United States Patent (19) 11 Patent Number: 4,473,446

Locke et al. 45 Date of Patent: Sep. 25, 1984

54 CHROMIC ACID-FLUORIDE ANODZNG 58) Field of Search ................. 204/32 R, 56 R, 38A
SURFACE TREATMENT FOR TITANUM
56) References Cited
75 Inventors: Melvin C. Locke; Joseph A. Marceau; U.S. PATENT DOCUMENTS
Kevin M. Harriman, all of Seattle,
Wash. 3,959,091 5/1976 Moji et al. ........................ 204/38 A
(73) Assignee: The Boeing Company, Seattle, Wash. Primary Examiner-R. L. Andrews
Attorney, Agent, or Firm-Christensen O'Connor,
(21) Appl. No.: 456,265 Johnson & Kindness
22 Filed: Jan. 6, 1983 (57) ABSTRACT
Related U.S. Application Data A method for surface treating titanium articles prior to
adhesive bonding by anodization in a chromic-hydro
63 Continuation of Ser. No. 259,374, May 1, 1981. fluoric acid bath at a low anodizing potential of greater
51 nt. C.3 .... ... C25D 11/02; C25D 1 1/34 than one volt but less than five volts,
52 U.S. C. ................................ 204/32.1; 204/56 R;
204/38. A 12 Claims, No Drawings
 1.
4,473,446
2
CHROMIC ACID-FLUORIDE ANODIZING DETAILED DESCRIPTION OF THE
INVENTION
SURFACE TREATMENT FOR TITANUM
The process of the present invention will produce
This is a continuation of the prior application Ser. 5 titanium oxide coatings on titanium articles that, when
No. 259,374, filed May 1, 1981, the benefit of the filing incorporated into an adhesively bonded system, provide
dates of which are hereby claimed under 35 USC 120. environmentally stable bonds superior to those obtained
BACKGROUND OF THE INVENTION with otherwise identical systems in which the titanium
coating was produced by other methods. The process of
Adhesively bonded titanium structural components 10 the present invention varies from the prior processes,
have been used in aircraft applications for many years. especially that disclosed in the aforementioned patent
Service experience with these bonded structures has issued to Moji and Marceau, in that the anodizing po
been varied with frequent failures from debonding of tential is maintained well below those levels heretofore
titanium articles. The bonding failures can be attributed thought necessary. Additionally, the titanium article
to the different interfacial structures of the variously 15 can be subjected to an alkaline etching step in a rela
treated adherends. Numerous titanium surface treat tively mild alkaline etching solution prior to anodiza
ments have been developed and used to promote adhe tion. Alternatively, or additionally if desired, the tita
sion and reduce the number of bonding failures. Among nium article can also be subjected to an acid pickling
these are the method of anodizing titanium to promote step prior to anodization.
adhesion disclosed in the U.S. Pat. No. 3,959,091, issued
20 Wedge, peel, and lap shear tests conducted in accor
to Y. Moji and J. A. Marceau. Although the latter treat dance with standard methods show no, or only a very
ment significantly increases the bond performance of small amount of, oxide-metal interfacial failure when
titanium articles, under certain conditions an apparent the conditions of the present invention are observed.
brittle oxide failure weakness still occurs near or at the 25 The oxide characteristics produced by the low-voltage
metal oxide primer interface. anodization produce oxide coatings that are uniformly
porous and have a columnar structure. This oxide is
It is therefore an object of the present invention to very receptive to polymeric adhesive materials. Fur
eliminate this apparent brittle oxide failure weakness thermore, the process of the present invention is rela
while maintaining the bonding strength of titanium tively simple to practice in a production situation and
articles at or equal to that achieved by the method dis 30 does not require etching with a strong alkaline solution
closed in the aforementioned patent. maintained at high temperatures.
Titanium articles to be anodized in accordance with
SUMMARY OF THE INVENTION the present invention include titanium alloys, such as
In accordance with the foregoing objects and other the alloy Ti-6Al-4V. The titanium articles are prefera
objects that will become apparent to one of ordinary 35 bly precleaned with a conventional vapor degreaser or
skill in the art after reading the ensuing specification, solvent cleaner. This precleaning step removes the oil
the present invention provides a process for forming a and other water-insoluble materials from the titanium
porous adhesion-promoting oxide coating on a titanium surface prior to an alkaline cleaning step.
article by anodizing the article at a relatively low anod 40 After precleaning, the article is subjected to a mild
ization potential. Prior to anodization the surface of the alkaline etch at moderate temperatures. A suitable,
titanium article can be etched with a relatively mild commercially available, alkaline etching solution that
alkaline etching solution maintained at a temperature of can be utilized in accordance with the present invention
from about 160' F. to about 220 F. The titanium article is sold under the trade name "Kelite 235' and is manu
is immersed in the solution for a period of at least about factured by Allied-Kelite Product Division, The Rich
15 minutes and preferably from about 18 to about 20 45 ardson Company, Des Plains, Ill. It is preferred that the
minutes. Alternatively, the titanium article can be im alkaline etchant be mixed with water to produce a mild
mersed first in a mild alkaline cleaning solution fol alkaline solution having a pH preferably in the range of
