSPHOROUS AND USUALLY 0.25-10 VOLUME PERCENT OF SURFACE ACTIVE AGENT, AND THEN DRYING THE RESULTING CONTACTED SURFACE. THE SOLUTION IS TYPICALLY MORE ACID THAN A CONVERSION COATING SOLUTION AND WEAKER IN ACID STRENGTH THAN A PICKLING BATH. RESULTING ARTICLES EXHIBIT WELDABILITY AND ENHANCED PAINT ADHESION.

Description

United States Patent 3,578,510 CONDITIONING FERROUS METAL SUBSTRATES Bert E. Palm, Mentor, and William Wayne Warner, Painesville, Ohio, assignors to Diamond Shamrock Corporation, Cleveland, Ohio No Drawing. Filed May 17, 1968, Ser. No. 729,931 Int. Cl. C23f 7/10, 7/26 U.S. 'Cl. 148-6.].6 Claims ABSTRACT OF THE DISCLOSURE To prepare a ferrous metal surface for receiving a hexavalent-chromium-containing coating composition there is first established on the surface a partially autogenous, loose dry powder. This powder is formed by contacting the surface with an aqueous solution having a pH below about 2.6 and containing up to about 30 weight percent of an acid of phosphorous and usually 0.25-10 volume percent of surface active agent, and then drying the resulting contacted surface. The solution is typically more acid than a conversion coating solution and weaker in acid strength than a pickling bath. Resulting articles exhibit weldability and enhanced paint adhesion.

BACKGROUND OF THE INVENTION Phosphate coatings and hexavalent-chromium-containing coatings are two types of conditioning treatments for metal surfaces which provide corrosion protection and may offer augmented paint adhesion. Generally, the phosphate coatings are tightly adhering to the metal substrate and are porous coatings which provide a good base for paint. Usually, such phosphate coatings fail to provide enhanced corrosion protection and therefore often receive a very dilute rinse, for example a chromic acid rinse, to increase the corrosion resistance of the finished product. These chrome rinses typically contain less than five grams per liter of hexavalent chromium, expressed as CrO and preferably less than 1 gram per liter of CrO Such surface protection techniques have been shown, for example, in British Pat. 1,054,356.

On the other hand, hexavalentchromium-containing coatings for metal substrates are applied to metal surfaces from liquid compositions containing hexavalent chromium often supplied by chromic acid, to typically impart some corrosion resistance to the metal surface. Such coatings have been shown, for example, in British Pat. 1,033,399 and in U.S. Pats. 2,768,104; 2,777,785; 2,846,342; 2,901,385; 2,902,390; 3,063,877; 3,346,522; and 3,382,081. The coating compositions may contain some trivalent chromium, or the coatings can form trivalent chromium compounds during application and/or curing. Coatings thus prepared tend to be non-porous and somewhat amorphous in nature. They often provide good corrosion protection, but paint adhesion to such coatings is usually inferior to the like adhesion exhibited by the phosphate coatings. Moreover, if hexavalent-chromiumcontaining coatings are applied over conventional phosphate undercoatings, the paint adhesion properties of the phosphate coating is usually downgraded to that of the hexavalent-chromium-containing coating.

The process of the present invention overcomes such problems by first contacting the metallic ferrous substrate with an aqueous phosphate coating solution. However, the coating solution is unconventional, partly since it is maintained at a pH below about 2.6. Most importantly, the applied coating solution is permitted to dry and thereby form a deposit of loose, poorly adhering powder, which has heretofore been avoided in the practice of coating metal substrates. This deposit from the phosphate solution is totally unsatisfactory as a basis for direct painting and offers no improvement in adhesion for directly applied paint or in corrosion resistance over conventional phosphate coatings.

However, it has been found that such a conditioning provides a superior surface treatment for subsequently applied heXavalent-chromium-containing coatings. Although not meaning to be bound to any particular theory it appears that the conditioning prepares a ferrous-ferric phosphate substance on the substrate surface and, for convenience, such surfaces are sometimes referred to herein as a tertiary-iron-phosphated surface. When the surface is permitted to dry, a loose powder is apparent at the surface, which powder can be readily removed by manual rubbing of the surface with a cloth. This loose powder, ostensibly, is at least partially autogenous in its formation, i.e., is formed by an interaction of the phosphate solution with the ferrous substrate.

