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Paper des Monats
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Samples of differently heat treated high alloyed stainless injection-pipe steels AISI 420 X46Cr13, AISI 420J X20Cr13 as well as X5CrNiCuNb16-4 AISI 630 were kept at T=60 °C and ambient pressure as well as p=100 bar for 700 h - 8000 h in a CO2- saturated synthetic aquifer environment similar to possible geological on-shore CCS-sites in the northern German Basin. Corrosion rates and scale growth are lowest after long term exposure for steels hardened and tempered at 600 to 670 °C and pits - indicating local corrosion- decrease in diameter but increase in number as a function of carbon content of the steel. Martensitic microstructure is preferred with respect to this particular CCS-site.
Damage analyses on two heat exchanger units showed that in both cases inappropriate flow conditions of media caused very different failure mechanisms that resulted in irreparable damage. The first incident was the breakdown of an unalloyed steel condenser, which operated in a coal-fired power plant. A considerably high number of tubes successively leaked. Metallography identified lines of segregation in the microstructure of the tube walls, thus, giving evidence that both uniform corrosion and erosion corrosion caused by low-pressure wet steam were the root cause. The second incident was the breakdown of a recuperator made from chromium–nickel steel due to mechanical damage to tubes and baffle. This unit operated as part of a pilot plant to regain heat from the drying process of sewage sludge. It turned out that soiled vapour caused clogging of the cross-sectional area and therefore accelerating the flow velocity of the vapour. This inappropriate operating condition caused the tubes to oscillate so severely that they even banged together. Abrasive wear especially at the intersection through the holes of the baffle damaged the tubes and the whole unit irreparably.
The Stolt Rotterdam tanker ship, filled with nitric acid, was unloading at the Krefeld-Uerdingen terminal on the Rhine River in November 2001. Nitric acid ran out of the stainless steel tanks into the ship's hull and damaged the structural steel. The ship then caught fire and sank at the terminal. Large quantities of nitrogen oxides were emitted, which drifted towards a residential area. Additionally, a mixture of acid and water was introduced into the Rhine River. The area became hazardous to people and the natural environment; however, the imminent danger for the residents and the environment was reduced because of the protective measures against pollution and the good teamwork between local authorities, fire brigades and the Bayer AG company. The acid was drained off of the ship using a controlled discharge of the acid into the river. Through the use of this measure, the ship was salvaged, limiting the pollution to the area and removing the health hazards to the people and the environment. After the ship was salvaged, the river police, along with assistance from the BAM Federal Institute for Materials Research and Testing, impounded the ship and selected corroded parts for further examination. Some of these components have been examined in detail by the BAM. These material investigations contributed to the elucidation of the origin of the leak in the floor panel. The cause of damage was deemed to be a result of a construction flaw at the enamelled valve on the load and discharge pipes.
The results from the comparative corrosion testing using the original structural steel and 60% acid revealed that the valves began to leak at least 8 h prior to the incident at the terminal.
Investigation on defect morphology and corrosion behaviour of TiMgN hard coatings on steel substrate
(2014)
Nitride hard coatings, such as Titanium nitride and chromium nitride coatings are widely used for applications focussing on wear protection and decorative purposes. Also these coatings are often applied on low alloyed steel substrates. The inevitable contact of the coated components with the environment through pores and coating defects bears the danger of corrosion of the steel substrate followed by further delamination of the coating and a loss of function of the component. A new attempt to increase the corrosion behaviour of those physically deposited hard coating systems is the implementation of magnesium. The positive effect of magnesium could be verified already electrochemically. Performed experiments on TiMgN have shown that corrosion resistance could drastically improved with increasing Mg content of the TiMgN up to 30 at% compared to steel substrates with applied titanium nitride hard coatings. Besides the clarification of the electrochemical and phenomenological effect of the magnesium on the corrosion behaviour, an additional aim of this work is the investigation of the coating defects, defect morphologies and their influence on the local corrosion behaviour. For this the incorporation of the magnesium and its effects on the coating material, the influence of coating growth related defects and the effect of the coating structure on the corrosion behaviour should be experimental examined by using innovative surface and material analysis methods such as FIB and TEM. Furthermore the evolution of the coating defects should be investigated by combing confocal microscopy with a new developed exposure test method using the electrochemical indication test KorroPad. The function of the KorroPad test, which was developed and patented [1] at the BAM for the detection of corrosion sensitive steel surfaces by indicating dissolution of iron ions [1], allows the detection and identification of critical coating defects for further microscopic investigation. Additional to that the KorroPad test simulates an accelerated exposure test by simultaneous absence of the disadvantages of typical short time exposure test like salt spray tests. Thus it allows a monitoring and analysis of the evolution of the critical coating defects and their influence on the local corrosion behaviour and the overall corrosion mechanisms. The results of this work should contribute to the development of new hard coatings with improved corrosion protection properties and also to a better understanding of the corrosion mechanisms of coated steel substrates.