lowed by an acid pickle in a conventional nitric acid 9.0 to 10.0. The Kelite 235 solution can, for example, be
hydrofluoric acid pickling bath. mixed with water in amounts ranging from 10 to 15
Thereafter, the article is removed from the alkaline 50 ounces per gallon of water to produce the desired etch
etching solution or pickling bath, water rinsed, and ing solution. During the cleaning step, the alkaline solu
anodized in an aqueous solution comprising fluoride tion is maintained at a temperature ranging from 160 to
ions and an oxidizing electrolyte. The pH of the anodiz 220 F., but preferably in the range of 160' to 180° F.,
ing solution is maintained at less than about 6.0 while making the solution much easier to handle than prior art
the temperature of the solution is maintained at at least
55 processes that have required very strong alkaline etch
about 50 F., and preferably from 60' F. to 70 F. The ing solutions to be maintained at higher temperatures.
anodizing potential is maintained at greater than one The titanium article is immersed in the alkaline etching
volt and less than five volts. The fluoride ion concentra solution for at least about 15. minutes and preferably
tion is maintained at a level that results in a current 60 from 18 to 20 minutes. When the etching step is finished,
the titanium article is removed and rinsed with hot
density of from about 0.5 to about 3.0 amperes per water for about five to about eight minutes.
square foot. The article is anodized for at least five If the titanium article has been subjected to heat treat
minutes and preferably from 20 to 22 minutes. Once the ment or forming processes, scales sometime build up on
anodizing step has been completed, the article is re the article. These scales must be removed prior to bond
moved from the anodizing solution, rinsed with water, 65 ing by subjecting the titanium article to an acid pickling
and dried. It is important that the article be rinsed with step in a nitric acid-hydrofluoric acid pickling bath.
water within about two minutes from the cessation of Prior to pickling, the titanium article is preferably first
anodizing current. immersed in a conventional alkaline cleaning solution of
 3
4,473,446
4.
sufficient high concentration and temperature to pro adhesive cure cycle of 90 minutes at 250 F. and 50 psi
duce mild etch of the titanium. The alkaline cleaning was employed. Conventional peel tests were conducted
solition is preferably of the phospho-silicate type. A on various specimens in accordance with the conditions
typical alkaline cleaner will contain about 30% sodium set forth in the accompanying Tables I and II. The peel
metasilicate, about 35% caustic soda, about 9% soda 5 tests were conducted in accordance with ASTM
ash with the balance being sodium tripolyphosphate. D1781.
This pickling step is conventional and is disclosed, for Results of peel tests conducted on specimens anod
example, in the patent referenced above. Once the acid ized at various voltages from one volt to six volts are set
pickling step is completed, the titanium article is again forth in Table I. The current density was maintained at
rinsed with water. O one amp/square foot while the solution is agitated.
Thereafter, the titanium article is anodized in accor Constant current density was achieved by the addition
dance with the present invention. The anodizing bath of fluoride ions. It is clear that at voltage ranging from
| generally comprises an aqueous solution of fluoride ions two to about five volts a desirable cohesive failure mode
and an oxidizing electrolyte. The basic composition of was obtained while at less than two volts and greater
the anodizing solution is known in the art and disclosed 15 than five volts, undesirable adhesive failure developed.
in the above-referenced patent. A typical and preferred At less than two volts, adhesive failure occurred at the
bath comprises chromic acid and hydrofluoric acid. primer-oxide interface probably due to thin oxide or
The anodization is conducted at temperatures from 50 improperly developed oxide. At greater than five volts,
to about 80' F., but preferably in the range of from 60' adhesive failure occurred within oxide due to apparent
F. to 70 F. The anodizing bath preferably contains 20 brittle oxide.
about 5% chromic acid, although this concentration of Table II sets forth similar results for peel tests con
chromic acid is not critical. The fluoride ion concentra ducted on specimens that were anodized for twelve
tion is adjusted to result in a current density ranging minutes. It is seen that again cohesive failure is achieved
from 0.5 amperes per square foot to about 3 amperes per at anodization voltages of about two to four volts, while
square foot, but preferably from about 0.75 to 1.75 am 25 the adhesive failure begins to develop within the oxide
peres per square foot. The solution is continuously agi (due to brittle oxide) at anodization voltages of five
tated. The anodizing potential is maintained in accor volts. Wedge and lap shear tests also conducted on the
dance with the present invention at greater than one specimens indicated that the bonds formed between
volt and less than five volts, preferably from three volts titanium articles when pretreated in accordance with
to less than five volts, and most preferably from about 30 the present invention were as strong or stronger than
3.5 volts to about 4.5 volts. The anodization, step is those pretreated by prior art methods such as that of the
conducted for at least about 5 minutes and preferably aforementioned patent.
from about 20 to 22 minutes.
Once the anodization step is completed, the anodizing The foregoing invention has been disclosed in con
