INTERGRANULAR CORROSION AND STRESS - CORROSION CRACKING

25.4 INTERGRANULAR CORROSION AND STRESS - CORROSION CRACKING

Intergranular corrosion of titanium (and various of its alloys) occurs in fuming nitric acid at room temperature (3 - to 16 - h tests). Addition of 1% NaBr acts as an inhibitor [30] . Similar corrosion of commercially pure titanium occurs in

methanol solutions containing Br 2 , Cl 2 , or I 2 ; or Br − , Cl − , or I − [31] . Water acts as an inhibitor.

INTERGR ANUL AR CORROSION AND STRESS- CORROSION CR ACKING

The 6% Al, 4% V – Ti alloy has failed by stress - corrosion cracking (S.C.C.) in liquid N 2 O 4 within 40 h at 40 ° C (105 ° F) [32] . A slight excess of NO (or the presence of H 2 O) inhibits such failure. Various titanium alloys, including 8% Al, 1% Mo, 1% V – Ti (8 – 1 – 1) heated in air in contact with moist sodium chloride (e.g., from fi ngerprints) at 260 ° C (500 ° F) or higher, undergo S.C.C. (or intergranular corrosion?) usually along grain boundaries [33 – 35] . Pure titanium is resistant to this type of failure.

B. F. Brown et al. [36] showed that titanium alloys in seawater, despite otherwise excellent corrosion resistance, tend to undergo transgranular S.C.C. when a sharp surface fl aw is present. It has been observed that aqueous solu-

tions of Cl − , Br − , and I − are unique in causing S.C.C. of commercial titanium high in oxygen content (0.2 – 0.4% O) and of various alloys including the 8 – 1 – 1 alloy. On the other hand, F − , SO 2 4 − , OH − , S 2 − , NO − 3 , and ClO − 4 do not cause failure and, to the contrary, may inhibit crack propagation in some alloys sus- ceptible to S.C.C. in distilled water (e.g., about 100 ppm KNO 3 is effective) [37, 38] . Various anions of the group just listed also act as inhibitors of S.C.C. in the presence of halide ions; in this respect, their behavior simulates the effect of extraneous anions in the case of austenitic stainless steels (see Section

19.2.6 ). Beck [38] found that cathodic polarization of the 8 – 1 – 1 alloy to − 0.76 V (S.H.E.) prevents failure in the presence of Cl − , Br − , or I − . Leckie [39] reported protection of 7% Al, 2% Nb, 1% Ta – Ti alloy in 3% NaCl at − 1.1 V (S.H.E.) and also at − 1.3 V, at which hydrogen evolution was copious. Successful anodic pro-

tection against S.C.C. of the 8 – 1 – 1 alloy was reported [38] in the presence of Cl −

but not in the presence of Br − or I − .

Failure of the 8 – 1 – 1 alloy also occurs in pure methanol (CH 3 OH). Interest- ingly, adding a small amount of Cl − to distilled water or to methanol did not increase velocity of crack propagation, but less potassium nitrate (10 ppm) was needed to inhibit cracking in methanol as compared to water [37] . The stressed alloy was also found to be sensitive to cracking in the pure nonaqueous solvents

CCl 4 and CH 2 Cl 2 . The 8 – 1 – 1 alloy is a mixture of mostly α (hexagonal close packed) and some β phase (body - centered cubic), with observed cracks proceeding across grains of the α alloy but with the β phase failing by ductile fracture. Heat - treatment pro- cedures and composition changes (e.g., lowering the aluminum content) that favor the β phase increase resistance to S.C.C. However, the composition of the phase is also a determining factor, because the β phase of several other titanium alloys is found to be susceptible to S.C.C. [40] . The sensitive effect of alloy struc- ture, the specifi city of the environment, and the effects of extraneous anions and applied potential are similar to effects observed in the stainless steels (see Section

8.3 ), suggesting similarities in the mechanisms. Nevertheless, the mechanisms of cracking of titanium alloys are still being discussed, with debate continuing to take place regarding the relative importance of anodic dissolution and hydrogen reduction processes.

TITANIUM

In brief, titanium is resistant to the following:

1. Seawater, 0.0008 mm/y, 4.5 - year exposure, including high - velocity condi- tions (0.005 mm/y, 42 m/s) [41] . Known to resist pitting and crevice attack for fi ve years and longer. High oxygen concentration in titanium may induce susceptibility to S.C.C.

2. Wet chlorine ( > 0.9% H 2 O, less in fl owing gas) [42] . In dry Cl 2 , titanium may ignite.

3. Nitric acid, all concentrations and temperatures up to boiling, but not to fuming HNO 3 .

4. Oxidizing solutions, hot or cold (e.g., CuCl 2 , FeCl 3 , CuSO 4 , and K 2 Cr 2 O 7 ).

5. Hypochlorites. Titanium is not resistant to the following:

1. Aqueous hydrogen fl uoride.

2. Fluorine.

3. Hydrochloric and sulfuric acids, except when dilute, or in moderate con- centrations when inhibited by oxidizing metal ions (e.g., Fe 3+ and Cu 2+ ) or other oxidizing substances (e.g., K 2 Cr 2 O 7 and NaNO 3 ) or when alloyed with platinum or palladium.

4. Oxalic, > 10% formic, anhydrous acetic [43] acids.

5. Boiling CaCl 2 , > 55% [44] .

6. Concentrated hot alkalies, or dilute alkalies plus H 2 O 2 .

7. Molten salts (e.g., NaCl, LiCl, and fl uorides).

8. High - temperature exposure to air, nitrogen, or hydrogen. Oxidation in air occurs above 450 ° C (840 ° F) with formation of titanium oxides and nitrides. Ignition temperature decreases with increased air pressure sometimes causing localized combustion of titanium - alloy blades in the compressor section of gas - turbine engines [45] . Titanium hydride forms rapidly above 250 ° C (480 ° F); it forms at lower temperatures when hydrogen is cathodi- cally discharged. Absorption of oxygen, nitrogen, or hydrogen at elevated temperatures leads to embrittlement.