Environmentally Assisted Cracking: Overview of Evidence for an Adsorption-Induced Localised-Slip Process,

The history of dynamic straining in stress corrosion cracking studies and the evolution of the slow strain rate test (SSRT) are reviewed. Smooth and notched specimens; the importance of strain rate, electrode potential, and other environmental factors; the evaluation of test results; and comparisons to other techniques are addressed. The SSRT’s application in research for oil and gas sour service, buried natural gas pipelines, ethanol transportation, nuclear power, low pressure turbines, and mechanism studies is summarized and its usage by material, industry, and geographic region quantified. Standard test procedures are compared and improvements suggested. The more recent use of cyclic loading is discussed and areas for future study proposed.

The Slow Strain Rate Stress Corrosion Cracking Test—A 50 Year Retrospective

An investigation has been made of the propagation of transgranular stress-corrosion cracks in monocrystals and polycrystals of Admiralty Metal tested in ammoniacal solutions. Studies were made of the topography and orientation of the fracture surfaces, the acoustic emission during crack propagation, and the stress dependence of the crack velocity. Cracking was found to occur on parallel but displaced {110 facets, and the steps between the fracture facets also were crystallographic in nature, consisting of alternating segments of two {111 planes. The overall direction of the steps was found to be perpendicular to the crack front, and examination of opposing fracture surfaces indicated that the surface features are matching and interlocking. While not generally resolvable with the SEM, crack-arrest markings could be detected on the fracture faces under certain conditions, and this, together with the occurrence of discrete acoustic-emission events, indicated that propagation is discontinuous. Based on these and other observations, it is suggested that propagation occurs by discontinuous cleavage on {110 planes and that the steps are produced by shear on {111 planes. Possible mechanisms of embrittlement are discussed.

The propagation of transgranular stress-corrosion cracks in admiralty metal

Hydrogen environment-assisted cracking (HEAC) of Monel K-500 is quantified using slow-rising stress intensity loading with electrical potential monitoring of small crack propagation and elastoplastic J-integral analysis. For this loading, with concurrent crack tip plastic strain and H accumulation, aged Monel K-500 is susceptible to intergranular HEAC in NaCl solution when cathodically polarized at −800 mVSCE (E
 A, vs saturated calomel) and lower. Intergranular cracking is eliminated by reduced cathodic polarization more positive than −750 mVSCE. Crack tip diffusible H concentration rises, from near 0 wppm at E
 A of −765 mVSCE, with increasing cathodic polarization. This behavior is quantified by thermal desorption spectroscopy and barnacle cell measurements of hydrogen solubility vs overpotential for planar electrodes, plus measured-local crevice potential, and pH scaled to the crack tip. Using crack tip H concentration, excellent agreement is demonstrated between measurements and decohesion-based model predictions of the E
 A dependencies of threshold stress intensity and Stage II growth rate. A critical level of cathodic polarization must be exceeded for HEAC to occur in aged Monel K-500. The damaging-cathodic potential regime likely shifts more negative for quasi-static loading or increasing metallurgical resistance to HEAC.

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Measurement and Modeling of Hydrogen Environment-Assisted Cracking in Monel K-500

Advances in the theory and practice of stress corrosion cracking (SCC) are reviewed for the period 1965–1990. The proceedings of two landmark conferences are used as a basis for discussion: Ohio State University (1967) and Kohler, WI (1988). The discussion is developed around the following topics: metal–environment combinations, testing, fractography, metallurgical aspects, electrochemical aspects and crack chemistry, mechanisms, and prediction and mitigation. It is concluded that the main developments since 1967 are the recognition of the lack of specificity of SCC environments, the use of slow strain rate and fracture mechanics testing, quantitative SEM fractography, studies of grain boundary structure and composition, transient electrochemistry of bare metal surfaces, measurement and modelling of crack chemistry, elaboration of several SCC models, including slip-dissolution and film induced cleavage, and mitigation by alloy development or anodic protection.

