Handbook Corrosion (1992) WW Part 8 ppsx

Hydrochloric acid 10% 35 95 Ti-5Al-2.5Sn . . . 340 650 Ti-8Al-1Mo-1V Red Fuming Nitric Acid. The first reported observation of titanium SCC occurred in red fuming nitric acid (Ref 156). Cracking was observed for commercially pure titanium in a room-temperature environment containing 20% NO 2 . Later investigators indicated that SCC occurred in as little as 6.5% NO 2 with less than 0.7% H 2 O. Intergranular cracking was observed on smooth specimens, which indicates the high sensitivity to SCC in this medium. In addition to commercially pure titanium, cracking was observed on Ti-8Mn and Ti-6Al-4V, even in red fuming HNO 3 without free NO 2 . Work on inhibitors showed than 1.5 to 2.0% H 2 O completely inhibits SCC. It was also shown that 1% NaBr inhibits SCC (Ref 156). The work on water as an inhibitor is especially interesting because it is of importance in a number of other environments that promote SCC. Nitrogen Tetroxide. As titanium use in aerospace increased, it was found that titanium alloys were highly resistant to corrosion attack in nitrogen tetroxide (N 2 O 4 ), an oxidizer used with hydrazine rocket fuels. Unfortunately, SCC was rather dramatically revealed in an explosion during proof testing of a Ti-6Al-4V storage vessel. The vessel that exploded had been exposed to N 2 O 4 at 40 °C (100 °F) at a stress level of 620 MPa (90 ksi). Testing revealed that titanium alloys would crack in NO-free N 2 O 4 (that is, N 2 O 4 with excess dissolved oxygen, or red N 2 O 4 ) but would not crack in NO-containing N 2 O 4 (oxygen free, or green N 2 O 4 ) (Ref 127). Methanol. Prior to the discovery of SCC in N 2 O 4 , methanol was found to cause stress cracking of titanium alloys. Methanol and NaBr were shown to be extremely corrosive to titanium, and in some cases, they promoted intergranular SCC of smooth specimens (Ref 157). Soon after this discovery, it was shown that methanol/HCl and methanol/H 2 SO 4 mixtures also caused SCC of commercially pure titanium once again on smooth specimens (Ref 158). Stress-corrosion cracking in methanol was dramatically rediscovered when a Ti-6Al-4V pressure vessel exploded during proof testing with methanol. This led to a flurry of research to discover the nature of this cracking phenomenon and the metallurgical factors that promoted cracking. Two types of SCC are observed in methanol solutions and are characterized by the failure mode exhibited. In the first type, intergranular fracture is evident. This type of fracture is common for commercially pure titanium and titanium alloys, such as Ti-13V-11Cr-3Al, that are exposed to methanol containing a halide ion, such as Cl - or Br - . Susceptibility to SCC measured as time to failure of smooth samples indicates that: • Increasing the halide content decreases time to failure • Water additions to a critical level decrease time to failure • Higher halide concentrations increase the critical level of water for maximum susceptibility to SCC • Water levels beyond the critical level reduce and can inhibit cracking susceptibility Details of this failure mode are presented in Ref 113, 114, 115, 116 and 159, 160, 161. The effects of both cathodic and anodic polarization have also been investigated. Anodic polarization increases the susceptibility of titanium to SCC in methanol/halide mixtures. On the other hand, cathodic polarization dramatically reduces SCC susceptibility, as shown in Fig. 34. Potentials more negative than -250 mV versus Ag/AgCl prevent cracking in methanol (Ref 161). Fig. 34 Time to failure versus applied current and water content for cold-rolled and annealed Ti-6Al- 4V stressed to 75% of yield strength in a methanol/HCl mixture. For 0.08% H 2 O, 0.15% H 2 O, and 0.20% H 2 O, there was no failure in time shown. Metal ion additions have also been examined and found to affect methanol SCC. In general, additions that have altered the cathodic reaction, such as palladium, chromium, iron, and gold, have accelerated cracking. This is shown in Fig. 35 for palladium. Fig. 35 Effect of palladium on time to failure and ele ctrode potential for commercially pure titanium in methanolic solutions The other fracture mode, typical of highly alloyed titanium, is characterized by transgranular -phase cleavage. Alloys such as Ti-8Al-1Mo-1V typify this failure mode. In tests using precracked specimens, cracking changes from intergranular in stage I cracking (SCC initiation, Fig. 8) to transgranular in stage II (SCC propagation). Most of the and - alloys susceptible to SCC in neutral aqueous solutions (discussed later) exhibit this mode of failure. In contrast, titanium alloys, such as