Corrosion and erosion-corrosion behaviour of lean duplex stainless steels in marine and oilfield environments

Lean duplex stainless steels are becoming attractive for applications in oilfield and marine environments due to their economic advantages, very good mechanical properties and relatively good corrosion resistance. One such application is in the production of the carcass of flexible pipes. However, materials selection for such oilfield applications becomes more complex as a result of the interactions between corrosion and erosion. Much effort has been directed towards the study of erosion-corrosion behaviour of carbon steels and other passive alloys. However, the subject of erosion-corrosion of lean duplex stainless steels is still rarely reported. Moreover, data available in the literature on the localized corrosion resistance of the lean duplex stainless steels are limited to alkaline environments. Efforts have been made in this thesis to add to the existing data and to the understanding of the subject of localized corrosion and erosion-corrosion resistance of lean duplex stainless steels UNS S32101, UNS S32304 and UNS S82441 in oilfield environments. The lean duplex alloy UNS S32101 has been studied in detail because of its combination of high strength and good corrosion resistance. This research also compared the corrosion and erosion-corrosion resistance of lean duplex stainless steels with standard austenitic stainless steels UNS S30403 and UNS S31603 as well as duplex stainless steel alloy UNS S32205. Aerated 3.5% NaCl and synthesized CO2-saturated oilfield brines were considered as the corrosion media. Extreme erosion-corrosion conditions were simulated to design for severe environments often encountered in sand-containing oilfield pipeline systems. Breakdown potentials, under static conditions, were found to be more positive in the aerated 3.5% NaCl than the CO2-saturated oilfield brine solution. Also, lean duplex stainless steels and standard austenitic stainless steels exhibited similar resistance in both environments. X-ray Photoelectron Spectroscopy (XPS) analysis of the passive film indicated higher chloride incorporation in the CO2-saturated oilfield brine. This, in addition to lower pH of the CO2-saturated oilfield brine appeared to be the reason why the breakdown potential was more negative in this environment. Erosion-corrosion results showed that lean duplex stainless steels, UNS S32101 and UNS S32304, have higher resistance to pure-erosion damage than UNS S30403 and UNS S32205; better erosion-corrosion resistance than UNS S30403 austenitic stainless steel; and equivalent erosion-corrosion resistance to UNS S32205 standard duplex stainless steel. There was also a correlation between the erosion-corrosion resistance of the alloys and the sub-surface crystallography, microstructure and phase transformation. This, together with repassivation kinetics of the passive film, may be used to explain the erosion-corrosion behaviour of UNS S32101 and UNS S30403 in the oilfield slurry

Flow-induced corrosion behaviour of low alloy steel in the presence of mono-ethylene glycol

Abstract: The flow-induced corrosion behaviour of welded low ally steel in 1 wt. % NaCl environment containing monoethylene glycol at 0oC and 25 oC respectively, was investigated. A modified submerged impinging jet rig was adopted to simulate the low temperature environment. Varying speeds between 1 and 3 m/s were used to study the degradation mechanism of the welded steel. Also, the ratio of MEG: 1 wt. % NaCl was kept at 1:4 for the MEG/NaCl solution. Static corrosion condition was also studied and compared with the flowing conditions. Results indicated that the alloy degraded with observable weight loss in pure 1wt. % NaCl at both temperatures whereas there was very negligible weight loss in all the samples in MEG/NaCl solution

Fabrication of forced air cool austempered ductile iron and exploring its corrosion behaviour in a simulated mine water

Abstract: The production of austempered ductile iron (ADI) with uniform microstructure and properties is constrained by the austempering process vis-à-vis the quenching medium. This is as a result of the stringent operating parameters with costly facilities. This limitation has restricted the application of ADI, despite its inherent mechanical and chemical properties. An emerging technology of overcoming this limitation is by austempering with force air cooling equipment, which is accessible, available and cost-efficient. This work characterizes the behaviour of the forced air cool ADI in simulated mine water due to the strategic importance of the mining industry in the global economy. The study establishes the influence of sample section thickness on the corrosion performance. The sample’s thickness were 5, 15, and 20 mm. Electrochemical experiments were performed on the forced air cool ADI at atmospheric pressure and room temperature with method such as open circuit potential (OCP). The post-corrosion analyses were performed using X-ray diffractometry (XRD) and field emission scanning electron microscopy (FESEM). The research highlighted that small section thickness has a more favourable performance compared with larger section. Consideration is also accorded to the capability of the ADI in the studied environment