Thereafter, as an initial surface treatment, the loose powder appears to alter the manner in which the hexavalent-chromium-containing coating deposits, and the powder orients or induces the coating toward a crystalline film. Without the tertiary-iron-phosphated surface, the coating tends to be more amorphous, i.e., has retarded crystallinity. However, as a result of the surface treatment of this invention, and after curing of the subsequently applied coating, the coated surface provides a base for paint adhesion which is enhanced over conventional phosphate coatings. Moreover, the coated surface permits weldability of the ferrous substrate and additionally provides enhanced corrosion resistance superior to that obtained from conventional phosphate coatings with or without chromic acid rinses.

Heretofore, phosphate solutions of low pH have been applied to metal substrates prior to applicationpf hexavalent-chromium-containing coating compositions for enhancing adhesion of later applied paints. For example, such procedure has been taught in U.S. Pat. 3,370,992. However, such low pH solutions contain oxidizing agents and are employed merely for etching the metal substrate, thus the substrate is maintained in a moist condition for application of the hexavalent-chromium-containing coating composition. Also, after the etch, the surface is typi cally rinsed before application of the bonding coat. However, with or without the rinse, this treatment of the metal surface deleteriously affects the weldability of the final coated substrate, hence such substrates are not deemed suitable for subsequent welding operation.

It has also been possible heretofore to approach the problem of protecting metal surfaces by first using merely a conventional phosphate pretreatment followed then by an unusually heavy chromic acid rinse, as for example the practice taught in U.S. Pat. No. 2,882,189. Although lacquer adhesion to such coatings is apparently acceptable, and thus these coatings have ostensibly found acceptance in the food container art, such coatings are achieved by sacrificing weldability and are thus distinguished from the coatings of the present invention, for example, not only by the conventional phosphate pretreatment, but also by their performance.

SUMMARY OF THE INVENTION Broadly, the present invention relates to a method of treating a ferrous metal substrate for coating with a hexavalent-chromium-containing coating composition, wherein the resulting coated article exhibits weldability and enhanced paint adhesion, which method comprises establishing on the surface of such substrate a partially autogenous, dry powdery residue in an amount of at least about 5 milligrams per square foot of the surface, by contacting such surface with an aqueous solution maintained at a pH level of below about 2.6, which solution contains up to about 30 percent by weight of at least one acid of phosphorous, and drying the resulting contacted surface.

The invention is further directed to the method for producing a corrosion-resistant coating on a ferrous metal substrate by applying, to the resulting contacted surface prepared by the method described immediately hereinabove, a hexavalent-chromium-containing coating composition and curing such composition.

This invention is further directed to the products prepared by the methods described above.

It is to be understood that by the use of the term one acid of phosphorous, such acid can be phosphoric acid, but can also be one or more of metaphosphoric, orthophosphoric, and pyrophosphoric acids, as well as polyphosphoric acids such as tripolyphosphoric acid. Additionally, such acid can also be one or more of hypophosphorous, pyrophosphorous, and phosphorous, some of which can be readily converted to phosphoric acid by the addition of the acid to water, usually accompanied by heating. Also, the surface for conditioning is a ferrous metal substrate which is meant to include, for example, iron, stainless steel, and steel such as cold rolled steel.

After the ferrous substrate is contacted with the acidic contacting solution, also called herein the aqueous solution, and the surface is dried, a loose powder is established on the ferrous metal substrate. For convenience, this powder is also referred to herein as a powdery residue. Also for convenience, the condition of this surface containing the powdery residue is referred to herein as a first treated ferrous metal surface, which is to be distinguished from the eventual coated metal substrate which contains the hexavalent-chromiumcontaining coating composition. This latter composition is often referred to herein simply as the coating composition.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The aqueous solution can contain suspended substituents, but in its simplest form is merely prepared as a solution of the acid and additional ingredients in water. The acid, e.g., phosphoric, is provided in sufiicient amounts to establish the pH level of the solution below about 2.6 and thereby enhance the formation of the loose, powdery deposit. Advantageously, for efiicient powder formation, sufficient acid of phosphorous is present to maintain the pH level of the solution between about 1.5 and about 2.5. Typically, this acid content can be up to about 30 percent by weight of the solution but is advantageously, for economy, maintained at less than about weight percent, e.g., 0.25 weight percent. Preferably for economy the acid is simply phosphoric acid and is present in an amount between about 0.5-15 weight percent.

