Fraser King

Characteristics of Near-Neutral-pH Stress Corrosion Cracks in an X-65 Pipeline

Abstract Two patches of near-neutral-pH stress corrosion cracks (SCC), as developed on a pipeline steel during service, have been characterized. The two patches can be characterized as containing e...

Microstructure Dependence of Stress Corrosion Cracking Initiation in X-65 Pipeline Steel Exposed to a Near-Neutral pH Soil Environment

Abstract A study was carried out to understand mechanisms of stress corrosion crack initiation in an X-65 pipeline steel exposed to a near-neutral pH soil environment under a mechanical loading condition typical of a pipeline operating in the field. Microcracks initiated on the polished surface of the X-65 pipeline steel after long-term exposure at open-circuit potential in a near-neutral pH synthetic soil solution. It was found that these microcracks were initiated mostly from pits at metallurgical discontinuities such as grain boundaries, pearlitic colonies, and banded phases in the steel. Strong preferential dissolution was observed along planes of the banded structures in the steel. Selective corrosion at these metallurgical discontinuities is attributed to the anodic nature of those areas relative to the neighboring steel surface. Consistent with previous observations, no increased susceptibility to crack initiation was found at physical discontinuities mechanically introduced into the surface of ste...

Precyclic-Loading-Induced Stress Corrosion Cracking of Pipeline Steels in a Near-Neutral-pH Soil Environment

Abstract Two pipeline steels of different grades were used to study the effect of cyclic loading prior to corrosion exposure upon the initiation of stress corrosion cracks in a near-neutral-pH soil...

Container Materials for the Storage and Disposal of Nuclear Waste

A wide range of alloys have been considered as candidate container materials for the storage and disposal of nuclear waste. The goal of the majority of national nuclear waste management programs is the ultimate disposal of the waste, although, depending upon the strategy being followed, disposal may come only after an extended period of storage. The management strategy depends on the nature of the waste, with intermediate level waste (ILW) generally being stored for a longer period before disposal than is the case for higher activity wastes, such as high-level waste (HLW) from reprocessing activities or spent fuel (SF). This review describes the corrosion issues associated with the storage and disposal of both ILW and HLW/SF. Various factors enter into the decision of which material to select for the container, of which the corrosion behavior in the expected service environment is only one. The corrosion behavior of the container material(s) is closely tied to the nature of the environment to which the co...

Model for the Microbiological Corrosion of Copper Containers in a Deep Geologic Repository

A model has been developed to predict the impact of microbiological processes on the long-term corrosion behaviour of copper containers in a deep geologic repository. The model accounts for a range of aerobic and anaerobic microbial processes. Various factors expected to limit the extent of microbial activity in the repository, such as the lack of water, evolving redox conditions, and the nutrient-poor environment, are taken into account in the model. Amongst other effects, the model predicts that microbial activity will not occur close to the container in the presence of highly compacted bentonite buffer material.

Characterizing Pressure Fluctuations on Buried Pipelines in Terms Relevant to Stress Corrosion Cracking

Pressure records for high pressure hydrocarbon pipelines that have previously shown to have occurrences of trans-granular stress corrosion cracking (tSCC) were supplied by ten Canadian Energy Pipeline Association (CEPA) member companies. These pressure records were analyzed by performing six methods of pressure cycle counting described in the ASTM standard E1049-85. A comparison was made of the magnitude and frequency of these counted pressure cycles at compressor/pump discharge and suction locations as well as intermediate locations along the pipeline. An attempt to relate the derived pressure cycles to tSCC growth was made by applying previously published “superposition” type cumulative damage models in calculating tSCC growth rates. Information about the relative importance of fatigue growth in the total tSCC lifecycle as well as the ability to distinguish tSCC susceptible areas on a pipeline based on differences in pressure cycles was gained. Characteristic pressure cycle spectra were developed from the supplied pressure data which describes the different modes of pipeline operation including liquid or gas hydrocarbon transportation as well as mainline or lateral operation which may be of use in further research efforts in this area.Copyright

A Modeling Approach to High pH Environmentally Assisted Cracking

A model is described for predicting the incidence of high-pH SCC on gas pipelines. The model is mechanistic and is based on a film rupture mechanism of crack growth. Based on field pressure cycle data, the model determines the crack tip strain rate and the occurrence of film rupture events at the crack tip during operating pressure cycles. Crack aspect ratio data were obtained from field data. Probabilistic distribution functions were assigned to input parameters and a Monte Carlo method was used to produce probabilistic crack growth rate distributions. The model grows a crack to failure while considering the effects of temperature and potential.Copyright

Environmentally assisted cracking of pipeline steels in near-neutral pH environments

Publisher Summary Environmentally assisted cracking (EAC) in dilute bicarbonate, near-neutral pH soil solutions occurs at the free corrosion potential underneath coatings that shield the pipe from cathodic protection. The transgranular quasi-cleavage crack appearance is an indication of a role for hydrogen in the crack growth mechanism. The mechanical driving forces for crack growth can be described by the stress intensity factor (K) and stress intensity factor range ∆K. Threshold values for K and ∆K vary with the type of steel involved microstructure, and environment. Cyclic plastic deformation is critical for crack growth. No crack growth is observed under constant load conditions up to 110% of the specified minimum yield strength. Mechanical overloads, crack deflection, and crack closure phenomena reduce the effective ∆K driving force. Work hardening has a similar effect by reducing the strain at the crack tip. Neighboring cracks in dense crack colonies can shield each other from the applied stress, thereby decreasing crack growth. In contrast, residual tensile stresses, and stress intensifiers increase the local K and ∆K, resulting in faster crack growth.

Hydrogen Permeation Behavior of X-70 Pipeline Steel in a Near-Neutral pH Soil Environment

This paper reports the hydrogen permeation behavior of an X-70 pipeline steel in a synthetic near neutral pH field solution under both galvanostatic and potentiostatic conditions. The hydrogen flux through the steel exposed to the solution is influenced by the chemical reactions at the steel surface on the charging side. Results from tests done under potentiostatic and galvanostatic conditions were compared to predictions based on different boundary conditions assumed in solving Fick’s diffusion equations. Constant concentration boundary conditions gave the best fit. It was also found that the diffusible hydrogen generated in a near neutral pH soil environment is likely to be 3 to 10 times lower than the minimum hydrogen concentration required to initiate hydrogen blistering in pipeline steels.Copyright

The role of hydrogen in EAC of pipeline steels in near-neutral pH environments

Publisher Summary The mechanism of near-neutral pH cracking is poorly understood. Both dissolution and hydrogen are believed to be involved. Cracks in the field often display extensive corrosion of the crack walls and are filled with corrosion products. A pertinent role of hydrogen is well accepted, with the main indicators being the quasi-cleavage crack appearance and the observation that hydrogen enters the steel in hydrogen permeation tests where the steel is exposed to groundwater solutions. Low-strength steels are not prone to hydrogen embrittlement under a constant load but severe cracking can occur when the stress intensity factor (K) is increased at a slow to modest rate. The effect of strain has been attributed to enhanced hydrogen uptake and to hydrogen damage within the plastic zone that results from active plastic deformation. For the corrosion of steel in near-neutral pH groundwater solutions, the anodic dissolution mechanism probably involves a rate-determining oxidation step of the intermediate FeOH. Although this is not a bond-breaking step, the reaction rate may be affected by the presence of the deuterium isotope, thereby affecting free corrosion potentials.