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Corrosion behavior of titanium in formic acid, oxalic acid, andformaldehyde
Tadafumi Kurogi, DDS,a Shigeru Nakamura, DDS,a Marie Koike, DDS, PhD,a Motohiro
Terano, DDS,a Kunihiro Hisatsune, PhD,b and Hiroyuki Fujii, DDS, PhDaabDivision of Removable Prosthodontics and Management of Oral Function, and Division of Dental andBiomedical Materials Science, Department of Development and Reconstructive Medicine, Course of
Medical and Dental Sciences, Graduate School of Biomedical Sciences, Nagasaki University,
Nagasaki, Japan
Purpose: The purpose of the present study is to evaluate the influence of formic acid, oxalic acid, andformaldehyde on corrosion behavior of cast titanium.
Materials and Methods: Cast titanium specimens were immersed in 128 mmol/L formic acid, oxalic acid, andformaldehyde, and in 64 mmol/L oxalic acid for three weeks. Color change, weight loss, surface chemicalbinding state, and solubility of titanium were determined after immersion in the four solutions, and the changes
in properties of the titanium surface were examined after immersion in 128 mmol/L oxalic acid.
Results: The discoloration of titanium specimens was significantly greater in formic acid than in either oxalic
acid or formaldehyde. Both the weight loss and amount of titanium ions dissolved from specimens in oxalic acid
were greater than the corresponding values after immersion in formic acid and formaldehyde. The superficial
titanium oxide layer in post-immersion specimens was thickest in formic acid, followed by oxalic acid andformaldehyde.
Conclusion: The oxalic acid corroded titanium. The results may have important clinical implications forconditions such as titanium allergy and incompatibleness. (Int Chin J Dent 2006; 6: 1-7.)
Key Words: corrosion, formaldehyde, formic acid, oxalic acid, titanium.

Introduction
Titanium is a useful biomaterial exhibiting chemical stability and higher level of biocompatibility, and it is
implemented widely in dental restorations and medical implants.1,2 However, titanium dissolves in lactic acidand shows discoloration to a golden-yellow or purplish-blue color when immersed in formic acid.3 There havealso been clinical reports regarding the discoloration and corrosion of titanium dentures.4,5 Much remains to be
known about the elution and discoloration of titanium in vivo.
Formic acid, which causes the discoloration of titanium, is a deoxidizing organic acid that possesses aldehyde
and carboxyl groups. It is considered to be strongly acidic, even among the carboxylic acids, and it is one of theacids promoting the growth of bacteria in the oral cavity. Formaldehyde possesses the aldehyde group, has achemical structure similar to that of formic acid, undergoes oxidization to form formic acid, and is contained incertain dental medications. Moreover, oxalic acid, a dimer of the carboxyl group with oxidation-reduction
potential, is an organic acid that is contained in many edible plants and forms formic acid when heated at hightemperatures6 and corrodes titanium.7 A metabolic byproduct synthesized from glyoxylic acid, etc. in the body,oxalic acid is also present in saliva.8 As these facts indicate, titanium may come in contact with these substancesin the oral cavity, and the observation of titanium’s reaction thereto is essential to assess the properties oftitanium as a biomaterial. The purpose of the present study is to conduct a comparative study on the reaction oftitanium to formic acid, formaldehyde and oxalic acid.

Materials and Methods
Immersion solution
Formic acid, formalin (40% solution of formaldehyde and water), and oxalic acid (Wako Pure Chemical

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Industries Ltd., Osaka, Japan) were prepared to serve as immersion test solutions. The three solutions were
adjusted with distilled water to achieve the same mole fraction concentration as 1% lactic acid (128 mmol/L)
used in previous study.3 Since oxalic acid is a dimer of the carboxyl group, a 64 mmol/L solution was also
prepared. After adjustment, the pH of the solutions was 2.50 for formic acid, 4.00 for formalin, 1.33 for 128
mmol/L oxalic acid, and 2.55 for 64 mmol/L oxalic acid.
Preparation of titanium specimens
Specimens were cast from pure titanium (JIS2, Morita Corp., Tokyo, Japan; Ti) in approximately 15x20x13
and 5x5x1 mm sizes according to the manufacturer’s instructions. Five large and three small specimens wereprepared for each immersion solution. The surface of the cast specimens was air-abraded using alumina powderof 50 µm in diameter. Next, the cast specimens were ground with #800 wet silicon carbide paper, subjected toultrasonic cleaning in acetone for 15 minutes to remove grease, and stored in a desiccator. The large specimens
were then used to measure changes in solubility, color and weight, while the small specimens were used to
examine the chemical binding state of the specimen surface.
Immersion of specimens
Each specimen was entirely immersed in 10 mL of immersion solution contained in 50 mL centrifuge tubes,
and lids were securely fastened on each tube. For each of the four immersion solutions, five tubes containing
large specimens and three tubes containing small specimens, or a total of 32 tubes, were placed in an isothermalbath and shaken at a rate of 80 times/minute at 37ºC for a period of three weeks. After three weeks, thespecimens were removed from the tubes, subjected to cleaning in distilled water for 15 minutes and ultrasoniccleaning in acetone for 15 minutes, dried in a desiccator and used for the respective measurements.
Observation and analysis
Discoloration: The surface color of each dried specimen was examined macroscopically.

