Hot Salt Stress Corrosion Cracking Research Articles

The effect of NaCl-induced corrosion on Ti60′s hot salt stress corrosion cracking

The hot salt stress corrosion cracking (HSSCC) behaviour of Ti60 alloy at 600 °C in moist air has been studied, focusing on NaCl-induced corrosion as an embrittlement mechanism. NaCl-induced active corrosion was proven to happen at the crack front. This corrosion process promoted the loss of metal and the ingress of O, leading to quick O embrittlement of the alloy. In the alloy, the β phase can somewhat inhibit HSSCC due to its less sensitivity to corrosion-promoted O ingress. Phase boundaries have strong corrosion sensitivity to NaCl-induced corrosion. Thus, phase boundaries are prone to cracking.

AgCl-induced hot salt stress corrosion cracking in a titanium alloy

The mechanism of AgCl-induced stress corrosion cracking of Ti-6246 was examined at 500 MPa and 380 °C for 24 h exposures. SEM and STEM-EDX examination of a FIB-sectioned blister and crack showed that metallic Ag was formed and migrated along the crack. TEM analysis also revealed the presence of SnO2 and Al2O3 corrosion products mixed into TiO2. The fracture surface has a transgranular nature with a brittle appearance in the primary α phase. Long, straight and non-interacting dislocations were observed in a brittle appearance fractured primary α grain, with basal and pyramidal traces. This is consistent with a dislocation emission view of the cracking mechanism.

Hot Salt Stress Corrosion Cracking Study of Selective Laser Melted Ti-6Al-4V Alloy

The hot salt stress corrosion cracking (HSSCC) behavior of the selectively laser melted (SLM) Ti-6Al-4V alloy in the as-built and heat-treated conditions was studied at 300 and 400 °C. The HSSCC resistance of very susceptible as-built alloy can be significantly increased through annealing at 750 °C, losing only a small amount of ultimate tensile strength (UTS). The improvement in the HSSCC resistance is brought out by a transformation of martensite into lamellar α + β. The absence of equiaxed α found in the wrought alloy seems to help in increasing the HSSCC resistance of SLM Ti-6Al-4V alloy at high strength level.

Hot salt stress corrosion cracking by silver chloride in Ti-6Al-2Sn-4Zr-6Mo

Hot salt stress-corrosion cracking (HSSCC) of titanium alloys has been found in the presence of chloride and other halide salts. Here, a series of AgCl HSSCC tests on Ti-6246 were conducted using a two-point bending rig at 380-500°C to determine the boundary conditions of stress and temperature. Rapid failure occurred after 24 hours above 440 °C, whilst the trigger stress of HSSCC was determined to be 400 MPa at 380 °C. Energy dispersive X-ray during scanning transmission electron microscopy (STEM-EDX) suggested the formation of metallic silver and chlorides of active alloying elements Al, Sn and Zr. The transgranular fracture surface is expected to be linked to an underlying hydrogen embrittlement via hydrogen charging from the corrosion reactions.

The mechanisms of hot salt stress corrosion cracking in titanium alloy Ti-6Al-2Sn-4Zr-6Mo

Hot salt stress corrosion cracking in Ti 6246 alloy has been investigated to elucidate the chemical mechanisms that occur. Cracking was found to initiate beneath salt particles in the presence of oxidation. The observed transgranular fracture was suspected to be due to hydrogen charging; XRD and high-resolution transmission electron microscopy detected the presence of hydrides that were precipitated on cooling. SEM-EDS showed oxygen enrichment near salt particles, alongside chlorine and sodium. Aluminium and zirconium were also involved in the oxidation reactions. The role of intermediate corrosion products such as Na2TiO3, Al2O3, ZrO2, TiCl2 and TiH are discussed.

Effect of long term exposure and hydrogen effects on HSSCC behaviour of titanium alloy IMI 834

Abstract Hot salt stress corrosion cracking (HSSCC) behaviour of alloy IMI 834 was studied to examine the effect of long-term exposure and the role of hydrogen. Studies were conducted by exposing the salt-coated specimens at 300, 350 and 400 °C, at different stress levels for 1000 h, and in the unstressed condition; and the role of hydrogen in HSSCC was studied by using X-ray diffraction. Although the specimens passed 1000 h of hot salt exposure test at stress levels higher than their yield strength, at these temperatures, the post-exposure tensile tests showed a significant loss in ductility and the presence of micro cracks. The study shows the presence of hydrides, which are responsible for HSSCC.

