Corrosion of general oil-field grade steel in CO2 environment - An update in the light of current understanding

In this review, we discussed the current mechanistic understanding and effects of key parameters on corrosion of carbon and low alloy steel (CLAS) in CO2 environments. In particular, we emphasized on the current understanding related to the mechanism of early stage nucleation of siderite that has evolved from the outcomes of in-situ synchrotron-based X-ray studies under various modes. We also highlighted the effect of the most important environmental and metallurgical factors affecting the corrosion behavior of CLAS. Finally, we addressed the aspects of material chemistry and micro-alloying necessary for achieving the most effective and economic materials system for mitigating corrosion in CO2 environment.Scopu

Effect of Trace 100 vppm H2S on the Corrosion Behaviour of Plain Carbon and Microalloyed Steels in a Predominant Sweet Environment in High Flow Regime

We investigate the effects of the presence of trace (100 vppm) H2S on the corrosion behaviour of plain carbon steel and its various micro-alloyed counterparts in a CO2 saturated (sweet) brine (0.5 M NaCl) environment, in a high flow regime (1000 RPM), at 80oC in a slightly acidic environment (pH 6.6). Potentiostatic current transients indicate that the presence of trace amount of trace H2S in a predominantly sweet regime, where the partial pressure ratio of CO2 and H2S (pCO2:pH2S) is ∼10000:1, shows a very different corrosion behaviour for both plain carbon steels and as well as micro-alloyed steels. In presence of trace H2S, current density starts dropping much earlier compared to H2S free standalone CO2 environment. Trace amount of H2S also induces faster passivation of the corrosion scale, especially for alloys with relatively high Mo (0.7 wt.%) and Ni (1.4 wt.%) content, suggesting that Mo and Ni have a strong effect in presence of trace H2S. On the basis of available literature, we speculate that the effects observed in presence of trace H2S is due to the formation of Mackinawite which forms on the steel surface immediately via solid state reaction and micro-alloying with some specific elements catalyzes the formation of mackinawite and/or assists formation of more stable sulfide phase(s), causing a faster current drop and passivation. Modeling of the hypothesis is currently in progress. Keywords: Micro-alloying, CO2 corrosion, Flow effect, RDE, Plain carbon steel, Cr-Mo-Ni steel. Figure: Potentiostatic current transient for various plain carbon and micro-alloyed steels. Condition - pH: 6.6, Temp: 80oC, Flow: 1000 RPM, @ anodic over potential: Open Circuit Potentials (OCP) +150 m

Effect of Trace 100 vppm H2S on the Corrosion Behaviour of Plain Carbon and Microalloyed Steels in a Predominant Sweet Environment in High Flow Regime

We investigate the effects of the presence of trace (100 vppm) H2S on the corrosion behaviour of plain carbon steel and its various micro-alloyed counterparts in a CO2 saturated (sweet) brine (0.5 M NaCl) environment, in a high flow regime (1000 RPM), at 80oC in a slightly acidic environment (pH 6.6). Potentiostatic current transients indicate that the presence of trace amount of trace H2S in a predominantly sweet regime, where the partial pressure ratio of CO2 and H2S (pCO2:pH2S) is ?10000:1, shows a very different corrosion behaviour for both plain carbon steels and as well as micro-alloyed steels. In presence of trace H2S, current density starts dropping much earlier compared to H2S free standalone CO2 environment. Trace amount of H2S also induces faster passivation of the corrosion scale, especially for alloys with relatively high Mo (0.7 wt.%) and Ni (1.4 wt.%) content, suggesting that Mo and Ni have a strong effect in presence of trace H2S. On the basis of available literature, we speculate that the effects observed in presence of trace H2S is due to the formation of Mackinawite which forms on the steel surface immediately via solid state reaction and micro-alloying with some specific elements catalyzes the formation of mackinawite and/or assists formation of more stable sulfide phase(s), causing a faster current drop and passivation. Modeling of the hypothesis is currently in progress. Keywords: Micro-alloying, CO2 corrosion, Flow effect, RDE, Plain carbon steel, Cr-Mo-Ni steel. Figure: Potentiostatic current transient for various plain carbon and micro-alloyed steels. Condition - pH: 6.6, Temp: 80oC, Flow: 1000 RPM, @ anodic over potential: Open Circuit Potentials (OCP) +150 mVqscienc

Usefulness of In-Situ Synchrotron Study on Scale Formation during CO2 Corrosion of Mild Steel: A Review