Then aspectrophotometer (CM-503i, Minolta Co. Ltd., Tokyo, Japan) was used to measure the color at four sites
selected randomly on each specimen, the mean measurement being considered the color value of the specimen.
The light source, visual field angle and diameter were set at D65, 10º and 3 mm, respectively, and the color
difference (∆E*ab), that is, the difference between the measured color value and a standard value, wasdetermined according to the L*a*b* colorimetric system.
Amount of dissolved ions: The amount of ions dissolved in each immersion solution was measured five times
for each tube using inductively coupled plasma-atomic emission spectrometry (SPS-1700HVR, Seiko
Instruments Inc., Chiba, Japan). The value was determined by dividing the measured amount by the surface areaof the specimen, and by obtaining the amount of dissolved ions for each unit of area.
Weight changes: Specimens were weighed using an electronic analytical balance (AEM-5200, Shimadzu Co.,
Kyoto, Japan; readability=0.001 mg). The weight of the abraded, ground and dried specimens before immersion
was compared with that of the cleaned and dried specimens after immersion, and the weight change per unit ofarea was determined by dividing the measured weight difference by the area of each specimen.
Depth analysis and microgeogram of the surface: We examined the chemical binding state of the surface ofthe small Ti specimens after immersion using an X-ray photoelectron spectrometer (ESCA-850, Shimadzu Co.,
Kyoto, Japan). Etching with argon ions was conducted repeatedly in the apparatus, and the depth of thechemical binding state was analyzed.3 MgKα (acceleration voltage, 8 kV; ion current, 30 mA) was used as theexcited X-ray source for ESCA, and etching was conducted at 1 kV, 20 mA for 30 s. The specimen used for2

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analysis was the specimen judged by the corresponding author to have, among the three specimens, the closest
approximation to a medium surface color on the basis of macroscopic evaluation.
To assess the extent of roughness and loss of gleam, we also examined the surface state of the specimens
immersed in 128 mmol/L oxalic acid using a scanning electron microscope (S-3500N, Hitachi Co. Ltd., Tokyo,
Japan).
Statistical analysis: We employed the one-way analysis of variance (ANOVA) for the statistical analysis.
When a difference appeared to be significant, we also conducted multivariate variance analysis using Fisher’s
P.L.S.D.

Results
Discoloration
Fig. 1 shows variation of the color in large specimens before (control) and after immersion. The illustrationon the far left shows a specimen prior to immersion (control). The four rows of five illustrations show, from top
to bottom, the specimens immersed in 128 mmol/L formic acid, 128 mmol/L formaldehyde, and 128 mmol/L
and 64 mmol/L oxalic acid solutions.

In the 128 mmol/L oxalic acid specimens, weak golden-yellow

discoloration was noted in all except the specimen second from the right, which showed a loss of gleam but nomacroscopic discoloration. All of the specimens immersed in formic acid solution turned a golden-yellow color.
The discoloration, however, was not necessarily uniform, some of the specimens showing a “mottled” or“shifting” pattern of discoloration.

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Fig. 1. Photographic presentation of titanium specimens.
The illustration on the far left shows a specimen prior to immersion (control). The four rows of five illustrations
show, from top to bottom, the specimens immersed in 128 mmol/L formic acid, 128 mmol/L formaldehyde, 128
mmol/L oxalic acid, and 64 mmol/L oxalic acid solutions. Golden-yellow discoloration is evident in all of the formic
acid specimens. In the 128 mmol/L oxalic acid specimens, weak golden-yellow discoloration was noted in all
except the specimen second from the right, which showed a loss of gleam but no macroscopic discoloration.
Fig. 2. Discoloration of Ti specimens after immersion (color difference between controls and post-immersion
specimens). Vertical axis, color difference ∆E*ab; Horizontal axis, four immersion solutions. The numbers andlines at the top of the graphs indicate the mean value and standard deviation, respectively. Asterisks, p<0.001.

