鎂合金絲狀腐蝕動態(tài)生長過程的原位研究
發(fā)布時間：2018-09-13 16:31
【摘要】：鎂合金具有許多優(yōu)異的性能,但化學性質(zhì)活潑,易發(fā)生腐蝕破壞。鎂合金部件的腐蝕失效幾乎全部是由局部腐蝕引起的,絲狀腐蝕作為一種常見的局部腐蝕形態(tài),破壞性和隱患性非常大。如果鎂合金的絲狀腐蝕得到抑制,它的使用安全性將得到極大改善,這就需要對鎂合金絲狀腐蝕的機制有清楚的認識。本文以Mg-3Zn和ZG21兩種鎂合金為研究對象,使用掃描電子顯微鏡(Scanning Electron Microscope,SEM)、掃描振動電極技術(shù)(Scanning Vibrating Electrode Technique,SVET)、X射線光子能譜分析(X-ray Photoelectron Spectroscopy,XPS)、電化學測試和浸泡等實驗方法確定了微觀結(jié)構(gòu)和腐蝕介質(zhì)對腐蝕絲的萌生和發(fā)展的影響,明確了鎂合金絲狀腐蝕發(fā)展的控制因素,并澄清了腐蝕絲沿水平方向發(fā)展的原因。通過研究微觀結(jié)構(gòu)和腐蝕環(huán)境對絲狀腐蝕的影響發(fā)現(xiàn),第二相比較小的鎂合金,它的腐蝕屏蔽作用比較小,導致腐蝕絲遇到第二相后會穿過第二相繼續(xù)發(fā)展。絲狀腐蝕的發(fā)展主要與Cl~-有關(guān),加入F-,或提高pH值能延緩腐蝕絲的出現(xiàn)。通過SVET原位觀察,發(fā)現(xiàn)絲狀腐蝕過程中的腐蝕微電池隨腐蝕的發(fā)展是動態(tài)變化的;外加陽極電位,表面有較多腐蝕絲出現(xiàn),且外加的陽極電位越正,腐蝕越嚴重。相反,外加陰極電位,沒有腐蝕出現(xiàn)。由此確定,鎂合金絲狀腐蝕生長過程為陽極控制。對比了在NaCl溶液中的絲狀腐蝕和Na_2SO_4溶液中的點蝕,發(fā)現(xiàn)Cl~-會在腐蝕絲的絲頭處聚集,而SO_4~(2-)在點蝕坑底部聚集,這與在兩種溶液中形成的表面膜有關(guān)。Cl~-的穿透性比較強,會在表面膜比較薄的地方優(yōu)先吸附,導致腐蝕萌生。腐蝕萌生后,會使該處膜層破壞失效,并生成很疏松的腐蝕產(chǎn)物,腐蝕產(chǎn)物具有毒化效應,大量的Cl~-會更易在該處聚集。相對于破損膜層下面的鎂基體,破損膜層周圍的完好膜層處也會更容易聚集一些Cl~-,因此腐蝕更易沿著完好膜層水平擴展。然而,在膜層破損區(qū)域的分布并不均勻,會出現(xiàn)某處Cl~-濃度高的情況,腐蝕就會沿著Cl~-濃度高的方向向前擴展,一旦腐蝕絲向某個方向擴展后會形成絲尾,Cl~-很容易沿著由疏松腐蝕產(chǎn)物組成的絲尾處向絲頭處傳輸,而未被腐蝕的區(qū)域Cl~-相對更難傳輸?shù)浇z頭處,因此腐蝕絲就會沿著絲尾的方向向前生長,最終導致腐蝕絲沿水平方向發(fā)展。
[Abstract]:Magnesium alloys have many excellent properties, but their chemical properties are lively and easy to corrode. Almost all the corrosion failure of magnesium alloy parts is caused by local corrosion. As a common local corrosion form, filamentary corrosion is very destructive and hidden. If the filiform corrosion of magnesium alloy is restrained, the safety of magnesium alloy wire will be greatly improved, which requires a clear understanding of the mechanism of magnesium alloy wire corrosion. In this paper, two kinds of magnesium alloys, Mg-3Zn and ZG21, are studied. By means of scanning electron microscope (Scanning Electron Microscope,SEM), scanning vibrating electrode (Scanning Vibrating Electrode Technique,SVET), X-ray photon spectroscopy (X-ray Photoelectron Spectroscopy,XPS), electrochemical measurement and immersion, the effects of microstructure and corrosion medium on the initiation and development of corrosion wire were determined. The controlling factors for the development of magnesium alloy wire corrosion were clarified, and the reasons for the development of the corrosion wire along the horizontal direction were clarified. By studying the effect of microstructure and corrosion environment on the filamentous corrosion, it is found that the magnesium alloy with smaller second phase has less corrosion shielding effect, which leads to the further development of the second phase after the corrosion wire encounters the second phase. The development of filamentous corrosion is mainly related to Cl~-. Adding F-or increasing pH value can delay the appearance of corrosion wire. In situ observation by SVET, it is found that the corrosion microcells in filamentous corrosion process change dynamically with the development of corrosion, and that there are more corrosion wires on the surface of the applied anode potential, and the more positive the additional anode potential, the more serious the corrosion is. On the contrary, no corrosion occurs when the cathode potential is applied. Therefore, the corrosion growth process of magnesium alloy wire is controlled by anode. By comparing the filamentous corrosion in NaCl solution with pitting corrosion in Na_2SO_4 solution, it is found that Cl~- aggregates at the wire head and SO_4~ (2-) aggregates at the bottom of the pitting pit, which is related to the penetrability of the surface film formed in the two solutions. It will preferentially adsorb on the surface where the film is thinner, resulting in corrosion initiation. The corrosion initiation results in the failure of the film and the formation of very loose corrosion products. The corrosion products have toxic effect and a large number of Cl~- will easily accumulate in the area. Compared with the magnesium substrate beneath the damaged film, it is easier for the intact film to gather some Cl~-, around the damaged film, so the corrosion is easier to expand along the level of the intact film. However, the distribution of the damaged area of the film is not uniform, and there will be a situation where the concentration of Cl~- is high, and the corrosion will spread along the direction of the high concentration of Cl~-. Once the corroded wire expands in a certain direction, it forms the tail of the wire, which is easily transported along the tail of the loose corrosion product to the wire head, and the Cl~- in the uncorroded area is relatively difficult to transmit to the wire head. Therefore, the corrosion wire will grow forward along the tail of the wire and eventually lead to the development of the corrosion wire along the horizontal direction.
