ENVIRONMENTAL BEHAVIOR

2435

Critical Review

ENVIRONMENTAL EFFECTS ON TITANIUM AND ITS ALLOYS H.B. Bomberger, Jr., Wright-Patterson Ai~ Force Base, Ohio USA U. A. Meyn, USA A. C. Fraker, National Bureau of Standards, Washington, DC USA

Introduction

Annual costs for corrosion have been estimated to be as high as 80 billion dollars in the United States [l] and 1,365 million pounds in the United Kingdom (2]. Such massive losses stimulate world-wide interest in more efficient use of materials. It is, therefore, encouraging to note that within the last few years several hundred publications have appeared on the behavior and use of titanium and titanium alloys in a great variety of aggressive environments.

Because of the metal's generally good performance, the chemical and allied industries have been providing the ·largest non-aircraft market for titani­um. This market employs about 35% of the current United States titanium production and it has been growing at about 12% per year [3]. The percent­age of titanium used in such applications, however, has been larger in some other countries.

Environments

With extensive testing over the past 35 years, the behavior of titanium is well understood under most.conditions (2,4,5,6,7,8). The metal resists all natural environments including food products, body fluids, natural wastes, and sea, mine and brackish waters; neutral and moderately acid and alkaline solutions; most oxidizing agents; reducing agents, including acids, provid­ing an oxidizing agent or a heavy metal ion or some other inhibiting agent is present; and most organic compounds.

Limited resistance is observed in several environments including air above about 650°C, dry halogen gases and certain other active gases; chloride salts in air at temperatures above abou·t 400°C; hot or concentrated uninhibited non-oxidizing acids and acidic salt solutions; a few organic acids, including oxalic and hot or concentrated formic and trichloroacetic acids; fluoride ions in solution at concentrations above about 10 ppm; hot concentrated caustic solutions; strong oxidizing agents, including oxygen gas, concentrated hydrogen peroxide and fuming nitric acids; and molten salts and metals. Titanium may also absor.b hydrogen under reducing condi­tions and become embrittled. The effect of air and other gases at elevated temperatures has been studied extensively (5,8,9,10).

Although titanium is vulnerable to a number of strong chemicals, it is interesting to note that the metal can be made immune to many of these environments by one or more means.

2436

Inhibitors --------Many process solutions contain intentional and unintentional chemical agents which can inhibit the corrosion of titanium.- Only a few species tend to have a negative influence, including fluoride (11] and oxalate ions, and high concentrations of hydrogen ions under reducing conditions.

Dissolved air, chlorine and other oxidizing agents and muitivalent ions in the process and from corrosion products can extend the usefulness of tita­nium to higher solution concentrations and temperatures. For example, as little as 0,01 to 1.0 weight percent of an oxidant or a heavy metal ion (such as Cr, Cu, Ni, Fe, Mo, V) can profoundly retard corrosion by hydro­chloric, sulfuric and other acid solutions (4, 12]. Aromatic organic chemicals can have a similar effect in sulfuric acid (13], Traces of sili­con [14] and chromium (4,15] inhibit attack in nitric acid at elevated temperatures, Small amounts of water are quite effective in chlorine gas (16] fuming nitric acid [4] and hydrogen gas (17]. Arsenic (18] selenium and tellurium [19] compounds also promote passivity. Effects of ferric ion on the corrosion resistance of pure titanium are shown in Fig. l (20].

120

110

100

90

BO . . a . 70 c_

~

~ BO

~ 40

Weight '*' HCL

Fig. 1: Effect of ferric ions on the resistance of unalloyed Ti in HCl. Corrosion rates )0.127 mm/y to right of each curve (20].

Surf ace Conditions

When resistance is marginal and when it depends on an inhibitor, pitting and crevice corrosion can be troublesome. Under restricted conditions, slow depletion of an inhibitor can result in localized attack, unless precautions Bre taken. Examples of such problems involved titanium in hot chloride salt solutions (21, 22] and wet chlorine gas (23], Multivalent ions and oxidizing conditions are effective as long as they are available but the slow accumula­tion of halide and hydrogen ions is detrimental (24,25,26], However, use of copper (27], PdO/Ti02 (28] and nickel in crevices can inhibit attack, Oxidation of _crevice surfaces before assembly (22] and use of compatible corrosion-resistant alloys (such as Ti-0 •. 15Pd) in the crevice areas (4, 29] also have been effective,

The nature of the surface can influence the behavior of titanium in aggres­sive environments. Foreign deposits, especially embedded iron particles, do result in local attack in some acidic chloride solutions (30], High concentrations of hydrogen in the metal appear to increase the corrosion rate and may be a contributing factor to ·autocatalytic corrosion [4].