Ti-11.5Mo-6Zr-4.5Sn, exhibit intergranular fracture in stage II. Stage II is the region in which crack velocity is essentially independent of stress intensity (SCC propagation). In general, halide additions increase crack velocity in stage II. It has also been reported that additions of sulfuric acid and acetic accelerate cracking. Application of cathodic potential between -1.5 and -1.0 V versus SCE has been shown to prevent crack initiation (stage I). Anodic potentials appear to increase crack velocity. As indicated earlier, small water additions prevent methanol SCC initiation. This effect is shown in Fig. 36. Little work has been performed on the effect of temperature on methanol SCC; however, the data available indicate that crack velocities increase with temperature. Fig. 36 Effect of bromide and chloride additions on SCC of cold- rolled and annealed commercially pure titanium stressed to 75% of yield strength in methanol/water solutions. Arrows indicate no failure in time shown. Titanium alloys known to be susceptible to aqueous SCC (discussed later) are also susceptible to methanol SCC, and the alloys most susceptible to seawater are also most susceptible to methanol. In addition, the alloys that are susceptible to SCC in distilled water are not beneficially affected by the inhibiting effect of water (Fig. 37). Fig. 37 Effect of water on crack velocity for Ti-8Al-1Mo-1V in methanolic solutions Other Alcohols. Very little work has been performed on SCC in alcohols other than methanol.The studies that have been reported on alloys other than commercially pure titanium show that certain - alloys, such as Ti-6Al-4V, may be susceptible to cracking in anhydrous ethanol. Cracking in ethylene glycol has also been reported for Ti-8Al-1Mo-1V. Other work indicates that cracking susceptibility significantly diminishes as the number of carbon atoms in the alcohol increases. Halogenated Hydrocarbons. No testing of commercially pure titanium has been performed in common hydrocarbons. However, widespread use of titanium alloys in the aerospace industry has prompted considerable study of SCC in halogenated hydrocarbons common to aerospace processing. Stress-corrosion cracking of certain titanium alloys has been identified in the following hydrocarbons: • Carbon tetrachloride • Methylene chloride • Methylene iodide • Trichloroethylene • Trichlorofluoromethane • Trichlorofluoroethane • Octafluorocyclobutane In most of these environments, precracked specimens (category 2) are required to identify SCC. Carbon tetrachloride (CCl 4 ) SCC was first noted in Ti-8Al-1Mo-1V (Ref 162, 163, 164, and 165). The threshold stress intensity was approximately the same as that observed for SCC in 3.5% NaCl. Crack velocities in CCl 4 were approximately ten times faster than velocities in methanol. Studies on dynamically loaded smooth specimens (category 3) also showed that Ti-5Al-2.5Sn was susceptible to SCC in CCl 4 at stresses approaching the tensile strength of the alloy. The other hydrocarbons identified were found to cause cracking in Ti-8Al-1Mo-1V and Ti-5Al-2.5Sn, alloys known to be susceptible to SCC in distilled water (Ref 165, 166). No other alloys were found to be similarly affected. Freons include any of a number of fluorinated hydrocarbons commonly used as refrigerants. Titanium alloys Ti-8Al- 1Mo-1V and Ti-5Al-2.5Sn have been found to exhibit threshold stress intensities in commercial freons below air threshold stress intensities (Ref 166). The alloy Ti-6Al-4V was also identified as susceptible when exposed in the solution-treated and aged condition. Hot Salts. In the late 1950s, cracking of a titanium alloys was discovered during routine creep testing. The failure was eventually traced to chlorides from fingerprints on the creep specimen. These findings were reproduced in several laboratory studies for a host of titanium alloys. Nearly all titanium alloys were found to be susceptible to this cracking phenomenon (termed hot salt cracking) with the exception of the commercially pure grades of titanium. With this discovery, a great deal of concern was expressed with regard to the multitude of existing applications similar to this laboratory environment. However, after much investigation, it was found that no failure in the field could be attributed to hot salt cracking. Several complete descriptions of host salt cracking can be found in th literature (Ref 167, 168, 169, 170, 171, 172, 173, 174, and 175). Hot salt cracking is primarily influenced by temperature, stress, time, and the alloy itself. Cracking is observed in the temperature range from 285 to 425 °C (545 to 800 °F). In general, susceptibility increases with stress and/or