Alternatif akım (A.A) empedans tekniği ve bitüm kaplamalara uygulanması koşullarının yapı ve özellikler üzerindeki etkisi

Abstract

Bu çalışmada, son yıllarda geniş bir uygulama alanı bulan alternatif âkım (A. A) empedans tekniği detaylı ola rak tanıtılmış ve korozyon alanındaki uygulamalarına ör nekler verilmiştir. Deneysel çalışmalar için üç farklı kaynaktan temin edilen (A.B ve C kodlu) Bitüm numuneleri ile kaplı ikişer adet çelik levha kullanılmıştır. Deneyler % 3'lük NaCl çözeltisinde, oda sıcaklığında ve sürenin fonksiyonu ola rak gerçekleştirilmiştir. Numunelerin önce serbest potan siyelleri ölçülmüş sonra 1D -0.03Hz aralığında, 5m.\/-gen- lik ile uygulanan alternatif akım-voltaja karşı empedans davranışları tespit edilmiştir. Deneylerde "model 37BEG ve G Princeton Applied Research Corporation?' A. A empedans ölçüm sistemi kullanılmıştır. Bu sistem potansiyostat (model 273), Lock-in amplifikatör (Model 5301), bilgisa-- yar (Apple II) ve yazıcıdan oluşmaktadır. Deney sonuçları A numunesinin, numune çözeltiye dal dırıldıktan sonraki ilk gün içinde bozunmaya başladığını göstermiştir. 72 saatlik çözeltide tutma süresinin so nunda kaplama yüzeyinde yaklaşık % 50'lik bir korozyon alanı oluştuğu gözlenmiştir. Aynı zamanda kaplamanın bozunmamış kısımlarınında su emişi A. A empedans verileri ile tespit edilmiştir. B ve C numuneleri çözeltiye daha dayanıklım gözükmelerine rağmen B numunesi çözeltiye dal dırıldıktan 73 saat, C numunesinin ise 30 saat sonra bo zunmaya başlamıştır. B numunesinde 120 saatte ve C nu munesinden de 72. saatte korozyon ürünü birikmesi görül müştür. Deney sonunda B(40B.saat) ve C(2BB.saat) numune lerinin yüzeyinde A numunesine nazaran daha az aktif ko rozyon alanı oluşmuştur. C numunesinin bozunmamış kısım larında karıncalanma da gözlenmiştir. Bu numunelerle daha önce yapılan 15 gün süreli BS 3900:Part 3: 1 9B6-IS0-1 9B4 »e uygun deneylerde A numune si deneyi başarılı olarak geçemez^ken, B ve C numune leri deney koşullarında direnç göstermişlerdir. Alter natif akım (A. A) empedans deneyleri ise, bu numunelerle ilgili kantitatif değerler vererek standardın sağlaya madığı bilgiler ile numunelerin aslında yeterli dirençte olmadıklarını kanıtlamıştır. Alternatif akım (A. A) empe dans ölçümleri organik kaplamalarla ilgili olarak kısa sürede kantitatif değerler verebilme üstünlüğüne sahip tirler.

