US3905837A - Method of treating titanium-containing structures - Google Patents

Abstract

A method is disclosed for the removal of metallic iron inclusions from the surfaces of chemical process equipment, particularly electrolytic cells, prepared from titanium. This method involves the treating of the titanium alloy surface with an acid solution to remove metallic iron inclusions from those surfaces of the titanium that are subject to attack by halides.

Description

United States Patent 1191 Du Bois 1 51 Sept. 16, 1975 METHOD OF TREATING TITANIUM-CONTAINING STRUCTURES [75] Inventor: Donald W. Du B015, Corpus Christi,

Tex.

[73] Assignee: PPG Industries, Incl, Pittsburgh, Pa.

[22] Filed: Jan. 4, 1974 [21] Appl. No.: 430,990

Related US. Application Data [62] Division of Ser. No. 239,991, March 31, 1972, Pat.

52 us. c1 148/6.l4 R; 117/49; 252/142; 134/41 1511 1111. c1 C23g 1/02 {58] Field of Search 148/614 R; 117/49; 134/3, 134/41; 252/142, 143, 144, 145

[56] References Cited UNITED STATES PATENTS 2,827,402 3/1958 Albcrs et a1. 252/145 2,830,536 4/1958 Wood et al. 134/41 2,856,275 10/1958 Otto 148/614 R 3,457,103 7/1969 Keller et al. 148/6.14 R 3,817,844 Kendall 134/3 Primary E.\'ami r 1erRalph S. Kendall Assistant EX'dmirzer-Charles R. Wolfe, Jr. Attorney, Agent, or FirmRichard M. Goldman [57] ABSTRACT *9 Claims, N0 Drawings METHOD OF TREATING TITANIUM-CONTAINING STRUCTURES This is adivision, of application Ser. No. 239,991, filed Mar. 31," 1972, and now US. Pat. No. 3,836,410.

BACKGROUND OF THE INVENTION In the construction of chemical process equipment for use in environments where free halogens are present, such as electrolytic cells for the production of chlorine by the electrolysis of brines, titanium is frequently used as a material of construction. This is because of the tendency of titanium to form a corrosionresistant film under oxidizing conditions. However, such titanium reaction vessels are particularly subject to attack under conditions of oxygen depletion at crevices such as welds, joints, laps, filletts, and the like. This type of corrosion is characterized as crevice corrosion."

attempts to prevent crevice corrosion or to substantially reduce the effects of it have typically focused on various organic coatings on the titanium, inorganic coatings on the titanium, and the use of titanium alloys. One particularly satisfactory titanium alloy is an alloy of titanium and nickel. Such a titanium alloy and its use in halide solutions is disclosed in US. Pat. No. 3,469,975 to Bertea et al. for Method of Handling Crevice Corrosionlnducing Halide Solutions". The alloy disclosed by Bertea et al. is a titanium alloy containing up to about 5 per cent nickel, at least about 0.3 per cent cobalt, and about 2.0 per cent molybdenum. Further disclosed therein is the corrosionresisting effect of small additions of cobalt and molybdenum to titanium. While such titanium alloys exhibit marked crevice-corrosion properties when tested as coupons in heated brine, it has been found that after various types of mechanical working and fabricating operations, their susceptibility to crevice corrosion suffers a marked increase.

SUMMARY OF THE INVENTION It has now surprisingly been found that the presence of unalloyed surface iron, even in amounts as low as 5 parts per million, results in a marked increase in susceptibility to crevice corrosion of titanium and titanium alloys. So sensitive to the presence of iron is the alloy, that iron inclusion sufficient to materially increase susceptibility to crevice corrosion will be introduced to the alloy during mechanical working. Such inclusions are normally the result of various metal working and fabricating processes quite apt to be used. 7

It has further been found that the crevice corro sionresistant properties of such worked and fabricated titanium and titanium alloys may be maintained substantially undiminished if, subsequent to the working and fabricating processes, particular care is taken to remove metallic iron inclusions therefrom.

According to this invention, metallic iron inclusions are removed by treatment of the worked or fabricated surface with an aqueous liquid composition containing two acids, one of which is an oxidizing acid and the second of which is capable of reacting with iron to form soluble iron salts.

Best results are obtained if the treatment of the worked or fabricated titanium or titanium alloy surface is continued until the amount of iron inclusions remaining in the surface is low enough that the susceptibility of the article tocrevice corrosion is substantially reduced. Such a concentration of metallic iron inclusions is typically shown by the negative response of the surface of the article to standard tests for metallic iron.

