Corrosion Behaviour of Proprietary Micro-alloyed Steels

Oil and gas industry is a strategic industry being of central importance for global economy as a vital source of energy for all other sectors of engineering industry. This vital role is being challenged as the important reserves of oil and gas which are left to be discovered and produced are mainly concentrated in difficult locations which are under severe operational conditions of high pressure, high temperature and aggressive environments. Therefore the materials and the technology required for exploration, production and transportation must possess appropriate properties considering the excessive cost of replacements or failures in these difficult-to-access locations. Carbon steel because of its tremendous and inherent properties such as low cost; easy to shaping, forming and machining; good weldability; wide availability; its properties have been studied in depth and are well understood; good mechanical properties etc is preferred over all other conventional engineering materials and has found extensive use in the oil and gas industry. However, carbon steels are susceptible to corrosion attack particularly in harsh oilfield environments thus affecting the capital expenditure (CAPEX), operational expenditure (OPEX) and health, safety and environment (HSE). Regardless of these disadvantages, carbon steel still remains the material of choice because no alternative materials have adequately exhibited better properties for oil and gas applications. Therefore efforts are being continuously made to improve the properties of carbon steel through diverse physical, chemical and thermo-mechanical processes with the aim of developing new materials which are stronger, lighter, and more resistant to corrosion. The quest to mitigate the corrosion susceptibility of carbon and low alloy steels for effective use as structural materials in oil and gas industry particularly in challenging locations and under severe conditions has led to the development of micro-alloyed steels. For the past six decades when micro-alloying was generally accepted, micro-alloy steels have become an indispensable class of structural materials providing desirable combinations of properties such as strength, toughness, formability, weldability, and corrosion resistance at affordable cost. This excellent combination of properties can be attributed to the presence of fine grain structures occasioned by the addition of small quantity of alloying elements, controlled processing technologies and appropriate heat treatments. Most of the elements located at the left of iron in the periodic table are considered as strong carbide-formers. These elements have found extensive application in micro-alloying of steels. Among these elements, Zr and Hf have not received comparative application like the other neighbouring elements such as Ti, V, Nb, and Cr. A comparative and systematic assessment of the corrosion behaviour of three proprietary micro-alloy steels designated as (i) ASTM A694-F65 forged steel bar (F65) (ii) improved collapsed grade P110 IC pipe (P110) and (iii) high collapsed sour service grade TN95 HS pipe (TN95) were conducted in different media/electrolytes using different environmental parameters, electrochemical techniques (Linear polarization resistance (LPR), Tafel polarization (TP), Electrochemical impedance spectroscopy (EIS) and potentiostatic polarization (PP)), weight loss measurements and surface analyses. API 5L X65 steel grade (X65) was used as reference sample. The results of the preliminary investigations showed that the corrosion rates of these micro-alloy steels using different electrochemical techniques and in all the environments corroborated each other and revealed a ranking order of F65 < X65 < P110 < TN95. The main reasons for this behaviour were microstructure and chemical composition of the respective microalloy samples. The two worst corrosion rate steels (P110 IC and TN95 HS) were selected for alloying with different quantities (0.1, 0.2 and 0.4 wt%) of Zr and Hf with a view to improving their corrosion resistance. The analysis of the alloyed specimens showed microstructural modification exhibiting grain refinement with increase in alloying elements (Zr and Hf). This was attributed to the formation of Zr- and/or Hf-base intermetallic precipitates. These precipitates have the ability to pin down grain boundaries thus limiting grain growth which resulted in improved mechanical properties such as increased hardness and tensile strength without any negative impact on toughness of the alloyed samples. The alloyed steels were subjected to corrosion tests in simulated produced water (SPW) saturated with 99.98% CO2 at 250C and pH 6.5. The results showed that the specimens alloyed with Zr and Hf exhibited superior corrosion resistance than the unalloyed and the reference samples with the corrosion rate of the alloyed steel decreasing with increase in the quantity of alloying elements. This signified that Zr and Hf has suppressive effect on electrochemical reactions thus leading to passivation with the resultant improvement in corrosion resistance ranging from 3% - 35% for Hf addition and 27% - 44% for Zr addition in TN95. This can be attributed to higher dissolution with the corresponding formation of more intermetallic precipitate of Zr in Fe than Hf.