Steel in concrete is protected by the alkaline pore water environment and the resulting formation of a protective passive layer against corrosion. Adverse environmental conditions due to carbonation or chloride ingress can destroy the passive layer on the steel surface. Corrosion processes starting in those areas lead to uniform corrosion or local corrosion like pitting corrosion. In comparison to uniform corrosion pitting corrosion is a form of increased local corrosion and thus leads to a progressive reduction in cross-section of the reinforcing steel. The corrosion products are first absorbed by the pores of the concrete matrix, without causing visible external changes at the concrete surface.
The development of anti-corrosive pigments for the most diverse coating Systems is extremely time consuming and expensive, owing to the necessary climatic testing, such as testing under salt spray exposure. To accelerate work on a new zinc-free pigment with improved protection, modern, electrochemical investigation methods have been used successfully and verified using traditional tests.
Development of anti-corrosive pigments for diverse coating systems is both time-consuming and costly, due to the necessary climatic testing, such as testing under salt spray exposure. To accelerate work on a new zinc-free pígment with improved protection, modern, electrochemical investigation methods have been used successfully and verified using traditional tests.
Corrosion resistance of duplex stainless steel (SS) (UNS S32101) and super austenitic SS (UNS NO8031) (Alloy 31) was investigated in crevice conditions in a saline brine. Rubber was used to form a metal/non-metal-crevice confguration. UNS NO8031 did not exhibit any signs of pitting corrosion even by applying potentials much more positive than the corrosion potential.
In order to extend the lifetime of buildings and constructions at the macro scale it is necessary to understand the damage processes of building materials at the micro scale. In particular, durability of reinforced concrete structures is one of the most important equirements for construction planning and restoration of buildings. Therefore degradation mechanisms were reproduced on laboratory specimens.
CT (Computed Tomography) is commonly used for non-destructive microstructural defect analysis for recurring tests on concrete specimens. In this work a few examples of CT applications on cementitious materials (including cement paste, mortar and concrete specimens) will be presented.
Firstly, in order to quantify the degradation processes, specimens analysed were damaged by corrosion due to carbonation and due to chloride ingress. Particular focus has been set to the analysis of cracks.
An automated crack detection tool, developed by Zuse Institut Berlin (ZIB) and BAM in ZIBAmira, has been applied for quantitative analysis of crack parameters and 3D visualization of cracks.
Furthermore the distribution of corrosion products has been evaluated inside the cement matrix and visualized in 3D data sets.
Another important factor for the ageing stability of concrete is the interfacial transition zone (ITZ). The ITZ consists of a layer of cement paste (20 to 40 μm) over every aggregate where porosity is generally increased in comparison with the bulk. This zone could be a preferential zone for transfer of aggressive species. To visualize the ITZ, a small sample of mortar with a diameter of 10mm has been prepared and scanned using the industrial μCT setup at BAM with a spatial resolution of 5μm voxel size. In addition the extracted surface of aggregates could be used for load simulations. We finally show how CT examination of drilled samples taken from building materials in conjunction with laboratory experiments is helpful for further evaluations of damage processes in concrete.
