US2440969A - Protectively coated ferrous metal surfaces and method of producing same - Google Patents

Description

MG-F811;)

Patented May 4, 1948 z, 11-; 1

CROSS" REFERENCE PROTECTIVELY COATED rnnnous- METAL.

summons AND warrior) or raonpomc SAME Victor Charles Jjohn Nightingal-l, deceaseit, late of Heidelberg, Victoria, Australia,- by. Euth- Valencia Nightingall, Aubrey Mitchell Old, and Charles Montefiore Gladstone Nightingall, ex;- ecutors, Melbourne, Victoria, Australia, assignors to Di-Met Proprietary Limited, West Melbourne, Victoria, Austr toria, Australia No Drawing.

This invention relates to protective coatings for metal surfaces and, more particularly, to hard protective coatings for the surfaces of ferrous metal articles. The invention contemplates a novel method of producing such coatings on a ferrous metal surface and the resulting. article having on its surface. such a hard protective coating.

Protective coatings for metal surfaces must meet several requirements if they are tovbe conl requirements, it fails to qualify as a protective.

coating. This is particularly true with respect to protective coatings for ferrous metal articles where a single flaw in the. coating will permit widespread corrosion and even disintegration of the ferrous article beneath the coating.

The method of the present invention contemplates the provision of a hard. protective coating on aferrous metal surface in such. manner that the coating meets each 01 the aforementioned requirements. The method of producing such a coating on a ferrous metal surface in accordance with the invention comprises applying to the surface a coating 01. a composition comprising finely divided metal l ig zlnc and finely divided lead oxide incorporated in an aqueous solution containing an alkali silicate and sodium Bicarbonate, and prefera y a o oracic ac and drying the applied coating. The coating obtained by this method is a extremely hard, protective, abrasion-resistant, adherent and water insoluble. It is particularly effective as a protective corrosion-resistant coatzinc dust of pigment particle size as free of zinc oxide as possible.

alla, a company or Application January 7 1948,; Serial No. 1,072 a 10 Claims. (Cl'. 11 7427) The quantity of finely-divided lead oxide used in making up the composition of the invention may range from about to 2 by weight 1 the zinc used. The lead oxide promotes drying or insolubilization or the coating composition and makes possiblecomplete drying and lnsolubilizartion of the coating by simple air-drying, The lead oxide further appears to increase the hard.- ness and abrasion resistance 0': the dry coating. About 5% lead oxide by weight of-the zinc used appears to be the smallest effective amount of lead oxide; An excess of lead oxide above about 20% by weight of the zinc unduly dilutes the zinc and tends to impair the corrosion-resistant ffectiveness of the coating. Particularly effective results have been obtained using amounts of lead oxide ranging from about to by weight oi the zinc. Although the leadoxide is preferably added in the form of red lead (Pb304) because of its commercial availabiiity and favorable, cost,

litharge (PhD) and lead suboxide (PhzO) have mcessf y w hin the above-men tioned rangesJ The alkali silicat solution may, be made up of h otassium silicate, lithium slli cate o suc s icates, in e arm ofan all olution having a specific gravity of approximately 13 ii; e., around by weight of alkali silicate). Elfective coating compositions may be made up with alkali silicate solutions of both higherand lower specific gravities, and satisfactory results have been obtained in practice with solutions containing from about 20% to by weight or the alkali silicate. The sodium silicate, for example, may be of any of the commonly marketed ratios of NazO to S102, which generally range from 1:1..7. to 1:3..9, but particularly satisfactory drying cfiara'cteristlcs are obtained with NazO to $10: ratio betwefinabout 112.4 and To the solution of alkali silicate is added uyi b icarbonate in amount ranging from about 0.1% to about. 0.8% by weight based onfth'e weight of thealkalisilicate solution.v The actual amount of sodium bicarbonate required it; the