【學位授予單位】：沈陽工業(yè)大學
【學位級別】：碩士
【學位授予年份】：2017
【分類號】：TG178
本文編號：2241718
[Abstract]:Magnesium alloys have many excellent properties, but their chemical properties are lively and easy to corrode. Almost all the corrosion failure of magnesium alloy parts is caused by local corrosion. As a common local corrosion form, filamentary corrosion is very destructive and hidden. If the filiform corrosion of magnesium alloy is restrained, the safety of magnesium alloy wire will be greatly improved, which requires a clear understanding of the mechanism of magnesium alloy wire corrosion. In this paper, two kinds of magnesium alloys, Mg-3Zn and ZG21, are studied. By means of scanning electron microscope (Scanning Electron Microscope,SEM), scanning vibrating electrode (Scanning Vibrating Electrode Technique,SVET), X-ray photon spectroscopy (X-ray Photoelectron Spectroscopy,XPS), electrochemical measurement and immersion, the effects of microstructure and corrosion medium on the initiation and development of corrosion wire were determined. The controlling factors for the development of magnesium alloy wire corrosion were clarified, and the reasons for the development of the corrosion wire along the horizontal direction were clarified. By studying the effect of microstructure and corrosion environment on the filamentous corrosion, it is found that the magnesium alloy with smaller second phase has less corrosion shielding effect, which leads to the further development of the second phase after the corrosion wire encounters the second phase. The development of filamentous corrosion is mainly related to Cl~-. Adding F-or increasing pH value can delay the appearance of corrosion wire. In situ observation by SVET, it is found that the corrosion microcells in filamentous corrosion process change dynamically with the development of corrosion, and that there are more corrosion wires on the surface of the applied anode potential, and the more positive the additional anode potential, the more serious the corrosion is. On the contrary, no corrosion occurs when the cathode potential is applied. Therefore, the corrosion growth process of magnesium alloy wire is controlled by anode. By comparing the filamentous corrosion in NaCl solution with pitting corrosion in Na_2SO_4 solution, it is found that Cl~- aggregates at the wire head and SO_4~ (2-) aggregates at the bottom of the pitting pit, which is related to the penetrability of the surface film formed in the two solutions. It will preferentially adsorb on the surface where the film is thinner, resulting in corrosion initiation. The corrosion initiation results in the failure of the film and the formation of very loose corrosion products. The corrosion products have toxic effect and a large number of Cl~- will easily accumulate in the area. Compared with the magnesium substrate beneath the damaged film, it is easier for the intact film to gather some Cl~-, around the damaged film, so the corrosion is easier to expand along the level of the intact film. However, the distribution of the damaged area of the film is not uniform, and there will be a situation where the concentration of Cl~- is high, and the corrosion will spread along the direction of the high concentration of Cl~-. Once the corroded wire expands in a certain direction, it forms the tail of the wire, which is easily transported along the tail of the loose corrosion product to the wire head, and the Cl~- in the uncorroded area is relatively difficult to transmit to the wire head. Therefore, the corrosion wire will grow forward along the tail of the wire and eventually lead to the development of the corrosion wire along the horizontal direction.
【學位授予單位】：沈陽工業(yè)大學
【學位級別】：碩士
【學位授予年份】：2017
【分類號】：TG178
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