Sixteen laboratories have performed electrochemical noise (EN) measurements based on two systems. The first uses a series of dummy cells consisting of a 'star' arrangement of resistors in order to validate the EN measurement equipment and determine its baseline noise performance, while the second system, based on a previous round-robin in the literature, examines the corrosion of aluminium in three environments. All participants used the same measurement protocol and the data reporting and analysis were performed with automatic procedures to avoid errors. The measurement instruments used in the various laboratories include commercial general-purpose potentiostats and custom-built EN systems. The measurements on dummy cells have demonstrated that few systems are capable of achieving instrument noise levels comparable to the thermal noise of the resistors, because of its low level. However, it is of greater concern that some of the instruments exhibited significant artefacts in the measured data, mostly because of the absence of anti-aliasing filters in the equipment or because the way it is used. The measurements on the aluminium samples involve a much higher source noise level during pitting corrosion, and most (though not all) instruments were able to make reliable measurements. However, during passivation, the low level of noise could be measured by very few systems. The round-robin testing has clearly shown that improvements are necessary in the choice of EN measurement equipment and settings and in the way to validate EN data measured. The results emphasise the need to validate measurement systems by using dummy cells and the need to check systematically that the noise of the electrochemical cell to be measured is significantly higher than the instrument noise measured with dummy cells of similar impedance.
Condensates from the gas stream in simulated CO2 transport pipelines have been identified during the experiments in the laboratory. Because of their acidic origin the corrosion resistance of pipeline steels used for CCS (carbon capture and storage) technology might be limited. Over the last years it has become clear that the amount of water and acid building constituents in the CO2 stream has to be controlled very well. In this work, condensates formed in experiments using gaseous CO2 containing high amounts of water, NO2 and SO2 were analyzed, replicated, and used for extensive electrochemical experiments. These highly acidic condensates were enriched with CO2 and then applied to characteristic steels planned to use in the CCS transport chain. Even high alloy steels are susceptible to localized corrosion under these conditions. The results implicate that condensation of aggressive acid droplets has to be avoided or the locations where condensation takes place have to be controlled extensively.
Case studies
(2013)
In this paper, some quite different survey tasks are described, and it can be seen that corrosion surveys will follow a certain scheme of data acquisition (first NDT measurements, than detailed testing on suspect areas), but the individual scope of on-site measurements may have a vast variety according to the local circumstances, and the interpretation always has to refer to a wider set of information than the data readings only.
The safety assessment of railway axles is based on a two-stage approach: fatigue strength design and regular inspections which, in terms of a general safety philosophy refer to safe-life and damage tolerance concepts. Starting with a recent failure case, a broken axle of a German high speed train, a discussion is presented on issues of both safety levels. These include ideas for finite life design, the treatment of in-service effects on the fatigue strength due to flying ballast damage and corrosion pits, the effect of corrosion on fatigue crack initiation and propagation, potential effects of non-metallic inclusions in steels, the way to detect them by quality control measures and reliability aspects of non-destructive testing with respect to the detection of fatigue cracks. Proposals are made how the safety level could be further improved.
In corrosive environments such as CCS bore holes or geothermal power plants the materials loaded cyclically are also exposed constantly to the highly corrosive hot thermal water. The lifetime reduction of (X46Cr13, AISI 420C) is demonstrated in in-situ-laboratory experiments (T=60 °C, geothermal brine: Stuttgart Aquifer flow rate: 9 Nl/h, CO2). S-N plots, micrographic-, phase-, fractographic- and surface analysis were applied to obtain sustainable information on the corrosion fatigue behavior. Maximum number of cycles (here 12.5 x 106 cycles to failure) is reached at σa =173 MPa. No typical fatigue strength exists and passive corrosion fatigue is most likely failure cause.
Bei der Qualitätsbewertung von Schneidwaren aus nichtrostenden martensitischen Chrom-Stählen stellt die Überprüfung der Korrosionsbeständigkeit ein wichtiges Kriterium dar. Allgemein üblich sind Wechseltauchversuche mit relativ hohem zeitlichem und auswertungsbedingtem Aufwand, die noch dazu zerstörend sind und teilweise fragwürdige Ergebnisse liefern. In den letzten Jahren wurden neue Ansatze der Prüfung geschaffen, die eine nahezu zerstörungsfreie Kurzzeit-Korrosionsprüfung erlauben. Es wird gezeigt, dass mit dem elektrochemischen Rauschen sehr empfindlich Einflüsse des Härtens und der Oberflächenbehandlung auf die Korrosionsbeständigkeit nachgewiesen werden können, wodurch eine schnelle und objektive Qualitätsbewertung innerhalb des Fertigungsprozesses und der Endkontrolle von Produkten möglich ist. Das für die Prüfmethodik ausgenutzte Phänomen des elektrochemischen Rauschens zielt darauf ab, den Beginn der Korrosion empfindlich und schnell zu detektieren, ohne die Prüfbedingungen maßlos zu verschärfen und Produkte damit zu zerstören. Neben der Funktionalität der Prüfung stehen aber auch die Anwenderfreundlichkeit und somit die Akzeptanz bei den Herstellern im Vordergrund. Aus diesem Grund wurde zusätzlich ein Indikatortest (KorroPad) als praxistaugliche Alternative zum elektrochemischen Rauschen angewendet. Die Ergebnisse zeigen eine gute Übereinstimmung beider Methoden und ermöglichen eine effektive betriebliche Qualitätsüberwachung von Schneidwaren.