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Al~hough surface fouling occurs, it does not appear to promote local attack under normal sea water conditions. Furthermore, 'fouling of titanium can be controlled with chlorine and mechanical treatments [31,32,33].

Anodic treatments are known to enhance resistance to most aggressive envi­ronments and especially if the treatment is continuous [34,35,36,37]. Being an oxidizing treatment it promotes the growth and stability of the surface oxide film. Surprisingly complex shapes, i.ncluding heat exchang­ers, have been protected by applying a small positive voltage [36].

Thermal oxidation in .air for a few minutes at moderate temperatures [e.g., 650°C) has an effect similar to anodizing for reinforcing the surface film. Good results have been claimed for such treatments [38,39] but it may be well to note they are not self-repairing if damaged. Furthermore, exces­sive times and temperatures can cause surface embrittlement.

The easily repaired natural oxide film on the surface helps to account for the metal's resistance to erosion corrosion [40].

Thin surf ace plating of gadolinium oxide and platinum is now widely used to enhance the anode characteristics of titanium [41,42,43,44]. Other treat­ments, including plasma-sprayed iron-, nickel- and cobalt-ferrites have been considered for the same purpose [45]. As noted later, palladium has been implanted in the surface to improve corrosion resistance.

Alloying

For most corrosion-resistant applications, the composition of commercial titanium products is not an important consideration. Most cor.11ne rcial grades have excellent resistance .to a wide variety of chemicals. Many studies indicate, however, that other elements can have important positive or negative effects on the performance in the more aggressive environl'!ents. This is especially so where the. metal has marginal resistance.

Iron is a common impurity which has little solubility in the alpha phase. If it exceeds about 0.05%, a beta or eutectoid phase appears. such phases can provide nuclei for localized corrosion by lowering the pitting poten­tial [4,46,47] in chloride solutions. Iron has also been associated with preferential weld corrosion [48] and pitting in sodium. bromide -[49]. Weld corrosion has also been related to the microstructure and to the oxygen content [SO]. Although rather uncommon, such problems with iron have been avoided by using titanium with iron levels below about 0.07%.

Intentional alloying has been a fruitful means for extending the usefulness of titanium into the more aggressive environments. Early work by Stern and others [51,52,53] has shown that small additions of palladium and other platinum metals will improve the resistance to uninhibited low-pH sol~tions

2438

at moderate temperatures and concentrations. These alloys appear to be. unaffected by most oxidizing environments. The most useful commercial a Uoy of this type is Ti-0. 15 to 0. 20Pd. With a smal L amount of corrosion, the palladium tends to replate on the surface where it passivates the tita­nium. A similar effect can be achieved by surface plating and by implant­ing small amounts of palladium into the surface (54,55].

Nickel, along with molybdenum, was found to be an inexpensive substitute for valladium. The alloy Ti-0.8Ni-0.3Mo is considered to be significantly better than unalloyed titanium in non-oKidizing solutions but not as effec­tive as the Ti-0.15Pd alloy (20].

Alloying additions of molybdenum, tantalum,· niobium and zirconium were found useful in non-oxidizing environments [ 56, 5 7, 58 J. Of these elements, molybdenum and tantalum are especial Ly effective. The Ti-30 to 40Ho and Ti-50Ta beta alloys have eKcellent resistance to the non-oxidizing acids but corrode under oxidizing conditions. Some effects of alloying are illustrated in Figs. 2 [20], 3 [51] and 4 [59].

110

100

00

0

: 80

: Q

j 70

! 80

00 Tl-0.2Pd

40 3 Mo

30 Unalloyed Tl

10 20 30 40 50

Welgl'll'llllizS~

Fig. 2: Resistance of Ti and alloys to aerated H2S04. Corrosion rates to right of each curve >0.127 mm/y (20].

80

Acidic Chi or Idea

Tantalum

Zirconium

Haatelloy B

Tl-15 lo 20 Mo a noble metal

Ti-15 10 20 Mo

Ti &noble met._i

Titanium

Ha1telloy C

- ________ ~8.!1!,fl_!!:Y ~ - - _____ - ______ .,.