Despite the availability of the detailed knowledge about the chemical, electrochemical and transport processes associated with the formation of ferrous carbonate during CO2- corrosion of mild steel, information available about the mechanism of early nucleation stages of FeCO3 (siderite) film formation was only speculative. In depth knowledge related to the early stage nucleation of siderite scale and its subsequent development and gradual growth were revealed only recently by judicious exploration of in-situ synchrotron based experimentation. Moreover, application of in-situ synchrotron x-ray diffraction technique rendered ability to monitor the gradual real time growth of FeCO3 scale with the efficacy of simultaneous controllability of the corrosion conditions electrochemically. This work shades light on how in-situ synchrotron study revealed apparent mysteries related to formation & growth of siderite scale during CO2- corrosion of mild steel. Keywords: In-situ synchrotron, CO2-Corrosion of mild steel, Nucleation mechanism during CO2- corrosion. Figure 1 (a) and (b) illustrate the photograph and schematic of experimental set up for in-situ synchrotron X-ray diffraction experiments using the powder diffraction Australian beam line. Figure 2 shows the anodic current as a function of time in a potentiostatic test in the NaCl solution at pH 6.3 (pCO2 = 1bar) and an applied potential of ? 500 mV (vs SCE), together with the intensities of the Fe and FeCO3 signals from the in-situ synchrotron XRD [1]. There is a clear peak in the current transient, with the XRD results showing FeCO3 formation during the increasing portion of this transient and then a rapidly decreasing rate of FeCO3 formation as the anodic current decreases towards the end of the test, indicating the growth of a protective surface scale. In these studies, it is shown that siderite only forms once the critical super-saturation is exceeded within a defined boundary layer and that the steel microstructure has critical role in developing a surface texture within which the critical super-saturation could develop. This idea of siderite nucleation in solution with a critically saturated boundary layer close to the metal surface is unlike the heterogeneous nucleation and growth phenomenon which occurs directly on the surface via solid state formation [1]. Establishment of this ideology was able to resolve the apparent thermodynamic anomaly observed in practice. One of the apparent thermodynamic anomalies in CO2- H2S mixed system is couched on the fact that in spite of the higher (?3 time) aqueous solubility of H2S compared to CO2 under similar temperature and pressure conditions, the formation of iron sulfide has been observed to be faster and predominant over iron carbonate under most parametric conditions even at a trace level of H2S concentration in CO2-H2S system. This fact indicates the formation of iron sulfide and iron carbonate films to be regulated by different mechanisms. In-situ synchrotron study revealed the early stage nucleation of iron carbonate (siderite) film and indicated that to be a gradual process giving rise to the formation of crystalline siderite phase only after the critical supersaturation stage is reached. Given that, there is significant time lag between the start of supersaturation and critical supersaturation limit, there is a possibility for iron sulfide formation during that time lag, as it forms by solid state formation mechanism which is a faster process. It is the in-situ synchrotron study which made it possible to illustrate the appreciable time lag between the start of supersaturation and critical supersaturation limit. Ingham et al. [2] used in-situ synchrotron small- and wide-angle X-ray scattering (SAXS and WAXS) to demonstrate that the formation of crystalline siderite (FeCO3) during the corrosion of steel in CO2-saturated brine is actually preceded by the formation of a colloidal precipitate and an amorphous surface layer, both assumed to be amorphous ferrous carbonate. Grazing incidence SAXS showed that upon the application of an anodic potential, film forms instantaneously and then a separate population of particles develops in the later stage followed by the formation of the ultimate crystalline FeCO3, observed by WAXS. Ingham et al. [2] interpreted these observations in terms of crystal nucleation within the amorphous surface layer. This observation was speculated to bear a significant consequence on the morphology of the corrosion scale and hence its protectiveness. However, it would be important to understand the effects of local pH change as well as variation in temperature in this gradual formation and development of siderite scale and then to find out a quantitative correlation of this process with the mentioned parameters. The fact of stable, adherent and protective siderite film formation in presence of trace amount Cr3+ was known [3, 4]. However the associated mechanism was not clearly understood until in-situ synchrotron x-ray diffraction study revealed a clearer picture of the mechanistic aspect of the expedited siderite film formation in presence of traces of Cr3+[2, 5]. This recently done in-situ synchrotron study made it clear that traces of Cr3+ in the solution significantly expedites the precipitation rate of the colloidal precursor and thus accelerate the appearance of the crystalline scale through its catalyzing influence on the nucleation process by modulating the local pH level at the steel surface and thus reducing of the critical supersaturation for precipitation. Another interesting and critical factor in developing surface texture within which the critical supersaturation can be developed is steel microstructure. Consequently, steel microstructure must have intimate relationship with the corrosion process and morphology of the scale as microstructure can change the diffusion conditions at the steel surface affecting the local supersaturation of siderite. However, in-depth mechanistic information related microstructural effect on adherent and protective scale formation was hardly available in the literature. It is recommended to perform in-situ synchrotron X-ray diffraction experiment in order to make a detailed investigation of such a phenomena. Ko et al [6] recently conducted such a study in order to investigate the effect of microstructure and boundary layer conditions on CO2 corrosion of low alloy steels. This recent investigation clearly demonstrated that the nucleation of crystalline scales onto the surface of steels under CO2 corrosion at elevated temperature is critically dependent on the initial surface roughness, microstructure-related surface roughness developed during corrosion. This study also indicated the interdependence between microstructure and chromium-enhanced siderite nucleation. However, finding out the quantitative effects of surface roughness on the initial nucleation process (rate) of the scale as well as the stability of the scale would be a good addition to this study.Qscienc

Effects of microstructures on hydrogen induced cracking of electrochemically hydrogenated double notched tensile sample of 4340 steel