While the possibilities of improving corrosion resistance (especially galvanic corrosion) by alloying are limited, surface finishing of magnesium alloys is the alternative for improving corrosion resistance. Due to the low corrosion potential of Mg and the danger of galvanic corrosion in the case of a coating defect, the choice of coatings is limited. This chapter will summarize the corrosion behavior of magnesium alloys and the metallurgical possibilities to improve the corrosion resistance of the alloys and reviewing critically the most commonly used surface treatments and coatings for magnesium.
The main problem limiting the application of magnesium alloys as biodegradable implant material is its high degradation rate. In order to slow down the corrosion rate an extrusion process and specific coating systems based on plasma-chemical oxidation (PCO) and organic dip coating with poly(ʟ-lactid-co-caprolacton) (PLLC) were applied on Mg–1Ca magnesium alloy. The additional PLLC coating is used to delay the start of substrate corrosion, while the purpose of the PCO coating is to decrease the substrate corrosion rate. The corrosion behaviour was investigated in synthetic body fluid (SBF) through measurement of the hydrogen evolution rate in long term tests and polarisation and electrochemical noise measurements in short term tests. The results showed significant differences between the cast and extruded alloys and a decrease of the corrosion rate due to corrosion product formation. The combination of both coating systems resulted in a significant delay of metal substrate corrosion and all coating systems showed good correlation between short and long term tests. The combination of the three investigation methods provides the possibility to gain more information about the degradation behaviour and break down of protective coatings.
An important possibility to improve the corrosion behaviour of magnesium alloys is the application of protective coatings. The quality of such coatings depends mainly on the pretreatment and the exposure conditions after pretreatment. Since magnesium surfaces change much faster under atmospheric conditions than those of almost any other technical material, it is necessary to pay special attention to this particular feature. The activity of acid-pickled surfaces of the magnesium alloys AZ31 and AZ91 in dependence on the exposure time and the humidity conditions was investigated with electrochemical noise (EN) measurements. In addition to pickling, plasma chemical vapour deposition processes open new possibilities for an economical, as well as ecologically quite safe, pretreatment. The results of EN investigations after acid pickling as well as specific plasma oxidation treatments of the two magnesium alloys after exposure to air with different humidities are presented.
The process chain for Carbon Capture and Sequestration (CCS) includes tubing for injection of CO2 into saline aquifers. The compressed CO2 is likely to contain specific impurities; small concentrations of SO2 and NO2 in combination with oxygen and humidity are most harmful. In addition, CO2 saturated brine is supposed to rise in the well when the injection process is interrupted. The material selection has to ensure that neither CO2 nor brine or a combination of both will leak out of the inner tubing. In this comprehensive paper the investigated materials range from low-alloy steels and 13% Cr steels up to high-alloy materials. Electrochemical tests as well as long term exposure tests were performed in CO2, in brine and combination of both; pressure was up to 100 bar, temperature up to 60 °C. Whereas the CO2 stream itself can be handled using low alloy steels, combinations of CO2 and brine require more resistant materials to control the strong tendency to pitting corrosion. The corrosion behavior of heat-treated steels depends on factors such as microstructure and carbon content. For different sections of the injection tube, appropriate materials should be used to guarantee safety and consider cost effectiveness.
