Accelerating corrosion in a laboratory set-up for corrosion-fatigue of offshore steels

Corrosion-fatigue is a dangerous failure mechanism that is not yet fully understood. Structures subjected to corrosion-fatigue are over conservative in design, which is economically unfavourable. To counter this, representative laboratory experiments simulating the corrosion-fatigue conditions of an offshore structure should be performed. Lab testing is, for obvious reasons, performed at frequencies much higher than these of wave and wind actions. However, this means that corrosion needs to be accelerated in the same manner. In this work two different ways to accelerate corrosion were selected, namely temperature and oxygen content adaptation. S-N curves were determined in different test conditions in order to evaluate the damage evolution. It has been found that high temperatures and high levels of oxygen content will result in earlier failure. The fracture surfaces are somewhat different than fracture surfaces obtained due to fatigue in air. More crack initiation sites can be observed and the fracture surface is generally rougher due to corrosion

Testing methodologies for corrosion fatigue

Offshore constructions are subjected to cyclic loading conditions. This situation is combined with the corrosive nature of the surrounding environment. It is of actual concern whether the combined effect is more damaging or not than the superposition of each effect independently. This literature review first introduces the reader to corrosion fatigue. Thereafter a critical comparison of some typical lab-scale fatigue corrosion test setups is given. Special emphasis is devoted to the instrumentation of the setup. This is followed by a design criteria summary which will be used to design a new corrosion fatigue test set-up for evaluating the fatigue properties of steel components in sea water environment

Fatigue crack initiation and growth on a steel in the very high cycle regime with sea water corrosion

The authors acknowledge Arts et Métiers ParisTech and Foundation Arts et Métiers for the financial support of P.C. Paris’ stay at LAMEFIP. They acknowledge Vicinay Cadenas S.A. for its financial support, and both the PCP France-Mexique and the CONACYT for their financial support too.This paper is devoted to the effect of corrosion on the gigacycle fatigue strength of a martensitic–bainitic hot rolled steel used for manufacturing offshore mooring chains for petroleum platforms. Smooth specimens were tested under fully reversed tension between 1E6 and 1E10 cycles in three testing conditions and environments: (i) in air, (ii) in air after precorrosion and (iii) in air under real time artificial sea water flow. The fatigue strength at greater than 108 cycles is reduced by a factor more than five compared with non-corroded specimens. Fatigue cracks initiate at corrosion pits due to pre-corrosion, if any, or pits resulting from corrosion in real time during the cyclic loading. It is shown that under sea water flow, the fatigue life in the gigacycle regime is mainly governed by the corrosion process. Furthermore, the calculation of the mode I stress intensity factor at hemispherical surface defects (pits) combined with the Paris–Hertzberg–Mc Clintock crack growth rate model shows that fatigue crack initiation regime represents most of the fatigue life.PCP France Mexique, Conacyt, Arts et Métiers ParisTech (professeur invité Paul C Paris

Corrosion fatigue of high strength fastener materials in seawater

Environmental effects which significantly reduce the fatigue life of metals are discussed. Corrosion fatigue is a major concern in the engineering application of high strength fasteners in marine environments. The corrosion fatigue failure of an AISI 41L4O high strength steel blade to hub attachment bolt at the MOD-OA 200 kW wind turbine generator was investigated. The reduction of fatigue strength of AISI 41L4O in marine environments and to obtain similar corrosion fatigue data for candidate replacement materials was studied. The AISI 4140, PH 13-8Mo stainless steel, alloy 718 and alloy MP-35N were tested in axial fatigue at a frequency of 20 Hz in dry air and natural seawater. The fatigue data are fitted by regression equations to allow determination of fatigue strength for a given number of cycles to failure