Quantitative fractographic characteristics of 4340 steel is demonstrated for a grain size range of 10-100 ?m and hardness range of 41-52 HRC. Double-notched tensile samples were electrochemically charged in-situ with hydrogen in 0.5 m H2SO4+5 mg/l As2O3 solution for 0-40 min charging time. Hydrogen induced fracture initiations were analyzed by novel metallographic investigation of the "unbroken" notch while the overall fractographic behaviors were examined by the scanning electron microscopic imaging of the fracture surfaces of the actually broken notch. Effect of hydrogen was predominantly manifested as intergranular fracture for the harder samples and quasi-cleavage fracture for the softer counterparts. 10-40 ?m samples showed the maximum intensity of the hydrogen induced fracture features (intergranular and/or quasi-cleavage) close to the notch which gradually reduced with increasing distance from the notch. The largest grained samples (100 ?m) however showed brittle behavior even in absence of hydrogen with similar intensity of percent fracture features at all distance from the notch, while presence of hydrogen intensified the overall percent brittle fractures with their intensities being highest close to the notch. Finally, the brittle fracture characteristics of the hydrogen embrittled samples were shown to be distinguishably different from that of the liquid nitrogen treated samples of same grain sizes and hardnesses.Scopu

Local supersaturation and the growth of protective scales during CO2 corrosion of steel: Effect of pH and solution flow

By correlating in-situ synchrotron X-ray diffraction measurements with electrochemical measurements using a rotating disc electrode, we demonstrate the critical dependence on the local supersaturation of the kinetics of formation of a protective crystalline scale on the surface of carbon steel during CO2 corrosion in brine at elevated temperature. We show that the total current is the sum of a current due to dissolution of iron and a current due to growth of a crystalline layer. We show that the dissolution current and the surface supersaturation are controlled by the thickness of an initially-formed amorphous layer. As in earlier work at room temperature, we infer that the amorphous layer dissolves as a carbonato-iron complex with surface concentration of the dissolving species determined by the electrode potential, and speculate on the importance of the chemistry of this dissolution reaction in determining the corrosion result. We construct a simple transport-reaction model, which shows that the supersaturation is determined by the precipitation rate constant of colloidal FeCO3 and by the product of the current for Fe dissolution and the diffusion boundary layer thickness. Using this model, we show crystal growth rate varying quadratically with supersaturation at pH 6.8 and linearly at pH 7.3. The effects of electrode potential, surface roughness, microstructure and flow are simply to change supersaturation by changing the current density per unit projected area flowing through the amorphous initially formed layer. Variation of brine concentration has no effect. We illustrate the sensitivity to solution flow of the crystallinity of the final scale. We show that siderite is the first crystalline product and that chukanovite follows, with a delay time that decreases with increasing pH. The ratio of chukanovite to siderite is low at sufficiently high pH and increases with decreasing pH, possibly through a maximum. From the results, we advance ideas concerning the importance of local microenvironments and local fluctuations in mass-transport rate. 2017 Elsevier LtdScopu

Erosion Behavior of API X120 Steel: Effect of Particle Speed and Impact Angle

The dry erosion behavior of API-X120 pipeline steel was investigated, under the erosive interaction of aluminum oxide particulates, in a range of speed (43–167 m·s−1) and impact angle (30°–90°). Erosion behavior is characterized by surface profile measurement, weight loss measurement, and surface morphology analysis by SEM/EDX. Optical profilometry revealed that the eroded area increased with elevating speed of particles while the penetration depth increased with the increases in impact angle as well as particle speed. Percent weight loss and normalized erosion rate indicated that the lower impact angles and higher speeds led to higher materials loss and erosion. SEM analyses on various combinations of impact angles and particle speeds demonstrated the predominant erosion mechanism under those specific conditions; attributed to the intensity of the resolved components of the momentum vector horizontal or normal to the target metal surface under those conditions

Time-resolved photocurrent spectroscopic diagnostics of electrically active defects in AlGaN/GaN High Electron Mobility Transistor (HEMT) structure grown on Si wafers

Time-resolved photocurrent (TRPC) spectroscopy with a variable-wavelength sub-bandgap light excitation was used to study the dynamics of the decaying photocurrent generated in the heterostructures of the AlGaN/GaN high electron mobility transistors (HEMTs) layers. In AlGaN/GaN HEMTs, reliability of the device is degraded due to the prevalence of current collapse. It is recognized that electrically active deep level defects at the surface/interfaces and the bulk in the HEMTs layers can contribute to the unwanted current collapse effect. Therefore, it is of great importance to analyze the deep level defects if the reliability of the HEMTs device is to be improved. In this research, TRPC spectroscopy was used to elucidate the origin and nature of the deep level defects by analyzing the time evolution of the photocurrent decay excited at different wavelengths of light. The two devices that show similar characteristics for wavelength-dependency on photocurrent generation were chosen, and TRPC spectroscopy was conducted on these devices. Although the two samples show similar characteristics for the wavelength-dependency on photocurrent generation, they exhibited dissimilar time-dependent photocurrent decay dynamics. This implies that TRPC spectroscopy can be used to distinguish the traps which have different origins but have the same de-trapping energy.Scopu