本文關鍵詞： 不銹鋼 硝酸 晶間腐蝕 電化學阻抗譜 Mott-Shottky曲線 流動腐蝕　出處：《北京化工大學》2017年博士論文　論文類型：學位論文

【摘要】：304和304L不銹鋼在硝酸介質中具有優(yōu)異的耐蝕性,因此,被廣泛地應用在硝酸生產裝置和管道中。但是不銹鋼管道的焊縫腐蝕失效和局部的沖刷腐蝕失效,一直是困擾我國硝酸生產裝置的問題。本文針對晶間腐蝕和流動腐蝕這兩個問題,探究了不同條件下304不銹鋼在硝酸中的腐蝕機理。首先,分析了不銹鋼硝酸輸送管道的焊縫發(fā)生腐蝕失效的原因,并利用熱處理方法模擬不銹鋼焊接過程中的受熱過程,研究了加熱溫度和時間對不銹鋼在硝酸中的耐蝕性影響規(guī)律,解釋不銹鋼焊縫熱影響區(qū)金屬的敏化行為。通過極化曲線、電化學阻抗譜、Mott-Shottky曲線和浸泡失重實驗,研究了 304不銹鋼在硝酸中腐蝕特性隨硝酸濃度、溫度和浸泡時間變化規(guī)律。此外,通過自主研制的管流式流動腐蝕實驗裝置對不銹鋼在流動條件下腐蝕電化學行為進行了研究。通過以上研究和實驗,主要得出以下結論:(1)三通母材和直管母材表面發(fā)生較為均勻的晶間腐蝕。三通母材和直管母材的Cr和Ni含量基本相同,且各自的腐蝕電流密度和阻抗值差別不大,兩種母材的耐蝕性良好。但三通母材的碳含量是直管母材碳含量的1.8倍,在焊接的熱影響區(qū),兩種材料的耐蝕性差別較大,熱影響區(qū)的材料的腐蝕速率明顯大于母材,三通一側熱影響區(qū)的腐蝕速率顯著大于其他區(qū)域。碳含量是影響304不銹鋼焊接接頭在硝酸中耐蝕性的關鍵因素。碳含量越高,焊接過程中熱影響區(qū)的不銹鋼越容易發(fā)生敏化,導致耐蝕性顯著降低。(2)當加熱溫度處在500-900℃時,即使短暫時間的加熱也會導致304L不銹鋼晶間腐蝕敏化度的顯著增大。當加熱溫度在650-750℃C區(qū)間時,304L不銹鋼的晶間腐蝕敏化度達到最大值。加熱溫度低于500℃C或者高于900℃C時,不銹鋼在硝酸中的晶間腐蝕敏化度的變化很小,耐蝕性不會顯著降低。通過電化學阻抗譜的變化可以區(qū)分經(jīng)過不同敏化度的不銹鋼試樣,晶間腐蝕敏化度高的試樣的反應電阻Rct變小,不銹鋼鈍化膜的完整性和保護性降低。電化學阻抗測試和DL-EPR測試得到的熱處理對304L不銹鋼耐蝕性的影響規(guī)律基本吻合。304和304L不銹鋼在經(jīng)650℃C熱處理后,鈍化膜表面界面電容和雙電層電容達到最大,在硝酸介質中其鈍化膜的保護能力變差,耐蝕性明顯下降。且隨保溫時間的增大,其在硝酸中的腐蝕裂紋內的溶液電阻與腐蝕界面的電荷轉移電阻也逐漸減小。(3) 304不銹鋼在不同溫度和濃度硝酸中的腐蝕機理基本相同。隨著硝酸溫度和濃度的增大,304不銹鋼在硝酸中的自腐蝕電位呈逐漸增大的趨勢,鈍化區(qū)的范圍逐漸減小,維鈍電流密度逐漸增大,腐蝕速率逐漸增大,不銹鋼在硝酸中的腐蝕越趨于活化腐蝕,不銹鋼發(fā)生過鈍化的趨勢越明顯。硝酸溫度越高,維持304不銹鋼鈍化膜的穩(wěn)定狀態(tài)越困難。隨著硝酸濃度的增大,反應電阻逐漸減小,鈍化膜的溶解速度增大,鈍化膜對不銹鋼基體的保護作用逐漸減弱。Mott-Schottky曲線分析發(fā)現(xiàn),不銹鋼表面的鈍化膜呈現(xiàn)P型半導體特性。隨著硝酸濃度的增大,線性區(qū)的斜率逐漸增大,載流子的濃度先減小后增大。隨著溫度的升高,線性區(qū)的斜率逐漸變小,載流子濃度逐漸增大,平帶電位負移。隨著浸泡時間的增加,304不銹鋼容抗弧半徑逐漸減小,鈍化膜對金屬基體的保護性能逐漸降低。不銹鋼表面的鈍化膜在硝酸中的溶解速度在浸泡初期較小,隨著浸泡時間的增大,鈍化膜的溶解速度先以較快的速度增大,最后達到穩(wěn)定值。304不銹鋼的表面在硝酸中形成的鈍化膜的成分主要為Fe-Cr-Ni等金屬化合物,即σ相。(4)不銹鋼在流動硝酸中存在鈍化行為,在浸入流動硝酸后不銹鋼表面的鈍化膜不斷發(fā)展,4h后鈍化膜逐漸趨于穩(wěn)定。在流動硝酸中,不銹鋼的腐蝕速率隨著溫度、流速和沖擊角的增大而逐漸增大。隨著溫度的升高,鈍化膜的厚度逐漸減小,致密和完整程度變低。硝酸溫度增大,鈍化區(qū)范圍變窄,過鈍化電位變小,不銹鋼在流動硝酸中的鈍化行為不穩(wěn)定,更容易發(fā)生過鈍化,導致晶間腐蝕的發(fā)生。隨著流速的增大,鈍化膜的厚度逐漸減小,鈍化膜對不銹鋼的保護作用逐漸減弱。速度對不銹鋼的自腐蝕電位、鈍化區(qū)間范圍和過鈍化行為影響不大,流速增大,鈍化電流密度增大,說明流速增大,鈍化膜的溶解速率增大。不銹鋼在流動硝酸中的耐蝕性,隨著沖擊角的增大逐漸變小。沖擊角的變化,主要影響不銹鋼在硝酸中致鈍電位。致鈍電位和鈍化電流密度的增加,導致鈍化膜受到破壞或者出現(xiàn)缺陷時的自我修復能力變弱。不銹鋼在硝酸中的流動腐蝕特性主要受其表面靜壓力分布情況的影響,在靜壓力大的區(qū)域,沖刷腐蝕較為嚴重。
[Abstract]:304 and 304L stainless steel in nitric acid medium has excellent corrosion resistance, therefore, is widely used in the production of nitric acid equipment and pipeline. But the failure of weld corrosion of stainless steel pipe and local erosion corrosion failure, has been plagued by nitric acid production equipment in our country. The problem of the intergranular corrosion and corrosion of the two flow the problem, explore the corrosion mechanism under different conditions of 304 stainless steel in nitric acid. First, analyzed the causes of corrosion failure of weld stainless steel nitrate pipeline, and the heat treatment method of heating process to simulate the welding process of stainless steel, corrosion resistance of stainless steel in nitric acid of heating temperature and time of interpretation stainless steel weld heat affected zone. The sensitization of metal polarization curves, electrochemical impedance spectroscopy, Mott-Shottky curve and immersion weight-loss experiment, studied 304 stainless steel in Corrosion characteristics of nitric acid with nitric acid concentration, temperature and soaking