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The purpose of this investigation was to determine the corrosion characteristics of Incoloy alloy 825 in 1.0 to 15.0 normal sulfuric and 0.5 to 3.0 normal hydrochloric acids at 25 to 60°C in both nitrogen-saturated and air-saturated acids, using potentiostatic polarization techniques. Anodic polarization curves were obtained by changing the potential of an Incoloy 825 test electrode and measuring the resulting current. In both acids, there is no difference in the polarization characteristics in air-or nitrogen-saturated solutions. In 1.0 to 15.0 normal sulfuric acid at 25 to 60°C, the alloy spontaneously passivates and does not exhibit a significant active region. In 0.5 normal hydrochloric acid at 25°C, Incoloy 825 is spontaneously passive, but is active at higher concentrations. At 25°C, as the hydrochloric acid concentration increases from 1.0 to 3.0 normal,the critical current density increases from 260 to 5900 microamperes per square centimeter. At 40°C, as the acid concentration increases from 0.5 to 2.0 normal, the critical current density increases from 25 to 28,000 microamperes per square centimeter. In 1.0 normal hydrochloric acid at 25°C, the corrosion rate calculated by weight loss measurements for potentiostatically controlled Incoloy 825 agrees closely with the corrosion rate calculated from polarization current densities, and indicates that the elements dissolve in the proportions present in the alloy. Potentiostatically passivated Incoloy 825 in 1.0 normal hydrochloric acid at 25°C does not exhibit a stable passive condition, reverting to the active state in less than 12 hours. / M.S.

The effect of crack tip strain rate (CTSR) on environmentally assisted cracking was studied for alloys AA-7075 (Al-Zn-Mg-Cu) and AA-8090 (Al-Li-Cu-Mg) in the artificially aged condition. Fatigue pre-cracked double cantilever beam (DCB) specimen were employed with the crack plane parallel to the rolling plane. The cracking behaviour under monotonic and cyclic loading conditions was investigated in aqueous sodium chloride solutions with and without additions of sodium chromate as a corrosion mhibitor. CTSR values were described in terms of K-rate ∆K/∆t (ie. dK/dt) as a measured average over the loading period of a fatigue cycle. This allowed a comparison with CTSR's of monotonically increasing load or constant load tests. At frequencies ≤1 Hz, the load was applied with a triangular wave form. A high frequency of 30 Hz was obtained by sinusoidal loading. Expressed as K-rate, CTSR values were varied over 7 orders of magnitude from 10⁵MPa√m/s to 10² MPa√m/s. Stress intensities investigated were mainly around region II values with respect to SCC K-log(da/dt) behaviour. At low K-rates, real time crack velocities (da/dt) measured under monotonic slow loading or constant load conditions were comparable to crack velocities obtained with cyclic loading experiments. As the K-rate was increased from low values, typical of constant load experiments, the real time crack velocities decreased. This was caused by plasticity induced crack growth retardation effects and a decrease in crack tip film rupture events during the unloading part of a cycle. The crack propagation rate decreased until minimal crack advance increments per cycle were dictated by mechanical parameters acting on a hydrogen embrittled crack tip region. Under monotonic loading conditions region II crack velocities were not influenced by an increase in K-rate which was explained with a mass transport controlled cracking process. Tests with alloy 7075 at intermediate K-rates and a high R-ratio of 0.78 allowed a crack tunnelling mechanism to operate. This overcame the plasticity induced crack growth retardation and, therefore, cracks propagated at the same rates as during low K-rate tests where no retardation phenomena were encountered. Scanning electron microscope investigations revealed a striated intergranular fracture surface of alloy 7075 if tested at K-rates above the transition value to K-rate independent crack propagation rates. Individual striations could be matched on opposing fracture surfaces and the striation spacing corresponded to the average crack propagation increment per cycle. The striations, therefore, were formed as part of the crack advance during every fatigue cycle. At the lower K-rates no striations were present but micro tear ridges could be found on the intergranular fracture facets indicating that dissolution processes alone did not cause the intergranular crack advance. Alloy 8090 did not reveal significant changes in fractography over the entire K-rate range investigated, except at the highest K-rates where small interlocking steps could be detected on some opposing transgranular fracture surfaces. In general, however, the crack path at all K-rates was mainly intergranular with dimpled fracture facets. Alloy 8090 exhibited a high resistance to SCC with fatigue pre-cracked DCB specimen. Therefore, to obtain crack velocity values with low K-rate monotonic loading tests very long test durations would have been necessary. It is concluded that the transition from intergranular SCC to intergranular CF occurs at a critical K-rate. Below the critical K-rate crack velocities are not increased by cyclic loading. Instead crack growth retardation effects can result in lower real time crack velocities than those typical for constant load tests at comparable stress intensities but much lower K-rates. / Applied Science, Faculty of / Materials Engineering, Department of / Graduate