In addition to the acid of phosphorous, this solution often contains from about 0.25 percent up to about percent by volume of a surface active agent. Such an agent augments the cleansing of the surface as the aqueous solution is contacting the ferrous substrate and further assists in achieving a uniform coating on the surface. Usually, above about one-quarter percent by volume of such agent is required to provide desirable uniform contact of the solution with the substrate. However, above about 10 percent by volume of such agent is uneconomical. Advantageously, for best economy and efiicient cleaning, the agent is present in the aqueous solution in an amount of between about 3-7 percent by volume.

Suitable surface active agents are hydroXyl-containing hydrocarbon others which include the alkyl ethers of alkylene glycols, such as butyl ether of propylene glycol, the oxyalkyl ethers of alkylene glycols, e.g., l-butoXyethoXy- Z-propanol, fatty alcohol polyoxyalkylethers, alkylphenol polyoxyalkylethers such as polyoxyethylated nonylphenols, and polyalkylene glycols, e.g., tetraethylene glycol. Other suitable surface active agents which may be used include products from pine wood distillation, e.g., pine oil,

as well as products prepared from waste sulfite liquors such as lignin sulfonic acids.

Enhanced salt spray resistance for final coated articles, that is, those whereupon the loose powdery residue is first formed, the coating composition is subsequently ap plied and cured, and additional paint topcoating is provided, can often be achieved by supplying the aqueous solution with up to about 10 grams per liter of metallic ions. Above about 10 grams per liter of such ions often does not provide for suflicient enhancement of the salt spray resistance to make further ion concentration desirable. Typically, for augmented salt spray resistance as well as economy, the aqueous solution contains between about 4-6 grams per liter of such ions.

Metallic ions can or have been used for enhancing salt spray resistance include calcium, ferrous, cobaltous, manganous, nickelous ions and their mixtures. However, caution should be taken when introducing such ions into the dispersion to avoid the commensurate introduction of chloride and sulfate ions, since these ions can act to retard salt spray resistance.

Generally, the ferrous substrate is contacted with the aqueous solution, for example by dipping such substrate into the aqueous solution, for a period of between about 2-30 seconds. Contact times of less than about two seconds can provide treated substrates which are not uniformly wetted thus leading to a non-uniform powdery deposit. Contact times between the ferrous substrate and the aqueous solution of greater than about 30 seconds are usually not economical. Preferably, for best economy as well as uniform wetting, the substrate is contacted with the aqueous solution for a period of between about 2l0 seconds. This preferred contact time will typically provide for a subsequent amount of powder of between about 5-50 milligrams per square foot on the ferrous substrate, which is a desirable amount of powder as is more fully described hereinbelow.

After contacting of the ferrous substrate with the aqueous solution, the formation of the loose, powdery deposit can be suitably obtained by simply air drying the substrate. However, advantageously, for efiiciency, the drying time is reduced by employing some forced drying technique such as infrared drying, oven drying, or forced air drying, some of which can achieve substrate temperatures of up to 400 F. Usually, when ostensible bubbling at the surface accompanies forced drying, because of rapid evaporation of volatile materials contained in the solution, such substrate temperature is maintained below about 200 F. until forced drying can be accomplished without such surface turbulence. The presence of a loose, powdery surface deposit may become readily apparent, even with air drying at normal temperatures of about 65-75 F., within about 30 seconds, or slightly longer. With forced drying techniques, the surface can become ostensibly dry within a period of less than about 5 seconds.

Sufiicient acid of phosphorous is present in the aqueous solution and/or ,sufiicient contact time between solution and substrate is permitted, with longer contact times usually being employed for weaker solutions, to provide the dry surface with at least about 5 milligrams per square foot of the powdery residue, but generally with not above about milligrams per square foot of such residue. Less than about 5 milligrams per square foot can be insufficient to enhance the adhesion of paint topcoats, while above about 100 milligrams per square foot of the powder is usually uneconomical. Advantageously, for augmented paint adhesion, excellent corrosion resistance, and best economy, a loose powder of between about 5-50 milligrams per square foot is achieved on the ferrous substrate. Such advantageous amounts are readily obtainable from solutions containing the preferred amount of about 05-15 weight percent of the acid of phosphorous and contacting the ferrous substrate for the preferred time of about 2-10 seconds.

Where high temperatures have been employed in drying the contacted ferrous substrate, such substrate can be advantageously cooled to a temperature below about 200 F. prior to application of the bonding coating, and thereby generally avoid excessively rapid evaporation of the volatile components of the coating composition which can result in a discontinuous, applied film.