current is turned off and the titanium article removed 35 junction with a preferred embodiment and variations
from the anodizing bath. The article is then rinsed with thereof. Various changes and substitutions of equiva
cold water for at least about five minutes and therafter lents can be effected by one of ordinary skill in the art
hot air dried at a temperature from 140 to 160' F., for after reading the foregoing specification without de
example. It is very important that the article be rinsed parting from the general concepts disclosed herein. It is
within about two minutes after the cessation of the therefore intended that the scope of Letters Patent
anodizing current to prevent destruction of the oxide granted hereon be limited only by the definition con
coating by the anodizing solution. Once the titanium tained in the appended claims and equivalents thereof.
article is dried, it is preferred that it be primed within TABLE I
about 72 hours. A suitable polymeric primer is sold PEEL TEST
under the trade name "BR-127' by the American Cyan 45 Voltage Variation
amid Company. Thereafter, the articles can be bonded (10 Amp/ft) -
to similarly preconditioned articles or other articles Voltage
R.T.
b/in Failure Mode
Ave.
lb/in
with conventionally available polymeric adhesives such
as those sold under the trade names “FM-73' and "FM volt 16 Adhesive 17.4
300' by the American Cyanamid Company. 50 16 Adhesive
16 Adhesive
EXAMPLE 23 Adhesive
16 Adhesive
The surface treatment of the present invention was 2 volts 37
33
Cohesive
Cohesive
34.2
experimentally conducted upon several sample articles. 31 Cohesive
The articles were composed of a titanium alloy (Ti-6Al 55 36 Cohesive
4V) produced in accordance with MIL-T-9046. The 34 Cohesive
test specimens were surface treated in accordance with 3 volts 37 Cohesive 35.0
the present invention by first vapor degreasing, thereaf 33
35
Cohesive
Cohesive
ter subjecting to a mild alkaline etch with Kelite-235. 33 Cohesive
No acid pickle was employed. The articles were anod 37 Cohesive
ized immediately after being removed from the alkaline 4 volts 34 Cohesive 33.2
etch bath and water rinsed. All process conditions were 30 Cohesive
36 Cohesive
maintained within the preferred ranges in accordance 34 Cohesive
with the present invention unless otherwise noted. 32 Cohesive
After the articles were dried, they were primed, coated 65 5 volts 33 Cohesive 33.0
with an adhesive, and joined to similarly prepared arti 31
36
Cohesive
Cohesive
cles. The adhesive was then cured. The adhesive em 32 Cohesive
ployed was FM-73 while the primer was BR-127. An 33 Cohesive
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4,473,446
6
TABLE I-continued 50 F., the fluoride ion concentration of said solu
PEEL TEST tion being such as to result in a current density of
Voltage Variation from about 0.5 amperes per square foot to about
(10 Amp/ft) 3/0 amperes per square foot, the anodizing poten
R.T. Ave. tial of said solution ranging from greater than one
Voltage b/in Failure Mode b/in
volt to less than five volts, and
6 volts 13 Adhesive 13.0 rinsing said aqueous anodizing solution from said
12 Adhesive
13 Adhesive article with water.
13 Adhesive
O 2. The process of claim 1 wherein said anodizing
14 Adhesive solution is rinsed from said article within two minutes
after the cessation of the anodizing current.
TABLE II 3. The method of claim 1 wherein said anodizing
PEEL TEST
potential ranges from about three volts to less than five
15
volts.
Voltage Variation
(1.0 Amp/ft2) 4. The method of claim 3 wherein said anodizing
(Anodized 12 min). potential ranges from 3.5 volts to 4.5 volts.
R.T. Ave., 5. The method of claim 1 wherein said anodizing
Voltage b/in Failure Mode b/in temperature ranges from about 50 F. to about 80° F.
2 volts 37
33
Cohesive
Cohesive
34.2 20 6. The process of claim 5 wherein said anodizing
31 Cohesive
temperature of said anodizing solution ranges from
36 Cohesive about 60' F. to about 70 F.
34 Cohesive 7. The method of claim 1 further comprising the step
3 volts 32 Cohesive 33.8 of:
33 Cohesive
35 Cohesive 25 immersing said titanium article in an acid pickling
33 Cohesive bath prior to anodizing said article.
4 volts
36
36
Cohesive
Cohesive 33.6
8. The method of claim 1 wherein said anodizing
33 Cohesive solution comprises a solution of chromic acid and hy
35 Cohesive drofluoric acid having a pH of less than about 6.0.
29 Cohesive 30 9. The method of claim 1 further comprising:
5 volts
35
35
Cohesive
Cohesive 31.8
prior to anodizing said article, immersing said article
16 40%. Cohesive in a mild alkaline etching solution maintained at a
38 98% Cohesive temperature of about 160 F. to about 220 F. for a
35 Cohesive
35,
period of at least about fifteen minutes, and thereaf
35 95% Cohesive tet rinsing said etching solution from said article
with water.
The embodiments of the invention in which an exclu 10. The method of claim 9 wherein said alkaline etch
sive property or privilege is claimed are defined as ing solution is maintained at a temperature of from 160
follows: to about 180' .
1. A method of forming a porous adhesion-promoting 40 11. The method of claim 1 wherein said current den
oxide coating on a titanium article comprising the steps sity ranges from about 0.75 to 1.75 amperes per square
of: foot.
anodizing said titanium article in an aqueous anodiz 12. The method of claim 1 wherein said titanium
ing solution comprising fluoride ions and an oxidiz article is anodized for a period of from twenty to
ing electrolyte for at least five minutes, the anodiz twenty-two minutes.
ing temperature of said solution being at least about 3k k sk ck sk