Within the project COORAL (German acronym for ''C02 purity for capture and storage") studies on pipeline steels exposed to circulating supercritical impure C02 have been carried out. For this purpose, a loop consisting of compressors, flow meter and two autoclaves was constructed. In order to simulate the real conditions in pipelines, impurities such as H20, CO, S02, N02 and 02 were added to the C02 stream before compression. Exposure experiments were carried out with Steel specimens placed in the autoclaves. Carbon Steel L360NB, pure iron X20Cr13, X46Cr13, X2CrMnNiN22-5-2 and alloy 31 have been exposed to circulating (flow rate 4 L/min) supercritical impure C02 for one week at 60 °C and 10 MPa Surface analysis and weight loss experiments in order to determine the corrosion products and the corrosion rates showed that the impurities cause corrosion problems. Slight general corrosion by L360NB and soft iron was observed. The initiation of pitting corrosion was observed at the surfaces of the materials X20Cr13, X46Cr13, X2CrMnNiN22-5-2. No visible signs of corrosion have been observed on alloy 31.
The degradation behaviour of an Mg-1Ca alloy is investigated in vitro to figure out the possibilities of influencing the degradation behaviour of such an alloy by applying heat treatment as well as the use of a coating system based on plasma-chemical oxidation. It is shown that an optimised solution annealing (T4-heat treatment) can reduce the degradation rate while an additional ageing between 240 and 300 °C (T6-heat treatment) increases it. A coating generated by plasma-chemical oxidation reduces the degradation rate in the immersion test. Its effect is depending on the former heat treatment of the Mg-1Ca alloy as well as on the parameter during plasma-chemical oxidation.
Carbon capture and storage (CCS) includes processing of supercritical carbon dioxide (scCO2). The carbon dioxide (CO2) stemming from flue gases contains acid forming impurities, especially in the case of coal fired power plants. In the present work, the mobility and reactivity of acids in supercritical scCO2 was investigated. The corrosive attack of low alloyed carbon steel (material 1.0484) by water (H2O) alone in scCO2 was negligible. Nitric acid (HNO3) was very mobile and corrosive towards the carbon steel while sulfuric acid did not migrate through the scCO2 to react with the steel surface. Hydrochloric acid in scCO2 was very mobile and reactive towards both carbon steel and high alloyed test equipment. Gravimetric analyses revealed a severe material loss after corrosion in the presence of HNO3. Thickness measurements showed localized material losses.
The appropriate strength of steels used for saline aquifer carbon capture and storage sites (CCS) is usually achieved by applying heat treatments. Thus, heat treatment influences the corrosion resistance for injection pipe steels with 13% chromium and different carbon content: 1.4034/X46Cr13 and 1.4021/X20Cr13 in CO2 saturated saline aquifer water at 60 °C, 1 and 100 bar. X46Cr13 shows better corrosion resistance with respect to corrosion rate, number of pits and maximum intrusion depth. Low corrosion rates are obtained for steels with martensitic microstructures exposed to supercritical CO2 at 100 bar, whereas normalized steels show better corrosion resistance at ambient pressure.
IIW-2326
Austenitic and austenitic-ferritic CrMnNi-stainless steels are suitable materials in the transport and automotive industry due to their high corrosion resistance and high strength that allows weight and cost savings. This study focuses on the laser weldability of a commercial lean duplex and an austenitic high manganese stainless steel. The impact of different laser sources, i.e. a 5 kW CO2- and a 4 kW Nd:YAG-laser, and of the main process parameters on the resulting weld quality will be investigated. One important aspect will concern the appearance of weld defects such as pores and hot cracks. The factors causing such internal imperfections will be analysed in order to find effective methods for preventing them. Weld microstructure and the associated corrosion and mechanical properties will be assessed with different techniques and adequate process parameters for high quality welds will be determined. The advantages and limitations of the applied welding processes will be evaluated for future applications.
A joint research project was accomplished by ibac and BAM with the aim to develop a numerical model
of cathodic protection (CP) of reinforced concrete with a special focus on CP of the rear reinforcement
layer. The model was intended to include the effects of chemical alterations within the concrete and the
steel concrete interface, which are induced by long-term application of CP. The investigations presented
in this paper focus on the migration of chloride ions due to small electric fields as applied during cathodic
protection of steel in reinforced concrete structures and its impact on polarisation characteristics. A
comparatively new method, laser induced breakdown spectroscopy (LIBS), was used to determine
chloride profiles on laboratory specimens in order to investigate one of several model parameters to
describe cathodic protection of the rear reinforcement of reinforced concrete structures. These
investigations are described elsewhere, [1].
The paper focuses on long term polarisation tests and their impact on the charge depending cathodic
polarisation behaviour of laboratory specimens.