temperature and does not occur below 260 °C (500 °F) or above 540 °C (1000 °F). Cracking is normally characterized by extensive branching and is not necessarily associated with the regions of highest stress intensity; therefore, category 2 specimens are not required to initiate cracking. Indeed, it is often difficult to initiate cracks in precracked notches. Statically loaded beam specimens (category 1) have been used in most of the laboratory investigations. The alloys that are most susceptible to hot salt cracking are alloys with more than 3% Al, such as Ti-5Al-2.5Sn. Commercially pure titanium is apparently immune (Ref 176). Alpha-beta alloys are less susceptible to cracking, although alloys with high aluminum contents are most susceptible. Apparently, the least resistant titanium alloy is Ti-8Al-1Mo-1V. Alloys with higher molybdenum content, such as Ti-4Al-3Mo-1V, are most resistant (Ref 169). The combined effect of time, temperature, and stress is shown in the Larsen-Miller diagram in Fig. 38 for several alloys. From Fig. 38, it is clear that alloy type and microstructural condition are important. Fig. 38 Larsen-Miller plot for hot salt cracking of several annealed - titanium alloys. T is temperature (°R), and t is exposure time (hours). Oxygen has been reported as necessary for hot salt cracking. At least one study has shown that cracking will not occur in Ti-5Al-2.5Sn when the environmental pressure is reduced below 10 m (Ref 169). Although the role of water (moisture) has not been clearly established, it appears that water is also a necessary environmental component in the cracking process (Ref 170, 171). Chloride, bromide, and iodide salts have all been shown to produce similar cracking. Fluoride and hydroxide salts have not. The cation associated with the salt has also been reported to affect cracking susceptibility. The severity of attach has been shown to increase as follows (Ref 170, 171): MgCl 2 > SrCl 2 > CsCl > CaCl 2 > KCl > BaCl 2 > NaCl > LiCl Table 28 lists titanium alloys in order of their susceptibility to hot salt cracking. This list is taken from Ref 177 and has not met with unanimous agreement. Table 28 Relative resistance of titanium alloys to hot salt cracking Ti-5Al-2.5Sn Ti-7Al-12Zr Ti-5Al-5Sn-5Zr Ti-8Al-1Mo-1V Ti-8Mn Least resistant Ti-5Al-5Sn-5Zr-1Mo-1V Ti-6Al-4V Ti-6Al-6V-2Sn Ti-5Al-2.75Cr-1.25Fe Ti-13V-11Cr-3Al Moderately resistant Ti-4Al-3Mo-1V Ti-2.25Al-1Mo-11Sn-5Zr-0.25Si Ti-2Al-4Mo-4Zr Ti-8Mo-8V-2Fe-3Al Ti-11.5Mo-6Zr-4.5Sn Most resistant Cracking is normally intergranular in nature, but it depends largely on alloy type. Alpha alloys exhibit both transgranular and intergranular fracture, depending on whether the material was annealed above or below the transus, respectively. Alpha-beta alloys exhibit predominantly intergranular fracture (Ref 171, 173, 176, and 178). From a practical standpoint, hot salt cracking appears to be a phenomenon that is restricted to the laboratory. As indicated earlier, no in-service failure has been attributed to hot salt cracking. The likely reason for this is the critical relationship among environment, stress level, and alloy type. Unless all of the conditions are met simultaneously and for extended time, cracking will not occur. Molten Salts. It would appear that Ti-8Al-1Mo-1V is the only titanium alloy tested for SCC in molten salt environments. Cracking has been observed in pure chloride and bromide eutectic melts at temperatures between 300 and 500 °C (570 and 930 °F). In general, increasing temperature increases crack velocity. Cathodic protection has been observed to inhibit or stop cracking. Nitrate salts below 125 °C (255 °F) do not induce cracking even when Cl - , Br - , or I - anions are present. At higher temperatures in pure molten nitrates, cracking can occur only when halides are present (Ref 164). Liquid/Solid-Metal Embrittlement. Several metals, both in liquid and solid form, have been found to induce cracking in contact with titanium alloys (Ref 127, 164, 179, 180, 181, 182, 183, 184, 185, 186, 187, 188, 189, and 190). The first reported incidence stemmed from a cracked compressor disk in contact with cadmium-plated steel bolts (Ref 179). Initial speculation hinted that the exposure temperature may have been above the melting point of cadmium, leading to liquid-metal embrittlement. However, later work found that cracking would occur well below the melting point of cadmium (Ref 186), such as at room temperature for Ti-6Al-4V. Those metals known to cause cracking of titanium alloys include cadmium, mercury, zinc, and certain silver brazing alloys. The titanium alloys that are known to be susceptible to cracking in cadmium