ABSTRACTAs an important infrastructure for oil and gas transportation, the safe and stable operation of long‐distance pipelines is significant for guaranteeing national energy security and economic development. However, pipelines are susceptible to corrosion due to various factors. Therefore, it is crucial to take effective corrosion protection measures. Pulse current cathodic protection technology, as an advanced pipeline corrosion protection technology, is superior to traditional cathodic protection technology. This paper mainly summarizes the research progress of pulse current cathodic protection technology for long‐distance pipelines in recent years. It briefly discusses the protection mechanism, characteristic parameters, challenges, and development trends of pulse current cathodic protection technology, which provides a useful reference for the further promotion and application of pulse current cathodic protection technology for long‐distance transport pipelines.

Binary transition metal borides have an important application prospect in aerospace field because of their high melting point, high hardness and high thermal conductivity, but their intrinsic brittleness and poor oxidation resistance seriously restrict their high temperature application in extreme environment. MAB phase is a new ternary transition metal boride obtained by introducing IIIA and IVA atoms into binary boride cells, which shows many excellent properties like ceramics and metals. In all the MAB phases, MoAlB has become the research focus due to its excellent fracture toughness, oxidation resistance, damage tolerance and machinability. Thus, the preparation methods and some basic properties of the MoAlB powder, bulk and coating, including physicochemical properties, mechanical properties, friction and high temperature oxidation resistance, were summarized in this paper. Additionally, the modification method and mechanism of MoAlB materials were discussed in order to clarify the key problems and possible solutions in the preparation and application of MoAlB materials, and the potential application fields of MoAlB materials were also presented.

Based on the corrosion rates under high-temperature and high-pressure, both the corrosion behaviors of TC4 titanium alloy tubes in neutral and acidic (containing CO2 and H2S) environments and the thermodynamic stability of their passivation films were investigated by in-situ electrochemical tests combined with molecular dynamics simulations and first-principles calculations. The results show that the corrosion of TC4 titanium alloy at 180 celcius is mild. And the corrosion reaction of TC4 titanium alloy in different environments is controlled by an anodic reaction process. Comparing with the neutral environment, TC4 titanium alloy in CO2 and H2S environment present smaller electrochemical corrosion kinetic resistance. Among three environments, TC4 titanium alloy shows the worst corrosion resistance in H2S environment. Cl-, HCO3-and HS-, all have strong charge interactions with the positive-charged Ti atoms of TiO2(110) passivation surface. By altering Cl- concentration and temperature, the existence of H2S and CO2 will reduce the binding energy between Cl- and TiO2(110) surface, i.e., the thermodynamic stability of the TiO2 passivation film became worse.

硬质涂层的制备及改性是当今涂层领域的研究热点之一,随着科学技术的迅速发展及工业要求的不断提高,对硬质涂层材料、性能及其制备技术则提出了更高的要求.本文主要从硬质涂层制备、组元微结构设计及其强化/退化机理、高性能硬质涂层三方面展开,介绍了硬质涂层领域的一些热点研究方向及其目前存在的问题.主要包括以下内容:发展新的沉积系统和涂层制备新技术,实现一些硬质涂层的低温沉积、低压沉积和高速率沉积,以及亚稳相的制备等;基于对Veprek制备的nc-TmN/a-Si3N4纳米复合涂层结构模型和理论解释的质疑,提出纳米晶/非晶复合涂层的研究重点;对比了氧元素在不同的涂层体系中所扮演的角色及其作用机制;总结了低应力厚硬质涂层的结构设计方法、对应机理及存在问题;归纳了一些高性能硬质涂层所需要具备的性能特点及实现方法,如高韧性、高抗裂纹扩展能力涂层,高热稳定性、抗氧化性涂层,超硬耐磨自润滑硼化物涂层.

Boron carbide thin films were deposited on (100) silicon substrates at ambient temperature via. pulse dc magnetron sputtering. Various frequency and duty cycles were applied to the hot-pressed B4C target in order to understand their influence on the structure and mechanical properties of the B4C films. X-ray Energy dispersive spectrum, Raman spectroscopy and Transmission electronic microscopy were used to characterize the composition and microstructure of the films. Nanoindenter was employed to measure the hardness and modulus. The film toughness was evaluated by a microindentation method. The results show that both pulse frequency and duty cycle significantly affect the B/C atomic ratio and then hardness and modulus in the boron carbide films. However, the amorphous structure of the films was maintained when the frequency and duty cycle changed. The maximum hardness of 29GPa and modulus of 247GPa combined with relative high toughness (3.3MPam1/2) were achieved under 50kHz frequency and 30% duty cycle. In addition, there was no evidence to prove that the graphite phase existed in the B4C films although exceeded C concentration was detected.