Alternative current (A.C) impedance technique has became popular in characterization of the performance of organic coatings. The importance of this technique in characterization of organic coating performance arouses from the fact that they can be used as short term tests for determination of the parameters effecting coating performance such as coating structure, environment, base metal surface characteristics. Furthermore by using these techniques it is possible to achieve more detailed information on kinetics and mechanism of reactions taking place at the double layer, by modelling the equivalent circuit of the surface-solution interface. In this study theoretical background of A.C. impe dance techniques was investigated in detail and the literature related with the applications of this technique to corrosion and coating performance were reviewed and the performance of three different bituminous coatings were studied experimentally by using A.C. impedance : technique. The bituminous coatings studied were from three different commercial origin. The two of them were white spirit solvent based (B and C) and the third was gasoline solvent based and contained sodium silicate additive (A). The coatings were applied on carbon steel substrate plates (50x50x2mm). Before application of the coating the substrates were wet ground up to 220 grit with SiC grinding paper then degreased with xylol. Coatings were applied by an applicator to give a dry film thickness of 30+ 5 jjm. The coated specimens were dried 7 days at room temperature. Vll After drying jthe specimens mere clamped between PVC plates using neoprene seals giving an exposed area of 20 cm2;. A PVC tube of 70mm diameter was used as the cell. This tube was glued on top of the upper PUC plate. Experiments were conducted open to air, at room temperature. The test solution was 3 % sodium chloride solution. Test solutions were prepared by using dis tilled water and reagent grade chemicals. A.C. impedance measurement system consisted of a EG&G Model 273 Potentiostat, Model 5301 Lock in Ampli fier which is controlled through an Apple lie computer by using M 36B A.C impedance softwere (developed by EG&G Inc.). This software was also used in analyzing the A.C. impedance spectra obtained. The low frequency and high frequency spectrum was acquired by using fast Fourier transform (FFT) and lock- in techniques, respectively. Experimental parameters were: * A.C amplitute : 5 ml/ 5 * Frequency range: 10 - 0.03 Hz. * D.C. Potential : Open circuit A.C. impedance measurements were carried out daily in a 15 day test period. After achieving the time dependent A.C. impedance specturm of the speciments, the data were plotted in Bode, Nyquist, Z' - wZ" forms and from these plots resistances (charge transfer R. and pore Rjv-, pore), capacitance (coating C and double layer Cöl) were cal culated. P vxn The time dependent results of A.C. impedance experiments in relation with the performance of three different bituminous coatings used in this study can be summarized as follows: For coating A: Charge transfer resistance mas -, 3x10 ohms after 1 hour and it fell down to 1.3x10 ohms in 72 hours. Pore transfer resistance was 1.8x10 ohms after 1 hour and it fell down to 9.5x10 ohms in 72 hours Double layer capacitance was 1.6 uf after 1 hour and it reached to 245 uf after 72 hours. Coating capacitance was 0.6 nf after 1 hour and it reached to-95 nf after 72 hours. For coating B: Charge transfer resistance was -, 3.6x10 ohms after 1 hour and it fell down to 6.6x10 ohms in 240 hours. Pore transfer resistance was 1.0x10 ohms after 1 hour and it fell down to 1.2x10 ohms in 240 hours. Double layer capacitance was 0.2 uf after 1 hour and it reached to 41 uf after 240 hours. Coating capacitance was 0.3 nf after 1 hour and it reached to 34 nf after 120 hours. It fell down to 1.7 nf after 240 hours. c For coating C: Charge transfer resistance was 8.5x 10 ohms after 1 hour and it fell down to 3x10 ohms in 216 hours. IX Pore transfer resistance was 1.5x10 ohms after 1 hour and it fell down to 1.4x10 ohms in 216 hours. Double layer capacitance was 0.6 uf after 1 hour and it reached to 6 uf after 216 hours. Coating capacitance was 0.6 nf after 1 hour and it fell doun to 0.3 nf in 25 hours, and then reached to 2.5 nf after 216 hours. The results showed that coating A had lost its protective properties after a period that can be des cribed as hours that can easily be seen from the decrease of the resistances which are very low in comparison with the resistance values of protective coatings namely: min. 10 ohms. The results also showed that coating B had lost its protective properties after a period that can be describeb as "longer than that of coating A". Altough coating B had better protective properties than coating A, the initial polarization resistance of coating B was also very low in comparison with the resistance values of protective coatings namely: min. 10 ohms. For coating C the results showed that it had lost its protective properties after a period that can be described nearly as same as that of coating B, longer than that of coating A. The initial polarization resistance of coating C was also very low in comparison with the resistance values of protective coatings namely: min. 10 ohms. The impedance characteristics of coating C such as resistances and capasitance agree with those of coating B rather than those of coating A. The results of this investigation has shown that the performance of all of the investigated coatings were not satisfactory. Coating A lost its protective properties in hours, coatings B and C though showed better performance than A, they can not be described as satisfactory. "The equivalent circuit model of these coatings agreed with a two time constant model. The possibility of obtaining different charac teristics of coatings, such as resistances, capacitan ces uıith A.C. impedance technique uhich are the main parameters to characterize coating performance, makes this technique promising both for short term corrosion tests and mechanistic studies.