Mechano-chemical Effect In Chloride Corrosion Of 304 Stainless Steel

As the installations in petrochemical, power generating and metallurgical industries are developing towards high temperature, high pressure and large-scale applications, corrosion-induced accidents have been a common occurrence, especially when the components are subjected to complex environments and applied loads. For a long time, extensive research efforts have been taken on the corrosion problem with applied stress. However, it remains as a significant challenge due to the complexity and diversity of the interaction between stress and corrosion. In the thesis, the corrosion behavior of 304 stainless steel in chloride solution was investigated. The effect of uniaxial plastic deformation on active dissolution, the effect of local deformation on active dissolution and the effect of multiaxial cycle loading on corrosion property were studied theoretically and experimentally. The main contents and conclusions of the thesis are as follows:The stress corrosion cracking sensitivity in the chloride solution with different acid concentration was firstly studied. The stress corrosion cracking mechanism was analyzed. Test results showed that, in the environment of 0.63mol/L NaCl+0.5 mol/L HCl solution,304 stainless steel had a relatively high sensitivity to stress corrosion cracking.Electrochemical tests under constant loading was conducted to study the effect of hydrogen and applied stress on anodic active dissolution. Test results revealed that the plastic deformation had a strong effect on active dissolution and the synergistic effect between stress and dissolved hydrogen only slightly promoted the process of active dissolution. On the basis of thermodynamics and electrochemistry, a theoretical model which reflects the effect of applied stress on active dissolution was proposed in terms of the stored strain energy and electron work function. A good agreement between the prediction and test results which revealed that the stress corrosion cracking mechanism of 304 stainless steel in acid chloride solution was not hydrogen-facilitated anodic dissolution.A mathematical model for simulating a stressed metal surface with a pit in which active dissolution occurs had been developed. An active dissolution mechanism was assumed and extended to the pit surface by including a multiaxial stress state dependent bare metal dissolution current density. The influence of applied tensile stress, pit radius and temperature was addressed. Through accounting for the thermal activation energy and the stress state in pit bottom, the distribution of solution potential and species concentration was predicted for different applied tensile stresses. The effect of multiaxial cyclic loading on corrosion property of 304 stainless steel was studied by use of proportional loading and non-proportional loading. It was found that, compared with proportional loading, the continuous rotation of the principle stress plane during multiaxial non-proportional loading blocked the formation of pits. Furthermore, the lack of stable dislocation structure during the non-proportional loading enhanced the additional hardening of material which strongly reduced the fatigue life of specimen while the corrosion environment only played a promotional role.