DETAILED DESCRIPTION OF THE INVENTION According to this invention, titanium or titanium alloy structures subject to corrosion in oxygen defi cient, halogen-containing environments may be rendered resistant to crevice corrosion by the removal of metallic iron inclusions from the titanium.

Titanium structures are preferred for use in the processing of halides and halogens. Such structures include apparatus for the desalinization of brines and brackish water, electrolytic cells for the production of chlorate, chlorine, and other halides, and various other chemical processing equipment. Titanium and its alloys are preferred materials of construction because of their tendency to form a corrosion-resisting film under oxidizing conditions. This film protects the underlining titanium material from further corrosive or oxidative attack. However, titanium structures are attacked at laps, filletts, crevices, compression fittings, and the like. They are also" attacked under gaskets at edges of compression fittings, joints, laminations, and the like. This phenomena is referred to as crevice corrosion."

According to US. Pat. No. 3,469,975 to Bertea et al. for Method of Handling Crevice Corrosion-Inducing Halide Solutions," it is known that small amounts of alloying materials significantly increase the resistance of titanium structures to crevice corrosion. Such alloying materials include nickel, cobalt, and molybdenum. In the crevice corrosionresistant alloys of Bertea et al., the quantity of nickel'present is from about 1.0 per cent to about 5 per cent; the amount of cobalt present is from about 0.3 percent to about 5 per cent; and the amount of molybdenum present is in the range of about 2 per cent. Alloyed iron, up to about 0.1 weight per cent may also be present in the alloy. Additionally, alloys of titanium with the precious metals, such as 0.2 per cent palladium alloysof titanium, have been found to be remarkedly resistant to crevice corrosion. Typical crevice corrosion-resistant alloys useful in the practice of this invention also include titanium alloys containing nickel, cobalt, molybdenum, niobium, aluminum, and tantalum. These alloying elements may be present singly as in Ti-Ni", Ti-Co, and Ti-Mo, or in combinations as in Ti-ALNb-Ta-Mo, and Ti-Al-Nb-Mo. In such alloys useful with the process of this invention, titanium is the major constituent, being or or more weight per cent of the alloy. Whenever titanium is referred to herein it will be understood to include alloys having titanium as a-rrlajor constituent. It has been found that such materials as described hereinabove are particularly resistant tocrevice corrosion when tested in the form of coupons where the crevices are formed by laboratory compressive means. However, these same materials suffer a significantly reduced resistance to crevice corrosion after fabrication into chemical processing apparatus.

It has now been found that the reduced resistance to crevice corrosion of normally crevice corrosionresistant titanium and titanium alloys is caused by the presence of any metallic iron inclusions in the alloy sufficient to cause pitting. This is generally about 5 parts per million of unalloyed, metallic iron. Such metallic iron inclusions are introduced during the manufacturing and fabricating steps. They may be introduced during the i'olling ,bendih'g forging, forming, and shaping a second acid capable of forming soluble iron salts.

Preferablyfthe'acid treatment is continueduntil the concentration of metallic iron in the alloy surface is reduced below that level at which crevice corrosion is initiated. This is usuallybelow about 5 parts per million on the surface and, for practical purposes, below the" level below which standard wet chemical analytical tests for iron are negative.

The aqueous liquid composition used in the practice of this invention contains, as its chemically active ingredients, two acids. The first acid is an oxidizing acid. By oxidizing acid is meant an acid that contains oxygen and that is capable of reacting with titanium to form an insoluble oxide surface on the titanium. Nitric acid is the preferred oxidizing acid. Perchloric acid and chromic-acid may also be used. The second'acid is an acid capable of reacting with iron to form soluble salts of iron. Suitable acids include the halo acids; hydrofluoric acid, hydrochloric acid, and hydrobromic acid. Hydrochloric acid is the preferred halo-acid. Hydrofluoric acid may also be used in the practice of this invention, although care must be taken to stop the treatment of the surface before the hydrofluoric acidbegins to solubilize the titanium. Hydrobromic acid may also be used in the practice of this invention although the higher cost of hydrobromic acid may render it'less attractive than hydrochloric acid. Suitable acids also include strong organicacids capable of giving up hydronium ions, such as the tri-halogenated acetic acids, trichloroacetic acid and trifluoroacetic acid. Satisfactory results are also obtained with sulfuric acid.

The liquid composition used in the practice of this invention typically contains from about 5 to about 20 volume per cent of the oxidizing acid and, preferably, from about 7 /2 to about 15 volume per cent of the total liquid composition. When volume per cents are referred to herein, such volume per cents are based on the volumes'of the original reagents prior to mixing and do not include effects. When the oxidizing acid is nitric acid, the concentration of nitric acid is preferably from about 7 /2 to about 15 volume per cent.

