US2310239A

US2310239A - Corrosion resistant coating for metal surfaces

Info

Publication number: US2310239A
Application number: US416552A
Authority: US
Inventors: George W Jernstedt
Original assignee: Westinghouse Electric and Manufacturing Co
Current assignee: CBS Corp
Priority date: 1941-10-25
Filing date: 1941-10-25
Publication date: 1943-02-09
Anticipated expiration: 1960-02-09
Also published as: FR919414A; GB560848A; NL61411C; BE472616A; IT429344A; ES177918A1; GB560847A; FR940964A; BE472617A; IT429345A; CH261381A; ES177919A1; CH268171A

Description

Feb.,9, 1943. G. w. JERNs-rEDT 2,310,239

CORROSION RESISTANT COATINGS'FOR METAL SURFACES Filed oct'. 2s, 1941 s sheetssheet 1 INVENTOR 6M .ifea/ l Feb. 9, 1943. G. w. JERNsTEDT 2,310,239 v CORROSION RESISTANT COATINGS FOR METAL SURFACES Filed Oct. 2 5, 1941 3 Sheets-Sheet-2 Feb 9 1943- G. w. JERNsTl-:DT 2,310,239

' CORROSION RESISTANT COATING'S FOR` METAL SUFACES Filed Oct. 25, `1941 3 vSheens-Shee't. 3

Patented Feb. 9, 1943 q `amazes CORROSION RESISTANT COATING FOR METAL SURFACES George W. Jernstedt, Bloomfield, N. J., assignor to Westinghouse Electric & Manufacturing Company, East Pittsburgh, Pa., a corporation of Pennsylvania Application October 25, 1941, Serial No. 416,552

produced by simply subjecting the metal surface 8 Claims.

This invention relates to the art of producing corrosion resistant coatings on the surfaces of ircn. zinc and other metals and alloys.

This invention relates ymore particularly to the production of phosphate coatings upon the surfaces of metals. Such phosphate coatings are of great economical importance in the preparation of the metal surfaces for the reception of an organic finish.

The object -of this invention is to provide a preliminary treatment for metal surfaces to accelerate the formation of corrosion resisting coatings thereon.

.Another object of this invention is to provide a composition capable of activating metal surfaces to improve the process of forming corrosion resisting coatings thereon. v

A further object of the invention is to provide a treatment for metal surfaces to .produce protective phosphate coatings in a rapid and economical manner.

Other objects of the invention will, in part, be obvious and will, in part, appear hereinafter.

For a fuller understanding `of the nature and objects of the invention, reference may lbe had to the following detailed description taken in conjunction with the accompanying drawings, in which: v

Figure 1 is a schematic view of one form of process for accomplishing the invention;

Fig. 2 is a fragmentary sectional view of a metallic member carrying a protective coating produced by the treatment described herein.

Fig. 3 is a photomicrograph at 300X of zinc plated steel with ordinary phosphate treated surface;

Fig. 4 is a .photomicrograph at 300X of zinc plated steel treated according to.the invention.

Fig. 5 is a photomicrograph at 300X of steel with ordinary phosphate surface treatment; and

Fig. 6 is a photomicrograph at 300X of steel treated according to the invention.

I'he treatment of metallic surfaces with solutions or compositions which, under appropriate conditions, will deposit or form upon the metallic .surface a protective phosphate coating is practiced extensively. These phosphate coatings protect the underlying metal from corrosion. Furthermore the phosphate coatings are desirable since they constitute excellent surfaces for the successful application of organic, finishes. The phosphate coatings occur as crystalline deposits to which'organic finishes will bond and adhere more tenaciously than to the bare metal surface.

In the prior art, phosphate coatings have been to a solution containing phosphoric acid and phosphates. The time involved in treating the metal surface in order to secure a satisfactory protective coating usually has been reckoned in hours, but within recent years the time of treat. ment, has been reduced to a fraction of anzhour, for example, 15 minutes.

According to the invention, it has `c'een discovered that the time necessary for the reaction .between the metal surface and a composition capable of developing the phosphate coating thereon depends to a great extent upon the relative activity of the metal surface to the composition. When treated with an activating solution as will be described herein, the bare metal surface has imparted to it improved characteristics whereby it is activated sol that the normal or vmodified phosphate coatingv compositions will form a phosphate lm or coating thereon with great rapidity, for example, in a minute or less.