The growth of small corrosion fatigue cracks in alloy 2024

The corrosion fatigue crack growth characteristics of small surface and corner cracks in aluminum alloy 2024 is established. The damaging effect of salt water on the early stages of small crack growth is characterized by crack initiation at constituent particle pits, intergranular microcracking for a less than 100 micrometers, and transgranular small crack growth for a micrometer. In aqueous 1 percent NaCl and at a constant anodic potential of -700 mV(sub SCE), small cracks exhibit a factor of three increase in fatigue crack growth rates compared to laboratory air. Small cracks exhibit accelerated corrosion fatigue crack growth rates at low levels of delta-K (less than 1 MPa square root of m) below long crack delta-K (sub th). When exposed to Paris regime levels of crack tip stress intensity, small corrosion fatigue cracks exhibit growth rates similar to that observed for long cracks. Results suggest that crack closure effects influence the corrosion fatigue crack growth rates of small cracks (a less than or equal to 100 micrometers). This is evidenced by similar small and long crack growth behavior at various levels of R. Contrary to the corrosion fatigue characteristics of small cracks in high strength steels, no pronounced chemical crack length effect is observed for Al by 2024 exposed to salt water

Boundary selectivity of crack paths in corrosion fatigue of stainless steel

Stress corrosion and corrosion fatigue cracks are frequently very branched and there have been extensive attempts to define the characteristics of crack-stopping features. EBSD has been used to examine the full length (~8mm) of a corrosion fatigue crack in stainless steel. The grain boundary character distribution of the cracked boundaries is compared to that of the rest of the material and observations presented on the effect of grain boundary character on the choice of crack path at grain boundary junctions of different configurations and orientations with restect to the principle stres

Corrosion inhibitors for water-base slurry in multiblade sawing

The use of a water-base slurry instead of the standard PC oil vehicle was proposed for multiblade sawing (MBS) silicon wafering technology. Potential cost savings were considerable; however, significant failures of high-carbon steel blades were observed in limited tests using a water-based slurry during silicon wafering. Failures were attributed to stress corrosion. A specially designed fatigue test of 1095 steel blades in distilled water with various corrosion inhibitor solutions was used to determine the feasibility of using corrosion inhibitors in water-base MBS wafering. Fatigue tests indicate that several corrosion inhibitors have significant potential for use in a water-base MBS operation. Blade samples tested in these specific corrosion-inhibitor solutions exhibited considerably greater lifetime than those blades tested in PC oil

Etude des mécanismes d'interaction, au cours du procédé d'emboutissage à chaud, entre les surces atmosphériques d'hydrogène et les aciers à haute résistance revêtus d'Al-Si

