US2813813A - Process for forming protective phosphate coatings on metallic surfaces - Google Patents

Description

United States Patent PROCESS FOR FORMING PROTECTIVE PHOS- PHATE COATINGS ON METALLIC SURFACES Hellmut Ley, Heinz Keller, Heinrich Anton Karl Kopp, Werner Rausch, and Heinrich Fleischhauer, Frankfurt am Main, Germany, assignors to Parker Rust Proof Company, Detroit, Mich., a corporation of Michigan No Drawing. Application December 11, 1953, Serial No. 397,778

Claims priority, application Germany December 17, 1952 5 Claims. (Cl. 1486.15)

The present invention relates to a process for forming protective chemical coatings on the surfaces of metallic articles and more particularly relates to processes which are adapted to form phosphate or mixed phosphate-oxide coatings on ferrous surfaces.

In the customary processes which are utilized in forming protective chemical coatings on metal surfaces such as zinc phosphate, manganese phosphates and iron phosphate coatings, it is conventional to preliminarily clean the metal surface to be coated and to contact that cleaned metal surface with a hot, aqueous, acidic solution of the selected phosphate either by dipping or by spraying and maintaining the contact between the solution and the surface for a sufficient time to form the desired coating. In the preliminary preparation of the metal surface, it has been customary to remove heavy oxide coatings by immersion in molten or fused alkali metal hydroxides and to follow with a series of acidic pickles formed from mineral acids or combinations thereof. After pickling, the metal surfaces are rinsed and are ready for the reception of the protective chemical coating. Even when the phosphate coating solutions contain oxidizing agents, immersion procedures usually require 5 to minutes time to obtain the desired coating. The preparation, maintenance and handling of both alkaline and acidic pickling baths is well-known to be highly undesirable. Moreover, the bare corrosion resistance of such chemical coating is known to be relatively poor and if corrosion protection is important, it is desirable to subsequently treat the metal surface by oiling or with a chromic acid rinse.

The primary object of the present invention is to provide a process for forming protective chemical coatings on metallic surfaces in a reduced period of time, which coatings exhibit superior corrosion resistance in comparison to that obtained with coatings produced by heretofore conventional procedures.

Another object of the present invention is to provide a process for forming protective chemical coatings on ferrous surfaces which eliminate the necessity for utilizing conventional acidic pickling procedures prior to the formation of the protective chemical coating.

Still another object of this invention is to provide a process which enables step-wise deformation operations to be performed on ferrous articles requiring annealing between successive drawing operations, with the minimum of additional steps in the formation of protective chemical coatings after each annealing step. Other objects and advantageous features of the invention will become apparent upon considering the present disclosure in its entirety.

In accordance with the present invention, it has been discovered that the above and related objects may be accomplished by heating the metal article to be coated to a temperature in the range of 100 C. to 400 C. prior to contacting the same with the coating solution, and contacting the article to be coated in its heated con- ICC dition with the aqueous acidic phosphate solution. Whereas in the past it has been customary to operate phosphate coating solutions at temperatures ranging from about 100 F. to about 200 F. and to immerse ar ticles to be coated in such solutions for about 5 to 10 minutes, it has been found that superior phosphate coatings may be formed in accordance with the method of the present invention after a contact period of about one minute. As an example, a low carbon steel having a light oxide coating, induced from the atmosphere, was pickled to smoothness with an aqueous sulfuric acid solution (15% by 'volume), rinsed and pre-heated in hot water to C. and then introduced into an aqueous acidic zinc phosphate coating solution at a temperature of 95 C. and allowed to remain therein for 5 minutes. A portion of the same steel was heated to 200 C. and immersed in the same zinc phosphate coating solution in its heated condition and allowed to remain in the solution for 5 to 10 seconds. Both coated samples were treated with a chromic acid rinse and subjected to a salt spray mist of 3% sodium chloride for 20 hours and observed for rusting. The first sample was found to have 60% of its surface covered with a medium thickness of rust. The second sample was found to have only 40% of its surface covered with a very light coating of rust.

The heating of the metallic article to be coated may be done in a mild oxidizing atmosphere if desired and the resultant coating which forms after contacting the oxide coated surface with the phosphate coating solution will evidence the improved corrosion resistance which is characteristic of this invention so long as the oxide coating which is originally induced is relatively thin, tightly adherent, and not the flaky oxide scale which is induced by oxidation for extended periods of time at elevated temperatures. Such oxide coatings may be formed intentionally in atmospheres containing controlled proportions of oxygen or in air, when both the time and temperature are regulated, or may inherently be present from a previous treatment such as a mild annealing or warm-rolling or coiling procedure.

