US3497440A - Process for the coating of metallic surfaces - Google Patents

Description

United States Patent 4 Int. 01. C231; 11/00,-n32b 15/04 US. Cl. 204-181 Claims ABSTRACT OF THE DISCLOSURE Process for modifying the surface characteristics of a metallic substrate (e.g. to limit corrosion of aluminum, magnesium or iron), wherein the substrate is electrically coated with a surface-active agent from an aqueous bath to form a deposit on the substrate consisting predominantly of the surface-active agent. Upon withdrawal of the substrate carrying the deposit fromthe bath, it may be baked at an elevated temperature (80 C. to 200 C.). Preferably the surface-active agent is an anionic or cationic surfactant and the substrate is electrically poled to attract said surfactant, the concentration of said surfactant in the bath ranging between substantially 1 and 100 g./l., the pH of the bath ranging between substantially 3 and 14.

Specification This application is a continuation-in-part of my application Ser. No. 373,970, filed June 10, 1964, now US. Patent No. 3,424,663 issued Jan. 1969.

My present invention relates to a process for the coating of metallic surfaces, e.g. surfaces of metals and metal alloys, to provide corrosion resistance, to modify the metallic surface to accommodate another coating operation and to alter the appearance, texture or physical or chemical characteristics of a metallic surface.

In the aforementioned copending application, I have described and claimed a process for the electrophoretic deposition of a synthetic resin upon a metallic body wherein the resin is dispersed in an aqueous medium to form negatively polarized particles which are attracted toward a surface of a metallic substrate immersed in the electrolyte and poled anodically with respect to a counterelectrode. In order to prevent the interaction of the resin particles with the metal ions released by the anode into the solution, the cations released by the metallic surface are converted into anions by interaction with a complexing agent having the necessary affinity for the metal involved. The synthetic-resin particles are solid particles of one or more high-molecular ewight polymers, e.g. formed by addition, substitution or condensation polymerization, while the liquid medium may further include conductive particles depositable on the substrate concurrently with the synthetic-resin particles so that the resin layer does not act as an insulator modifying the electrophoretic current flow. In other words, these conductive particles maintain the electric current flow between the collecting surface of the substrate and the liquid medium or phase. It may be noted that these particles can be the ionic particles mentioned above whose efiective electrical charge has been reversed in polarity by complexing or complex formation. The substrate can be composed of a metal which is anodizable concurrently with deposition of the synthetic resin so that an oxide "ice layer is formed on the collecting surface simultaneously with deposition of the synthetic resin. The arrangement is especially advantageous for aluminum substrates since the oYide layer may function in part as a complex former to tie up metal ions as noted earlier. The anodizing step may also impart the desired color to the substrate. Additionally, the synthetic resin dispersion can include particles of a coloring agent or pigment which are deposited as part of the resin layer. When the substrate is rendered anodic a is usually the case (as a consequence of the generally negative charge carried by the resin particles), anionic dyestuff may be employed. Cationic dyestuffs can be used when the substrate has a negative charge.

The aforedescribed method has been found to be highly advantageous and desirable for the protective coating of metal and metal alloys. In some cases, however, the adhesion of the coating may not be sufiiciently strong or the nature of the coating may be undesirable for a particular type of anticorrosion protection.

It is, therefore, the principal object of the present invention to provide an improved method of coating metallic surfaces so as to render them corrosion-resistant or provide a basis for further coatings adapted to adhere strongly to the surfaces.

A more specific object of this invention is to provide an improved method of electrically coating metals and metal alloys with organic substances.

These objects and others which will become apparent hereinafter, are attained in accordance with the present invention by solubilizing in an electrochemical medium one or more organic surfactants, i.e. tensides or surfaceactive agents, and connecting the substrate as an anode or cathode, the polarity being chosen so as to be opposite the polarity of the large-molecular portion of the solubilized surface-active agent. I have found that it is thus possible to deposit upon the substrate from solution, a coating of a surface-active agent which appears to bond chemically to the metallic surface and provide a selfcontained anticorrosion protection or constitute a base for the deposition of other materials, e.g., the electrophoretically deposited resins mentioned earlier. Preferably, the surface-active agent is then baked upon the substrate at a temperature well above ambient, preferably C. to 200 C.

