Successful use of special stainless steels in nitrogen-based fertilizer plants

The modern urea plants are using a stripping process to reduce the energy consumption. The potential, very severe corrosion problems have been mastered by special grades and strict process control. The different grades are reviewed. After more than 20 years operation the results are very satisfying. In ammonia plants, an often forgotten corrosion problem is found in the heat exchangers. After about 3–6 years failures caused by chloride continuing cooling water may appear on carbon steel or low alloyed austenitic stainless steels. Corrosion mechanism is reviewed. Duplex stainless steels has been used with success and the properties are presented. An example of life-cycle cost shows that duplex stainless steel is an inexpensive way to solve chloride related corrosion problems. Condensation and evaporation of nitric acid in heat exchangers in nitric acid plants cause severe corrosion on conventional stainless steel grades. Low impurity in combination with high chromium content has proved to result in stainless steel with improved corrosion resistance.     

Biocorrosion of stainless steel grade 304L (SS304L) in sugar cane juice

The main objective of this research study is to assess the behaviour of stainless steel AISI 304L in contact with sugar cane juice. Stainless steel grade 304L is largely used in the sugar cane industry but the acidic nature of sugar cane juice pose a serious challenge in maintaining the life span of vital components.Sugar cane juice is acid having a pH value of around 5.6 at extraction. This acidic property is accounted for by the presence of a variety of acids namely: aconitic, citric, malic, oxalic, glycolic, mesaonic, tartic, succinic, fumaric and syringic in sugar cane juice.In addition to these acids there are approximately 50 different kinds of microorganisms present in the green cane and which are very active. These microorganisms will act as a contributor to a rather quick drop in pH (pH∼3.1) of the sugar cane juice once extracted.The more so, several minerals like water, salt, sulphate and silica are also present throughout the process line and are other contributor in the surface degradation wear mechanism.Faced with all these adverse elements, it is therefore fundamental to investigate thoroughly in the wear corrosion mechanism and biocorrosion on stainless steel grade 304L.To evaluate the mass loss, several corrosion experimentations were carried with the help of a potentiostat both in a sterilized juice and none sterilized juice. From these experimentations, it has been noted that corrosion was present on the surface of the disc (SS304L) in contact with none sterilized juice in the form of pitting while no corrosion wear was observed in the case of sterilized juice.The presence of biofilm was also observed on the sample disc surface. Biofilm formed on the surface of the sample disc was transferred to a potato dextrose agar (PDA) plate by stamping the disc in a circular sequence on the plate in sterilized condition in view to measure the density and resistance of the film. It was noted through this stamping protocol that the biofilm was very resistant due to the fact that after 12 stamping time we still observe the trace of the biofilm on the PDA plate. Microorganisms present in the biofilm have growth on the PDA plate and isolation of each colony was carried out in view of their identification.     

Die Bedeutung von Spalten zwischen Rohren und Rohrboden für das betriebliche Verhalten von Wärmeaustauschern

Importance of crevices formed between tubes and tube plate for the operational behaviour of heat exchangers . It must be guaranteed by construction and manufacture of heat exchangers that primary and secondary medium are completely separated from each other. When this requirement is fullfilled, the operational use of heat exchangers can be impaired by corrosion reactions within the crevice formed between tube and tube plate which may result in corrosion damage. The various techniques which are in use to connect tubes and tube plate and which are described in the present report, must be valued with respect to the tightness of the connection as well as to the formation of crevices between tubes and tube plate. Corrosion resistant copperbase alloys and stainless steels are the most important materials which are in use for the construction of heat exchangers. The mechanisms of crevice corrosion with unalloyed and low alloy carbon steels, stainless steels, and mixed connections between tube and tube plate with these materials are described in detail. Crevice corrosion may be caused also by the formation of galvanic cells between materials of differing electrochemical response. Furthermore, the concentration of aggressive media in crevices between tubes and tube plate can lead to corrosion damage of heat exchanger tubes. For the service operation of heat exchangers without any hazard of corrosion damage in crevices between tubes and tube plate, such crevices must be avoided by proper construction and manufacture. As a model for suitable measures to avoid crevices, the manufacture of steam generators for PWR's is described.     