silicate solution to eficct'the formation of an in; sol ble com lex silicate in accordance with the invention varies with the amount of free alkali in the silicate solution which must be neutralized by the bicarbonate before the unconsumed bicarbon- EXAMINER ate is available to produce the desired complex silicate. The amount of free alkali to be neutralized depends to some extent upon the ratio of NazO to SiOz in the silicate and upon the concentration of the silicate solution. The amount of free alkali to be neutralized also appears to vary with the amount of zinc present in the composition, and, of course, depends upon whether boracic acid is used in preparing the coating composition as hereinafter more fully discussed. Approximately g lj sodiumbicarbona te appears to be the ggal1est efl'ectiveamount which can be used in accordance with the invention, and this lower limit can be obtained only by the use of the maximum permissible amount of boracic acid. Amounts of sodium bicarbonate substantially in excess pf q.il% by weight of the sillcate solution can be jgleraiesl, either in the presence or absence of boracic acid, without precluding the formation of an insoluble coating when the applied composition is dried. However, such excessive amounts of sodium bicarbonate are detrimental to the appearance of the coating because they promote the formation of a bloom on its surface and are detrimental to the effectiveness of the coating because they tend to impart porosity thereto.

The sodium bicarbon ate may be added to the silicate Edition in the solid formifijflin the form of an agu eous solutionflhle"latte? form is preferred because it facilitates complete dissolution and uniform dispersion of the sodium bicarbonate in the alkali silicate solution. The bicarbonate solution is advantageously one which is saturated with the bicarbonate at the temperature of the silicate solution to which it is added (generally room temperature). For example, the addition of about 12 parts by volume of a sodium bicarbonate solution saturated at 60 F. to 100 volumes of an alkali silicate solution having a specific gravity of about 1.3 is equivalent to the addition of about 0.8% of sodium bicarbonate by weight of the silicate solution. Smaller quantities of the bicarbonate solution may be added to the silicate solution when less free alkali is present to be neutralized by the bicarbonate as explained hereinabove.

Boracic acid is preferably also added to the alkali silicate solution in an amount sumcient to react with the bulk of the free alkali in the solution but insufficient to cause precipitation of silicic acid. To the extent that the Q alkali is neutralized by theboracicacid, the required amount of sodium bicarbonate may be reduced in accordance with the principle heretofore explalned. For example, when about 0.2% by weight of boracic acid based on the weight of the solution is added to a sodium silicate solution having a specific gravity of about 1.3 and a No.20 to $102 ratio of about 1 :2.4, the amount of sodium bicarbonate required in accordance with the invention may be reduced to about 0.2% by weight .of the silicate solution. Larger or smaller amounts of boracic acid may be used with advantage, the only limitation being that it be not added in amount sufllcient to cause precipitation of silicic acid from the silicate solution. The boracic acid may be added in the solid form, although it is preferred to add the boracic acid in the form of a substantially saturated aqueous solution in order to facilitate dissolution thereof in the silicate solution. In addition to its effect in reducing the amount of sodium bicarbonate required, the inclusion of boracic acid in the coat- 7 heres to the metal surface to which it has been ing composition efiects a substantial increase in the hardness of the dry coating. a

In forming a hard protective coating on a metal surface in accordance with the invention, the metal surface such as iron or steel is preferably first cleaned to remove all scale, rust, dirt, and the'like. The metal surface may be cleaned in a pickling bath such as a solution of phosphoric acid or sulphuric acid, followed by appropriate rinsing. The surface may also be cleaned by sand blasti buffing, or other mechanical means. rhe mm with the composition comprising the finely divided zinc and lead oxide incorporated in the aforesaid aqueous silicate solution. The composition may be applied to the metal surface either by dipping, brushing, spraying, or the like. The dipping procedure produces a relatively heavy coating which varies in thickness with the viscosity of the coating composition. The brushing and spraying procedures are more readily controlled and can .be used to provide any coating thickness ranging from very thin coatings to heavy coatings of excellent physical characteristics.

The composition applied to the metal surface is then dried under conditions such as to substantially dehydrate the coating composition. This result may be accomplished by permitting the applied composition to dry for a period of at least twenty-four hours, and preferably for a period of about 36 to 48 hours, in an atmosphere of normal temperature and humidity. Excessive moisture in the atmosphere, as evidenced by high humidity, or rain or dew, impedes the drying step and interferes with the production of a hard, protective coating. The drying may be expedite with particularly eflective results by heating the applied coating to a temperature of about to 5091B. for a period of about 2 to 12 hours, the heating period depending upon the heating tern perature used.