The corrosion-resistant, hexavalent-chromium-containing coating compositions often contain chromic acid as the hexavalent-chromium-providing substance. But such chromium can be supplied by a salt such as ammonium dichromate, e.g., as taught in U.S. Pat. 2,846,342, or by sodium or potassium salts as shown in U.S. Pat. 2,559,812, or by substances such as calcium, barium, magnesium, zinc, cadmium, and strontium dichromate as hown for example in U.S. Pat. 2,901,385 and/or British Pat. 1,033,399. Additionally, the hexavalent-chromium-providing substance might be a mixed chromium compound, i.e., include trivalent chromium compounds as shown in U.S. Pat. 3,185,596. Although the coating compositions might contain as little as about 0.25 weight percent of hexavalent chromium, expressed as CrO and may contain as much as about 400 grams per liter of composition of hexavalent chromium, expressed as CrO such compositions typically contain from several weight percent up to about 30 weight percent of hexavalent chromium, expressed as CrO In addition to a hexavalent-chromium-providing substance, these coating compositions contain a reducing compound which is typically a polyalcohol or orgamc acid and many of these useful reducing compounds have been shown for example in U.S. Pat. 2,559,812; 2,901,385; and 2,777,785. The reducing agent or component is usually present as a single compound which is often organic but can be an inorganic substance, such as potassium iodide, or a hypophosphite reducing agent as shown in U.S. Pat. 2,846,342. Organic reducing agents may be very low molecular weight agents such as formaldehyde, disclosed in U.S. Pat. 3,063,877 or such high molecular weight materials as polyacrylic acid compounds as taught in U.S. Pat. 3,185,596. The reducing agent can be the solution media of the coating composition, as shown in U.S. Pat. 2,927,046 and such reducing components may be preformed and stored prior to use, as taught for example in U.S. Pat. 3,346,522. The reducing agents may not be added to the coating composition directly, but rather supplied to a metal surface already containing an applied chromic acid solution, i.e., the agents are applied during drying of the chromic acid solution, on the metal surface as shown in U.S. Pat. 2,768,103. Combinations of reducing agents are disclosed, for example, in U.S. Pat. 3,382,081.

Additional substances which may be included in such coating compositions include phosphoric acid or a compound of phosphorous. These phosphate-containing coating compositions may, conveniently, be referred to as chromate-phosphate coating compositions. Other useful compounds often found in hexavalent-chromium-containing coating compositions are manganese compounds which are useful for extending the useful life of the coating bath, as disclosed in U.S. Pat. 2,777,785, or pigmentary substances as disclosed in British Pat. 1,033,399, as Well as resinous materials which have been shown in U.S. Pat. 3,346,522. Additional useful coating composition components may include organic and inorganic acids to maintain composition acidity as taught in British Pat. 972,072 as well as surface active agents.

Substantially all of the hexavalent-chromium-containing coating compositions are water based but other liquid materials are used, and typically these are alcohols, e.g., tertiary butyl alcohol, and this particular alcohol has been used in conjunction with high boiling hydrocarbon solvents to prepare the liquid medium for the coating composition, as taught in U.S. Pat. 2,927,046. Often the coating compositions are solutions but may be dispersions 6 which can contain dispersed pigments, or contain a water dispersible reducing agent such as water dispersible polyacrylic acid compounds.

These coating compositions are usually applied to a metal surface either by dipping the article into the coating composition or by spraying the composition onto the metal surface, which surface can be a preheated metal surface to assist in the curing of the coating, as taught in U.S. Pat. 2,846,342. However, the coating composition may be used as an electrolytic bath to coat a metal surface employed as a cathode in the bath, as shown in British Pat. 972,072. Although the first treated surface may exhibit a loose, powdery residue if dried, it may nevertheless be carefully placed into such a bath without deleterious removal of such residue. Moreover, such residue will not be significantly dissolved by such a bath even within greatly extended coating times of ten to fifteen minutes. The coating composition may be applied to the metal surface after an etch, e.g., a nitric acid etch, as taught in U.S. Pat. 2,768,103, or the reducing agent may be applied after the application of the hexavalent-chromium-containing solution and during drying of such solution on the metal surface, as mentioned hereinabove. The coating composition may be applied from a heated bath, for example one heated up to 200 F. as taught in U.S. Pat. 2,768,104. Moreover, after application and curing of the composition the heated metal may be desirably quenched in a solution of chromic acid in a solution of chromic acid in water as taught in U.S. Pat. 2,777,785.