A graphic illustration on the color difference observed in the specimens after immersion (mean and SD) ispresented in Fig. 2. The color difference (∆E*ab) between the specimens before and after immersion in formic
acid solution was an average 17.7 (SD 4.25), a value that was significantly higher than that observed in the otherthree solutions (p<0.001). With regard to the five specimens immersed in 128 mmol/L oxalic acid, four showedpale yellow discoloration and one a loss of gleam or pale gray color, the color difference being 4.7 (SD 0.73) inthese specimens. The specimens immersed in the two remaining solutions were still silvery white in color, the3

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color difference being 2.5 (SD 0.78) and 2.3 (SD 1.28) for formaldehyde and 64 mmol/L oxalic acid,respectively. The extent of discoloration in these specimens was so low that it was impossible to discern
macroscopically.
Dissolution
Fig. 3 shows the concentration of Ti ions dissolved in the immersion solutions. The concentration of Ti ionsin the 128 mmol/L and 64 mmol/L oxalic acid solutions was 1.963 (SD 0.847) and 0.778 (SD 0.294) ppm/cm2,
values that were significantly higher than those observed in formic acid and formaldehyde. In fact, the Ti
concentration observed in the formic acid solution was only 0.009 (SD 0.007) ppm/cm2, and that observed informaldehyde solution was so low as to be undetectable.
Weight loss
Fig. 4 presents the weight changes observed in the specimens. A slight increase was observed after immersion
in formic acid solution (0.0009±0.0005 mg/cm2), while a decrease was observed in the 128 mmol/L and 64
mmol/L oxalic acid solutions (0.0262±0.0121 mg/cm2 and 0.0116±0.0049 mg/cm2, respectively). The weightchange in the formaldehyde solution was negligible. A statistically significant difference was observed between
the formaldehyde solution and the 128 mmol/L oxalic acid and formic acid solutions (p<0.05).

Fig. 3. Dissolution of Ti ions into immersion solution2(ppm/cm ). Asterisk, p<0.05; Asterisks, p<0.001;
Cross, Undetectable level.

Fig. 4. Weight loss of Ti specimens after immersion
2(mg/cm ). Asterisk, p<0.05.

Depth analysis with ESCA
Fig. 5 presents the results of the analysis of chemical binding state of surface of small specimens after
immersion using ESCA. On the first (superficial) layer and several lower layers of all specimens, peaks wereobserved near 459.1 eV and 464.8 eV,9,10 binding energies that correspond to titanium oxide. This layer wasabsent, and peaks appeared at 453.8 eV and 459.9 eV9,10 corresponding to metallic Ti, at the 74th layer in
specimens immersed in formic acid, followed by the 60th layer in specimens immersed in 128 mmol/L oxalic
acid, the 30th layer in specimens immersed in formaldehyde, and the 26th layer in specimens immersed in 64
mmol/L oxalic acid. The small specimens showed similar surface discoloration to the large specimens, and theappearance of a “mottled” pattern was observed slightly.
Microscopic surface stateFig. 6 presents SEM images of a specimen before (control) and after immersion in 128 mmol/L oxalic acidsolution (3rd row, 2nd column in Fig. 1). The post-immersion specimen surface shows an irregular series ofgrooves and ridges scattered with flat depressions and pinholes.

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Fig. 5. ESCA images. The results of depth analysis with ESCA.
From among the three small specimens immersed in each solution, the specimen judged by macroscopic
examination to have the closest approximation to a medium surface color was selected for analysis. Vertical axis,
duration of etching; Horizontal axis, binding energy. The thin arrows indicate the position of metallic Ti (453.8 eV
and 459.9 eV), while the thick arrows indicate the position of titanium oxide (459.1 eV and 464.8 eV).
Fig. 6. SEM images.
Left image is a specimen before (far left in Fig. 1), and right image is a specimen after immersion in 128 mmol/L
oxalic acid solution (3rd row, 2nd column in Fig. 1).