The concentration of the second acid isp'referably from about 10 to about 40 volume per cent. When the second acid is hydrochloric acid, good results are obtained at from about to about 40 volume per cent of hydrochloric acid. Best results are obtained at about to about volume per cent hydrochloric acid. When the second acid is hydrofluoric acid, particularly satisfactory results are yielded in the rangeof from about 5 to about 10 volume per cent hydrofluoric acid.

In a preferredexemplification of this invention, nitric acid is theoxy geri-containing' acid, and hydrochloric acid is the second 'acid'. According to this exemplification, liquid composi tions having particularly satisfactory iron removal properties are provided in the range of from 'about'5 to about IS-volu'mepercent; nitric acid and from about 15 to abouti40 volume per cent hydrochloric acid. The preferred liquid composition .of this' exemplification contains from about 7 to about 15 volume per cent and preferably about 10 volume per cent nitric acid, and from about 20 to about 40 volume per. cent hydrochloric acid, and preferably about 30 volume per cent hydrochloric acid.

Lower concentrations of the second acid, for example, less than about 10 volume pencent hydrochloric acid or lower than about 1 volume per cent hydrofluoric acid may be used to remove iron inclusions from titanium materials. However, such low concentrations of the second acid result in unnecessary long periods of treatment, for example, inn excess of about 30 to about 45 minutes. Similarly, particularly high concentrations of the second acid, for example, liquid compositions of 40 volume per cent hydrochloric acid and 10 volume per cent nitric acid also require in excess of 45 minutes at 25C. to 30C. to remove substantially all of the iron. I

When the second acid is hydrofluoric acid, liquid compositions containing, for example, .10 volume per cent nitric acid'and in excess of about;;10 volume per cent nitric acid and in excess of about 10 volume per cent hydrofluoric acid, result in solubilization of the titanium.

The temperature of the liquid composition is such as to keep it a liquid; that is, between the freezing temperature and reflux temperature. Particularly satisfactory results are obtained at temperatures of from about 5C. to about 50C. Temperatures above about 50C. do not result in any significantly increased rate of removal of iron inclusions or in decreased treatment time. Furthermore, such temperatures give rise. to problems related with solubilization of the titanium. Accordingly, there is no incentive to go to temperatures above about 50C., although such temperatures are included within the scope of this invention. Temperatures below about 5C. result in a significantly increased times for the removal of the iron inclusions and, therefore, no incentive exists to go to temperatures below about 5C. although such temperatures are also included within the scope of this invention.

The time of treatment varies from about 5 minutes or less up to about 4.5 minutes or even longer. As described hereinabove, the time of treatment is a function ofthe concentrations and proportions of acids in the liquid composition and the temperature of the liquid composition. For example, at high temperatures the time necessary to, obtain substantially complete removal-of iron'as determined by standard colorimetric tests is less than the time necessary to obtainthe same degree of iron removal at lower temperatures. Similarly, in a liquid composition containing 10 volume per cent nitric acid and 20 volume per centhydrochloric acid, the time required to obtain substantially complete iron removal is less than the time required to obtain an equivalent degree of iron removalw ith liquidcompositions containing significantly. more or; significantly less hydrochloric acid.. J, a

In a further exemplification of thisinvention, reduced treatment times may-.be obtained :abrading the titanium alloy surface prior, to treatment with the acid compositions. The titanium alloy surface of the apparatus may be. abradedby anysoff the methods normally used for the abrasivecleaping of metal surfaces,

forexample;a power driven wire wheel may be used to remove-surface inclusions of metallic iron. In the case removed by the abrasive will bemoreareadily removed by the liquid composition than the iron inclusions dee;

posited during the manufacturing process. Alternatively, sandpaper or sand blasting may be usedias a suit able abrasive method for thepreliminaryremoval of iron inclusion prior to theuseof the acid composition.

While the invention has been described and illustrated with respect to certain alloys of titanium it is to be understood that the tendency of titanium articles to suffer crevice corrosion may also be reduced by the use of the acid composition method of this invention either where .other titanium alloys or unalloye d titanium itself are used. In order that those skilled in the art may more completely, understand the present invention and the preferred methods by which'the same may be carried out, the following specific examples are offered:

EXAMPLES [THROUGH VIII In each of Examples 1 through VIII, a 1 inch by 2 inch by 0.008 inch titanium coupon was used for testing. An analysis of the titanium coupons prior to testing showed a nickeLco'ntent of 1.42 per cent and an iron content of 0.007 weight per cent. Each coupon was then contam inated with iron on o'nesurface. This was done by clamping the coupon in a vise 'on a drill press. A blunt iron rod was inserted in'the drill press bit, the drill press turnedon, and' the iron rod pressed onto the coupon.

