| In this work, the crack growth behavior of X70pipeline steel in a near-neutral pH solution was investigated. The effects of the solution pH, Cl-concentration, cathodic polarization, alternating current (AC) interference, plastic strain and hydrogen on the electrochemical and stress corrosion cracking (SCC) behavior of X70steel were analyzed. The anodic dissolution behavior of the crack tip was simulated by a galvanic corrosion method, and the non-steady electrochemical process was discussed.The results showed that the stress corrosion crack propagated perpendicular to the stress direction under the cyclic load, with a crack growth rate of4.28×10-3mm/cycle, resulting in a low pH and high Cl-concentration environment around the crack tip. During the propagating process, the pH of the crack tip is about4.0and the concentration of Cl-reaches4mol/L. Under the protection of low cathodic polarization, crack growth rate decreased resulting from the effect of hydrogen induced ductility (HID). While under more negative cathodic potentials, the crack growth rate increased due to the hydrogen embrittlement (HE) effect. Under all the experimental conditions, the crack growth behavior is in accordance with the stress corrosion fatigue mechanism (SCF) which was the combined effect of stress corrosion cracking and corrosion fatigue.The decrease of pH from6.8to4.0enhanced the electrode reaction process, especially the cathodic reactions. The SCC sensitivity decreased slightly with the reduction of solution pH from6.8to5.5, and then increased obviously as the solution pH decreased from5.5to4.0. The enlargement of the Cl" concentration also facilated the corrosion of X70steel, and promoted the SCC sensitivity as it amplified100times. The combined effects of solution acidate and accumulation of Cl-further accelerated the SCC process. During the corrosion process, the ferrite grains dissolved preferentially while the carbon rich structures such as grain boundaries, carbides and M/A islands remained intact. The inclusions including Al-Mg-O-S-Ca and Si-rich particles were selectively attacked, resulting in the formation of corrosion pits. During the SCC process, cracks also tended to initiate at the Al-Mg-O-S-Ca and Si-rich inclusions.The cathodic polarization (CP) altered the charge transfer process and SCC sensitivity of X70steel in near-neutral solution by affecting electrode reactions. Under the cathodic protection at nobler potentials than-775mV, SCC sensitivity of the steel decreased. While with the shift of cathodic potential to more negative values, SCC sensitivity increased attributing to the combined effect of anodic dissolution (AD) and hydrogen embrittlement (HE). Under cathodic protection, AC interference transformed the corrosion potential from its designed value. Under a noble CP potential, AC shifted the DC potential negatively and accelerated the anodic dissolution of X70steel. In addition, the SCC sensitivity was increased by AC interference. While when the CP potential was sufficiently negative, the DC potential of the steel was shifted positively and the cathodic reaction was promoted. The marginal region of the electrode was corroded more severely compared with the center area due to the "skin effect" of AC.Plastic deformation changed the density and distribution of dislocations, the surface roughness and consequently accelerated the corrosion process of X70steel. The plastic strain also increased the micro-hardness, the yield strength and the hydrogen adsorption sites, which promoted the adsorption and permeation of hydrogen and improved the concentration of hydrogen in X70steel. Moreover, the hydrogen inside the steel accelerated the anodic dissolution of the heavily deformed X70steel. The simultaneous enhancement of plastic strain on the anodic dissolution and the hydrogen adsorption resulted in the increase of SCC sensitivity with increasing of the plastic strain.By using galvanic corrosion method, the relationship between crack tip dissolution and plastic strain is obtained and the crack tip dissolution current density can be expressed as a function of plastic strain and strain rate: i=i0+(5.13+1gε)△ε. Then the crack growth rate was calculated by Faraday’s law which was two orders lower than that under cyclic load. It might be caused by the ignorance of newly exposed fresh metal at the crack tip and the effect of hydrogen.Considering the synergistic effect of various factors including newly exposed fresh metal at the crack tip, crack tip environment, plastic strain and hydrogen, a crack tip anodic dissolution rate calculation formula was proposed. Basing on the synergetic model of hydrogen and stress, this formula introduced the cofficient of the crack tip fresh metal surface and the crack tip environment for the first time and it was significant for predicting the the crack growth rate. |
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