Thermal conductivity studies on Si/SiC ceramic composites

Ceramic heat exchangers are increasingly used in many nuclear power plants. Silicon carbide has been treated as a promising material for heat exchanger application since it has good thermal conductivity and corrosion resistance. In this work, four different types of Si/SiC ceramic composites were prepared by liquid silicon infiltration technique. Thermal conductivities of these ceramic composites at different temperatures are measured by the laser flash thermal conductivity method. Results show that the presence of free carbon and voids are notably affecting the thermal conductivity of these materials.     

Corrosion performance of conventional (ASTM A615) and low-alloy (ASTM A706) reinforcing bars embedded in concrete and exposed to chloride environments

Conventional reinforcing steel is used in the majority of reinforced concrete structures. In general, steel reinforcement meeting ASTM A615 specifications has been the predominant reinforcement used for these structures. Low-alloy reinforcing steel (ASTM A706) was developed and is being marketed to improve ductility and weldability deficiencies associated with the ASTM A615 reinforcement. Several State Highway Agencies have adopted the use of these low-alloy reinforcing steels. Limited research has been performed on the corrosion characteristics of the steel reinforcement meeting ASTM A706 specifications. This paper presents results from a laboratory study on the critical chloride threshold, macrocell corrosion rates, and mass loss testing for ASTM A706 and ASTM A615 reinforcing steels embedded in concrete and exposed to chloride solution. Results from this study indicate that ASTM A706 reinforcing steel exhibits lower critical chloride threshold levels and higher corrosion rates than ASTM A615 reinforcing steel when embedded in cementitious materials.     

奥氏体钢盐雾腐蚀研究

本文采用盐雾腐蚀方法评价奥氏体钢的抗盐雾腐蚀性能,利用金相、XRD对材料的组织进行了分析,对该材料在盐雾环境中的腐蚀形态进行了观测,结果表明该材料在盐雾环境中的腐蚀形态以点蚀为主,并提出在盐雾环境中使用该材料的热处理方法,以提高该材料的耐盐雾腐蚀性能.     

换热器表面复合涂层耐腐蚀与导热性能

为了提升换热器表面耐腐蚀性且不影响换热器的换热效果，以石墨烯，石墨粉末，环氧树脂等为原料，制备应用于换热器表面的耐腐蚀高导热石墨烯复合涂层。经硫酸腐蚀实验、导热实验、结合强度实验的测试，结果表明：涂层的耐腐蚀性能随石墨烯含量的增加而提升，当石墨烯质量分数达到0.06%时，涂层的腐蚀速率达到最低值0.2338mg/(cm2·h)，其耐腐蚀性能远强于304不锈钢的1.5 mg/(cm2·h)；涂层的导热性能随石墨粉含量的增加而提升，且当石墨质量分数为8%时达到最大值35.848 W/(m·K)；涂层的结合强度达到ASTM等级：5B。     

The Influence of Heat Treatment and Process Parameters Optimization on Hardness and Corrosion Properties of Laser Alloyed X12CrNiMo Steel

Martensitic stainless steels are used in the production of steam turbine blades but their application is limited due to low hardness and poor corrosion resistance. Laser surface alloying and heat treatment of X12CrNiMo Martensitic stainless steel was conducted with the aim of enhancing hardness and corrosion properties. A Rofin Sinar Continuous Wave Nd: YAG solid-state laser was used to alloy the specimens. The electrochemical and hardness properties were studied using potentiodynamic polarization technique and Vickers micro hardness tester. The microstructures of the as-received, post-heated and pre-heated specimens were investigated by a Scanning Electron Microscope (SEM) and Optical Micrograph (OM) respectively. From the experimental results, the post-heated specimens exhibited the highest hardness property as compared to all other specimens. There was also significant improvement in the corrosion resistance of the post heated specimen compared to all other specimens and the substrate as evidenced by higher polarization resistance and lower corrosion rates. From the analysis of grey relational grade model, the significant laser processing parameters were identified. The results showed the influence of laser power and scanning speed on the corrosion rate, hardness and alloyed depth. The predicted results were found to be in good agreement with the experimental results.     

Threshold chloride concentration for stainless steels activation in concrete pore solutions

The threshold chloride concentration for stainless steels activation in concrete environments depends not only on chemical composition of the steel and on pH level of concrete pore solution but is also considerably affected by the superficial state of the steel. The presence of scales leads to significant decrease of corrosion resistance. This study evaluates the corrosion resistance of various stainless steels exposed to pore solutions of fresh concrete by means of electrochemical methods. All tested materials with bare surface, except for martensitic chromium steel FeCr12, proved to be completely resistant to fresh concrete pore solution containing chlorides. If the surface is scaled, the identically high resistance cannot be expected even for the most alloyed stainless steels.     