Inasmuch as the pre-heating of the surface to be coated and the contact thereof with the coating solution while in its heated condition enables the use of any thin, tightly adherent oxide coating which is on the surface and eliminates any necessity for the conventional acidic pickling procedures, it will be appreciated that the greatest advantage is gained when this process is employed. It is possible, however, to obtain the benefits which accompany the introduction of the article to be coated into the coating solution at an elevated temperature even when the article is coated with a heavy flaky oxide coating. In this case, the article should be heated to a temperature above 400 C. in air or a controlled oxidizing atmosphere, and then subjected to a reducing atmosphere to eliminate the scale and leave the metal surface in a metallicly clean condition, and if the temperature employed for the reducing atmosphere treatment is higher than 400 C., the article should be cooled below 400 C. in an inert atmosphere so that the metallicly clean surface Will be maintained clean. The article can then be contacted with the phosphate coating solution while at a temperature of C. to 400 C. to form the superior corrosion-resistant coatings of this invention.

The particular phosphate coating solution which is selected for use in the process of this invention is not particularly important. Any aqueous acidic phosphate solution which is known to form a protective phosphate coating on metallic surfaces may be used, including zinc phosphate, manganese phosphate, iron phosphate and alkali metal phosphates. One of the most important end uses for the coatings of this invention is for the protection of the metal surface during metal deformation operations,

such as tube or wire drawing, and the selection of the best phosphate coating solution will depend upon the requirements of the particular application. All the above types of phosphate solutions, with or without the conventionally employed oxidizing agents, are suitable for this end use.

In the drawing of metallic articles such as tubes or wires, it is conventional to employ a protecting phosphate coating in conjunction with a lubricant, which lubricant usually contains some fatty material. The present invention enables the utilization of such fatty material in forming an oxide coating by heating, in a controlled atmosphere where necessary, and eliminates the necessity for solvent removal of the fatty material and conventional pickling procedures prior to the formation of a new protective phosphate coating. All that is necessary is to oxidize the fatty material to a thin, tightly adherent oxide coating and introduce the oxide-coated article at a temperature of between 100 C. and 400 C. in the phosphate solution.

. The contact between the metal to be coated and the coating solution may be made by dipping the article in the solution or by spraying the solution on the article. In

either case, the solution may be cold, at room temperature or preliminarily warmed. It is preferred that the article have a temperature between 200 C. and 300 C. when it is contacted with the phosphate coating solution since these conditions produce the most corrosion resistant coatings in the shortest periods of time.

The following examples illustrate the invention in greater detail.

Example I Low carbon steel wire was annealed in air at temperatures of 200 C. and 400 C. and while at these temperatures was drawn through an aqueous zinc phosphate solution containing 22 grams/liter P205, 35 grams/liter NO: andl9.5 grams/liter zinc. For comparative purposes, other portions of the same wire were heated in air to 200 C. and 400 C. and allowed to cool. The same phosphate solution was heated to 95 C. and the cooled sections immersed therein for 5 minutes and withdrawn. The wires were then coated with the same rust resisting oil coating. All of the sections of wire were subjected to identical salt spray tests in 3% sodium chloride mist for hours and checked for corrosion. The wires treated in accordance with the method of this invention showed trace quantities of rust on about 40% of their surface. The wire sections treated by immersion for 5 minutes showed light rust over the entire surface and much heavier rust over about 40% of the surface.

Example II Low carbon sheet steel having some fatty material on its surface was cleaned with trichloroethylene and dried. An aqueous zinc phosphate solution was prepared having 11.5 grams/liter Zinc, 14 grams/liter nitrate and 13 grams/liter P205 and the temperature of the solution was raised to 100 C. and the cleaned metal immersed in the heated solution for 5 minutes and withdrawn, the phosphate coated surface being then treated with oil. Another. portion of the same sheet metal was first wiped with trichloroethylene, then heated to above 450 C. in a controlled atmosphere furnace which contained a mild oxidizing atmosphere, and the oxidized surface was then treated in a reducing atmosphere of hydrogen at 450 C. until the surface became clean. The metal was then transferred to an inert atmosphere cooling chamber where the temperature was decreased to approximately 400 C. and the article was then immersed in the same aqueous phosphate solution as used before for one minute and withdrawn. The coated metal was then oiled in a manner similar to the oiling procedure used in connection with the article resulting from the 5 minute immersion. Both articles were then subjected to a salt spray test of 92 hours duration in a chamber employing a 3% sodium chloride fog spray for 15 minutes out of each hour. The inspection of the article resulting from the 5 minute immersion showed that the entire surface of the panel was covered with rust and that about 60% of the surface was covered with an extra heavy layer of rust whereas the article resulting from the method of this invention showed very light rust over the entire surface and only about 20% of the area having a medium layer of rust thereon.