The chemical reactions which take place with in the applied coating composition during the drying stage are obscure, although there is evidence that a mixture of silicates or complex silicates, or both, are formed. It appears that the lead oxide has a catalytic action which is particularly important when dehydration is effected at normal temperatures. Additionally, however, the lead oxide improves the properties of the coatings obtained either by air-drying or by heating to atemperature of 180 F. or higher. It is believed that carbon dioxide released from the bicarbonate reacts to produce carbonate of zinc or other zinc compounds which in turn react with the sodium silicate or silicic acid precipitated by the carbon dioxide to form a complicated zinc silicate. These suggested reactions, however, represent only the first stage of the formation of the ultimate coating, as further advantageous but obscure changes occur slowly over a period of one or two years or more. The boracic acid incorporated in the solution reduces the alkalinity thereof and thus the carbon dioxide subsequently released from the bicarbonate is more fully available to react with the zinc, since only a minor portion of the bicarbonate is then required for completely neutralizing the alkalinity of the silicate solution. The boracic acid presumably reacts with the free alkali to form borax which latter, it is thought, has a useful fluxing eflect.

The tenacity with which the dried coating adan om applied suggests that some reaction with the ferrous metal surface takes place, such as the formation of an iron silicate. Whatever reactions actually take place, th e applied composition forms a protective, hard, abrasion-resistant, adherent coating by the end of the drying period, that is, by the time the applied composition is substantially dehydrated. It must be understood, however, that the dehydration referred to herein comprises the removal of free moisture:

from the applied composition and not necessarily the complete removal of water chemically combined with the other ingredients of the coating composition.

The following specific examples .will serve to further illustrate the practice of the invention:

An aau c mq utlqn-2f. sqdi in s i ate d sodium rightful: 'was prepared'bf'dissoiving in wat Fa sufiicierft quantity of sodium silicate having a I& Q. i2-.ii ?a.r.aii0.. f ifi fiiifi fi form a solution having a specific gravity "of approximately 1.3, and by then adding thereto a wudso ut ap sai in h carh te. saturated at 60 F.) inthe proportion'of'12 parts by volume of the sodium bicarbonate solution to 100 parts by volume of the silicate solution. This addition corresponds to about 0.8% sodium bicarbonate by weight of the silicate solution. Sufiicient boracic acid was then incorporated in the solution to'siibs tahtially completely remove free alkali therefrom while avoiding precipitation of silioic acid. About pints (Imperial) of this solution weighing about 9 pounds were placed in a mechanical mixing machine and about 20 pounds of very fine ginc dust substantially free of zinc oxide were added.' The mixture was then worked in the mixing machine for about 4 minutes, after which approximately 10% of red leadaby weight of the solution (i. e., about 1 pound) was added, and mixing was continued for an additional 4 to 10 minutes. The compounds of sodiuni'blcarbonate were added to isoo'poundmtfesiliatesoliition by addinl 12.

gallons (Imperial) of a solution onsodium bi; carbonate saturated at 60 F. to 100 gallons (1m- 5 periai) 5f the sodium silicate solution. About 20 position was promptly applied to a metal surface with a brush and the applied coating was allowed to dry in the air at temperate humidity and temperature conditions for a period of about 36 hours. The resulting coating was hard, insoluble, adherent, and abrasion-resistant and fully protected the metal surface against corrosion by outdoor atmospheric conditions.

An a Pifi Q i i p lg 13 32. $1 .93.??? as paredliaving a specific gravity of approximately 1.3 and a l'a2'O to sip ratio of 1:2 .4 To 100 gallons (Imperial) oi thissolution were added 2.5 pounds of boracic acid, and 2.5 pounds of sodium bicarbonate, both in the form of saturated" solutions at'room temperature. About 6 pints (Imperial) of the resulting solution, weighing approximately 10 pounds, were placed in a mechanical mixer and pounds of commercial aing dust were added. After a few minutes interval 2 pounds of rgd lead were added and mixing was continued for an additional period of several minutes. The coating obtained by airdrying the composition applied to a steel surface was hard, insoluble, adherent and abrasion-resistant and fully protected the steel surface against corrosion.

