Improving corrosion resistance of Al‐11mass%Si alloy through a large number of ECAP passes

Ultrafine‐grained (UFG) Al‐11mass%Si alloy, processed by multi‐pass equal‐channel angular pressing (ECAP) at 573 K, was investigated on corrosion behavior in 0.6 M NaCl solution. Potentiodynamic polarization tests and scanning electron microscopy observation showed that a large number of ECAP passes resulted in lower corrosion current density, more positive corrosion potential, and rather smooth corroded surface with shallow corrosion pits. The uniform distribution of fine secondary‐phase particles on UFG Al matrix weakened the susceptibility to pitting corrosion while inhibited general microgalvanic reactions. The present results indicate that grain refinement of aluminum matrix to the UFG state and uniform redistribution of broken particles (including eutectic silicon and secondary phases), via severe plastic deformation at elevated temperature undergoing dynamic recrystallization, can significantly improve the corrosion resistance of Al alloys, besides the known exceptional mechanical advantages. The simple and effective ECAP procedure makes UFG Al alloys more attractive for high strength structural application in corrosive environment.     

Long term corrosion characteristics of metallic materials in marine environments

Abstract

Long term corrosion test data in marine environments are essential data to guide the design and application of marine engineering. Corrosion test results, over a 16 year period from October 1986 to October 2002, using eight kinds of metallic materials are reported in this paper. Materials were exposed in the splash zone, tidal zone, and full immersion zone of four corrosion test stations located in Qingdao, Zhoushan, Xiamen and Yulin respectively, and therefore the corrosion behaviour of various ferrous and non-ferrous materials in these marine environments has been obtained. The corrosion of carbon steel in the seawater fits the following general relationship: D=A+K(t–1). The variation in the general corrosion rates for the different carbon steels in seawater is relatively small and consistent. However, the pitting corrosion rates for the different carbon steels are all significantly different. The marine splash zone is about 0–2˙4 m above the mean high water level of the seawater. The corrosion peak was located about 0˙6 to 1˙2 m above the mean high water level. The order of the corrosion potential of stainless steels with the corrosion resistance of stainless steels is consistent in seawater. The corrosion of copper was found to be serious in the full immersion zone of the Yulin sea area. The effective lifetime of the cathodic protection for a wrapped aluminising layer of the hard and ultrahard aluminium may be 16 years.     

Role of climatic conditions on corrosion characteristics of structural steels

Corrosion behaviour of low alloy (LASS) and plain carbon (PCSS) structural steels exposed in different types of climatic conditions and nature of rust formed on their surfaces have been studied after 2 years of exposures. The test sites were chosen to represent four types of environments who strongly influence the corrosion of metals and alloys. They include (a) humid-saline, (b) humid-saline-urban (c) humid-industrial and (d) plain dry-urban environments. Mass loss, Raman spectroscopy, scanning electron microscopy and electrochemical impedance spectroscopic studies have been performed to study the corrosion behaviour and characterise the nature of rusts formed on these steels. Mass loss measurement technique has been used to determine the loss of thickness of steels during their atmospheric and salt spray exposures. Results indicate that the corrosion rate of steels is strongly influenced by the climatic conditions prevailing at the exposure sites. The presence of SO₂ and salinity in the environments change the structure and protective properties of rust formed on the steels’ surface. Electrochemical impedance and cyclic polarisation studies of the steels in simulated environments have been performed to understand the mechanism of corrosion in different climatic conditions.     