Another advantage obtained by the activation of metal surfaces is that metal surfaces which previously did not form satisfactory phosphate coatings, successfully acquire phosphate good corrosion inhibiting coatings upon activation. As is well known, zinc does not form good protective phosphate coatings when coating compositions are applied, but with activation zinc will acquire coatings capable of meeting extreme corrosion requirements.

When metal surfaces are activated by a pretreatment, as disclosed in this invention, the surto that secured on unactivated surfaces.

It is well known to those skilledin the art that zinc, galvanized iron and electroplated Zinc cannot be treated with uniformly satisfactory and consistent results with conventional phosphate solutions. Numerous attempts to modify the phosphate solutions in'order to produce an adequate protective phosphate coating on zinc have been unsuccessful from a commercial standpoint.

, POSE.

-di'um phosphate will activate metal surfaces.

In contrast to the prior art dimculties with zinc, zinc surfaces after activation may be subjected to the phosphate coating solutions with phenomenally rapid production of crystalline deposits of highest quality.

In the practice of this invention, metal surfaces to be subsequently subjected to a treatment capable of producing crystalline phosphate coatings are activated by pretreatment with an activating solution. The pretreatment comprises the application of a solution of disodium phosphate containing additions of small quantities of titanium as a compound. In the absence of titanium, the disodium phosphate has negligible activating effgit.

In preparing a satisfactory disodium phosphate and titanium pretreatment, the following process has been found to be successful. Ihe titanium compound is added to ortho-disodium phosphate in water solution. Preferably soluble titanium compounds are employed for additions to the disodium phosphate. Titanium tetrachloride, titanium trichloride, titanium hydroxide, titanium nitride and titanium potassium oxalate have been employed for this purpose with highly satisfactory results. Less soluble compounds of titanium such as titanium carbide. titanium dioxide and titanium potassium fluoride however, have been added to disodium phosphate and al1 have rendered the disodium phosphate capable of activating metal surfaces. The solu. tion of disodium phosphate with the titanium compound addition is preferably evaporated to dryness at temperatures above 60 C.

It has been found that disodium ortho-phosphate is the only material which is satisfactory in producing the desired activation of metal surfaces. Dipotasslum phosphate and both monosodium phosphate and trisodium phosphate, for example, were substantially useless for this pur- 'Ihe only compound, apart from disodium phosphate, of many tested that exhibited some activating characteristics was sodium pyro phosphate (Na4P20r). Disodium phosphate has been found to be the most satisfactory for activating metal surfaces.

The dried disodium phosphate-titanium compound composition as prepared above may contain from 0.005% up to 20% by weight of titanium radical. The dried composition is dissolved in water to produce a. solution suitable for use in activating treatment, having from 0.1% to 2% of disodium phosphate and generally from 0.005% to 0.05% of titanium ions in solution. Titanium may be present in larger amounts with beneficial results. However, it has been found that from 01% to a saturated solution of diso- A 1% aqueous solution of disodium phosphate with 0.01% titanium radical appears to be the optimum concentration. The pH at this concentray tion is from 8.0 to 8.5.

Referring to Fig. 1 of the drawings, there is shown a schematic process which may be followed in applying the activating pretreatment to metal surfaces including the subsequent treatment in producing the' coated metal surface.

Metal whose surface has been cleaned thoroughly is introduced into tank I0. At I is an apparatus. either a tank in which the metal is to be dipped in the activating solution I2, or a booth in which the activating solution is sprayed upon the members being treated. The disodium phosphate and titanium solution I2 need be applied for only 10 seconds to the metal surface to con- Percent Iron 0.03 to 0.30 Zinc phosphate 0.04 Sodium nitrate 0.2 to l Phosphoric acid to produce a 20 point solution and comprising 1:8 of total phosphate content.

Remainder water.

In 30 seconds or less a highly effective phosphate coating will be deposited on the zinc surfact. Longer applications of the phosphate solution are also successful.

In the prior art, unactivated metal surfaces required treatment with a phosphate solution for 15 to 20 minutes. For example, in applying phosphate coatings to automobile bodies, several thousand gallons of solution were sprayed upon the metal before an adequate amount of phosphate coating was formed. In addition, the coatings were non-uniform and it was desirable to paint such coatings as soon as possible. When pre-activated, metal surfaces may be sprayed or dipped in the phosphate solution 16, for a period of time of the order of one-half minute, to produce and extremely uniform and lne phosphate coating.