Stress corrosion cracking: 1965–1990

Alloy 260 brass specimens under stress were exposed at room temperature to 15 N aqueous ammonia solution with 8 g/1 of cupric copper predissolved. This environment causes tarnishing of the brass surface and intergranular stress corrosion cracking. Scanning electron microscopy, energy dispersive X-ray analysis, and Auger electron spectroscopy were employed to study fractography, corrosion product composition and distribution within the stress corrosion crack, and fracture surface chemistry characteristic of stress corrosion cracking in this system. A thin oxidized film was detected by Auger spectroscopy at the leading edge of the propagating crack. With continued exposure to the corrosive environment, deposits form on the fracture surface, then coalesce to form a continuous tarnish film that is depleted of zinc. No bulk depletion of zinc was detected in the alloy at the stress corrosion crack leading edge. No evidence of noncrystallographic crack arrest marks was found on the intergranular fracture surface.

Stress corrosion cracking of alpha brass in a tarnishing ammoniacal environment: Fractography and chemical analysis

The susceptibility of Monel 400® to embrittlement by mercury and by hydrogen was measured as affected by thermal treatment. Specimens were annealed at 900°C and either quenched or furnace cooled. Auger analysis revealed phosphorus segregated to the grain boundaries; furnace cooling causes the boundary concentration to be about twice that of the quenched material. Tensile tests were conducted with specimens (1) in air, (2) coated with mercury, or (3) cathodically precharged with hydrogen. Fractures in air were completely ductile, and ductility was not affected by thermal treatment. Fractures of mercury-coated and hydrogen-charged samples were brittle and intergranular; elongation to fracture increased significantly with increasing concentration of grain boundary phosphorus. The results are discussed in terms of additive and interactive mechanisms.

The Influence of Grain Boundary Phosphorus Concentration on Liquid Metal and Hydrogen Embrittlement of Monel 400

Admiralty brass (Cu-Zn-Sn) is shown to be susceptible to stress corrosion cracking (SCC) in copper sulfate solutions. Fracture surfaces of the resultant transgranular cracks are characteri...

Stress Corrosion Cracking of Admiralty Brass in Aqueous Copper Sulfate

A set of straightforward experimental techniques are described for the examination of slow strain rate stress corrosion cracking (SCC) of sheet deforming under nearly all multiaxial deform...

Slow Strain Rate Stress Corrosion Cracking under Multiaxial Deformation Conditions: Technique and Application to Admiralty Brass

The slow strain-rate stress corrosion cracking (SCC) of Admiralty brass sheet in an aqueous 0.1M CuSO4 solution has been studied over a range of strain paths from uniaxial to equibiaxial tension. At a constant bulk solution pH and open circuit potential, the brass undergoes transgranular SCC characterized by cleavage-like cracks propagating on a macroscopic plane normal to the maximum principal strain axis for all strain paths. The average crack growth velocity is also independent of deformation path. It is thus concluded that the mechanism of transgranular SCC in this system does not depend on multiaxial strain path for the range of stress states examined. However, the fracture strain data show that the slow strain-rate SCC of the brass sheet results in ductility losses which are much larger in equibiaxial tension than in either uniaxial or plane strain tension. This behavior is attributed to stress corrosion cracks acting as linear imperfections whose presence causes failure of the uncracked ligament by a form of localized necking and whose influence is dependent not only on time but also on strain path.

L. A. Heldt

Papers

Stress corrosion cracking of alpha brass in a tarnishing ammoniacal environment: Fractography and chemical analysis

The Influence of Grain Boundary Phosphorus Concentration on Liquid Metal and Hydrogen Embrittlement of Monel 400

Stress Corrosion Cracking of Admiralty Brass in Aqueous Copper Sulfate

Slow Strain Rate Stress Corrosion Cracking under Multiaxial Deformation Conditions: Technique and Application to Admiralty Brass

The influence of deformation path on the slow strain-rate stress corrosion cracking of admiralty brass sheet