Corrosion behaviour of experimental copper–antimony–molybdenum carbon steels in industrial and marine atmospheres and in a sulphuric acid aqueous solution

Low alloyed carbon steels are used in several applications as in automotive, home appliances and civil industries. Sb-bearing steels have been developed to withstand acid condensation, mainly to exhibit corrosion resistance to sulphuric acid aqueous solutions. This work is aimed at studying the corrosion resistance of three experimental low alloyed carbon steels with additions of copper, antimony and molybdenum using the electrochemical impedance spectroscopy (EIS) in a sulphuric acid aqueous solution, and field tests in industrial and marine atmospheres. The field tests showed the mass loss of antimony–molybdenum carbon steels was higher compared to that of other steels. The alloyed carbon steels with copper and antimony additions showed the highest atmospheric corrosion resistance evaluated by using field tests in industrial and marine environments. The molybdenum-bearing steels showed the highest corrosion resistance in a sulphuric acid solution, measured by using the EIS.     

The influence of alloying elements on the corrosion behaviour of ferritic steels in simulated combustion atmospheres

The low‐chromium steels (Cr content 0.7–1.4 wt.%) are usually being used in low temperature boiler applications where the environments are not so aggressive as in waste‐fire boilers. To improve their mechanical properties and corrosion resistance several alloying elements have been used. In the present study the new low‐chromium steels modified with Si, Ni and Ce have been investigated. These steels were compared with the common boiler steel – 2.25Cr1M. The multi‐sample exposure tests were made in moist air (8% O₂ + 15% H₂O) with and without 200 ppm SO₂ or 2000 ppm HCl addition at the isothermal temperature of 500°C. Corrosion products were analysed by Scanning Electron Microscope with Energy Dispersive Spectrometer (SEM/EDS) and XRD techniques. The results of the study show that addition of Ni and Si to the steels studied could improve their corrosion resistance, but only in moist air atmospheres. In SO₂‐containing atmospheres the corrosion resistance seems to remain the same, when in HCl‐containing atmospheres Ni and Si additions seem to have a negative effect on the corrosion behaviour. As could by seen from SEM analysis, the scales formed, on the steels with above elements, are more adherent to the metallic core. The cerium was added to the studied materials mainly to improve their mechanical properties. This alloying element seems to increase the corrosion rate of materials, especially in atmospheres containing Cl.     

Susceptibility to corrosion damage of pipeline steels under coating disbondments

This study provides an experimental investigation on the corrosion behaviour of three carbon steels used for pipeline application. The susceptibility of these materials to corrosion damage was analysed in order to simulate its service conditions particularly under disbonded coating. Monitoring of open‐circuit potential (E_free), polarization resistance (R_p) and measuring of the weight loss during immersion time were used to evaluate the corrosion behaviour of the studied materials. All the corrosion experiments were performed in two aqueous solutions: natural seawater and synthetic one (3 wt% NaCl solution). The morphology of the corrosion products was examined by optical microscopy. The results obtained from electrochemical tests have shown different behaviour for the studied steels into the retained corrosive environments: more stable potentials (E_free), higher R_p‐values with large fluctuations evolution were found in natural seawater. The gravimetric measurements have also shown a continuous variation of the weight loss throughout the exposure period in the sodium chloride solution. However, it seemed that a passive behaviour was observed in natural seawater. A little difference was observed between all the studied steels in terms of corrosion kinetics. The steel, having the little ferritic grain size, seems to be more resistant to corrosion damage. Qualitatively, a porous and non‐adherent oxide film was observed on the corroded surface in the synthetic solution; while, the rust layer, which is formed in the natural seawater, has acted as a barrier of corrosion process. Finally, all the results obtained from both electrochemical tests and weight loss measurements were in reasonably good accordance. The important common point that can be concluded was that all the tested materials seem to be more suitable for natural seawater than 3 wt% NaCl solution. Also, they are not recommendable to be used in an environment where chloride attack is possible and important.     