After application of these coating compositions to a metal substrate, the preferred temperature range for the subsequent heating, which is also often referred to as curing and which may be preceded by drying such as air drying, is from about 200 F., as taught for example in U.S. Pat. 3,185,596, but more typically from about 212 F. at a pressure of 760 mm. Hg, up to about 300 C., i.e., about 572 'F., e.g. as taught in British Pat. 972,072. Such an elevated substrate temperature may be attained by preheating the metal prior to application of the coating composition as shown in U.S. Pat. 2,846,342. However, such curing temperatures do not often exceed a temperature within the range of about 450-550 F. to avoid charring or other adverse coating effects as taught in U.S. Pat. 2,777,785. At the elevated curing temperatures the heating can be carried out in as rapidly as about 2 seconds or less but is generally conducted for several minutes at a reduced temperature to provide the most corrosion-resistant and adherent coatings. Resulting coating weights may be as low as about 1.5 to 3 milligrams per square foot, or be as heavy as about 200 milligrams per square foot, but are typically within the range from about 5 to about milligrams per square foot.

Before starting the treatment of the present invention it is, in most cases, advisable to remove foreign matter from the metal surfaces by thoroughly cleaning and de-greasing. De-greasing may be accomplished with known agent for instance, with agents containing sodium metasilicate, caustic soda, carbon tetrachloride, trichlorethylene, and the like.

After baking the coating composition on the ferrous substrate it can be topcoated with any suitable paint i.e., a paint, primer, including electrocoating primers, enamel, varnish, or lacquer. Such paints can contain pigment in a binder or can be unpigmented, e.g., generally cellulose lacquers, rosin varnishes, and oleoresinous varnishes, as for example tung oil varnish. The paints can be solvent reduced or they can be water reduced, e.g., latex or watersoluble resins, including modified or soluble alkyds, or the paints can have reactive solvents such as in the poly esters or polyurethanes. Additional suitable paints which can be used include oil paints, including phenolic resin paints, solvent-reduced alkyds, epoxys, acrylics, vinyl, including polyvinyl butyral and oil-wax-type coatings such as linseed oil-paraffin wax paints. The paints can be applied as mill finishes.

7 The following examples show ways in which the invention has been practiced but should not be construed as limiting the invention. In the examples all temperatures are in degrees Fahrenheit unless otherwise specifically stated. In the examples the following procedures have been employed.

PREPARATION OF TEST PANELS Bare steel test panels (typically 4" x 12, and all being cold rolled, low carbon steel panels) are typically prepared for subsequent treatment by immersing in water which has incorporated therein 24 ounces of cleaning solution per gallon of water. The cleaning solution usually is 75 percent by weight of potassium hydroxide and 25 percent by weight of tripotassium phosphate. The water containing the cleaning solution is maintained at a temperature of about l60-l80 F. After this cleaning treatment the panels are rinsed with warm water, usually while scrubbing with a soft bristle brush.

MANDREL TEST BENDING (ASTM-D 522) The conical mandrel test, abbreviated hereinafter for convenience to the CM. test, is carried out by the procedure of ASTM test D-522. Briefly, the testing method consists in deforming a paint-coated metal panel by fastening the panel tangentially to the surface of a conical steel mandrel and forcing the sheet to conform to the shape of the mandrel by means of a roller bearing, rotatable about the long axis of the cone and disposed at the angle of the conical surface, the angle of deformation or are travel of the roller bearing being approximately 180. Following the deformation, at strip of glass fiber tape coated with a pressure-sensitive adhesive is pressed against the painted surface on the deformed portion of the test panel and is then quickly removed. The coating is evaluated quantitatively according to the amount of paint removed by the adhesive on the tape, in comparison with the condition of a standard test panel.

REVERSE IMPACT STRENGTH In the reverse impact test, a metal ram of specified weight, in pounds, with a hemisperical contact surface is allowed to drop from a predetermined height in inches onto the test panel. Panit removal is measured qualitatively or quantitatively on the convex (reverse) surface. In the qualitative measurement the impacted surface is merely observed by visual inspection and comparative panels, i.e., those subjected to the same impact in inch-pounds, are rated according to a numerical scale presented herein below. Quantitative measurements are expressed in inchpounds and the figure presented is the maximum amount, from at least two determinations, withstood by the coating without any removal to bare metal.