Depending upon the polarity of the substrate, I may deposit a surface-active agent of opposite polarity, e.g. anionic and cationic surface-active agents. Among the anionic surfactants or anion-active surface active agents I have found most satisfactory, are the fatty-acid condensation products (e.g. fatty acids condensed with aminoalkanesulfonic acid, with glycerol and naphthalene and sulfonated, with lower alkylhydroxysulfonic acids, with methyltaurine, etc.-see pages ff. Schwartz-Perry, Surface Active Agents, volume I, Interscience Publishers, N.Y., 1949), salts of alkyl sulfosuccinates (e.g. sodium oleyl, cetyl or stearyl sulfosuecinate), salts of ricinus oilsulfuric acid half ester (turkey red oil), alkyl sulfates (e.g. the alkyl sulfates of 8 to 20 carbon atoms, especially octyl, decyl, lauryl, dodecyl, myristyl, tetradecyl, cetyl, stearyl and oleyl sulfates), alkyl phosphates (e.g. the alkyl phosphates of 8 to 20 carbon atoms), alkyl sulfonates (see pages 82l10, Surface Active Agents, supra), and alkylaryl sulfonates (see pages 111 if, Surface Active Agents, supra).

Among the cationic or cation-active surface active agents, I have found that best results are obtained with alkyl pyridinium salts and alkyl ammonium salts (see pages 156 if, Surface Active Agents, supra). Even nonionic surface active agents may be used under specific conditions, these being preferably polyoxyalkylene addition products.

According to a more specific feature of this invention, the surface active agent is present in the deposition bath in a concentration of 1 to 100 grams/ liter while the pH value of the bath is maintained between 3 and 14. For iron and iron alloys the pH of the bath is preferably above 8, while for aluminum, magnesium and their alloys, the pH range should be 6 to 14. When it is desired to form a water insoluble coating upon the metal, the surfaceactive agent and deposition conditions (under anodic or cathodic polarity) are selected such that the surfactants form water-insoluble compounds with the metal at the surface. It is also possible, however, to burn the coating, or bake the coating, onto the surface to effect an interaction which increases the strengih of the bond or merely promotes more permanent adhesion. Finally, it may be pointed out that an insoluble coating can be formed by depositing alternate layers of cationic and anionic surfaceactive agents; in the latter case, the polarity of the substrate is switched between cathodic and anodic polarities. Alternatively, the subsequent layer or layers can be applied after electrical deposition of a prior layer, by chemical (nonelectrical) deposition or by simple mechanical coating. The bond between cationic and anionic surfaceactive layers is sufficiently strong to render the final coating substantially insoluble.

The electrochemical coating is carried out with direct current or with alternating current and with direct current upon which alternating current is superimposed. It

has been found that the potential across the deposition bath may range effectively from 2 to 600 volts but preferably ranges up to 250 volts. At the commencement of the deposition, current densities of 0.05 to 80 amperes/ decimeter (a./dm. are used but preferably ranges up to 20 amperes/decimeter Coating times may range from 2 to 60 seconds. It has also been observed that uniformity of coating increases with agitation of the substrate in the bath or relative to the bath.

The bath from which the surface-active agent is deposited, in accordance with the present invention, may include one or more complex formers (comp-lexing agents) as described in my copending application mentioned earlier and which are capable of complexing the metal ions passing into solution from the substrate. It has also been found desirable to limit or prevent oxygen and hydrogen evolution at the anode or cathode, respectively, by introducing into the bath compounds having a tendency to react therewith. When the overvoltage is such that oxygen may be released at the anode, and uniform coating of this anode is intended, I introduce an oxygenaccepting agent, generally hydrazine, to reduce the propensity toward oxygen evolution. Conversely, when coating of the cathode is desired and hydrogen may be evolved, hydrogen acceptors such as peroxides are introduced into the bath.