Example III An aqueous zinc phosphate solution was prepared containing 10 grams/liter P205, 27 grams/liter zinc and 6 grams/liter chlorate. Low carbon steel preliminarily degreased was immersed in this aqueous zinc phosphate solution at 100 C. for 5 minutes and withdrawn. Another portion of the same sheet steel was heated to 400 C. m a mild oxidizing atmosphere and at this temperature 1mmersed in the same solution for one minute and withdrawn. Without applying oil or otherwise treating the coating for the enhancement of the corrosion resistance, both portions of the coated steel were subjected to a 3% sodium chloride fog spray for a 3 hour period. At the end of this time, the immersion process portion of the sheet steel was found to be completely covered with a medium thickness layer of rust. The portion of the steel treated in accordance with the method of this invention was found to have 20% of its surface completely free of rust and the other covered with only a very light layer of rust.

What is claimed is:

1. A method for forming a chemical protective coating on metallic surfaces which comprises the steps of heating the metallic surface in an oxidizing atmosphere to a temperature above 400 C. to form an oxide coating thereon, heating the oxide coated surface in a reducing atmosphere to remove the said oxide coating and form a metallicly clean surface, cooling said surface to a temperature in the range of C. to 400 C. in an inert atmosphere, and contacting the said article in heated condition with a phosphate coating solution for a time suiiicient to form a phosphate coating on said surface.

2. A method for forming a chemical protective coating on ferrous surfaces which comprises the steps of heating the ferrous surface in an oxidizing atmosphere to a temperature above 400 C. to form an oxide coating thereon, heating the oxide coated surface in a reducing atmosphere to remove the said oxide coating and form a metallicly clean surface, cooling said surface to a temperature in the range of 200 C. to 300 C. in an inert atmosphere, and contacting the said article in heated condition with a phosphate coating solution for a time suflicient to form a phosphate coating on said surface.

3. A method for forming a protective chemical coating on metallic surfaces which have previously been mechanically deformed which comprises the steps of heating the said surface to a temperature above 400 C. slowly cooling said surface in an oxidizing atmosphere to a temperature in the range of 100 C. to 400 C., and contacting the said surface while in the heated condition with an aqueous acidic phosphate solution for a time sufficient to form on said surface a coating which is at least partially a phosphate coating, and thereafter mechanically deforming said surface.

4. A method for forming a protective chemical coating on ferrous surfaces which have previously been mechanically deformed which comprises the steps of heating the said surface to a temperature above 400 C. slowly cooling said surface in an oxidizing atmosphere to a temperature in the range of 200 C. to 300 C., and contacting the said surface while in the heated condition with an aqueous acidic phosphate solution for a time sufiicient to form on said surface a coating which is at least partially a phosphate coating, and thereafter mechanically deforming said surface.

A method for forming a protective chemical coating on ferrous surfaces which comprises the steps of forming a phosphate coating on said surface, contacting said phosphate coated surface with a lubricant containing fatty material, mechanically deforming said surface, heating said mechanically deformed surface in an oxidizing atmosphere, and contacting the said re-heated surface with an aqueous acidic phosphate solution for a time suflicient to form on said surface a coating which is at least par tially a phosphate coating.

6 References Cited in the file of this patent UNITED STATES PATENTS 1,761,186 Baker et a1. June 3, 1930 2,543,710 Schmidt et a1 Feb. 27, 1951 2,588,234 Henricks Mar. 4, 1952 OTHER REFERENCES Metals Handbook, 1948 edition, page 6.

Claims (1)

1. A METHOD FOR FORMING A CHEMICAL PROTECTIVE COATING ON METALLIC SURFACES WHICH COMPRISES THE STEPS OF HEATING THE METALLIC SURFACE IN AN OXIDIZING ATMOSPHERE TO A TEMPERATURE ABOUT 400*C. TO FORM AN OXIDE COATING THEREON, HEATING THE OXIDE COATED SURFACE IN A REDUCING ATMOSPHERE TO REMOVE THE SAID OXIDED COATING AND FORM A METALLICLY CLEAN SURFACE COOLING SAID SURFACE TO A TEMPERATURE IN THE RANGE OF 100*C. TO 400*C. IN AN INERT ATMOSPHERE, AND CONTACTING THE SAID ARTICLE IN HEATED CONDITON WITH A PHOSPHATE COATING SOLUTION FOR A TIME SUFFICIENT TO FORM A PHOSPHATE COATING ON SAID SURFACE.