Study Of Early Stage Current Fluctuation Of Pitting For304Stainless Steel

Pitting corrosion is one of the main factor that give rise to failure ofstainless steel, because the pitting damage is highly concentrated, thedistribution of pits are randomly around the surface of material, thepropagation direction is the direction of gravity and the pits are usually smalland hardly to be noted. Therefore, the study of the early stage of pitting isnecessary for prediction and preventing of pitting damage. In this thesis, slowrate potentiodynamic polarization, potentiostatic polarization, SEM analysiswere used to study the influence of chloride, surface roughness, PO₄^3-andWO₄^-2on metastable pitting potential E_m, stable pitting potential E_b,nucleation rate of metastable pitting, metastable pitting peak current andmetastable pitting growth rate K. The main conclusions are as follows:（1） According to the concentration range of chloride ion, thedistribution of metastable pitting potential E_mand stable pitting potential E_bboth follow Gaussian distribution. Chloride concentration increases, both E_mand E_bdecrease, the nucleation rate of metastable pitting and stable pittingdecrease. The distribution of metastable pitting peak current follows extremevalue distribution. The pits with suitable growth rate may propagate to stable pits, but those pits with fastest or slowest growth rate may disappear.（2）According to the surface roughness range, the distribution of E_mandE_bboth follow Gaussian distribution. E_mand E_bdecrease with the increasingof surface roughness, the probability of nucleation of metastable pitting andstable pitting are increased. The distribution of metastable pitting peak currentfollows extreme value distribution. The corrosion pits grow in the direction ofabrasion grooves.（3）According to the concentration range of phosphate added intochloride solutions, the distribution of E_mand E_bboth follow Gaussiandistribution. With the increasing of phosphate concentration, both E_mand E_bincreased, the nucleation of metastable pitting and stable pitting are inhibited.（4）The effects of PO₄^3-on nucleation and growth of metastable andstable pitting were investigated by cycling potentiodynamic polarization. BothE_mand E_bincrease with the increasing of PO3-4concentration. HigherPO₄^3-concentration results in lower average growth rate and peak current ofmetastable pitting, hence inhibits the nucleation of stable pitting. However, asPO₄^3-concentration increases, the repassivation potential of stable pitting Epdecreases, indicating that the repassivation of the pit is retarded. The reason isexplained by the deposition of phosphate film around the pit mouth, whichmay promote the growth stability of the pit.（5）The effects of WO₄^2-on the nucleation and growth of metastableand stable pit was also studied. With the increasing of WO₄^2-concentration, E_mand E_bincrease, average growth rate and peak current of metastable pittingdecrease, hence the probability of transformation from metastable pit to stablepit is increased. Epdecreases with the increasing of WO₄^2-, indicating that therepassivation of stable pits is retarded.