| Pressure equipments, such as boilers, pressure vessels, pipelines, are the maincomponents of special equipments, widely used in energy, petrochemical, metallurgy, food,medicine and other fields, and with explosion risk. Pressure equipments, especially pressurevessels and pipelines, are working in the corrosion environments of medium, and also in highstress state of the inner (outer) pressure load in the using process. The interaction of stress andcorrosion environment is the normal working state for materials of pressure vessels andpipelines, which should easily cause material damages until failure, and lead to structuralfailure, leakage, and explosion accident. Therefore, it is necessary to pay attention to andstudy the damages of pressure vessels and pipelines faced in the working state to evaluate itsdamage process and protect its safety. In this paper, the Strain Corrosion Damage is proposedfor the first time on the basis of the working status of materials used in pressure vessels andpipelines, which is the common damage types of the pressure vessel and pipeline materials,and conducted a systematic theoretical and experimental research.Three of the most typical and commonly used materials were chosen in this paper,Q235B, Q345R and L390, and the strain corrosion damage tests and electrochemical test weredone respectively in the ASS salt spray environment and NS4solution under different stress(strain) levels. Meanwhile, microstructure analysis of corroded samples, SEM and EDSanalysis of corrosion surface were conducted. Through the analysis of corrosion morphology,corrosion weight loss ratio, corrosion weight loss rate and localized electrochemicalcharacteristics of materials under the same corrosion environment and different stress (strain)levels, the strain corrosion damage behavior, mechanism and evolution of the three kinds ofmaterial were investigated. The strain corrosion damage mechanism was discussed throughthe microstructure, SEM and EDS of corrosion surface. And the strain corrosion damageevolution law of Q345R and Q235B was found different through comparative analysis ofcorrosion weight loss and electrochemical data analysis. And it was put forward that the mechanical sensitivity correction factor of material to fixed the mechanical chemicaldescription of different materials.At the same time, a practical and simplified strain corrosion damage unit volume modeland an improved porosity model based on microscopic damage mechanics were establishedaccording to the application of damage mechanics method, the metal material mechanicalchemical effect theory and strain corrosion test data and mechanism. Through the two damagemodel, the metal material strain corrosion damage evolution process was described, and thestrain corrosion damage life assessment method was investigated, which realized theprediction and evaluation of pressure vessels and pipelines strain corrosion damage.Research of strain corrosion damage on Q235B indicated that, its main strain corrosiondamage morphology was pitting corrosion. The higher the stress level of the sample, the moreserious of the pitting corrosion degree. Along with the corrosion time prolonged, pittingdensity first increased and then decreased, pitting size increased gradually, ratio of depth todiameter decreased. The corrosion weight loss ratio with time of Q235B was a linearrelationship, and higher stress levels, larger slope. The corrosion weight loss rate with thecorrosion time first decrease rapidly, and inclined to be smooth, and then was the leveldevelopment. Based on this, it was put forward in this paper that the data table of stablecorrosion weight loss rate of Q235B with the different stress levels which should be used toestimate the project conveniently for engineering application. The corrosion weight loss ratioand corrosion weight loss rate with stress levels were increased. Electrochemical test resultsshowed that, the negative shift of corrosion potential of loading sample was bigger, corrosiontended to increase. The corrosion potential distribution of notched loading sample wascorresponding with the stress distribution in the notch area, the higher the stress, the higherthe corrosion potential (corrosion current density) and corrosion speed. Macrostructureanalysis of corroded samples, SEM and EDS of sample corrosion surface showed that, themechanism of Q235B strain corrosion damage started from grain boundary, and then grainsfell off or dissolved. The existence of deep holes on the pits of corrosion surface showed that,it had the tendency of pitting corrosion of strain corrosion damage of Q235B sample in theASS salt spray environment.Research of strain corrosion damage on Q345R indicated that its pitting was not obvious.Similarly, the corrosion weight loss ratio with time of Q345R was a linear relationship, andhigher stress levels, larger slope. The strain corrosion damage of Q345R tending to be stableneeded longer time, and the final stable corrosion weight loss rate was lower, and corrosion resistance was stronger compared to Q235B in the ASS salt spray environment. The corrosionweight loss ratio and corrosion weight loss rate with stress levels were increased.Electrochemical test results showed that, the negative shift of corrosion potential of loadingsamples was bigger, corrosion tended to increase. The corrosion potential distribution ofnotched loading sample had a corresponding trend with the stress distribution in the notcharea. Macrostructure analysis of corroded samples, SEM and EDS of sample corrosionsurface showed that, the mechanism of Q345R strain corrosion damage started from grainboundary, and then grains fell off or dissolved.Based on comparative analysis of the Q235B and Q345R corrosion weight loss andelectrochemical data, the stress sensitivity of different materials was different on corrosionbehavior, it was put forward that to add the mechanical sensitivity correction factor tocorrect the chemical mechanical relationship of materials in engineering.The strain corrosion test of L390showed that, it appeared less pitting corrosion. Thecorrosion weight loss ratio with time of L390was a linear relationship, and higher stresslevels, larger slope. The strain corrosion damage of L390tending to be stable needed longertime, and the final stable corrosion weight loss rate was lower. Corrosion weight loss ratio andthe corrosion weight loss rate were significantly accelerated trend with the stress levels. Thisshowed that the sensitivity of L390on the stress level was bigger. Electrochemical test resultsshowed that, the negative shift of corrosion potential of loading sample was bigger thanQ235B and Q345R, corrosion tended to increase. The corrosion potential distribution ofnotched loading sample was corresponding with the stress distribution in the notch area, thehigher the stress, the higher the corrosion potential (corrosion current density) and corrosionspeed. Macrostructure analysis of corroded samples, SEM and EDS of sample corrosionsurface showed that, the mechanism of L390strain corrosion damage started from grainboundary, and then grains fell off or dissolved.Based on the research of damage mechanics methods of the mechanical damage and thecorrosion damage, according to the mechanical chemical effect theory and strain corrosionmechanism, a practical, simplified element volume model of strain corrosion damage wasestablished in the basis of the results of the experimental data analysis, and checked with theexperimental results. Then, an improved porosity model based on meso-damage mechanics was established according to the mechanical chemical effects of thermodynamics andelectrochemistry equation. And the unit volume model only needed a few of tests or literaturedata to estimate strain corrosion damage and service life, which was easy for practicalapplication. The improved porosity model can be applied to describe the elastic and plasticstrain corrosion damage. And the model was modified according to the experimental results,which made it more consistent with the actual process of strain corrosion damage. |
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