Titanium alloys in stress-corrosion cracking

A.J. Hatch, //.VP. Rosenberg, E.E Erbin Effects of environment on cracking of titanium alloys , in Stress Corrosion Cracking in Titanium, ASTM STP397. pp. 122-136 (1996)... 

Schutz, R. W., Stress Corrosion Cracking of Titanium Alloys, in Stress-Corrosion Cracking—Materials Performance and Evaluation, ASM International, Metals Park, OH, July 1992, pp. 265-297. 

G. Martin Investigation of long-term exposure effects under stress of two titanium structural alloys", in Stress Corrosion Cracking of Titanium, ASTM, STP396, pp. 95-120, (1966). 

III aged sheet (8 h, 510 °C, or 950 °F) in 3.5%NaCl solution showed a total lack of stress-corrosion reaction under stress equal to 80% of the tensile yield stress. (AFML-TR-70-252, Oct 1970). On the other hand, various degrees of immunity or susceptibility have been reported for Beta III materials where precracked specimens were exposed under stress to salt water (see table). In general, the susceptibility of titanium alloys to stress-corrosion cracking in aqueous media is influenced by the type and concentration of species in solution, the pH, temperature, and viscosity of the solution, and the metal potential in the solution. See Techmcal Note 2 Corrosion for general information. 

Sandoz, G., In Stress Corrosion Cracking in High Strength Steels and in Titanium and Aluminium Alloys, Ed. B.F. Brown, Naval Research Laboratory, Washington, pp. 79-145, (1972)... 

Schmitt [52] reviewed the effect of elemental sulfur on corrosion of construction materials (carbon steels, ferric steels, austenitic steels, ferritic-austenitic steels (duplex steels), nickel and cobalt-based alloys and titanium. Wet elemental sulfur in contact with iron is aggressive and can result in the formation of iron sulfides or in stress corrosion cracking. Iron sulfides containing elemental sulfur initiate corrosion only when the elemental sulfur is in direct contact with the sulfide-covered metal. Iron sulfides are highly electron conductive and serve to transport electrons from the metal to the elemental sulfur. The coexistence of hydrogen sulfide and elemental sulfur in aqueous systems, that is, sour gases and oils, causes crevice corrosion rates of... 

Steel is the most common constructional material, and is used wherever corrosion rates are acceptable and product contamination by iron pick-up is not important. For processes at low or high pH, where iron pick-up must be avoided or where corrosive species such as dissolved gases are present, stainless steels are often employed. Stainless steels suffer various forms of corrosion, as described in Section 53.5.2. As the corrosivity of the environment increases, the more alloyed grades of stainless steel can be selected. At temperatures in excess of 60°C, in the presence of chloride ions, stress corrosion cracking presents the most serious threat to austenitic stainless steels. Duplex stainless steels, ferritic stainless steels and nickel alloys are very resistant to this form of attack. For more corrosive environments, titanium and ultimately nickel-molybdenum alloys are used. 

Resistance to stress-corrosion cracking Commercially pure titanium is very resistant to stress-corrosion cracking in those aqueous environments that usually constitute a hazard for this form of failure, and with one or two exceptions, detailed below, the hazard only becomes significant when titanium is alloyed, for example, with aluminium. This latter aspect is discussed in Section 8.5 under titanium alloys. 

The stress-corrosion cracking hazard for titanium alloys containing aluminium is significantly higher than that obtaining for commercially pure titanium, and in addition to stress-corrosion cracking in methanol and red... 

Stress-corrosion cracking occurs in titanium alloys in a number of environments, although the number of failures that have occurred under service conditions is very small. Because of the widespread use of titanium alloys in aeroplanes and space vehicles and their increasing use in marine applications it is important that the possibilities of service failures should be removed. As a result a considerable and increasing amount of work has been done on this subject over the last decade as indicated in a recent extensive survey. ... 

Many titanium alloys are susceptible to stress-corrosion cracking in aqueous and methanolic chloride environments. 

In common with many of the alloy-environment systems described so far, if the alloy is not susceptible to stress-corrosion cracking under constant stress or stress intensity, then little or no effect of environment on fatigue crack growth is observed. In these cases, frequency, R ratio and potential within the passive or cathodically protected ranges for titanium have no effect on growth rates. 

If chlorinated solvents are used with titanium surfaces, they must be completely removed prior to bonding. Chlorinated solvents give rise to stress corrosion cracking in the vicinity of welds. Welding of titanium often occurs in the same plant as adhesive bonding, and it is sometimes done on the same parts. So the best practice is to avoid the use of chlorinated solvents completely. Several airframe manufacturers that fabricate titanium alloys no longer permit the use of chlorinated solvents. 

Majority of titanium alloys are resistant to SCC stress-corrosion cracking has been observed in absolute methanol, red fuming nitric acid, nitrogen tetroxide, liquid, and metals of Cd and Hg and halide media67... 

The environments, along with the cracking modes of zirconium and titanium, are given in Table 4.88. It is obvious from the table that zirconium alloys are susceptible to stress-corrosion cracking in a variety of environments. It is necessary to subject the weld to heat treatment in order to lower the stress in the weld. The most serious problem encountered in the nuclear applications is delayed hydride cracking in addition to stress-corrosion cracking, particularly in Zr-2.5% Nb alloy. 

M.J. Blackburn, W.H. Smyrl, J.A. Feeney and B.F. Brown (eds.), Stress-Corrosion Cracking on High Strength Steel and in Titanium and Aluminum Alloys, Naval Research Laboratory, Washington, DC, 1972, pp. 245-363. 

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