水源热泵用换热设备常用金属的腐蚀性能

目前有关金属腐蚀的研究多集中在探讨影响腐蚀的因素以及防腐蚀相关技术,有关换热器金属腐蚀经济性的文献较少。本文采用挂片失重试验,结合电镜分析,分别研究了碳钢10^#、不锈钢316L、紫铜T2和铝合金LF21在海水和污水中的腐蚀速率、电偶腐蚀速率以及微观腐蚀形貌;并运用模糊综合评价法进行评价,为合理选择换热设备金属材料提供依据。结果表明,根据金属均匀腐蚀耐蚀性十级标准,不锈钢316L、紫铜T2与铝合金LF21都属于耐蚀金属,不锈钢316L在静止和流动水环境中,均具有最好的耐蚀性;在严格避免与电位较正金属偶合使用的前提下,铝合金LF21比碳钢10^#和紫铜T2耐蚀,其经济性也较为优秀;碳钢10^#虽耐蚀性差但胜在价格优势,是实际工程最常用的金属材料。     

In situ corrosion study of stainless steels in an MSF desalting plant

The corrosion resistance of different stainless steel grades have been studied as regards localized attack under service conditions. Test racks with welded samples with artificially applied crevices were inserted in three different flash chambers. Materials used for splash plates, brine piping and. condenser tubes were also tested.Beinge a pilot plant, various process conditions were tested. Hence, the materials have been exposed to different corrosive conditions, including stagnant seawater during shut-down periods.The results show that stainless steels of type 316L and higher alloyed exhibit good corrosion resistance, even in brine at high temperature viz pipes from the brine heater.     

Praktische Hinweise zur Wahl des Werkstoffs von Maschinen und Apparaten

Materials for production plants must be corrosion resistant and mechanically stable. Additionally, the plant hygiene and product hygiene may represent key aspects, depending on the product. Typical materials for the equipment in the chemical, pharmaceutical and food industries are high‐quality stainless steels as well as special metals, super alloys, non‐metals and plastics. To avoid contaminations, the stainless steel equipment is polished on the product contact surfaces. Chemical resistant special materials are used as massive material or often applied on a steel base.     

Corrosion behavior of ferritic stainless steel alloyed with different amounts of niobium in hydrochloric acid solution

The corrosion behavior of stainless steel alloys containing corrosion-resistant elements was investigated. Ferritic stainless steel (FSSs) electrodes were synthesized by applying a scan rate of 1 mV s⁻¹. Stainless steels were used unalloyed and alloyed with about 0.5, 1, and 3 wt% elemental Nb. The samples were obtained from casting and forging. The samples were classified into three groups. In the first group, samples were unhomogenized and remained in production condition. In the second and third groups, samples were exposed to homogenization at 1,100 °C for 30 min or 180 min, respectively, and then quenched. The corrosion performance of the FSSs was investigated in 0.3 M HCl acid solution using electrochemical impedance spectroscopy (EIS). Corrosion resistance was calculated using the Stearn–Geary equation. SEM investigations of samples immersed in 0.3 M HCl acid solution for 60 and 360 min were performed. SEM micrographs showed generalized pitting. Consequently, it was determined that niobium has a beneficial effect on the corrosion resistance of FSS since niobium reacts with carbon to form stable carbides.     

氟塑料-石墨板式换热器在铝溶胶生产中的应用

介绍了氟塑料 石墨板式换热器在铝溶胶生产中的选择和应用。并与其他类型换热器作了对比,它具有强耐腐蚀性和高效换热性及密封性等优越性,使得换热器使用寿命显著提高     

The influence of microstructure on the corrosion rate of various carbon steels

This paper presents a fundamental study of the influence of carbon steel microstructure on the corrosion rate. Subsequently, the corrosion performance of various grades of carbon steels were evaluated in stirred autoclaves under elevated carbon dioxide and temperature conditions. Corrosion and penetration rates were determined via mass loss and optical microscopy, respectively. It was found that the corrosion rate of carbon steel line pipe is influenced by microstructure. More specifically, a relationship between localized corrosion susceptibility and the presence of pearlite bands in the steel microstructure was found. However, no correlation was evident between minor elemental concentrations (i.e., Ni, Cr, Mo) and corrosion resistance. It has been proposed that the corrosion stability of the various microstructures may arise from variations in the distribution of carbon bearing phases within the steel. In the banded ferrite/pearlite structure, the carbon-bearing phase (pearlite) is distributed in layers whereas in the other structures the carbon-bearing phases are much more evenly distributed. This study reports on the corrosion resistance of carbon steels in relation to their chemical and physical properties.     