III

An aqueous solution of sodium silicate having a Nazqtgfiiguatio of 1 was prepared havpounds of glue dust and 2 pounds of red l ead} were incorporatedin 6 pints (Imperiafi of the" silicate-bicarbonate solution and the resulting composition was applied with a brush to a steel surface. The coating was allowed to dry in the open atmosphere and formed a hard. insoluble,

abrasion-resistant and corrosion-resistant coatmg on the steel surface.-

The aqueous solution of alkali silicate contain ing either sodium bicarbonate alone or sodium bicarbonate and boracic acid is stable .over a long period of time; Upon the addition of the metallic zinc and lead oxide to the silicate solution, however, the mixture becomes unstable in that it tends to set. The time required for this setting to become objectionable ranges from about 10 minutes in .an'atmosphere at a temperature substantially in excess of to about 10 hours in a cold atmosphere. Accordingly, the solid ingredients of the composition (the finely divided zinc and the lead oxide) should not be incorporated in e silicate solution until just before the composition is to be applied .to the metal surface to be protected. In general, the coating composition of the invention-should be applied to the ferrous metal surface within about 15 to 45 minutes after incorporation of the metallic zinc and lead oxide in the silicate solution. For this reason, it is expedientto prepare the silicate solution in large quantities but to add the solid ingredients to only so much of the silicate solution'as will make up a volume of coating composition which can be applied to the ferrous surface within the aforementioned time limit.

The zinc ang lead one em r'be addedssnareiel to t e silicate so ution or theyrnaybe combined in the desired proportion for incorporation in the silicate solution as a single addition. Thus, the coating composition is advantageously packaged for field and other use in the form of two complementary units (one the liquid vehicle and the other the solid ingredients) adapted, when their contents are admixed, to produce the composition of the invention in condition for immediate use.

It will be understood also that, if desired, pigments and/or fillers may be included in the above mentioned coating mixtures either to provide increased body or *to' modify the appearance, whether as to-color or otherwise, of the resultant coatings.

This application is a continuation-in-part of an application for Letters'Patent of the United States, filed March 18, 1947, Serial No. 735,534, now abandoned, which application was a continuation-impart of an'application for Letters Patent of the United States, filed July 3, 1945, Serial No. 603,116, both of said applications having been filed by Victor Charles John Nightingall.

It-isclaimed:

1. The method of producing a hard protective coating .on a ferrous metal surface which comprises applying to the surface a coating of a composition comprising finely divided metallic zinc and finely divided lead oxide incorporated in an aqueous solution of an alkali silicate and sodium bicarbonate, the aqueous solution containing about 20% to 40% by weight of the alkali silicate and about 0.1% to 0.8% by weight. of soing a specific gravity of'about 1.3. About 10.5 '"Ldium bicarbonate, the zinc being present in 7 amount of about 100% to 350% by weight of said solution and the lead oxide being present in amount of about to 20% by weight of the zinc, and drying the applied coating.

2. The method of producing a hard protective coating on a ferrous metal surface which comprises applying to the surface a coating of a composition comprising finely divided metallic zinc and finely divided lead oxide incorporated in an aqueous solution of an alkali silicate and sodium bicarbonate, the aqueous solution containing about 30% by weight of the alkali silicate and about 0.2% to 0.8% by weight of sodium bicarbonate, the zinc being present in amount of about 140% to 280% by weight of said solution and the lead oxide being present in amount of about 10% to by weight of the zinc, and drying the applied coating.

3. The method of producing a hard protective coating on a ferrous metal surface which comprises applying to the surface a coating of a composition comprising finely divided metallic zinc and finely divided lead oxide incorporated in an aqueous solution of an alkali silicate and sodium bicarbonate, the aqueous solution containingabout by weight of the alkali silicate and about 0.8% by weight of sodium bicarbonate, the zinc being present in amount of about 200% by weight of said solution and the lead oxide being present in amount of about 10% by weight of the zinc, and drying the applied coating.