. 0

u 0 z

Zirconium Ha1talloy C

No. 20 alloy

-1- Monal

lnconel

316 atalnleH ateel

304 etalnleaa atael - Reducing __.

Fig. 3: A comparison of Ti alloys with other materials in oxidizing and re­ducing solutions with or without el­and with decreasing pH. Each material resists environments below its line [ 51 J •

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Tl

Tl-0.2Pd 20

20

10

10 _ Hli.!!,_RHl•lanc_e Low.Re~ _

(Sea Water) --- --50 100 150 200 250

Temperature Degrees C

Fig. 4: Effect of temperature and chloride ion on the pitting resistance of Ti and Ti-0.2Pd. Resistance decreases with increasing temperature [59].

Applications

Titanium is used extensively in corrosion applications where it is not only cost effective but also where it is convenient to use. For example many corrosion problems with alternate materials are related to poor water quality, and especially to the presence of chlorides. Titanium equipment, however, permits the direct use of such waters without special treatments. Consequently many new and rebuilt heat exchangers and condensers for power, desalination and chemical process plants have been constructed with welded thin-walled titanium products. The choice of titani>.im materials for con­denser tubing has been discussed by a number of authors with emphasis on the corrosion resistance of the titanium alloys [60-63]. Galvanic corro­sion can occur if titanium materials are-coupled with less ~orrosion resistant metals. Condenser tubing failures, causes, mechanisms. and re­commendations for prevention have been discussed [64]. Good heat transfer· rates and high resistance to erosion corrosion [65], stress corrosion, pitting and nonbiological fouling have been reported. Also, it is often not only desirable but convenient to use titanium as a substitute for other less versatile materials. The majority of the metal requirements in many chemical plants could be met with titanium and titanium alloys [66].

Stainless steels have been largely replaced by titanium in many plants pro­ducing and using nitric acid, wet chlorine, chlorine dioxide, chlorates, hypochlorites and chlorides. A considerable amount of titanium is used for the oxidation of many ·organic compounds. Such processes include the nitric acid oxidation of xylene to produce terphthalic acid for the production of synthetic fibers, and the oxidation of ethylene to produce acetyaldehyde [3J. Some of these processes involve both reducing and oxidizing condi­tions and may require the use of alloys such as Ti-0.15Pd or Ti-0.8Ni-0.3Mo.

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Processing .i.n brine environments at temperatures up to at least 175°C is a major use of titanium. Uses e><tend to chlor-alkali plants for the production of wet chlorine, caustic, chlorates, perchlorates, hypochlorites, wet chlorine dioxide ~nd other chlorine chemicals [3].

Apparently the largest single industrial application for titanium involves the use of coated anodes for the production of chlorine. A thin active surface film, consisting of platinum, gadolinium oxide or some other material can provide very efficient dimensionally-stable anodes. Such anodes are also used for the production of chlorates, oxygen and hydrogen; for cathodic protection, fuel c~ls, water purification, electrolytic winning of metals, electrochemical machining and other applications [3,41, 42,44,67].

Additional electrochemical applications include the use of supporting racks for anodizing aluminum parts and the use of titanium anode baskets for electroplating nickel, silver, gold, zinc and other metals. The high ohmic and corrosion resistance of titanium's surface film makes these operations attractive. Coated anodes are used for recovering metal values for leach liquors, including copper, nickel, cobalt, gold, silver, zinc and manga­nese. A large application is in the copper industry where titanium starter sheets are used for the cathodic deposition of refined copper [68].

Other metallurgical applications for titanium components include the leach­ing of copper, nickel, uranium, molybdenum, cobalt, zinc, manganese, gold, silver and other metal ores. The acid solutions employed are very corro­sive but various metallic ions in the solutions inhibit attack and make such operations practical [3,20].

The power industries employ titanium extensively as heat exchangers and auxiliary equipment. Other applications are expected to extend such usage, including turbine blading [69], geothermal developments [70], various heat recovery systems [71J, hydrogen storage methods [72] and ocean thermal energy conversion plants [73].

Corrosion fatigue resistance of stainless steels and Ti-6Al-4V alloy for steam.turbine blade use has been studied by a number of investigators (74] and [75]. The Ti-:6Al-4V alloy was compared with 403 and 17-4PH stainless steels. Results showed that the Ti-6Al-4V had the highest fatigue strength. Environmental effects which were present in all tests were less with the Ti-6Al-4V alloy except in solutions containing silicates [76].