Corrosion behavior of a superduplex stainless steel in chloride aqueous solution

Super duplex stainless steels (SDSS) have been widely used as structural materials for chemical plants (especially in those engaged in phosphoric acid production), in the hydrometallurgy industries, and as materials for offshore applications due to their excellent corrosion resistance in chloride environments, compared with other commercial types of ferritic stainless steels. These alloys also possess superior weldability and better mechanical properties than austenitic stainless steels. However, due to their two-phase structure, the nature of which is very dependent on their composition and thermal history, the behavior of SDSS regarding localized corrosion appears difficult to predict, especially in chloride environments. To improve their final properties, the effect of the partition of the alloying elements between the two phases, and the composition and microstructure of each phase are the key to understanding the localized corrosion phenomena of SDSS. This paper concerns the effects of the SDSS microstructure and heat treatment on the SDSS corrosion resistance in aqueous solutions, containing different amounts of NaCl at room temperature.     

Corrosion resistance of a cast steel overpack for high-level radioactive waste disposal in Japan

The corrosion rate and stress corrosion cracking (SCC) susceptibility of a cast steel are studied for its application to metal containers (namely overpacks) for geological disposal of high-level radioactive waste. Specimens for corrosion tests are cut from a prototype overpack manufactured by full-scale casting. Casting defects are widely distributed in the prototype overpack; however, the flat-bottom hole equivalent diameters for all defects detected by an ultrasonic test are 3.6 mm or less, which is relatively small. Forged steels and rolled steels are also tested for comparison of their corrosion properties with the cast steel. The corrosion rates are obtained by immersion tests in bentonite saturated with synthetic seawater under anaerobic conditions at 80°C for up to 1 year. The corrosion rate for the cast steel calculated by the weight loss during the experiments is close to that for the forged steels and rolled steels. The SCC susceptibility is examined using slow strain rate tests in a 1.5-mol L⁻¹ carbonate–bicarbonate solution, in which the occurrence of high-pH SCC is often reported for carbon steels. The SCC susceptibility increased with the increase in the carbon content of the products; however, there are no clear differences between casting and forging.     

Über den Einfluß der Bewitterung auf das Korrosionsverhalten unlegierter und niedriglegierter Stähle

Influence of weathering on the corrosion behaviour of unalloyed and low-alloy steels Accelerated weathering tests with plain carbon steel and low-alloy steels show that the influence of weathering is of considerable importance for the atmospheric corrosion. Attention has to be paid that the steel surfaces are never wet over a longer period of time. The faster the steels will be dry, the better is the long time behaviour of these materials. Frequent short wet periods, followed by dry phases lead to the formation of an increasingly better protective layer. By favorable weathering conditions the influence of the composition of the alloys is comparatively small. Metallographic and electrochemical tests further show that the corrosion protection is not only due to the formation of a compact macroscopic layer. Local passivation effects are significant as well.     

The case for ultrahigh-carbon steels as structural materials

Ultrahigh-carbon steels (UHCSs) are low-alloyed plain carbon steels containing 1–2.1% carbon. These steels have remarkable structural properties when processed to achieve fine ferrite grains with fine spheroidized carbides. They can be made superplastic at intermediate temperatures. Further, they can be made hard with compression toughness and strong with good tensile ductility at ambient temperatures. Contrary to conventional wisdom, UHCSs are ideal replacements for currently used high-carbon (0.5–1 % carbon) steels because they have comparable ductility but higher strength and hardness. In this article, examples of structural components formed from fine-grained spheroidized UHCSs are illustrated, and other potential structural applications are reviewed. These steels can be laminated with other metal-based materials to achieve superplasticity, high impact resistance, exceptionally high tensile ductility, and improved fatigue behavior.     

