NS4 Solution Research Articles

Stress Corrosion Cracking Behavior of X60 Pipe Steel in Soil Environment

ABSTRACTStress corrosion cracking (SCC) susceptibility of API X60 pipeline steel in a soil solution by slow strain rate tests (SSRT), and surface fracture analysis was investigated. The SSRT were performed at strain rate of 25.4 × 10-6 mm/sec in a glass autoclave containing the soil solution called NS4 with pH of 3 and 10 at room temperature and 50°C. Both anodic and cathodic polarization potentials of 200 mV referred to Ecorr was applied. The results of ratio reduction area (RRA), time to failure ratio (TFR) and elongation plastic ratio (EPR) indicate that X60 pipeline steel was susceptible to SCC at pH 3 and cathodic polarization of -200 mV at room temperature and 50°C. Scanning electron microscopy (SEM) observations of these specimens showed a brittle type of fracture with transgranular appearance. The SCC process and mechanism of X60 steel into NS4 solution was hydrogen based mechanism. With the different applied potentials the dominance of SCC process changes. At low pH the temperature effect on SCC susceptibility is more noticeable at 20°C. However at high pH this effects changes, being the steel more susceptible to SCC at 50°C.

Sensitivity of pipelines with steel API X52 to hydrogen embrittlement

The following cases of hydrogen influence on pipeline metal were considered: gaseous hydrogen under internal pressure in notched pipes and electrochemically generated hydrogen on external pipe surface from soil aqueous environment. The burst tests of externally notched pipes under pressure of hydrogen and natural gas (methane) were carried out after the pipe has been exposed to a constant “holding” pressure. It has been shown that even for relatively “soft” test conditions (holding pressure p = 20 bar and ambient temperature) the gaseous hydrogen is able to penetrate into near surface layers of metal and to change the mechanism of local fracture at notch. The sensitivity to hydrogenating of given steel in deoxygenated, near-neutral pH NS4 solution under soft cathodic polarisation was studied and the assessment local strength at notches in pipeline has been made for this conditions. Here, the relationship between hydrogen concentration and failure loading has been found. The existence of some critical hydrogen concentration, which causes the significant loss of local fracture resistance of material, was also shown.

Stress Corrosion Cracking of Pipeline Steel in Soil Environments of Eastern China

Stress corrosion cracking (SCC) behaviors of pipeline steel X70 in various near-neutral pH soil environments with characteristics of eastern China have been studied through electrochemical measurements and slow strain rate tests (SSRT) at various electrode potentials in four solutions containing different typical soils in eastern China as well as in NS-4 solution. The SCC susceptibility in the four soil solutions was generally higher than that in NS-4 solution. There was a general trend that SCC susceptibility increased with decreasing the potential, suggesting that hydrogen induced cracking probably plays a key role in the cracking at least at low potentials and the parameters of cathodic protection in engineering should be carefully optimized to avoid SCC.

Mechanical and Environmental Influences on Stress Corrosion Cracking of an X-70 Pipeline Steel in Dilute Near-Neutral pH Solutions

Abstract Slow strain rate tests (SSRT) were used to investigate stress corrosion cracking (SCC) behavior of an X-70 pipeline steel in a soil solution and NS-4 solution, both saturated with 5% carbo...

Crack Initiation and Early Propagation of X70 Steel in Simulated Near-Neutral pH Groundwater

Abstract The near-neutral pH environmentally assisted cracking (EAC) initiation and propagation behavior of X70 pipeline steel was investigated in NS4 solution deaerated with N2/5% carbon dioxide (CO2) under the low-frequency cyclic load. The test results show that an increase in the applied stress level promotes the crack initiation. Based on the crack initiation sites, the cracks are divided into two types, pit cracks and non-pit cracks. Non-pit crack initiation is found to be the dominative cracking mechanism on the original external pipe surface, but as the tests progress, the contribution of pit cracks becomes significantly pronounced. The pits acted as the crack initiation sites are related to the localized dissolution at/around the nonmetallic inclusions. Therefore, it is expected that the EAC initiation resistance of steel can be improved by reducing the inclusion content in the steel. The crack propagation rate is found to be essentially independent of the applied stress level. By comparing the c...

Effects of Environmental Factors on Stress Corrosion Cracking of Pipeline Steels

Effects of some environmental factors on stress corrosion cracking (SCC) of pipeline steel X-70 both in near-neutral pH environments, including NS4 solution and several solutions containing main types of soil in the eastern part of China, and in high pH solution were studied by means of electrochemical measurement and slow strain rate testing (SSRT). The anodic polarization curves showed different features in near-neutral pH and high pH solutions in terms of active-passive transition behavior. In near-neutral pH solutions, the cracking mode was transgranular with the feature of quasi-cleavage, the susceptibility to SCC increased with decreasing potential, pH and temperature as well as increasing CO2, indicating a dominant mechanism of hydrogen induced cracking (HIC). In high pH solutions, the cracking behavior was similar to that in near-neutral pH solutions when the specimens were polarized at cathodic potentials, but quite different at anodic potentials. A comparison of the electrochemical behavior with the SCC potential region indicated a dominant SCC mechanism associated with anodic dissolution (AD) of X70 in high pH solution at anodic potentials. A preliminary experimental potential (E)-pH-SCC diagram has been established for X70 in near-neutral pH environments.