In recent years, the demand for alternatives to fossil fuels has risen dramatically, and ethanol fuel has become an important liquid fuel alternative globally. The most efficient mode of transportation of petroleum-based fuel is via pipelines, and due to the 300% increase in ethanol use in the U.S. in the past decade, a similar method of conveyance must be adopted for ethanol. Low-carbon, low-alloy pipeline steels like X52, X60, and X65 comprise the existing fuel transmission pipeline infrastructure. However, similar carbon steels, used in the ethanol processing and production industry, were found to exhibit stress corrosion cracking (SCC) in ethanol service. Prior work has shown that contaminants absorbed by the ethanol during distillation, processing or transport could be the possible determinants of SCC susceptibility; 200 proof ethanol alone was shown not to cause SCC in laboratory studies. To ensure the safety and integrity of the pipeline system, it was necessary to perform a mechanistic study of SCC of pipeline steel in fuel grade ethanol (FGE). The objective of this work was to determine the environmental factors relating to SCC of X65 steel in fuel grade ethanol (FGE) environments. To accomplish this, a systematic study was done to test effects of FGE feedstock and common contaminants and constituents such as water, chloride, dissolved oxygen, and organic acids on SCC behavior of an X65 pipeline steel. Slow strain rate tests (SSRT) were employed to evaluate and compare specific constituents' effects on crack density, morphology, and severity of SCC of X65 in FGE. SCC did not occur in commercial FGE environments, regardless of the ethanol feedstock. In both FGE and simulated fuel grade ethanol (SFGE), SCC of carbon steel was found to occur at low water contents (below 5 vol%) when chloride was present above a specific threshold quantity. Cl- threshold for SCC varied from 10ppm in FGE to approximately 1 ppm in SFGE. SCC of carbon steel was inhibited when oxygen was removed from solution via N2 purge or pHe was increased by addition of NaOH. During SSRT, in-situ¬ electrochemical measurements showed a significant role of film rupture in the SCC mechanism. Analysis of repassivation kinetics in mechanical scratch tests revealed a large initial anodic dissolution current spike in SCC-causing environments, followed by repassivation indicated by current transient decay. In the deaerated environments, repassivation did not occur, while in alkaline SFGE repassivation was significantly more rapid than in SCC-inducing SFGE. Composition and morphology of the passive film on X65 during static exposure tests was studied using X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM). Results showed stability of an air-formed native oxide under static immersion in neutral (pHe = 5.4) SFGE, and dissolution of the film when pHe was decreased to 4.3. XPS spectra indicated changes in film composition at high pHe (near 13) and in environments lacking sufficient water. In light of all results, a film-rupture anodic-dissolution mechanism is proposed in which local plastic strains facilitates local breakdown of the air-formed oxide film, causing iron to dissolve anodically. During crack propagation anodic dissolution occurs at the crack tip while crack walls repassivate preserving crack geometry and local stress concentration at the tip. It is also proposed that SCC can be mitigated by use of alkaline inhibitors that speed repassivation and promotes formation of a more protective Fe(OH)3 film.

Cold rolled carbon steel 1018C is widely used in pressurized fuel pipelines. For those structures, stress corrosion cracking (SCC) can pose a significant problem because cracks initiate late in the lifetime and often unexpectedly, but grow fast once they get started. To ensure a safe operation, it is crucial that any damage can be detected before the structural stability is reduced by large cracks. In the early stages of SCC, microstructural changes occur which increase the acoustic nonlinearity of the material. Therefore, an initially monochromatic Rayleigh wave is distorted and measurable higher harmonics are generated. Different levels of stress corrosion cracking is induced in five specimens. For each specimen, nonlinear ultrasonic measurements are performed before and after inducing the damage. For the measurements, oil coupled wedge transducers are used to generate and detect tone burst Rayleigh wave signals. The amplitudes of the received fundamental and second harmonic waves are measured at varying propagation distances to obtain a measure for the acoustic nonlinearity of the material. The results show a damage-dependent increase in nonlinearity for early stages of damage, indicating the suitability for this nonlinear ultrasonic method to detect stress corrosion cracking before structural failure.