The petroleum and associated industries become a major user of such components. drilling [77], production and treatment and well logging [79].

employ some titanium and could Applications involve offshore

of petroleum and sour gases [78]

Availability of titanium has made certain new processes attractive and practical. An example of this is the use of highly corrosive bleaches for

2441

the pulp and paper industry. Environments include high concentrations of chlorine, chlorides, chlorine dioxide and caustic at elevated temperatur~s [l,80]. The rapid displacement bleaching methods require e><tensive usage of titanium hardware [81].

Titanium is employed in the different metal forms in numerous other appli­cations. Some of the most important of these are for water purification plants [82], deep sea exploration [ 83, 84] and waste control, including the wet air oxidation of sewage sludge [85]. Certain alloys, including Ti-0.8Ni-0.3!1o, show promise for storing nuclear wastes [86]. Results after 434 days of exposure of synthetic ground water at 25°C show titanium to have a low corrosion rate which decreases with time [87]. This corrosion resistance is adversely affected by increasing temperature and/or the presence of sulfur [88]. Titanium should also perform well in flue gas scrubbers even if fluorides are present if desulfurization is achieved with calcium or magnesium compounds [89]. 'The resulting fluorides are essen­tially insoluble in normal pH ranges and· are therefore harmless.

The high strength to weight ratio, corrosion resistance to saline environ­ments, and biocompatibility [90,91,92] of titanium materials make pure titanium and selected alloys useful for surgical implants. The use of titanium as surgical implants has increased in recent years as more elec­trochemical, biocompatibility and mechanical property data have become available regarding titanium's suitability for this purpose. "Titanium Alloys in Surgical Implants" was the subject of an ASTM Symposium, pub­lished in reference [93]. 'The Ti-6Al-4V alloy is the one most widely used for orthopedic implants, although other titanium alloys have potential for use after appropriate testing. For example, the TiNi memory alloy with its high strain recovery is used in dentistry for bracing teeth [94, 95, 96]. The elastic modulus of Ti-6Al-4V alloy, 124 GPa, is closer to that of bone, 16 GPa, than that of 316L stainless steel, 200 GPa or the Co-Cr-Mo alloy, 248 GPA. The corrosion fatigue life of Ti-6Al-4V exceeds those of 316L stainless steel and Co-Cr-Mo when tested under conditions of torsional loading [97]. 'The rotating bending fatigue strength of Ti-6Al-4V is 600 to 660 MPa which compares favorably with that of hot forged Co-Cr-Mo, 500 to 880 MPa [98] and e><ceeds the minimum requirements of 400 MPa [99].

Porous coatings (to help tissue adherence) can be applied to titanium sub­strates by sintering wires or spheres to the surface or by arc plasma spraying [100,101,102]. Other porous titanium implants can be made by a powder metal sintering process [103] to produce void ~~tal composites. Geometric configurations and possible contamination during processing could cause variations in titanium's excellent corrosion resistance to body fluids, and these porous materials should be tested further. Corrosion fatigue behavior also could be altered by .. these processes and should be monitored for each type of production process.

The high corrosion resistance of titanium and its alloys in nearly neutral saline solutions (such as body fluids) is due to the rapid formation of a protective surface oxide film. 'This oxide film has been determined to.fonn as a series of oxide compositions ranging from Ti06 in a gaseous environ­ment [104] to Ti203 in saline solutions [105]. Ti-6Al-4V has a breakdown

2442

potential of 2.0 volts (SCE) whereas the 316L breaks down between 0.2 and 0.3 volts and the Co-Cr-t1o breaks down at 0.47 volts [106]. Repassivation data show that titanium continues to have film formation without complete dissolution at voltages exceeding 7.0 volts.

Cracki!_!_)i Eff~ct~

Titanium alloys exhibit degradation of Load carrying capacity in a wide variety of environments [107], but because of their great corrosion and pitting resistance, this may only appear in pre-cracked materials [108]. The susceptibility parameters include a threshold stress, Oth• or ductility parameter (reduction in area, RA, for example) for smooth or notched speci­mens, a threshold stress intensity parameter, Kth or Krth• for pre-cracked specimens [109], and crack growth rates (da/dt). It is important to real­ize that sustained load crack growth (SLC) can occur in completely inert environments [110] at stress intensity parameter values (K or K1) well below the fast rising load fracture toughness (Kc or Kic) [see ASTM E-399]; therefore stress corrosion cracking (SCC) susceptibility should refer to comparison with SLC thresholds, rather than with Kc or Kic.