CRO SS-HATCH This test, otherwise abbreviated herein as the Xi-I. test, is conducted by scribing, through the coating to the metal panel with a sharp knife, a first set of parallel lines one-eighth inch apart. A second, similar set of lines, is then scribed on the panel at right angles to'the first set. Following this, a strip of glass fiber tape coated with a pressure-sensitive adhesive is pressed against the painted surface on the scribed portion of the test panel and is then quickly removed. The coating is evaluated qualitatively according to the amount of paint removed by the adhesive on the tape, in comparison with the condition of a standard test panel.

COIN ADHESION A freshly minted, i.e., uncirculated, nickel coin is firmly secured in vise-grip pliers; the pliers are manually held in a position such that a portion of the rim of the nickel coin contacts the coated substrate at about a angle. The nickel coin is then drawn down across the panel for about two inches. The type of coating flaking and/or chipping is evaluated qualitatively by visual ob; servance, and panels are compared with the condition of a standard test panel.

PAINT FILMS The paint films (topcoats) referred to in the examples are either a commercial white alkyd enamel topcoat or a commerical water-based red oxide electrocoat primer which are applied to the panels by dip-coating, for the enamel, and by electrocoating. The alkyd paint is prepared from a modified alkyd resin based upon a system of partially polymerized phthalic acid and glycerine. The paint contains 50 weight percent solids and has a viscosity of 50 seconds as measured on a No. 4 Ford cup at 70 -F. The electrocoating primer is a water emulsion, pigmented with red oxide of iron, and has a resin system derived from polycarboxylic acids plus a water miscible glycol ether coupling agent. The amine employed in the emulsion is diethylamine. The primer initially contains about 40 weight percent solids but is diluted before use at a volume ratio of primer to water of about 1:33.5.

In some of the following tables, the eflicien-cy of the coating systems are qualitatively evaluated on a numerical scale from 1-5 as follows:

(1 Film integrity very poor.

(2) Retention of film integrity less than that of control.

(3) Film integrity of control.

(4) Degree of retention of film integrity above that of control.

(5) Film integrity exceptionally good for the test used.

Where the observed condition of a test panel is slightly better than one of the numerical values but nevertheless poorer than the next higher value would indicate, a plus sign is used to so indicate the condition. Similarly a minus sign indicates a condition slightly poorer than one of the numerical value, but better than the next lower value.

EXAMPLE 1 A cold rolled, low carbon steel coil 4 inches in width is employed for preparing phosphate coated panels. A portion of the coil is run at 12 feet per minute through a phosphoric acid bath maintained at 168 F. and containing 21 points of free phosphoric acid per liter of the bath. Acidity expressed in points is the number of milliliters of 0.1 N base required to titrate a 10-ml. sample of the bath to the phenolphthalein end point. The portion of the panel after treatment in the bath is permitted to air dry at room temperature to obtain a loose, powdery residue on the steel substrate. In Table I below, panels containing this phosphate treatment are designated as Phos-l. These panels contain an average coating Weight of the loose powdery phosphate coating of about 37-38 milligrams per square foot.

Another portion of the coil is fed at 12 feet per minute through a phosphoric acid bath maintained at F. and containing 15 ml. of free acid per liter of the bath. This portion of the coil is also allowed to air dry at room temperature and test panels obtained from this portion of the coil are designated in Table I below as Phos-2. These panels contain an average coating weight of the loose powdery phosphate coating of about 36-37 mi ligrams per square foot.

A still further portion of the coil treated in the manner of the Phos-Z portion, except that in place of simple air drying at room temperature, this coil portion is dried with infrared lamps at a substrate temperature of 250 F. for less than about one minute. Panels obtained from this portion are designated in Table I below as Phos-3. These panels contain an average coating weight of the phosphate coating of about 28-29 milligrams per square foot.

The hexavalent-chromium-containing coating composition for the test panels is prepared in accordance with the teachings of US. Pat. 3,382,081, mentioned earlier,

and the panels containing such coating are shown in the table below as D-Bond panels. This coating is applied to the test panels by dipping same into a composition containing 40 g./'l. (grams per liter) of CrO g./l. of succinic acid, 7.5 g./l. of succinimide and 0.5 g./1. of polyoxyethylated nonylphenol. After dipping, the panels are removed, excess composition drained from the panel, and the panels are air dried at room temperature until the coatings are dry to the touch. Subsequently the panels are cured under infrared lamps for about one minute to a substrate temperature of 450 F. This treatment provides a coating residue on the panels of 50 milligrams per square foot.