50

55

60

65
 UNITED STATES PATENT AND TRADEMARK OFFICE
CERTIFICATE OF CORRECTION
PATENT NO. : 4,473,446
DATED September 25, 1984
INVENTOR(S) : Melvin C. Locke et al.
It is certified that error appears in the above identified patent and that said Letters Patent is hereby
corrected as shown below: Item A757:
Title Page, under "Inventors", add --Yukimori Moji--.
eigned and Sealed this
Fourteenth Day of May 1985
SEAL
Attest:

DONALDJ. QUIGG
Attesting Officer Acting Commissioner of Patents and Trademarks
 UNITED STATES PATENT AND TRADEMARK OFFICE
CERTIFICATE OF CORRECTION
PATENT NO. : 4,473,446
DATED September 25, 1984
NVENTOR(S) : Melvin C. Locke et al.
It is certified that error appears in the above-identified patent and that said Letters Patent is hereby
Corrected as shown below:

Column 3, line 37: "therafter" should be --thereafter--.

Column 6, line 4: "3/0" should be --3.0--
Column 6, lines 34 & 35: "thereaftet" should be --thereafter--.
eigned and evealed this
Fifth Day of March 1985
(SEAL
Attest:

DONALD J. QUIGG
Attesting Officer Acting Commissioner of Patents and Trademarks