Stainless steel thread forming screws and their susceptibility to stress corrosion cracking

Abstract

Thread forming fasteners incorporate both drilling and hole tapping features and are commonly used in the construction industry to fix together steel sheets of different material types. With this practical application in mind, fasteners manufactured from martensitic and austenitic stainless steels have been subjected to alternating corrosion conditions in accordance with test standards DIN 50021-SS and DIN 50018-K WF 2·0. The torque applied to the screws during these tests was controlled to place the fasteners under equal tensile loads, independent of their tensile strength. Thus, the results provided information on their relative susceptibilities to stress corrosion cracking. At the end of the tests, up to 80% of the martensitic stainless steel drilling and tapping screws had failed due to hydrogen induced stress corrosion cracking. The fasteners manufactured from austenitic materials withstood identical test conditions without any evidence of cracking or crack initiation. It is concluded that fasteners manufactured from modified martensitic stainless steel are more susceptible to stress corrosion cracking under the conditions of test than those made from cold worked austenitic stainless steels. This suggests that in practical applications the potential for catastrophic failure due to stress corrosion cracking could be considerably higher in modified martensitic fasteners in comparison with austenitic stainless steel fasteners, including those with hardened carbon steel drill points.     

Corrosion performance of titanium and titanium stabilised stainless steels

Abstract

Titanium and its alloys provide industry with a number of materials which are strong, light, and very corrosion resistant. In addition, titanium is added as an alloying constituent to some stainless steels to act as a stabiliser during welding. Over the past 30 years titanium alloys have been increasingly used in process industries, and wherever ‘nil corrosion’ is considered to be an essential design feature. The main drawback to titanium usage has been relatively high cost, but freedom from plant corrosion failures, reduced downtime for maintenance, and the increasing availability of titanium have made this metal and its alloys an attractive choice in recent years. Applications include process vessels, heat exchangers, marine fittings, offshore components, pump castings, and other applications where materials encounter a hostile service environment. Nevertheless, titanium and its alloys are still subject to some forms of corrosiveattack, such as galvanic corrosion, hydrogen absorption, erosion corrosion, and crevicecorrosion. Special welding procedures are also required, which, if ignored, can lead toserious problems. This paper outlines a number of recent investigations into some problems encountered in industrial and marine environments, where both titanium metal and titanium stabilised stainless steels have suffered unexpected corrosion attack. The case histories described illustrate that titanium may show unexpected corrosion problems if certain aspects of its corrosion behaviour are overlooked.     

Stress corrosion cracking and hydrogen embrittlement susceptibility studies on modified 9Cr-1Mo steel weldments in acidic and neutral media

Abstract

There is much scope for the development of new engineering materials for high temperature applications such as for tubing used in steam generation in fossil fuel and nuclear power plants and in petrochemical cracking units. The materials for such applications are often manufactured from mild steel and low alloy ferritic steels containing up to 9%Cr. Modified 9Cr-1Mo ferritic steel, prepared by incorporating vanadium and niobium, is one of the newer materials extensively used for high temperature applications. A study of weldments of this alloy and its susceptibility to stress corrosion cracking (SCC) and hydrogen embrittlement was carried out in acidic and neutral media. The anodic behaviour of modified 9Cr-1Mo under 90% proof stress was also examined. Mechanical properties of the material were assessed before and after SCC and hydrogen embrittlement tests. The SCC tests were made at various anodic potentials in 1M H₂SO₄ and showed that the welded alloy is not susceptible to SCC in all three critical zones. The alloy was, however, susceptible to SCC in chloride environments, namely a solution containing equal volumes of 1M NaCl and 1M MgCl₂. Fractographic analysis was carried out to determine the reasons for the failure of this alloy. The hydrogen embrittlement studies in 1M H₂SO₄ showed that the alloy is prone to delayed hydrogen cracking at more negative potentials. The fractographic examination using scanning electron microscopy revealed the presence of adsorbed hydrogen in microvoids which were responsible for the failure of the alloy. Other possible reasons for the failure of the alloy are also discussed.     