Discussion
The immersion solutions used in the present study were formic acid, which discolors the surface of Ti,3 andoxalic acid and formaldehyde, two chemical substances made from the functional group that synthesizes formic
acid. Formic acid is an organic acid that promotes the growth of resident bacteria in the oral cavity.11 Oxalic
acid is a metabolic byproduct of the oxidization of glyoxylic acid6 and an organic acid contained in large
quantities in foods such as spinach, celery, oxalis and cocoa.12 Formalin cresol, including formaldehyde, is usedin endodontics.13 Therefore, Ti dental restorations in the oral cavity may be exposed to these three chemical
substances. Needless to say, a variety of factors interact in the oral cavity, making it impossible, at the present
time, to recreate the exact intra-oral conditions under which Ti is affected by these substances. Consequently, inview of the fact that formic acid is composed of two relatively simple functional groups, we decided to examine
the corrosive effect of these functional groups on Ti in the present study. The concentration of immersion
solutions was set at the mole fraction concentration of 1% lactic acid (128 mmol/L),3 which is used in metal
discoloration,14 corrosion and immersion tests.15,16 We also prepared a 64 mmol/L solution of oxalic acidsolution to make the volume of the carboxyl group equivalent to that of formic acid.
It is a well-known fact that the corrosion of metals is affected by pH.3,17 The pH of the immersion solutions
used in the present study ranged from 1.33 to 4.00. The pH of many foods such as lemon juice and pickledplums also fall within this range,18 and some denture cleansing agents possessing the carboxyl group have a pHof 1 (Nishioka K. Personal communication). Aside from the effect of the buffering action that undoubtedly
occurs in the oral cavity, the immersion solutions used in the present study can be considered to replicate theconditions encountered by denture users in their daily life and to provide appropriate experimental conditions forstudies on the corrosion of Ti used as a dental biomaterial.
The surface color of Ti changes with the thickness of the oxidized layer on the surface of the specimen andalso with the angle of examination. These phenomena are attributed to the refection of light at the film-covered
Ti surface.19 In the present study, all of the five specimens immersed in formic acid solution, and all of the five5

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specimens immersed in 128 mmol/L oxalic acid solution except one that lost its gleam, showed golden-yellow
discoloration, and the extent of discoloration (Figs. 1 and 2) was greater the thicker the oxidized layer covering
the specimen surface (Fig. 5).
A significantly greater release of Ti ions was observed in the oxalic acid solution than in either the formic acidor formaldehyde solution (p<0.001: Fig. 3). In fact, the volume observed in 128 mmol/L oxalic acid solution(1.962±0.847 ppm/ cm2) was approximately four times that of lactic acid solution at the same concentration.20
Furthermore, the changes observed in the weight of immersion specimens (Fig. 4) and in the surface state
captured in SEM images (Fig. 6) provided evidence for the release of Ti ions in the oxalic acid solution or, inother words, for the occurrence of a corresponding level of hydrogen-induced corrosion. This would seem toindicate thinning of the oxidized layer on the surface of specimens after immersion in oxalic acid. Our results
showed, however, that the oxidized layer was thicker after immersion in both 128 mmol/L and 64 mmol/L oxalic
acid solutions (Fig. 5) compared with before immersion.3 This apparent contradiction can be explained by thefact that the surface of specimens immersed in oxalic acid solution under the conditions of the present study
underwent greater hydrogen evolution type corrosion and even greater oxygen diffusion type corrosion than thatobserved in either lactic acid or formic acid solution.3
In the case of formaldehyde, the release of Ti ions in immersion solution was so low as to be undetectable, theweight of specimens increased slightly, and the thickness of the oxidized layer was only slightly thicker (10thlayer)3 than that observed under the same conditions prior to immersion. These findings indicate that, although
the corrosion of Ti specimens in formaldehyde is similar to that observed in formic acid solution, the degree ofcorrosion is obviously lower than that in formic acid.
The formic acid, formaldehyde and oxalic acid (which is found in salivary stones8 as well as in certain
foods12) used in the present study are chemical substances that may come in contact with dental restorations in
the oral cavity.
In conclusion, titanium corrodes oxalic acid and both hydrogen evolution type corrosion and oxygen diffusion
type corrosion can occur titanium surface with chemical solutions of this study. This may have important
implications for clinical practice in titanium allergy and incompatibleness.21-24

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Correspondence to:Dr. Tadafumi Kurogi
Division of Removable Prosthodontics and Management of Oral Function
Department of Development and Reconstructive Medicine
Course of Medical and Dental Sciences, Graduate School of Biomedical Sciences, Nagasaki University
1-7-1, Sakamoto, Nagasaki 852-8588, Japan
Fax: +81-95-849-7694 E-mail: kurofumi@net.nagasaki-u.ac.jp
Received November 29, 2005. Revised December 19, 2005. Accepted December 26, 2005.
Copyright ©2006 by the International Chinese Journal of Dentistry.

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