Effect Of Deep Ocean On Critical Pitting Temperature Of 316L Stainless Steel

Critical pitting temperature (CPT) is an important parameter to evaluate pitting sensitivity of the austenitic stainless steels. It can be used to forecast the type of the metal corrosion. The effects of deep ocean environment on the CPT of based metal 316LSS (BM),weld metal(WM) and the as-welded sample (argon arc welding) were researched by response surface methodology (RSM). Finally it is of significance to establish the correlation between the microstructure and corrosion behavior, and provide the criterion for forecasting the critical pitting temperatures.The effects of three characteristic environmental factors of deep ocean hydrostatic pressure(HP), dissolved oxygen(DO) and temperature(T) on the corrosion resistance of 316L SS were studied by using potentiodynamic polarization test(POL). The statistics results of cumulative probability showed that: (1) the hydrostatic pressure made the pitting potential drop, which exacerbated the corrosion resistance of the material;(2) oxygen reduction was suppressed in anoxic environment; (3) Pitting potential under 4? rose to 1.5V nearly. It was concluded that the transpassive corrosion occured on 316L SS, combining with the corrosion microtopography and the potential-pH diagrams of iron and chromium.The conjunct impacts of HP and DO on the CPT of 316L SS were analyzed with RSM,which provided a linear theory model. There is no interaction between two factors, but they show the single factor effect simultaneously. HP was characterized by negative effect, while DO acted as the positive role.The microstructure, composition and phase constituents of the different organizations on the as-welded sample were analyzed by SEM-EDS. XRD and metalloscope. The BM grains near the fusion line didn't grow up without overheating phenomenon, therefore the heat affected zone is not obvious. Apart from three austenitic peak, a weak peak of 8-ferrite presented in the XRD spectrogram. The proeutectoid ferrite presented as skeletal shape in the austenite. WM contained a high content of Cr,and the micro-segregation existed between ferrite and austenite.The RSM analysis to the effect of HP and DO on the CPT results of WM and as-welded sample , also established the correlation between the microstructure of BM and WM, and corrosion behavior. The high content of Cr and beneficial effect of ferrite for dissolving impurity elements, suppressed the adverse micro-segregation influence. It ensured that the weld metal had superior corrosion resistance than BM, hence the CPT results of WM were greater. Pits that near the fusion line 2mm distance on the weld zone, had a larger pitting number per square centimeter, according to the statistical results of pits distribution.According different environmental factors' datas collecting from the northeast Pacific,Atlantic Ocean and the South China Sea, prediction for the critical pitting temperature curves of 316L SS,the WM and as-welded sample in the boundary conditions was completed. It is convenient for forecasting types of corrosion below various depths, and providing guidance for engineering marine structural steels.