应变比对复合细化超细晶纯钛低周疲劳行为的影响

采用等径通道弯曲挤压(Equal Channel Angular Pressing, ECAP)+旋锻(Rotary Swaging, RS)技术制备超细晶纯钛,细化后晶粒尺寸达到纳米级。在室温下对超细晶纯钛实施应变比分别为-1、-0.5、0.5的应变控制低周疲劳试验,通过TEM对微观组织观察。研究了应变比对材料循环硬化软化特性、循环应力应变关系及疲劳寿命的影响。研究结果表明,应变比增大使得超细晶纯钛循环硬化现象更为显著,应变比越大超细晶纯钛低周疲劳寿命越低。低周疲劳高应变比情况下亚晶晶粒尺寸小,数量多,阻碍位错运动,使得材料发生循环硬化。     

New stainless steels for sea-water applications. Part II: Comparison of corrosion and mechanical properties of laboratory and commercial ELI ferritic and superaustenitic stainless steels

This paper represents a follow-up to the first part of the work on new stainless steels for sea-water service. Four laboratory ELI (Extra Low Interstitial) ferritic stainless steels (types 25 Cr-4 Ni-4 Mo), two commercial ELI ferritic stainless steels (types 25 Cr-4 Ni-4 Mo? Ti and 26 Cr-2.5 Ni-3 Mo? Ti) and two highly alloyed austenitic stainless steels (types 20 Cr-25 Ni-4.5 Mo? Cu and 20 Cr-18 Ni-6 Mo? N) have been investigated. With a view to establish the performance of these new alloys in chloride containing environments, systematic electrochemical and laboratory exposure tests have been carried out to define how various factors affect its susceptibility to intergranular, pitting, crevice and stress corrosion. Tension tests were also performed. From the comparison of the localized corrosion resistance and mechanical properties it has been concluded that the laboratory Ti, Ti + Nb or Nb stabilized ELI ferritic stainless steels and the commercial type 25 Cr-4 Ni-4 Mo? Ti of analogous composition could be a valuable alternative to the more expensive highly alloyed stainless steel type 20 Cr-25 Ni-4.5 Mo? Cu which has been especially developed and already used for industrial sea-water applications.     

Application limits of stainless steels in the petroleum industry

Stainless steels have a great variety of potential applications in the petroleum industry, mainly as an alternative to carbon steel in corrosive environments. Within a number of media that can cause corrosion problems with these materials, only chloride solutions and hydrogen sulfide are of importance in oilfield service. A reliable tool that permits the proper selection of stainless steels has yet been missing. In order to provide engineering diagrams for this purpose, pitting and stress corrosion cracking (SCC) tests were performed. Specimens were exposed to NaCl solutions containing from 3 to 100,000 ppm Cl^? at temperatures from 40 to 200 °C. This test configuration was chosen to give a better representation of actual service conditions than accelerated standard test procedures do. Tested materials were the austenitic stainless steel grades 321, 316Ti (API LC30‐1812) and 254 SMO, and 22Cr duplex (austenitic‐ferritic) steel (API LC65‐2205). Based on an optical examination of the specimens, no‐risk regions of chloride concentration vs. temperature have been identified. Subsequently, service temperature limits have been deduced for each tested material. Thus, material failures by pitting and SCC can be prevented without overdesigning. The results of the testing series are applicable to all chloride environments without presence of H₂S, as they have to be handled by primary production equipment, as well as transportation and gas processing facilities.     

Herstellung und Eigenschaften stickstofflegierter,korrosionsbeständiger Stähle und Sonderstähle mit niedrigen Kohlenstoffgehalten

Production and properties of nitrogen alloyed, corrosion resistant steels and special steels with low carbon contents Alloying with nitrogen has favourable influence in particular on the mechanical properties of CrNiMo steels (X 2 CrNiMoN 17 12, materials No. 1.4406, X 2 CrNiMoN 17 13 5, materials No. 1.4439 und X 2 CrNiMoN 22 5, materials No. W.-Nr. 1.4462). This comes to bear when ambient temperature and low temperature strength and toughness are concerned. With respect to the corrosion behaviour the data concerning the effect of nitrogen are contradictory. It has become clear that nitrogen improves pitting corrosion resistance; this applies, however, only to pit initiation but not to pit growth. Stress corrosion cracking is not delayed by nitrogen but different results have been obtained with different media: while the duplex steel X 2 CrNiMoN 22 5 is attacked considerably faster than the corresponding nitrogen-free steel in 42% boiling magnesium chloride solution the time-to-failure of both steels are comparable in 30% boiling MgCl₂-solution. The nitrogen alloyed steels can be welded by all known welding procedures, provided fully austenitic welding rods are used.     

