積層製造316L不銹鋼在3.5 wt. %氯化鈉水溶液中抗蝕性研究

Abstract

積層製造(Additive manufacturing)為工業4.0自動化中重要的一環，其中以選擇性雷射熔融技術(Selective Laser Melting, SLM)最為成熟，在傳統合金的製造與應用上， 316L不銹鋼因抗腐蝕性佳而在過去數十年廣泛應用，因此許多學者開始針對積層製造的316L不銹鋼進行抗腐蝕性研究，提出製程快速的冷卻速率在材料中產生不同尺度的缺陷，包含孔洞、二次相、成分偏析、介在物等，皆可能降低SLM 316L不銹鋼的抗腐蝕性，然而缺陷在材料中尺寸重疊難以劃分，也因此難以釐清單一因素對抗腐蝕性影響。 本研究將透過微結構分析及電化學測量，利用相同的製程參數的SLM 316L棒材，系統性地釐清表面孔洞、介在物、二次相、成分偏析等缺陷對抗腐蝕性的影響。微結構分析上，阿基米得原理測量材料體孔隙率；光學顯微鏡及軟體Image J分析面孔隙率，溶液蝕刻後用電子顯微鏡(SEM)可觀察SLM細微結構特徵，包含融池及亞晶粒；X光繞射儀(XRD)檢測鍛造316L僅存γ相，而SLM 316LC含 γ相及少量的δ相；EBSD及TEM繞射圖確認了δ相分布在熔池邊界、γ相則在內部，但沒有對應的成分偏析；SEM觀察到鍛造316L中存在微米級MnS及富含鋁、矽、鈣的氧化介在物，而SLM中則為廣泛隨機分布的矽氧介在物，介於數百奈米至數個微米間，TEM則找到SLM中有奈米級氧化介在物及MnS。 抗蝕性分析利用動電位極化曲線(PDP)及循環電位極化曲線(CP)，使用的是三極系統，測試水溶液為3.5wt%氯化鈉及相同離子強度的2.84wt%硫酸鈉，測量範圍分別為1.767cm2及0.04cm2。量測完傳統鍛造316L表面具有明顯的孔蝕，孔蝕電位介於0.2-0.5V，在鈍化區間有介穩態孔蝕，因具微米級大小不等的孔蝕而無再鈍化電位。SLM 316L則針對表面孔洞率範圍0-4%、熔融不完全孔洞、不同鉻添加含量、δ相及氧化介在物等位置量測電化學，測量後表面及極化曲線上皆沒有孔蝕發生，證實以上缺陷皆非誘發孔蝕主因。透過循環動電位極化曲線進一步釐清過鈍化、間隙腐蝕及再鈍化行為，發現SLM高電位電流密度瞬間上升對應的電位為過鈍化電位，隨電流密度上升，與測試面積邊界處開始間隙腐蝕，間隙腐蝕的面積大小決定了再鈍化電位高低。 結合微結構及電化學測量結果，在3.5wt%氯化鈉水溶液中，傳統鍛造316L中微米級的MnS為誘發孔蝕主因，而SLM 316L則因製程細小化MnS，奈米級的MnS降低了誘發孔蝕的機率，整體提升SLM 316L不銹鋼之抗腐蝕性，但硫化錳出現在SLM表面的機率、硫含量達一定程度、尺寸達一定大小仍有可能誘發孔蝕。

Additive manufacturing is an important part of Industry 4.0 automation, among which Selective Laser Melting (SLM) is the most mature technology. When it comes to manufacturing and application of traditional alloy, 316L stainless steel (SS) has well-known high corrosion resistance. It has been used widely for decades, so many scholars began to study the corrosion resistance of SLM 316L SS. Researchers proposed that rapid cooling rate during manufacturing induced different scale defects such as porosity, second phase, element segregation, inclusion many reduce the corrosion resistance. However, overlapping scale of defects are difficult to clarify the impact of single factor on corrosion resistance. This study uses microstructural analysis and electrochemical measurements to investigate the influence of SLM 316L surface pores, inclusions, second phase, element segregation on the pitting corrosion mechanism. The material microstructure analysis involves measuring the volume porosity of the material body according to Archimedes principle, as well as OM photographs on the surface of the material, then software Image J for surface porosity analysis. SEM is used to observe the surface structure features such as melting pools and subgrains after chemical etching. XRD is used to detect different phases that wrought 316L only has austenite, while SLM 316L has austenite and a small amount of δ ferrite. Combined EBSD and TEM results, which shows austenite distribute inside melting pool while δ ferrite on the boundary. But there is is no corresponding element segregation. SEM observed that there are micron-sized MnS and enriched in Al, Si and Ca oxide inclusion in wrought 316L while SLM 316L randomly distributed silicon oxide inclusion in the range of several hundred nanometer to several micron. TEM observed both nano-sized MnS and oxide inclusions in SLM 316L. The corrosion resistance analysis involves potentiodynamic polarization (PDP) curve and cyclic potential polarization curve(CP). This was done by a three-electrode system, the test specimen is served as the working electrode (WE), platinum as the counter electrode (CE), and a calomel electrode (SCE) as the reference electrode (RE). The test solution used is 3.5wt% sodium chloride (NaCl) and 2.84wt% sodium sulfate (Na2SO4), with the same ionic strength, and measurements are taken in a large area (1.767cm2) and a small area (0.04cm2). In the case of wrought 316L, the electrochemical results show an obvious pitting potential, and the curve oscillates in the passivation area, indicating the presence of metastable pitting. Because of stable pits, there is no repassivation potential in wrought 316L. However, in the case of SLM 316L, different surface porosity (0~4%), different morphology of porosity, δ phase and inclusions are analyzed. The anodic polarization curve and tested surface shows there is no induced pitting observed at high potential. The transpassivation, crevice corrosion and repassivation behavior were further clarified through CP test. It was found that the potential correspond to the point of instant increase current density in SLM 316L is transpassivation potential. The repassivation potential was determined by the level of crevice corrosion at the edge of the test area. Based on the microstructure and electrochemical measurement, we have concluded that the presence of micron-sized manganese sulfide inclusions in wrought 316L is responsible for the formation of metastable pitting/stable pitting in the 3.5wt% sodium chloride solution. In contrast, due to the miniaturization of MnS by additive manufacturing, the nano-sized MnS reduce the probability of inducing pitting, which improve the corrosion resistance of SLM 316L. However, if the following condition are reached, the probability of manganese sulfide appears on the surface, certain level of the sulfur content and the size is big enough, may still induce pitting in SLM 316L.