Abstract
應用電化學技術，建立各工程合金在地熱水中之實驗電位-pH 圖。研究結果供作評估API J-55鋼、304、410不鏽鋼及鈦合金在地熱水中的耐蝕性優劣。重要研究結論如下：?鈦合金在各種環境條件下有優異的抗孔蝕能力。?304 不鏽鋼由於含有較高的鉻合金元素，所以抗孔蝕能力比410 不鏽鋼佳。?低合金鋼最容易受到均勻腐蝕。將不鏽鋼、鈦合金及銅合金等冷凝器材料置於興達火力發電廠進水灣進行海生物腐蝕試驗。並應用SAM（scanning Auger microscope）表面分析技術，探討微生物影響腐蝕之機構。採用電漿氮化（plasma nitriding）及電漿輔助物理氣相沈積（PEPVD）兩種電漿表面技術來處理鋼材表面。表面處理過的材料再應用電化學技術探討其在各種水溶液中的腐蝕行為。電漿氮化鋼在水溶液中的腐蝕研究有以下重要結論：?SAE 4140經電漿氮化後，置於空氣中，氧會慢慢擴散進入ε-（Fe,Cr）2-3N及 r1-（Fe,Cr）4N 等氮化物的孔隙中，形成一厚度大於9000A 的緻密氧化氮化物層（oxy-nitride layer ）。此一表層可以阻絕材料基地與環境間之直接接觸，從而降低其腐蝕速率。?逐層磨去氮化物層後，以XRD （X-ray diffraction ）分析其相組成，再配合Fe-N-Cr三元相圖，可以求得氮和鉻的深度組成關係及各種相的深度分佈情形。?SAE 4140 經電漿氮化後，其電位-pH 圖的均勻腐蝕區大幅縮小，其鈍態區相對變大；上述第?項結論可能是造成此一改變的主要因素。?氮化的SAE 4140的電位-pH 圖中，出現一迅速溶解區（PH:0?4，電位：-400mV?+100mV（SCE）。經過水溶液化學分析及熱力學計算，證實此現象是氮自氮化物中溶出而形成NH4+所引起。?電化學試驗結果顯示，氮化後維持氮化物產生鈍態所需之最小濃度由16M 降至8M。XPS 配合AES 的分析結果顯示，氮化SAE 4140 在8N至16N HNO3中產生鈍態之原因除了上述?所述外，可進一步說明如下：Fe的優先溶解造成N 和Cr的表面濃度增加，從而形成一多層鉻氧化物、鐵氧化物及氮化物之鈍態結構。孔隙中溶解的氮原子形成NH4+可以防止孔隙酸化效應，因此氮化SAE 4140 在低氯離子濃度水溶液中具有相當程度之抗孔蝕能力。經PEPVD 處理之氮化鈦塗層在硫酸溶液中之腐蝕研究有以下結論：?在0.1N-10N硫酸溶液中，TiN塗層的腐蝕速率隨濃度增加而增加；在10N-36N硫酸液中，腐蝕速率隨濃度增加而降低。?動力學的研究結果建議：在0.1N-10N 硫酸溶液中之陰極反應因HSO4之促進作用而加速。?在20-36N 濃度範圍，腐蝕速率很低，可以用一吸附模式加以說明。?XPS 試驗結果配合熱力學數據及理論Pourbaix 圖，可以鑑定腐蝕生成物並決定其腐蝕反應方程式。///////ABSTRACTFour engineering alloys of API J-55 steel, Type 304 and 410 stainlesssteels and titanium alloy were assessed in a low salinity geothermalsystem in Ching-Shui, Taiwan. By using electrochemical polarizations, theexperimental Pourbaix diagrma for each alloy have been constructed, inshich domains of immunity, general cor-rosion, passivation, and pittingare defined. The titanium alloy was completely re-sistant to pitting underall test conditions. the type 304 stainless steel was more resistant topitting than type 4`0 stainless steel due to a higher Cr content in theformer alloy. The low alloy steel of API J-55 was more susceptible togeneral corrosion and could only be readily passivated at pH value greaterthan 11.8.The biological corrosion of condenser tubes has been investigated byscanning Auger microprobe (SAM) techniques. A duration test was conductedat the inlet bay of Hsin-Da Power Plant in Taiwan to study the roles ofmacro-organisms and micro-organisms in facilitating or inhibiting themicrobiologically influenced corro-sion (MIC) of metal alloys in seawater.A significant amount of fouling organisms was grown on the attachmentplate submerged in the field for 556 and 92 days. Copper toxicity resultedin little fouling but slight corrosion still occurred on the copper plate.Titanium was found to be 100% fouled by the fouling organisms. Stainlesssteel plate was found to have the worst MIC corrosion among the testedsamples. SAM results indicate that sodium, oxygen and sulfur accumulatedon the MIC regions of the sample surfaces covered by the foulingorganisms, and chlorine and nitrogen were distributed in the regionscompensating for the local distribu-tions