Reference will be made to commercial purity (CP) Ti, a alloys (little or no i3), a-jl alloys (some 13), 13 alloys (mostly metastable 13) and to 13-u alloys, which are metastable 13 alloys thermomechanically processed to contain a"'il mixtures. References [ 107, 111] contain further information.

Much information on environmental effects has been published in previous reviews [107,112-116]. Figure S(a) shows schematically the SCC or SLC behavior of specimens loaded then held at a pre-determined load until failure occurs. The rapidity of approach to the threshold (oth or Keh) is a measure of initiation and growth kinetics, which decrease in the order A > B > c. In the pre-cracked case highly susceptible alloys (A) fail within minutes, less susceptible alloys (B) within many minutes or a

"O 11 a r--J'" a "' I

"O I CJ I A 0 ...J I

t1 K K

Fig. 5: Schematic representation of sub-critical cracking behavior. a) Initial stress (o) or stress intensity factor (K) vs time to failure. b) Crack growth rates (da/dt) vs K for SCC case. c) Same as (b) for inert-en~ironment SLC case for low hydrogen concentra­

tion (Cl) and high concentration (C2).

few hours, whereas curve C may indicate failure from SLe, rather than sec, as indicated by slower kinetics despite identical thresholds [117]. Figure 5( b) shows sec growth rate behavior. Region I (dashed) occurs in very active environments such as acidified chloride solutions, methanolic solu­tions, liquid metals, and H2 gas, and no true Kth may exist in this case. Neutral aqueous environments cause only Ila and II behaviors, with a mini­mum initial crack rate and true Kth• Microbranching occurs in Ila, and macrobranching may occur in II if cracking is not severely anisotropic [107]. Region III is mainly due to onset of mechanical overload. Figure 5(c) shows inert environment SLe behavior; no true region II (constant growth rate) exists [118,119]. If a constant loading or straining rate is maintained during testing (slow strain rate technique [120, 121]) behavior shown in Fig. 6 is, observed. Curve l, showing reduction of ductility or in Kth over a critical strain rate range, occurs in neutral aqueous solutions (120,121] or in inert environments due to internal hydrogen embrittlement (HEC and SLC) (122]. Curve 2 is typical of environments causing region I cracking [120].

.s::

:.:: a c

a: 0

> ~

...J

~ (.) 2 :::> 0

S T R A I N R A T E 0 R T-1

Fig. 6: Effect of load or strain rate d£ /dt) on ductility or minimum K to start crack growth.

High values of Krth or low values of da/dt can result from inadequate specimen thickness, pre-cracking or initially loading in air instead of the environment, using step loading techniques, or loading up too slowly [107, 109]. The "decreasing K" test apparently provides results comparable to conventional techniques, avoiding the latter three problems (109,123], and side-grooves can circumvent problems due to inadequate thickness [110, 109,123]. Microbranching may cause arrest after initial crack growth, giving misleading estimates of Kth (124,125]. Degree of environmental cracking susceptibility in non-corrosive environments (neutral aqueous solutions, for example) increases from unnotched through sharp notched to pre-cracked specimens. Apparent unnotched susceptibility may be caused by mechanical factors: for example cleavage cracks in an oxygen-contaminated surface layer caused sec of Ti-SAl-2.SSn weldments in trichloroethylene [ 126].

2444

Cracking of unnotched Ti alloys can occur in red fuming nitric acid, Nz04, methanol, and methanol with Cl-or HCl at low water contents; addition of l-Z% HzO inhibits such cracking. Un-notched SCC also occurs in contact with acidified aqueous halides (except fluorides), methanol vapor, HCl vapor, Br vapor and in Clz (at 450°C), and in Hz gas [ 107 ,127]. Cracking in methanol and Brz vapors is .inhibited by Oz.