time. In addition, the electrochemical corrosion behavior of stainless steel in flow conditions were investigated by self-made tube flow corrosion test device. Through the above research and experiments, the main conclusions are as follows: (1) the uniform intergranular corrosion of three the parent material and pipe base material surface occur. Cr and Ni content of three parent material and straight base material is basically the same, and the corrosion current density and the impedance values of the respective difference, the corrosion resistance of two kinds of parent material is good. But the carbon content of three parent material is 1.8 times straight carbon content of the base material. In the heat affected zone of welding, corrosion resistance difference of two kinds of materials is larger, the heat affected zone of the material corrosion rate was significantly greater than that of the parent material, effects of three side heat corrosion rate area was significantly larger than in other regions. Carbon content of 304 stainless steel welding joint The key factor in the nitric acid corrosion resistance. The carbon content is high, the welding process of stainless steel heat affected zone is more prone to cause sensitization, corrosion resistance decreased significantly. (2) when the heating temperature is 500-900 degrees, even a short time heating can lead to significant increase of 304L stainless steel intergranular corrosion sensitivity degree when. The heating temperature of 650-750 degrees in the range of C, 304L stainless steel intergranular corrosion sensitivity degree reaches the maximum value. The heating temperature is below 500 DEG C or higher than 900 DEG C, the change of stainless steel in nitric acid in the intergranular corrosion susceptibility degree of corrosion resistance is very small, not significantly reduced. By changing the electrochemical impedance spectra can be distinguished by stainless steel samples with different degree of sensitivity, Rct reaction resistance to intergranular corrosion sensitivity of the high degree of sample decreases, integrity and protection of the passive film on stainless steel to reduce heat. By electrochemical impedance test and DL-EPR test in 30 Influence of corrosion resistance of 4L stainless steel and 304L stainless steel in.304 basically is 650 DEG C after C heat treatment, surface passivation film capacitor and electric double layer capacitor reaches the maximum in the medium of nitric acid in protecting its passivation film poor corrosion resistance decreased significantly. And with the holding time increasing, it decreases gradually the charge in nitric acid corrosion in the cracks and corrosion resistance of the solution interface transfer resistance. (3) the corrosion mechanism of 304 stainless steel in different temperature and concentration of nitric acid is basically the same. With the increase of nitrate concentration and temperature of 304 stainless steel in nitric acid, the corrosion potential increased gradually, the scope of the passivation region decreases, the passivation current density increases, the corrosion rate increases, corrosion of stainless steel in nitric acid more activated corrosion, passivation