of sodium, oxygen and sulfur. Itwas proposed that the development of a thick macro-fouling mass will casuethe anaerobic zones to devlop on the metal surface, wherein the MICprocesses are accelerated.The sulfuric acid corrosion behavior of TiN on stainless steel, preparedby plasma enhanced physical vapor deposition(PEPVD), has been studied byusing the electrochemical techniques indluding dynamic polarization, opencircuit poten-tial (OCP) and eletrochemical impedance spectrscopy(EIS)measurements. It is found that the corrosion of TiN strongly dependson the concentration of sulfuric acid.In 0.1-10N H2SO4, the corrosion rateincreases as H2SO4 concentration is increased. Kinetics study proposesthat the cathodic reaction is stimulated by the H2SO4 kons in 0.1-10 NH2SO4. In 20-36 N H2SO4, the corrosion rates are much lower than those ofin 0.1-20 N solution. This result suggestes that TiN is spontaneouslypassivated in such concentrated solution. An XPS analysis andthermodynamic data have been combined to identify the chemical states ofcorro-sion products and to determine the corrosion reactions.The effects of nitriding on corrosion was investigated by performingelectro-chemical tests on toth nitrided and untreated SAE 4140. Afternitriding, the cor-rosion resistance is improved significantly in HNO3 andNa2SO4 aqueous envi-ronments. The minimum HNO3 concertration forpassivating the 4140 steel was ronments. The minimum HNO3 concentrationfor passivating the 4140 steel was found to be reduced from 16M to 8 M.Areaction model is proposed to explain the beneficial effect of nitride oncorrosion resistance. It is concluded that N and Cr (an alloying element)act synergisticaly to form a ense protective layer which is responsiblefor the corrosion resistance. Characteriation of the surface layers by AESand XPS reveals that the protective layer is composed of γ'-(Fe,Cr)4N, ε- (Fe,Cr)2-3 N and CrN in the inner layer, Fe2O3, Cr2O3 together withnitrides in the middle layer and nitrides, γ'-FeOOH,and Cr(OH)3H2O in theouter-most layer.Two experimental potential-pH diagrams are also constructed and comparedin terms of the effect of nitrided nitrogen on the corrosion behavior ofthe steel. Some prominent results are drawn as follows:(1) After nitriding, the domain of general corrosion of 4140 steel isreduced and its passivation, on the contrary, is broadened. Theinvorporation of nitrided nitrogen in Fe2O3 to strengthen the passivity of4140 steel is responsible for the enlargement of the passivation region.(2) An impediment to the anodic dissolution of the nitrided 4140 steel hasbeen observed and is accounted for the enrichment of nitrogen atoms on theactive sites.(3)A subdomain in general corrosion with a range of potentials from -0.4Vto 0.1V(SCE) and a range of pH from 0 to 4 for the nitrided 4140 steel wasfound to be corresponding to the rapid dissolution of the oxy-nitridedlayer.