sec of .nainly pre-cracked alloys is observed in other environments: ethylene glycol, various hydrocarbon liquids, CCl4, various chlorinated and chloro-fluoro hydrocarbons, hydraulic fluid and hydrazine, as well as in neutral aqueous halides (except fluorides) and dry molten halide salts [107,113]. Unusually susceptible alloys, such as Ti-5Al~Z.5V or Ti-8Al-1Mo-1V aged at low temperatures or Ti-13V-11Cr-3Al in the 100% S phase condition do not require pre-cracks, and show pre-cracked sec in distilled HzO. Other alloys do not experience SCC in aqueous solutions without Cl-, Br-, or 1- ions (107,lZH] and growth rates increase with tt1eir concentration [ 107]. Atmospheric humidity causes some SCC effects [ 1Z9,130], but may not reduce Kth or increase growth rates [ 131], possibly because of crack arrest due to microbranching [1Z3,1Z4]. SCC can be inhibited by oxidizers such as N03-, S04-, Cro4=, FeCl3 or Brz [107,13Z] or by solution dearation [1Z8J.

The electrochemic;:il potential of Ti alloys in aqueous solutions is vari­able, but becomes rapidly more negative on initiation of SCC propagation [ 133]. Cathodic polarization in neutral or alkaline (but not strongly acidic) solutions reduces and even stops propagation due to SCC in neutral or basic solutions, although SLC may occur if Kr is above its threshold [ 107 ,133]. Growth rates increase with increasingly anodic polarization; anodic protection occurs, but only if applied before crack growth initiates [ 107 ,134]. The addition of F-ion or strong acidification prevent cathodic protection, enabling cathodic polarization acceleration of SCC, apparently by enabling hydrogen absorption at the crack tip (107,135]. Crack rates increase with temperature; the region II activation energy is about Z3 kJ/mole in aqueous solutions (107,136].

Hot salt SCC (HSSCC) takes place under stress in contact with halide salts at temperatures from 300°C to 1100°C (115,137,138]. Post-exposure embrittlement can occur, which dissipates with time (138]. No effects occur during unstressed exposure [138,139,140]. These effects are associated with very high hydrogen concentrations [138,139,141,14Z] which increase with Al or S stabilizer content. Alkali chlorides an4 AgCl are most aggressive; bromides and iodides are less severe; fluorides, other salts and hydroxides have no effects, and severity does not correlate with salt melting point (138,139,143]. Activation energy for failure is 63 kJ/mole; for crack propagation it is Z85 kJ/mole (142,144]. sec in dry molten halide salts, except for temperature, is similar to SCC in aqueous halides (113].

Hydrogen Effects

Hydrogen embrittlement includes impact embrittlement, slow strain rate embrittlement (SSRHE), sustained load cracking, and SCC caused by Hz gas

2445

[107,113,1Z2, 145-151]. Impact ernbrittlement is mainly observed in C.P. Ti and dilute a alloys, but can occur in any alloy given sufficient hydrogen content. F)nbrittlemen't arises from hydrogen contamination during processing or environmental exposure and exists prior to straining, and is therefore considered irreversible. Ductility (or toughness Kc) decreases more or less sharply above some minimum strain rate which decreases with increasing temperature.

SSRHE is best documented for a-e alloys, but can occur in most Ti alloys. Embrittlement increases with hydrogen content, and peaks over a range of low strain rates (lo-4 to lo-1/s) as in Fig. 6, curve 1, roughly from ZOOK to 350K [145-lSZ]. Static cracking of notched (stress rupture) or pre­cracked (SLC) specimens shows similar temperature effects [153]. SLC occurs with as little as 8 ppm (weight) hydrogen, and the threshold Kth decreases and .crack rates increase with hydrogen content, Fig. Sc [118, 119,131,154,155]. Region II (constant growth rate) cracking is not observed, with some possible exceptions [113,130,155]. sec effects of moisture in air [1Z9,130] and purely mechanical crack extension [156] can cause confusion in interpretation of SLC results. SSRHE effects occur in fatigue, reducing fatigue life or increasing crack growth rates at very low frequencies or with a dwell period at maximum cyclic load [157-16Z].

Gaseous hydrogen embrittles un-pre-cracked alloys under stress [163-165] and a critical strain rate range for embrittlement is observed [lSZ]. Region II crack growth rates in a-e alloys having some continuity of 8 phase increase with temperature (activation energy -Z3 KJ/mole) and pres­sure, and Kth increases with loading rate [166,167]. a matrix microstruc­tures showed no dependence of rates on Hz pressure. Fatigue is also strongly affected by Hz gas [168,169]. Gas purity is critical to the occurrence of cracking effects [lSZ,168].