of stainless steel had a more obvious trend. Nitrate with higher temperature, maintain 304 Steady state stainless steel passivation film more difficult. With the increase of nitric acid concentration, reaction resistance decreases, the dissolution rate of passive film increases, the protective effect of passivation film on stainless steel substrate decreased.Mott-Schottky curve analysis, the passivation film on the surface of stainless steel P type semiconductor characteristics. With the increase of nitric acid concentration, the slope of the linear region gradually increased, the carrier concentration decreased and then increased. With the increase of temperature, the slope of the linear region becomes smaller, the carrier concentration increases gradually, the flat band potential negative shift. With the increase of immersion time, 304 stainless steel capacitance arc radius decreases and the protection performance of passivation film on metal substrate decreased. The dissolution rate of passive film the surface of stainless steel in nitric acid. In the early immersion period, with the increase of immersion time, the dissolution rate of passive film with fast speed increases, and finally achieve the The passivation film formed on the surface of.304 stainless steel in nitric acid stable value is the main component of Fe-Cr-Ni metal compounds, namely the sigma phase. (4) there are stainless steel passivation behavior in the flow of nitrate, the continuous development of the passivation film on the surface of stainless steel after immersed in flowing nitrate, 4h passivation film is becoming more and more stable. In the flow of nitric acid the corrosion rate of stainless steel, with the increase of temperature, velocity and impact angle increases. With the increase of temperature, the passivation film thickness decreases, dense and complete degree is low. The nitrate temperature increased, passive range narrows over the passivation potential is smaller, the passivation behavior of stainless steel in nitric acid flow instability. More prone to passivation, leads to intergranular corrosion. With the increase of flow rate, film thickness decreases and the protective effect of passivation film on stainless steel decreased. The corrosion potential of stainless steel speed, The range and passivation passivation behavior has little effect on the velocity increases and the passivation current density increases, indicating the velocity increases and the dissolution rate of passive film increases. The corrosion resistance of stainless steel in nitric acid in the flow with the increase, the impact angle decreases. The changes of impact angle, the main impact of stainless steel in nitric acid passivation potential increase. Passive potential and passive current density, leading to self repair ability of the passivation film is damaged or faulty becomes weak. Influence of flow induced corrosion properties of stainless steel in nitric acid is mainly controlled by the distribution of surface static pressure, static pressure in large area, erosion corrosion is more serious.

【學位授予單位】：北京化工大學
【學位級別】：博士
【學位授予年份】：2017
【分類號】：TQ111.2;TQ050.9

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