4. The method of producing a hard protective coating on a ferrous metal surface which comprises applying to the surface a coating of a composition comprising finely divided metallic zinc and finely divided lead oxide incorporated in an aqueous solution of an alkali silicate containing sodium bicarbonate and boracic acid, the aqueous solution containing about 20% to by weight of the alkali silicate, about 0.1% to 0.8% by weight of sodium bicarbonate, and the boracic acid being added to the solution in amount sufldcient to react with the bulk of the free alkali in the solution but insuflicient to cause precipitation of silicic acid, the zinc being present in amount of about 100% to 350% by weight of said solution and the lead oxide being present in amount of about 5% to 20% by weight of the zinc, and drying the applied coating.

5. The method of producing a hard protective coating on a ferrous metal surface which comprises applying to the surface a coating of a composition comprising finely divided metallic zinc and finely divided lead oxide incorporated in an aqueous solution of an alkali silicate containing sodium bicarbonate and boracic acid, the aqueous solution containing about 30% by weight of the alkali silicate, about 0.2% to 0.8% by weight of sodium bicarbonate, and the boracic acid being added to the solution in amount sufilcient to react with the bulk of the free alkali in I the solution but insufllcient to cause precipita- 8 taining sodium bicarbonate and boracic acid, the aqueous solution containing about 30% by weight of the alkali silicate, about 0.2% by weight of sodium bicarbonate, and about 0.2% by weight of boracic acid, the zinc being present in amount of about 200% by weight of said solution and the lead oxide being present in amount of about 10% by weight of the zinc, and drying the applied coating.

7. A ferrous metal article having on the surface thereof a hard protective coating resulting from the application thereto and the drying thereon or a composition comprising finely divided metallic zinc and finely divided lead oxide incorporated in an aqueous solution of an alkali silicate and sodium bicarbonate, the aqueous solution containing from about 20% to 40% by weight of the alkali silicate and from about 0.1% to 0.8% by weight of the sodium bicarbonate, the zinc being present in amount of about to 350% by weight of said solution, and the lead oxide being present in amount of about 5% to 20% by weight of the zinc.

8. A ferrous metal article having on the surface thereof a hard protective coating resulting from the application thereto and the drying thereon of a composition comprising finely divided metallic zinc and finely divided lead oxide incorporated in an aqueous solution of an alkali silicate and sodium bicarbonate, the aqueous solution containing about 30% by weight of the alkali silicate and about 0.8% by weight of the sodium bicarbonate, the zinc being present in amount of about 200% by weight of said solution, and the lead oxide being present in amount of about 10% by weight of the zinc.

9. A ferrous metal article having on the surface thereof a hard protective coating resulting from the application thereto and the drying thereon of a composition comprising finely divided metallic zinc and finely divided lead oxide incorporated in an aqueous solution of an alkali silicate, sodium bicarbonate and boracic acid, the aqueous solution containing from about 20% to 40% by weight of the alkali silicate, from about 0.1% to 0.8% by weight of the sodium bicarbonate, and an amount of boracic acid sufilcient to react with the bulk of the free alkali in the solution but insufficient to cause precipitation of silicic acid, the zinc being present in amount of about 100% to 350% by weight of said solution, and. the lead oxide being present in amount of about 5% to 20% by weight of the zinc.

10. A ferrous metal article having on the surface thereof a hard protective coating resulting from the application thereto and the drying thereon of a composition comprising finely divided metallic zinc and finely divided lead oxide incorporated in an aqueous solution of an alkali silicate, sodium bicarbonate and boracic acid, the aqueous solution containing about 30% by weight of the alkali silicate, about 0.2% by weight of the sodium bicarbonate, and about 0.2 by weight of boracic acid, the zinc being present in amount of about 200% by weight of said solution, and the lead oxide being present in amount of about 10% by weight of the zinc.

RUTH VALENCIA NIGHTINGALL,

AUBREY MITCHELL OLD,

CHARLES MONTEFIORE GLADSTONE NIGHTINGALL, Executors of the Estate of Victor Charles John Nightinoall, Deceased.