Liquid metal embrittlement (LME) and solid metal embrittlement (SME) occur under tensile stresses with or without pre-cracks [107,170,171]. Region II crack rates in Hg can exceed 100 mm/s [107]. SME occurs in contact with Cd, Ag, Au, and possibly Zn [107,171], at temperatures as low as 0.36 of the melting point (for Au). Intimate contact is required, and for SME is achieved by high contact pressures which disrupt the.interfering oxide layer; best results can be achieved by testing in vacuum [171]. A ductile­brittle transition temperature is observed, especially for LME [171,17Z].

Corrosion Fatigue

Figure 7 schematically illustrates environmental effects on fatigue. Inert environments are vacuum or dry argon, and curves ·n illustrate behavior in environents not causing sec such as air, or at cyclic frequencies above about 10 Hz. Most Ti alloys show little effect of sea water or salt solu­tion compared with humidity in air under conditions where SCC plays no part [173-176]. Fatigue strength may be strongly affected in environments causing unnotched sec [173], at any frequency. sec effects occur above the cyclic SCC threshold at low cyclic frequencies or with a dwell at maximum cyclic load, as illustrated in Fig. 7, curves fZ and f3 [173,174]. A

2446

z

60 "' " '-"

f 3 0

1,

LOG(~K)

Fig. 7: Fatigue behavior. a) Cyclic stress range t::.o vs cycles to failure (Nf) of smooth or notched specimens. f1, fz, f3 are decreasing cyclic frequencies. b) Crack growth rate per cycle (da/dN) vs stress intensity factor range (t::.K).

reverse frequency effect occurs at low growth rates [174]. These effects can be unden;tood ir1 terms of an sec-fatigue superposit.i.on model [177].

Fracture t1odes ---------Ambient sec region I and possibly Ila crack growth in aqueous chlorides, methanolic solutions and Hg are intergranular, especially in C.P. Ti, but 1aay become transgranular cleavage as alloy ccontent increases [ 107 ,137, 171,178,179]. Initiation at Kth from fatigue precracks may be intergranu­lar whereas re-initiation at Kth from SCC "pre-cracks" may be cleavage [ 180]. Region II growth is usually cleavage on planes oriented 15° from { 0001} of a phase with flutes in a and a-tl alloys, and on { 001} a planes in a and ~-a alloys. a cleavage in Hg is { 0001} and 10° from { 0001} [ 107, 114,170,181,182]. Intergranular cracking occurs in some aged a alloys, possibly because of grain boundary a formation [113]. SSRHE, SLC and Hz gas sec can cause a cleavage (low H activity) and a-tl · interface cracking (high tf activ.i.ty) in varying proportions depending on composition and microstructure [167,144,122,148,118,183,l84]. Interface cracking facets show "terraces" or arrest marks indicative of incremental growth [118,169]. HSSCC and SME and elevated temperature LME occur by mixed intergranular and cleavage modes [107,137,171]. sec or SLC processes during fatigue cause fracture modes similar to static modes above.

Metallurgical Factors

The ambient SCC and HSSCC susceptibility of a and a-tl alloys are increased by Al, O, and Sn additions, and by aging to produce ordering and az phases (Ti3Al or Ti3Sn), principally because they induce coarse, planar slip [107, 112-116,140-142]. Al may also weaken the protective passive film (185]. C.P. Ti (Ti-40A and Ti-SOA) is immune. Large grains or Widmanstatten a colonies promote sec; thick continuous a phase can increase resistance by impeding cleavage through colonies of a particles. Certain a eutectoid

2447

stabilizers (Si, Fe, Mn, Cu) and hydrogen also may increase susceptibility, either by increasing B phase susceptibility or by influencing microstruc­tures produced by thermo-mechanical processing.· Hydrogen may also increase SCC susceptibility. Similar rules apply to 13 or 8-a alloys, respecting ordering effects, grain size, coarse slip, eutectoid S stabilizers, and such with respect to S phase. S-a alloys designed around isomorphous i3 stabilizers, without w phase or eutectoid compounds, such as Ti-3Al-8V­ocr-4Ho-4Zn, are essentially immune to aqueous sec [186]. Strongly textured cx--jl alloys show low SCC resistance when.{ 0001} a planes are oriented for cleavage; B processing provides a dual benefit of random texturing and providing alternating a and B layers [ 107, 116]. SSRHE sus­ceptibility is usually increased by O, most eutectoid B stabilizers (except Cr) and by B processing or B annealing, and Al additions and isomorphous B stabilizers increase hydrogen tolerance [148,149,151,187]. Hz gas cracking resistance also seems increased by increase in S content [152), and both this and SLC resistance are lessened by B processed (Widmans tat ten) micro­s t ructures [167,154,156], at ordinary hydrogen pressures and high concen­trations, but the inverse is true at low pressures and ordinary concentra­tions ll67,l54,188,L89]. Metallurgical effects on cyclic sec should be similar to the static case. Generally, fatigue strengths in the non-SCC case are greater for cx--jl alloys than for a alloys, and large-scale micro­structures such as result from B processing are deleterious [173, 190]. Corrosion fatigue crack propagation resistance at low cyclic stres·s inten­sities is improved by such microstructures given psuedo-random te><turing [ 19 l] •

