Study Of Fatigue Reliability On H₂S Corroded Metallic Materials

Corrosion fatigue of the material is caused by the synergy of corrosive media andalternating stress, it is widely present in the engineering such as petrochemicalindustry, nuclear industry, aerospace and marine vessels. With the rapid developmentof the industry, production safety and equipment security requirements are alsoincreasing, and the corrosion fatigue problem is increasingly important in engineering.Corrosion fatigue, which is related to mechanics, electrochemistry, materials scienceand other disciplines, is very complex because of the variability of the corrosiveenvironment and the service conditions. Thus the long-term development frameworkfrom2006to2020of our Country has put the "Interaction of material service and theenvironment, Performance evolution, Failure mechanisms and Life predictionprinciple" as a major strategic need of basic research projects, expect to solve thematerial environment service problems.Based on the above background, studies on the corrosion fatigue issues aredeveloped in this paper. For the corrosion fatigue contains two different problems,corrosion and fatigue respectively. Thus the corrosion fatigue study can be firstdivided into two independent studies, corrosion study and fatigue study, and thencarry out a comprehensive study. In order to solve the problem of corrosion fatigueand its reliability in engineering, there are three parts of the research, the first is thestudy of the fatigue reliability, the second is the study of the corrosion assessment,and the third is the study of the corrosion fatigue and its reliability.In fatigue reliability studies, based on the assumption that the logarithmic fatiguelife follow the normal distribution, a conditional probability density distributionsurface （CPDDS） of material is established by constructing the functions μ（S） andσ（S）. Where μ（S） is the function between the logarithm of fatigue life and the stresslevel, σ（S） is the function between the standard deviation of logarithmic fatigue lifeand the stress level. The CPDDS can reflect the fatigue properties of the materialmore comprehensively, such as the S-N curve, p-S-N curve and the theoretical fatiguelimit. The theoretical fatigue limit is the stress level at which the material failure is asmall probability event within the required service time. The definition of thetheoretical fatigue limit broke the conventional definition of the fatigue limit. A newmethod for researching the performances of the material fatigue is proposed in thisstudy, while the method can also be used in the study of the corrosion fatigue and itsreliability. In order to assess the corrosion degree of the material after the H₂S corrosion, thelayer by layer micro-hardness test method is proposed. By monitoring the hardnesschanging with the thickness of the polished layers after the H₂S corrosion, thestratification phenomenon is proved. The material will induce the corrosion layer andthe hydrogen embrittlement affected layer after the H₂S corrosion, wherein thesolution concentration plays little role on these stratifications, the etching time playsan important role on these stratifications. For the C45steel used in this paper, thethickness of the hydrogen embrittlement affected layer will reach the maximum after48h immersion, the thickness of the corrosion layer will reach the maximum after96himmersion. The mechanical model which expresses the hardness changing with thesolution concentration, etching time and the layer thickness is proposed in this part,and the physical definition of the model parameters is also given. The proposed modelprovides a new research method for the hydrogen damage study.In studies of the corrosion fatigue and its reliability, a new research method ofthe corrosion fatigue in engineering is proposed by the combination of the corrosionassessment and the fatigue reliability research, and the qualitative explanation of thecorrosion fatigue mechanism is given. The corrosion fatigue test of C45steel isresearched in this part, the study found the material stratifications after the corrosionhave an important impact on the fatigue behavior. The corrosion layer can easily leadto the micro-crack initiation and accelerate the crack propagation because of thepresence of pitting, resulting in the weakening of anti-fatigue properties, causing thereduction of the theoretical fatigue limit; the hydrogen embrittlement affected layerbecause of the hydrogen damage will lead to the increase of the material failurerandomness, resulting in the material fatigue life dispersibility increases, causing areduction in material‘s reliability life. The above findings provide a theoreticalreference for the fatigue strength based reliability design or the fatigue life basedreliability design, and the valid test data for life maintenance program development ofthe serving equipment and materials in engineering.