Monitoring accelerated corrosion in chloride contaminated concrete with ground penetrating radar
(2012)
During the compression of emission gasses into deep geological layers (Carbon Dioxide Capture and Storage, CCS) CO2-corrosion will become a relevant safety issue. The reliability of the steels used at a geological onshore CCS-site in the Northern German Bassin 42CrMo4 (1.7225, AISI 4140) used for casing, and the injection pipe steels X46Cr13 (1.4034, AISI 420 C), X20Cr13 (1.4021, AISI 420 J) as well as X35CrMo17 (1.4122) is demonstrated in laboratory experiments. Samples were kept in a synthetic aquifer environment at T = 60 °C. This corrosive environment is then saturated with technical CO2 at a flow rate of 3 l/h. Microstructures were characterized by X-ray diffraction, light microscopy, scanning electron microscopy, and energy dispersive X-ray analysis, after a series of heat treatments (700 h to 2 years). The non-linear isothermal surface corrosion behaviour of the steels reveals surface corrosion rates around 0.10.8 mm/year, when obtained by mass gain. Severe pit corrosion (pit heights ca. 4.5 mm) are only located on the injection pipe steels. Main phases of the continuous scales are siderite FeCO3 and goethite α-FeOOH. The formation of the non-protective layer is likely to form via a
transient Fe(OH)2-phase.
Manganese alloyed stainless steels represent a cost-effective alternative to conventional CrNi- stainless steels due to strong fluctuations of the market prices for nickel seen during the last years. In CrMnNi steels, nickel is partially replaced by lower-cost manganese and small amounts of nitrogen for stabilization of the austenitic phase. This also brings benefits regarding the mechanical properties, as it results in an increased material strength. Laser beam welding of such materials was investigated for direct comparison with standard CrNi steels. Main emphasis was laid on finding adequate process parameters to achieve a stable welding process and obtain a good weld quality. Two different laser sources, a 4.4 kW Nd:YAG and a 5 kW CO2 laser, were used to weld 1.5 mm stainless steel sheets in continuous wave mode. A high-Mn austenitic (1.4376) and a lean duplex (1.4162) steel, as well as the standard austenitic (1.4301) and duplex (1.4362) grades were selected as test materials. Both butt and lap joint configurations were studied. Experiments were carried out systematically, varying the welding speed, laser power and focal point position in order to determine adequate process windows. The influence of the shielding gas type and flow rate on the process stability and the weld quality were investigated. The effects of weld edge preparation on the weld appearance and quality levels attained were also examined. The obtained welded joints were subjected to radiographic tests for detection of internal imperfections. Also a metallurgical characterization of the samples regarding the resulting phase composition or balance and hardness depending on the welding process parameters was conducted. Furthermore, tensile and potentiodynamic tests were performed to evaluate the mechanical and corrosion properties, respectively. The results provide an insight into the advantages and limitations of the laser beam welding process for joining high-manganese alloyed stainless steels. Conditions for the production of defect-free and corrosion-resistant welds having good mechanical properties could be determined.
Basic research on the corrosive effect of flue gases has been performed at the BAM Federal Institute for Materials Research and Testing (Germany). Conditions at both high and low temperatures were simulated in specially designed experiments. Carburization occured in flue gases with high CO2 content and temperatures higher than 500 °C. In SO2 containing flue gases sulphur was detected in the oxide scale. At lower temperatures no corrosion was observed when gases with low humidity were investigated. Humidity higher than 1500 ppm was corrosive and all steels with Cr contents lower than 12% revealed corroded surfaces. At low temperatures below 10 °C a mixture of sulphuric and nitric acid condensed on metal surfaces. Acid condensation caused severe corrosion. Humidity, CO2, O2, and SO2 contents are the important factors determining corrosion. Below 300 °C acid condensation is the primary reason for corrosion. Low humidity and low temperatures are conditions which can be expected in the CO2 separation and treatment process. This work includes major conditions of the flue gas and CO2 stream in CCS plants and CCS technology.