Mechanisms ------LME and SME occur by adsorbtion-induced processes [171,19Z]. Hydrogen embrittlement cracking (HEC) mechanisms are evidently responsible for HSSCC [ 137-142], and certainly so for impact HEC, SSRHE, SLC and Hz gas cracking. cx-B interface cracking caused by SSRHE, SLC or Hz gas is very likely due to hydrogen absorption (Hz gas case), diffusion to the· maximum stress zone under stress gradient accelerated by dislocation sweep-in, followed by an increment of crack growth due to a critical hydrogen concentration· causing precipitation of hydrides or other hydrogen-rich phases [lZZ,132,146,161, lb6,167 ,193]. This incremental growth model is supported by the arrest marks or terraces and existence of fracture surface hydrides [166,118,194]. a cleavage in such cases initiates within a grains near the interface [131], possibly because of basal or near basal hydride precipitation [ 195,196], or because of hydrogen modified deformation-induced cracking [193,197]. No evidence for incremental cleavage growth within facets exists [131,198], thus most of the cleavage may be purely mechanical, hydrogen serving only to initiate it. Some evidence for fracture surface hydrides exists for a cleavage [ 198] but because cleavage occurs with only a few ppm (weight) of hydrogen, the generality of a hydride mechanism is questionable [ 131,193].

lntergranular sec is probably caused by active path processes' n 99 ,zoo l, although some evidence exists for a HEC mechanism for methanol-HC1-H20 systems [ZOl]. Active path cracking is unlikely for transgranular cleavage sec [199,ZOO]. Hydrogen may be the critical species: sec does not occur in chloride-containing dimethyl sulfoxide, which solvates IP" ions [ Z02]; crack tip conditions are conducive to H generation [108,l34,135,Z03J; a cleavage

2448

for SCC and SLC are similar [ l 98J; SSRHE caused by methanol-HCl pre­exposure causes identical cleavage+ flutes [!99J; hydrides are found on sec surfaces [198], and there ls some autoradiographic evidence for tritium in fracture surfaces from sec in tritiated solutions [204]. On the other hand, SLC cleavage planes are 0° and 10° from (0001) of a versus 15° for SCC and fluting is not typical for SLC [131,205]; CCl4 SCC is identical as to behavior and cleavage mode with aqueous SCC and involves no hydrogen reactions [107,l!0,181,l82,206J; cathodic polarization stops or does not increase sec rates unless pH is <<l or F"" or other depassivators are in solution [107,!35J; and 2 investigators found no evidence for tritium or hydrogen absorption after sec [207,208], contrary to reference [204]. Other possible critical species and mechanisms have been discussed [ 112, !13,ll4J, such as Cl and various localized dissolution schemes, without satisfactory conclusion. Lynch (209], following Oriani (210], proposed a hydrogen adsorption cracking model which avoids the problem that H diffu­sion in a is far too slow to follow sec growth rates [113,114,199]. The observation that SCC cleavage initiates at a particle edges, rather than within as is true of HEC/SLC [131J, and that Hg causes very similar cleav­age [182,209J support such a model. Some recent results suggest further experimental work in identifying hydrogen's role in SCC. Aqueous SCC causes s01ne intergranular or interfacial secondary cracking in an a-a alloy, whereas CCl4 does not (128], a possible indication of a very weak hydrogen fugacity [ 167 J. A discontinuity in aqueous SCC growth rates has been found at about 310K, the temperature range of the FCC + FCT transition of y Titt2 [2l!J. The meaning of this latter observation is very unclear, but further study seems warranted.

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