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After 24 h of free corrosion, the impedance was lowest in the sample without pigment, which reveals the higher activity of this system, and another time constant appeared at the high frequencies. Fitting of the spectra was made using the equiv-alent circuit in Fig. 8, where the high frequency impedance (1 kΩ cm2) is due to the electrolytic solution at the scribe.RHF and QHF describe the high frequency process, and Rct and Qdl correspond to the charge transfer resistance and the double layer capacitance, respectively. Finally, a component describing the mass transfer impedance was considered in some cases in order to account for a small tail appearing below 0.1 Hz. The high frequency process had a resistance of 5–10 kΩ cm2, as shown in Fig. 9, whereas the Y0 for this process has a value of roughly 10−8 to 10−5 Fcm−2. This capacitance is too high for a polymer film, which generally has values in the range of 10−10to 10−8Fcm−2.Further, in the presence of chromate Y0 decreases with time, a behaviour that is the opposite to the known evolution of coating capacitance values (which tend to increase due to water uptake). This high frequency time constant can how-ever be explained by the formation of the passivating film, of either chromium hydroxide or zinc phosphate. The formation of this film starts in the first minutes of exposure, possibly with the precipitation of a monolayer, and grows afterwards, during the first 24–48 h. The low frequency process gives Rct = 20–80 kΩ cm2 and is maximum for the chromate. The double layer CPE grows with time, revealing the growth of the active area underneath the coating, i.e., delamination. The ratio of Y0 with and without pigment is roughly 0.1, mean-ing that the loss of adhesion was weaker in samples with pigment. The delamination rate, estimated from the CPE, was minimum in the epoxychromate coating, followed by the epoxyphosphate and finally by the clear coat. This was the same rate that was determined by visual inspection of the samples at the end of the test. The SVET was also used to study the ionic current distribution above the scribed samples, for the three coatings in 0.1 M NaCl. The evolution of the current maps and the current density values have not show any significant differ-ences among the specimens with the various coatings. As an example, Fig. 10 shows SVET measurements of the scribed HDG sample, coated with the clear epoxy paint. In the first 2–3 days of immersion there was a clear separation between the cathode and the anode along the scribe ( Fig. 10 a) and there were no signs of delamination in the micro-video im-age. For longer exposure times, however, the anodic activity along the scribe increased and the signs of cathodic activity disappeared from the map, as delamination progressed away from the scribe ( Fig. 10 b) |
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金蟲 (著名寫手)
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RXMCDM: 金幣+1 2014-05-13 13:39:48
RXMCDM: 金幣+1 2014-05-13 13:39:48
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僅供參考 不帶顏料的樣品自然腐蝕24小時之后電阻最低,顯示了這一系統(tǒng)的較高活性，另一個時間常數(shù)出現(xiàn)在高頻段。 使用圖8的等效電路進行光譜擬合，其中的高頻阻抗(1kΩcm2)歸因于劃線處的電解液。 RHF和QHF描述高頻過程，Rct和Qdl分別對應于電荷轉移電阻和雙層電容。 最后，表示傳質電阻分量有時被認為是為了說明于低于0.1 Hz處出現(xiàn)一個小拖尾的原因。 如圖9中所示這種高頻過程具有一個5–10 kΩcm2的電阻,而這一過程的Y0約為10?8到10?5 Fcm?2。。 這種電容，通常值為10?10至10?8Fcm?2，對于高分子膜來說過高。 而且，在鉻酸鹽存在下Y0隨時間而減小,一個與已知的涂層電容值變化（這往往由于吸水而增大）相反的特性。 然而這種高頻率時間常數(shù)可以用或者是氫氧化鉻，或者是磷酸鋅鈍化膜的形成來解釋。 這種薄膜的形成始于暴露的第一分鐘,也許會有單層的沉淀，后來在最初24–48小時期間逐漸成長。低頻過程得出Rct = 2080 kΩ_cm2,為鉻酸鹽最大值。雙層CPE隨時間而逐漸加大,表明涂層下活動面積的增加，亦即,剝離。帶有和不帶有顏料Y0的比值約為0.1,這意味著帶有顏料的樣品的粘附力的損失較小。由CPE測定,剝離比例最少的是環(huán)氧鉻酸鹽涂層,其次是環(huán)氧磷酸鹽，最后是透明涂層。這與在試驗結束時樣品的目測檢查測定的結果相同。 SVET也用來對上述劃線樣品的于0.1 M NaCl中的三個涂層的離子電流分配的研究。 電流圖和電流密度值的變化沒有顯示出各種涂層樣品之間任何顯著差異。 例如,圖10顯示涂有透明環(huán)氧樹脂涂料的劃線HDG樣品的SVET測定結果。 在浸入開始的2–3當中陰極和陽極之間沿著劃線（圖10a）有一個明確的分離，在顯微視頻圖像中沒有任何剝離的跡象。 然而，暴露時間更長時沿著劃線陽極活度持續(xù)增加并且陰極活動性有從圖上消失的跡象,正如遠離劃線剝離的發(fā)展那樣（圖10 b） |
4樓2014-05-12 11:08:35
RXMCDM: 不想應助就算了！ 2014-05-12 09:16:51
|
After 24 h of free corrosion, the impedance was lowest in the sample without pigment, which reveals the higher activity of this system, and another time constant appeared at the high frequencies. Fitting of the spectra was made using the equiv-alent circuit in Fig. 8, where the high frequency impedance (1 kΩ cm2) is due to the electrolytic solution at the scribe.RHF and QHF describe the high frequency process, and Rct and Qdl correspond to the charge transfer resistance and the double layer capacitance, respectively. Finally, a component describing the mass transfer impedance was considered in some cases in order to account for a small tail appearing below 0.1 Hz. The high frequency process had a resistance of 5–10 kΩ cm2, as shown in Fig. 9, whereas the Y0 for this process has a value of roughly 10−8 to 10−5 Fcm−2. This capacitance is too high for a polymer film, which generally has values in the range of 10−10to 10−8Fcm−2.Further, in the presence of chromate Y0 decreases with time, a behaviour that is the opposite to the known evolution of coating capacitance values (which tend to increase due to water uptake). This high frequency time constant can how-ever be explained by the formation of the passivating film, of either chromium hydroxide or zinc phosphate. The formation of this film starts in the first minutes of exposure, possibly with the precipitation of a monolayer, and grows afterwards, during the first 24–48 h. The low frequency process gives Rct = 20–80 kΩ cm2 and is maximum for the chromate. The double layer CPE grows with time, revealing the growth of the active area underneath the coating, i.e., delamination. The ratio of Y0 with and without pigment is roughly 0.1, mean-ing that the loss of adhesion was weaker in samples with pigment. The delamination rate, estimated from the CPE, was minimum in the epoxychromate coating, followed by the epoxyphosphate and finally by the clear coat. This was the same rate that was determined by visual inspection of the samples at the end of the test. The SVET was also used to study the ionic current distribution above the scribed samples, for the three coatings in 0.1 M NaCl. The evolution of the current maps and the current density values have not show any significant differ-ences among the specimens with the various coatings. As an example, Fig. 10 shows SVET measurements of the scribed HDG sample, coated with the clear epoxy paint. In the first 2–3 days of immersion there was a clear separation between the cathode and the anode along the scribe ( Fig. 10 a) and there were no signs of delamination in the micro-video im-age. For longer exposure times, however, the anodic activity along the scribe increased and the signs of cathodic activity disappeared from the map, as delamination progressed away from the scribe ( Fig. 10 b) |
2樓2014-05-11 21:49:59
3樓2014-05-12 08:25:44
5樓2014-05-12 18:52:56
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