A
W O R K B O O K
F E A T U R E
by Mars G. Fontana, Ohio State University
Stress Corrosion in Titanium and Its Alloys Titanium alloys difficult to crack by stress cor-
rosion even with unfavorable heat treatments
AN
EXTENSIVE and recent investigation of stress corrosion of titanium and titanium alloys showed that these materials possess excellent resistance. Specimens were difficult to crack even when the materials were given unfavorable heat treatments to produce localized attack and also when environments that should be conducive to local corrosion were selected. Table I lists the compositions studied. 4-55 is commercial titanium. A-110AT and C-110M are commercial alloys, and the last is a special.
Method of Stressing
I n the constant load tests used predominantly in this study, the specimen is loaded a t two points and has the maximum stress over a substantial length or area of the specimen. The apparatus is shown in Figure 1. The end supporting blocks and the center loading block are made of Alundum brick. The vertical rods are loaded by a dead weight consisting
of a can containing lead shot. A loaded strip specimen and a duplicate specimen without load are run simultaneously in each heat-resistant glass container. The heating bath is kept at 33" C. During the test, the glass disks are covered with plastic sheet to minimize evaporation of the corrodent and also to permit less temperature fluctuation. I n general, high stresses Ivere used to induce cracking. Heat Treatment
Metallographic examination showed that the as-rolled microstructures did not appear to be favorable for intergranular corrosion because they \\.ere fine grained and homogeneous. I t was decided to heat treat the alloys so that a more or less continuous path of anodic phase might be produced. Stress corrosion may occur transgranularly but the obvious approach seemed to be promotion of intergranular attack. Figure 2 is a brief summary of equi-
librium diag-ram types that should be useful in follo\Ying the heat treatments used. These are as follo\vs: 1. .4-.53. This is an all-alpha alloy. Solution heat treat (beta field) for hour at 1750" F., and furnace cool to room temperature. The purpose here was to give impurities a chance to come out of solution at grain boundaries. 2. A4-110AT alloy. This is an all-alpha alloy. Heat in the beta field at 2000" F. for 2 hour, and then furnace cool to yield a coarse structure, and possibl!. grain-houndar)- precipitation. 3. C-110M allo)-. Heat I,,? hour in the beta field, furnace cool to 1300" F., hold 1 hour, and cold water quench. Intergranular alpha !vas expected here. 4. 3 M n complex alloy. Both 30-p.p.m. hydrogen and 260-p.p .In. alloys \vex included here. Solution treat for ' / 2 hour a t 1600" F., cold Lvater quench, re-solution treat for 1 hour at 1300" F. in the alpha-beta field, and cold water quench. 3 . 3 M n complex. Solution treat for 1 hour at 1300" F., cold wa-
Table I. Compositions of Titanium and Titanium-Base Alloys Studied
% C
N 0
H Mn A1 Sn Cr Fe Mo
V
AC1lOAT llOM 0 . 1 max. 0 . 0 7 0.12 0 . 2 max. 0 . 0 1 0.03 A-6P
... ... ... ... ... ... ... ... ...
. , , , .. ......
...
7.3
... ... ...... ...... ...... ......
6.8 2.2
$Mn 0.02 0.008 0.09
0.003; 0.026b 3.36
... ...
1.18 0.91 1.08 0.98
Commercial titanium, nominal composition. Split heat; one low and one high hydrogen. a
Figure 1.
View of apparatus VOL. 48, NO. 9
SEPTEMBER 1956
59 A
m d
CORROSION
A Workbook Feature
‘P
T
CY
Stabilizing
AI, 0, N, C
I Alloy ConfrnT-
t
0 Stabilizing
T
Ma, V (p isomorphous) Mn, Fe, Cr, Ni, Cu, Si, H (eutectoid)
Neulral Sn,
Zr
A l l o y ContPnt--c
Figure 2.
Three principal types of alp ha- beta alloys
ter quench, and age for 24 hours at 900” F. 6. 3 M n complex. Solution treat for 1 hour at 1300” F., furnace cool to 1050”F., and then air cool. 7. For treatments 4, 5, and 6 above, two specimens of C-l10M alloy were included for comparison. In carrying out the above treatments, a 24-hour aging treatment at 900” F. was added to item 4. All of the alloys, except A-llOAT, were heat treated in a special tube fur-
Figure 3. Commercial titanium with grain boundary precipitate
60 A
Figure 4. A-1 10AT alloy, large grains, all-alpha
nace reserved for titanium alloy work. The furnace was provided with a dried argon atmosphere. Results of attempts to produce “susceptible” structures are illustrated in the next four figures. Figure 3 shows commercial titanium heat treated to allow grain boundary precipitation. Figure 4 is the structure of ‘4-110AT heat treated to produce large grains with maximum accumulation of impurities at the grain boundaries. This is an all-alpha structure. Figure 5 depicts the C110M alloy with alpha phase at the grain boundaries and in the matrix grains. Figure 6 shows the high hydrogen 3 M n complex with the hydrogen-rich phase at the grain boundaries. Environments
Cracking
Since little information on stress corrosion of titanium is reported in the literature, much time was devoted to devising and testing corrosives that might induce cracking. Red fuming nitric acid causes cracking but it was not included here because of the several hazards involved. The mechanism of these hazards is not understood and is being studied elsewhere. The basic approach to the environment aspect consisted of selecting a corrosive that would attack the metal and then adding to the environment a passivating agent. The idea is to obtain partial passivity so that attack on the active sites or weak spots would be accelerated because of galvanic effects and the unfavorable area ratio (large cathodic area and small anodic area). For example, a microstructure may contain alpha
5. C- Figure 6. 110M alloy with complex
Figure
duplex structure
INDUSTRIAL AND ENGINEERING CHEMISTRY
(titanium rich) grains with beta (lower titanium content) phase in between. Since oxidizing agents passivate titanium, it was thought that a combination of a n oxidizing compound and a nonoxidizing acid might passivate the alpha grains and selectively attack the beta boundary phase. Examples of such environments are, ( u ) H&304-HCl-K2Cr%07-Hz0 and ( 6 ) H2S04-KBr03HzO solutions. Other reagents used are nitric acid and fluoride compounds. A large amount of corrosion data was obtained, but the results were generally unsuccessful as far as cracking was concerned. Hydrochloric acid attacks titanium readily but even very small amounts of nitric acid tended to completely passivate the specimens.
3 Mn
with grain boundary precipitate
Some complete cracking was finally obtained. Strips of A-11OAT cracked through after 103 hours of exposure when loaded to 90% of the proportional limit and immersed in 10% hydrochloric acid at 35” C. Figure 7 shows a specimen which had not cracked completely. This is intergranular cracking in the beta phase because of stress corrosion. Surface cracking (not complete failure) was obtained on A-55 material in 1Oy0 hydrochloric acid at 35” C., (Figure 8). The specimen was formed into a “hairpin” and then the ends were pulled together by means of a bolt. Here again the attack and cracks are in the beta phase. This study was conducted by Vincent D. Barth in connection with a thesis for a Master’s degree at T h e Ohio State University.
Figure 7. Stress corrosion of A1 1 OAT alloy
Figure 8. Surface crack in A-55 alloy