include commercially pure titanium (ASTM grade 3) with more than 0.2% oxygen, Ti-4Al-4Mn, Ti-8Mn, Ti-31V-11Cr-3Al, Ti-6Al-4V, and Ti-8Al-1Mo-1V. It is likely that most other titanium alloys are susceptible, but have not been tested. Alloys tested and found to crack in mercury include commercially pure titanium (ASTM grade 4, 0.3% oxygen), Ti- 8Mn, Ti-13V-11Cr-3Al, Ti-6Al-4V, and Ti-8Al-1Mo-1V. As with cadmium, other alloys are probably susceptible, but have not been tested. Zinc, in both solid and liquid form, has been reported to cause cracking of titanium alloys. However, there is conflicting evidence in the literature as to whether this is actually the case. Silver and silver brazing alloys have been shown to cause cracking in titanium alloys that are particularly sensitive to SCC. These alloys include Ti-8Al-1Mo-1V, Ti-5Al-2.5Sn, and Ti-7Al-4Mo (Ref 187, 188). As with cadmium, both solid and liquid forms of silver may produce cracking. Susceptibility for Ti-6Al-4V is considered to be above 345 °C (650 °F). Aqueous Environments. Under certain metallurgical conditions, several titanium alloys have been shown to be susceptible to SCC in distilled water. These include Ti-8Al-1Mo-1V, Ti-5Al-2.5Sn, and Ti-11.5Mo-6Zr-4.5Sn. Microstructural variation for each alloy affects the degree of susceptibility. For example, mill-annealed Ti-8Al-1Mo-1V is less susceptible than step-cooled Ti-8Al-1Mo-1V. Testing has been performed with category 2 type specimens where crack velocity and threshold stress intensity are determined. In these alloys, the degree of susceptibility is highly dependent on heat treatment. Test results in neutral-pH environments with category 2 type specimens indicate that titanium alloys exhibit a threshold stress intensity, K ISCC , below which cracks will not propagate. The individual effects of ionic species, concentration, potential, pH, and so on, have been extensively studied and are discussed below. Ionic Species. The anions Cl - , Br - , and I - are the only species shown to promote and/or induce SCC in titanium alloys. The few alloys susceptible to cracking in distilled water become more susceptible, while alloys that are not susceptible in distilled water may become susceptible when these species are present. Anions such as , , OH - , F - , Cr 2 , and may reduce sensitivity. In general, cations do not alter SCC sensitivity. However, oxidizing cations, such as Fe 3+ or Cu 2+ , can increase K ISCC in more susceptible alloys. This effect is analogous to anodic polarization, which is discussed in the section "Potential and pH" in this article. Concentration. As shown in Fig. 39, increasing the concentration of anions that promote SCC generally increases crack velocity (Ref 164, 191, and 192) and decreases K ISCC . Additions of and to distilled water can completely inhibit SCC in alloy/heat-treatment combinations that are moderately susceptible, such as mill-annealed Ti-8Al-1Mo-1V. More susceptible combinations, such as step-cooled Ti-8Al-1Mo-1V, are not similarly affected. [...]... Hydrochloric acid +2.5% NaClO3 10.2 80 0.009 +5.0% NaClO3 10.2 80 0.006 +0.5% CrO3 5 38 nil +0.5% CrO3 5 95 0.031 +1% CrO3 5 38 0.0 18 +1% CrO3 5 95 0.031 +0.05% CuSO4 5 38 0.040 +0.05% CuSO4 5 93 0.091 +0.5% CuSO4 5 38 0.091 +0.5% CuSO4 5 93 0.061 +1% CuSO4 5 38 0.031 +1% CuSO4 5 93 0.091 +5% CuSO4 5 38 0.020 +5% CuSO4 5 93 0.061 +0.05% CuSO4 5 Boiling 0.064 +0.5% CuSO4 5 Boiling 0. 084 Hydrochloric acid Hydrochloric... 0.003 Ammonium fluoride 10 Room 0.102 Ammonium hydroxide 28 Room 0.003 28 100 nil Ammonium nitrate 28 Boiling nil Ammonium nitrate + 1% nitric acid 28 Boiling nil Ammonium oxalate Saturated Room nil Ammonium perchlorate 20 88 nil Ammonium sulfate 10 100 nil Ammonium sulfate + 1% H2SO4 Saturated Room 0.010 Aniline 100 Room nil Aniline + 2% AlCl3 98 1 58 >1.27 Aniline hydrochloride 5 100 nil 20 100 nil Antimony... 35 4.45 0.1 Boiling 0.10 1 Boiling 1 .8 Hydrochloric acid + 4% FeCl3 + 4% MgCl2 19 82 0.51 Hydrochloric acid + 4% FeCl3 + 4% MgCl2 + Cl2 saturated 19 82 0.46 Hydrochloric acid, chlorine saturated 5 190 . to induce cracking in contact with titanium alloys (Ref 127, 164, 179, 180 , 181 , 182 , 183 , 184 , 185 , 186 , 187 , 188 , 189 , and 190). The first reported incidence stemmed from a cracked compressor. cause cracking in titanium alloys that are particularly sensitive to SCC. These alloys include Ti-8Al-1Mo-1V, Ti-5Al-2.5Sn, and Ti-7Al-4Mo (Ref 187 , 188 ). As with cadmium, both solid and liquid. titanium is similar to that of the 18- 8 stainless steels (Ref 211). Therefore, the galvanic effects of titanium on active metals are quite similar to those for 18- 8 stainless, as observed in salt