The chromate-phosphate coating for the test panels is prepared generally in accordance with the teachings of U.S. Pat. 2,846,342 mentioned above and the panels so coated are referred to in the table below as C-Bond coated panels. This coating is obtained by dipping the panels into a bath containing 40 g./1. of CrO 13 g./l. of zinc oxide, 12 g./l. of concentrated phosphoric acid,

and 7 g./l. of ethylene glycol, distilled water being used for the bath medium. After the panels are removed from the bath they are air dried and are cured for 5 to 6 minutes in a convection oven at a metal substrate temperature of 325 F.

Bare steel test panels, employed for comparative purposes are prepared in the manner described earlier and are designated in the table below simply by the notation Bare Steel. Additional comparative test panels, designated in the table below as Bonded" test panels are commercial panels containing an average weight of about 40 50 milligrams per square foot of a tightly adhering, corrosion inhibiting iron phosphate substrate coating and exhibit no powdery residue prior to application of the chromate or chromate-phosphate coating. Such Bonded panels have met with general acceptance as a standard for performance when evaluating corrosion inhibiting phosphate coatings in, for example, the automotive and household appliance industries. All C-Bond coated and D-Bond coated panels are topcoated with the electrocoat primer mentioned above and are tested for coin adhesion in the manner described hereinbefore. The results of this test are set forth in Table I below.

TABLE I Panel: Coin adhesion C-Bond/bare steel Poor. C-Bond/Phos-l Excellent. D-Bond/Phos-l Excellent.

C-Bond/Phos-2 Excellent. D-Bond/Phos-2 Excellent.

C-Bond/Phos-3 Excellent. D-Bond/Phos-3 Excellent.

C-Bond/bonded Fair. D-Bond/bonded Fair-good.

As can be readily seen from the above table, the phosphate treatments provide enhanced paint adhesion not only when compared with coated bare steel substrates, but also on comparison with coated substrates containing the commercial, non-powdery iron phosphate coatings.

EXAMPLE 2 The D-Bond coating of Example 1 is used to coat bare steel panels, Phos-3 treated panels, and bonded panels which have been described in Example 1. These D-Bond coated panels are then topcoated with the alkyd enamel as well as Phos-2 panels which have been described in Example 1 are coated with the C-Bond coating of EX- ample 1. These resulting panels are then topcoated with the electrocoating primer mentioned hereinbefore.

Panels selected at random are then subjected to either the coin adhesion, or cross-hatch, or reverse impact test described above. The results of these tests are set forth in Table II below; the reverse impact test in the table is abbreviated for convenience simply as R.I. M. In Table 10 II the panels containing the Bonded coating have been nominated as the standard test panels and the results for these panels thus have a numerical grading of 3, since all panels are qualitatively evaluated in accordance with the table set forth hereinabove.

TABLE II X.H RIM 3+ 3 D-B 0nd plus enamel topeoat over- B onded Phos-2 subsubstrate strate C-Bond plus electrocoat primer 0ve1' Bonded substrate Phos-2 substrate B are steel Coin adhesion. 3 3

EXAMPLE 3 Bare steel panels, Phos-2 treated panels, and bonded panels all of which have been described in Example 1 are coated with a hexavalent-chromium-containing coating composition prepared in accordance with the teachings of U.S. Pat. 2,777,785, which coating composition is referred to hereinafter for convenience as the H-Bond coating. This coating composition contains 30 g./l. of CrO 15 g./l. of cane sugar, and 1 g./l. of potassium permanganate, with the balance being distilled water. The coating bath is maintained at F., the panels are dipped into the bath for about 2 seconds, and then air dried followed by curing under infrared lamps for 10 seconds at a substrate temperature of 250-300" F. After curing the panels are quenched in a solution of 5 grams per liter CrO maintained at 200 P.

All panels are topcoated with the electrocoated primer mentioned above and then subjected to the reverse impact test. Testing of two panels for each coating system discloses that the Bare Steel/H-Bond/electrocoat, as well as the Bonded/H-Bond/electrocoat, systems both show initial coating failure on the impacted surface at 10 inch pounds. By comparison, the Phos-2/H-Bond/electrocoat system withstands up to 1'6 inch-pounds before incipient coating failure on the impacted surface.

EXAMPLE 4 Bare Steel panels, Phos-2 treated panels and Bonded panels as described in Example 1 are coated in the manner of Example 3 with the H-Bond coating of Example 3. Additionally, other like panels are coated with a hexavalent-chromium-containing coating composition prepared in accordance with U.S. Pat. 3,063,877, mentioned hereinabove, which coating composition, for convenience, is referred to herein as the A-Bond coating. This coating composition contains 10 g./l. of CrO 1.5 g./l. of formaldehyde expressed as HCHO, and 0.59 g./l. of phosphoric acid, with the balance distilled water. The coating bath is maintained at room temperature, i.e., about 75 F., and panels are dipped into the bath for 90 seconds, withdrawn, and cured under infrared lamps for two minutes with a maximum achieved substrate temperature of 400 F.

