FeCrNiCoMn高熵合金及FeCrNiCo中熵合金在3.5wt%氯化鈉水溶液中的腐蝕性質研究

Abstract

高熵合金為多種元素以等比例混和而成的一種新世代材料。為了探討其腐蝕性質，選定等比例組成之FeCrNiCoMn以及FeCrNiCo兩種合金，並針對Mn的有無是否對抗蝕性質產生影響來做研究，其中304L不銹鋼也會做為對照組來一同比較。 在極化曲線中，FeCrNiCoMn沒有出現鈍化區，且與先前的文獻相比之下在較低電位就發生孔蝕；反觀FeCrNiCo則是出現了寬廣的鈍化區。若以Cr含量與FeCrNiCoMn相近的304L不鏽鋼來做測試，卻發現304L不鏽鋼不但會出現鈍化區，且孔蝕電位也明顯較高。經由SEM以及EDX的分析，發現熱軋均質化的FeCrNiCoMn以及FeCrNiCo試片含有大量介在物，其成分多為S、O的化合物，且在諸多文獻中都顯示MnS溶解後可能抑制周圍底材的鈍化行為。此外，由XPS分析的結果得知在FeCrNiCoMn的鈍化膜中含有高比例的Mn氧化物，因此合理推測材料中的Mn會與Cr產生競爭氧化，使得鈍化膜中的Cr2O3含量下降而造成孔蝕電位的降低。 為了清除MnS等有害的介在物，本研究所使用的表面改質配方包括複合酸洗-鈍化配方以及硝酸酸洗-鈍化配方，且發現前者能夠有效移除大部分O、S的化合物，而後者則是僅能移除S的化合物。此外，由於酸洗-鈍化配方能讓試片表面生成富含Cr2O3的鈍化膜，經酸洗-鈍化後的試片在電化學測試中都呈現了良好的抗蝕性質，但在F離子的負面影響下，複合酸洗-鈍化配方對於抗蝕性的提升會略遜於硝酸酸洗-鈍化配方。 最後的章節本研究以SEM和EDX Mapping分析極化至孔蝕電位的試片，發現FeCrNiCoMn以及304L不鏽鋼的孔蝕起始位置皆為MnS介在物。而在酸洗-鈍化後的試片中，由於大部分的孔蝕起始位置都被移除，因此孔蝕的數量有著明顯的下降。

High entropy alloy (HEA) is a new generation material composed of multiple elements in equal proportions. To investigate the corrosion properties of HEA, FeCrNiCoMn and FeCrNiCo alloys were chosen in this study, focusing on whether Mn element can have an influence on corrosion resistance. Simultaneously, 304L stainless steel would become the control group in all of the experiments. The results showed that no passive region was exhibited in the FeCrNiCoMn polarization curve, and pitting occurred at a much lower potential compared to previous studies, on the other hand, FeCrNiCo exhibited a wide passive region in the polarization curve. In 304L stainless steel, the passive region could be exhibited in the polarization curve, and pitting potential was obviously higher than that of FeCrNiCoMn even though both of them had nearly the same concentration of Cr. By SEM and EDX analysis, a substantial number of inclusions composed of sulfide and oxide existed in the FeCrNiCoMn and FeCrNiCo specimens. Moreover, many research have reported that MnS dissolution might inhibit the passivation behavior around the inclusion. By XPS analysis, the results showed that a high percentage of Mn and Cr oxides were contained in the passive film, so it was reasonable to presume that Mn competes with Cr in the oxidation reaction so that Cr_2 O_3 decrease in the passive film and leads to a much lower pitting potential. To eliminate the harmful MnS inclusion, surface treatments including Multiple Pickling-Passivation (MP-P) and Nitric Acid Pickling-Passivation (NP-P) were carried out; the former effectively removed most of the sulfide and oxide, but the latter could solely remove sulfide. Besides, the superior corrosion resistance in the electrochemical test might result from Cr_2 O_3 enrichment in the passive film by surface treatments, but under the negative effect of fluoride ion, MP-P had less improvement in corrosion resistance compared to NP-P. In the final chapter, SEM and EDX Mapping were used to analyze the specimens which were polarized to pitting potential, confirming that MnS was the pitting initiation site for both FeCrNiCoMn and 304L stainless steel. Owing to remove most of the initiation sites of pitting, pitting numbers dropped largely after performing surface treatments.