The metal forms largequantities of gaseous bubbles when the solution is first applied to the activated surfaces. It is believed that the bubbles consist mainly of hydrogen gas. After a. fraction of a minute, the bubbling ceases and the reaction is substantially complete. It is generally desirable to prolong the application of the solution to the metal surface for a short period of time beyond the point at which the gaseous bubbles cease to be given oil'.

The extremely rapid formation of a protective phosphate coating at this stage of the process constitutes one of the unexpected results of the invention. In particular, zinc surfaces acquire an almost velvety and clean-appearing surface of very fine crystalline nature.

In treating ferrous metals, for example-stampings of all types, a solution for use in receptacle i4 more suitable than the one given above for zinc is suggested:

Manganese 4phosphate -pounds-- 1% Phosphoric acid 83% .-do 5 Sodium nitrate dn 1/2 Cupric nitrate ounce-- 1/4 Water to make one gallon.

Therefore, a non-uniform coating may result.

The oxidizing agent will immediately react with the bubbles of nascent hydrogen and remove them from the surface of the. metal. Otherl oxidizing agents, such as sodium or potassium nitrate, are suitable for this purpose.

Zinc phosphate may be introduced to replace a part of the manganese phosphate in the solution for treating ferrous metals` Generally, the presence' o'f one or more of the group consisting of zinc, copper and manganese greatly expedites the reaction of the ferrous metal with the phosphate solution.

Numerous other solutions capable of producing phosphate coatings upon metal are known to the art and their reaction with activated metal surfaces is equally feasible to produce a protective coating.

After an adequate crystalline phosphate coating has been produced upon the metal surface, the metal surfaces are rinsed with water I8 contained in receptacle 20. For best results, the surfaces are subjected to a sealing treatment at 22 with chromic acid solution 2l. The chromic acid 24 consists of an Iaqueous solution of 71A, ounces of Cr(OH3) per 100 gallons of water. The water rinse 20 is not necessary with the sealing treatment of chromic acid. Only a few seconds application of the chromic acid solution is necessary to secure a sealing of the phosphate coating to improve the corrosion resistance. Y

The metal, Aafter a sealing treatment at 22, may be passed through a drying oven 26 to completeiy dry the surface for subsequent operations, such as applying of an organic finish.

Alternatively, it has been discovered that the water rinse and drying oven stcps in the process may be dispensed with if the chromic acid solution 24 is heated-to a temperature of vabout 190 F., or even higher, suicient heat being imparted by the hot chromic acid solution to the metal so that after removal from the solution, the metal dries automatically in air.

Microscopic examination of the coated metal surface reveals a structure somewhat as shown in Fig. 2. The base metal has a 'distinct and separate layer 32 which is of substantially uniform thickness, and appears to be keyed to the metal 30. The coating 32 is extremely durable and will withstand both chemical and mechanical abuse. remove the coating 32. Such coating is uniformly deposited over all the recesses and other irregularities in the surface of metal-body 30.

Referring to Figs. 3 to 6, there are shown photomicrographs at a. magnification of 300X of sam'ples of material treated with prior art phosphate solutions alone, and treated as disclosed herein.

Fig. 3 shows a coarse, crystalline surface deposit of phosphates-on zinc plated steel. This was produced by applying a phosphate-phosphoric acid solution alone. The deposit is nonuniform withv large spaces between crystals. Such a surface deposit produces an unsatisfactory protective coating.

The coarse nature of the phosphate deposit in Fig. 3 requires the application of two or more coatings of organic finish to produce a smooth surface. The organic finish does not adhere any `too well to the large crystals and its life is relatively short.

For comparison, Fig. 4 shows a similar zinc plated steel member as in Fig. 3, with a phosphate coating produced by a predip in a disodium phosphate-titanium solution followed by an application of the same phosphate coating solution as Abrasion or scratching is necessary to empioyed alone for the Fig. 3 coating. The miiform and fine grained phosphate deposit tn Fig. 4 is an outstanding feature. There are no open spaces in Fig. 4 as exist in Fig. 3.