Understanding the “blue spot”

Abstract During hot component fatigue tests there have been two cases of low life crack initiation of gas turbine rotating parts manufactured from the Titanium alloy Ti-6246. Both exhibited a small (~ 0.1 mm) elliptical ‘blue spot’ at the origin. Through validated striation count work and fracture mechanics it was established that fatigue had propagated with a near-nil initiation life. Early investigation suggested that the ‘blue spot’ was possibly a region of stage 1 fatigue growth, and was therefore a material behaviour concern with potential implications for service. During an investigation of a later cracking incident in this alloy, subsequently shown to have resulted from stress-corrosion cracking (SCC), near-identical fractographic characteristics to that seen in the “blue spot” were found that subtly differentiated it from stage 1 fatigue. Also, similar ‘blue spots’ have since been identified on Ti6246 Laboratory hot LCF test specimens and found to have been due to contamination by NaCl, through the application of focussed long-term EDX examination and other novel chemical analyses techniques. By the application of those techniques, fractography, and comparison against these specimens, Rolls-Royce and Imperial College London have collaborated to show that the original two component ‘blue spots’ were subtle examples of NaCl-related Hot Salt Stress-Corrosion Cracking (HSSCC). Such cracking has not been found to occur in service components, due to air pressure within the engine, and the effect is therefore confined to Laboratory and component tests at near-atmospheric pressure or below.

Environmentally assisted fatigue crack nucleation in Ti–6Al–2Sn–4Zr–6Mo

An unexplained feature was observed at the fatigue crack origin of a number of α/β titanium specimens tested at 450°C in the low cycle fatigue regime. The origin was discoloured blue but this was not a result of temper colouration; this feature sometimes resulted in large reductions in fatigue lives. A number of specimens were examined to determine the cause and formation mechanism of these “blue spots.” This feature was associated with elevated oxygen and chloride levels and the presence of sodium. A mechanism based on hot-salt stress-corrosion cracking is proposed and the implications for service components are discussed.

The stress-corrosion cracking susceptibility of near-α titanium alloy IMI 834 in presence of hot salt

The hot salt stress corrosion cracking (HSSCC) behaviour of a near α titanium alloy, IMI 834, was studied using slow strain rate test method in the temperature range 250–450°C. The alloy was found to be susceptible to HSSCC at 300°C and above; and the susceptibility increased with an increase in the test temperature. The secondary cracks grew exponentially with temperature. HSSC cracks initiated at α/β interfaces and grew across the primary α grain and along the α/β interfaces.

Effect of process conditions on structure and properties of silver-tin oxide contacts : B.P.Joshi et al. (University of Baroda, Baroda, India.)

The H.S.S.C.C. (Hot-Salt-Stress Corrosion-Cracking) of certain titanium alloys (Ti 5 Al-2.5 Sn; Ti-6 Al-4 V; Ti-8 Al-1 Mo-1 V; Ti-13 V-11 Cr-3 Al) was investigated both by means of exposure experiments with U-bend specimens and by means of electrochemical experiments with a macro-couple model proposed by the authors of this paper. With this model it has been possible to confirm that there is an electrochemical mechanism for this form of corrosion and to perfect a system capable of revealing the incubation period of cracks as a function of environmental, metallurgical and mechanical factors with good reproducibility and accuracy.

Effet d'un grenaillage de précontrainte sur la résistance à la corrosion sous contrainte de l'alliage de titane Ti 6246

Hot salt stress corrosion tests performed on a specific test bench have revealed a suceptibility of the tested material to hot salt stress corrosion cracking. This alloy may have to face the applied stress corrosion conditions in service when it is used to realise gas turbine compressors. It is, therefore, necessary to apply a surface treatement to this material that protects it from the corrosion effect. The benefical effect of shot peening is evaluated in this paper. We see that this surface treatment provides a protective effect that remains efficient up to 500 °C.

Isothermal oxidation behavior of alpha-2 titanium aluminide alloys

Monolithic alpha-2 alloys are being considered for applications in gas turbine engines at temperature between 593 and 705C. The alpha-2 alloys have better creep and oxidation resistance than conventional Ti alloys in this temperature range, but concerns over low temperature mechanical properties, interstitial embrittlement and hot salt stress corrosion cracking have limited the application of these alloys. Protective coatings address environmental concerns, while improved processing and chemistry may improve mechanical properties. The compositions of alpha-2 alloys have evolved over time to obtain a better balance of properties. Two prospective alpha-2 compositions are Ti-25Al-10Nb-3V-1Mo and Ti-24.5Al-12.5Nb-1.5Mo. This work characterizes the isothermal oxidation behavior of these two alloys in the 593-705C temperature range and discusses observed differences. The isothermal oxidation behavior in air of Ti-24.5Al-12.5Nb-1.5Mo was superior to Ti-25Al-10Nb-3V-1Mo and T-6242 in the temperature range of 593-705C. The 12.5-1.5 alloy was better because an external alumina scale formed which grew more slowly than the TiO[sub 2]-based scales observed on the 10-3-1 and Ti-6242. The alumina scale formed on the 12.5-1.5 alloy because interstitials like O, N and C lower the thermodynamic activity of Ti to levels where Al[sub 2]O[sub 3] formation is favored. In the absence of microstructural effects, the V inmore » 10-3-1 prevented continuous external alumina from forming.« less

Hot Salt Stress Corrosion Cracking of a Titanium Alloy: The Phenomenon in View of Aero Gas Turbine Operating Conditions

Hot Salt Stress Corrosion Cracking of a Titanium Alloy: The Phenomenon in View of Aero Gas Turbine Operating Conditions

Hot-salt-stress-corrosion cracking of titanium alloys

The H.S.S.C.C. (Hot-Salt-Stress Corrosion-Cracking) of certain titanium alloys (Ti 5 Al-2.5 Sn; Ti-6 Al-4 V; Ti-8 Al-1 Mo-1 V; Ti-13 V-11 Cr-3 Al) was investigated both by means of exposure experiments with U-bend specimens and by means of electrochemical experiments with a macro-couple model proposed by the authors of this paper. With this model it has been possible to confirm that there is an electrochemical mechanism for this form of corrosion and to perfect a system capable of revealing the incubation period of cracks as a function of environmental, metallurgical and mechanical factors with good reproducibility and accuracy.