Influence of Processing and Heat Treatment on Corrosion Resistance and Properties of High Alloyed Steel Coatings

Corrosion and abrasive wear are two important aspects to be considered in numerous engineering applications. Looking at steels, high-chromium high-carbon tool steels are proper and cost-efficient materials. They can either be put into service as bulk materials or used as comparatively thin coatings to protect lower alloyed construction or heat treatable steels from wear and corrosion. In this study, two different corrosion resistant tool steels were used for the production of coatings and bulk material. They were processed by thermal spraying and super solidus liquid phase sintering as both processes can generally be applied to produce coatings on low alloyed substrates. Thermally sprayed (high velocity oxygen fuel) coatings were investigated in the as-processed state, which is the most commonly used condition for technical applications, and after a quenching and tempering treatment. In comparison, sintered steels were analyzed in the quenched and tempered condition only. Significant influence of alloy chemistry, processing route, and heat treatment on tribological properties was found. Experimental investigations were supported by computational thermodynamics aiming at an improvement of tribological and corrosive resistance.     

Deformation behaviour of intermetallic NiAl–Ta alloys with strengthening Laves phase for high-temperature applications IV. Effects of processing

The properties of the intermetallic NiAl–Ta–Cr alloy IP75 with strengthening Laves phase were studied as a function of alloy processing procedure: investment casting, hot extrusion of cast material, hot isostatic pressing (HIP), powder injection moulding (PIM) of pre-alloyed powder, and isothermal forging of HIPped material. Powder-metallurgically processed materials show finer microstructures and correspondingly reduced brittle-to-ductile transition temperatures (BDTT), lower yield stresses at all temperatures and lower creep resistances at high temperatures than cast materials. The lowest BDTT was obtained for isothermally forged material, whereas the highest yield stress was observed for remelted cast material. The effects of processing on the mechanical behaviour can be used for adjusting the property spectrum to specific applications. IP75, which is attractive for high-temperature applications because of high strength at temperatures above 1000 °C in combination with tolerable brittleness at room temperature as well as high corrosion and thermo-shock resistance, is the subject of an ongoing development aiming at applications in stationary gas turbines.     

Korrosionsverhalten nitrocarburierter Stähle

Corrosion behaviour of nitrocarburized steels Diffusion coatings have been used increasingly to modify surface properties of various machine components for several applications. In general, these coatings are used to improve the wear behaviour. Frequently, it is desirable that the corrosion behaviour will be improved at the same time. In this investigation, the corrosion behaviour of various diffusion coating-substrate-combinations has been studied. The coatings developed by three different nitrocarburizing processes, namely saltbath, gas and plasma nitrocarburizing, were conducted on five various steel substrates. These substrates were St 52-3, Ck 45, 42 CrMo 4, 30 CrNiMo 8 and × 20 Cr 13. The thickness of the compound layers and their porosity were measured using optical microscopy. The structure of the compound layers was characterized using an X-ray diffractometer and their surface roughness by a stylus profilometer. The corrosion test was carried out using a salt-water spray test. The predominant corrosion mechanisms have been evaluated using scanning electron microscopy. It was found that the employed nitrocarburizing processes have improved the corrosion resistance of all tested coating-substrate-combinations comparing with that of the base materials. In general, the corrosion resistance increases with an increase of the compound layer's thickness and after an oxidation process. The best coating-substrate-combinations to improve the corrosion behaviour were salt-bath nitrocarburized tempering steels, whereas the thin layers of the plasma nitrocarburized specimens were the worst.