Pitting corrosion analysis of stainless steel by boundary element method with strain-dependent polarization curve

Abstract

This study aims at evaluating the corrosion rate at the first stage of stress corrosion cracking by a numerical simulation. The stress corrosion cracking starts with a pitting corrosion which appears from a damaged portion of passive film induced by plastic deformation. From micromechanical standpoint, the stress and strain are concentrated around the grain boundaries due to the heterogeneity of microstructures; therefore, the plastic slip occurs mainly around the grain boundaries and generates a fresh surface without passive film. This produces a microcell and affects the macroscopic polarization curve. We obtained this polarization curve of stainless steel from the open-circuit tensile tests associated with the microscopic electrostatic simulations. Moreover, this paper shows the two-dimensional formulation for coupling analysis of elastic stress and electrolytic potential. Both fields are solved by the boundary element method with the discontinuous quadratic element. The strain-dependent polarization curve is used as a nonlinear boundary condition of the potential problem. First, the elastic problem is solved to obtain the surface strain which governs the polarization curve on the surface. Next, the potential problem is solved to obtain the current density on the surface which determines the corrosion rate. Since each node has two corrosion rates in different directions coming from the neighbouring elements, we average these two rates and directions, so as to conserve the volumetric reduction rate unchanged. After moving the nodes as a result of corrosion during the time step, we return to the stress analysis and iterate this procedure during the interested period of time. We demonstrate a corrosion pit growth from a small hemi-elliptic surface defect and show the availability of the proposed method.

Technical Issues to Realize Stress-Corrosion Simulation

“…この場合，流体の速度場 v を解いた後に，式(20)と式( 19) を式( 18)の制約条件と式(15)の境界条件の下で解けばよ く，あとは適切な時間積分スキームを用いて，繰り返し計算 を行えばよい．式(20)を定常流れにおける拡散と泳動に適 用して，境界要素法で解析した例がある (23) ． 極反応速度に影響があるとの報告 (28) もあり，実験により低 い応力でも分極曲線が変化したとの報告 (29) もある．また， ステンレス鋼の分極曲線が塑性変形により変化することも実 験で示されている (30) ．更に引張試験と同時に自然電位を測 定すると，塑性変形と共に自然電位が大幅に低下すること が，ステンレス鋼 (31)(32) やアルミニウム合金 (33)(34) で示され (20) ．更に新生 面と皮膜面の分極曲線がわかっていれば，新生面と皮膜面の 面積比を固定することで，数値計算により全体の分極曲線を 評価できる (20) (40) ．図はオーステナイト系ステンレス鋼の 分極曲線を境界要素法で評価した結果である (39) (20) ．Wang ら (21)…”