Environmental Crack Growth under Variable Amplitude Loading of Pipeline Steel

Abstract An investigation was carried out on API X60 steel specimens with surface cracks to study environmentally assisted crack growth rates in simulated groundwater (NS-4 solution). The environmental conditions were similar to those for buried natural gas pipelines in service, and loading was aggressive. The maximum stress intensity factor at the tip of the crack on the surface ranged from 20.6 MPa√m to 31.4 MPa√m. A periodic underload was applied to pre-cracked cantilever bending specimens. Tests were carried out for a period of 40 days at a frequency of either 1 cycle or 40 cycles per day. Crack growth rates were obtained for two stress ratios (R = minimum/maximum load) of 0 or 0.5 under both constant and variable amplitude loading conditions. Depending on loading conditions, the crack growth varied from 0 μm to 2,640 μm. However, the majority of crack growth measured was <100 μm. The experimental results showed that there was an acceleration in growth following an underload. The growth was higher whe...

The role of hydrogen in the process of stress corrosion cracking of pipeline steels in dilute carbonate-bicarbonate solution

Stress corrosion cracking (SCC) in buried highpressure pipelines occurs on the outside surface where ground water is in direct contact with pipelines through disruptions of the protective coating. Instances of SCC have been related to an electrolyte composed of alkaline carbonate–bicarbonate [1–3]. SCC of pipeline steels in high concentration of carbonate–bicarbonate solution (pH of about 10) has been well studied [4–7]. However, recent research [8] has shown that the chemistry of the solution beneath coating disbondments on pipelines is concerned with much lower carbonate– bicarbonate concentrations, and hence lower pH (about 7). SCC is transgranular in low-pH solution and is intergranular in high-pH solution [9–11]. Only a few studies have focused on the SCC of the pipeline steels in dilute carbonate–bicarbonate solutions in the presence of chloride ions. Since the low pH of solution facilitates hydrogen accumulation in stressed region in steel, hydrogen plays an important role in SCC [12]. It is important to clarify whether hydrogen could diffuse into the materials and its distribution status around the crack tip, if the dissolved hydrogen has influence on SCC of the pipeline in NS-4 solution. The research described in this paper was carried out with the following objectives: firstly to determine hydrogen reduction and enrichment near the crack tip, and secondly to investigate effects of hydrogen on the anodic SCC of the pipeline steels in dilute carbonate–bicarbonate solution. The materials used were pipeline X-80 and X-52 supplied by IPSCO, Canada. The nominal compositions are listed in Table I. Compact tension (CT) specimens were prepared from the pipeline steels and the dimensions are shown in Fig. 1. A sharp notch with a radius of 0.1 mm was made in the specimen along the longitudinal direction. All specimens were ground with up to 1000 grit emery paper before testing. The specimens were loaded by a wedge with thickness of 2.50 mm. The load was calibrated by a load–extension curve measured using a materials-testing system. NS-4 solution, the typical solution in coating disbondment area in Canadian pipelines [8], was used in this study to simulate coating disbonding condition on pipelines in the field. The composition of NS-4 solution is listed in Table II. The pH values of the solution were adjusted to 5 and 3 by adding H2SO4 to the solution to simulate the solution condition within the crack tip during SCC. Before measuring the hydrogen distribution in the steel, the specimens were immersed in NS-4 solutions of various pH for 72 h. Hydrogen pre-charging was conducted in 1 N H2SO4 ‡ 250 ppm As2O3 solution for 24 h. Measurements of hydrogen distribution were carried out using secondary-ion mass spectroscopy (SIMS). A primary ion beam of Ar, an accelerating voltage of 15 kV and a beam current of 1310y6 A were used. The spot diameter of the ion beam was

Electrochemical polarization and stress corrosion cracking behaviours of a pipeline steel in dilute bicarbonate solution with chloride ion

The electrochemical and stress corrosion cracking behaviour of Grade 550 (X-80) pipeline steel with coating disbandments was studied in a range of bicarbonate solutions using a potentiodynamic polarization technique and slow strain rate test. The shape of the polarization curves were found to vary with bicarbonate concentration. Increase in bicarbonate concentration results in a wider passive region and a more noble breakdown potential. Low concentrations of chloride ion in bicarbonate greatly affect the polarization characteristics and can cause the elimination of passivity in specific solutions. Tensile specimens were pulled at constant strain rate \\ ́ ge = 10 −6/sec. under potential control ranging from −350 mV to −900 mV (SCE) in NS-4 solution (dilute bicarbonate solution with chloride ion). Fracture surface of the specimens were examined under scanning electro microscopy. Overall fracture is ductile dimple, however, transgranular stress corrosion cracking has been found in the edge of specimen for the sample with potential controlled at −800 mV, −750 mV and −650mV (SCE).