Thereafter, each coupon was inserted-into an acid solution at a temperature of 25 to 30C. as described hereinafter. Each coupon wasremov'edfrom the acid solution after minutes of immersion for the purpose of testing the surface for iron content. If the coupon showed the presence ofsurface iron, it was reimmersed in the acid solution for 5 minutes and tested-again. This was continued until the test-forsurface iron was negative.

The surfaee irons content was determined according to the method of ASTM A-380-57, paragraph 7, (c)( l), by the presence or absence of a darkblue color. .According to this method on aqueous indicator solution containing grams of potassium ferric cyanide (K Fe(CH) and milliliters of concentrated ,(70 weight per cent) nitric acid per liter prepared. Each time a coupon was removedfrorn the acid solution, it wasrin'sed with distilled water andseyeral drops of indicator solution were applied to' the surface. Iron was considered present if the indicator turned dark blue on the surface of the titanium coupon within 30 seconds. i

EXAMPLE I A titanium alloy coupon prepared and contaminated withiron as described above was inserted in a 10 volume per cent solution of nitric acid. The coupon was tested every 5 minutes of immersion for the presence of iron for 30 minutes. After 30 minutes there was still sufficient iron contamination present on the surface to turn the indicator dark blue.

EXAMPLE II A titanium alloy coupon prepared and contaminated 6 with iron as described above was inserted in a 1 volume per cent solution of hydrochloricacid. The coupon was removed from the acid after every 5 minutes ofimmersion and tested, for the presence of' ir on contamination as describedhereinabove; After3 O'minutesQt here was still sufficient ironcontarnination'on the surface to turn the indicator dark blue within seconds 'a s described hereinabove.

i XAMPLE III A'titanium alloy coupon prepared and contaminated as described hereinabove was inserted in a solution.

containing 10 volume per cent nitric acid and 10 volume per cent hydrochloric acid. The coupon was removed and tested forthepresence of surface iron contamination after-every 5 minutesof immersion .as described hereinabove. After. 30 -minutes there was still enough iron contamination on the surface to turn the.

indicator dark bluewithin. 30 seconds as described hereinabove.

EXAMPLE iv A titanium alloy coupon was prepared and contaminated with iron as described hereinabove. It was inserted in a solution containing 10 volume per cent nitric acid and 20 volume per cent hydrochloric acid. The coupon was removed from the acid mixture and tested for the presence of surface iron contamination after every 5 minutes of immersion as described hereinabove. After 20 minutes; the surface'contamination of iron was reduced sufficiently that the indicator did not turn blue within 30 seconds.

. EXAMPLE V a A titanium alloy coupon prepared and contaminated with iron as describedhereinabove was inserted in a'solution'containing 10 volume per cent nitric acid and 30 volume per cent hydrochloric acid. After every 5 minutes of immersion the coupon'was removed from the solution and tested for thepresence of iron as described hereinabove. After 25 minutes there was insuf ficient iron contamination remaining on the surface of the coupon to change the indicator to dark blue within 30 seconds.

EXAMPLE v1, l

EXAMPLE VII A titanium alloy coupon was prepared and contaminated with iron as described hereinabove. The coupon was inserted in a mixture of 10 volume per cent nitric acid and 5 volume per cent hydrofluoric acid. The coupon was removed and tested for surface iron contamination after every 5 minutes of immersion as described hereinabove. After 20 minutes of immersion in the acid mixture there was no longer sufficient surface iron contamination remaining on the coupon to change the indicator blue within 30 seconds.

EXAMPLE IX A titanium alloy coupon was prepared and contaminated with iron as described hereinabove. The coupon was brushed with a wire brush sanding wheel for approximately minutes at normal hand pressure. Thereafter the coupon was washed with distilled water and immersed in a solution of volume per cent nitric acid and 20 volume per cent hydrochloric acid. The coupon was removed after every 5 minutes of immersion to test for iron contamination as described hereinabove. After 5 minutes of immersion there still remained on the surface of the coupon sufficient iron contamination to turn the indicator blue within 30 seconds. After 10 minutes of total immersion time, there was insufficient iron remaining on the surface of the coupon to turn the indicator blue within 30 seconds.