An improved hot-press method with the sintering temperature just beneath the melting point of copper was applied to prepare copper matrix composites. Cu-based hybrid composites reinforced with most TiB2 particles and a small amount of TiB whisker have been in-situ synthesized from the primary Cu, Ti (or TiH2) and B powder mixtures. The reinforcement in TiB2(-TiB)/Cu composites often appears as bubble-like cluster, which is believed to result from the local melting of Cu-Ti intermetallic compound forming in pre-sintering stage. Subsequently, the formation mechanism of the reinforcement was identified as the in-situ reaction occurring at the original site of Ti (or TiH2) by diffusing B from Cu matrix into Cu-Ti liquids. As compared with Cu-Ti-B system, the composites fabricated from Cu-TiH2-B powder mixture have more favorable microstructures and thus better overall performance.

Fe/VC multilayer coatings with a fixed bilayer period (Λ=8nm) and variable Fe fraction (ΛFe/Λ ranging from 0.6 to 0.9) were deposited on Si substrates via dc magnetron sputtering. X-Ray diffraction and transmission electron microscopy showed the nanoscale layer structure of these coatings. High-resolution transmission electron microscopy imaging revealed the formation of coherent interfaces between Fe and VC (100). Throughout the entire range of Fe fractions investigated in this study, the hardness was enhanced over the rule-of-mixture trend line. Even at Fe fraction of 0.9, the hardness value was 16.3GPa, enhanced by about 80% over the rule-of-mixture value of 9GPa. While all Fe/VC coatings investigated in this work have statistically the same hardness, Fe/VC coating with Fe fraction of 90% has almost twice the fracture toughness of the other Fe/VC coatings. We attribute this enhanced toughness to the favorable generation of dislocations in the Fe layers and their activation by coherency strain emanating from Fe/VC interfaces. This study also demonstrates that H/E and H3/E2 are not good proxies for the toughness of these coatings.

It is desirable to increase the hardness of protective coatings for reduced abrasive wear and to increase the toughness for improved wear performance due to fatigue and formation of flaws or cracks. Unfortunately, there is an inverse relationship between hardness and toughness: the harder the coating, the lower the toughness. Using W/VC multilayer coatings, we demonstrated that it is possible to reverse this hardness–toughness trend. These coatings were synthesized by DC magnetron sputtering with bilayer periods of about 10nm and different thickness fractions of W. They are crystalline, with hardness and toughness exceeding that of pure VC (25GPa and 1.0MPa-m1/2). In particular, the W/VC multilayer coating with thickness fraction of 90% W achieves hardness of 28.5GPa, comparable to many ceramics-based hard coatings, and toughness of 7.5MPa-m1/2, similar to many nanocrystalline metals. This investigation shows that one can synthesize coatings as hard as ceramics and as tough as metals. These results also indicate that H/E and H3/E2 (H=hardness and E=elastic modulus) are not good proxies for coating toughness.

Most hard coatings are based on ceramic materials and are generally brittle. It is desirable to have coatings that are both hard and tough. Here, we review several strategies that can be employed to increase coating toughness while maintaining hardness. Various nanocomposite and multilayer coatings (Ti/TiB2, FeMn/TiB2, Fe/VC and W/VC) were synthesized to explore three such toughening strategies: coherency strain, transformation toughening, and nanograined metals. Practical methods used to measure coating toughness in this work were presented: scratch testing, nanoindentation, and modified Vickers. Results demonstrate that coating systems that exploit these strategies show significantly enhanced toughness compared with those that do not. In particular, the strategy of using nanolayers of a metal with high elastic modulus alternating with spacer layers much thinner than the metal appears to be the most effective. In principle, one can reach hardness values up to 10% of the elastic modulus, while attaining toughness comparable to most nanocrystalline metals. Given that most metals with high elastic moduli are refractory materials, such coatings may also be useful for high-temperature applications.

The objective of this research is to characterize the elastic modulus, hardness, fracture toughness, and structure of multilayer coatings consisting of alternating nanolayers of Fe1−xMnx and TiB2 coatings (x=0, 0.18, and 0.35). These coatings were synthesized by dc magnetron sputtering. X-ray diffraction showed that Fe0.82Mn0.18 contains 79% bcc and 21% fcc phases, while Fe0.65Mn0.35 contains close to 100% fcc phase. The hardness of these multilayer coatings was found to exhibit a small enhancement (~2GPa) over the rule-of-mixture values. The most striking finding is that the fracture toughness of Fe0.82Mn0.18/TiB2 is about twice of that for Fe0.65Mn0.35/TiB2 and Fe/TiB2 with comparable hardness. In addition, Fe0.82Mn0.18/TiB2 exhibits a quasi-elastic response in nanoindentation experiments. Given that Mn addition to Fe is known to result in the formation of metastable fcc phases at room temperature (with the degree of metastability controlled by the Mn content) and that the fcc phase may transform to the bcc phase under stress, such a transformation is likely to play a role in the increased toughness and quasi-elastic nanoindentation response observed in these coatings and may provide a strategy in the synthesis of hard coatings with improved toughness.