In lightweight car body shell mass production, due to requirements on vehicle weight reduction and carbon dioxide emissions, joining of advanced high strength steels (AHSS) with different joining procedures and especially hybrid bonding techniques is becoming more and more important. One of these hybrid bonding techniques is the combination of resistance spot welding and adhesive bonding called weldbonding. One of the important advantages of weldbonded joints in comparison to resistance spot welded joints are the enhanced mechanical properties. To guarantee sufficiently high quality conditions regarding the strength of the weldbonded joints, the influences of the applied adhesive systems and of different base metal combinations are studied. This is carried out for both non-corrosive and corrosive environments and for the choice of different joining parameters settings. In particular, the mechanical behaviour of the weldbonded joints is investigated under quasi-static, impact and fatigue loads. Furthermore, the energy absorption of the weldbonded joints for both non-corrosive and corrosive environments is studied. It is shown that the weldbonded joints possess higher mechanical strengths in all load cases (quasi-static, impact and fatigue). Corrosive attack affects weldbonded joints, and the quasi-static strength is reduced. Resistance spot welded joints are not affected by the corrosive attack, but even after several weeks of corrosive attack, the quasi-static strength of weldbonded joints remains higher than that of resistance spot welded joints.
Surface treatments usually are used to modify the appearance and properties of stainless steel surfaces. Simultaneously, the corrosion resistance of the stainless steel surface being related intrinsically to the spontaneous formation of a passive layer also will be affected. In this respect, the influence of different surface treatments on the corrosion resistance of Type 304 (UNS S30400) stainless steel to pitting corrosion has been evaluated by means of potentiostatic electrochemical noise measurements and surface characterization. Typical industrial treatments including mechanical grinding, glass blasting, and pickling were taken into account. Additionally, special consideration was put on the effect of aging conditions of the passive layer after surface treatments, in particular on the relative humidity. Benefits and limitations of the different applied surface treatments concerning protectiveness of the stainless steel surface were determined and discussed.
When emission gasses are compressed into deep geological layers (CCS) CO2-corrosion of injection pipe steels is a relevant safety issue. The reliability of the steels used at the geological onshore CCS-site at Ketzin, Germany, is demonstrated in 2 years laboratory experiments under an equivalent corrosive environment at ambient pressure (T=60 °C, aquifer water, CO2-flow rate of 3 l/h). Corrosion kinetics and microstructures were characterized using samples of the heat treated steel 42CrMo4 (casing), and samples of the martensitic stainless steel X46Cr13 (injection).
Single-pass solid-state laser welding of plates in the thickness range of 10 to 20 mm became possible with the invention of the fibre laser. This new technique provides excellent beam quality at powers as high as 20 kW or more, and has proved applicable in several industrial applications. By replacing conventional methods with the fibre laser, it is possible to avoid multiple-pass welding that requires time-consuming bevelling. The high energy density of the fibre laser beam also reduces the heat input and consequently the distortion. However, the rapid solidification and cooling associated with laser welding can cause imbalance of the microstructure of duplex stainless steel weldments, where excessively high ferrite contents may reduce the corrosion resistance and the ductility of the material. The solution is normally to add nickel-based filler wire and to increase the heat input. By using a hybrid welding process where the laser beam and the gas metal arc (GMA) process act in a common process zone, filler metal can be added to the molten pool at higher heat input and at the same time, higher welding speed and deeper penetration can be achieved. In this work, 13.5 mm thick 2205 (EN 1.4462, UNS S31803) was fibre laser-GMA hybrid welded in a single-pass using 14 kW of laser power and ISO 22 9 3 N L as filler wire for the GMA process. The resulting welds were free from defects, with smooth surfaces and full penetration. The investigation examines the weld metal microstructure and the effect on corrosion resistance and mechanical properties. The option to add nickel foil, when hybrid welding, was also investigated, as comparison, and the effect on austenite formation was evaluated.