The fine grained phosphate coating in Fig. 4 has beensatisfactorily coated with a single coating4 or'organic finish to produce a smooth, commercial outer surface. Due to its better corrosion-proonng organic finishes last longer when applied to the Fig. 4 surface. The bonding oi' the finish tothe `metal is enhanced by the fine grained phosphate deposit. y

Fig. 5 is a photomicrograph at a. magnification of 300X of an ordirmry steel member with the usual type of phosphate coating produced thereon by the application of a solution of phosphates and phosphoric acid alone. Not only is the protective phosphate coating coarse grained, but its protection is limited. lIt is customary to require that organic finishes be applied to ferrous metals within a few hours after the phosphate The eectiveness of the predip in producing a o fine grained, highly protective phosphate is ev'ldent. The Fig. 6 phosphate coating showsthe same superiority over lthe prior art coatings as does the zinc plated member subjected to the predip.

While occasionally in prior art practice protective coatings have been secured whose appearance to the naked eye appears to correspond to that produced by the present process involving an activating pretreatment, corrosion tests have revealed a decided difference in the quality of the coatings with better corrosion resistance produced by the invention in this case. Ordinary zinc plating on a steel base will turn completely white after 12 hours in a steam chest at 140 F., due to the corrosion of the zinc. The same type of zinc plated member subjected to a standard phosphate `composition treatment, without an activating pretreatment, exhibits an improved resistance to corrosion in the steam chest: After 1.2 hours a few White spots will be apparent on the surface of such metal. When zinc plated members similar to those tested above were vgiven an activating pretreatment, followed by a phosphate surface treatment as detailed in Fig. l, at least 14 days elapsed before any sample showed asign of corrosion in the steam chest operating at F. Generally the members are capable of withstanding a month in the steam chest before any extensive degree of corrosion is evidenced in activated material.

When the three types of material were additionally coated with lan organic finish, an almost equivalent proportionality in the resistance to corrosion was obtained. Samples to which an" activating pretreatment was applied to zinc prior to a phosphate treatment lasted over 25 times es long at 140 F.'ln a steam chest before failure of the organic finish was observed, as compared to the other types of material.

It will be seen by this process that not only has the processing time been cut down to a minimum, as compared to the vtime required in the l prior art to produce aphosphate coating, but

also coatings of a superior corrosionresistance are deposited or formed on the metal. Furthermore, certain metals which have been treated prior art in which two or more applications of organic iinish have previously been necessary.

Since certain changes may be made in the above invention and dierent embodiments of the invention may be made Without departing from the scope thereof, it is intended that all matter contained in the above-described disl closure or taken in connection with the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

I claim as my invention:

1. A composition which when in aqueous solution is capable of activating ferrous and zinc metal surfaces to provide for the application. of protective phosphate coatings which comprises disodium phosphate and more than 0.005% by weight of titanium, the titanium being present as a compound.

2. A composition'which when in aqueous solution is capable of activating ferrous and zinc metal surfaces to provide for the application of protective phosphate coatings which comprises 80% and cver of disodium phosphate and from 0.02% to by weight of titanium, the titanium being present as a compound.

3. An aqueous solution for use in activating ferrous and zinc metal surfaces comprising from 0.1% to 2.0% of disodium phosphate and 0.005% to 0.05% of titanium, the titanium being present as a compound.

4. An aqueous solution for activating ferrous and zinc metal surfaces to accelerate the formation of corrosion resisting coatings thereon comprising disodium phosphate and at least one t1- tanium compound.

5. In the process of treating ferrous and zinc metal surfaces prior to producing thereon corrosion resisting .phosphate coatings, the step comprising applying to the metal surface an aqueous solution of disodium phosphate and at least one titanium compound to activate the metal surface.

6. In the process of treating ferrous and zinc metal surfaces prior to producing thereon corrosion resisting phosphate coatings, the step comprising applying to the metal surface an aqueous solution containing 0.1% to 2% disodium phosphate and from 0.005% to 0.05% titanium to activate the metal surfaces, the titanium being present as a compound which may be ionized.

7. 'Ihe process of treating ferrous and zinc metal surfaces to provide corrosion resisting coatings thereon which comprises, in combination, applying a solution of disodium phosphate and titanium ions to activate the metal surface and subsequently applying a solution of phosphoric acid, phosphates, and an oxidizing agent to the activated metal surface to produce a protective phosphate coating thereon.

8. The process of treating ferrous and zinc metal surfaces to provide corrosion resisting coatings thereon which comprises, in combination, applying an aqueous solution having from 0.1% to 2.0% disodium phosphate and 0.005%

to 0.05% titanium, the titanium being present as a compound, to activate the metal surface and applying a solution of phosphoric acid, phosphates, and an oxidizing agent to the activated metal surface to produce a protective phosphate coating thereon.

GEORGE W. JERNS'I'EDT.