Wear behaviour of cast aluminium silicon (Al-Si) alloy in dot 4 brake fluid

Abstract : The frictional sliding behaviour of cast Al-Si alloy was investigated with and without DOT 4 hydraulic brake fluid lubrication/environments. Cast Al-Si specimen was produced from wrought alloy and the specimen surface was metallurgically prepared. The as- cast product was characterized with aid of micro-hardness tester, the chemical composition was analysed by X-ray fluorescence (XRF), while X-ray diffraction (XRD) was used to identify phases present, microstructures and surface examinations were done on a high resolution metallurgical Optical Microscope. The properties of the polished and worn sample surfaces were examined by Scanning electron microscope (SEM) equipped with Energy dispersive X- ray (EDS) facilities. A 6mm diameter alumina ball under varying normal loads (3-10 N) was rubbed against the surface of the cast Al-Si coupon specimen. The dry and wet sliding wear behaviour was assessed based on the CoF-sliding time behaviour. From the results, the average CoF values of 0.9064 for dry and 0.2038 for wet contacts were obtained. Comparatively, 1.03E-06 and 4.21E-07 minimum wear intensities; and 467290 and 132170 wear resistances were obtained for dry and wet contacts respectively. Behaviour of cast Al-Si in DOT4 were compared with its performance in DOT3 regarding the hydraulic brake system application

Corrosion Resistance of Precipitation-Hardened Al Alloys: A Comparison between New Generation Al-Cu-Li and Conventional Alloys

The corrosion resistance of conventional (AA2024-T3, AA6082-T6 and AA7050-T7451) and the new generation (AA2050-T84, AA2098-T351, AA2198-T8, and AA2198-T851) precipitation-hardened alloys has been studied and compared using electrochemical and non-electrochemical approaches. The AA6082-T6 was the most resistant alloy followed by the new generation Al-Cu-Li alloys, except the AA2050-T84. All the alloys exhibited pseudo-passivity, except for the AA2024-T3 alloy which presented the highest number of pitting sites per cm2 and also exhibited the most insidious form of corrosion amongst the alloys tested. However, the alloy with the highest corrosion depth was the AA2050-T84 alloy followed by the AA2024-T3 and AA7050-T7451 alloys. Intergranular corrosion was associated with rapid rates of penetration. In addition to the microstructural features of the alloys before corrosion, the modes of localized corrosion in the alloys were also influenced by evolving microstructural features (such as re-deposited Cu) during corrosion

Mechanical properties of dual phase steel quenched in bitumen medium

The mechanical properties of a dual phase steel treated in different intercritical windows and subjected to bitumen as the quenching medium was investigated. The viscosity of the quenchant was altered at several rate, the morphology of phases formed was characterized. Also, the quenched samples produced ferrite-martensite phases obtained at different intercritical windows. The tensile and hardness behaviour of the structures was examined; and the microstructures were characterized. It was observed that samples intercriticaly heat treated at 790°C and held for 60mins, bitumen quenched at 125°C, offers the highest impact toughness, while its counterpart at 730°C soaked for 30mins, bitumen quenched at 125°C offered the least impact toughness value. Sample intercriticaly treated at 790°C and held for 45mins in a 150°C quenching, offered the peak hardness factor. However, samples at 790°C held at 45mins and bitumen quenched at 150°C possessed relatively the best combination of tensile properties, hardness and impact energy. The water quenched samples at 790°C held for 60mins has the peak hardness value. The micrographs as well showed a uniformly distributed dual phase structure of ferrite and martensite at various volume fractions

Green inhibitors for corrosion protection in acidizing oilfield environment

AbstractTafel extrapolation method has been employed to determine the corrosion resistance of super austenitic stainless steel (UNS S31254) in a CO2-saturated acidizing oilfield environment. Green inhibitors (Tridax procumbens and Chromolaena odorata extracts) were employed for protection of the alloy in the environment. Over 90% inhibition efficiency was achieved at inhibition dosage of 100ppm for one of the inhibitors. Results showed that both inhibitors are effective for protecting the alloy in the acidizing environment. The result of the adsorption isotherm showed that the adsorption of the extracts on the surface of UNS S31254 is a monolayer and consistent with the Langmuir isotherm