During the compression emission gasses in deep geological layers (Carbon Capture and Storage, CCS) CO2-corrosion will become a relevant safety issue. The reliability of the steels used at the geological onshore CCS-site at Ketzin, Germany, (heat treated steel 42CrMo4 (1.7225, AISI 4140) used for casing, and the martensitic stainless injection pipe steels X46Cr13 (1.4034, AISI 420 C), X20Cr13 (1.4021, AISI 420 J), X35CrMo17 (1.4122)) is demonstrated in 1 and 2 years laboratory experiments. Samples were kept in a synthetic aquifer environment similar to the geological CCS-site at Ketzin, Germany at T=60 °C. This corrosive environment is then saturated with technical CO2 at a flow rate of 3 l/h. Microstructures were characterized by X-ray diffraction, light microscopy, scanning electron microscopy, and energy dispersive X-ray, after a series of heat treatments (700 h to 2 years). Due to very slow mass loss at extended exposure times to CCS-environment one year is sufficient to predict stable surface corrosion rates from laboratory experiments. The non-linear isothermal surface corrosion behaviour of the steels reveals surface corrosion rates around 0.1 to 0.8 mm/year, when obtained by mass gain. The loss of the base material is higher when calculated from the corrosion layer magnitude due to the unpredictable local corrosion attacks. Severe pit corrosion (pit heights ca. 4.5 mm) are only located on the high chromium steels. Main phases of the continuous scales are siderite FeCO3 and goethite α-FeOOH. The formation of the non-protective layer is likely to form via a transient Fe(OH)2-phase.
Five stainless steel alloys were tested and compared to find a test method for the qualification of stainless steel used in drinking water systems. Determination of the pitting/critical potentials from the conventional cyclic polarization results was shown to be difficult when stainless steels have high Mo and Cr content. The electrochemical impedance data indicated that results received with the conventional methods do not necessarily reflect the steady state. The low frequency time constant determined from the impedance measurements showed that the anodic polarization should be conducted with very slow scan rate to get information about the mechanism. The steady state investigations were compared with the conventional electrochemical results to propose an electrochemical-based route as a standard qualification method. The qualification method was tested to be valid by a European round-robin-test.
Alumosols in combination with well dispersed corundum were successfully used to form up to 2.5 ìm thick coatings on the Ni-base alloy SC16. These coatings withstood heat treatments at 700 °C in a water vapour containing atmosphere. The heat treatment caused formation of delta-alumina in the coating and diffusion of chromium and titanium into the coating.
CO2-corrosion of injection pipe steels is a relevant safety issue when emission gasses are compressed in deep geological layers (CCS). The reliability of the steels used at the geological onshore CCS-site at Ketzin, Germany, is demonstrated in laboratory experiments under an equivalent corrosive environment (T=60 °C,p=1-60 bar, aquifer water, CO2-flow rate of 3 l/h, 700 h8000 h heat treatment). Corrosion kinetics and microstructures were characterized using samples of the heat treated steel 1.7225 (AISI 4140, 42CrMo4) used for casing, and samples of the martensitic stainless injection-pipe steel 1.4034 (AISI 420, X46Cr13).
In engineering storage sites for Carbon Sequestration the CO2-induced corrosion of injection pipe steels is a relevant safety issue when emission gasses are compressed in deep geological layers. The reliability of the steels suitable for the geological onshore CCS-site (Carbon Capture and Storage) at Ketzin, Germany, is demonstrated in laboratory experiments in equivalent corrosive environment (T = 60 °C, p = 160 bar, aquifer water, CO2-flow rate of 3 L/h, 7008000 h exposure time). Corrosion kinetics and microstructures were characterized using specimens of the heat treated steel 42CrMo4 (1.7225) used for casing, and specimens of the martensitic stainless injection-pipe steel X46Cr13 (1.4034).
Although the complex changes at the steel-concrete interface due to cathodic polarisation are widely acknowledged to have a beneficial influence concerning the cathodic protection (CP) of steel in concrete, some questions concerning the repassivation of carbon steel in consequence of cathodic polarisation are still not satisfactorily clarified. In the recent literature, some indications are presented that repassivation occurs after a certain time of polarisation. Therefore, the investigations discussed in this paper aim to clarify, to what extent the re-passivation of carbon steel due to cathodic polarisation occurs, and if the ennoblement of OCP is a sufficient indication for repassivation. In a first step, the corrosion state of five nominal equal test specimens was determined by electrochemical impedance spectroscopy (EIS). After determining the initial corrosion state by evaluating the charge transfer resistance and the polarisation resistance, respectively, the specimens were polarised cathodically. Impedance data were recorded before, during and after polarisation. The impedance data were evaluated by equivalent circuit fitting with special attention to charge transfer resistances and the impact of diffusion on the corrosion and polarisation behaviour. The results indicate that the reduction of oxides and oxygen diffusion during cathodic polarisation has strong impact on the systems behaviour and that repassivation effects occur after switching off the polarisation current and during depolarisation, respectively.