Erosion studies of the iron boride coatings for protection of tubing components in oil production, mineral processing and engineering applications
Introduction
Erosion and erosion-corrosion, which widely occur in production and processing of heavy oil, oil sand, in mining operations and mineral processing, create sufficient losses in industry. These losses are dealt with equipment failure and unpredictable production shutdowns, necessary replacement of the damaged components, product contamination and possible environmental impact. Some problems associated with erosion of steels in oil & gas and mineral processing applications are summarized in a few papers [[1], [2], [3], [4]]. The synergistic action of erosion and corrosion, which are the common service situations, creates more severe degradation of the materials compared to the “individual” actions (i.e. when erosion or corrosion acts independently). The total loss W, in this case, can be expressed asW = E + C + Δwhere E – erosion loss, C – corrosion loss, and Δ – synergistic ingredient [1,[5], [6], [7], [8]].
The resistance to wear, corrosion and wear-corrosion and degradation mechanisms depend on structure, composition and properties of the materials, being in service, nature of processing media (abrasive and corrosive), service/application conditions and design and geometric features of the components [5,[7], [8], [9], [10], [11], [12], [13], [14], [15]]. In erosion and erosion-corrosion application conditions, character and hydrodynamics of flows (e. g. turbulence), velocity, angle of impact, type, properties (e.g. hardness) and particle size of abrasive medium (erodent), presence of fluids (slurry erosion) and their characteristics, corrosive medium composition and concentration, pressure and temperature, strongly affect the materials performance. The erosion data of structural materials, including metals, ceramics, composites, polymers and coatings, are related to the testing conditions and testing apparatus employed [2,5,[15], [16], [17], [18], [19], [20]]. The testing methods employed different equipment, such as a blasting unit, as the simplest for dry test conditions, as well as different “pot” testers [[21], [22], [23], [24]], slurrty jet erosion testing devices [2,4,[20], [21], [22], [23], [24]], Coriolis erosion tester [2,15,17,25,26], slurry whirling arm ring tester [16,26] and some others [14,[27], [28], [29], [30], [31]], can be considered for the dry erosion and slurry erosion testing. However, there is no generally accepted and standardized method for testing and evaluating erosion resistance of the materials. The methods based on ASTM G76-13 [32], originally designated only for dry erosion testing, as well as ASTM G73-10 (2017) [33], may be considered for slurry erosion only after their modifications, and even after that they have some limitation, e.g. related to the size and shape of test samples (it is difficult to test tubular shape sections of large diameters).
The components of equipment of downhole oil production and transporting systems in oil sand tailings slurry handling and tailing in mineral and mining operations (piping, tubing, casing, centralizers, elbows, reducers, cyclones, different valves, chokes, nozzles and many others) are commonly made of carbon- and low-alloy steels. Because of severe service conditions, they experience a high level of degradation and failure. In order to reduce the failure problems of steel components, either more advance materials need to be employed, or steel should be protected. The use of highly expensive stainless steels and Ni- and Co-rich alloys with high corrosion resistance instead of carbon- and low-alloy steels reduces the corrosion failures. However, even in this case, the abrasion- and erosion-related failures cannot be eliminated because these expensive steels and alloys have lower hardness than processing materials (e.g. silica sand and many others). Surface engineering, e.g. special hard inorganic coatings on steels, can be the effective route to protect steel components (including those made of low-cost carbon steels) against wear and corrosion and to provide service at temperatures 200–350 up to 500 °C (which presently became a common situation at downhole oil production and many mineral processing and power generation applications). In this case, in particular, for tubular components with big lengths (up to 10 m) and high length-to-diameter ratios, the coating technology should provide protection of inner or inner and outer surfaces. The coatings should have high hardness, which needs to be significantly greater than the processing materials (e.g. sand), high chemical inertness, dense structure and sufficient thickness, i.e. they should remain integrity during the service in the mentioned conditions for long time. Different coating processing options, such as electroplating and electroless technologies, thermal spray, microwave cladding, friction stir processing, plasma-assisting technology, PVD, CVD and some other processes may be employed for steel products protection [[34], [35], [36], [37], [38], [39], [40], [41], [42], [43]]. However, many of them are not very efficient against continuous erosion actions of hard processing media or cannot be effectively employed for protection of components with complex shapes and inner surface of long tubing, as well as cyclones, vortex and similar parts (such as PVD, thermal spray, cladding, friction stir technologies). As opposed to them, the coating technologies, such as CVD or based on the CVD principles may be very prospective, when hard and chemically inert materials are formed [18]. They allow formation of the protective layer on various surfaces of complex shape components.
