不锈钢硝酸输送管道的腐蚀行为及特性研究

发布时间：2018-02-26 10:31

本文关键词： 不锈钢硝酸晶间腐蚀电化学阻抗谱 Mott-Shottky曲线流动腐蚀　出处：《北京化工大学》2017年博士论文　论文类型：学位论文

【摘要】：304和304L不锈钢在硝酸介质中具有优异的耐蚀性,因此,被广泛地应用在硝酸生产装置和管道中。但是不锈钢管道的焊缝腐蚀失效和局部的冲刷腐蚀失效,一直是困扰我国硝酸生产装置的问题。本文针对晶间腐蚀和流动腐蚀这两个问题,探究了不同条件下304不锈钢在硝酸中的腐蚀机理。首先,分析了不锈钢硝酸输送管道的焊缝发生腐蚀失效的原因,并利用热处理方法模拟不锈钢焊接过程中的受热过程,研究了加热温度和时间对不锈钢在硝酸中的耐蚀性影响规律,解释不锈钢焊缝热影响区金属的敏化行为。通过极化曲线、电化学阻抗谱、Mott-Shottky曲线和浸泡失重实验,研究了 304不锈钢在硝酸中腐蚀特性随硝酸浓度、温度和浸泡时间变化规律。此外,通过自主研制的管流式流动腐蚀实验装置对不锈钢在流动条件下腐蚀电化学行为进行了研究。通过以上研究和实验,主要得出以下结论:(1)三通母材和直管母材表面发生较为均匀的晶间腐蚀。三通母材和直管母材的Cr和Ni含量基本相同,且各自的腐蚀电流密度和阻抗值差别不大,两种母材的耐蚀性良好。但三通母材的碳含量是直管母材碳含量的1.8倍,在焊接的热影响区,两种材料的耐蚀性差别较大,热影响区的材料的腐蚀速率明显大于母材,三通一侧热影响区的腐蚀速率显著大于其他区域。碳含量是影响304不锈钢焊接接头在硝酸中耐蚀性的关键因素。碳含量越高,焊接过程中热影响区的不锈钢越容易发生敏化,导致耐蚀性显著降低。(2)当加热温度处在500-900℃时,即使短暂时间的加热也会导致304L不锈钢晶间腐蚀敏化度的显著增大。当加热温度在650-750℃C区间时,304L不锈钢的晶间腐蚀敏化度达到最大值。加热温度低于500℃C或者高于900℃C时,不锈钢在硝酸中的晶间腐蚀敏化度的变化很小,耐蚀性不会显著降低。通过电化学阻抗谱的变化可以区分经过不同敏化度的不锈钢试样,晶间腐蚀敏化度高的试样的反应电阻Rct变小,不锈钢钝化膜的完整性和保护性降低。电化学阻抗测试和DL-EPR测试得到的热处理对304L不锈钢耐蚀性的影响规律基本吻合。304和304L不锈钢在经650℃C热处理后,钝化膜表面界面电容和双电层电容达到最大,在硝酸介质中其钝化膜的保护能力变差,耐蚀性明显下降。且随保温时间的增大,其在硝酸中的腐蚀裂纹内的溶液电阻与腐蚀界面的电荷转移电阻也逐渐减小。(3) 304不锈钢在不同温度和浓度硝酸中的腐蚀机理基本相同。随着硝酸温度和浓度的增大,304不锈钢在硝酸中的自腐蚀电位呈逐渐增大的趋势,钝化区的范围逐渐减小,维钝电流密度逐渐增大,腐蚀速率逐渐增大,不锈钢在硝酸中的腐蚀越趋于活化腐蚀,不锈钢发生过钝化的趋势越明显。硝酸温度越高,维持304不锈钢钝化膜的稳定状态越困难。随着硝酸浓度的增大,反应电阻逐渐减小,钝化膜的溶解速度增大,钝化膜对不锈钢基体的保护作用逐渐减弱。Mott-Schottky曲线分析发现,不锈钢表面的钝化膜呈现P型半导体特性。随着硝酸浓度的增大,线性区的斜率逐渐增大,载流子的浓度先减小后增大。随着温度的升高,线性区的斜率逐渐变小,载流子浓度逐渐增大,平带电位负移。随着浸泡时间的增加,304不锈钢容抗弧半径逐渐减小,钝化膜对金属基体的保护性能逐渐降低。不锈钢表面的钝化膜在硝酸中的溶解速度在浸泡初期较小,随着浸泡时间的增大,钝化膜的溶解速度先以较快的速度增大,最后达到稳定值。304不锈钢的表面在硝酸中形成的钝化膜的成分主要为Fe-Cr-Ni等金属化合物,即σ相。(4)不锈钢在流动硝酸中存在钝化行为,在浸入流动硝酸后不锈钢表面的钝化膜不断发展,4h后钝化膜逐渐趋于稳定。在流动硝酸中,不锈钢的腐蚀速率随着温度、流速和冲击角的增大而逐渐增大。随着温度的升高,钝化膜的厚度逐渐减小,致密和完整程度变低。硝酸温度增大,钝化区范围变窄,过钝化电位变小,不锈钢在流动硝酸中的钝化行为不稳定,更容易发生过钝化,导致晶间腐蚀的发生。随着流速的增大,钝化膜的厚度逐渐减小,钝化膜对不锈钢的保护作用逐渐减弱。速度对不锈钢的自腐蚀电位、钝化区间范围和过钝化行为影响不大,流速增大,钝化电流密度增大,说明流速增大,钝化膜的溶解速率增大。不锈钢在流动硝酸中的耐蚀性,随着冲击角的增大逐渐变小。冲击角的变化,主要影响不锈钢在硝酸中致钝电位。致钝电位和钝化电流密度的增加,导致钝化膜受到破坏或者出现缺陷时的自我修复能力变弱。不锈钢在硝酸中的流动腐蚀特性主要受其表面静压力分布情况的影响,在静压力大的区域,冲刷腐蚀较为严重。
[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.

【学位授予单位】：北京化工大学
【学位级别】：博士
【学位授予年份】：2017
【分类号】：TQ111.2;TQ050.9

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