不锈钢和低碳钢在高浓度相变储能溶液体系中的腐蚀行为

文章研究了10号钢、20号钢、314和316L不锈钢在高浓度三水醋酸钠和饱和硫酸铵铝两种相变储能溶液体系中的腐蚀行为,采用PS-268B电化学工作站测试四种钢材在这两种体系中的极化曲线。实验结果表明,316L不锈钢在高浓度醋酸钠溶液体系中耐蚀性能最好,自腐蚀电流密度为6.89×10-5 A·cm-2,与20号钢(0.855 A·cm-2)相比,少了四个数量级。316L不锈钢在饱和硫酸铵铝溶液体系中耐蚀性能最好,自腐蚀电流密度为0.025 A·cm-2,仅为20号钢(0.133 A·cm-2)的五分之一。对钢种进行归纳,综合比较,不锈钢在两种溶液中的耐蚀性能比低碳钢优异。     

Electrochemical investigation of the corrosion behavior of chromium-modified carbon steels in water

Electrochemical measurements were performed on four steels with different chromium (Cr) contents in simulated CANDU reactor coolant at room temperature to investigate the influence of chromium content on the corrosion behavior of the steels. The addition of chromium to carbon steel can promote passivation. The passive film significantly decreases the corrosion rate, and thus improves the corrosion resistance of the steels as chromium content is increased. Repassivation kinetic experiments indicate that there is a more rapid repassivation rate and a more stable passive film on steels containing a higher concentration of chromium. The slip-oxidation model was used to calculate the crack growth rate (CGR) on the steels. The 304L stainless steel has the highest CGR with an increase in potential of the steels studied. Chromium modification of steels can improve the resistance to corrosion. However, its influence on cracking and crack growth must be taken into consideration. The 2.5%Cr-1%Mo steel gave the best overall results when considering both the corrosion and the cracking behavior of the steels studied.     

炼厂循环水中不锈钢孔蚀电位的影响因素分析

通过对某炼厂生产装置中腐蚀较为严重的不锈钢换热设备进行调研、分析,确定该换热设备腐蚀失效的主要原因是由于循环冷却水垢下腐蚀造成的不锈钢点蚀穿孔。选取几种典型的不锈钢材料,模拟出循环水在换热设备的现场腐蚀环境,利用动电位极化的方法测定及对比其各自的点蚀电位,对上面所选取的材料通过改变腐蚀介质的氯离子浓度、pH值等几个主要影响因素,进而在同条件下对各种因素对不锈钢点蚀的影响进行评价。     

Corrosion behaviour of new stainless steels reinforcing bars embedded in concrete

The activities concerned with the evaluation, repair and restoration of structures are estimated to amount to 35% of the total volume of the work in the building sector and this continues to increase. The corrosion of rebars in the reinforced concrete structures (RCS) is the main reason for their degradation, so the use of reinforcing stainless steels seems to be one of the possible solutions with most probabilities of solving this problem. In this work, in order to demonstrate the advantages of using reinforcing stainless steels, the corrosion behaviour of AISI 304 and 316 reinforcements embedded in concrete slabs (C35/45 and C60/70 concrete) with two chloride contents are compared with three low-cost and low-Ni austenoferritic stainless steels and with the conventional carbon steel. The lower chloride contamination selected in this research, was enough to cause the corrosion in the active state of the carbon steel reinforcements, whereas the highest one exceeded the expected contamination in the natural environments, including sea media. The metallic materials remaining in the passive state can be considered, from the point of view of corrosion resistance, adequate as reinforcements in the RCS.     

换热器专用钢管08Cr2AlMo的性能及应用

着重介绍了换热器专用钢管 0 8Cr2AlMo的耐腐蚀性能、力学性能 ,以及 0 8Cr2AlMo钢管与 16Mn管板的焊接工艺以及在炼油厂的应用