I have also found that certain surfactants give unusually eifective (strongly bonded uniform and tough) coatings with certain metals as will be apparent hereinafter. For example, for the coating of aluminum and magnesium and their alloys, the anionic surface-active agent designated commercially as turkey red oil (i.e. the sodium salt of ricinus-oil sulfuric-acid half ester) may be used with excellent results. The same metals can be coated as effectively under cathodic conditions with the cationic surfaceactive agent octadecyloxymethylenepyridiniumchloride. In these cases, it has been found that fluoride complexing agent (potassium fluoride) affords a further improvement in the character of the coating. Thus potassium fluoride may be added as a complexing agent in an amount of, say, 1 g./l. to 20 g./l. while the pH is maintained at its preferred value of 6 to 14.

The present invention also has the advantage that organic substances of high or low molecular weight and 1 Pp. 202 ff., Surface Active Agents, supra.

of simple organic or metallo-organic structure and inorganic substances may be carried by the surface-active agent onto the substrate and incorporated in the coating. Suitable substances for this purpose may be oils, fats, waxes, synthetic resins and pigments. These substances should be present in a proportion less than that of the surface active agent. The surface-active agents have been found to have a high affinity for such adjuvants and may impart to them mobility in an electric field, electrical coating using these substances being otherwise diflicult. Moreover, the present system has the advantage that the electrical energy serving to coat the substrate may in part be converted to heat so that the temperature a the interface of the bath and substrate may be relatively high and promote fiowability of the coating, thereby imparting a lacquer-like appearance to the coating. The addition of plasticizers and flowability modifiers may further increase the smoothness of the coating.

EXAMPLES (A) Preparation of substrate Each of the tests 1-6 was carried out with a sheet metal square plate (see the appended table) of a thickness of 1 mm. and side length of mm. The iron plates were 0.05% carbon steel while the aluminum plates were of 99.7% purity and of the type designated soft.

Prior to coating, each plate was cathodically degreased in an aqueous bath with a composition of 15 g./l. sodium hydroxide, 5 g./l. sodium carbonate, 5 g./l. sodium cyanide, balance water. The counterelectrode (anode) was stainless steel (Midvale V2A stainless) with a composition approximately of 0.06 to 0.25% carbon, less than 0.5% manganese, 8 to 9% nickel, 17 to 19% chromium, balance iron. The degreasing parameters were as follows:

Interelectrode potential volts 10 Current density a./dIn. 8 Treatment period seconds 60 Bath temperature C 20 Bath volume liter 2 Following the d-egreasing treatment, the sheet-metal plates were thoroughly rinsed with water.

(B) Coating of aluminum and iron plates with surfaceactive agent Examples 1-6 below show the invention as applied to the coating of various surface-active agents upon alumium and iron plates under varying conditions. The table provides, for each of these examples, a summary of the relating parameters and conditions including the nature of the coating process (i.e., whether the substrate is poled anodically and cathodically), the type of surface-active agent, the characteristic of the surface-active agent (whether anionic, cationic or nonionic), the concentration of the surface-active agent in the bath and, when a two-stage treatment is used, the concentration of the different types of surfactant in the respective baths, the nature and concentration of any complexing agent or other additive, the pH of the bath, the electrical conductivity of the bath, the coating temperature, the applied voltage, the starting current density, the coating time, the baking temperature and baking time for setting the coating, and the final coating thickness.

EXAMPLE 1 An aluminum plate is coated with the anionic surfaceactive agent turkey-red oil (sodium salt of ricinus-oil sulfuric-acid-half ester).

EXAMPLE 2 Anodic coating of a higher-alkyl phosphoric-acid ester (anionic surface-active agent) upon iron using hydrazine as oxygen acceptor.

EXAMPLE 3 Cathodic coating of a cationic fatty-acid condensation product (see pages 172 ff. of Surface-Active Agents, supra), on iron.

anionic and cationic surfactants, said substrate being electrically poled to attract said surfactant, the concentration of said surfactant in said bath ranging between substantially l and 100 g./l., the pH of said bath ranging between substantially 3 and 14.