EXAMPLE x A titanium alloy coupon was prepared and contaminated with iron as described hereinabove. The coupon was sanded with normal 120 220 mesh fine emery cloth at hand pressure for approximately 5 minutes. Vertical and horizontal strokes were applied randomly. Thereafter the surface of thecoupon was washed with distilled water and inserted in a solution containing 10 volume per cent nitric acid and 20 volume per cent hydrochloric acid. The coupon was removed from the solution after every 5 minutes of immersion to test iron contamination as described hereinabove. After 5 minutes of total immersion and after 10 minutes of total immersion sufficient iron contamination remained on the surface of the coupon to turn the indicator solution blue within 30 seconds. After minutes of total immersion there was insufficient iron contamination remaining on the surface of the coupon to turn the indicator blue within 30 seconds.

EXAMPLE Xl A titanium alloy coupon is prepared and contaminated as described hereinabove. A solution containing 10 per cent nitric acid and 20 per centsulfuric acid is prepared. The coupon is inserted inthe'acid solution. After every 5 minutes of immersion in the solution the coupon is removed from the solution, rinsed with distilled water, and tested for the presence of surface iron contamination. After 30 minutes of total immersion there is insufficient surface iron contamination to turn the indicator blue within 30 seconds.

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.

l claim:

1. A method of removing metallic iron inclusions introduced during fabrication from fabricated, scale-free titanium structures intended for use in crevice corrosion susceptible environments which method comprises I contacting the titanium structure with an aqueous acid composition comprising a first acid capable of oxidiz- 'ing titanium which acid is chosen from the group consisting of nitric acid, chromic acid, and perchloric acid, and a second acid chosen from the group consisting of HF, 'HCl, l-lBr, HI, H CCI COOH, and CF COOH, capable of forming soluble iron salts; and maintaining the acid composition in contact with titanium until the surface concentration of iron in the titanium is reduced below five parts per million.

2. The method of claim 1 wherein thetitanium is in an alloy comprising nickel.

3. The method of, claim 1 wherein the oxidizing acid isHNO I 4. The method of claim 3 wherein the acid composition comprises from about 5 to about 15 volume per cent HNO 5. The method of claim-l wherein the second acid, capable of forming a soluble iron salt, is HCl.

6. The method of claim 5 wherein the acid composition comprises from about 15 to about 40 volume per cent HCl.

7. The method of claim 1 wherein the said. aqueous acid composition comprises nitric acid and hydrochloric acid. ""8. The method of claim 7 wherein the acid composition comprises from about 5 to about 15 volume per cent nitric acid and from about 15 to about 40 volume per cent hydrochloric acid. I

9. Titanium treated by the method of claim 1.

Claims (9)

1. A METHOD OF REMOVING METALLIC IRON INCLUSIONS INTRODUCED DURING FABRICATION FROM FABRICATED, SCALE-FREE TITANIUM STRUCTURES INTENDED FOR USE IN CREVICE CORROSION SUSCEPTIBLE ENVIRONMENTS WHICH METHOD COMPRISES CONTACTING THE TITANIUM STRUCTURE WITH AN AQUEOUS ACID COMPOSITION COMPRISING A FIRST ACID CAPABLE OF OXIDIZING TITANIUM WHICH ACID IS CHOSEN FROM THE GROUP CONSISTING OF NITRIC ACID, CHROMIC ACID, AND PERCHLORIC ACID, AND A SECOND ACID CHOSEN FROM THE GROUP CONSISTING OF HF, HCI, HBR, HI, H2SO4, CCI3COOH, AND CF3COOH, CAPABLE OF FORMING SOLUBLE IRON SALTS, AND MAINTAINING THE ACID COMPOSITION IN CONTACT WITH TITANIUM UNTIL THE SURFACE CONCENTRATION OF IRON IN THE TITANIUM IS REDUCED BELOW FIVE PARTS PER MILLION.

2. The method of claim 1 wherein the titanium is in an alloy comprising nickel.

3. The method of claim 1 wherein the oxidizing acid is HNO3.

4. The method of claim 3 wherein the acid composition comprises from about 5 to about 15 volume per cent HNO3.

5. The method of claim 1 wherein the second acid, capable of forming a soluble iron salt, is HCl.

6. The method of claim 5 wherein the acid composition comprises from about 15 to about 40 volume per cent HCl.

7. The method of claim 1 wherein the said aqueous acid composition comprises nitric acid and hydrochloric acid.

8. The method of claim 7 wherein the acid composition comprises from about 5 to about 15 volume per cent nitric acid and from about 15 to about 40 volume per cent hydrochloric acid.

9. Titanium treated by the method of claim 1.