Dans l’industrie automobile, le défi permanent d’allègement en vue de diminuer la consommation de carburant est profitable tant au niveau économique qu’écologique. Pour permettre cette diminution de masse sans compromettre pour autant la sécurité des passagers, il est donc essentiel de développer des nuances d’aciers toujours plus résistantes. Néanmoins, ces aciers doivent également garder une certaine ductilité nécessaire à leur mise en forme, mais aussi fondamentale pour la sécurité des passagers puisque ces pièces doivent pouvoir dans une certaine mesure absorber l’énergie en cas de crash ou d’intrusion. Pour répondre à cette demande, une solution en plein essor concerne l’emboutissage à chaud d’aciers au bore (tel l’USIBOR1500P© commercialisé par ArcelorMittal). Ce procédé consiste à austénitiser l’acier puis à le placer dans une presse, permettant une mise en forme facile et précise ainsi qu’une trempe menant à une microstructure martensitique de très haute résistance. De plus, ces aciers pour emboutissage à chaud sont souvent revêtus d’un alliage Al-Si (AluSi – 90% Al, 10% Si) afin d’effectuer ce procédé sous atmosphère non-contrôlée en évitant l’oxydation pendant le traitement thermique et en assurant une bonne résistance à la corrosion perforante. Néanmoins, cette haute résistance peut être défaillante suite à une fragilisation du matériau par l’hydrogène atmosphérique. En effet, si ce phénomène de fragilisation est connu depuis longtemps dans de nombreux autres domaines (nucléaire, soudage, …), il devient de plus en plus critique lorsque la résistance intrinsèque de l’acier augmente. La quantité tolérable d’hydrogène dans l’acier doit alors être maintenue à un niveau toujours plus faible. Le comportement particulier du revêtement Al-Si complique encore la problématique de ces aciers emboutis à chaud : il présente la particularité d’être perméable à l’hydrogène à haute température, et de ne plus l’être à température ambiante. L’hydrogène peut donc pénétrer dans la microstructure mais il y reste bloqué à l’ambiante. Dans ce contexte, le premier objectif consistera à étudier et comprendre les mécanismes régissant, lors du traitement thermo-mécanique d’emboutissage à chaud, les interactions entre un acier à haute résistance revêtu d’Al-Si et les sources atmosphériques d’hydrogène (hydrogène moléculaire ou vapeur d’eau). D’autre part, le second objectif consistera à trouver une solution (revêtement additionnel ou introduction de pièges à hydrogène) permettant de réduire la quantité d’hydrogène entrant dans l’acier au cours de sa mise en forme à chaud. Afin de comprendre les mécanismes d’entrée de l’hydrogène dans le matériau, différents sous-systèmes seront considérés, du point de vue des sources d’hydrogène atmosphérique et de celui des matériaux interagissant avec ces sources. Tout d’abord, les sources d’hydrogène sont d’une part la vapeur d’eau (source active dans le procédé industriel) et le dihydrogène (pour comparaison). La vapeur d’eau deuterée sera également utilisée afin d’assurer une atmosphère contrôlée en évitant le problème de dissociation de l’eau en dihydrogène dans le four. Ensuite, plusieurs sous-systèmes seront considérés pour l’acier au bore revêtu. En effet, lors du traitement thermo-mécanique, le revêtement en contact avec la source d’hydrogène évolue. À température ambiante, le revêtement initial est un alliage Al-Si solide ayant faiblement réagi avec le fer lors de son dépôt. Lorsque la température croît, ce revêtement repasse d’abord par la phase liquide puis forme des intermétalliques Al-Fe-Si. À ces trois sous-systèmes s’ajoute également l’acier nu, point de comparaison par rapport à l’acier revêtu. Pour ce faire, les premières expériences concernent des chargements gazeux sous atmosphères contrôlées d’un acier nu. Des analyses de désorption thermique sont ensuite réalisées. Elles consistent à envoyer un flux de diazote à température croissante dans une chambre contenant le matériau-cible. L’hydrogène contenu dans le matériau est alors emporté par le diazote vers un spectromètre de masse. Des informations sur la quantité d’hydrogène dissous sont ainsi obtenues et comparées pour les différentes sources d’hydrogène atmosphérique, différents points de rosée, ainsi que différents temps de chargement gazeux

Corrosion-fatigue lifetime of Aluminium–Copper–Lithium alloy 2050 in chloride solution

The fatigue behaviour of Aluminium–Copper–Lithium 2050 alloy under two metallurgical states (T34 and T84) was studied in air for healthy and pre-corroded samples in a 0.7 NaCl solution. The results were compared to those obtained during fatigue–corrosion tests performed in a similar chloride medium. Preliminary corrosion tests demonstrated that the T34 metallurgical state was susceptible to intergranular corrosion, while the T84 metallurgical state was susceptible to intragranular corrosion. Fatigue life tests in air on pre-corroded samples revealed a significant decrease in fatigue life related to the presence of corrosion defects before the cyclic solicitation. A strong effect of the first minutes of immersion in corrosive media was evidenced on fatigue life behaviour. The fatigue–corrosion tests revealed that the T34 metallurgical state was more affected by fatigue–corrosion in terms of fatigue life than the T84 metallurgical state. This observation can be explained by the increased propagation of intergranular corrosion enhanced by the cyclic solicitation

An evaluation method for corrosion fatigue life of steel structure considering mechanical factors

Steel structures in corrosive environment are often subjected to coupling effect and damage caused by corrosion and fatigue. This paper proposed a new assessment method to study corrosion fatigue life of steel structure, including the effect of cyclic loading and corrosion damage. Based on mechanical factors, the corrosion depth of structure under cyclic loading at different time intervals was defined by a mathematical model for corrosion damage. A finite element model was established to calculate structure damage. Finally, the cumulative damage could be obtained by Miner guidelines to assess the fatigue life. Comparing traditional methods, the coupling effect of corrosion and fatigue were taken into account by this new method. According to this new method, the results showed that the calculated corrosion rate was faster, and the corrosion fatigue life shorter. Corrosion fatigue could cause more damage to structure than was expected. Furthermore, this method was convenient and practical for assessing/estimating the corrosion fatigue life of normal steel structure