The average coating weight after application and cur ing of the coating composition, is between about 312 milligrams per square foot for the Phos-2/A-Bond system and about 75-114 milligrams per square foot for the Phos-2/H-Bond coating system. All parts are topcoated with the electrocate primer disclosed earlier and then panels are selected at random for separate testing under the reverse impact, conical mandrel, and coin adhesion tests. Results of such tests are shown in Table 'III below 1 1 wherein the systems containing the bonded basecoat are used as controls and therefore given a rating of 3 for all tests. 1 J V,

As will be readily seen from the results presented in Table II, the Phos-Z treatment provides a superior coating system for the coating composition residues which have been subsequently topcoated, by sometimes maintaining, but usually by improving the physical characteristics of the finished topcoated panels.

When panels are tested in accordance with the test presented in the examples hereinabove, which test panels have been treated in accordance with the concepts of this invention but, before application of the hexavalent-chrornium-containing coating compositions the powdery residue obtained from the phosphate treatment is removed by manual brushing of the ferrous substrate with moderate pressure, such panels generally provide results from coin adhesion, reverse impact, and the like, which are merely comparable to the results exhibited by the bare steel panels. These results are essentially always the poorest results reported.

It is to be understood that, although the invention has been described with specific reference to particular embodiments thereof, it is not to be so limited, since changes and alterations therein may be made which are within the full intended scope of this invention as defined by the appended claims.

What is claimed is:

' 1. The method for producing a corrosion-resistant coating on a ferrous metal substrate, wherein the resulting coated substrate exhibits weldability and enhanced paint adhesion, which method comprises:

(1) establishing on the surface of said substrate a partially autogenous, dry powdery residue in an amount of between about 5-100 milligrams per square foot, by contacting said surface with an aqueous solution maintained at a pH level below about 2.6, which solution comprises up to about 30 percent by weight of at least one acid of phosphorous;

(2) drying the resulting contacted surface, thereby preparing a first treated surface;

(3) applying to said first treated surface a hexavalentchromium-containing coating composition for metal substrates containing hexavalent-chromium-providing substance and reducing compound therefor; and

(4) heating said substrate at a temperature, and for a period of time, suflicient to vaporize volatile substituents from said coating composition and deposit on the substrate a composition residue.

2. The method of claim 1 wherein said drying of the resulting contacted surface is conducted at a temperature not substantially above about 400 F. and for a period of time up to about 30 minutes, and the substrate is subsequently cooled to a temperature below about 200 F. prior to application of said coating composition.

3. The method of claim 1 wherein said coating composition residue is the residue remaining after heating an applied hexavalent-chromium-containing coating composition, containing between about 025-400 weight percent of hexavalent chromium expressed as CrO at a temperature not substantially above about 300 C. and for a time of at least about two seconds, and said residue is present on said substrate surface in an amount not substantially in excess of about 200 milligrams per square foot.

4. A coated, corrosion-resistant ferrous metal substrate exhibiting weldability and enhanced paint adhesion at the surface thereof, which coated substrate comprises:

(1) a partially autogenous, dry powdery residue in an amount of between about 5l00' milligrams per square foot of metal substrate; and

(2) the coating composition residue obtained upon heating an applied corrosion-resistant, hexavalentchromium-containing coating composition for metal substrates containing hexavalent-chromium-providing substance and reducing compound therefor, at a temperature, and for a period of time, sulficient to vaporize volatile substituents from said coating composition and deposit on said partially autogenous residue said coating composition residue;

wherein said partially autogenous residue is the residue remaining after contacting the metallic surface of said ferrous substrate with an aqueous solution maintained at a pH level below about 2.6, which solution comprises up to about 30 percent by weight of at least one acid of phosphorous, and drying the resulting contacted surface.

5. The coated metal substrate of claim 4 wherein said coating composition residue is present in an amount not substantially in excess of about 200 milligrams per square foot of substrate surface.

References Cited UNITED STATES PATENTS 2,318,656 5/1943 Thompson 148-616 2,725,310 11/1955 McBride -r l486.15X

RALPH S. KENDALL, Primary Examiner US. Cl. X.R. 148-315