EXAMPLE 4 5 4. The process defined in claim 3 wherein said substrate Coating of iron with a nonionic condensation product is electrically coated in said bath with a surfactant of one of polyoxyethylene and polyoxypropylene using hydrazine the canon: and amplnc y i compnsmg the as Oxygen acceptor. step of thereafter coating said deposit with a layer con- EXAMPLE 5 sisting predominantly of a surfactant of the other type.

5. The process defined in claim 4 wherein said sur- In the first step, the anionic Surfactant disodium octa factant of said other type is deposited from said bath onto decylsulfosuccinate is anodically deposited upon the alumi- 'z ipfi d fi d l 5 h num substrate. In the second step, the cationic surfacef e ii nc 6 fsur' active agent octadecyloxymethylenepyridiniumchloride is ii g g ell'atype 1S elccmc y deposlte mm deposited upon the cationically connected substrate anod- Sal at on O Sal Su i icany coated in the first step 7. The process defined 1n cla1m 1 wherein said substrate is anodically coated with said surface-active agent, further XAMPLE 6 comprising the step of adding an oxygen acceptor to said bath.

A11 alumlnum Plate 15 coatecl lf Wlth Q 8. The process defined in claim 1 wherein said substrate octadecylsulfosucclnate y q Coatlng While 1n the is cathodically coated with said surface-active agent, fur- Q f Stage Q catlf'mlc octadecyloxymethylenether comprising the step of adding a hydrogen acceptor t0 pyridiniumchloride is deposited from the bath in the abid b h sence of an applied voltage to chemically coat the previ- 9. The process defined in claim 1 wherein said substrate 3! deposlted aIllOIllC Coatlngis aluminum, magnesium or an alloy thereof and said TABLE Coating 0i Aluminum Iron Iron Iron Aluminum Aluminum. Nature of substrate Anodic Anodic Cathodicnu Anodic/cathodic Anodic. Surfactant A.-." B C lsttstepFE, 2nd E+F. Nature of surfactant Anionic Anionic Cationic Nonionic Ar ii iiick, Anionie E, Surfactant concentration so 5 50 2 s tiif iiifl 5071 Bath additive 6 g./1. K2F2 18 g./l/. h 3 }g%1ne, 18 g./l. hlyldgtgne, pH ofbath s12 12:15 s41 mi i' a Bath conductivity 12,000. 15,000- 12,000

(p.890. cmr Bath temperature C.) 15-40.... 15-40.-. 15-20 Working potential (v0lts) 50 25 50 100 Initial current density 8 5 1 E (a/dmfl). Coating time (see) 5 15 10 5 5 E=5; F=3. Baking temperature C.) 160. Baking time (min.) 5 Coating thickness (,4) 6-8 A=turkey red oil; B=alky1phosphate; C=fatty-acid condensation product; D=polyoxyethylene-polyoxypropyleneladduct; E=disodiu1noctadecyls ulfosuccinate; F 0ctydecyloxymethylenepyridiniumchloride.

In all cases, the coating was adherent uniformly and free from discontinuities, providing excellent corrosion protection to the metallic surface. The coating was served as an excellent base for other coating substances, as previously described.

I claim:

1. A process for modifying the surface characteristics of a metallic substrate, comprising the steps of immersing said substrate in an aqueous bath containing a surfaceactive agent, and electrically coating said substrate with said surface-active agent from said bath to form a deposit on said substrate consisting predominantly of said surface-active agent.

2. The process defined in claim 1, further comprising the step of Withdrawing said substrate carrying said deposit from said bath and baking said deposit onto said substrate at an elevated temperature.

3. The process defined in claim 1 wherein said surfaceactive agent is selected from the group which consists of UNITED STATES PATENTS 4/1968 Gentles et al 204-181 OTHER REFERENCES Fink, Transactions of the Electrochemical Society, vol. 94 (1948), pp. 309, 317 and 321.

HOWARD S. WILLIAMS, Primary Examiner US. 01. X.R. 117-415