Hot Salt Stress Corrosion Cracking of Titanium Alloys: An Improved Model for the Mechanism

Abstract An improved model explains the hot salt stress corrosion cracking (HSSCC) of titanium alloys. Basically, corrosion produced hydrogen from a secondary cathodic reaction is responsible for the premature failure which can occur when tensile stressed titanium alloys are exposed to chloride containing hot salt environments. The electrochemical model is consistent with all observations and data which have been reported and verified by earlier HSSCC investigators. Examination of the mechanism provides an explanation why titanium alloy components in aircraft gas turbine engines probably have not failed by HSSCC in service. Microstructural factors are involved in the cracking process. Optically observable microstructures probably should not be used in discussing the relative susceptibility of titanium alloys to HSSCC.

The effect of microstructure on the hot salt stress corrosion susceptibility of titanium alloys

It was the purpose of this study to identify what metallurgical processes could be applied to commercial structural titanium alloys to increase the hot salt stress corrosion (HSSC) threshold stress and therefore increase their range of application. Toward this purpose Ti-6A1 and Ti-6A1-4V were evaluated as a function of microstructural variables. Specifically, it was shown that both increasing amounts of cold work and increasing grain size decrease HSSC resistance of Ti-6A1. Also, for Ti-6A1-4V it was shown that preferred orientation can have a profound effect on the HSSC resistance. Crack initiation time, crack growth rate, and stress rupture life were evaluated in Ti-6A1-4V as a function of applied stress at 727 K. These results indicate that HSSC cracking can be described by a critical resolved shear stress criterion, and that increased high temperature creep resistance and decreased room temperature notch rupture strength combine to increase HSSC susceptibility and embrittlement.

An electrochemical model for hot-salt stress-corrosion of titanium alloys

An electrochemical model of hot-salt stress-corrosion cracking of titanium alloys is proposed to correlated the diverse observations made concerning this process. According to the model, an oxygen-concentration cel exists between the exposed surface of the corrosion specimen and shielded areas, such as under salt crystals and at crack tips. Oxygen is reduced at the exposed cathode regions, while substrate dissolution occurs at the shielded anodes with the formation of complex halides. The hydrolysis of these halides is the source of hydrogen that subsequently embrittles the substrate. The model is consistent with observations made concerning the distribution of various ions and absorbed hydrogen in the vicinity of stress-corrosion cracks. The model is also consistent with the, effects of salt composition and various surface additives on the susceptibility to stress-corrosion cracking as well as the effects of alloying and coatings.

Effect of Initial Hydrogen Content of a Titanium Alloy on Susceptibility to Hot Salt Stress Corrosion

The Ti-8Al-1Mo-1V alloy was tested in four conditions: mill annealed (70 ppM H), duplex annealed (70 ppM H), vacuum annealed to an intermediate (36 ppM) and a low (9 ppM H) hydrogen level. Material annealed at 650 C (duplex condition) exhibited resistance to hot-salt stress corrosion superior to that exhibited by material in the mill-annealed condition. Reduction of the alloy hydrogen content from 70 to as low as 9 ppM did not influence resistance to hot-salt stress corrosion embrittlement or cracking.

Hydrogen in hot-salt stress corrosion cracking of titanium-aluminum alloys

Experimental studies show that hydrogen produced during the corrosion of titanium alloys by halide salts and subsequently absorbed by the titanium is responsible for cracking of these alloys. Autoradiography and proportional β counting were used to detect the diffusion of hydrogen into the metal matrix. Similar failure modes were observed in fractographic examinations of surfaces produced by hot-salt cracking and surfaces of precracked specimens pulled to failure in high pressure hydrogen gas. Stressed titanium-aluminum alloys cracked under low-energy proton bombardment and show that failure can be caused by the effects of stress and hydrogen alone. Similarities to hot-salt cracks were noted, and the approximate concentrations required to initiate and propagate cracks were calculated.

Tensile Hot Stage for Metallographic Examination

An elevated temperature, controlled atmosphere, and metallographic stage for observing flow and fracture on a microscopic scale is described. A load cell in series with the test specimen allows measurement and control of the applied loads. The specimen is enclosed in a sealed chamber so that the test environment can also be controlled by regulating the composition and pressure of the chamber atmosphere. Elevated temperatures are obtained with a Nichrome or tungsten heater, the selection depending on the temperature and atmosphere required. The apparatus has been used to study the hot salt stress corrosion cracking of titanium alloys.