Among different CVD coating processing options [35,36,44,45], the boronizing has a high potential. Boronizing (boriding) is a thermal diffusion process based on the CVD principles [[44], [45], [46]], and it provides the formation of hard and chemically inert iron borides on various steels and ferrous alloys. In this process, gaseous boron species are formed at high temperatures in specially formulated mixes of certain inorganic powders (which include B-containing ingredients), and these B-rich species deposit onto the preheated steel substrates with diffusion of boron atoms into the steel structure with subsequent formation and growth of iron borides Fe2B and FeB on the steel surface [[44], [45], [46], [47], [48], [49], [50]]. Due to the diffusion nature, the boronizing process and, therefore, the structure and thickness of the formed iron boride coatings can be managed by varying temperature and time, substrate surface preparation, creation of the gaseous conditions favorable for the B formation and by some other factors [46,47,[49], [50], [51]]. Iron borides have high hardness (about 8–10 times greater than bare steels and ferrous alloys), high chemical inertness due to short and strong covalent Fe–B bonds and rather high energy of crystalline lattice (or enthalpy) and melting point [50,[52], [53], [54]]. This coating becomes the “integral” part of steel components with no mechanical interface between the iron boride layers and the steel substrate with minimal possibility of delamination. Because of that and due to versatility of the technology, boronizing can be successfully employed for protection of steel complex shape components and inner surface of tubes, cyclones, etc. against wear and corrosion in different industrial applications, including oil and gas and mineral processing, [49,50].
Due to significant difference in structure and properties of bare steels and the boronized coatings, the rate of materials removal during mechanical and corrosive actions and the mechanisms of their damage and degradation should be very different. Thus, steels are not hard but ductile, while the iron boride coatings have significantly higher hardness (higher than steels and higher than the processing materials, like sand and many other minerals) but lower ductility and toughness. The chemical nature and structural bonds between the major elements in the considered materials (Fe with alloying elements for steels vs. Fe–B for the crystalline coating) are also totally different. Because of that, the formation of micro- and macro-defects and cracks, their nature and propagation, which are responsible for material degradation and removal, will strongly affect their performance at the continual erosion-corrosion actions.
Some wear properties of boronized steels, such as abrasion resistance and withstanding to adhesive wear and rod wear and their combination with corrosion action, have been evaluated in different publications [47,49,[55], [56], [57], [58], [59], [60], [61], [62], [63]]. However, it is very important to study the behavior of the boronized steels in dry erosion and slurry erosion conditions and to analyze the factors affecting their performance. In fact, the information related to the erosion and erosion-corrosion behavior of boronized steels is very limited, and only a few publications can be mentioned [12,[64], [65], [66], [67]]. The limited data obtained using different procedures does not allow to understand the performance of the boronized coatings at erosion conditions and the erosion mechanism. In particular, there were no studies considering erosion of boronized steels in the environments simulating downhole oil production, oil sand and some other mineral processing situations when large volumes of slurries containing hard abrasive particles, such as sand, impinge the material at high velocities. The aim of the present work is to evaluate the dry erosion and slurry erosion resistance of the iron boride coatings obtained through the thermal diffusion process in the conditions partially simulating the mentioned applications, even creating the “extreme” testing situations, and to compare the behavior of this coating with “traditionally” used bare steels. For example, in the majority of downhole oil production and oil sand transporting situations, the erosive flows of sand (in fact, erosive-corrosive flows) impinge the surface of tubing and related components at rather low angles (usually at 5–20°). In this work, special testing units designed and fabricated at Tallinn University of Technology (TalTech) were employed. In order to explore the possible degradation mechanism during erosion at different conditions, extensive examinations of surfaces and structures of the considered materials subjected to the testing should be conducted. These studies should have a significant importance for industry and should be utilized for selection and design of components designated for oil production, oil sand processing and some other mineral processing and engineering applications.
Section snippets
Materials and processing
Carbon steels J55 and L80 widely used for the tubing and piping systems in oil production and mineral processing have been selected for the experimental works. The materials chemistry according to the Mill Test Reports (MTR) is shown in Table 1. Both materials have similar chemical compositions, but L80 is the steel with higher mechanical properties.
The test samples were cut from steel bars or standard tubing with 3.5″OD (~89 mm) and thickness of ~6.5 mm or tubing with ~220 mm OD and thickness
Results and discussion
The obtained boronized coating on carbon steel has a well-consolidated structure consisted of two layers (Fig. 6). The inner zone (which is closer to the substrate) mostly consists of the Fe2B phase, and the outer zone mostly consists of the FeB phase. Despite different crystalline lattice parameters of the boride phases, both layers have similar “saw-tooth” morphology, which is formed due to optimization of the pack composition and process parameters, i.e. the FeB phase grows on the Fe2B
Conclusion
Erosion studies, e.g. in different dry and slurry erosion conditions at low impingement angles, which simulate some situations in downhole oil and gas production, oilsand and mineral processing, power generation and engineering applications were conducted for the iron boride coatings obtained on carbon steels through the thermal diffusion process and compared to bare carbon steels. All testing demonstrated superior performance of the iron boride coatings over carbon steel.
While the erosion
Funding
This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors in Canada. The work conducted at TalTech (by MA) was supported by the Estonian Ministry of Education and Research (SS427, M-ERA.NET DURACER18012, PRG643).
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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